An Ecological Approach for
Reviewing CWA Section 404
Compensatory Mitigation
Projects in EPA Region 4
XV EPA
United States
Environmental Protection
Agency
Version 1.1
October 2023
EPA-904-D-23001
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An Ecological Approach for Reviewing CWA Section 404
Compensatory Mitigation Projects in EPA Region 4
Version 1.1
October 2023
William Ainslie, D. Eric Somerville, and Ashley Monroe
Wetlands and Streams Regulatory Section
Oceans, Wetlands and Streams Protection Branch
Water Division
EPA Region 4
Citation: Ainslie, W., D.E. Somerville, A. Monroe. 2023. An Ecological Approach for Reviewing
CWA Section 404 Compensatory Mitigation Projects in EPA Region 4. U.S. Environmental
Protection Agency, Region 4, Atlanta, GA. EPA-904-D-23001.
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Acknowledgements
Many dedicated and committed individuals contributed to this project from both the private and
public sectors. Given the breadth of topics discussed, this document benefitted tremendously from
the input of an interdisciplinary cadre of experts who provided their editorial and technical
expertise to take this effort from early concepts to the current product. Without their sustained
help this task would have been difficult, if not impossible, to accomplish. In particular, we would
like to thank Brian Topping of EPA Office of Water for his support of the project and early review
comments. We would also like to thank the project's technical reviewers for providing critical input
at every step of the process. Specifically, Dr. Eric Stein (SCCRWP), the late Dr. Rob Brooks (Penn
State University) and Dr. Curtis Richardson (Duke University) who provided constructive and
encouraging comments on the wetland aspects of the document. For the stream portion of the
document, Will Harman (Stream Mechanics) facilitated the review process and, along with Kevin
Tweedy (Ecosystem Planning and Restoration), provided experienced insights from a practitioner's
viewpoint. Dr. Brian Bledsoe (University of Georgia), and Dr. Art Parola (University of Louisville)
provided many cogent comments, suggestions for additional investigation, references and many
hours of discussion on topics related to stream ecology and restoration. The authors would also like
to thank Julia McCarthy (McCarthy Ecology) for her insights, thorough and thoughtful review, as
well as her expertise and diligence in editing and contributing to the document to make it more
readable and broadly applicable. This effort could not have been completed without their
participation and contributions.
Disclaimer: This document organizes technical and programmatic information to facilitate the
efficient review of compensatory mitigation proposals within the context of the Clean Water Act
(CWA) Section 404 program. It is not intended, nor can it be relied upon, to create any rights
enforceable by any party in litigation with the United States. Anyone may decide to use the
questions and information provided in this document, or not. The statutory provisions and
regulations described in this document contain legally binding requirements. This document does
not substitute for those provisions or regulations, nor is it a regulation itself. Thus, it does not
impose legally binding requirements on U.S. Environmental Protection Agency (EPA), the U.S. Army
Corps of Engineers (USACE), States, or the regulated community, and may not apply to a particular
situation based upon the circumstances. Any decisions regarding a particular compensatory
mitigation proposal (i.e., bank, in-lieu fee or permittee responsible) will be made based on the
statute and regulations. Therefore, interested parties are free to raise questions and objections
about the substance of these documents and the appropriateness of the application of these
documents to a particular situation.
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Table of Contents
Executive Summary 8
1.0 Introduction 15
2.0 Overview of the Approach 17
2.1 Background 17
2.2 The Mitigation Rule 18
2.3 The Six Questions Every Mitigation Reviewer Should Ask 19
2.4 Addressing Uncertainty and Risk 22
3.0 What is the Current Aquatic Resource Class? 25
3.1 Regulatory Context 25
3.2 Wetland Classification 26
3.2.1 Rationale 27
3.2.2 Determining Wetland HGM Class 28
3.2.3 Using Soil Information in HGM Classification 30
3.3 Stream Characterization 31
3.3.1 Rationale 32
3.3.2 Watershed and Landscape Context 33
3.3.3 Flow Permanence 34
3.3.4 Geomorphic Context 35
4.0 What is the baseline condition? 41
4.1 Regulatory Context 43
4.2 Watershed-scale Baseline Assessment (Wetlands and Streams) 44
4.3 Use of Rapid Assessments in Evaluating Baseline Condition 50
4.4 Wetland Baseline Condition (Site-scale) 51
4.4.1 Wetland Assessment 52
4.4.2 Wetland Site Sampling Considerations 57
4.5 Stream Baseline Condition (Riparian and Site-scale) 59
4.5.1 Stream Assessment 60
4.5.2.1 Hydrologic Condition Analysis 63
4.5.2.2 Hydraulic Condition Analysis 67
4.5.2.3 Geomorphic Condition Analyses 70
4.5.2.4 Physicochemical (Water Quality) Condition Analysis 74
4.5.2.5 Biological Condition Analysis 75
5.0 What is the target condition? 78
5.1 Regulatory Context 79
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5.2 Goals and Objectives 80
5.2.1 Wetland Goals and Objectives 81
5.2.2 Stream Goals and Objectives 82
5.3 Reference 83
5.3.1 Reference Approaches Useful in Compensatory Mitigation 86
6.0 How will the mitigation design achieve the target condition? 90
6.1 Regulatory Context 91
6.2 Wetland Design and Implementation to Achieve Target Conditions 91
6.2.1 Hydrology 92
6.2.2 Soils 94
6.2.3 Vegetation 96
6.3 Stream Design and Implementation to Achieve Target Conditions 97
6.3.1 Addressing Hydrologic Connectivity 100
6.3.2 Addressing Hydraulics and Sediment Transport 101
6.3.3 Addressing Channel Geomorphic Processes 104
6.3.4 Restoring Physicochemical Attributes 110
6.3.5 Restoring Biological Attributes Ill
7.0 How will progress toward the target condition be assessed? 114
7.1 Regulatory Context 116
7.2 Performance Standards 117
7.3 Monitoring 119
7.4 Wetland Performance Standards and Monitoring 121
7.5 Stream Performance Standards and Monitoring 126
8.0 How will the site be managed to sustain the target condition? 131
8.1 Regulatory Context 132
8.2 Maintenance Plan 132
8.3 Adaptive Management Plan 133
8.3.1 Examples of Conditions Requiring Adaptive Management 133
8.3.2 Beaver 134
8.4 Financial Assurances Plan 134
8.5 Long-Term Management Plan 135
8.6 Site Protection Mechanism 135
9.0 Literature Cited 136
Appendix A. EPA Region 4: Information to Consider for Wetland Mitigation Projects 152
Appendix B. EPA Region 4: Information to Consider for Stream Mitigation Projects 157
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Figures
Figure ES-1. Information needed to address the six questions in the Approach for wetlands and
streams, with the relevant section in the document discussing background and rationale.
Figure 1. Relationship of the twelve elements required by the Mitigation Rule for a complete
mitigation plan with the six basic questions fundamental to the Approach. Note that the
determination of credits (Element #5) is not directly addressed in the Approach.
Figure 2. Approach for reviewing CWA Section 404 compensatory mitigation projects in Region 4.
Figure 3. Stream Evolution Model (SEM) stages (reproduced from Cluer and Thome, 2014).
Figure 4. Spatial scales, relevant stream functions and corresponding indicator parameters for those
functions. Stream functions and categories from Fischenich (2006); reproduced from David et
al. (2021).
Figure 5. Reference domains of HGM regional guidebooks in the southeast and elsewhere in the
United States (reproduced from ERDC HGM website).
Figure 6. Stream Functions Pyramid overview (reproduced from Harman et al., 2012).
Tables
Table 1. Twelve elements required by the Mitigation Rule to be included in a mitigation plan.
Table 2. Summary of review objectives of each of the six questions in the Approach.
Table 3. Hydrogeomorphic wetland classes at the continental scale (after Smith et al., 1995).
Table 4. Key to typical HGM classes in the southeastern U.S (after Klimas et al., 2004 and Wilder et
al., 2013).
Table 5. General descriptions of stream types and their criteria for Level 1 classification (Rosgen,
1996). Note that sinuosity and slope (provided as typical ranges) are secondary classification
factors and should not limit the use of specific stream type.
Table 6. Channel evolution model and stream evolution model stages with description of reach-
averaged characteristics (Cluer and Thome, 2014) and corresponding Rosgen classes (Johnson
et al., 2015).
Table 7. HGM regional guidebooks developed in and applicable to assessing the functions of
wetlands in the listed subclasses in the southeastern U.S.
Table 8. Potential functions performed by HGM wetland classes in the southeast U.S. (R = Riverine;
MF = Mineral Flat; OF = Organic Flat; S = Slope; D = Depression; LFr = Lacustrine Fringe; TFr =
Tidal Fringe).
Table 9. Typical impairments affecting wetland classes of the southeast U.S. (after Wilder et al.,
2013). (R = Riverine; MF = Mineral Flat; OF = Organic Flat; S = Slope; D = Depression; LFr =
Lacustrine Fringe; TFr = Tidal Fringe.)
Table 10. List of primary stream and riparian functions and their interrelationships (Fischenich,
2003; 2006). Reproduced from Somerville (2010).
Table 11. Common stressors affecting stream hydrologic regime (after Johnson et al., 2015).
Table 12. Common anthropogenic stressors contributing to total sediment regime in streams (after
Johnson et al., 2015).
Table 13. Field indicators of channel incision, aggradation, and bank erosion (after Phillips, 2013)
that can help identify SEM/CEM stage.
Table 14. Common stressors affecting stream stability (after Johnson et al., 2015).
Table 15. Common stressors affecting stream biota (after Johnson et al., 2015).
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Table 16. Examples of potential methods to improve wetland water components by HGM class.
(ET= Evapotranspiration: a combination of the evaporation and transpiration terms of the
water budget equation representing water losses to the atmosphere).
Table 17. Potential stream process restoration methods and design considerations to address
stream problems (after Roni and Beechie, 2013).
Table 18. Restoration actions (Roni and Beechie, 2013) or practices (Bledsoe et al., 2016) typically
used to alleviate common stream impairments along with examples of indicators which can
be used to assess the progress of restoring physical (hydrologic, hydraulic, geomorphic, water
quality) and/or biological processes and functions.
Boxes
Box A. Recommended information to inform HGM wetland classification.
Box B. Recommended information to address stream characterization.
Box C. Recommended information to evaluate watershed-scale stressors.
Box D. Recommended information to evaluate wetland baseline condition.
Box E. Recommended information to evaluate stream baseline condition.
Box F. Recommended information to identify appropriate target condition.
Box G. Recommended information to address appropriateness of wetland mitigation plan/design.
Box H. Recommended Information to address appropriateness of stream mitigation plan/design.
Box I. Recommended information to address performance standards and monitoring.
Box J. Information to assess adequacy of wetland and stream project management.
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Executive Summary
The planning and implementation of wetland and stream compensatory mitigation proposals is
complex and fraught with uncertainty and risk due to natural and anthropogenic factors. This
uncertainty and risk have been highlighted by various scientific studies dating back to the 1980's. In
2001, the National Research Council (NRC) in their study entitled Compensating for Wetland Losses
Under the Clean Water Act concluded that the Clean Water Act (CWA) Section 404 Program was not
achieving the goal of No Net Loss of wetland functions through the use of compensatory mitigation
and provided a series of recommendations to improve project and program performance. Although
the NRC did not address stream mitigation, other studies done subsequently indicate similar
challenges for stream mitigation projects and program performance (ELI et al., 2016).
To address shortcomings and update aspects of aquatic resource compensatory mitigation in the
CWA Section 404 program, the USACE and EPA jointly promulgated the Compensatory Mitigation
for Losses of Aquatic Resources; Final Rule (Mitigation Rule) in 2008. These regulations provide a
sound regulatory basis for establishing policies and equivalent information requirements for
compensatory mitigation provided by mitigation banks, in lieu fee programs, and permittee-
responsible mitigation proposals. However, despite improvements in the mitigation proposals,
there is still a considerable amount of natural and programmatic variability which continues to
contribute to uncertainty and risk associated with wetland and stream compensatory mitigation
projects throughout the country. This document outlines an approach (referred to herein as the
Approach) which attempts to lower uncertainty and risk associated with compensatory mitigation
projects by recommending more detailed and focused information during the mitigation project
review in the form of aquatic resource classification, specific identification of stressors, quantifiable
objectives, explicit design criteria connected to the quantifiable objectives, and post-project
monitoring and long-term management.
This Approach represents a distillation of the
12 elements found in the Planning and
Documentation, Ecological Performance
Standards, and Monitoring and Management
sections (40 CFR §230.94-230.97) of the
Mitigation Rule into six basic questions to
facilitate regulatory review of wetland and
stream mitigation proposals. The objectives of
the Approach are to frame the 12 elements in
a logical and ecologically meaningful way, and
to provide reviewers a consistent, ecologically
based approach to mitigation project reviews
so they can effectively evaluate any given wetland and/or stream project proposal for its
sustainable ecological potential. The goal is to promote ecological sustainability and resilience of
compensatory mitigation projects and enhance EPA's role as a member of USACE District
Interagency Review Teams (IRTs). It is not the intent, or within the ability of this document, to
provide a complete treatise on the detailed technical and policy aspects of reviewing aquatic
resource compensatory mitigation plans. Nor is it our intent to supersede USACE District templates
for presenting information in mitigation banking, in-lieu fee and permittee-responsible mitigation
documents.
The structure of this document follows the sequence of questions, or steps, that are proposed to
provide an ecological perspective on compensatory mitigation plan review and the links of those
Central themes of this Approach:
• Focusing on appropriate site selection,
• Incorporating goals and objectives
connected to an explicit target condition,
• Monitoring specific, objective ecological
performance standards,
• Developing sustainable and resilient
wetland and stream mitigation projects,
and
• Using reference data or discrete sites in
project planning and evaluation.
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steps to the Mitigation Rule. These questions consider regional variations in conditions, functions,
and services, and address how to apply equivalent standards and criteria to compensatory
mitigation in streams and wetlands. The questions focus on classification, baseline and target
condition, implementation plans, monitoring, performance and long-term management. Each
section of the Approach addresses one of the six questions for both wetlands and streams, and
includes detailed rationale, links to the Mitigation Rule, and recommended technical information
and resources to help frame and provide useful information in answering each question.
To efficiently utilize this document, Figure ES-1 provides a breakdown of each question, the section
it is in, and the essential wetland and stream information recommended to address each question.
In addition, composite lists of all the information recommended by the Approach can be found in
Appendix A for wetlands and Appendix B for streams.
As outlined in Figure ES-1, the six questions that frame the review of any potential compensatory
mitigation site are:
(1) What is the current class?
(2) What is the baseline condition?
(3) What is the target condition?
(4) How will the mitigation design achieve the target condition?
(5) How will progress toward the target condition be assessed?
(6) How will the site be managed to sustain the target condition?
Question 1: What is the current class? (Aquatic resource classification)
For the purposes of this Approach, classification is a means of organizing and understanding
complex information about streams and wetlands to bring a semblance of order to natural
variability, to promote better communication in the management of aquatic resources, to focus
information gathering and ensure appropriate ecological site context. To this end, classification is a
critical first step in the mitigation project review process to provide a conceptual understanding of
wetland and stream ecological processes, and fundamentally set the stage for the effective
implementation of the proposed mitigation project.
Aquatic resource classification should be provided early on in mitigation project planning, as it
supports site selection, goals and objectives and other required elements. For wetlands, a project
proposal should identify the appropriate wetland class(es) on site, preferably using the
Hydrogeomorphic (HGM) wetland classification, as it establishes a link between landscape position
and hydrology to wetland processes and functions and is widely recognized as an appropriate
wetland functional classification upon which to base assessment and mitigation planning.
Classification should be supported by hydrogeomorphic (i.e., geomorphic position in the landscape,
water sources, and hydrodynamics) and soils data, including landscape information, Web Soil
Survey data, current HGM regional guidebook descriptions and keys, and any other available
information.
For streams, project proposals should classify/characterize streams using a combination of
approaches to focus information for mitigation planning. Given that fluvial systems are constantly
adjusting and evolving in response to changes in sediment supply, channel and floodplain
geometry, floods, groundwater hydrology, vegetation, woody debris, fauna (both natural (e.g.,
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Figure ES-1. Information needed to address the six questions in the Approach for wetlands and streams,
with the relevant section in the document discussing background and rationale.
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beaver) and introduced (e.g., livestock)), as well as other historic and contemporary natural and
anthropogenic factors that affect the stream system, one or more classification approaches will be
needed to provide ecological context. This Approach outlines several methods for characterizing
streams, including ecoregion, valley type, Strahler stream order, flow permanence, Rosgen
classification, and stream evolutionary stage. Flow permanence provides information on water
source and flow duration while valley type, stream order and ecoregion provide information about
landscape position in the watershed, and geology. Rosgen stream class provides information about
the reach's geomorphic condition, and stream evolution stage conveys information about a
channel's response to disturbance and/or where that channel is along a geomorphic and ecologic
disturbance gradient.
Question 2: What is the baseline condition? (Baseline assessment/monitoring)
This question focuses on the information needed to characterize the baseline condition and the
level and source of stressors affecting the proposed mitigation site. Establishing the baseline
condition and identifying stressors acting on the proposed site provides insight into factors
requiring amelioration to re-establish function. These factors would then need to be addressed
and/or considered as limitations in the mitigation project.
A site's baseline condition, or current functional capacity, is affected by factors operating at a
watershed scale as well as those operating at the site or reach scale. Evaluating information at a
watershed scale considers stressors and land uses outside the project boundaries that can affect
the site's restoration potential, including surrounding land uses, landscape connectivity, and
hydrologic potential (i.e., frequency, duration, magnitude, and timing of flows).
Stressors acting at a site-specific scale can also affect the condition and restoration potential for a
mitigation site, and the level and sources of degradation should be assessed as part of baseline
characterization. Site specific rapid assessments could be used in the baseline assessment but
should be able to link current condition and/or function to the stressors impacting those functions
and should be supplemented by site specific and watershed data.
Important elements of a baseline assessment include evaluation of watershed and reach or site-
scale stressors, and any necessary measurements to characterize the existing conditions on site and
inform project design/performance standards. For wetlands, baseline assessments should include
site-specific vegetation, soils and hydrologic data. For streams, baseline assessments should include
characterization of site-scale hydrologic, hydraulic, geomorphic, physicochemical and biological
processes. Baseline data represent the starting condition of the site that will be compared to
monitoring data in the future to ascertain the degree to which site conditions improve as a result of
the mitigation project (i.e., level of performance). This information should also be compared to
either existing reference data or reference data collected by the mitigation provider to establish the
site's relationship with a reference population.
Question 3: What is the target condition? (Goals and objectives/reference)
The intent of this question is to understand the target condition of a site. Target condition, or the
projected endpoint, establishes the functional expectations for the site (goals), specific actions
(measurable objectives) which will be implemented and tracked to assess progress towards
achieving goals, and identifies the reference(s) used to inform target condition. Goal statements are
used as the enunciation of the vision for the project, and are broad statements of the intended
outcome, or expectations of the mitigation project, including a list of the functions to be provided
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by the mitigation site. Objectives represent the actions needed to achieve the goal. Objectives
should be SMART (i.e., Specific, Measurable, Achievable, Relevant, and Time bound) so that
progress on the project can be effectively tracked. These objectives should then be tied to the
performance standards. Reference comparisons are used to provide a benchmark for the end point,
and reference standard condition data are often derived from analog comparisons with high quality
sites, regional curves or other empirical equations, analytical models, and regional indices that
correspond to the appropriate aquatic resource class and/or landscape setting.
Important elements to inform target condition include clear, function-based expectations for the
outcome of the project (goals) given aquatic resource class(es) and the level of disturbance and
stressors acting upon the site, quantifiable actions (objectives) needed to ameliorate the stressors
and achieve target condition, and relevant reference comparisons. Establishing target condition is
highly dependent on identifying the watershed and/or site-scale problems that may be affecting
functional capacity. For example, in streams, watershed-scale stressors may affect hydrology and
sediment routing that may explain changes in stream condition; and this information needs to be
translated into goals and objectives that inform an appropriate design (Roni and Beechie, 2013).
Question 4: How will the mitigation design achieve target conditions? (Mitigation workplans)
This step ensures mitigation work plan/designs describe the proposed actions that will alleviate the
impact of stressors and improve ecosystem functions at the mitigation site consistent with the
stated goals and objectives and within site and watershed constraints. Details of any mitigation
work plan will vary on a case-by-case basis and should be tailored to address the identified
objectives.
Important elements of an effective mitigation plan include appropriate consideration of site
conditions, landscape context, and site constraints due to surrounding land use and watershed
conditions; and an understanding of the causes and levels of degradation, the effects a particular
restoration method or approach might induce, as well as how to make modifications or refinements
to the plan (i.e., adaptive management). Specific methods or approaches proposed in the workplan
should be informed by baseline conditions and function-based goals and objectives that are
appropriate given the aquatic resource class, landscape potential, watershed and site constraints,
and logistics and have the potential to move the site from its current condition to the proposed
target condition. Some restoration approaches are more commonly used than others (e.g., Natural
Channel Design (NCD), planting specific forested species), and some providers utilize methods
because of their familiarity with the method as opposed to the ability of the method to address a
stressor. All methods, or sets of methods, should be considered so long as they have the potential
to achieve the desired result.
Question 5: How will progress toward the target condition be assessed? (Performance standards
and monitoring)
This question highlights information needed to determine the ecological success of a mitigation
project, including performance standards and monitoring plans. Performance standards are based
on the specific goals and objectives of the mitigation project, and monitoring is used to determine if
a project is meeting its performance standards. Each mitigation project will have a potentially
unique set of specific performance standards that are applicable for the class or subclass of aquatic
resource and are tied to the specific mitigation project goals and objectives. Generally,
performance standards reflect a range of wetland functions (e.g., wetland hydrologic storage,
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biogeochemical cycling and plant and animal habitat) and/or stream functions (e.g., stream
hydrologic connection, hydraulic maintenance, geomorphologic form) by measuring field
parameters which are sensitive to changes brought about by the mitigation plan and tied directly to
a project objective.
To help determine the effectiveness of a provider's performance standards, reviewers should
evaluate whether they generally address the questions of: "what is measured"; "how is it
measured"; "when is the level of performance required met" and "Does the proposed performance
standard effectively represent a target condition or function?" Performance standards are
appropriate when they are informed by a reference comparison, described using clear and concise
wording, scientifically defensible, and supported by data collection methods that are appropriately
timed, repeatable and with clearly defined error rates.
Given the inherent variability and highly dynamic nature of streams, it is difficult to establish a
static set of performance standards for a given point in time (e.g., achieve reference planform by
year five). The choice of performance standards depends on the results of the classification,
baseline assessment, the objectives of the project, and the actions required to achieve the
objectives. The ultimate goal of any stream compensatory mitigation project is to enhance,
establish, or re-establish hydrologic, hydraulic, geomorphic, physicochemical, and biological
function to the stream proposed as compensatory mitigation. Again, there are a wide variety of
methods which can be brought to bear on problems a stream may be having. Therefore, measuring
the ability of any method(s) to improve stream functions will depend on the information collected
previously in this Approach, the parameters chosen, and the availability of comparable reference
stream information for each function.
Monitoring can be split into three phases: baseline, implementation, and effectiveness monitoring.
Baseline monitoring is addressed during the baseline condition assessment for both wetlands and
streams. Implementation monitoring is typically accomplished by comparing as-built plans with the
100% design drawings and design criteria presented in the mitigation plan to verify the project was
actually constructed as designed and approved by applicable permitting authorities. Effectiveness
monitoring evaluates the project's effect on resource indicators (e.g., wetland habitat conditions or
stream geomorphology) and whether the project achieved its goals and objectives and target
condition. Effectiveness monitoring is directly related to performance standards, and will inform
credit releases, project closeout and whether adaptive management measures are needed.
Generally, monitoring plans should clearly and concisely describe the parameters to be assessed;
where and how often each will be measured; specific methods and/or protocols for measuring each
parameter; the resources (e.g., time, money, equipment, and expertise) needed; how data will be
analyzed and interpreted with respect to objectives and performance standards; the level of
statistical rigor to determine treatment effectiveness; and how data will be made accessible and
understandable for decision-making regarding achievement of performance and/or adaptive
management. Clarity in the monitoring plan is important since different parties may be involved in
monitoring a site over the long-term. Personnel changes, on both the provider's team as well as
among the IRT, dictate that monitoring plans be clearly written and organized to promote accurate
interpretation at any point during the life of the project. A 10-year monitoring period may be
warranted for aquatic resource compensatory mitigation projects that develop more slowly (e.g.,
forested wetlands and/or riparian areas), unless performance can be demonstrated in a shorter
period of time.
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Question 6: How will the site be managed to sustain the target condition? (Maintenance plans,
adaptive management plans, financial assurances, long-term management plans, and site
protection instruments)
This question outlines administrative components of a mitigation plan, including site protection
mechanisms and financial assurances, as well as maintenance plans, long-term management plans
and adaptive management plans.
The maintenance plan accounts for activities that will need to take place on the project site for the
duration of the monitoring period and should support the goal of making the compensatory
mitigation site ecologically sustainable and resilient. Maintenance plans describe and outline a
schedule of maintenance requirements (e.g., maintain or replace monitoring equipment, replant
dead vegetation, maintain access routes to monitoring stations, and those required for field
inspections) to ensure the continued viability of the resource once initial construction is
completed. Reviewers should ensure that the maintenance plan is included and complete.
Preparing for adaptive management means developing a management strategy that anticipates
likely challenges, but provides flexibility to accommodate unforeseen changes, associated with the
project and provides for the implementation of actions to address those challenges. Adaptive
management is considered a hedge against the risk, uncertainty, and dynamic nature of
compensatory mitigation projects (i.e., will what the provider is undertaking work?), and guides the
process by which modifications to those projects are implemented to optimize performance. Plans
should be linked both to a provider's and reviewer's experience with potential problems and/or
monitoring results indicating that attainment of performance standards has been impeded, and
how such problems could be rectified. Adaptive management plans should address a range of
issues, including but not limited to climate (e.g., flood, drought, and hurricanes), trespass, invasive
species issues, and beaver.
Financial assurances require providers to set money aside for contingencies and are an important
mechanism for managing risk of project failure, including failure to complete the project, to meet
performance standards, or to maintain the project. Holding all forms of compensatory mitigation to
equivalent standards, these financial assurance plans are typically worked out between the
mitigation provider and the USACE and cannot be modified without prior notice to the USACE.
Long-term management plans outline and describe how the compensatory mitigation project will
be managed after performance standards have been achieved to ensure the long-term
sustainability of the resource. This is a very important aspect of the compensatory mitigation
program since many ecosystem functions will likely not fully develop within the typical monitoring
period. Therefore, the long-term management plan should account for the continued maturation of
the compensatory mitigation site and development of stream and/or wetland functions.
The site protection mechanism is a description of the legal arrangements and instrument, including
site ownership, that will be used to ensure the long-term protection of the compensatory
mitigation project site (40 CFR §230.97(a)). The long-term protection of compensation sites should
be arranged through appropriate real estate instruments, such as conservation easements,
restrictive covenants, or transfer of title to public or private land managers. The real estate
instrument should restrict or prohibit incompatible uses (e.g., clear cutting, mineral extraction, all-
terrain vehicle access).
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1.0 Introduction
Aquatic resource compensatory mitigation is seldom
straightforward. Ecosystem restoration projects have
been characterized as exercises in approximation (NRC,
1992). Many, if not all projects are burdened with
uncertainty due to vagaries in project implementation
and the inherent complexity, dynamics, and variability of
natural ecosystems. Project implementation can vary as
a result of the methods used to carry out the project, as
well as the implementation procedures unique to each
U.S. Army Corps of Engineers (USACE) District (e.g.,
Standard Operating Procedures (SOPs), credit
calculations, and service area assignments) and State. This document outlines an approach
(herein referred to as the Approach) to address general aspects of ecological requirements within
the context of the Mitigation Rule to provide a consistent means of guiding EPA's ecological review
of Clean Water Act (CWA) Section 404 compensatory mitigation plans. This Approach was initially
developed for use in EPA Region 4 (Kentucky, Tennessee, North Carolina, South Carolina, Georgia,
Florida, Alabama and Mississippi), and contains regionally specific references and examples. The
questions in the Approach, however, can be applied nationally with appropriate consideration of
regionally specific references, policies and resource constraints.
This document describes an ecological approach intended to assist EPA staff in their role as
contributing members of Interagency Review Teams (IRT), and lays out a technical, objective
framework based on the purpose, goals, and requirements of the CWA and the Section 404(b)(1)
Guidelines, as clarified and enhanced by the Mitigation Rule. The Approach provides a process by
which EPA staff can and should consider the watershed context and site-specific elements spelled
out in 40 CFR §230.93(c) in an ecological context and will aid the consistent review of mitigation
proposals in regulatory settings, including mitigation banking prospectus and instruments, in-lieu
fee (ILF) instruments and compensation planning frameworks, ILF mitigation plans, and permittee-
responsible mitigation plans. In addition, the Approach may support the compilation of EPA
comments on individual CWA Section 404 permit actions, development of regional conditions for
Nationwide Permits and/or regional general permits, and formulation and review of mitigation
plans for enforcement actions. Finally, this Approach could also be used to inform development
and/or improvement of (SOPs, guidelines, or agreements at the USACE District, state, or local level
for compilation and evaluation of mitigation proposals.
Specific objectives of this Approach are to:
• Evaluate stream and wetland projects using the same review process;
• Use aquatic resource classification to inform appropriate mitigation site selection;
• Ensure that mitigation project designs incorporate important ecological processes;
• Ensure that mitigation project designs account for hydrologic and ecologic landscape
connections, (i.e., the watershed context); and
• Ensure that mitigation plans include performance standards and monitoring tied directly to
project objectives.
Neither this document, nor the Approach itself, establish new requirements in addition to those of
the CWA and regulations promulgated thereunder, or any other applicable statute. Nothing herein
Underlying this Approach is EPA
Region 4's commitment to the
Section 404 (b)(1) Guidelines (40
CFR §230) and the sequential
requirement to first avoid, and
then minimize impacts to aquatic
resources to the maximum extent
practicable before proposing
compensatory mitigation (40
CFR §230.91).
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shall be deemed to expand or restrict the authorities of EPA. The Approach does not create or alter
any legal rights, requirements, or benefits, nor is it intended to address all factual scenarios that
may arise on a case-by-case basis. This ecological approach is presented as an effort to improve the
outcomes of compensatory mitigation for aquatic resources, and as such will likely be revised as
more experience is gained. It is our intent that this will improve compensatory mitigation
approximations and lead to the design of more successful and sustainable projects that lessen
impacts associated with the Section 404 program, and better address the uncertainty associated
with those projects.
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2,0 Overview of the Approach
2.1 Background
In 1990, with the signing of the Mitigation Memorandum of Agreement, EPA and USACE agreed to
define mitigation as avoidance, minimization and compensation, although the focus since that time
has been primarily on compensation (Hough and Robertson, 2008). A report on the status of the
science behind wetland creation and restoration in 1989 concluded, among other things that:
project goals were often unclear; monitoring was uncommon; success varied with the type of
wetland and target functions; site conditions were often improper for the anticipated aquatic
resource; and problems with site success were often due to off-site impacts (Kusler and Kentula,
1989). Although this report is over 30 years old, the issues raised continue to persist.
In 2001, the National Research Council (NRC) published a review and critique of the CWA Section
404 wetland compensatory mitigation program's record in meeting the No Net Less goal originally
set by the interagency National Wetlands Policy Forum in 1988 and adopted by the President
George H.W. Bush administration in 1990. Their report summarized existing studies of wetland
compensatory mitigation sites nationwide and concluded that the program had not been successful
in meeting the No Net Loss goal. The NRC outlined numerous concerns with the CWA Section 404
compensatory mitigation program and determined that many compensatory mitigation sites failed
to adequately replace wetland functions lost as a result of impacts authorized by the CWA Section
404 program.
Generally, implementation of wetland mitigation projects in the southeastern U.S. has not been
effective at replacing lost wetland functions and could be improved by injecting more ecologically
based siting, design, and performance standards into the process; and by using reference sites to
help assess the development of conditions on compensatory mitigation sites (NRC, 2001; Morgan
and Roberts, 1999; Rheinhardt and Brinson, 2000; Reiss et al., 2007). Moreno-Mateos et al. (2012)
undertook an analysis of wetland restoration projects across the globe and found that many had
failed to restore functions, some even after decades of recovery time.
This Approach is predicated upon a number of central themes that emerged from the 2001 NRC
report, as well as wetland and stream restoration literature. These themes are:
• A focus on appropriate site selection,
• Incorporation of specific goals and objectives connected to an explicit target condition,
• Adequate monitoring of specific, objective ecological performance standards,
• Development of sustainable and resilient wetland and stream mitigation projects, and
• Use of reference data or discrete sites in project planning and evaluation.
While the NRC was not charged to evaluate stream restoration as a part of the CWA Section 404
compensatory mitigation program, more recent efforts by the Environmental Law Institute, Stream
Mechanics and the Nature Conservancy (ELI et al., 2016) identified several gaps in implementing
stream mitigation programs, including the need to integrate functional lift into the mitigation
program, more guidance on the watershed approach and linking project goals to watershed goals,
the need for improved data for site selection, more guidance on monitoring for adaptive
management and improving program success, aligning flexibility versus prescribed approaches, and
a need to better align regulator and ecological goals. Bernhardt et al. (2005) compiled data on over
37,000 stream restoration projects nationwide in a National River Restoration Science Synthesis
database. Subsequent review and assessment of that data found that 20% of the projects had no
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stated goals and only 10% of project records included any form of assessment or monitoring
(Bernhardt et al., 2007). The authors concluded that a comprehensive assessment of stream
restoration progress in the U.S. was not possible with the piecemeal information currently
available- an assessment also reached by the Government Accountability Office in reports to the
U.S. Congress in 2002 and 2003 (GAO, 2002; 2003).
2.2 The Mitigation Rule
In their 2001 report, the NRC made several technical, programmatic, and policy recommendations
aimed at enhancing compensatory mitigation proposals, which lead to more ecologically successful
mitigation projects and improved project tracking. Many of these recommendations informed the
promulgation of a new CWA regulation in 2008 to address compensatory mitigation.
In 2008, the USACE and EPA promulgated regulations at 33 CFR §320 and 40 CFR §230, respectively,
to improve the CWA Section 404 compensatory mitigation program (Compensatory Mitigation for
the Losses of Aquatic Resources; Final Rule; Mitigation Rule). The Mitigation Rule lays out the policy
and information required by the USACE and EPA when determining the appropriateness of
compensatory mitigation for the CWA Section 404 program. It also sets policy for ensuring that the
sequencing provisions of the Section 404(b)(1) Guidelines are followed (i.e., avoid, minimize, and
compensate), and it provides a preferential hierarchy of mitigation types or mechanisms to satisfy
mitigation requirements: mitigation banks (banks), in-lieu fee (ILF) programs, and permittee-
responsible mitigation (PRM) projects. The Mitigation Rule also establishes equivalent
requirements, performance standards and criteria for use by all types of compensatory mitigation
to improve the quality and success of mitigation projects.
The Mitigation Rule discusses general compensatory mitigation requirements, including location,
type, and amount of compensation, the use of preservation, buffers, and riparian areas as
compensation, and the relationship with other federal programs (33 CFR §332.3; 40 CFR §230.93).
Further, the Mitigation Rule incorporates the recommendation of the NRC report, and other
scientific literature to use a watershed approach for selecting compensatory mitigation sites.
Fundamental considerations in a watershed approach include watershed scale; landscape position
and resource type; habitat requirements of important species; trends in habitat loss or conversion,
sources of watershed impairment; and current development trends.
In addition, the Mitigation Rule adopts recommendations that compensatory mitigation projects
have measurable, enforceable ecological performance standards and requires regular monitoring
for all types of compensation to verify achievement of stated objectives. Detailed mitigation plans
must be provided for both wetland and stream mitigation projects, and all projects must include
provisions for long-term management, long-term site protection mechanisms, financial assurances,
and identification of the parties responsible for specific project tasks.
The Mitigation Rule establishes 12 items, or elements, that must be included in every mitigation
plan, regardless of whether it is a bank, ILF project, or PRM site (40 CFR §230.94(c)). These 12
elements comprise the substantive information that the permittee (often a proposed Bank Sponsor
or ILF Sponsor) must provide in order to explain and justify their plan for a proposed mitigation
project and serve as the basis for project review by the USACE District Engineer and the Interagency
Review Team (IRT) agencies (Table 1).
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Table 1. Twelve elements required by the Mitigation Rule to be included in a mitigation plan.
Element
Description
Objectives
§230.94(c)(2)
A description of resource type and amount to be provided; the method of provision; and the
manner in which the resource functions of the compensatory mitigation project address the
needs of the geographic area of interest (i.e., watershed, ecoregion, physiographic
province, or other geographic area of interest).
Site Selection
§230.94(c)(3)
Description of factors considered during the site selection process.
Site Protection
Instrument
§230.94(c)(4)
Description of the legal arrangements and instrument, including site ownership, to be used
to ensure long-term protection of the site.
Baseline Information
§230.94(c)(5)
Description of the ecological characteristics of the proposed compensatory mitigation site
and, in the case of an individual permit application, the impact site.
Determination of
Credits §230.94(c)(6)
Description of the number of credits to be provided by the mitigation site, along with a brief
explanation and rationale for the determination.
Mitigation Work Plan
§230.94(c)(7)
Detailed written specifications and work descriptions for the compensatory mitigation
project, including but not necessarily limited to the geographic boundaries of the project;
construction methods, timing, and sequence; source(s) of water, including connections to
existing waters and uplands; methods for establishing the desired plant community; plans
to control invasive species; grading plan, including elevations and slopes; soil
management; and erosion control measures. Mitigation work plans are required for both
wetland and stream projects, and additional relevant information may be required for
stream projects (e.g., planform geometry, channel form, and design discharge).
Maintenance Plan
§230.94(c)(8)
Description and schedule of maintenance requirements to ensure continued viability of the
resource once construction is complete.
Performance
Standards
§230.94(c)(9) and
§230.95
Ecologically-based standards that will be used to determine whether the compensatory
mitigation project is achieving its objectives.
Monitoring
Requirements
§230.94(c)(10) and
§230.96
Description of the parameters to be monitored to determine if the compensatory mitigation
project is on track to meet its performance standards and whether adaptive management is
needed.
Long-Term
Management Plan
§230.94(c)(11) and
§230.97(d)
Description of how the compensatory mitigation project will be managed once the
performance measures have been achieved to ensure long-term sustainability of the
resource, including financial mechanisms to appropriately manage the site.
Adaptive
Management Plan
§230.94(c)(12) and
§230.97(c)
Management strategy to address unforeseen changes to site conditions or other
components of the compensatory mitigation project. The adaptive management plan will
guide decisions for revising compensatory mitigation plans and implementing measures to
address both foreseeable and unforeseen circumstances that adversely affect
compensatory mitigation success.
Financial Assurances
§230.94(c)(13) and
§230.93(n)
Description of the financial assurances that will be provided, and how they are sufficient to
ensure a high level of confidence that the compensatory mitigation will be completed in
accordance with its performance standards.
2.3 The Six Questions Every Mitigation Reviewer Should Ask
The Approach re-arranges, and in some cases re-sequences, the 12 elements required under the
Mitigation Rule into a series of six questions to facilitate a more logical sequence and progression
for project review and to frame the elements in an ecological context (Figure 1). For example, the
first element to consider in the Mitigation Rule is the objective(s) of the project. However,
understanding the type of aquatic resource, its current condition, and the impairments contributing
to that current condition are needed to identify and evaluate project objectives. Thus, these
questions are presented first in the Approach. Only then can the objectives of the mitigation plan
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be considered. The Approach aims to account for watershed context and regional variability while
providing objective and consistent comments between multiple USACE Districts and among all
mitigation types (banks, ILF, and PRM).
Requirements of the Mitigation Rule Six Questions
1, Objectives 1. What Is the current aquatic resource class?
1, Site Selection
3. Site Protection \ x "^-^7 What is the baseline condition?
4. Baseline Information ^
5. Determination of Credits V<\X\ " ^ 3« what is the Ur*« condition?
V
6. Mitigation Work Plan
\ "V 4. How will the mitigation design achieve
7. Maintenance Plan x target conditions?
\
8. Performance Standards —. \
9. Mnnitnrinv ..
9. Monitoring — _ .. _>v'-Lr-r.:x 5. How will progress toward the target
\ condition be assessed?
10. Long-term Management
V
11, Adaptive Management
6. How will the site be managed to maintain
12. Financial Assurances the target condition?
Figure 1. Relationship of the twelve elements required by the Mitigation Rule for a complete mitigation
plan with the six basic questions fundamental to the Approach. Note that the determination of credits
{Element #5) is not directly addressed in the Approach.
The Approach is an amalgamation of current ecological concepts and techniques drawn from the
scientific literature and current practice, with an emphasis on application in aquatic resources in
the southeastern U.S. (EPA Region 4). The Approach is rooted in six basic questions to be asked of
all proposed mitigation projects:
(1) What are the current aquatic resource class(es) within the proposed mitigation site?
(2) What is the baseline condition of these resources?
(3) What is the site's projected target condition?
(4) How will the mitigation design achieve the target condition (i.e., what key interventions are
necessary to achieve the target condition)?
(5) How will progress toward the target condition be assessed?
(6) How will the site be managed to sustain the target condition in perpetuity?
Gardner et al. (2009) noted that while the Mitigation Rule represents significant progress in
implementing compensatory mitigation in the CWA Section 404 program, its effectiveness will
depend on implementation in the field. By focusing the information required by the Mitigation Rule
in a more resource-type context and connecting the information provided by proposed mitigation
project providers and/or IRT agencies in a series of logical steps, the Approach could provide
consistency to project reviews, and lead to improved aquatic ecosystem enhancement,
establishment and restoration within the context of CWA Section 404 compensatory mitigation. By
first classifying the aquatic resources on site, the subsequent questions build on the classification
and expand the understanding of watershed and site scale functions and stressors and how they
may influence project design and ecological success.
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The fundamental construct of the Approach is illustrated in Figure 2. Essentially, when a site is
proposed, reviewers will want to first understand its current aquatic resource type, which provides
information about aquatic resource functions that should be assessed as part of the baseline
assessment and also provide the basis for goal and objective setting. Methods used for assessing
baseline condition should identify site impairments and stressors relative to appropriate reference
conditions, which affect the restoration potential of the proposed site and inform the development
of project-specific goals and objectives. A mitigation plan is devised to address project goals and
objectives, such as to alleviate impairments and stressors, and improve or return the expected
functions characteristic of the aquatic resource type given the watershed constraints. Within the
mitigation plan, a monitoring plan should be provided which identifies the methods and intensity of
sampling to establish that the target conditions, as measured by performance standards, have been
met and to verify the site has achieved its ecological objectives (e.g., that impairments and
stressors have been remedied or alleviated). If performance standards have not been met, then
adaptive management is implemented to correct the aspect of the original plan which proved
ineffective. Once the site has achieved, or is on a trajectory to achieve, the target condition, long-
term management to maintain the site can begin.
Proposed site
Long Term Monitoring and Management
Figure 2. Approach for reviewing CWA Section 404 compensatory mitigation projects in Region 4.
It is important to not only ask a question, but also to understand the basis, or rationale for asking.
The latter enables interpretation of the information presented to answer the question. Given the
variability between stream and wetland sites, watershed conditions, effects of perturbations,
experience of the mitigation provider, etc., a great deal of information will likely be needed to fully
and adequately answer the six basic questions in the Approach and assess the adequacy of that
information to support any given proposal. Understanding the rationale for why a question is being
asked (Table 2) will help to discern what information is appropriate to address the
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question/elements of the Mitigation Rule. Each of the six questions is posed not only to consolidate
information from the Mitigation Rule, but also to explicitly establish an ecological basis for
compensatory mitigation review of both wetland and stream compensatory mitigation projects.
Table 2. Summary of review objectives of each of the six questions in the Approach.
Question
Rationale (i.e., why are we asking?)
What is the current aquatic resource
class?
Organize natural variability; facilitate communication; focus information
gathering; and ensure appropriate placement of site in landscape.
What is the baseline condition?
Identify landscape- and site-scale stressors affecting site; and understand
existing site condition and functional capacity within the context of reference
aquatic resources.
What is the target condition?
Establish vision/goals for the site and objectives outlining how to achieve
those goals; place target condition in the context of existing condition,
reference aquatic resources, and appropriate reference standard resources.
How will the mitigation design
achieve the target condition?
Design projects to ameliorate landscape- and site-scale stressors affecting
the site and be sustainable (recognizing that some stressors may not be able
to be addressed).
How will progress toward the target
condition be assessed?
Monitor efficient measures of performance that indicate functional lift; ensure
sample locations and timing of sampling is representative of the whole site,
the processes acting on the site, and the effects of the mitigation action; and
ensure monitoring approaches can inform ecological performance standards
and the need for adaptive management.
How will the site be managed to
sustain the target condition?
Managing site conditions after mitigation plan implementation to allow site to
mature, be sustainable and continue to contribute ecological function to the
watershed; and adaptively manage if needed.
This Approach does not specifically address the production, value, costs, release, approval, or
accounting of credits. The determination of credits is a significant aspect of the Mitigation Rule (see
40 CFR §230.98(o)) and the current CWA Section 404 compensatory mitigation program. There is
wide variation in credit determination methods among USACE Districts, with some using
area/length-based ratios to determine the amount of compensatory mitigation needed to offset
impacts and others incorporating condition and/or functional assessments and/or estimates of
functional lift (ELI et al. 2016). These approaches are largely determined and established by USACE
District policy, with input from the IRTs, and are typically standardized within a District to decrease
uncertainty for the regulated community and increase consistency among District project
managers. Because this Approach focuses on determining ecological suitability, evaluating site and
watershed ecological conditions and site-specific planning, it only becomes relevant to credit
determination where District-specific approaches incorporate information on existing and/or
proposed site conditions or estimates of functional lift in their credit determination methods.
2.4 Addressing Uncertainty and Risk
This document attempts to lower uncertainty and
risk associated with compensatory mitigation
projects by recommending more detailed and
focused information from the mitigation providers
in the form of better resource classification,
specific identification of stressors, quantifiable
objectives, explicit design criteria connected to the
This approach recommends more
ecologically focused information
about the site's aquatic resource
class, stressors, mitigation objectives,
plans, monitoring and performance,
and long-term management.
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quantifiable objectives, and post-project monitoring and long-term management. An important
aspect of the Mitigation Rule, and aquatic resource restoration in general, is adaptive management.
In the context of uncertainty and risk, adaptive management may help to relieve the providers of
some of the responsibility to foresee and guard against possible adverse outcomes at the project
planning and design stage. Monitoring, assessment, and adaptive management make it possible to
identify and evaluate unexpected developments before they become problems and take the steps
necessary to control or prevent the risk of project failure.
Uncertainty is inherent in all mitigation projects. Despite the number of times a particular resource
type is mitigated, or a particular method is used, there will always be uncertainty surrounding the
outcomes of a project. Uncertainty is divided into natural variability, referring to the randomness
observed in nature, and knowledge uncertainty, which refers to our limited understanding of a
physical system and our ability to measure and model it (Skidmore et al., 2011).
Natural variability is inherent to natural systems and therefore will be difficult to reduce. With this
in mind, the reviewer should recognize that there will always be a certain level of uncertainty in all
mitigation projects and should discuss ways to accommodate this uncertainty with the mitigation
provider. Some examples of ways a project might reduce uncertainty and risk associated with
natural variability include:
• Using reference sites to better characterize the natural variability of the aquatic resource.
• Providing additional space within a riparian corridor for hydrological and morphological
adjustments without damaging or destroying valuable habitats, species, ecosystems, people,
or property;
• Removing or redesigning any artificial constraints (roads, bridges, culverts, and bank
protection works, etc.) in the project area/reach with additional capacity to allow for
unpredicted changes in the site's hydrodynamics, flow, or sediment regimes due to, for
example, climate or land use changes; and
• Including post-project monitoring and adaptive management of unforeseeable
developments if and when they occur (Skidmore et al., 2011).
Knowledge uncertainty can be reduced by paying careful attention to:
• Precision and accuracy of field measurements (i.e., measurement error);
• Gaining in-depth understanding of the operating processes and functions of the aquatic
system (i.e., uncertainty decreases as knowledge of the system increases);
• Adequately characterizing the symptoms and causes of problems manifested in the aquatic
system (e.g., is the aquatic resource suffering from watershed-, or site-scale stressors?);
• Selecting appropriate monitoring parameters to represent current condition, identify design
criteria, and objectively document project success; and
• Understanding the limitations (i.e., limited representativeness of natural processes and
limited predictive capacity) of models and indicators used in the project. It is important for
the reviewer to understand, and have thorough discussions with the provider and others,
about the level of uncertainty in input data, analyses, and outputs of models. Many models
include error estimates which should be understood and accommodated in the mitigation
planning process.
Risk is broadly defined as the product of the chance a particular event will occur (probability), and
the impact the event would cause (consequence), if it did occur. Risk and uncertainty are not the
same thing: uncertainties may be large, but many of them pose little risk to outcomes of the
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project. Therefore, reviewers should consider whether risks are unacceptable before asking the
provider to reduce uncertainty. For example, not calculating the changes in velocities and shear
stresses as a result of channel reconfiguration and placement of instream structures may result in
failing structures or the stream abandoning the structures. The reviewer may consider these
anticipated results unacceptable and ask for the provider to address the uncertainties. For
wetlands, restoring hydrology to former riverine wetlands requires reasonable certainty in
predicting the hydrology in adjacent channels and shallow groundwater levels. If the risks
associated with a project appear unacceptable, the reviewer needs to request that the provider
include information, like that listed above, in the mitigation and the adaptive management plans to
lower the risk.
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3,0 What is the Current Aquatic Resource Class?
For the purposes of this Approach, classification is a means of organizing and understanding
complex information about streams and wetlands to bring a semblance of order to natural
variability, to promote better communication in the management of aquatic resources, to focus
information gathering and ensure appropriate ecological site context. To this end, classification is a
critical first step in the mitigation project planning process to focus conceptual understanding of
wetland and stream ecological processes, and fundamentally set the stage for the effective review
of the proposed mitigation project. By first understanding the functional class(es) of aquatic
resources on a potential mitigation site, the provider can better understand and communicate the
site-specific and watershed characteristics/processes that exert the most influence on the functions
being performed, or capable of being performed by that resource type on that site.
Aquatic resource classification should be provided early on in mitigation project reviews, as it
supports site selection, development of mitigation goals and objectives, and other required
elements. When reviewing and commenting on early stages of mitigation plans or proposals, for
example during pre-application meetings or at the prospectus stage, reviewers should consider
whether the appropriate aquatic resource class(es) has been identified, and, if not, whether
sufficient information is available to inform aquatic resource classification. Data to inform
classification should be requested as early as possible. For wetlands, this information includes
hydrogeomorphic (i.e., geomorphic position, water sources and hydrodynamics), soils and
vegetation data. For streams, this information may include stream order, valley type, flow regime,
and/or classification via established methods (e.g., Rosgen, stream evolution models). This section
provides the reviewer with background information on recommended classification approaches in
wetlands and streams and the underlying data and information used to inform these classification
approaches.
3.1 Regulatory Context
While there is not a specific regulatory requirement for classifying aquatic resources, using a
functional approach to determine aquatic resource type informs rule requirements for objectives
(40 CFR §230.94(c)(2)) and site selection (40 CFR §230.94(c)(3)) as well as informing ecological
performance standards (40 CFR §230.95) and monitoring (40 CFR §230.96). The Mitigation Rule
requires a description of the resource type(s) when developing objectives for the project,
specifically stating that objectives should include "[a] description of the resource type(s) and
amount(s) that will be provided, the method of compensation (i.e., restoration, establishment,
enhancement, and/or preservation), and the manner in which the resource functions of the
compensatory mitigation project will address the needs of the watershed, ecoregion, physiographic
province, or other geographic area of interest."
Further, the Mitigation Rule stipulates that selected sites must be ecologically suitable for providing
the desired aquatic resource functions (40 CFR §230.93(d)). Using a functional classification to
identify aquatic resource types informs a number of ecological suitability factors, including
characterizing the "hydrologic conditions, soil characteristics, and other physical and chemical
characteristics;" as well as "watershed-scale features, such as aquatic habitat diversity, connectivity
and other landscape functions," (40 CFR §230.93(d)(1)). Ecological performance standards may be
based on reference aquatic resources that represent the range of variability exhibited by the
"regional class of aquatic resources" (40 CFR §230.95(b)); and the content and level of monitoring
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must be commensurate with not only the scale and scope of the mitigation project but also to
inform whether the project is meeting its performance standards.
The biotic and abiotic variability in aquatic resource conditions presents a challenge for IRTs to
isolate the most applicable technical considerations necessary to effectively review mitigation
plans. Addressing this challenge can be aided by taking a functional approach to classifying
wetlands and streams. Classifying wetland and stream types in ways that make connections to
ecological functions is advantageous to the planning, implementation, and review of compensatory
mitigation proposals, informs site selection and ecological suitability, and facilitates development of
relevant, function-based objectives.
3.2 Wetland Classification
When a potential mitigation site is proposed,
the review process should begin by considering
the site's landscape context, what (if any)
water sources affect the site and how water
moves through the site (Box A). Wetland
classification schemes that describe ecological
units based on similar characteristics are
useful, as they can link a specific site to a
wetland class with known ecological functions
using site-specific information on landscape
position, hydrologic sources, soils, and
vegetation. For this, the hydrogeomorphic
(HGM) classification of wetlands (Brinson,
1993) is particularly well-suited and is the
preferred method in this Approach. HGM
classification combines site-specific landscape,
hydrologic, and soil information and places a site into a broader ecological context that can inform
baseline assessment, setting target conditions, monitoring, and performance standards. Soils data
support classification by providing the relationship between landforms, hydrology and wetland soil
development, a key element of successful wetland restoration or enhancement efforts (see Section
3.2.3 below).
Ideally, reviewers should ensure the provider classifies a proposed wetland mitigation project
hydrogeomorphically to at least the class level and in many cases the subclass level, where possible
(see Tables 3 and 4 below, as well as local regional HGM guidebooks). This classification should be
supported by landscape information, Web Soil Survey data (e.g., Soil Web online, or Soil Web.kml
in Google Earth), current HGM regional guidebook descriptions and keys, and any other available
information. Additional information on technical resources for HGM and soils are provided in the
Sections 3.2.2 and 3.2.3 below. However, despite the preference for HGM classification, other
wetland classification methods (see below) could be used if available information for HGM is
insufficient and the alternate method is well documented and provides information on how the site
functions.
Key Points of Classification:
• Classification aids communication
between provider and reviewer and
focuses review on key processes and
functional attributes.
• HGM is the recommended wetland
classification due to its consideration of
landscape position, water source and
hydrodynamics.
• For streams, a combination of
classification approaches may be
needed to characterize a stream's
watershed and landscape context, flow
permanence and geomorphic context.
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Box A. Recommended information to inform HGM wetland classification
Hvdroqeomorphic classification:
• What is the HGM classification of the existing wetlands on site?
o Geomorphic position. Data source?
o Water sources. Data source?
o Hydrodynamics. Data source?
o Applicable HGM regional guidebook? Rationale?
• Does the site classification suggest that a different wetland type would be appropriate?
Soils information to support classification:
• Geomorphic position
• Flooding (frequency, timing and duration)
• Ponding (timing, frequency and duration)
• Seasonal high-water table (depth and timing)
3.2.1 Rationale
The following section provides the rationale for functionally classifying wetlands using the HGM
classification (Brinson, 1993) and discusses other potential classification methods which could be
used to augment, or in lieu of, HGM. HGM establishes a link between landscape position and
hydrology to wetland functions and is widely recognized as an appropriate wetland functional
classification upon which to base assessment and mitigation planning (Smith et al., 1995). It has
been shown to be effective in differentiating wetland types based on geomorphic setting and
hydrology (Cole et al., 1997; Gwin et al., 1999; Shaffer et al., 1999; Cole and Brooks, 2000; Cole et
al., 2002). In addition, the basic premise of the HGM classification is a useful construct for wetland
restoration (e.g., Bedford, 1999; Gwin et al., 1999; NRC, 2001; Zedler, 2006; Galatowitsch and
Zedler, 2014) and is a wetland functional classification designed for use in the CWA Section 404
program (Clairain, 2002). There are currently numerous HGM regional guidebooks applicable to the
southeastern U.S. that discuss prevalent wetland subclasses, the functions they perform, and the
common impairments affecting those functions. Further HGM classifications are based on reference
data (see Section 4.4). Using HGM classification focuses information on landscape setting (e.g.,
floodplains, slopes, depressions) and the hydrologic sources and processes that drive wetland
function (e.g., overbank flooding, surface ponding, and shallow groundwater saturation), making it
particularly useful to categorize and communicate wetland types for wetland mitigation planning.
Other wetland classification systems can also provide information on landscape position and
hydrology and may be useful in the review of wetland mitigation proposals. The Cowardin
classification (Cowardin et al., 1979), perhaps the most widespread wetland classification system in
the country, is used by the U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI) to
periodically inventory and monitor trends affecting wetland resources in the U.S. The classification
is hierarchical and described by the Federal Geographic Data Committee as being primarily based
on systems or complexes "that share the influence of similar hydrologic, geomorphic, chemical, or
biological factors," (FGDC, 2013).
However, the NWI lacks descriptors needed for estimating wetland functions. To address this, an
enhancement to include hydrogeomorphic-type modifiers for landscape position, landform, water
flow path, and waterbody type (LLWW) has been developed and utilized for mapping in some parts
of the U.S. (Tiner, 2010; 2013). These enhanced modifiers require remote sensing or aerial photo
interpretation to incorporate into existing NWI maps (hence the term NWI+). There have been
efforts to develop keys to the LLWW modifiers and apply them to NWI maps in some parts of the
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country, although not in the southeast U.S. However, where this enhanced NWI (NWI+) is available,
the keys and map products could provide useful information for supporting an aquatic resource
type determination but would require site-specific data to verify conditions detected via remote
sensing.
3.2.2 Determining Wetland HGM Class
HGM classification involves the determination of a wetland's geomorphic setting, the source(s) of
water that supply the site's hydrologic regime, and the hydrodynamics of the water source(s),
which indicate the power and direction of water movement entering and exiting the site.
Geomorphic setting refers to the landform and position of the wetland in the landscape, while
water source refers to the primary origin of the water that sustains wetland characteristics, such as
precipitation, floodwater, or groundwater. Hydrodynamics refers to the level of energy with which
water moves through the wetland and the direction of water movement. These three abiotic
factors largely dictate the functions wetlands perform and the level to which they are performed.
Brinson (1993) identified five HGM wetland classes that
were later expanded to the seven classes described by
Smith et al. (1995) (Table 3). Classifying wetlands into
one of the seven wetland classes differentiates
wetlands based on function and reduces the range of
natural variation in function compared to all wetlands
generally. However, as it was intended for use as an
impact assessment protocol for the CWA Section 404
program, HGM classification requires an additional
level of resolution to adequately differentiate
anthropogenic disturbances (i.e., impacts) from natural
disturbances. Wetland subclasses are differentiated by applying the three HGM classification
criteria along with other characteristics (e.g., geomorphic characteristics and/or plant communities)
at finer geographic scales (Smith et al., 1995). For example, Wilder et al. (2013) classified alluvial
forested wetlands of the Coastal Plain into four subclasses: riverine mid-gradient; riverine low-
gradient; slope headwaters; and connected depressions. Smith and Klimas (2002) recognized seven
subclasses in the Yazoo Basin of Mississippi, namely riverine overbank, riverine backwater, flats,
connected and isolated depressions, isolated and connected fringe. Other major subclasses present
in the southeastern U.S. include mineral soil wet pine flats (i.e., bunchgrass/ pine savanna,
cypress/pine savanna, and switchcane/pine savanna (Rheinhardt et al., 2002)), headwater slopes
(Noble et al., 2007), Tennessee flats and seasonally inundated depressions (Noble et al., 2013),
Everglade flat wetlands (Noble et al., 2002) and Gulf of Mexico tidal fringe wetlands (Shafer et al.,
2007).
The descriptions of HGM wetland classes in Table 3 point out the variety of conditions, particularly
hydrologic, under which different wetland classes can occur. Using these descriptions alone makes
differentiating wetland classes difficult, although Table 3 provides a beginning. In addition to
wetland class descriptions in Table 3, a simplified key to HGM wetland classes in the southeastern
U.S. is presented in Table 4, which can help determine wetland class. Keys and descriptions of
wetland classes and subclasses for a number of reference domains can be found in HGM regional
guidebooks (see Section 4.4.1. for a list of HGM regional guidebooks for the southeast).
Functional aspects of the HGM
wetland classification are:
• Geomorphic position: Where a
wetland sits in the landscape;
• Water source: Where the water
comes from before it enters the
wetland; and
• Hydrodynamics: How much
energy the water carries when
it enters the wetland.
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Table 3. Hydrogeomorphic wetland classes at the continental scale (after Smith et al.» 1995).
HGM Class
Definition
Depression
Geomorphic setting: Topographic depressions (i.e., closed elevation contours) allowing accumulation of
surface water. May have any combination of inlets and outlets or lack them completely.
Water sources: Precipitation, overland flow, streams, or groundwater/interflow from adjacent uplands.
Hydrodynamics: Vertical fluctuations ranging from diurnal to seasonal. Water flows from higher elevations
toward the lowest point of the depression. May lose water thru evapotranspiration, intermittent or perennial
outlets, or infiltration to groundwater.
Examples: Cypress domes.
Tidal Fringe
Geomorphic setting: Along coasts and estuaries, where influenced by sea level.
Water sources: Primarily tides and secondarily groundwater discharge and precipitation. Seldom dry for
significant periods.
Hydrodynamics: Frequently flooded with water table elevations controlled mainly by sea surface elevation.
At interface between tidal fringe and riverine classes, bidirectional flows from tides dominate over
unidirectional flow controlled by riverine floodplain slope. Water is lost thru tidal exchange, overland flow to
tidal creek channels, and evapotranspiration.
Examples: Spartina alterniflora salt marshes.
Lacustrine
Fringe
Geomorphic setting: Adjacent to lakes where lake water elevation maintains the water table in the
wetland. In some cases, these wetlands consist of a floating mat attached to land.
Water sources: Additional sources of water are precipitation and groundwater discharge, the latter
dominating where lacustrine fringe wetlands intergrade with uplands or slope wetlands.
Hydrodynamics: Surface water flow is bidirectional, usually controlled by water-level fluctuations resulting
from wind or seiche. Water is lost through flow returning to the lake after flooding and by evapotranspiration.
Slope
Geomorphic setting: Normally occur on sloping land ranging from slight to steep but can occur in nearly
flat landscapes if groundwater discharge to the wetland surface is dominant. Distinguished from
depressional wetlands by lack of closed topographic depression and predominance of the
groundwater/interflow water source.
Water sources: Predominantly groundwater or interflow discharging at the land surface. Precipitation is
often secondary.
Hydrodynamics: Dominated by downslope unidirectional water flow and water lost primarily by saturated
subsurface flows, via low-order streams, and/or by evapotranspiration.
Example: Fens
Mineral Soil
Flats
Geomorphic setting: Interfluves, extensive relic lake bottoms, or large floodplain terraces with poor vertical
drainage due to impermeable layers.
Water sources: Precipitation. Virtually no groundwater discharge, which distinguishes them from
depressions and slopes.
Hydrodynamics: Vertical fluctuations, slow lateral drainage, and low hydraulic gradients. Lose water by
evapotranspiration, overland flow, and infiltration to underlying groundwater.
Example: Pine flatwoods
Organic Soil
Flats
Geomorphic setting: Flat interfluves, as well as where depressions have become filled with peat to form a
relatively large flat surface. Organic soil flats, or extensive peatlands, differ from mineral soil flats in part
because their elevation and topography are controlled by vertical accretion of organic matter.
Water source: Dominated by precipitation.
Hydrodynamics: Vertical fluctuations, slow lateral drainage, and low hydraulic gradients. Lose water by
evapotranspiration, overland flow, and seepage/infiltration to underlying groundwater.
Example: Portions of the Everglades and northern Minnesota peatlands.
Riverine
Geomorphic setting: Floodplains and riparian corridors in association with stream channels.
Water source: Overbank flow from channel or subsurface hydraulic connections between the stream
channel and wetlands. Additional sources may be interflow, overland flow from adjacent uplands, tributary
inflow, and precipitation.
Hydrodynamics: During overbank flows, surface flows down the floodplain may dominate hydrodynamics.
Headwater riverine wetlands often intergrade with slope wetlands, depressions, poorly drained flats, or
uplands as the channel (bed) and bank disappear. Perennial flow is not required. Lose surface water via
floodwater return to channel and surface runoff to the channel during rainfall events. Subsurface water is
lost by discharge to the channel, movement to deeper groundwater (for losing streams), and evaporation.
Example: Bottomland hardwoods on floodplains.
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Table 4. Key to typical HGM classes in the southeastern U.S (after Klimas et al.» 2004 and Wilder et al.»
2013).
1. Wetland is located on floodplain of a stream and principal water source is the stream flooding
Riverine
Wetland is not located on floodplain of a stream and principal water source is not the stream
2
2. Wetland is not in a topographic depression, nor is it impounded
4
Wetland is in a topographic depression or it is impounded
3
3. Wetland is associated with a water body that is ponded or less than 2-m deep in most years
Depression
Wetland is associated with a water body that has permanent water more than 2-m deep in
most years
4
4. Wetland is associated with an estuarine system and subject to tidal influence
Estuarine Fringe
Wetland is not associated with an estuarine system and subject to tidal influence
5
5. Wetland is fringing a reservoir
Lacustrine Fringe
Wetland is not fringing a reservoir
6
6. Topography is sloping with upslope watershed; principal water source is groundwater
discharge or subsurface flow
Slope
Topography is not sloping with no upslope watershed, and principal water source is not
groundwater discharge or subsurface flow
7
7. Topography is flat (0-2% slope) with no upslope watershed, dominated by organic soils,
principal water source is precipitation
Organic Flat
Topography is flat (0-2% slope) with no upslope watershed, dominated by soil indicators of
vertical water movement, principal water source is precipitation
Mineral soil flat
3.2.3 Using Soil Information in HGM Classification
Soil information can be used to augment the classification key above and to better understand the
relationship between landforms, hydrology and wetland soil development, the key elements of
successful wetland restoration and enhancement efforts. The formation of wetland soils and the
sources of water that maintain hydric soils and wetland hydrology are directly related to the depth,
duration, and direction of movement of water in a wetland (Bruland and Richardson, 2005a).
Landscape position, coupled with the frequency, duration, and timing at which water flows
vertically or laterally through the soil profile, ponds, or flows on/across the surface of the soil,
affects the formation and characteristics of the soil as well as the functions a wetland performs.
Interpretating soil survey data not only helps with hydric soil determinations, but also corroborates
HGM classification and places soil information into a context for understanding processes
important to wetland ecological functions (USDA NRCS, 2011).
Valuable sources of information for understanding the hydrologic regime associated with hydric
soils include:
• Web Soil Survey (https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm);
• Soil Web online (https://casoilresource.lawr.ucdavis.edu/soilweb-apps/ or the SoilWeb.kml:
https://casoilresource.lawr.ucdavis.edu/soil_web/kml/SoilWeb.kmz);
• Google Earth Pro
All three of these resources present data contained in the Soil Survey Geographic Database
(SSURGO), which can be used to classify a wetland area to an appropriate HGM class or subclass
(USDA NRCS, 2011). Data derived from SSURGO is particularly useful because of the objectivity of
the database (i.e., it was not generated to determine wetland class); the ubiquity of soil map
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coverage; and the presence of information pertaining directly to geomorphic setting, water source
and hydrodynamics.
Soil taxonomy, landform(s), groundwater saturation (depth to seasonal high-water table), ponding
and flooding frequency and duration information are provided for the mapped soil units. Landform
can be used to corroborate the wetland geomorphic position identified in Table 3, while the depth
to seasonal high-water table and the duration and frequency of ponding and flooding are all
indicators of predominant water source. For example, the Waverly soil series is a common wetland
soil in western Kentucky and Tennessee. It is listed as a hydric soil on the Kentucky and Tennessee
State Hydric Soil List due to its aquic suborder (Fluvaquentic Endoaquept), exhibiting hydric soil
indicators (criteria 2) and due to being frequently flooded (criteria 4). It is typically found in a
floodplain landform with a seasonally high-water table of approximately 10 in (23 cm), no ponding,
but with a frequent flood regime. This information lends support and additional information to the
classification of low gradient riverine wetlands described in the regional guidebooks developed for
western Kentucky (Ainslie et al., 1999) and western Tennessee (Wilder and Roberts, 2002). Other
examples and description of using soil interpretations to support HGM classification can be found in
Technical Note 3, Soil Hydrodynamic Interpretations for Wetlands (USDA NRCS, 2011).
The use of dichotomous keys (e.g., Table 4; Smith et al., 1995; Brooks et al., 2011), and soil
information from the Web Soil Survey can indicate if a site is located in a particular geomorphic
setting (e.g., riverine, slope, flat) and has mapped hydric soils with associated water features that
indicate predominant soil hydrodynamics (USDA 2011). This information could be used by the
provider to assign HGM class(es) to the site and set functional expectations for the proposed
mitigation project. For instance, in the example above, Waverly soils found along a floodplain
indicate a riverine wetland setting. In addition, the water sources are expected from seasonal high-
water tables and overbank flooding. By contrast, slope wetland soils would have saturated soils
with no flooding or ponding and flats soils would tend to have high water tables, perhaps pond, but
not flood. Although the soil information from the Web Soil Survey may be general and categorical
for any particular soil series in the restoration context, it augments HGM classification by providing
site level information on typical water sources and hydrodynamics expected with the mapped soil
series. This is important information to have as the review process proceeds.
3.3 Stream Characterization
In like fashion to wetland classification, the expectation of the Approach is that stream mitigation
sites will be classified to aid in understanding the site-specific and watershed characteristics and
processes that exert the most influence on the functions being performed, or capable of being
performed at that site. However, unlike HGM classification for wetlands, there is not a widely
accepted functional classification for streams. Thus, the information below does not outline a
discrete functional classification for streams, but rather lists several classification approaches that
can provide pertinent information to characterize aspects of a site's watershed, hydrology and
geomorphic context which will influence the assessment of stream function for baseline, target and
reference condition assessment.
Reviewers should ensure a proposed stream project site is characterized using function-based
approaches that provide the watershed, hydrologic and geomorphic information listed in Box B.
Classification approaches, such as Rosgen stream class, stream order, flow regime, valley type, and
stream evolution models (SEM), provide insight into important aspects of stream function at the
beginning of the mitigation plan review process. Stream order and valley type provide information
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about landscape position in the watershed; ecoregion information provides landscape context; and
flow permanence provides information on hydrology. Rosgen stream type provides information
about the reach's current geomorphic condition, while SEM provides information about a channel's
response to disturbance, its degradational status or where that channel is along a geomorphic and
ecologic disturbance gradient. Characterization using these approaches provides landscape,
geomorphic, water source, as well as disturbance-related information.
Each aspect of the stream characterization should be supported by the appropriate landscape
information, field data, and other relevant information provided as part of the proposal.
Information on flow permanence, morphology and slope, and watershed position of the project
reach will provide context to subsequent baseline assessment and target condition steps of the
Approach. Descriptions of several stream classification approaches are provided in the sections
below as examples of useful information to characterize a stream mitigation site. However, any
information which can provide watershed, hydrologic and geomorphic context to a stream
mitigation proposal can be used in addition to, or in lieu of, the information outlined below.
Box B. Recommended information to address stream characterization.
What is the stream's watershed/landscape context?
•
Where does the stream sit in the watershed?
o e.g., determine Strahler stream order (ensure appropriate mapping resolution) and
watershed size
•
What is the current valley type (e.g., very confined, confined, partially confined, unconfined)?
o Data sources? How was valley type determined?
•
In what ecoregion does the site occur?
What
is the flow permanence?
•
Is the stream reach perennial, intermittent, or ephemeral? Data sources?
What is the geomorphic context?
•
What classification approach(es) was/were used?
•
Were all relevant data provided to support classification? (e.g., entrenchment ratio, width/depth
ratio, sinuosity, bed materials, slope, number of channels)
•
What are the stable stream types for the given valley type? How does that compare to the existing
stream type?
•
What is the Stream/Channel Evolution stage?
o Model used?
o Justification for stage?
o Field indicators noted?
3.3.1 Rationale
There are a wide variety of stream classification systems developed to address various scientific and
management purposes. David et al. (2021) provides a summary of classification approaches,
including classification by flow regime, drainage network, geomorphology, riparian vegetation,
other ecological factors, as well as approaches that integrate several of these together. In an effort
to communicate effectively with IRT agencies and mitigation providers while also considering
classifications useful to the mitigation of streams, it is recommended that a combination of
approaches be used to characterize the watershed and landscape context (e.g., Strahler stream
order, valley type, ecoregion), flow permanence, and geomorphic context (e.g., Rosgen
classification, and stream evolution stage) and to focus further review considerations. Any stream
classification system must be used with an understanding that fluvial systems are constantly
adjusting and evolving in response to changes in sediment supply, channel and floodplain
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geometry, floods, groundwater hydrology, vegetation, woody debris, fauna (e.g., beaver, livestock),
as well as other historic and contemporary natural, and anthropogenic factors that affect the
stream system. Streams will respond differently to stressors and restoration approaches depending
on underlying processes.
Process-based approaches to stream classification could also be considered, although there are few
that are widely used, perhaps in part due to the lack of agreement on which spatial, temporal,
structural, and functional attributes are most important for classification. Gordon et al. (2004)
provides an overview of ecological, geomorphological, hydrological, water quality-based, and
combined physical-chemical-ecological classification models which are used in different parts of the
world to address different physical, chemical, biological, and ecological aspects of streams.
Examples of process-based stream classifications in the U.S. include Whiting and Bradley (1993),
who developed a classification of headwater streams based on dominant physical processes;
Woolfe and Balzary (1996), who developed a channel classification to predict sedimentation and
erosion regimes; Schumm et al. (1984), who identified straight, meandering, and braided channels
and related both channel pattern and stability to modes of sediment transport in alluvial channels;
and Montgomery and Buffington (1997), who proposed a similar classification system for alluvial,
colluvial, and bedrock streams in the Pacific Northwest that addresses channel response to
sediment inputs throughout the drainage network.
3.3.2 Watershed and Landscape Context
Strahler stream order (Strahler, 1952) is a
widely accepted classification that provides
insight into the position of the stream in
the watershed. In this system, the first
intermittent or perennial streams in a
drainage area are assigned order one;
ephemeral streams are not counted. First order streams have no tributaries, with subsequent
stream orders proceeding downgradient and increasing in order as tributaries of equal order join to
form larger order streams. Assigning Strahler stream order is typically done from maps, which show
the stream network of any given watershed. Thus, the method is dependent on the level or
resolution of mapping to precisely determine the extent of, particularly, first order streams whose
channel features may not show up on larger scale maps. Typically, 1:24,000 scale maps or
information at better resolution, should be used to determine stream order. Stream order not only
places a mitigation site in a watershed context, but also informs mitigation site selection by
recognizing the distribution of stream orders where impacts and compensatory mitigation occur
within watershed drainage networks (i.e., mitigating for impacts within a given stream order with
streams of the same order and in similar watershed positions).
Valley type will affect the relative contribution of materials to the stream from hillslope processes.
Thus, the shape, or confinement, of a valley can be used to predict stream type (particularly
entrenchment), the extent of the floodplain and riparian zones, and the connectivity of valley side
slopes to the stream, which may all affect the interpretation of baseline assessment and target
condition information. Rosgen (1994) includes 22 valley types categorized by morphology and
geology. However, for the purposes of reviewing stream mitigation projects, valley types can be
organized into four categories of valley confinement (Johnson et al., 2015):
Functional aspects of stream classification are:
• What is watershed/landscape context?
• What is the flow permanence?
• What is the geomorphic context?
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• Very confined: Valleys are so narrow that only the fully entrenched stream types can exist.
In these valleys, there is no room for an appreciable floodplain to develop, and the streams
are intimately associated with the steep side slopes that define their edges. Valley bottom
width is less than 1.4 times the bankfull width of the stream.
• Confined: Valleys at more moderate levels of confinement, where the side slopes tend to be
more gradual or spaced farther apart, and there is room for narrow floodplains and riparian
strips alongside the channel. Valley bottom width is between 1.4 and 2.2 times the bankfull
width of the stream.
• Partially confined: Valleys that can be defined as those in which the belt width of the
stream is constrained on one side or the other by either bedrock or the valley wall. Valley
bottom width is between 2.2 and 7.0 times the bankfull width of the stream.
• Unconfined: Valleys are wide enough to accommodate broad floodplains and riparian
wetlands; streams in these valleys are generally buffered from hillslope processes. They are
generally lower-gradient than confined valleys and composed of finer-grained material
including alluvial or lacustrine fill. In unconfined valleys, lateral migration is not limited on
either side, and the natural stream encounters valley bottom edges only occasionally. Valley
bottom width exceeds 7 times the bankfull width of the stream.
Ecoregions are areas of similar climate, landform, soil, potential natural vegetation, hydrology or
other ecologically relevant parameters. Ecoregions represent areas where similar aquatic biotic
assemblages (e.g., fish, macroinvertebrates, algae, riparian birds) are likely to occur (Stoddard,
2004). Ecoregions have been used in federal and state efforts to assess and monitor biological and
water quality conditions in streams and wetlands. Reviewers should consider at least the Level 3
Ecoregion in which the project site occurs. This level of resolution provides a landscape description
of the site and a means of accessing existing federal and state biological and water quality
databases that could serve as a potential basis of comparison during the baseline assessment
(Question 2), targeting (Question 3) and performance monitoring (Question 5). In some cases,
differences in Level 4 ecoregions are important, as they have been shown to support statistically
different macroinvertebrate community structure in headwater streams (Somerville and Pond,
2022), and the ambient monitoring programs in numerous southeastern states base their
macroinvertebrate bioassessment indices on Level 4 ecoregions (e.g., Alabama, Georgia,
Tennessee).
3.3.3 Flow Permanence
Flow permanence is the degree to which flow and water persist in the channel. It is an important
ecological characteristic of streams because of its effect on stream water quality and biota. Stream
reaches that flow continuously (perennial) often support much different fauna than those where
flow is discontinuous for portions of the year (intermittent). Another major change in fauna occurs
not only when flow is discontinuous, but also in stream channels where pools dry for part of the
year. Ephemeral channels are those which only flow for a period of days in response to runoff, are
dry much of the year and tend to only sporadically support aquatic organisms.
For the purposes of this Approach, flow permanence should be characterized as: perennial for
streams that flow continuously during a year of normal rainfall, often with a streambed located
below the water table for most of the year; intermittent for streams that flow for only part of the
year, typically during a wet season when the streambed may be below the water table or when
melt water from snow provides sustained flow; or ephemeral for streams that flow only in direct
response to precipitation including rainstorms, rain on snow events, or snowmelt (Nadeau et al.,
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2015). These definitions coincide with those used by EPA in the development of regional stream
duration assessment methods (SDAMs) (https://www.epa.gov/streamflow-duration-assessment).
SDAMs can be used in lieu of long-term hydrologic records of streamflow duration, and rely on
regionally specific field indicators to determine streamflow duration. Regional SDAMs consider
differences in climate, geology and topography in developing robust regional hydrologic,
geomorphic and biological indicators that reflect a streamflow duration classification of perennial,
intermittent, or ephemeral. As these methods are developed, they can be used to characterize flow
permanence for this Approach and may be useful in the baseline assessment (Question 2). Links to
existing SDAMs, including local and regional SDAMs, and status of federal regionalization efforts
can be found at the EPA website listed above. Reviewers should consider how streamflow
permanence was determined and encourage the use of SDAMs when and where available.
3.3.4 Geomorphic Context
The Rosgen classification of streams (Rosgen, 1994; 1996) is an example of classifying streams
based on geomorphic channel patterns and has gained widespread use throughout the southeast
and much of the country. The channel information collected for Rosgen classification can be useful
to inform the review process and as a communication tool between mitigation providers,
practitioners, and regulators. Generally speaking, the Rosgen classification is similar to the HGM
classification in its description of geomorphic characteristics of the channel. However, unlike HGM,
which incorporates hydrology and hydrodynamics into the classification, Rosgen does not explicitly
incorporate hydrologic and hydraulic processes into the designation of classes; it has been
described by some authors as a form-based or descriptive classification system (Goodwin, 1999;
Buffington and Montgomery, 2013). Hence the recommendation to use in combination with other
classification approaches to characterize valley type, stream order, flow permanence, and channel
evolutionary stage.
In brief, the Rosgen system uses six morphological measurements (entrenchment, width/depth
ratio, sinuosity, number of channels, slope, and bed material particle size) for classifying a stream
reach into eight major Rosgen stream classes (Table 5). Entrenchment describes the relationship
between a stream and its floodplain (i.e., how accessible a floodplain is to the stream) and is
defined as the vertical containment of the stream and the degree to which it is incised in the active
floodplain. The width/depth ratio can have a significant effect on the mechanics of secondary flows,
sediment transport, and baseflow habitat, and is measured as the ratio of top width to mean depth
for the bankfull channel. Sinuosity is the ratio of stream length to valley length or, alternatively,
valley slope to stream slope. The bed material particle size used in the classification is the dominant
bed surface particle size, determined in the field by a pebble-count procedure (Wolman, 1954) or
as modified for sand and smaller sizes. Stream slope is measured over a channel reach of at least 20
bankfull widths in length (FISRWG, 2001). The number of channels differentiate single-thread
streams from streams that are braided or anastomosed with multiple channels. Reviewers should
be familiar with the general description and geomorphic aspects of the eight Rosgen stream classes
(i.e., width/depth ratio, entrenchment ratio, sinuosity, and slope) and the valley type to effectively
utilize or interpret the Rosgen classification.
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Table 5. General descriptions of stream types and their criteria for Level 1 classification (Rosgen, 1996).
Note that sinuosity and slope (provided as typical ranges) are secondary classification factors and should
not limit the use of specific stream type.
Stream
Type
General Description
Entrenchment
Ratio (ER)
W/D
Ratio
Sinuosity
Slope (ft/ft)
(%)
A
High relief. High energy/ debris transport
associated with depositional soils. Erosional or
depositional and bedrock forms. Entrenched and
confined streams with cascading reaches.
Frequently spaced, deep pools in associated
step/pool bed morphology. Very stable if
bedrock or boulder dominated channel.
<1.4
<12
1.0 to 1.2
0.04 to 0.10
(4-10%)
B
Moderate relief, colluvial deposition. Moderate
ER and W/D ratio. Narrow, gently sloping
valleys. Rapids predominate with scour pools.
Very stable plan and profile. Stable banks.
1.4 to 2.2
>12
>1.2
0.02 to 0.039
(2-3.9%)
C
Low gradient. Broad valleys with terraces in
association with broad floodplains, alluvial soils.
Slightly entrenched with well-defined
meandering channels. Riffle/pool morphology.
>2.2
>12
>1.4
<0.02
(<2%)
D
Low gradient. Broad valleys with alluvium,
steeper fans. Depositional, braided channel with
longitudinal and transverse bars. Active lateral
adjustment, with abundant sediment supply.
Convergent/ divergent bed features,
aggradational processes, high bedload and bank
erosion.
n/a
>40
n/a
<0.04
(<4%)
DA
Low gradient. Broad valleys with fine alluvium
and/or lacustrine soils. Anastomosed (multiple
channel) geologic control creating fine
deposition with well vegetated bars that are
laterally stable with broad wetland floodplains.
Very low bedload, high wash load sediment.
>2.2
Highly
variable
Highly
variable
<0.005
(<0.5%)
E
Broad valley/meadows with alluvial materials
and floodplains. Low gradient, highly sinuous
with stable, well-vegetated banks. Riffle/pool
morphology with very low W/D ratios.
>2.2
<12
>1.5
<0.02
(<2%)
F
Entrenched in highly weathered material. Gentle
gradients with high W/D ratio. Meandering,
laterally unstable with high bank erosion rates.
Riffle/pool morphology.
<1.4
>12
>1.4
<0.02
(<2%)
G
Gullies, step/pool morphology with moderate
slopes and low W/D ratio. Narrow valleys or
deeply incised in alluvial or colluvial materials,
i.e., fans or deltas. Unstable with grade control
problems and high bank erosion rates.
<1.4
<12
>1.2
0.02 to 0.039
(2-3.9%)
An important aspect of Rosgen's classification is the determination of the bankfull geometry (i.e.,
channel width and depth required to contain the bankfull discharge which fills an alluvial channel
up to the elevation of the active floodplain). The determination of bankfull geometry requires
effective identification of indicators in the channel that correspond to the bankfull discharge.
Bankfull indicators have been shown to be subjective and often difficult to accurately determine in
the field, especially in rapidly changing channels, because field indicators are often subtle and are
not valid if the stream is not stable and alluvial (FISRWG, 2001). In unstable streams, bankfull
indicators are often missing entirely. Identifying bankfull indicators (e.g., the first flat depositional
surface nearest in elevation to the baseflow water level in stable alluvial channels, and/or the
36
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average elevation of the highest surface of the channel bars) in the field and their correlation with
the appropriate stream discharge requires training and experience.
For the reviewer, understanding how bankfull was determined is of critical importance in
characterizing stream type both for existing condition and setting the target condition for design
plans. When a stream is classified using the Rosgen classification, the indicators used to determine
bankfull geometry and their correlation with appropriate flow conditions (i.e., bankfull flows) will
need to be clearly explained by the mitigation provider and considered in the review. Bankfull
discharge is often used in stream design as the design flow (see Section 6.3) upon which the
modified or reconfigured channel is sized. For more information on determination of bankfull, see
Rosgen (1996), FISRWG (2001), Harrelson et al. (1994), USDA NRCS (2007), and USACE (2020).
Stream evolution models (SEM) and channel evolution models (CEM) can also aid in
conceptualizing how channel morphology responds to disturbance (FISRWG, 2001). The
evolutionary trajectory of any given stream and floodplain system can be significantly different
based on the magnitude and size of bedload and suspended load, local bed and bank controls, and
repeated impacts (Table 6). These models add considerable benefit to any classification approach
and should be included to lend insight into stream morphological responses to disturbance.
The most notable channel evolution models (Schumm et al., 1984; Simon and Hupp, 1986) have
been in use since the 1980's to help inform management of impacts that have caused channel
instability. However, as Cluer and Thome (2014) point out, an unintended consequence of using
CEM is the perpetuation of the assumption that single-thread meandering channels represent the
natural configuration of dynamically stable alluvial streams and thus always represent appropriate
target morphology for establishment or restoration. Cluer and Thome (2014) instead maintain that
many pre-European settlement streams were anastomosed (i.e., lower energy, low width/depth
ratio channels comprised of cohesive bed and bank materials with lots of vegetation creating
vegetated islands (not bars)), and that the multi-channel, frequently inundated floodplain stream
configuration represents a natural and ecologically valuable stream-type. Such conditions in pre-
colonial North America have been attributed largely to the former abundance and distribution of
beaver (Castor canadensis) (Naiman et al., 1988; Wohl, 2021). As such, Cluer and Thome (2014)
proposed an update to the CEM concept, referred to as the Stream Evolution Model (SEM), which
combines stages outlined in Schumm et al. (1984) and Simon and Hupp (1986) and inserts a multi-
threaded anastomosed precursor stage to better represent pre-disturbance conditions in some
contexts (Figure 3). This precursor stage is equivalent to a Rosgen DA channel and represents an
ecologically rich stage of stream evolution where stream morphology and floodplain interactions
make these stream types stable (i.e., as channel depth decreases and floodplain widens) and
ecologically diverse (i.e., channel and floodplain become integrated habitat). Restoration of these
stream systems has gained increased attention throughout the U.S. in recent years and has been
referred to as valley restoration (e.g., Starr and Harman, 2015) or restoration using beaver dam
analogues (e.g., Pollock et al., 2015; Goldfarb, 2018; Davee et al., 2019).
37
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Table 6. Channel evolution model and stream evolution model stages with description of reach-averaged
characteristics (Cluer and Thorne, 2014) and corresponding Rosgen classes (Johnson et al., 2015).
CEM
(Schumm et
al., 1984)
CEM
(Simon and
Hupp, 1986)
SEM
(Cluer and
Thorne, 2014)
Description
Rosgen
Stream
Type
Anastomosing
Pre-disturbance, dynamically meta-stable network
of anabranching channels and floodplain with
vegetated islands supporting forested or
herbaceous wetland;
Da, Db
Single thread channels
I. Undisturbed
I. Pre-modified
1. Sinuous
Dynamically stable and laterally active channel
within a floodplain complex. Flood return interval 1-5
year range.
C, E
II. Constructed
2. Channelized
Re-sectioned land drainage, flood control, or
navigation channels.
Typically
F, B
II. Degradation
Degradation
3. Degrading
Incising and abandoning its floodplain. Featuring
head cuts, knick points or knick zones that incise
into the bed, scour away bars and riffles, and
remove sediment stored at bank toes. Banks
geotechnically stable.
G
3a. Arrested
Degradation
Stabilized, confined or canyon-type channels.
Incised channel in which bed lowering and channel
evolution have been halted because non-erodible
bed materials (bedrock, tight clays) have been
encountered and banks remain largely stable.
F, G
III. Rapid
Widening
IV.
Degradation
and Widening
4. Degradation
and Widening
Incising with unstable, retreating banks that collapse
by slumping and/or rotational slips. Failed material
is scoured away and the enlarged channel becomes
disconnected from its former floodplain which
becomes a terrace.
G
4-3. Renewed
incision
Further head cutting within incised channel.
G
IV.
Aggradation
V. Aggradation
and Widening
5. Aggradation
and Widening
Bed rising and aggrading, channel widening with
unstable banks in which excess load from upstream,
together with slumped bank material, build berms
and silts bed. Banks stabilizing and berming.
F
V.
Stabilization
VI. Quasi-
Equilibrium
6. Quasi-
equilibrium
Inset floodplain re-established, quasi-equilibrium
channel with two-stage cross-section featuring
regime channel inset within larger, degraded
channel. Berms stabilize as pioneer vegetation traps
fine sediment, seeds, and plant propagules.
C
VII. Late-Stage
evolution
7. Laterally
active
Channel with frequent floodplain connection
develops sinuous course, is laterally active and has
symmetrical cross-section promoting bar accretion
at inner margins and toe scour and renewed bank
retreat along outer margins of expanding meander
bends.
C, E
Single thread channels
8.
Anastomosing
Meta-stable channel network. Post disturbance
channel featuring anastomosed planform connected
to a frequently inundated floodplain that supports
forested or herbaceous wetlands bounded by set-
back terraces on one or both margins.
Da, Db
38
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VTflCf O
Aiuttoin«UA{
AflMtwooiicj
Wet Woodljnd
Grwd Wet!jn4
h«*v
A A A
mlAN
i Tt i t
i
Figure 3. Stream Evolution Model (SEM) stages (reproduced from Cluer and Thome, 2014).
The SEM, which is most applicable to low gradient (<2% slope), unconfined valleys, in humid
forested regions (Bledsoe, pers comm), differs from the two previous channel evolution models by
adding two successor stages to incorporate late-stage evolutionary changes (Table 6). This model
also emphasizes that stream evolution in disturbed streams is not linear, but rather cyclical and
explicitly recognizes that some streams may never evolve to some of these stages. For example,
Stage 2 represents a maintained channel and Stage 3s is a channel that has downcut to a resistive
layer at which degradation is either stopped (bedrock) or significantly slowed (cohesive clay).
Regardless of the model used, channel or stream evolution models aid in identifying the trajectory
from current condition, whether that be a general tendency for a stream and its floodplain to
progress toward a more stable configuration as the floodplain widens and the channel depth
decreases to a point when it is difficult to know where the channel ends and floodplain begins, or
whether degradation progresses until the channel functions completely separate from the
floodplain (Cluer and Thome, 2014). Adding the stream evolution model to the classification step is
advantageous, because it (FISRWG, 2001; Cluer and Thome, 2014):
• Provides a link between watershed land use change and reach scale morphology (see
Section 4.5 on determining stream baseline condition).
• Recognizes multi-threaded channels resulting from channel aggradation as a natural channel
form (Stages 0 and 8) which may only require minimal, if any, active work.
• Recognizes that a stream in an early degradational stage may be stabilized with grade
control measures before they cross a geomorphic threshold of bank failure (e.g., banks too
39
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high/ too steep; SEM stage 4) thus averting severe widening and massive sediment loading
to downstream reaches and preventing further degradation of that reach, as well as those
upstream.
• Recognizes that less intensive efforts may be required to improve stream reaches in Stages
6 and 7 than those in Stages 2 through 5 (see Section 6.5 on stream design and
implementation).
• Recognizes the role lateral stability plays in accelerating degradation and bank stability
problems in Stages 3s through 5, and that mitigation approaches must consider the lateral
instability as well as vertical.
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4.0 What is the baseline condition?
Baseline condition reflects the current functional capacity, or the degree to which a resource
performs a suite of specific functions, and is influenced by the level and source of stressors
affecting the proposed mitigation site. Establishing the baseline condition and identifying stressors
acting on the proposed site provides insight into factors requiring amelioration to enhance,
establish or re-establish functions. These factors would then need to be addressed and/or
considered as limitations in the mitigation project. In essence, this step of the Approach provides
information to evaluate site selection and the ecological suitability of a proposed mitigation site in
relation to the level and source of stressors affecting the site. This step also identifies initial site
data that will inform target condition and evaluate progress towards achieving performance
standards.
Site-specific baseline information should be provided early on in mitigation project reviews, as it
supports site selection, mitigation goals and objectives, workplans and other required elements. At
the prospectus or pre-application stage, some baseline data should be provided, including an
evaluation of both watershed and site-scale stressors and the results of any rapid assessments. At
this stage, baseline information should be sufficient to support the ecological suitability of the
mitigation site, inform project objectives and the mitigation activities proposed (e.g., what stressors
should be addressed) and should indicate where more detailed data or additional analyses may be
needed to inform design, performance standards and monitoring.
When reviewing and commenting on early stages of mitigation proposals, reviewers should provide
comments to highlight areas where additional data collection may be needed to finalize the
mitigation plan and should consider whether results of initial baseline analyses support site
selection, project objectives and proposed mitigation activities. Reviewers should also consider
requesting a site visit if one has not already been coordinated for the IRT. During review of
complete draft mitigation plans, reviewers will want to ensure all necessary baseline information
has been collected and included before the mitigation plans are finalized. The main opportunity to
provide these comments will be during the Public Notice comment period for individual permits or
during review of the draft prospectus and/or mitigation bank instrument or ILF project addendums.
Important elements of a baseline assessment include evaluation of watershed and reach or site-
scale stressors, results from approved rapid assessment approaches, and any necessary
measurements to characterize the existing conditions on site and inform project design and
performance standards. Baseline data represent the starting condition of the site that will be
compared to monitoring data in the future to ascertain level of performance (Questions 4 and 5).
This information should also be compared to existing reference data or reference data collected by
the provider to establish the site's relationship with a reference population (Question 4).
A site's baseline condition is affected by factors operating at a watershed scale as well as those
operating at the site or reach scale. For wetlands, stressors are typically evaluated at the sub-
watershed or catchment (i.e., area draining directly to the wetland) as well as at the site scale
(Sections 4.2 and 4.4). Stream stressors can be evaluated at a landscape, valley segment/type,
stream reach, and/or a site or habitat unit scale, all of which represent the spatial context for
various stream functions (Figure 4). However, to simplify this Approach, essentially three scales
(i.e., watershed, riparian, and reach) are described for streams (Sections 4.2 and 4.5), and all affect
the potential of a site to be restored. Riparian scale refers to the area up- and down-stream of the
project reach (reach scale) that encompasses the riparian area and is important for evaluating
41
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sediment dynamics that may influence the site being restored. Evaluation of information at
multiple spatial scales is important since impairments or stressors at the watershed, catchment,
riparian and/or site scale may limit the potential of a particular mitigation project to improve
ecosystem functions. For example, depressional wetlands located in an active agricultural field may
not support amphibians due to the input of sediment and chemicals, as well as being limited by the
adjacent habitat needed by certain species for dispersal. Similarly, an incised, entrenched channel
that will be maintained in a degraded stage due to active channel maintenance or watershed land
uses is likely to have limited capacity to restore full functional capacity. Such limitations should be
documented by the provider early and may limit the ecological suitability of a site for compensatory
mitigation.
SpataaD SteaD®
v'"*y'mRidm
HtinfemiWin|gr
Steep—Confined valley,
colluvial moderate gradient
valleys- -«»¦
Unconfined alluvial fan,
low gradient
Stream Functions
Maintain stream evolution processes
Energy management processes
Provide for riparian succession
Surface water storage processes
Maintain surface/subsurface connections and processes
General hydrodynamic balance
Sediment continuity
Maintain substrates and structural processes
Quality and quantity of sediments
Support biological communlltes and processes
Provide necessary aquatic and riparian habitats
Maintain trophic structure and processes
Maintain water and soil quality
Maintain chemical processes and nutrient cycles
Maintain landscape pathways
o
3
o
O
o
=y
CT>
Figure 4. Spatial scales, relevant stream functions and corresponding indicator parameters for those
functions. Stream functions and categories from Fischenich (2006); reproduced from David et al. (2021).
42
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Sampling and analytical methods, to the extent possible, should be standard accepted protocols
that lead to the consistent and repeatable production of data. The provider should identify these
objective and repeatable methods in planning documents before data collection begins. These
same methods should be used consistently throughout the life of the project to be comparable
between the baseline stage (pre-project) and the monitoring of performance (i.e., post project).
Likewise, the parameters measured during the baseline assessment should be the same as those
used to measure performance (e.g., plant composition, plant cover, soil organic matter, depth to
seasonal high-water table, and flood frequency). This section provides background information on
evaluating watershed and site-scale stressors, as well as technical information on parameters and
methods to assess condition in both wetlands and streams.
4.1 Regulatory Context
The Mitigation Rule explains that site selection should
include consideration of watershed needs and the
practicability of accomplishing self-sustaining aquatic
resource restoration, enhancement, etc., at the
proposed mitigation site (40 CFR §230.93(d)).
Watershed-scale features, such as aquatic habitat
diversity, habitat connectivity, stressors and other
landscape-scale functions, as well as the size and
location of the compensatory mitigation site relative to hydrologic sources and other ecological
features, should also be considered. These factors are components of a watershed approach to site
selection.
The Mitigation Rule explains that the goal of the watershed approach is to "maintain and improve
the quality and quantity of aquatic resources within watersheds through strategic selection of
compensatory mitigation sites," (33 CFR §332.3(c); 40 CFR §230.93(c)(1)). The watershed approach,
therefore, requires consideration of how the types and locations of potential compensatory
mitigation projects will sustain aquatic resource functions in the watershed and how surrounding
land uses will affect the ability of the potential site(s) to deliver those functions. Fundamental
considerations in a watershed approach include: landscape/geomorphic position (i.e., for wetlands
this refers to slope, depression, flat, etc.); resource type (i.e., wetland or stream); habitat
requirements of important species; trends in habitat loss or conversion; sources of watershed
impairment; and current development/land use trends. Gathering information at a watershed scale
is intended to broaden the geographic extent of information considered during project planning to
an area outside the project boundaries but within which land uses can affect the site's restoration
potential.
Baseline information is one of the 12 elements required in a mitigation plan. The Mitigation Rule
describes baseline information as the ecological characteristics of the proposed compensatory
mitigation project site including: historic and existing plant communities, historic and existing
hydrology, soil conditions and other site characteristics appropriate to the type of resource
proposed as compensation. Together, the Mitigation Rule requirements for baseline information
and the watershed approach for site selection lay the groundwork for this question (Figure 1).
The ultimate goal of a watershed
approach is to maintain and improve
the quality and quantity of aquatic
resources within the watersheds
through strategic site selection and
reconnection of these resources to
up-and downstream waters.
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4.2 Watershed-scale Baseline Assessment (Wetlands and Streams)
This section of the Approach provides a summary of potential tools, resources, and datasets
available to conduct a watershed-scale baseline assessment. A watershed assessment is intended
to provide enough information to understand the existing condition of the watershed, how well it
functions ecologically, and a site's mitigation potential. Ultimately, the information provided in the
mitigation plan should identify potential causes of impairments and potential actions. The NRC
(2001) noted that watershed structure governs the hydrologic and nutrient flows through the
watershed. This structure also drives the hydrologic processes that develop and sustain streams
and wetlands in the watershed, and thus establishes the relationship between wetland functions
and watershed position, or stream function within the watershed (EPA, 2015). Understanding the
linkages between the abiotic and biotic components of the ecosystem and the surrounding
landscape serves as the basis for not only assessing ecosystem functions, but also attempting to
replace/restore/enhance them (Richardson et al., 2011).
In light of the connection between wetland and stream ecosystems and their surrounding
watersheds (EPA, 2015), as well as requirements in the Mitigation Rule to evaluate proposed
compensatory mitigation sites in a watershed context, mitigation plans should include a watershed
assessment that identifies watershed-scale stressors and their potential influence on the site's
sediment and water quality functions, hydrologic potential and landscape connectivity (Box C). In
addition, mitigation plans should assess the habitat potential of a site based on its proximity to
biological refugia. Indicators, such as land use, hydrology, water quality, aquatic resources, habitat
connectivity and landscape connectivity, can be used to characterize these functions, and should be
assessed and reported by the mitigation provider.
Box C. Recommended information to evaluate watershed-scale stressors.
Watershed scale issues that must be addressed include:
Sediment and water quality functions
• Are land uses impacting sediment supply to the stream reach or wetland?
• Are land uses potentially impacting water quality from urban or agricultural runoff?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting sediment/organic matter transport and/or timing?
• How much of the proposed site is contiguous to natural land cover, indicating adequate buffers?
• Are any water quality impairments occurring in the contributing catchment or watershed?
Hydrologic potential
• What is the propensity for site hydrology and wetness?
• Are land uses altering runoff processes (e.g., flashy hydrographs) to the stream reach or wetland?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting flow characteristics (i.e., magnitude, duration, frequency, timing, rate of change)?
• Does soil data support wetland conditions?
Landscape connectivity
• Are land uses impacting the connectivity of stream and wetlands to adjacent habitats?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings) limiting
biota movement (i.e., longitudinal connectivity)?
• Are any impediments limiting flow access to floodplain (lateral connectivity) during flood events?
• Is the site located within an ecologically connected hub or corridor that identifies both aquatic and terrestrial
connectivity?
Can the mitigation provider ameliorate adverse landscape-scale conditions with onsite management to reduce or
remove the effects of those conditions on restoration site potential?
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Given the availability of remote sensing data and GIS models and/or layers, it is feasible for
mitigation providers to complete an evaluation of a proposed site's watershed, to identify the
effect various land uses have on the frequency, timing, duration and magnitude of rainfall runoff,
sediment, nutrients, organic material, and pollutants to the project site. Such an analysis may also
locate natural areas providing ecological connections (e.g., corridors, buffers, refugia) to the
project site and may indicate opportunities to reconnect relatively intact areas by restoring the
connecting reach or wetland. GIS data layers are available nationally, but many states might have
finer resolution or supplemental data available for use by providers to determine the level of
watershed stressors affecting a proposed site.
Examples of datasets representing currently available data, which can be used to describe stressors
on sediment and water quality, landscape connectivity, and hydrologic potential of proposed sites,
are listed here and described in greater detail below.
• Land Use: National Land Cover Data (NLCD), USGS Gap Analysis Project (GAP);
• Water Quality: EPA's How's My Waterway (HMW), Water Quality Portal (WQP);
• Hydrology: National Hydrography Dataset (NHD), Topographic Wetness Index (TWI);
• Aquatic Resources: USDA NRCS SSURGO, National Wetland Inventory (NWI), Potential
Wetland Soil Landscapes (PWSL);
• Habitat Connectivity: National Ecological Framework (NEF), Southeast Conservation
Adaptation Strategy (SECAS) Blueprint;
• Longitudinal Connectivity: National Inventory of Dams (NID), Southeast Aquatic Resource
Partnership (SARP) Barrier and Inventory Tool; and
• Elevation or Terrain: Light Detection and Ranging (LiDAR) remote sensing data.
Other resources to inform watershed assessments include:
• Various resources that describe the watershed approach specific to CWA Section 404, such as
The Watershed Approach Handbook: Improving outcomes and increasing benefits associated
with wetland and stream restoration and protection projects developed by the Environmental
Law Institute and The Nature Conservancy (ELI and TNC, 2014).
• The Watershed Index Online (WSIO): developed by EPA as a national watershed data library and
tool. The Indicator Library includes ecological and stressor indicators based on national
datasets. While the WSIO was developed to compare across watersheds, there are state-
specific Recovery Potential Screening tools that provide access to data across 300 indicators.
The indicators are based on data that include land use, wetland inventories, protected lands,
soils, road density, population density, runoff, and CWA NPDES data among many others.
• Historic and contemporary aerial photography, including GoogleEarth and EagleView.
• StreamCat (the Stream Catchment Database): assembled by EPA's Office of Research and
Development (ORD) to support the National Rivers and Streams Assessment (NRSA) and publicly
available (http://www2.epa.gov/national-aquatic-resource-surveys/streamcat). StreamCat
greatly reduces the specialized geospatial expertise needed by providers and project managers
to acquire landscape information for both catchments (i.e., the nearby landscape flowing
directly into streams) and full upstream watersheds of specific stream reaches (Hill et al., 2016).
Layers included in StreamCat representing natural components are: landcover, soils, lithology
(surficial geology), runoff, and topography. Layers representing anthropogenic factors include
roads, dams, mines, U.S. Census Data on population and housing density, urban and agricultural
land use, imperviousness, and EPA Superfund sites.
45
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• Water Resources Registry: state-specific preservation and restoration models displayed on an
interactive online mapping tool created with the collaboration of a team of federal, state, and
local partners. Spatial analyses and resulting models are generally developed for four main
categories: wetlands, riparian areas, terrestrial areas, and stormwater management control.
CAVEAT: Use of these data sources is not completely diagnostic of a site's potential, nor will it be
definitive for recommendations of concurrence or non-concurrence with mitigation proposals.
Additional watershed-scale assessment should be undertaken to further review the factors
identified by the above-referenced GIS data layers. Many states may have higher resolution data
available that can help analyze watersheds within a particular geographic region (e.g., Water
Resource Registry, LiDAR).
Sediment and water quality functions
Example of Landuse Data: NLCD categorizes land uses into broad categories that
differentiate anthropogenic land uses (e.g., agriculture, residential, and urban) from natural cover
(e.g., forest, meadows, and wetlands). The USGS GAP includes landcover data, protected areas
data, and species data. The land cover data includes detailed vegetation and land cover patterns for
the continental U.S. The protected areas, data and species data, have been incorporated into the
NEF data described below to identify habitat connectivity.
Interpretation: Land use/land cover data represents the effects, or potential effects of
anthropogenic activities on the site potential for a compensatory mitigation project. Anthropogenic
cover types (e.g., high, medium and low development intensity, cultivated cropland, and developed
open space) indicate conditions that affect the hydrologic and material balance into and out of the
wetland and/or stream. Land uses typically increasing sediment supply include: upland drainage;
deforestation; surface mining; urban development; and agricultural drainage. Decreases in
sediment supply may result from dams and river regulation; reforestation; mine reclamation; and
sediment management (Skidmore et al., 2011). Urban and agricultural areas typically decrease the
ability of rainfall to infiltrate thus increasing the amount and velocity of runoff as well as affecting
the timing of flows. Therefore, proposed compensatory mitigation sites adjacent to developed land
covers will be at higher risk of not achieving full functional replacement. Similarly, for streams, the
NLCD layer can be used to understand how surrounding land use might impact site potential by
altering the hydrology and sediment supply, which in turn may affect the stream's ability to support
reference quality biological communities. Many land uses reduce the hydrological resistance (i.e.,
surface roughness, storage, and infiltration) as water moves through the watershed, leading to
more rapid runoff, increased peak flows and decreased low flows. Erosion and the discharge and
volume of storm runoff in watersheds increase as rural or vegetated areas are converted to urban
or agricultural uses. Reviewers should recognize these effects and ask how the mitigation provider
intends to address these issues.
Example of Water Quality Data: Multiple mapping resources are available to identify water
quality impairments within watersheds, including the HMW tool by EPA
(https://mywaterway.epa.gov/) and the WQP developed by the National Water Quality Monitoring
Council (https://www.waterqualitydata.us/portal/). These tools provide the ability to map
waterways to identify sources of impairment as well as download more specific datasets.
Interpretation: Water quality impairments provide an indication of how human activities in
the watershed may be adversely affecting aquatic organisms. Waters may be impaired for specific
constituents, such as sediment, temperature or nutrients, or generally for aquatic life uses.
Determining impairments can provide information on the types of stressors occurring in the
46
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watershed, and whether they could be overcome by mitigation activities proposed at a project site.
For example, a watershed may be impaired for heavy metals from historic mining activities, which
will limit aquatic life use within the project reach. Unless the project proposes to ameliorate the
sources of these heavy metals, improvements to aquatic life may be limited. Alternatively, if there
are temperature impairments, restoration, establishment, or enhancement of canopy and shade
cover as part of a project may provide beneficial thermal refugia for aquatic organisms.
Hydrologic potential
Example of hydrology data: TWI data layer combines flow accumulation and direction from
the NHD with digital elevation models to indicate areas likely to accumulate water on the
landscape. This reflects not only potential wetlands, but stream channels as well. NHDPIus High
Resolution Data has additional attributes for catchment areas and catchment characteristics.
Interpretation: The TWI provides a basis for understanding the propensity for an area to be
wet. However, it does not represent groundwater presence, nor does it indicate duration of
wetness. The TWI is based on an analysis of land elevations and cover, aerial images, and NWI to
determine increasing probability of wetness (Wolock and Price, 1994). As TWI values increase, the
probability of encountering wetness supporting wetland hydrology also increases giving the
reviewer information about wetland site potential based on the probability that the site's
topography contributes to the accumulation of water.
Example of aquatic resource data: The PWSL was created by NRCS to combine SSURGO
data layers that indicate hydrologic and soil conditions indicative of wetlands. PWSL was developed
to help identify sites that have hydric soils as a dominant or named component but are not mapped
as NWI wetlands (Galbraith et al., 2011), and have the potential to be wetlands. Areas with
dominant hydric soils might not be identified as wetlands for a number of reasons such as: a) the
area was incorrectly mapped on the NWI maps, b) the area is missing wetland hydrology, c) the
area is protected from flooding, and/or d) the area is not dominated by wetland vegetation.
Interpretation: Sites that are missing hydrology or vegetation might yet be suitable sites for
mitigation based on the soil information in PWSL. This data also provides information about
flooding and ponding frequency, which can help determine the HGM classification of wetlands as
well as the site's potential to support wetlands.
Landscape Connectivity
Example of habitat connectivity: The NEF indicates landscapes with large accumulations of
natural habitat either in hubs or corridors. The NEF defines ecological hubs as priority ecological
areas consisting of at least 5000 acres of natural ecosystems, while corridors are derived from least-
cost pathway (i.e., least human disturbance pathway) analyses among hubs. Auxiliary connections
are manually derived adjustments for connected areas not captured in the computer-based least-
cost modeling. The NEF was developed from the Southeastern Ecological Framework (Durbrow et
al., 2001). The SECAS Blueprint is a living spatial plan that identifies important areas for
conservation and restoration across the Southeast. The Blueprint stitches together smaller
subregional plans into one consistent map incorporating the best available information about the
current condition of key species and habitats, as well as future threats.
Interpretation: NEF provides an indicator of ecological connectivity and provides useful
information about the larger landscape surrounding mitigation sites. The NEF and SECAS data
indicate ecological connections between natural habitats, as well as indicating the extent of up-
stream and down-stream riparian buffers. Connectivity of natural areas to a proposed mitigation
47
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site is an important aspect of maintaining wildlife habitat (Ainslie et al.,1999; Wigley and Lancia,
1998). Therefore, potential mitigation sites located in a hub or corridor, or that can be connected to
a hub or corridor have the potential added benefit of contributing to wildlife habitat at the
landscape scale. For streams, if a proposed reach is located within a hub or associated with
upstream or downstream corridors, it has greater site potential than a reach that has no associated
ecological connections, because it has riparian buffers that are either already established or, if not
established, can potentially be connected to existing upstream and downstream riparian areas.
Example of longitudinal connectivity data: The NID is a congressionally authorized database
documenting the current state of dams in the U.S. Specifically, dams included in the NID must have
one of the following: a high- or significant-hazard potential classification, equal or exceed 25 feet in
height and exceed 15 acre-feet in storage, or equal or exceed 50 acre-feet storage and exceed 6
feet in height. Last updated in 2016, it is maintained and published by USACE, and contains
information, collected from state and federal regulatory agencies about a dam's location, size,
purpose, type, last inspection, and regulatory facts. SARP barrier data includes the location of
smaller dams and road-stream barriers, as well as parameters that identify the amount of habitat to
be reconnected by barrier removal. SARP data can be found at https://connectivity.sarpdata.com/.
Interpretation: Dams can impact stream hydrology in many ways, such as: reduce peak
flood flows, increase low flows due to dam releases, increase duration of moderate flows, and
generally alter the stream's flow regime as compared to unimpaired pre-dam flows (Poff at al.,
2006). The change in downstream hydrology from a dam can reduce the sediment supply to
downstream reaches which may increase scouring in the channel. Reduced peak flows can affect
riverine wetlands that depend on overbank flows as a hydrologic source, as well as a mechanism for
the import and export of sediment, nutrients, and organic matter. Reviewers should identify dams
in the proximity of a proposed site and have the longitudinal, lateral, vertical, and temporal effects
on the site assessed.
Interpreting Watershed Baseline Assessments for Streams:
Like wetlands, stream functions are largely governed by the timing and delivery of water,
sediments, and nutrients from the watershed. Stream functions are also influenced by vegetation
and large wood that contribute organic material, as well as dissipating hydraulic energy on stream
beds, banks and floodplains. Decades of research has demonstrated many of the threats posed to
river and stream ecological integrity are a result of human actions occurring at the watershed scale
(Allan, 2004). These effects tend to accumulate within watersheds, both over time and in the
downstream direction, and it may take decades for the stream to recover, if ever (Harding et al.,
1998; Pond etal., 2014).
Stream condition research has identified threshold values or ranges at which the percent cover of
various land uses has strong correlations with various measures of stream condition (Allan, 2004).
For example, both Paul and Meyer (2001) and the Center for Watershed Protection (2003) provide
excellent reviews of the effects of urbanization (i.e., impervious surfaces) on the physical, chemical,
and biological conditions of streams. Many research studies summarized in these reviews have
clearly detected degradation of physical, chemical, and biological conditions in streams when the
proportion of total impervious surfaces in watersheds reaches 10 to 20 percent. Similarly, Petty et
al. (2010) found that ecological impairment to streams becomes almost ubiquitous in the central
Appalachian region when the spatial coverage of coal mining in watersheds exceeds 18 to 20
percent. Further, Petty et al. (2010) and Bernhardt et al. (2012) both found that biological and
water quality impairment of streams in this region can become apparent at mining intensities as
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low as 2 to 5 percent. Poff et al. (2006) found that increases in urbanization and agricultural
landcover increased peak flows and flow variability, while decreasing low flows in the southeastern
U.S.
Allan (2004) cites numerous studies documenting declines in water quality, habitat, and biological
assemblages as the extent of agricultural land use increases in watersheds. However, the effect of
agricultural land uses on streams is somewhat variable. Wang et al. (2011) found obvious declines
in stream habitat conditions and biotic integrity (via fish Index of Biotic Integrity) that were
apparent only when agricultural land use exceeded 50 percent in Wisconsin watersheds. However,
Walser and Bart (1999) found significant increases in stream sedimentation and declines in fish
diversity in Piedmont streams in the Chattahoochee River drainage basin (Georgia) when the
percent agricultural land use in the watersheds exceeded 20 to 30 percent. Finally, Braccia and
Voshell (2007) found that biological integrity, as measured by benthic macroinvertebrate metrics
and community assemblage composition, was highly related to the density of cattle grazing
adjacent to small streams in the Blue Ridge mountains of Virginia. Specifically, the number of
sensitive taxa declined markedly when the density of cattle exceeded only one cow per hectare
(Braccia and Voshell, 2007).
Activities affecting watershed-scale processes may be of such a nature that alterations to those
processes cannot be undone (e.g., urban or mined watersheds), and that due to the condition of
the watershed and the goals and objectives of a project, mitigation efforts focused only on a
wetland or stream reach would be largely ineffectual. In these cases, the site potential for
functional lift may be considered low or the project severely constrained. On the other hand,
watersheds that have been impacted, but have best management practices (BMPs) or other
remedies in place (e.g., watershed improvements, off-line stormwater management, and floodplain
lowering), may be considered to have more potential for siting a compensatory mitigation project
than those that do not because those BMPs may effectively ameliorate alterations to water,
pollutant, nutrient, and sediment flows reaching the wetland or stream mitigation site. Therefore,
assessment of land uses leading to stream impairment must take place at both the watershed and
the reach scale.
In addition to assessing watershed stressors, it is important to evaluate mitigation site proximity to
refugial habitats. Factors affecting the colonization of restored stream reaches by desired fish or
benthic macroinvertebrates include water quality conditions (e.g., Kowalik and Ormerod, 2006) and
the availability of a pool of organisms for colonization (Cornell and Lawton, 1992; Blakely et al.,
2006; Hughes, 2007; Lake et al., 2007; Tonkin et al., 2014). For example, Sundermann et al. (2011)
found that benthic macroinvertebrate assemblages improved in restored stream reaches only when
there were source populations of the desired taxa within 5 km of the restored sites. Based on their
evaluation of 21 stream restoration sites (ranging from 1 to 12 years old) and over 290 additional
sites in the stream network within 10 km of the restored reaches, Tonkin et al. (2014) found that
taxon pool occupancy rate (i.e., frequency and density of occurrence for a given taxa in near-vicinity
stream reaches) was the most important factor affecting the likelihood of colonization of that taxa
in the restored stream reach. The next most important factor cited by Tonkin et al. (2014) was
distance to nearest source, with the first kilometer away from the restoration reach particularly
important. Collectively these studies reinforce the importance of site selection and watershed
conditions to biological success of stream restoration and enhancement efforts. Proximity of a
proposed site to probable refugia should be considered by the mitigation provider and evaluated by
the mitigation reviewer.
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4.3 Use of Rapid Assessments in Evaluating Baseline Condition
Providers may include rapid aquatic resource (wetland and/or stream) assessments as part of the
baseline assessment of a project site and/or USACE Districts may have prescribed methods to
characterize current conditions. Rapid assessments can provide useful information by placing the
baseline condition of a stream or wetland into context based either in terms of the project site's
ecological condition (i.e., the relative ability of an aquatic resource to support and maintain a
community of organisms having a species composition, diversity, and functional organization
comparable to reference aquatic resources in the region (40 CFR §230.92)) or function (i.e., the
physical, chemical, and biological processes that occur in ecosystems (40 CFR §230.92)). Many
wetland and stream rapid assessment methods have been developed over the years (Fennessey et
al., 2007; David et al., 2021; Dorney et al., 2018; Somerville, 2010). Aquatic resource assessments
can be divided into two general types: condition or functional assessments. Generally, condition
assessments represent point-in-time assessments, or snapshots, of a stream or wetland's condition,
while functional assessments represent processes occurring over time and may provide more
insight into what functions are impaired and the stressors contributing to the impairment. Either
type of rapid assessment can be useful in a baseline assessment, particularly if calibrated to
reference sites that occur across a range of disturbances. These can be helpful to compare the site
to proposed target conditions, develop mitigation plans, and provide context to performance
standards.
Condition assessments measure the extent to which a site departs from full ecological integrity
using relatively simple field indicators to provide a measure describing the position of the site on a
continuum ranging from full ecological integrity (least impacted or reference condition) to highly
degraded (poor condition) (Fennessey at al., 2007). This relative score may be useful to give the
reviewer an overall indication of the site's condition but may lack the specificity to determine the
cause of impairments and/or information on ameliorating the impacts.
Functional assessments are considered better suited for application to compensatory mitigation
projects. Functional assessment methods utilize field indicators to characterize chemical, physical,
and biological functions and assess the functional capacity of wetlands or streams to perform these
functions. Indicators are typically chosen to represent characteristics and processes of the wetland
or stream that are responsive to impacts (e.g., vegetation changes, changes to flow regime,
geomorphology, habitat and secondary effects) and stressors. These indicators could be used to
detect change across typical post-construction mitigation monitoring timescales (e.g., 5-15 years)
and spatial scales over which the assessment method is being applied.
An example of a wetland functional assessment method useful for determining baseline condition
is the HGM approach. HGM explicitly links the characteristics and processes contributing to wetland
functions with field indicators that are measured against reference and aggregated into functional
capacity indices via basic logic multiple criteria assessment models. These assessment models are
basic logic models representative of "the relationship between attributes of the wetland ecosystem
and the surrounding landscape, and the functional capacity of the wetland," (Smith et al., 1995).
The HGM assessment approach accompanies the wetland classification completed in Question 1
and is discussed below (Section 4.4.1).
The Stream Quantification Tool (SQT) is an increasingly used stream assessment approach in the
stream mitigation program. The SQT is not a functional assessment, as it measures the point-in-
time condition of structural variables rather than relying on measurements of functions themselves
and does not link the variables via logic models to functions. As such, the SQT is considered a
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function-based assessment. What is novel about the SQT is that it aggregates multiple individual
variables into a single tool to calculate the loss and lift of aquatic resource functions related to
various impacts or restoration efforts. Therefore, the SQTs are more like a calculator than an
ambient stream assessment protocol. Parameters included in SQTs are quantitative and can rely on
rapid or more detailed data collection methods, and thus are more robust than qualitative rapid
stream assessment approaches. The SQT assessment approach can be used to support baseline
condition assessment and is discussed below (Section 4.5.1).
In instances when HGM, SQT or similar types of assessments are not available, other rapid
assessment methods may be used in the collection of baseline information. Reviewers can gauge
the effectiveness of a particular rapid assessment method by its ability to:
• Identify stressors acting on the site;
• Identify aquatic resource functions;
• Be applied to the variety of wetland and/or stream types in the region of use;
• Represent the functional capacity of a site calibrated against reference; and
• Provide field verification.
Regardless of the rapid assessment method used, most are limited in terms of resolution and
quantitative data, and often lack the ability to capture all the parameters that are relevant for
more-substantial projects (Somerville and Pruitt, 2004; David et al., 2021). In addition, rapid stream
assessments, which are often based on visual estimates of parameter condition or value, have been
shown to invite observer bias and suffer from poor objectivity and precision (i.e., repeatability)
(Hannaford and Resh, 1995; Roper and Scarnecchia, 1995; Poole et al., 1997; Kaufmann et al., 1999;
Somerville and Pruitt, 2004). Even field crew training does not consistently improve observer
variability among crew members using the same rapid assessment protocol on the same group of
streams (Hannaford et al., 1997). Stauffer and Goldstein (1997) also report that common rapid
stream assessments include many highly correlated parameters and thus overemphasize some
stream features and diminish the influence of others. Thus, assessing the baseline condition of a
proposed site with rapid assessment methods alone will likely not provide sufficient information to
ascertain watershed, riparian, and reach/site scale stressors or the current functional capacity of
the pre-project site. More detailed site data, either calibrated with reference data or empirically
derived, will likely be necessary (see Section 4.4.2 and 4.5.2).
4.4 Wetland Baseline Condition (Site-scale)
The following sections organize the discussion of wetland baseline condition into general wetland
assessment considerations, as well as technical information for assessing vegetation, soils and
hydrology. In addition to an evaluation of watershed stressors, reviewers will want to ensure a
baseline assessment includes results from any approved rapid assessment approaches, and/or any
necessary measurements to characterize the existing conditions on site, including site-scale
vegetative, soil and hydrologic characteristics and any local site-scale stressors present (Box D).
Remote data, such as information from hydrology and soils data layers (see Section 4.2), should be
verified at the site scale. Local disturbances/stressors should also be verified, and any additional
site-specific limitations should be inventoried and evaluated. These local, site-specific factors will
affect baseline condition; and they represent the wrongs that may be righted through the
implementation of the compensatory mitigation plan.
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Box D. Recommended information to evaluate wetland baseline condition.
Information below is in addition to, and complementary of, the results of the watershed analysis:
• Is the site in the reference domain of an existing HGM regional guidebook?
• Is the wetland of similar class and subclass to that in the HGM guidebook? List the wetland functions of the
site and how they have been impaired.
• If HGM guidebook is used, what are the results?
• Was a rapid assessment method used? If so, which one and what were the results?
• What stressors are present on the site (e.g., clear-cut, ditching, fill)?
• Has plot-level vegetation data been appropriately stratified and sampled at a sufficient intensity? If so, how
was the information interpreted? What effect have stressors had on vegetation?
• Has site-specific soil mapping and soil verification been completed? What effect have stressors had on soils?
• Have soil profile descriptions, notation of hydric soil indicators, and impacts to soil integrity (e.g., tilling,
plowing, bedding, land clearing, mining, and fill) been provided?
• Is there available hydrologic monitoring data, depending on HGM class and soils (e.g., surface water ponding,
shallow groundwater/water table data)? For what duration?
o Are there active stream stage recorders? For what period of record is data available?
o Are there rain gauges in proximity to the site? For what period of record is data available?
o Are there active ground water wells? How long have they been active and at what frequency/interval
is the data available?
• Have results from hydrologic monitoring been graphically represented with rainfall data?
• What effect have the stressors had on ground and/or surface water hydrology?
4.4.1 Wetland Assessment
Reviewers should evaluate whether the mitigation provider's baseline assessment approach
appropriately characterizes the wetland functions of the site. There is no expectation for reviewers
to conduct wetland baseline assessments using HGM guidebooks, or any of the other rapid
assessment protocols currently used. However, HGM or other rapid assessment approaches may be
included in a baseline assessment and reviewers should become familiar not only with HGM
classification but also with the rationale, structure, and basis for the functions, stressors, and
indicators of any method used. Pertinent HGM guidebooks can provide not only general
background information about the regional wetland subclass, but also functional capacity index
curves which can be used as estimates of indicator response to disturbance.
Wetland classification (Section 3.2.) provides useful information about the current hydrogeographic
context of a proposed mitigation site and a segue into assessing wetland baseline condition at the
site. Wetland functions, defined by Smith et al. (1995) as "...the normal activities or actions that
occur in wetland ecosystems, or simply, the things wetlands do," provide context and organize
pertinent factors that must be addressed in a mitigation proposal. In general, HGM guidebooks and
their associated reference datasets are a good starting point for gaining an understanding of the
functions of different wetland types and can be used to frame baseline data assessment to ensure
hydrology, soils, and vegetation data relate to wetland functions on site. All HGM regional
guidebooks are available at: https://wetlands.el.erdc.dren.mil/hgmhp.html.
Current HGM regional guidebooks in the southeast address most of the classes typically identified
(with the exception of lacustrine fringe) and describe many wetland subclasses over a large portion
of the southeast (Figure 5). HGM regional guidebooks for a variety of HGM wetland subclasses in
the southeastern U.S. are listed in Table 7. Major Land Resource Regions (MLRA) are designated by
NRCS based on similarities in physiography, geology, climate, water, soils, biological resources, and
land use and represent areas where HGM regional guidebooks could potentially be expanded or
extrapolated.
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HGM Guidebook Domains
AK North Slope*
Rocky Mountain Riverine Floodplain
Intermontane Praine Potholes
Northcentral & Northeastern Peatlands*
Praine Potholes
Rain Wator Basins
| Upper Oes Piames Basin Depressions
Wet Pine Mineral Flats
| W-KY Riverine
Highland Rim*
Appalachia'
Piedmont* (in development)
Southeastern Coastal Plain*
Lower Mississippi Valley*
| East TX Alluvial Valleys*
NW Gulf Tidal Fringe
MS Yazoo Basin*
AR Ozarks*
AR Delta*
AR Arkansas Valley*
AR Ouachitas & Crowleys Ridge* |
| AR Coastal Plain"
W-TN Low Gradient Rivenne
| MS-AL Tidal Fringe
| MS-AL Slope
SC Headwater Slopes
FL Low Gradient Blackwater Hardwood Forests
I Everglades Marl & Rocky Organic Flats
FL Cypress & Herbaceous Depressions
•Multiple subclasses included.
Figure 5. Reference domains of HGM regional guidebooks in the southeast and elsewhere in the United
States (reproduced from ERDC HGM website).
A general list of functions organized by HGM wetland classes (Table 8) can be used to understand
and describe hydrologic, biogeochernical, and plant and wildlife habitat functions of wetlands
within these hydrogeomorphic classes. There are some functions which are not performed by
certain wetland types due to their geomorphic position and/or water sources. For example, mineral
and organic soil flats do not perform a sediment retention function since that requires a flowing
water vector to carry the sediment into the wetland. Since flats are driven by precipitation, there is
not the flowing water energy, or hydrodynamics, necessary to carry sediment into the flat. Even in
situations where similar functions are performed by different wetland classes, they are not
performed at the same rate or in the same way. In the instance of surface water storage, riverine
wetlands store surface water delivered from overbank flooding; depressions store surface water
delivered by overland flow, precipitation, and/or subsurface flow; and flats store rainwater that
ponds on the wetland surface.
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Table 7. HGM regional guidebooks developed in and applicable to assessing the functions of wetlands in
the listed subclasses in the southeastern U.S.
HGM Regional Guidebook
State(s)
MLRA
HGM Subclass(es) Covered
Tidal Fringe Wetlands Along the
Mississippi and Alabama Gulf
Coast (Schafer et al., 2007)
AL, MS
S. Atlantic and Gulf
Coast Lowland
Forest and Truck
Crop Region
Tidal Fringe Wetlands
Forested Wetlands in the Delta
Region of Arkansas, Lower
Mississippi River Alluvial Valley
(Klimas et al., 2004)
AR
MS Delta Cotton
and Feed Grains
Region
Flat; Mid-gradient Riverine, Low-
gradient Riverine Backwater, Low-
gradient Riverine Overbank, Headwater
Depression, Isolated Depression, and
Connected Depression.
Flats Wetlands in the Everglades
(Noble et al., 2002)
FL
Florida Subtropical
Fruit, Truck Crop,
and Range
Flats
Low-Gradient, Blackwater Riverine
Wetlands in Peninsular Florida
(Uranowski, etal. 2003)
FL
Florida Subtropical
Fruit, Truck Crop,
and Range
Low-gradient Riverine
Depressional Wetlands in
Peninsular Florida (Noble et al.,
2004)
FL
Florida Subtropical
Fruit, Truck Crop,
and Range
Cypress and Herbaceous Depressions
Forested Wetlands in the
Mississippi Alluvial Valley (Murray
and Klimas, 2013)
KY, TN, MO,
AR, LA, MS
MS Delta Cotton
and Feed Grains
Region
Flat, Low-gradient Riverine Backwater,
Low-gradient Riverine Overbank,
Isolated Depression, and Connected
Depression
Forested Wetlands in Alluvial
Valleys of the Coastal Plain of the
Southeastern United States (Wilder
etal., 2013)
VA, NC, SC,
GA, FL, AL,
MS, TN, KY,
LA, AR, TX
South Atlantic and
Gulf Coast Lowland
Forest and Truck
Crop Region
Headwater Slope, Low-gradient
Riverine, Mid-gradient Riverine, and
Connected Depression.
Headwater Slope Wetlands on the
Mississippi and Alabama Coastal
Plains (Noble etal., 2007)
MS, AL
South Atlantic and
Gulf Coast Lowland
Forest and Truck
Crop Region
Headwater Slope
Wet Pine Flats on Mineral Soils in
the Atlantic and Gulf Coastal Plains
(Rheinhardt et al., 2002)
NC, SC, GA,
FL, AL, MS,
LA, TX
South Atlantic and
Gulf Coast Lowland
Forest and Truck
Crop Region
Mineral Soil Flats (Pine savanna)
Flat and Seasonally Inundated
Depression Wetlands on the
Highland Rim (Noble et al., 2013)
TN, KY, IN,
AL
East and Central
General Farming
and Forest Region
Flats and Depressions
Low Gradient, Riverine Wetlands in
Western Kentucky (Ainslie et al.,
1999)
KY
East and Central
General Farming
and Forest Region
Low-gradient Riverine
Low-Gradient Riverine Wetlands in
Western Tennessee (Wilder and
Roberts, 2002)
TN
South Atlantic and
Gulf Coast Lowland
Forest and Truck
Crop Region
Low-gradient Riverine
Selected Regional Wetland
Subclasses, Yazoo Basin, Lower
Mississippi River Alluvial Valley
(Smith and Klimas, 2002)
MS
MS Delta Cotton
and Feed Grains
Region
Flats, Riverine Backwater, Riverine
Overbank, Isolated Depression,
Connected Depression
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Table 8. Potential functions performed by HGM wetland classes in the southeast U.S. (R = Riverine; MF =
Mineral Flat; OF = Organic Flat; S = Slope; D = Depression; LFr = Lacustrine Fringe; XFr = Tidal Fringe).
Processes
Functions
R
MF
OF
s
D
LFr
TFr
Description
Hydrologic
Store Surface
Water
X
X
X
X
X
X
Capacity to temporarily store and convey surface water
from overbank flows, precipitation, or runoff from
adjacent uplands.
Maintain
Subsurface Water
X
X
X
X
X
Capacity to store and convey subsurface water from
interflow and groundwater.
Cycle Nutrients
X
X
X
X
X
X
X
Ability to convert nutrients from inorganic forms to
organic forms and back through a variety of
biogeochemical processes such as photosynthesis and
microbial decomposition.
Biogeo-
chemical
Remove/
Sequester
Compounds
X
X
X
X
X
X
Ability to permanently remove or temporarily immobilize
nutrients, metals, and other elements and compounds
imported from sources outside the wetland.
Sediment
Retention
X
X
Capacity to physically remove and retain inorganic and
organic particulates from the water column.
Organic Carbon
Export
X
X
X
X
X
Capacity of wetland to export dissolved and particulate
organic carbon produced within the wetland.
Biological
(Plants)
Maintain Plant
Habitat
X
X
X
X
X
X
X
Capacity to provide the environment necessary for a
characteristic plant community to develop and be
maintained. This includes both existing plant
community and the abiotic conditions required to
maintain the characteristic community.
Biological
(Wildlife)
Maintain Wildlife
Habitat
X
X
X
X
X
X
X
Ability to support the wildlife species that utilize
wetlands during some part of their life cycle.
Use of HGM Assessments: Relevant HGM assessment protocols can be used to perform a baseline
site assessment. Results from these assessments transform and combine discrete field data into
indices that relate a site's functional capacity to reference standard wetlands. Regional guidebooks
not only identify and describe a suite of functions performed by each wetland class and associated
subclasses, and characteristics and processes contributing to those functions, but also identify
common disturbances that impair them (Table 9). The characteristics and processes associated with
each function are represented by field measures or indicators, which are scaled against reference
standard conditions, as well as across a disturbance gradient (see Section 5.3 for more discussion
on reference). Having parameters scaled across a disturbance gradient incorporates the effects of
disturbance into index scores. Consequently, HGM regional guidebooks can provide important
insight on the types and effects of disturbance on particular wetland types, and they can be
valuable as baseline assessment protocols for proposed wetland compensatory mitigation sites.
Note that the geographic coverage of all possible HGM wetland subclasses by guidebook reference
domains is not complete in the southeast. Similar HGM models could be used; however,
assumptions of applicability of particular assessment models would have to be made or
demonstrated if a mitigation site were assessed outside of the stated reference domain. In these
cases, additional reference data needs to be collected, compiled, and incorporated into existing
reference data. The task of collecting appropriate additional data from reference sites would likely
fall on the mitigation provider with input from the IRT.
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Table 3. Typical impairments affecting wetland classes of the southeast U.S. (after Wilder et al.» 2013). (R =
Riverine; MF = Mineral Flat; OF = Organic Flat; S = Slope; D = Depression; LFr = Lacustrine Fringe; TFr = Tidal
Fringe.)
Impairment
Affected HGM Classes
Effects
R
OF
MF
s
D
LFr
TFr
1. Ditching and draining
X
X
X
X
X
X
X
Eliminates wetland and wetland habitats,
shunts pollution and sediment directly into
downslope stream.
2. Filling to convert to
cropland, silviculture, or
impervious surface (e.g.,
roads)
X
X
X
X
X
X
X
Eliminates wetland and wetland habitats.
3a. Excavating,
straightening, and/or
stabilizing channels to
move water downstream
quickly (e.g.,
channelization, levees)
X
X
X
Reduces or eliminates hydrologic connection
between channel and floodplain, lowers
water table, reduces denitrification potential,
eliminates sediment accumulation, and
eventually changes species composition in
channel and on floodplain.
3b. Timber removed or
selectively cut from
wetland; natural
succession allowed
X
X
X
X
X
Removes biomass, changes species
composition, decreases biodiversity,
temporarily increases carbon detrital pool
(from slash), and disrupts nutrient cycling and
sequestration until forest regenerates.
4. Conversion to
intensively managed
industrial silviculture
X
X
X
X
X
Changes species composition, reduces
biodiversity, and reduces detrital carbon pool.
5. Damming channel for
flood control, recreation,
waterfowl management,
and/or power
X
X
Reduces sediment aggradation downstream;
changes frequency, timing, and duration of
overbank flow events; and changes species
composition on floodplain.
6. Groundwater withdrawal
from contributing aquifer
(e.g., center pivot irrigation,
pulp mills)
X
X
X
X
X
Reduces water table and duration of
saturated conditions, perhaps changing
species composition.
7. Deadfall removed from
channel
X
Reduces instream habitat, decreases
residence time of flooding, and reduces
source of dissolved and particulate organic
matter.
8. Stormwater runoff
shunted directly to channel
(e.g., often from impervious
surfaces)
X
X
Increases flashiness of hydrologic regime,
incises channels which decreases duration of
overbank flow events, and increases pollution
loading.
9. Excessive cover (>25%)
of invasive species
X
X
X
X
X
Reduces biodiversity by reducing habitat
heterogeneity for animals, reduces native
plant species populations, and may alter
nutrient cycling.
10. Fire exclusion
X
X
Affects plant community diversity and
regeneration.
11. Damming by road
X
X
X
X
Disrupts wetland water budget and inundates
upslope area causing longer than normal
hydroperiod.
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4.4.2 Wetland Site Sampling Considerations
The purpose of the baseline assessment is to capture the level and sources of degradation on the
site. Applicable HGM regional guidebooks and other rapid assessments can provide useful
information for assessing baseline condition but should not be used without additional site-scale
vegetation, soils, and hydrology information. Rapid assessment methods are often not sufficient for
use as a monitoring protocol alone to detect incremental changes in vegetative community
succession, soil anaerobiosis or saturation, or increasing or decreasing hydroperiod that occur
during the monitoring time frame of a typical mitigation project. As such, more detailed site
hydrologic, soil, and vegetation baseline data will likely be needed, particularly where specific data
and parameters will be used to inform performance standards and post-project monitoring. For
instance, given the importance of hydrology to wetland functions, onsite hydrologic data is
recommended to augment HGM and/or condition assessment results. If rapid assessments are not
available, or not calibrated in a particular area, hydrology, vegetation, and soil
sampling/characterization will need to be done as part of the baseline assessment. In addition,
relevant stressors (e.g., ditches, levees, and fill material) should be identified. Reviewers should
ensure parameters and protocols for baseline data collection are consistent with parameters and
protocols for post-project monitoring (Section 7.3).
Elements of a Baseline Wetland Site Assessment
Vegetation sampling intensity within each
habitat/cover type (area covered by plots) should
be between 1-5% of the total area, although some
have recommended covering as much as 20% of
the sample area with plots (Stein, pers. comm.
2018). For example, if a proposed site's total
acreage is 100 acres, then sampling 5% of that area
would mean 2 acres of sample plots would be
needed. If the sample design includes using 1/10-
acre plots, then 20 plots would need to be sampled
and the data analyzed and interpreted. Ideally, the
number of plots required to adequately
characterize an area should be determined using a
species-area curve (Kent and Coker, 1992).
Sampling is deemed sufficient when the species-
area curve flattens out.
Wetland Sampling Considerations
• Detailed plot-level vegetation, site
specific soil mapping and hydrologic
data (e.g., shallow ground water/
water table data, stream stage, and
surface water ponding) should be
collected and reported.
• Sampling and analytical methods
should be standard acceptable
protocols that lead to the consistent
and repeatable production of data.
• Parameters and methods should be
consistent between the baseline
assessment and post-project
monitoring.
Vegetation data should, at a minimum, include (Stein et al., 2022):
• Vegetation cover
• Community composition and structure
• Physical disturbance of plant community
• Invasive plants present and total cover
• Age-stand distribution
• Evidence of recruitment (sapling species and abundance)
• Shoreline or littoral habitat for tidal fringe wetlands
Soil classification and mapping of the site should be required as part of baseline data collection and
assessment with the production of a site-specific soil map. Location and areal coverage of each soil
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series, as well as disturbances (e.g., compaction, excavation, A-horizon removal, and
contamination) should also be mapped during the soil characterization. Soil classification and series
verification is, and should be, typically done by certified soils scientists. In addition to the soil series
verification, information should be collected on microtopographic relief of the site, soil structure,
organic matter accumulation, and bulk density, along with documentation of hydric soil indicators.
Assessment of the current hydrologic regime should rely on groundwater wells and/or staff gauges
in concert with a rain gauge, installed onsite during the baseline condition assessment. Data from
groundwater wells and stream gauges should be used with an analysis of rainfall data, which is
compared to the standards for normal rainfall (Sprecher and Warne, 2000), to provide as much
hydrologic information as possible prior to developing a detailed mitigation plan. Ideally, baseline
data collection should include the information needed to quantify the wetland water budget in
terms of inputs (i.e., precipitation, surface, and groundwater inflows), outputs (i.e.,
evapotranspiration, surface runoff, and groundwater outflows) and change in storage (see Section
7.4). Rain gauges are necessary to correlate the magnitude of rainfall events to water levels in the
wetland. Ideally, a continuous recording rain gauge should be installed on the mitigation site, which
can be correlated with either the stream stage and/or ground water changes. However, if a long-
term, maintained rain gauge (NOAA, USGS, USACE, State, and university, etc.) is in close proximity
to the site, it can be substituted for an onsite gauge.
HGM class and the associated soil survey information can provide insight into placement of wells
and stage recorders. For instance, riverine wetlands typically experience overbank flooding, shallow
groundwater near the surface from upslope sources, and even some ponding after flood events.
Therefore, these wetlands will require hydrologic monitoring in the adjacent stream channel as well
as groundwater wells in the floodplain. Mineral and organic soil flats are predominantly rainfall
driven wetlands. No stage recorders would be needed in this wetland class, but the use of
groundwater wells would be needed to verify saturation or ponding. The same is true of slope
wetlands, which are predominated by groundwater flow downslope. No stage readings are
required, but groundwater wells would need to be arrayed perpendicular to the direction of flow.
Fringe wetlands may only require stage recorders to assess the level and extent of flooding, while
depression wetlands may require either surface water and/or groundwater monitoring to
determine water sources, frequencies, and durations (Noble, 2006).
Graphical depiction of water levels in relation to rainfall should be included in baseline assessment
and subsequent monitoring reports. Graphing stage-discharge relationships in streams, in relation
to overbank/flood elevations, and depths to shallow groundwater in relation to ground surface
elevations can be critical to understanding the hydrologic regime of any given site. Any hydrologic
monitoring devices (e.g., wells, crest gauges, and stage recorders) must be surveyed into common
elevation benchmarks so that stage and groundwater elevations are consistent across the network.
Reviewers should request and look for hydrologic monitoring early in the review process in order to
compare before-project and after-project water levels.
The baseline data collected in the field on a proposed mitigation site must not only be sufficient to
inform design, but it must also be technically accurate and thorough enough to provide a datum
from which post-compensatory mitigation project development of the ecosystem may be
compared. Areas considered to have high wetland restoration potential tend to be hydrologically
and ecologically connected to other habitats upslope and downslope. They have soils indicating
appropriate conditions for wetland re-establishment, and they provide habitat structure for
suitable niches for a variety of wildlife and plant species. Finally, they also do not have land uses
that might contribute to the site being contaminated with pollutants.
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4.5 Stream Baseline Condition (Riparian and Site-scale)
The following sections organize the discussion of stream baseline condition into the same five
categories of stream functions/processes (hydrologic, hydraulic, geomorphic, physicochemical and
biological) as Harman et al. (2012) (Figure 6) and provide technical information for assessing
relevant stream functions. Baseline assessments of streams should characterize functional capacity
and how that capacity is affected by stressors. Rapid assessments can be used in the baseline
assessment but should be able to link current stream condition and/or function to the stressors
impacting those functions, and will likely need to be supplemented with specific watershed and
reach-scale data characterizing hydrology, hydraulics, current and future channel form(s), water
quality, and riparian and instream biology, particularly where these data and parameters will also
be used to inform performance standards and post-project monitoring (Box E). Reviewers should
identify connections between the stressors identified in the baseline assessment to the factors
which are either addressed in the mitigation plan or treated as constraints on the project.
Geology
Climate
Figure 6. Stream Functions Pyramid overview (reproduced from Harman et al., 2012).
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Box E. Recommended information to evaluate stream baseline condition.
Information below is in addition to, and complementary of, the results of the watershed analysis.
Hvdroloaic (Summary of hydrologic analyses conducted, data sources and period of record)
• How were flood flows, low flows, and bankfull flows calculated? Was a stream gauge used to evaluate the
period of record for the site? If not, how were streamflow estimates made (e.g., Manning's equation, regional
regression equations, StreamStats, hydraulic models, runoff and streamflow models)?
• Was floodplain groundwater elevation established? Is the groundwater flowing across the floodplain towards
the channel? At what time of year?
• What watershed and site scale stressors affect the hydrology of the site?
Hydraulic
• Were hydraulic models used? If so which ones? Was the confidence in the model results explained?
• Were sediment transport models used? If so, which ones? Was the confidence in the model results explained?
• What watershed factors contribute to the sediment supply?
o Severe bank erosion in the channel network? Mass wasting?
o Channel incision upstream?
o High availability of coarse sediment in bed or banks?
o What flow and sediment retention areas exist in the upstream watershed? (e.g., reservoirs, ponds, low
head dams)
• How will the evolution of the upstream channels effect sediment supply? What is the CEM / SEM stage above
the reach? Below the reach?
Geomorphic
• Methods used to complete channel stability assessment? Results?
• What is the current planform; longitudinal profile; and dimension of the project reach?
• What are the causes of channel instability? How were they determined?
• How do channel impairments effect stream and floodplain habitat? Is the channel laterally constrained?
• Extent of channel alteration (e.g., enlargement, floodplain encroachment, or channelization) of the project
reach?
Phvsicochemical
• Are state surface water quality standards being met? If not, how often and under what conditions are they out
of compliance?
• What are the sources of nutrients and other pollutants in the watershed and are they effecting the site?
• Is this reach on the 303(d) list or other TMDL for any of the following impairments: sediment, nutrient, metals &
toxics, temperature, or flow modification?
• Can the components of existing water quality that are limiting biological productivity be addressed at the
mitigation site (e.g., mass wasting, flooding, streamflow, shade, vegetation, and soils)?
Biological
• Biological community parameters sampled? Protocol? Results?
• Are habitat features altered? Cause of those alterations?
• Are there high-quality sites upstream and downstream of the proposed site? Would this project connect the
upstream and downstream sites?
• Are there sources of beneficial organisms that can migrate from the upstream or downstream portions of the
watershed to repopulate the site? Proximity of those source areas to the mitigation site?
• Are there invasive species (animals, plants, and algae) that will migrate from the upstream or downstream
portion of the site that would affect long term characteristics of the site communities?
• What is the percentage and average width of the intact riparian area contiguous to the site?
4.5.1 Stream Assessment
Reviewers should evaluate whether the mitigation provider's baseline assessment approach
appropriately characterizes the stream functions of the site. There is no expectation for reviewers
to conduct stream baseline assessments using SQTs or other rapid assessment protocols currently
used (see Section 4.3). However, rapid assessment approaches may be included in a baseline
assessment and reviewers should become familiar with the rationale, structure, and basis for the
functions, stressors, and indicators of any method used. Applicable rapid assessments can provide
useful information for assessing baseline condition but should not be used without additional site-
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scale baseline data for relevant stream functions, particularly where specific data and parameters
will be used to inform performance standards and post-project monitoring.
Categorizing functions, or processes, provides context and organizes pertinent factors that must be
addressed in a mitigation proposal (Roni and Beechie, 2013; Beechie et al., 2010; and others).
Functions are defined as the physical, chemical and biological processes that occur in ecosystems
(Harman et al., 2012). Fischenich (2003; 2006) proposed a list of fifteen primary stream functions
and illustrated the relationships among them by showing which functions influence others (Table
10). From this work, Harman et al. (2012) organized categories similar to Fischenich's into a
pyramid to illustrate the hierarchical nature of stream functions, with lower-level functions
supporting higher-level functions (Figure 6).
Table 10. List of primary stream and riparian functions and their interrelationships (Fischenich, 2003;
2006). Reproduced from Somerville (2010).
Functions Directly Affected
Functions Indirectly
Affected
System Dynamics
1.
Stream Evolution Processes
2, 3, 4, 5, 6, 7, 8, 9, 11, 13, 14, 15
10, 12
2.
Energy Management
1, 3, 4, 6, 7, 8, 9, 11
--
3.
Riparian Succession
1,2, 4, 6, 7, 11, 14, 15
10, 12
Hydrologic Balance
4.
Surface Water Storage Processes
2, 5, 6, 7, 11, 13, 14, 15
1, 3, 8, 9, 10, 12
5.
Surface/Subsurface Water Exchange
3, 6, 11, 13
4, 10, 12, 15
6.
Hydrodynamic Character
1,2, 3, 4, 5, 7, 8, 9, 10, 11, 13, 14, 15
12
Sediment Processes and Character
7.
Sediment Continuity
2, 3, 4, 8, 9, 10, 11, 13
6, 12, 14
8.
Substrate and Structural Processes
1, 2, 5, 6, 7, 8, 11
3, 10, 12, 13
9.
Quality and Quantity of Sediments
1, 2, 3, 5, 7, 8, 11
6, 10, 12, 15
Biological Support
10.
Biological Communities and Processes
3, 11, 12, 13, 14
5, 6, 8, 9, 14, 15
11.
Necessary Habitats for all Life Cycles
10, 12, 15
12.
Trophic Structures and Processes
10, 13, 14
9
Chemical Processes and Pathways
13.
Water and Soil Quality
9, 10, 12, 14
3
14.
Chemical Processes and Nutrient
Cycles
9, 10, 12
2
15.
Landscape Pathways
10, 11, 12, 14
2
Hydrologic functions influence other processes which contribute to stream ecosystem integrity, and
largely operate at a watershed scale. Thus, there is considerable overlap in assessing a project's
watershed context and reach-scale hydrodynamics. Closely related to hydrology are stream
hydraulic processes, which in this context are largely interpreted as the forces affecting sediment
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dynamics (e.g., sediment capacity, supply, and transport). Hydraulic processes also operate at a
watershed scale with contributions of sediment being generated and transported from upstream to
downstream. Geomorphic processes, supported by hydrologic and hydraulic processes, contribute
to stream stability, channel complexity and form. Geomorphic processes operate largely at the
reach scale, although watershed processes also clearly affect channel form and stability. Supported
by these three categories are physicochemical (stream water quality) and biological (stream and
riparian biota) functions. Stream water quality processes (e.g., nutrient and organic carbon cycling,
pollutant transport, and temperature) also tend to operate at a watershed scale as, like sediment,
the transport of materials occurs from upstream to downstream. Like water quality, stream biology
is influenced by watershed-scale processes but is often evaluated directly via sampling at the reach
scale.
There is considerable interaction and feedback between processes which ultimately stems from the
inherent natural variability and diversity in stream systems. For example, hydraulic diversity (e.g.,
various water velocity regimes in the stream) is caused by variable water flows interacting with the
physical structure of the stream bed (bedform diversity) and banks (riparian vegetation and coarse
wood). This hydraulic diversity contributes to habitat for biota that utilize different areas of the
stream at various times to fulfil life cycle requirements (e.g., resting, shelter, feeding, and
reproduction), and also contributes to sediment flow and deposition within a reach. Ecological
complexity is a result of the considerable interaction between stream processes and functions
(Table 10).
Regardless of this complexity, a simplifying assumption is that functional capacity, or the degree to
which a resource performs a specific function, will be influenced by stressors. In other words, a
stream without site and/or watershed level stressors performs at full functional capacity, while a
stream with stressors is more limited in its capacity to function. Thus, the identification of stressors
and their resulting impact on functional capacity is an important component to baseline
assessment. This, combined with the organizational framework from the functional pyramid,
provides a simple and logical means to review baseline condition data and understand important
aspects of a site's function and ecosystem integrity. Stressors identified in this step of the Approach
should then be addressed in a project's goals and objectives, implementation, monitoring,
performance, and long-term management.
The SQT is an assessment tool used to characterize the functional capacity within the functional
categories outlined in the Stream Functions Pyramid (Harman et al., 2012). SQTs have been
developed for numerous states, including Colorado (USACE, 2020), Georgia (Somerville et al., 2021),
Michigan (Ml EGLE, 2020), Minnesota (MNSQT SC, 2020), North Carolina (Harman and Jones, 2016),
South Carolina (South Carolina Steering Committee, 2021), Tenessee (TDEC, 2018) and Wyoming
(USACE, 2018). SQTs characterize these stream functional categories by evaluating a suite of
indicators representing structural or compositional attributes of a stream using function-based
parameters and measurement methods. The SQT approach integrates multiple function-based
parameters and metrics across multiple assessment scales (i.e., watershed, riparian and reach) from
these functional categories into a reach-based index score using index values per metric derived
from reference curves compiled primarily from literature values and regional reference datasets.
The function-based parameters, metrics and measurement methods used in SQTs are generally
familiar to the stream mitigation community, including mitigation providers, practitioners and
regulators.
Scale of assessment: There are essentially three scales at which stream baseline conditions should
be assessed: 1) the contributing watershed (see Section 4.2), 2) the riparian zone and 3) the site or
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reach scale (Johnson et al., 2015). The riparian zone encompasses the area adjacent to the stream
and for a distance upstream and downstream of the project limits. This scale encompasses riparian
processes contributing to stream functions, as well as acknowledges the importance of conditions
upstream and downstream of a project site in either contributing or controlling the flow of
materials to, or within, the project reach (e.g., sediment inputs from upstream and/or aggradation
or degradation downstream of the project area). The site or reach scale is the stream reach
proposed for mitigation. While reviewing the baseline assessment of a stream mitigation plan, the
scale at which hydrologic, hydraulic, geomorphic, physicochemical, and biological processes and
functions operate is important to consider. Providers often focus on the reach to be restored and
may not fully consider functional aspects that originate beyond the boundaries of the project site.
4.5.2.1 Hydrologic Condition Analysis
Hydrologic assessments determine if the current flow regime of the site has been altered by
stressors, including land uses, barriers to flow, diversions, or flow augmentation. Changes in flow
regime are typically reflected in changes to magnitude, frequency, timing, and/or duration of flood,
low, and bankfull flows. Hydrologic analyses characterize the seasonal and interannual variation in
the timing and volume of streamflow. The magnitude and duration of flow supplied to a stream is
heavily influenced by the upstream watershed characteristics including the drainage area,
topography, bedrock geology, surface soils, and the drainage network. Alterations to the
contributing watershed will have a significant impact on flow and transported constituents to the
site. As discussed in Section 4.2.2, as land uses within a stream's watershed change, so do the
characteristics of the stream's hydrologic regime.
Elements of a Baseline Hydrologic Assessment
Important aspects of a baseline hydrologic assessment include characterization of a stream's flow
regime, including the magnitude (i.e., how much), frequency (i.e., how often), duration (i.e., how
long), and timing /seasonality (i.e., when) of important flows (e.g., extreme low flows, baseflow,
bankfull, floodplain inundating flows). The following discussion is meant to provide the reviewer
with basic background information on common approaches used to characterize flow regimes. The
discussion also provides background information on important flow thresholds that a provider
should consider when assessing the current hydrologic condition. Reviewers should evaluate
whether appropriate methods have been used to characterize streamflow, and whether important
flows have been accurately characterized. Note that the scale of analysis will be commensurate
with the scope of a compensatory mitigation project, and thus, much of this information may not
be needed for small projects.
Characterization of flow regime
It is important for the reviewer to understand the range of flows affecting a mitigation site, and
how they may be currently or potentially influenced by stressors (Table 11). The level of the
provider's analysis, and selected methods, will be influenced by the presence and intensity of these
stressors. For example, many stream mitigation projects are located in watersheds with increasing
impervious surface, often in urban areas. Flow variability due to imperviousness is often manifested
as rapid short-term variations in streamflow during runoff events, referred to as flashiness (Bledsoe
et al., 2017). Flashiness is strongly related to more frequent delivery of sediment to urban channels
due to increased stormflow runoff velocities associated with the increases in impervious surfaces. If
stressors linked to repeated, flashy flows are identified, bankfull and flood flows would be affected,
and results of the hydrologic analyses should illustrate this relationship.
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Table 11. Common stressors affecting stream hydrologic regime {after Johnson et al., 2015).
Common stressors
Examples
Explanation
Land
Forest thinning
Silviculture
Development
Increased runoff rate causing higher peak
discharge, increased flashiness.
Vegetation/ cover shift
Grazing
Agriculture
Development
Increased runoff rate causing higher peak
discharge, increased flashiness.
Impervious surface
density
Roof density
Roads
Pavement
Compacted soils
Increased runoff rate causing higher peak
discharge, increased flashiness, loss of
groundwater recharge associated with roads and
development
Groundwater pumping
Ag wells
Municipal wells
Aquifer depletion or water table depression that
limits groundwater recharge or exacerbates loss
Drainage
Dams, reservoirs
Large dams
Small dams
Artificially managed hydrology, attenuated peak
flows, evaporative losses
Diversions/ augmentation
Irrigation
Municipal
Intercept ditches
Off-channel
Water extracted from the watershed or stream
system via diversions and ditches
Widespread floodplain
disconnect
Floodplain encroachment
Channelization
Incision
Decreased aquifer recharge and diminished water
storage in the contributing watershed due to lack
of floodplain activation or loss of tributary wetlands
Widespread wetland loss
Various causes
Methods used in estimating streamflows at any given project site can vary but may include
measuring streamflow with stream gauge data; Manning's equation along with reach specific
channel geometry measurements; regional regression equations; hydraulic models; and /or runoff
and streamflow models (Cramer et al., 2012).
Gauge data- Ideally, the provider will be able to locate near-by long-term stream gauge data to
assess hydrologic variability at the reach. The United States Geological Survey (USGS) has a number
of sites with long-term stream gauge data which are typically available online
(https://waterdata.usgs.gov/nwis/rt). Other federal or state agencies may also have long-term data
from which a provider can obtain direct stream hydrologic measurements (e.g., USACE, U.S. Forest
Service, and Florida Water Management Districts).
Ungauged sites- Many, if not most, sites proposed for stream mitigation will be ungauged,
especially for lower order streams. Basic limitations in the current USGS streamflow gauging
network will continue to require stream mitigation designers to rely on predictions in ungauged
basins because, in many instances, continuous streamflow gauges are heavily biased toward
relatively large streams and rivers: 95% of streams have less than 3% of the gauges and over 93% of
stream length is represented with less than 1/3 of the gauges (Bledsoe et al., 2017). In ungauged
situations, the provider will have to estimate streamflow using one or a combination of several
methods. Some common methods for streamflow estimation are:
• On-site hydrologic measurements: To capture baseline and future stream hydrologic condition,
it is recommended the provider install stream gauging equipment (e.g., staff gauge, crest gauge
(Harrelson et al., 1994, USDA NRCS, 2020)), or a continuous recording device (e.g., water level
logger and water level recorder) as early in the project planning/review process as possible. In
addition, if groundwater is a potential source of low flows, then groundwater wells placed
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perpendicular to the direction of flow should also be established at the site. Collection and
analysis of stream and groundwater hydrologic data will assist in establishing the range of flow
conditions (e.g., low, bankfull, flood), flow durations and seasonality, as well as the lateral
extent and depth of flooding on the floodplain. Commensurately, just as in the wetland
hydrologic analysis (Section 4.4.2), analysis of rainfall records and establishment of normal
rainfall should accompany any hydrologic evaluation. Unlike most wetlands, however,
consultation of rainfall data collected by weather stations throughout the stream or river's
watershed is important to help correlate observed stream discharge or stage with rainfall.
• Manning's equation, coupled with channel geometry measurements, is probably the most
commonly used formula for basic hydraulic calculation in natural channels. In its most basic
form, the equation relates flow velocity to hydraulic radius, hydraulic roughness, and channel
slope. Manning's equation is often the basis for estimating bankfull discharge at ungauged sites.
Despite the wide use of Manning's, the estimate for channel roughness (n) significantly
influences the estimate of channel discharge and is difficult to estimate accurately, especially
over the entire length of a reach.
• Regional regression equations relate discharge to channel dimensions and watershed
characteristics. The equations vary by region and can be applied to ungauged basins within the
same region to estimate flood magnitudes at recurrence intervals ranging from the 2-year to
the 100-year events. Many of the regional regression equations estimating flood flows are
available from the USGS in the form of StreamStats, a web-based GIS application which allows
users to access, among other things, streamflow data. USGS regional regression equations are
often accompanied by error estimates, and it is not uncommon to have 30-40% error associated
with any given estimate.
• Runoff and streamflow simulation models can provide flood flow estimates where gauge
and/or regional regression equations are unavailable, or in areas where the equations are not
applicable, such as highly urbanized watersheds. These models predict streamflow based on
simulated runoff from storm events and other inputs. Common models include HEC-1,
developed by the USACE; and the NRCS TR-20 and TR-55. TR-20 provides an analysis of flood
events, while TR-55 presents simplified procedures to calculate storm runoff volume, peak
discharge, hydrographs, and storage volumes for floodwater reservoirs. Use of these flow
estimates, survey data from the channels, and one and two-dimensional hydraulic models can
provide sufficiently accurate data to assess channel and floodplain velocity and shear stress
during storm events (Parola, pers. comm.).
Recognizing that any given model, or technique may have associated error is not intended to
discourage the use of these models if the situation warrants. However, it is important to recognize
how and why the error in these flow estimates is produced and discuss how this might affect results
of a baseline assessment and potential stream design with the provider (i.e., all models are
erroneous, some are useful). Hydrologic analyses should be accompanied by information on the
models used to determine streamflow, the inputs and assumptions included with the model,
associated error estimates, and an interpretation of the results in their project site assessments.
Important flows to be assessed:
Once methods for estimating flows have been determined, there are threshold flows that are
important for the provider and the reviewer to evaluate. The provider should establish estimates of
the timing, magnitude, frequency and duration of low-flows, floodplain inundating flows, and the
bankfull flow. It is important to understand the range of flows, and/or potential flows delivered to
the reach by the watershed and to the extent possible optimize the interactions of flood flows with
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the adjacent floodplain. The range of flows evaluated in the baseline condition assessment will be
the range of flows used for the design.
Low Flow: Loss of channel flow and streambed drying are important ecological controls affecting
stream organisms. Therefore, assessment of the low-flow characteristics (e.g., base flows and
extreme low-flows) of the channel is critical, especially for headwater streams. Assessment of the
channel low-flow characteristics follows the determination of flow permanence, or whether the
channel is perennial, intermittent, or ephemeral. Transitions from each hydrologic regime should
be identified and documented. Often the transitions occur where there are changes in surface
topography, the thickness of alluvial or colluvial bedrock cover, or at locations of headcuts in
channel bed.
Assessment of a channel's low-flow characteristics may include an assessment of the adjacent
valley's groundwater system and its influence on stream low-flows. The groundwater system in the
stream valley plays a significant role in determining how much of the water supplied by the
watershed is valley groundwater and how much is low-flow surface water in the channel during
seasonal dry periods and during droughts. In instances where valley groundwater may be a
significant contributor to streamflow, reviewers should request an appropriate assessment of the
valley groundwater system to identify any impairment (e.g., ditches, increased impervious surface,
and groundwater pumping) to the low-flow regime and to provide a basis for how such an
impairment might be addressed in the mitigation plan.
Bankfull Flows: Bankfull stage is the water level at which flows begin to overtop the banks and
enter the floodplain. From a geomorphological standpoint, it is defined as the elevation of the
stream channel that "corresponds to the discharge at which channel maintenance is most effective,
that is, the discharge which moves sediment, forms or reforms bars, forms or changes bends and
meanders, and generally does the work that results in the average morphologic characteristics of
channels" (Castro and Jackson, 2001, cited in Saldi-Caromile et al., 2004). Generally, the bankfull
discharge frequency for perennial rivers in humid environments is likely to fall in the range of one
to two years using an annual maximum flood series (Soar and Thome, 2001). As discussed in
Section 3.3.4, it is important for the reviewer to understand how the provider verified bankfull
flows and calculated bankfull discharge.
Flood Flows: The timing, frequency, and duration of floodplain inundation is critical for many
morphological and ecological reasons. Generally, a higher level of ecological functionality results
from more interaction of channel overflows with the floodplain (Cluer and Thome, 2014; EPA,
2015). Flows that reach the floodplain typically dissipate shear stresses and stream power and,
along with increased roughness from riparian vegetation, often cause sediments carried by flood
waters to drop on the floodplain. In addition, organic debris is often moved into and out of
floodplains during flood events to support downstream organisms. Water quality enhancements
can occur, as well, if flood durations are sufficient to allow removal and sequestration of elements
and compounds.
If not provided in the draft mitigation plan materials, reviewers should request the provider
characterize the magnitude, frequency, duration, and timing of current flow(s) that inundates the
floodplain of the mitigation project site, along with the shear stress on the channel bed and banks
that correspond to that flow. The results of this analysis can then be used to compare the range of
flows designed to inundate the floodplain after the project is implemented. It is important that the
current range of flows be discussed to allow the reviewer to compare the degree of the impairment
to the intent of the mitigation.
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4.5.2.2 Hydraulic Condition Analysis
Hydraulics can be defined as the movement of water within a channel and the forces generated by
this movement. Hydraulic effects result in movement and deposition of sediment, erosion of
channel banks, and scour of the channel bed (Copeland et al., 2001; Saldi-Caromile et al., 2004;
Gordon et al., 2004). Diverse hydraulic conditions (e.g., varying patterns of depth, velocity, and
substrate in space and time) are created by the interaction between the inflowing water (i.e.,
hydrology), sediment regime, boundary materials, and the channel form (i.e., geomorphology).
Channel hydraulic conditions contribute to maintenance of stream energy and evolution processes
(Fischenich, 2006) as well as a range of channel morphologic conditions which contribute to a
diversity of fauna (Cramer et al., 2012; Cluer and Thome, 2014).
Elements of a Baseline Hydraulic Assessment
Important aspects of a baseline hydraulic assessment include hydraulic parameters that relate
channel conditions to flow and a characterization of stressors affecting sediment dynamics and
evolutionary stage. The following discussion includes basic background information on the types of
information that may be needed to assess the current hydraulic condition. Reviewers should look
for information on the hydraulic and sediment parameters described below.
Hydraulic parameters
Hydraulic conditions are typically evaluated using parameters that relate channel conditions (e.g.,
cross-sectional area, wetted perimeter, and hydraulic radius) to water flows in the channel (e.g.,
water surface elevation, energy gradient, water velocity, and shear stress). These parameters
represent the basic components of hydraulic calculations that will likely be performed during the
assessment and design of a stream mitigation project. Perhaps the most used hydraulic equation is
Manning's (see Section 4.5.2.1) which relates the flow of water to the slope, size, and roughness of
the channel. Slope, roughness, cross-sectional area, wetted perimeter, and hydraulic radius are
channel parameters that are typically manipulated in a stream restoration to accommodate various
water surface elevations, velocities, and associated shear stresses on the bed and banks. Although
the interactions of water flow and channel conditions are complex, these basic parameters should
be kept in mind when interpreting a provider's hydraulic assessment.
Hydraulic assessments can also be done using more complex 1-D, 2-D, or 3-D models (described in
Section 6.3.2) that include sediment transport. In-depth understanding of channel hydraulic
conditions (velocity distributions) is very complex, three dimensional, and influenced not only by
channel morphology and boundary conditions in the project reach, but also by conditions in the
reaches immediately up and downstream. This complexity makes detailed assessment of hydraulic
conditions challenging. A simple assessment might suffice for a provider, or reviewer, to use to
verify bed, bank, and floodplain conditions during the baseline assessment. However, more
complex analyses may need to be completed by the provider during the design phase of a proposed
project, particularly if project objectives relate to improved hydraulic conditions or changes in
hydraulic forces or sediment supplies. Likely, the provider will need to have engineering and/or
hydraulics expertise on the design team to adequately address stream hydraulics.
An assessment of baseline channel and floodplain (i.e., overbank flows up to the 100-year flood)
hydraulics is normally completed to provide information on the current channel and floodplain
elevations that would be used to compare with mitigation design flood elevations. This baseline
condition analysis is useful for assessing hydraulic parameters of the current channel such as bed
and bank shear stress, and channel flow velocity and depth. Survey data to inform a hydraulic
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assessment, such as cross-sections and a longitudinal profile, are also used to inform a geomorphic
assessment (see Section 4.5.2.3).
Sediment
Assessment of stream sediment dynamics is a complex, cost and time intensive undertaking and
often is not evaluated directly. However, it is important for the reviewer to understand the
sediment dynamics affecting a mitigation site, and how sediment may be currently or potentially
influenced by stressors (Table 12). A baseline assessment should indicate the potential hydraulic
and sediment transport problems needing to be addressed, and should provide basic information
on sediment sources, stressors affecting sediment transport, and the potential for a project reach
to experience changes in sediment supply or transport. The following provides background
information to help the reviewer interpret landscape and site scale indicators, either included in the
baseline assessment or observed during site visits, for potential sediment problems. Potential
problems noted as a result of the baseline assessment should be addressed in the provider's
mitigation design, performance and monitoring plans.
Table 12. Common anthropogenic stressors contributing to total sediment regime in streams (after
Johnson et al.» 2015).
Common Stressors
Examples
Explanation
Land
Forest thinning
Silviculture
Development
Increased susceptibility to surface erosion and rill formation;
consolidation of runoff flows; increased risk of bank and
mass erosion when occurring on steep unstable drainages
Vegetation/
cover shift
Grazing (including hoof
sheer)
Agriculture
Development
Roads
Road surfaces
Drainage ditches
Road cuts, Crossings
Surface erosion on unimproved roads, flow consolidation in
drainage/borrow ditches, mass erosion risk with cuts at the
toe of steep slopes, sediment transport barriers at
undersized crossings
Point sources
Stormflow effluent
Direct sediment input
Channel
Dams/reservoirs
Large dams
Small dams
Weirs
These are major stressors that can overwhelm all other
impacts. Large in-line dams and reservoirs typically trap all
incoming sediment. Small dams may pass some suspended
sediment, but still often interrupt all bedload transport.
Direct stability
impacts
Instream mining
Channelization
Reconstruction
Hardening
Straightening
Culverts/Crossings
Direct manipulation of channel leading to excess erosion
from lateral instability or incision and gully formation;
possible barriers to sediment transport
Indirect stability
impacts
Many causes of riparian
vegetation shifts
Indirect impacts that lead to channel instability and excess
erosion.
A common goal of stream mitigation projects is to attain a sediment flux sufficient to maintain
channel habitat and connection of flood flows to the floodplain. In an unconfined alluvial channel,
providers will try to balance a stream's sediment capacity (i.e., stream power sufficient to move
incoming sediment) with the sediment supply (i.e., the type and amount of sediment entering the
reach) in an effort to make them equivalent. Equilibrium conditions exist when the amount and
type of sediment moving into a reach can be efficiently transported out of the reach, thus resulting
in little, or no accumulation or excess transport of sediment in that reach.
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Sediment is produced in the contributing watershed via land erosion (surface erosion, mass
erosion, and point source discharges) and channel erosion on reaches upstream and tributary to
the assessment reach (Table 12). Some sediment enters the reach directly from valley side slopes,
but most is discharged to the reach from the contributing watershed as bedload and suspended
sediment in the stream. Watershed impacts affect sediment production, and major drainage
impacts such as dams affect the transport of that sediment to the reach (Johnson et al., 2015).
Presence of these stressors indicate the need to consider sediment capacity and supply in the
project documentation, particularly in the design phase.
Land erosion considers the amount of sediment produced in the watershed via surface and mass
erosion (Table 12). Surface erosion includes land uses that affect ground cover over significant
portions of the watershed area, particularly those that result in exposed or bare soil. Mass erosion
includes sediment supply impacts from natural disturbances such as forest fires, blow-downs, insect
infestations, or other natural processes.
Channel erosion considers the rate of sediment produced by channel erosion in the contributing
watershed (Table 12). Erosion is a natural process on all streams and tributaries, but the rate of
erosion can be severely altered by human impacts, particularly when alluvial streams become
incised to form gullies. In most watersheds, channel erosion is a much greater source of sediment
to streams than land erosion.
Some degree of sediment mobility is critical for the ecological health of a stream system. Physical
habitat is created and sustained through processes such as the maintenance of pools and riffles, the
formation of transient bars, side channels, and backwater areas, the deposition of spawning
gravels, and the flushing of fines from bed substrate. Sediment sorting through selective transport
creates spawning habitat and quality habitat for benthic organisms. The maintenance of pool-riffle
sequence morphologies and the effective sorting of bed materials exemplifies balanced conditions
of sediment caliber, hydraulic complexity, and transport energy that serve to generate and
maintain quality aquatic habitat.
In addition to the land and channel sources of sediment production, another aspect of sediment
dynamics is sediment transport. Within the contributing watershed, sediment transport is
concerned with the transport of sediment to and through the project reach. In-line dams and
reservoirs are the greatest stressor to sediment transport by trapping sediment and limiting its
distribution in the downstream system. However, dams and reservoirs may largely attenuate any
impacts from unnatural sediment production. Even in watersheds with greatly accelerated
sediment production, the reaches downstream from dams may be in a sediment-starved condition
because reservoirs are such efficient sediment traps. If dams are present upstream of a proposed
project, their effects on sediment transport must be considered in the project documentation,
particularly in the design phase.
In an unconfined alluvial channel at equilibrium, the balance of sediment capacity to sediment
supply is equivalent. Simply put, equilibrium conditions exist when the amount and type of
sediment moving into a reach can be efficiently transported out of the reach, thus resulting in little,
or no accumulation or excess transport of sediment in that reach. Note however that altering the
sediment capacity of a stream reach without also considering its sediment supply or the sediment
capacity of downstream reaches may have unintended consequences in both the mitigation reach
and downstream.
Reviewers can use the providers' assessment of CEM/SEM stage (Section 3.3.4) as an indicator of
sediment dynamics that may have to be addressed in the mitigation plan (Section 6.3). Reviewers
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can interpret the baseline assessment of CEM/SEM stage as context to more complex modelling of
sediment dynamics that likely will need to be completed during the mitigation planning and design
phase. Basically, as channels progress through the stages of channel evolution, the balance of
sediment supply (represented by stream bank failure) and sediment capacity (represented by
channel slope) provide information about trends in sediment dynamics of the channel (see Table 6).
Bank failure via mass wasting can be a significant sediment source to reaches downstream of the
failing banks. Therefore, the height of banks compared to the critical height is an indicator of
potential sediment supply (Watson et al., 2002). As the channel degrades, either through
channelization or incision, sediment coming into the reach is less than sediment moving out,
indicating an increase in the capacity of the reach to transport sediment, or a reduction in supply,
and inducing incision. Typically, when streams reach the stages of degradation and widening (Table
6), the bank heights exceed the critical bank heights and banks begin to fail (i.e., sloughing into the
channel) and contribute sediment to the onsite reach as well as those downstream. Following this,
the channel begins to aggrade and widen, indicating a situation where the sediment supply is
greater than the capacity to move the sediment through the reach. Hydraulically, as streams begin
to aggrade, channel slopes are reduced contributing to the stream not having the stream power to
move the amount of sediment coming into the reach. Eventually, the sediment contributed from
bank failure, reinforces the toe of the bank, and the banks cease failing and contributing to the
sediment supply.
CEM/SEM stage should be assessed both upstream and downstream of the proposed project reach
to place the project reach in context with conditions outside the project area. For instance,
upstream reaches going through degradation and widening, may be contributors of sediment to the
project reach. Providers should be aware of the potential for sediment inputs from upstream in
order to accommodate those inputs in the design, implementation, and post-construction phases.
At the downstream end of the reach, providers should be aware of potential headcuts moving up
the valley that may impact the project by causing incision, changing channel slope (due to incision),
increasing stream power, and transporting more sediment out of the reach than is being supplied
from upstream. On the other hand, where downstream reaches have aggraded, these reaches
could serve as grade control and diminish the stream power available to transport sediment,
leading to aggradation in the project reach.
4.5.2.3 Geomorphic Condition Analyses
Inflowing regimes of water and sediment are the drivers of geomorphic conditions within, and
beyond, a project reach. In the two previous sections, Hydrology (Section 4.5.1.1) and Hydraulics
(Section 4.5.1.2), assessing the baseline condition of water and sediment flows, respectively, were
discussed. While hydrologic and hydraulic processes contribute to and affect the geomorphic
condition of the channel, this section focuses on information to evaluate the range of channel
features, aggradation or degradational status, and whether the channel has stable boundary
conditions. Some information needed to assess the baseline condition of the project reach may also
have been compiled as a result of the classification of the channel (e.g., stream width/depth ratio,
entrenchment ratio, sinuosity, and slope, SEM/CEM) (Section 3.3.4).
Some key indicators of geomorphic instability include evidence of incision, aggradation, rapid
lateral migration, or planform metamorphosis. These processes can occur naturally, however,
excessive rates of change caused by historic and contemporary land use practices or by the effects
of past channel manipulation (e.g., enlargement, floodplain encroachment, and channelization)
constitutes a stream problem. Reviewers, as well as providers, must be mindful of the information
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collected during the assessment of watershed conditions, especially the potential influences of land
use effects on water and sediment flows when assessing the baseline geomorphic condition.
Elements of a Baseline Geomorphic Assessment
Important aspects of a baseline geomorphic assessment include morphology, departure from
expected stable conditions for the given stream type, determination of causes of channel change,
floodplain connection and presence of large wood. The following discussion is meant to provide the
reviewer with basic background information to determine if the provider has adequately assessed
the current geomorphic condition. Reviewers should look for information on the geomorphic
parameters described below. Descriptions of these common geomorphic parameters is largely
taken from Saldi-Caromile et al. (2004).
Morphologic conditions:
• Channel longitudinal profile - Channel profiles (elevation vs. distance plots) depict slope trends
on a stream system, as well as the shape and pattern of the channel bed (e.g., pools and riffles).
Channel slope is defined as the vertical fall of a stream over a given distance. It is typically
reported as a percentage (ft/ft) or as feet of drop per mile (ft/mile). Slope is typically
determined by surveying the channel thalweg, water surface, and elevation of bankfull
indicators. The longer the survey length, the more accurate the reach slope calculation will be.
• Channel planform - Channel planform is the form a stream takes as seen on a map (i.e., aerial
view). Other parameters that describe channel planform are the sinuosity, belt width,
wavelength, amplitude, and radius of curvature of an individual meander bend. Collectively,
these planform characteristics can be compared to historical conditions in order to assess
channel behavior over time, and to expected ranges of values for channels of the same type in
the same physiographic province. Radius of curvature is particularly important, as overly sharp
radii (i.e., tight bends) greatly increase the near-bank shear stress and erodibility and can lead
to channel instability.
• Channel cross-section - Channel cross-section reflects the two-dimensional view across the
channel, typically viewed in the downstream direction. A set of surveyed cross-section points
typically include terrace elevation, floodplain elevation, top of bank, bank toe, lower limit of
vegetation, and thalweg with enough intervening points to define the shape of the channel. The
ends of the cross-section should extend into the floodplain to define at least some of the
important peak flows (e.g., including the extent of the 100-year floodplain).
• Pools and riffles - Pools and riffles generally occur at relatively constant spacing in alluvial
streams. A pool-riffle sequence is a dynamic response of the channel to large-scale, non-
uniform distribution of three parameters: stream velocity, boundary shear stress, and sediment.
Generally, riffle spacings are on the order of five to seven times the channel width.
Channel stability: Channel stability is assessed by measuring excessive bank erosion, excessive
streambed erosion or scour, or excessive deposition. Here, excessive means outside the expected
range of variability for the given stream type and setting. If excessive erosion or deposition is
occurring, the channel is in a state of transition from one type to another, i.e., it is changing its basic
shape, pattern and/or longitudinal profile. Vertical instability (incision or aggradation) is often
coupled with lateral instability (excessive bank erosion and accelerated channel migration or
avulsion rates). Common indicators of channel incision and aggradation are listed in Table 13.
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Table 13. Field indicators of channel incision, aggradation, and bank erosion (after Phillips, 2013) that can
help identify SEM/CEM stage.
Channel degradation/ incision:
Channel aggradation
Mass wasting
Exposure or undercutting of bridge
pilings, boat ramps, docks, pilings, pipe
crossings, perched culverts, etc.
Burial or partial burial of channel and lower
bank vegetation, large woody debris,
and/or culvert outfalls
Concave bank profile or lower
profile
Failed revetements due to undercutting
Reduced bridge clearance
Absence of vegetative cover
Exposure of bedrock or material known
to be from a previous regime in bed
Island formation; relatively young islands
as indicated by vegetation and soil
characteristics
Isolation in channel of formerly
bank-attached features (e.g.,
bulkheads, docks, and signs)
Headcuts and knickpoints
Sand sheets
Exposed roots
Channel ledges or paleobanks
Crevasses and avulsions (local levee
damage or flow diversion)
Toppled trees (toward channel)
Obligate hydrophytes well above
normal water levels (perched ground
water)
Evidence of increased frequency of
overbank flow (e.g., increased floodplain
sedimentation, soil redox features,
vegetation changes, floodplain flow and
hydrologic indicators, erosional floodplain
stripping and increased discharge)
Encroachment on or toppling of
buildings, boat ramps, utility
poles, etc.
Riparian trees tilted back away from
river (wind throw)
Tributary aggradation
Scarps and failure surfaces
Evidence of reduced overbank flow
(e.g., reduced sedimentation, soil
formation, soil redox features, and
vegetation changes)
Increased tributary back-flooding,
indicators of floodplain or channel
aggradation along lower tributary reaches,
organic deposits near tributary mouths
Channel narrowing without evidence of
significant changes in discharge,
stream power or sediment supply
Tributary downcutting
Perched tributaries
Streams with vertical instability, or channel incision and/or aggradation, leading to Stages 2, 3, or 4
of Schumm's channel evolution model are likely to be considered unstable. Channel incision causes
reduced interaction between the stream and its floodplain, reduced spatial habitat heterogeneity,
greater temporal instability, reduced hydraulic retention, degradation of water quality, stream
channel enlargement, and shifts in the fish community structure (Doll et al., 2016). Vertical
instability may be initiated or exacerbated by a lowering in the base level of streams downstream of
the mitigation site, loss of grade control downstream due to the removal of structures (e.g.,
culverts, roads, dams), or channel narrowing due to human encroachment and channelization.
Vertical instability can be assessed with indicators such as bank-height ratio (BHR) and
entrenchment ratio (ER) calculated from channel cross-section data.
Lateral migration, or bank erosion, may be initiated or exacerbated by hardening, or stabilizing,
channel banks upstream or along the opposite bank, which may reduce the channel's capacity to
adjust locally, and may transfer the excess energy to an un-hardened area. Channel aggradation,
channel incision, removal of riparian and channel bank vegetation, excessive saturation of banks
during low flow periods due to irrigation, and rapid drawdown and saturation failures related to
dam releases can also contribute to bank erosion. Excessive lateral migration, or lateral instability,
can be assessed using indices that quantify near-bank shear stress and bank erosion potential, such
as the Bank Erosion Hazard Index (BEHI) (Rosgen, 2001), width to depth ratios calculated from
channel cross-section data, and measured bank erosion rates from surveys (bank pins, toe pins, or
cross-sections) or aerial photos.
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A field inspection, along with georeferenced photos of channel conditions of the site, is always
recommended to look for indicators of channel incision, aggradation, bank erosion through mass
wasting, and/or accretion as well as artificial constraints to alluvial processes like bank protection,
weirs, or grade control structures. The reviewer should be looking for signs that may indicate
instability of the channel and the local or watershed-level causes of the instability and discuss these
with the provider. Indicators used to estimate SEM or CEM stage in Section 3.3.4 (Table 6), as well
as those in Table 13, may also be useful in assessing channel instability as well as upstream and
downstream of the project reach.
Changes in stream geomorphology and channel stability will likely be the focus of design elements
in the mitigation plan. However, instability manifested at the reach scale could have causes at the
watershed, riparian and/or reach scale (as previously discussed). Stressors to the stream's energy
(i.e., hydrologic and hydraulic) regime, sediment supply, and boundary conditions could all
contribute to channel instability. Some common examples of these stressors and an explanation of
how they affect the channel are listed in Table 14. To the extent possible, a provider and/or the
reviewer should strive to identify the stressors acting on the channel to cause its instability, and
determine which stressors can be rectified (at the reach scale) and included in the mitigation plan
versus stressors that cannot and hence may become constraints (at the riparian/watershed scale).
Table 14. Common stressors affecting stream stability {after Johnson et al.» 2015).
Common Stressors
Examples
Explanation
Energy
Unnatural
hydrology
Water volume
Impacts to hydrology, particularly bankfull discharge
and peak flows directly alters energy.
Peak flows/floods
Bankfull discharge
Channel evolution
Channel change
Altered stream morphology changes hydraulics
including shear stress, stream power and work.
Sediment transport capacity and competence depend
directly on hydraulic characteristics of the channel.
Planform impacts
Branching
Sinuosity
Dimension
impacts
Entrenchment
Cross-sectional area
W/D ratio
Profile impacts
Bankfull slope
Localized gradient
Boundaries
Floodplain
disconnection
Floodplain access
Floodplains function as safety valves that limit stream
power by distributing large discharge over wide area
and provide wide area for channel to move and adjust.
Floodplain extent
Saturation duration
Riparian
vegetation
removal
Streamside vegetation
Riparian vegetation provides roughness, bank
protection, and soil-binding root mass. Mechanism for
rapid recovery following disturbance.
Riparian vegetation
Changes to biotic
drivers
Beavers, large woody
debris
Biotic drivers such as beavers and large woody debris
provide stabilization functions and a mechanism for
rapid recovery following disturbance.
Direct channel
impacts
Channel hardening
Hardened bed or banks or solid structures affects
stability and decreases ability to move or adjust.
Bridges, crossings
Sediment
Changes to
watershed supply
Land erosion
Changes to the amount, size, timing of sediment
delivery from the watershed is a direct impact to the
sediment side of Lane's balance, altering dynamic
equilibrium.
Channel erosion
Delivery
Changes to reach
supply
Degradation
Positive feedback; instability may be increased by rapid
sediment production on an unstable eroding reach.
Bank erosion
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Floodplain Connection: Another important aspect of geomorphic assessment is the connection of
the channel to its floodplain. The floodplain becomes part of the channel during flood flows.
Therefore, the ability of flood flows to access the floodplain during storm events and the extent and
condition of the floodplain to store and slow flood waters are important geomorphic and ecologic
factors (Cluer and Thome, 2014; EPA, 2015). If a channel is too incised, flood flows may be
contained within the stream channel. This will lead to increased shear stress on the channel bed
and further exacerbate incision. Floodplain encroachment by agricultural, urban, and/or residential
land uses, decreases the area available to store flood waters and alters the frictional resistance, or
roughness, provided by native vegetation on floodplains. The frequency and extent of the
floodplain connectivity, and the degree of anthropogenic encroachment on the floodplain should
be documented during the baseline assessment.
Large woody debris: Woody debris and several forms of coarse and fine particulate carbon play an
important role in channel morphology and ecology (EPA, 2015). Woody debris has important
effects on controlling bed grade and creating pools and riffles, as well as providing stable epifaunal
habitat at low flows. Large wood in streams represent large roughness elements that divert flowing
water and influence the scour and deposition of sediment in forested streams. Large wood in
stream channels results from trees that fall from banks or hill slopes. The quantity of woody debris
in the channel and on the floodplain should be measured to determine if conditions are adequate
for supply and retention of this habitat feature.
4.5.2.4 Physicochemical (Water Quality) Condition Analysis
The physicochemical condition of a stream is a major determinant of the quality of habitat for
instream aquatic organisms. Some solutes may be beneficial or necessary to support life within a
certain range of concentrations (e.g., dissolved oxygen and nutrients) while others have only
detrimental impacts above a certain threshold concentration. Where water quality is impaired,
restoration, establishment, or enhancement of physical habitat in the absence of water quality
improvement measures may provide minimal benefit, if any, to the instream aquatic biological
community (e.g., fish and macroinvertebrates). Water quality impacts may also affect the riparian
biological community, providing limited benefits for more mobile biota like birds and mammals.
Many of the factors affecting water quality in a particular reach emanate from the upstream
contributing watershed. Typically, the water quality parameters of concern include temperature;
pH; dissolved oxygen (DO); specific conductance; nutrients (phosphorus and nitrogen); sediment;
and sometimes metals (e.g., selenium), pesticides, herbicides, or fungicides. Sediment is a natural
constituent of stream ecosystems, as are nitrogen and phosphorus, however, abnormally high
levels of any of these constituents can result from anthropogenic disturbances and should be
recognized and, if possible, addressed in the mitigation plan.
Reviewers should ensure that any adverse water quality conditions that limit the biological
condition of the stream are identified by the mitigation provider early in mitigation project planning
and are characterized in the baseline assessment. If water quality is limiting the biological diversity
and abundance of the site, the mitigation plan should identify whether site-scale management will
ameliorate the water quality problems. In some cases, these adverse water quality conditions may
be caused by discreet point sources or spatially contained non-point sources that the project itself
may be able to address (e.g., stormwater and/or sediment detention). In other cases, degraded
water quality may be the result of widespread non-point sources over which the project proponent
or sponsor has little control or recourse (e.g., urbanization, agricultural, and mining land uses). The
latter situation may limit the ability of the proposed stream mitigation site to develop and sustain a
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productive and diverse aquatic biological community, which may in turn limit its ecological
suitability as a mitigation site.
Elements of a Baseline Physicochemical Assessment
Important aspects of a baseline physicochemical assessment include collection of temperature, pH,
DO, specific conductance, nutrients, and sediment data at the upstream end of the project as well
as at the downstream end. If there are known or suspected sources of metals or pesticides,
herbicides, etc., in the contributing watershed, these should be evaluated as well. Water quality
parameters should be collected using the sampling protocols and standards established by the state
in which the mitigation project is occurring, as this allows results to be compared with state
datasets and water quality standards. Each state has numeric or narrative water quality standards
which can be used to compare the water quality of these parameters to standards. Comparison of
water quality at the project reach to water quality standards will form the basis of discussions
about the provider being able to ameliorate water quality concerns with their project. The water
quality status of any stream in the U.S. can be checked by going to:
https://www.epa.gov/waterdata/hows-my-waterway.
4.5.2.5 Biological Condition Analysis
Given the overall objective of the CWA to restore, maintain and enhance the chemical, physical,
and biological integrity of the nation's waters, biological condition should be characterized as part
of a baseline assessment of any proposed stream restoration, establishment or enhancement site,
particularly when the objectives of a project relate to improving biological functions. As the top
level of the functional pyramid, a stream's biological community is largely influenced by the
hydrologic, hydraulic, geomorphic, and physiochemical conditions which operate at watershed,
riparian, and reach scales. In turn, biota, particularly riparian plant communities, contribute to the
geomorphic stability of, and the organic carbon/detrital inputs to, the channel. Stream biota utilize
the organic carbon inputs and contribute to the overall biochemical cycling within the stream
ecosystem. The complex interactions of these ecosystem components contribute to the faunal
diversity found in natural streams.
Elements of a Baseline Biological Assessment
Important aspects of a baseline biological assessment include characterization of the condition of
and stressors affecting riparian and floodplain vegetative communities and the instream faunal
communities (e.g., fish and/or benthic macroinvertebrates). The following discussion is meant to
provide the reviewer with basic background information to determine if the provider has
adequately assessed the current biological condition. Reviewers should look for information on the
following parameters.
Riparian Vegetation: The riparian vegetation community is a complex assemblage of plant species
that interact with each other and with abiotic factors to produce critical structural and functional
aspects of the stream ecosystem. Riparian vegetation provides the root structure and roughness
that stabilizes banks, channels, and floodplains while also providing a buffer to the stream from
nearby stressors. Riparian vegetation is the source of large woody debris that supports channel
complexity, stability, and structure, as well as the detritus that forms the basis of aquatic food
webs. It also provides cover, shading, and habitat for species with terrestrial life stages. Important
aspects of the riparian community to document in a baseline assessment are:
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• Vegetation structure (physiognomy) of canopy, shrub, and herbaceous layers (e.g., percent
cover, age-stand distribution, density, and canopy height);
• Species diversity, distribution and abundance, and recruitment (e.g., floristic quality index);
• Percent non-native species; and
• Riparian extent (e.g., buffer width, area).
Identifiable stressors affecting the riparian community which should be noted are:
• Tree and shrub removal - Direct manipulation of tree or shrub strata, altered light regime
for herbaceous stratum;
• Grazing - Species composition shift (grasses and grazing tolerant species), cover depletion,
bare ground, introduction of weedy species, biomass and nutrient export;
• Cultivation - Suppression of non-grass species, biomass and nutrient export;
• Hydrologic Impacts - Unnatural hydrology (e.g., wetting or drying from ditching, upstream
dams, levees, irrigation; see Section 4.5.2.1) which may favor some species over others. For
instance, causing native hydrophytes to drop out due to drying; and
• Invasive species - Introduced or escaped species which alter vegetation structure.
Instream Biota: Aquatic life represents the biological component of the natural infrastructure of a
stream, and is the main subject of stream ecology. Stream biota include microbes, macrophytes,
macroinvertebrates, fish, terrestrial mammals, and amphibians. These biota carry out biochemical
processing through a characteristic trophic structure which is assumed to take place in the absence
of stressors. A stream's biota is a core feature of reach condition and an important element to
characterize in a baseline assessment. A baseline assessment should consider the stressors that
may affect biological functions and should include site-specific data to characterize biota (e.g., fish
and/or macroinvertebrate surveys, biomass estimates, indices of biotic integrity).
Ecologically, diversity in channel dimensions and geometry, channel features (e.g., bars, bedforms,
islands, banks, riparian margins, and confluences), and substrate all contribute to habitat diversity
under a variety of flow conditions. As stream channel conditions decline, habitat diversity may
diminish. As such, reviewers should consider the results of the hydrologic, hydraulic and
geomorphic baseline assessments (e.g., channel cross-sectional dimensions, channel planform
patterns and attributes, longitudinal in-channel features (e.g., bedforms and spacing thereof),
characterization of channel substrate and floodplain elevations relative to the channel bed) when
reviewing the results of the baseline biological assessment to ensure that reported biological
condition aligns with baseline habitat conditions.
Stressors affecting instream biota include alterations of characteristic habitat, including any
physical, chemical, or biological features that affect the habitability of the reach by organisms
(Table 15; Johnson et al., 2015). Energy or food limitations may disrupt food webs, causing shifts in
characteristic biota. A common and easily identifiable biotic stressor is the presence of migration
barriers that limit access of specific organisms, primarily fish, to and from the reach. The presence
of exotic or non-native species affects the normal distribution of plants and animals, as well as the
balance of competition, predator-prey interaction, and symbiotic relationships among organisms
and populations leading to community-level changes. The loss of native species by extinction or
extirpation is similarly both a direct and indirect stressor on community assemblage and biotic
structure. Direct management of certain biotic components, especially game fish and pest
management are also common human impacts.
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Table 15. Common stressors affecting stream biota (after Johnson et al.» 2015).
Common stressors
Examples
Explanation
Chemical habitat
Temperature regime
Temperature or water quality limitations,
tolerance or preferred range exceedance
Water quality
Structural habitat
Stream morphology
Physical habitat structural limitations
Coarse scale (e.g., bed and bank
structure, LWD, jams and dams, and rock)
Fine scale (e.g., bed material size
distribution, fine sediment deposition and
scour, embeddedness, compaction, and
macrophyte cover)
Riparian tree and shrub removal
Energy/ food
limitations
Nutrient sources
Lack of available energy from lower trophic level
biomass
Organic matter (dissolved, coarse, and
fine particulate)
Trophic level impacts
Direct management
Stocking
Direct manipulation of biota
Culling / Control
Grazing
Cultivation
Migration Barriers
Crossings (e.g., culverts, bridges)
Barriers that limit organism access to and/or from
the reach. Note that these barriers may be a long
distance up-or downstream from the reach.
Dams/reservoirs
Temperature/chemical barriers
Non-native species
Introduced microbes
Non-native species (introduced and invasive) that
come to occupy natural and novel niches.
Aggressive invasives can overwhelm entire
functional guilds and have far-reaching effects.
Other invasives
Game species
Extirpation/
extinction
Native fish
Regional or global loss of characteristic species
Amphibians
Macroinvertebrates
Beavers
Site-specific data can be obtained by a variety of field methods including population sampling to
quantify number or biomass of specific organisms by species, age class, size class, functional guild,
or other specific grouping. Biological data collection is typically an intensive effort, and may involve
multiple sampling dates, paired sampling with a control reach and post-field sample processing.
Biological condition should be monitored using sampling protocols and standards established by
the state in which the mitigation project is occurring or by the agency(ies) responsible for
administering the mitigation program, as this allows results to be compared with regionally
applicable biological datasets and, sometimes water quality standards. When reviewing baseline
data, ensure data were collected within appropriate index periods and using approved methods.
Data are often used to calculate specific indices that quantify biotic community-level characteristics
such as diversity and richness; and specific parameters are available for quantifying functional
characteristics at the community level, such as indices of biotic integrity (IBI) and parameters of
condition based on macroinvertebrate diversity. For larger taxa, indirect measures of populations
are often measured instead of direct samples of organisms such as redd counts, scat counts, creel
samples, etc. (Johnson et al., 2015). As discussed in Section 4.2, it is important to document, the
location of, and the distance to, proximate intact biological communities that serve as recruitment
sources for the restored stream reach. These refugial aquatic habitats and their existing water
quality and aquatic organism population levels should be documented in the baseline assessment.
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5.0 What is the target condition?
The intent of this question is to understand the target
condition of the site, given the site's aquatic resource
class(es), watershed and site-scale stressors and baseline
condition. Stating this target condition is important
because it establishes the functional expectations for the
site (goals), the specific actions (objectives) which will be
implemented and tracked to assess progress towards
achieving the target condition and identifies the
reference(s) used to inform the target condition. A
realistic target condition serves as the basis for
developing workplans, performance standards,
monitoring, adaptive management, and long-term
management plans.
At the prospectus or pre-application stage, goals and objectives should be outlined in enough detail
to inform conceptual mitigation workplan elements (e.g., what stressors should be addressed,
whether areas are being established, restored, enhanced, or preserved). The statement of goals
and objectives should describe the target condition, with appropriate references proposed for
comparison. When reviewing and commenting on early stages of mitigation proposals, reviewers
should consider whether the target conditions and project-specific goals and objectives are logical
and achievable given the aquatic resource class(es) and existing baseline condition of the site
(including the presence of watershed and site-specific stressors). It would be advantageous for the
reviewer to inquire about the selection of reference sites early in the process to ensure that
reference data and/or sites are being considered by the provider and that the reference sites are
applicable (e.g., of the same class, comparable watershed size). If reference data collection is
required, early consideration of reference sites will better accommodate the time required to
collect, analyze and digest the data early in the planning process. Reviewers should also consider
requesting a field visit to available reference sites to provide additional perspective on the target
condition.
During review of complete draft mitigation plans, reviewers will want to ensure target conditions
are consistent with the actions proposed in the mitigation workplan, that they are informed by
appropriate reference datasets or analog sites, that they relate to identified performance
standards, and that appropriate methods and measurements are included in the baseline and
monitoring assessments to determine whether target condition has been achieved at project
closeout. The main opportunity to provide these comments may be during pre-application or draft
prospectus stages or the Public Notice comment period for individual permits, or during review of
the draft mitigation bank instrument or ILF project addendums.
Important elements in this step include clear, function-based expectations for the outcome of the
project (goals) given aquatic resource class(es) and the level of disturbance and stressors acting
upon the site, quantifiable actions (objectives) needed to ameliorate the stressors and achieve
target condition, and relevant reference comparisons (Box F). Establishing target condition is highly
dependent on identifying the watershed or site-scale problems that may be affecting functional
capacity. For example, in streams, stressors will affect the status of watershed controls, including
hydrology and sediment, resulting in changes in the watershed and stream corridor that may
explain changes in condition; and this information needs to be reflected in the goals and objectives
Key points for establishing target
condition:
• Establishing goals (statement
of functional expectations)
• Objectives: SMART
o Specific
o Measurable
o Attainable
o Relevant
o Time bound
• Establishing reference
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that inform an appropriate design (Roni and Beechie, 2013). This section provides the reviewer with
background information on developing goals and objectives, as well as technical information on
identifying reference comparisons in both wetlands and streams.
It is important to be clear about the time frame in which the stated target condition is expected to
be reached and the basis of this expectation. Often, regulatory time frames may expire before the
stated target condition is attained. For instance, a target condition of full wetland functional
replacement for a forested, riverine wetland may take several decades. Likewise, full functional
replacement and reaching equilibrium conditions in stream ecosystems may take time as the
channel and floodplain adjust after the project. Therefore, the time expected to reach any stated
target condition should be clearly stated. The rationale for estimating the trajectory of the site
should be explained. Target conditions at various points in time can be estimated by substituting
space-for-time using appropriate reference sites that represent different ages of development.
Box F. Recommended information to identify appropriate target condition.
• What are the goal statements outlining the expectations for the project considering the aquatic
resource class(es), baseline conditions and the identified stressors and impairments affecting the
site?
• Given the conditions in and around the site (from baseline assessment), will the proposed site support
the target functions/condition(s)?
• Are goals and objectives tied to specific functions?
o For wetlands, consider hydrologic, biogeochemical and habitat functions
o For streams, consider hydrologic, hydraulic, geomorphic, physicochemical and biological
functions
• Are the objectives:
o Specific to the aquatic resource class(es) on site?
o Measurable (as indicators of functions outlined above)?
o Achievable given current and projected site-specific and watershed stressors?
o Relevant and supportive of the goals of the project?
o Time bound to the monitoring and performance period of the project?
• What reference approach(es) was (were) used to establish target conditions)? Are there reference
aquatic resources, including reference standard sites or regional references (e.g., HGM regional
guidebooks), to which the target condition can be compared?
• Is (are) the reference site(s) the same class and representing the least-altered condition to which the
biology can be compared?
5.1 Regulatory Context
The Mitigation Rule indicates that mitigation project objectives must include a description of the
resource type(s) and amount(s) that will be provided, the method of compensation (i.e.,
restoration, establishment, enhancement, and/or preservation), and the manner in which the
resource functions of the compensatory mitigation project will address the needs of the watershed,
ecoregion, physiographic province, or other geographic area of interest. These project objectives,
as informed by the classification and baseline condition considerations, outlined in Sections 3.0 and
4.0, provide the basis for this step of the Approach (Figure 1). Objectives are foundational to
subsequent required rule elements, including developing ecological performance standards, which
relate directly to achieving project objectives.
Reference data are used to help establish realistic objectives for mitigation projects. The Mitigation
Rule defines reference aquatic resources as "a set of aquatic resources that represent the full range
of variability exhibited by a regional class of aquatic resources as a result of natural processes and
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anthropogenic disturbances." The preamble provides further clarification on how reference aquatic
resources relate to concepts of reference condition in the ecological literature, noting that "the
term 'reference standard' is used for the subset of reference aquatic resources that are the least
disturbed and exhibit the highest levels of functions."
5.2 Goals and Objectives
Mitigation providers sometimes characterize project objectives in terms of providing credits.
However, the Mitigation Rule describes mitigation objectives in terms of identifying the functions
that the proposed mitigation project will provide and the manner in which they will be provided.
The Approach adds goal statements as the place to identify which functions are expected to be
replaced, with objectives identifying the manner in which they are replaced. Therefore, clearly
enunciating project objectives tied to aquatic resource class and the re-establishment of functions
typical of that class is critical for tracking the mitigation project and its performance. After assessing
the stressors acting on the project site, the goals and objectives should address the problem cause
and context (Roni and Beechie, 2013).
Goals are broad statements of the intended outcome, or
expectations of the mitigation project, including the
functions provided by the mitigation site. Function-based
design goals are statements about why the project is
needed at the specific project site (ELI et al., 2016). Goals
should efficiently express the intent of a project and serve
as a fixed point for evaluating project elements (Skidmore
et al., 2011; Roni and Beechie, 2013). For example, a
project goal may be "restoration of a forested low-
gradient riverine wetland to provide hydrologic,
biogeochemical and plant and animal habitat functions
similar to reference standard wetlands of this wetland
type." This statement provides an expectation of the type of wetland and functions to be restored
which can be used to guide the project. An equivalent stream goal might be "to restore channel
processes needed for resilient and sustainable riparian and biotic habitat." This statement provides
an expectation that to restore sustainable riparian and biotic habitat, hydrologic and hydraulic
reconnection of the floodplain, as well as repair of instream channel functions, might be needed.
Objectives, by comparison, are designed to describe the actions needed to achieve the goal. They
are typically specific, measurable targets critical to the establishment of the functions that describe
the time necessary to reach the target. It is likely each goal may have multiple objectives. Well-
defined objectives form the basis for specific performance standards (see Section 7.0) that allow for
evaluation of the success or failure of the mitigation project. The objectives tie the actions planned
to the performance measures (i.e., specific level of measure attained) needed to evaluate if the
goals have been achieved.
SMART objectives are recommended because they can be efficiently tracked through the mitigation
project (Skidmore et al., 2011; Galatowitsch and Zedler, 2014; Roni and Beechie, 2013). Some
USACE Districts in the southeastern U.S. (e.g., Savannah and Wilmington) have adopted a similar
SMART approach for identifying objectives. For this Approach, SMART objectives are:
• Specific to the aquatic resource type and location (classification and watershed position);
Goals and objectives should
reflect the possibilities and
constraints of the site by setting
realistic expectations.
Both the provider and the IRT
should understand limitations
imposed by watershed
conditions and consider project
goals and objectives accordingly
to address the anticipated
outcomes of the project.
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• Measurable using parameters that can be monitored before and after project
implementation;
• Achievable given the current and projected site and watershed stressors;
• Relevant (clearly related) to the identified problem and supportive of the project goal; and
• Time-bound to the monitoring and performance period of the project.
It is important to recognize that it may not be possible to fully restore a wetland and/or stream
ecosystem to its pre-disturbance condition. Past and current land use, existing infrastructure,
invasive species, limited native species abundance and extinction, among other perturbations, may
prevent full ecosystem recovery from being achieved. In such systems, returning the system, or a
portion of the system, to an improved state of ecological functioning but not necessarily its pre-
disturbance condition may be the only achievable goal. Goals and objectives should reflect the
possibilities and constraints of the site, or its restoration potential, by setting realistic target
expectations. In the instance of a degraded wetland or stream that may never reach a pre-
disturbance condition, a goal should be to establish a self-sustaining ecosystem that is resilient in its
recovery response to its disturbance regime, and not one that will require repeat intervention by
humans (Saldi-Caromile et al., 2004; ELI et al., 2016).
The goals and objectives become the statements of target condition and are the elements needed
to inform workplans and mitigation activities. Reviewers and providers will likely have to work
together to produce effective goals and objectives that provide realistic target conditions for a site.
Classification, as well as the baseline condition of the site, need to be considered in formulating the
goals and objectives. Reference sites (discussed below) can also be useful in visualizing the target
condition. Differences between developing target conditions for wetlands and streams is discussed
below in Sections 5.2.1 and 5.2.2
5.2.1 Wetland Goals and Objectives
In general, the target condition of wetland compensatory
mitigation will be the restoration or enhancement of a
sustainable and resilient level of the functions performed
by the appropriate HGM class at the proposed site. HGM
classification and consultation of applicable regional
guidebooks facilitate description of project goals by laying
out the functions expected to be performed by a particular
wetland class or subclass. Understanding these functions,
the level of on-site and surrounding watershed impairment
and the restoration potential of the site should lead to
development of appropriate, realistic wetland goals and
objectives. Watershed and site-scale stressors should also be considered in the scoping of goals and
objectives. Importantly, wetland goals and objectives should be framed in the context of reference
standard conditions, not jurisdictional criteria.
Goals should be realistic and not set expectations of full functional replacement if the landscape
level factors affecting the wetland do not support some functions. For instance, a goal of "full
functional replacement of low gradient riverine wetland wildlife habitat functions" would likely be
unattainable if the riverine wetland was disconnected from habitat types up- and downstream of
the site and land uses prohibited reconnection. Understanding that habitat replacement, including
connection to a naturally vegetated corridor, may be unattainable for this particular wetland class
Goals and objectives for
wetland mitigation projects
should be expressed with a
clear understanding of where
the site sits along the
disturbance gradient
represented by similar reference
wetlands, and whether and how
the site can be returned to a
reference standard condition.
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in the specific landscape setting is important for determining appropriate goals. A more appropriate
goal would be "to restore onsite wildlife habitat characteristics that will provide usage by species
whose requirements can be met onsite."
Objectives need to reflect the characteristics and functions of specific wetland classes and the
stressors intended to be repaired. For instance, restoring overbank flooding to a mineral soil flat
wetland would not make sense, as mineral soil flats are not associated with streams and do not rely
on overbank flooding as a water source. Therefore, an appropriate goal for mineral soil flats would
be to re-establish the hydrologic regime typical of reference standard mineral soil flats (i.e.,
saturation and/or ponding). If the mineral soil flat site was impounded by a road, leading to
prolonged ponding of the site, an objective would be to decrease the ponding on the site to
reference levels of timing and duration. This objective supports the goal of re-establishing
hydrologic function to the wetland and is quantifiable (i.e., timing and duration can be measured).
A commensurate hydrologic goal for riverine wetlands would be to re-establish surface and
subsurface water storage functions typical of reference standard wetlands. The objective(s) could
then be to re-establish overbank flooding, and shallow groundwater, magnitude, frequency, timing
and duration typical of the reference standard wetland.
5.2.2 Stream Goals and Objectives
Setting target conditions for streams means recognizing stream classification characteristics (i.e.,
watershed and landscape context, flow permanence, and geomorphic context) in the context of
baseline conditions and relevant stressors at the project site. Target conditions will be informed by
the condition of the stream at the reach, riparian, and watershed scales. It is possible that target
conditions may only be projected to improve or restore certain categories of functions (e.g.,
hydrology, hydraulics, and geomorphology).
Goals should include the intention to improve processes that sustain and support natural
ecosystem functions and conditions, and emphasize resilience (i.e., capacity to recover from natural
disturbance) (Roni and Beechie, 2013; Palmer et al., 2007). More succinctly, goals should be
function-based (ELI et al., 2016). Ideally, goal statements will describe intended outcomes without
being too prescriptive about the means to those outcomes (Skidmore, 2011). For instance, goal
statements which might be considered too prescriptive include restoring historic planform
alignment, stabilizing stream banks or streambed, returning channel bed elevation to historic
elevation and grade, or restoring channel dimension, pattern, and profile. Each of these examples
drive the planning toward stabilization, reconstruction, or reconfiguration and may constrain the
options for achieving improved ecological conditions. Examples of more appropriate goal
statements might include:
• Increase recruitment of target species;
• Remedy direct or indirect human actions contributing to channel instability to address
sources of excess sediment and associated habitat and water quality degradation; or
• Restore and maintain dynamic channel processes to foster sustainable riparian and fish
communities.
Goals associated with the restoration of aquatic biota should be reasonable given site and
watershed constraints and the importance of site selection and watershed conditions to biological
success of stream restoration and enhancement efforts, particularly where water quality, proximity
to colonizing populations and probable refugia may be limited (Section 4.2). Where there are
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constraints to restoring biota, reviewers should also consider the appropriateness of habitat-
related goals or objectives.
Objectives may be either structural or functional, and in either case, should include an action, a
measurable, quantifiable target and the time frame required to reach the target (Roni and Beechie,
2013; Skidmore et al., 2011). A structural objective focuses on the distribution, abundance, and
physical condition of some element of the ecosystem and is measured as a point-in-time value.
Examples of structural objectives include "to restore native riparian vegetation to include at least
four native woody species within three years" or "to create a minimum of two acres per stream
mile of rearing fish habitat in the first year of restoration." A functional objective focuses on
processes that sustain an organism or environmental components and is measured as a change, or
a rate over time. Examples of these include "to establish fish passage to at least one mile of stream
within two years" or "to restore overbank flow to an average duration of five percent of the
growing season in three out of five years within ten years." Stream functions outlined by Fischenich
(2003; 2006) or Harman et al. (2012) can provide a general framework for identification of
applicable stream functions and potential goals, objectives and parameters that can be designed for
and monitored.
5.3 Reference
Assessing baseline condition, setting appropriate target
conditions, designing the project, and evaluating
compensatory mitigation performance all require a basis of
comparison, a benchmark, or a reference. At its most
inclusive, an ecosystem reference represents some target,
benchmark, standard, model, or template from which, or to
which, ecosystem structure, function, condition, biological
integrity, or relative health are compared (Smith et al.,
1995; Brinson and Rheinhardt, 1996; Miller et al., 2012). Ecosystem impacts are typically assessed
by comparing what is seen on the ground (current condition) to some standard (real or virtual)
representing a non-altered condition (reference). The Society for Ecological Restoration recognizes
the importance of reference sites in representing the target condition upon which restoration
designs are based and against which the progress of a site is evaluated (McDonald et al. 2016).
Reference conditions used in targeting, and subsequent design, monitoring and assessing
performance, can be derived from reference site data, historical information about the site to be
restored, existing or historical sites with similar structural characteristics, and/or predicted
conditions based on computer models (Merkey, 2005; Hawkins et al., 2010). Selecting reference
sites for comparison to mitigation sites must emphasize the similarities with the landscape-scale
attributes (e.g., landscape position), as well as the representativeness of the target physical,
chemical, and biological composition of the particular wetland or stream resource. For instance, it
would be inappropriate to choose a reference condition similar to the pre-mitigation degraded
biological or stream geomorphic conditions. This is one reason why classifying the aquatic resource
is critical. Without similar structural characteristics (e.g., landscape position, hydrodynamics,
geomorphology, slope, sediment grain size, hydroperiod, and salinity) between mitigation and
reference sites, comparison of functional characteristics, such as presence and abundance of
specific groups of organisms or nutrient cycling dynamics, is inappropriate (Brinson and Rheinhardt,
1996; Merkey, 2005).
Selecting reference sites for
comparison to mitigation sites
must emphasize the similarity
that defines the physical,
chemical and biological
composition of the particular
stream or wetland resource.
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Setting wetland target conditions conceptually is more straightforward than setting those for
streams, since the HGM guidebooks have already scaled reference data collected across a
disturbance gradient (i.e., regional index) that can be used to narrow natural variability and better
define a given wetland target condition. Streams, being more dynamic, exhibit more variability and
are more difficult to capture in a system where key landscape, physical, hydrologic, and biological
processes allow ecological classification based on functional similarities. Thus, setting stream target
conditions with appropriate reference streams involves understanding terminology and use of
different approaches. Reviewers should be aware of differences in reference terminology between
wetlands and streams, and the limitations in reference data approaches, including how such
limitations may affect not only the target condition of the project but also its design, performance,
and adaptive management.
Discussion of the use of existing reference data and/or collecting reference data for application to a
particular project with a provider and/or other IRT agencies, is facilitated by understanding some
concepts and terminology associated with ecosystem reference (Smith et al., 1995; Brinson, 1993;
Stoddard et al., 2006; Miller et al., 2012).
• A reference standard ecosystem is one or more existing, former or hypothetical ecosystems
that serve as a guiding image, or target ecosystem, for a mitigation project. Aquatic
ecosystems that represent the full range of variability are referred to as reference aquatic
resources in the Mitigation Rule, with reference standard sites representing the subset of
reference aquatic resources that are the least disturbed and exhibit the highest levels of
functions. Typically, the reference standard ecosystem is considered the best representation
of a particular class. A reference site can represent one whose functional capacity is very
degraded, or one within the range of natural variability, across a range of conditions
exhibited by a particular aquatic resource class.
For example, in wetlands, a reference standard wetland can be one of a group of wetlands
in minimally disturbed or least disturbed condition on the landscape that represent the
natural range of variability exhibited by that wetland subclass. Reference standard wetlands
also represent the highest level of functions across the suite of functions performed by the
particular wetland class. However, reference wetlands are wetlands of the same HGM class
that represent wetland conditions under an anthropogenic disturbance regime (e.g.,
vegetation removed, hydrology altered, and soils disturbed). In other words, reference
wetlands represent examples of a particular wetland class along a disturbance gradient. For
example, for riverine forested wetlands, a reference standard would be one with mature
timber, well stratified forest layers, intact hydrology and soils, and being part of a well-
connected corridor. A wetland that is of the same type but disturbed (e.g., clear-cut,
ditched, and/or soils disturbed) would be included as a reference wetland that exhibits
conditions along a disturbance gradient.
Within the stream assessment and restoration arena, the concept of reference standard, or
least-disturbed condition, is typically discussed using related but different terms depending
on the stream function being assessed.
• Reference condition, typically used in stream biological assessment, is the set of
quantifiable characteristics of the reference ecosystem that characterize the range of
natural variability under undisturbed conditions. In this manner, the term reference
condition is similar in concept to reference standard in representing undisturbed or least
disturbed biological condition. They can be developed using current or historical
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information, conceptual, empirical, or quantitative models (Nestler et al., 2010), or well-
documented professional judgment (Miller et al., 2012).
Reference condition categories (i.e., historical condition; minimally disturbed condition; and
least disturbed condition (Stoddard et al., 2006)) pertain not only to the site characteristics
of particular reference sites, but also to the landscape conditions in which these reference
sites are found. In the southeast U.S., it is highly unlikely to have a stream or wetland site
that would be considered pristine or completely unaltered by anthropogenic impacts. The
most likely reference condition will fall into either the category of minimally disturbed (i.e.,
really close to pristine conditions) or least disturbed (i.e., this is the best we could find)
(Miller, 2012). In other words, even within the same aquatic resource class, the reference
condition, or reference standard, could vary depending on the level of disturbance in and
around the sites used to define undisturbed conditions. The state of the contributing
watershed or ecoregion will indicate the type of reference conditions available for
comparison to the mitigation project which will affect setting mitigation targets, assessing
baseline condition and performance.
• Reference/reference reach, typically used in stream geomorphic assessments, is the portion
of stream segment that represents a stable channel within a particular valley type (Rosgen
1998). Reference reaches are typically used as templates for stream designs (see Section
6.3) and in the context of stream mitigation, typically represent only the geomorphic
function(s). The term reference and/or reference reach are often used by providers when
discussing Natural Channel Design (NCD) plans, and reflect the portion of stream from which
measurements for optimal channel dimensions were collected.
• A reference approach is a set of assumptions and techniques for characterizing and applying
reference ecosystem and reference condition data to practices associated with
compensatory mitigation. Reference approaches (discussed below) vary and may rely on
one, or more, of the reference condition categories described above.
The availability of regional reference data is not ubiquitous. In many cases, mitigation providers are
asked to supply project-specific reference data early in the mitigation planning process to compare
to baseline conditions, help to establish target conditions, assist in design and to guide setting
performance standards. In a mitigation proposal, reference sites and reference site data (from
regional datasets) should correspond to the appropriate wetland class and/or the appropriate
contributing watershed and stream setting to ensure an apples-to-apples comparison. Reviewers
should be clear on the use of reference terminology to understand the context and type of
reference data being used in the project. It is primarily through comparison of the mitigation site to
reference sites that providers and IRTs can document changes resulting from mitigation activities
versus those caused by natural variability.
Reference ecosystem information can be gathered using essentially four approaches: analog,
empirical, analytical, or using regional indices and condition gradients. The specific reference
approach selected for a mitigation project will depend upon the ecosystem type, the application of
the reference data (baseline assessment, target condition, design, and monitoring, etc.), and
especially the availability of information from which a reference standard can be specified. These
four approaches are not mutually exclusive and combinations of these can be, and often are, used
together. In fact, a hybrid approach, where different approaches are used for different components
of a design or as corroborative evidence, is commonplace in contemporary channel design practice
(Bledsoe, pers comm.). Therefore, reviewers should be aware of shortcomings of the various
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reference approaches and the data they produce and look for use of several reference approaches
to apply to the project. The reference approach(es) used, and the information gathered, should be
reviewed and agreed upon by the IRT.
5.3.1 Reference Approaches Useful in Compensatory Mitigation
Analog Approach
An analog approach to collecting reference information simply refers to sampling current, on-the-
ground sites of equivalent aquatic resource class, where data collected represents the structural
and functional characteristics associated with a healthy ecosystem. These analogs can be located
either on- or off-site of the mitigation area. On-site analogs can be wetland patches or stream
reaches, in close proximity to the mitigation area, that represent a set of characteristics that
represent one or more project features (Miller, 2012; Stein et al., 2022). For instance, forested
wetland patches with mature timber may be sampled as analogs for vegetation structure and
species composition, but because that patch is surrounded by agriculture, the site may not
represent the connectedness needed for good wildlife habitat. For streams, an analog reach may be
sampled for its geomorphic form but may not be appropriate for biologic reference due to
sustained water quality impacts upstream.
Looking for appropriate analog sites within or beyond a compensatory mitigation site's watershed
requires adherence to stream or wetland classification but offers flexibility to identify a more
ecologically representative reference for the system of interest (Miller et al., 2012). This might
involve, for example, identifying a stream reach of a similar type in a nearby watershed and similar
geomorphic setting that is fully functional and otherwise healthy. Data gathered from this
reference system can be used to evaluate conditions at the project site and serve as a guide for
project formulation and design.
For wetlands: Identified reference wetlands should be of similar HGM class and subclass to warrant
appropriate comparison with a proposed site (Bruland and Richardson, 2005b; 2005c). If sites fall
within the reference domain covered by an HGM regional guidebook, these guidebooks may be
used to describe prevalent functions, characteristics, and processes expected on the project site
(see regional index approach below). If sites do not fall within such reference domains, they will
require collection of additional reference data to provide a basis of comparison. Providers can use
aspects of the SSURGO database to determine the likelihood of particular sites being of similar
HGM class to the mitigation site, as discussed in Section 3.2.3. In addition, landscape information
from Section 4.2 and any other available watershed information can be used to select reference
sites with similar landscape settings to determine if a reference site represents a reference
standard or a more degraded condition along the disturbance gradient.
For streams: Typically, reference standard conditions for geomorphic channel designs in the
compensatory mitigation program have been obtained from reference reaches (analog approach)
in combination with empirical relationships (regional curves) and analytical methods using
computer models like HEC-RAS, River-2D, or FESWM (FHWA) (Hey, 2006; Bledsoe et al., 2016;
Yochum, 2016; Jennings, pers comm, 2018). The challenge in designing a geomorphically stable
channel is to identify a reference standard condition which is appropriate for the project site's
channel boundary materials and setting, as well as being able to accommodate current flow and
sediment regimes. Analog reference standard sites may also be used to provide biological data,
including vegetative communities, that represent the communities that occur in minimally or least
disturbed conditions.
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Finding appropriate analog reaches involves ensuring that key criteria including the valley setting,
boundary conditions, and inflowing loads of water and sediment are equivalent between the analog
and project reaches. The following criteria assess the appropriateness of an analog reach chosen by
a provider for use in stream design (Bledsoe et al., 2017). For an analog reach to be deemed
appropriate for channel design purposes it must have:
• Similar drainage area as the proposed restored reach (e.g., within 20%);
• Location - If on the same river, the analog reach is located upstream from the project reach;
• Same hydrophysiographic region (type, amount, timing and magnitude of precipitation
events) is the same as the restored reach;
• Similar channel type to the target project, given (1) prevailing historical channel type that
was previously stable (diagnose why departure occurred) in that location under current land
use; or (2) channel type is stable under same current land use, flow, and sediment supply in
analog reach;
• Stable channel - (i.e., CEM stage I or V per Schumm et al. (1984); Table 6) banks stable, no
evidence of trends in aggradation/degradation, planform change, etc. over approximately
50-year time-frame;
• Similar land use - The extent and nature of land use (e.g., curve number) is similar between
the two watersheds (within 20%);
• No flow/sediment regime alterations - Analog reach has no noteworthy tributaries, dams,
or intervening flow augmentations or extractions initiating changes in the flow and
sediment regimes.
Many of these same considerations are also important in the selection of biological analog
reference streams from which, for example, macroinvertebrates, fish, and/or riparian vegetation
data may be collected to inform target conditions or performance standards. In addition to the
above factors, historical watershed land use, water quality analysis and biological community
sampling should be completed at potential analog sites prior to selection. Contemporary watershed
land use may be a poor indicator of instream biological communities (Harding et al., 1998), and thus
historical land use should also be considered. Further, it can be difficult to ascertain if a high quality
biological reference has been selected without completing field sampling and analyzing the water
quality and biological community data from that stream. Water quality plays a critical role for
instream biological communities. Some adverse water quality conditions significantly affect in-
stream biota quality and may not be readily observable without measuring or analyzing water
quality (e.g., elevated specific conductance; see Boehme et al., 2016; Cormier et al., 2013). In-situ
measurement of water quality using appropriate techniques (e.g., properly calibrated
multiparameter sonde) should be included to investigate whether water quality could be a
biologically limiting stressor prior to full biological sampling in the field.
Regional Curve/ Empirical Approach
Empirical equations generally relate attributes of channel form (e.g., width, depth, flow velocity,
slope, meander wavelength, and amplitude) to independent parameters defining the channel-
forming flow (usually bankfull discharge), sediment regime, and the boundary conditions (bed
material size, valley slope, bank characteristics, and bank vegetation). In an empirical approach, the
functions, constants, and exponents are derived from local, regional, or global sets of experimental
or observed data. The approach is similar to the analog approach, with the difference being that
the dataset used for the method relies on a population of analogs to derive average values and
predictive formulas rather than an individual analog or reference reach. This is a logical extension of
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the analog approach, because it adopts the intuitive approach of replicating equilibrium conditions
observed in nature, but the values derived are more defensible since they are based on a larger
data set (e.g., regional curves) (Skidmore et al., 2011).
Regional curves are empirically derived, and typically plot the cross-sectional geometry of a subset
of stable streams in a region against their respective drainage areas. These regional relationships
between bankfull geometry and flow can be used to approximate bankfull channel geometry in
unstable channels where indicators are difficult to find or do not exist. These regional curves can
also be used to corroborate the reasonableness of bankfull channel geometry obtained for a
specific site. Verification of bankfull indicators with estimates of bankfull flow is typically
accomplished using regional curves (see McCandless and Everett (2002) for information on curve
construction).
Regional curves have been developed for many regions of the country (Somerville, 2010) and
reviewers should verify the source of any regional curves used as a reference in stream design.
There are a number of southeastern-specific regional curves developed in Alabama, Tennessee,
Georgia, North Carolina, South Carolina, and Florida. The USDA National Water Management
Center is also working with other federal, state, and local agencies to develop Regional Hydraulic
Geometry Curves throughout the country (see USDA Regional Hydraulic Geometry Curves):
https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/water/?cid=nrcsl43_015052.
Limitations of a regional curve/empirical approach are similar to those on the analog approach; that
is, the reference comparison(s) must be in equilibrium, and conditions in the project reach must
match those in the streams from which the hydraulic geometry equations were derived. The
empirical approach is appropriate where the causes and adverse impacts being addressed in the
project are localized (for example, being due to a poorly designed culvert, local channelization, or
unmanaged bank grazing and trampling), and where sediment and flow regimes are relatively
undisturbed. Even using appropriately derived empirical equations, the confidence in the precision
of these can be quite variable (Bledsoe et al., 2017). Therefore, it is important to have the provider
address how situations within the range of variation might be addressed after construction (e.g.,
within the adaptive management plan).
Analytical Approach
Analytical approaches differ fundamentally from analog and empirical relations in that they are
theoretical and require the quantification of the independent parameters governing alluvial
channel form (e.g., water and sediment flow and boundary resistance) (Skidmore et al., 2011).
Further, analytical approaches are capable of simulating dynamic process-response mechanisms in
unstable channels, whereas analog and empirical approaches assume equilibrium conditions and
therefore are unable to represent the processes and forms associated with channel adjustment.
Analytical or predictive design makes use of the equations for continuity, flow resistance, and
sediment transport to derive equilibrium channel dimensions for specified values of water
discharge, sediment supply, bed material, and bank characteristics. A good example of this
approach is the Copeland Method (Soar and Thome, 2001) which is incorporated into the HEC-RAS
models. However, in practice, the use of analytical methods is limited by the availability of the input
data required, the quality of that data, and scientific uncertainty in the models employed.
Regional index or condition gradient
A regional index is developed by characterizing a range or gradient of ecological condition that
represents regional values of ecosystem parameters of interest in ecological restoration.
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Aggregated values that enable discrimination between degraded and non-degraded sites are used
to build a reference index, set ecological condition gradients, and define functional endpoints.
Reference sites (e.g., poor, fair, good, and excellent) can be selected across the disturbance
gradient, using best professional judgement or derived quantitatively from among all the sampled
sites based on selected parameters.
Examples of regional indices include the HGM method for developing and applying indices for
assessment of wetland functions, Floristic Quality Indices (FQAI), or Indices of Biological Integrity
(IBIs) (Miller et al., 2012). Where sites are of similar HGM class/subclass and fall within the
reference domain of a regional guidebook (i.e., area from which reference data was collected and
to which the models apply), that data in whole or in part, can be used as a basis of comparison to a
mitigation site. HGM guidebooks will describe the reference domain in terms of climate, geology,
hydro-regime, soils, vegetation, and typical land uses. The demarcation and description of the
reference domain can be used to determine if the reference information in the guidebook can be
appropriately applied.
Where available, regional datasets can be used to develop ecological condition gradients from a
population of sampled sites. For example, water quality and biological data are available from state
water quality offices as well as from EPA's National Streams and Rivers Assessment (NRSA).
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6.0 How will the mitigation design achieve the target condition?
The intent of this question is to ensure mitigation work plans describe the proposed actions that
will alleviate stressors and improve ecosystem functions at the mitigation site, consistent with the
stated goals and objectives and within site and watershed constraints (Figure 1). There is not a one-
size-fits-all restoration method or technique that can be universally applied to all wetland or stream
restoration projects (Skidmore et al., 2011; Galatowitsch and Zedler, 2014; Stelk et al., 2017);
therefore, the provider must prepare a site-specific plan that provides a clear link between the
stressors causing the current wetland or stream impairment (baseline assessment), the goals and
objectives of the project and the proposed approach to address those problems.
Four restoration principles, originally proposed for streams but also applicable to
wetlands, that should be represented in every mitigation plan (after Beechie et al.,
2010):
• Wetland and/or stream processes degraded by anthropogenic activities should be
restoration targets;
• Mitigation designs and methods should be tailored to the ecological potential of the
site which is controlled by processes operating at watershed, riparian/catchment,
and reach/site scales.
• Scale of the project should match the scale of the problem. Given most
compensatory mitigation actions occur at the site or reach scale while many
processes acting upon that site or reach occur at broader watershed scales,
watershed-scale processes (Section 4.2) that cannot be addressed in the mitigation
project should be considered as constraints on the project;
• Mitigation plans should be consistent with the goals and objectives for the project.
Objectives will take time to be realized after implementation, thus there should be a
clear and quantitative link between management actions and the expected results.
Mitigation workplans, including project designs, are generally provided as part of a completed draft
mitigation plan, banking instrument or ILF instrument or addendum. Because a mitigation plan is
unique to each site, reviewers will want to ensure workplan elements are consistent with the
specific goals and objectives of the project and that proposed methods are appropriate for the site.
The main opportunity to provide comments on workplans will be during the Public Notice comment
period for individual permits or during review of the draft mitigation bank instrument or ILF project
addendums.
Important elements of an effective mitigation workplan include appropriate consideration of site
conditions, landscape context, and the constraints placed on the site by surrounding land uses; and
an understanding of the causes and levels of degradation, the effects a particular restoration
method might induce, as well as how to make modifications or refinements to the plan (i.e.,
adaptive management). Reviewers should consider the specific methods that are proposed in the
workplan to ensure they are informed by baseline conditions and function-based goals and
objectives; appropriate given the aquatic resource class, landscape potential, watershed and site
constraints, and logistics; and have the potential to move the site from its current condition to the
proposed target condition. Some restoration methods are more commonly used than others (e.g.,
NCD, planting specific tree/shrub species), and some providers may utilize methods because of
their familiarity with the method as opposed to the applicability of the method to the resource
class or setting, or its ability to address a stressor. The reviewer should remain open to consider the
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merits of any method, or set of methods, so long as they have the potential to achieve the desired
result. It is important to consider whether the proposed methods can effectively address the root
causes of degradation, are consistent with the ecological potential of the site, are appropriate given
the existing condition and scale of stressors or impairments, and whether they are consistent with
stated objectives (Beechie et al., 2010).
This section provides the reviewer with background information on important elements to be
included in wetland and stream workplans, as well as technical information on specific methods
and techniques commonly used in wetlands and stream restoration projects.
6.1 Regulatory Context
The Mitigation Rule requires that the mitigation work plan include written specifications and work
descriptions for the compensatory mitigation project, including, but not limited to: the geographic
boundaries of the project; construction methods, timing and sequence; source(s) of water,
including connections to existing waters and uplands; methods for establishing the desired plant
community; plans to control invasive plant species; the proposed grading plan, including elevations
and slopes of the substrate; soil management; and erosion control measures. For stream
compensatory mitigation projects, the mitigation work plan should also include other relevant
information, such as watershed size, proposed planform channel geometry, channel hydraulic
geometry (e.g., typical channel cross-sections for both pools and riffles), longitudinal profile and
channel bed forms, design discharge and riparian area plantings. Any given mitigation plan may
include any combination of the requirements in the Mitigation Rule, as well as additional
considerations that may be called for based on the type and level of impairment of the wetland
and/or stream.
6.2 Wetland Design and Implementation to Achieve Target Conditions
Methods and techniques described in the mitigation work plan should be appropriate to address
the level of degradation of the proposed site and address all factors necessary to move the site
from its current condition to the proposed target condition. To be self-sustaining, wetland
mitigation sites must have functioning hydrologic, soil biogeochemical, and biologic processes (i.e.,
water source, hydrodynamics, soil, and vegetation) (NRC, 2001). For both ecological and logistical
reasons, the evident degradation of the abiotic components of a site needs to be addressed before
biological treatments can be properly designed and/or implemented (Bruland and Richardson,
2005b; 2005c; Galatowitsch and Zedler, 2014; Craft, 2016; Stein et al., 2022). Only if abiotic
problems with topography, hydrology, soils, nutrients and/or contaminants can be resolved, will
there be a reasonable likelihood for successful restoration or enhancement of the biological
components of the ecosystem (e.g., target vegetation and animal/insect communities).
Specific methods proposed to address hydrologic, soil and vegetative impacts should be explained
and/or evaluated on the basis of the plan's purpose and rationale, its effect on the site, evidence of
its past effectiveness and full disclosure of assumptions and constraints (Box G). Generally,
stressors to wetland water sources (e.g., ditches, levees, and groundwater pumping) should be
characterized by wetland water budgets and addressed in the mitigation plan. Wetland soil
restoration needs should be identified based on the stressors affecting the site (e.g., compaction,
excavation, agriculture, and contamination) and the comparison of soil properties (i.e., bulk density,
texture, organic material, and nutrients, etc.) with reference standard sites and the Soil Survey.
Wetland vegetation plans should be compared to reference data and take into account how
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plantings will adjust to the growing season hydrologic regime (e.g., microtopography needs to be
incorporated into the plan to allow plants with different hydrologic tolerances, as well as
biogeochemical processes needing different hydrologic regimes, to occur on the site) and how
vegetation change over time. The following three sections address some aspects of hydrology, soils,
and vegetation that are important to be addressed in wetland mitigation plans.
Box G. Recommended information to address appropriateness of wetland mitigation plan/design.
The following questions should be addressed in context with the site classification (Section 3),
baseline watershed and site assessment (Section 4), and the goals and objectives for the project
(Section 5):
Hydrology
• Do the methods proposed address degradation of the wetland's water inputs, outputs, and storage
capacity as compared to reference wetlands of the same class?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore the hydroperiod of surface and/or groundwater typical for this wetland
type?
• What is the growing season hydroperiod during normal, wet, and dry years AND will this support a
vegetation community similar to reference standard?
Soils
• Do the methods proposed address degradation of the wetland's soils as compared to mapped soils
and reference?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore the soil characteristics (e.g., permeability, structure, hydric
characteristics, and bulk density) indicative of this wetland type?
Vegetation
• Do the methods proposed address degradation of the wetland's vegetation community as compared
to reference standard?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore a plant community (all strata) typical of the reference wetland class?
• Does the proposed method include a vegetative planting plan that accounts for variations in soil
types, site elevations, planting depths, and timing for planting?
6.2.1 Hydrology
Hydrology exerts the greatest influence on wetland structure and function. Thus, a proposed
mitigation wetland's water budget (i.e., inputs, outputs, and storage) should be carefully assessed,
and appropriate methods identified to restore the wetland's target hydrology. Wetland restoration
or enhancement projects should not merely aim to assure the minimal wetland hydrology criterion
(e.g., 14 days, five percent of growing season), but rather to restore the optimal hydrologic regime
of the target HGM wetland class and vegetative community. Adherence to a single, static
hydrology criterion for all wetland types ignores the fact that different wetland types possess
different hydroperiods. It is exactly this variability across wetland types that allows them to
function differently, thus enhancing the diversity and resiliency of wetland ecosystems on the
landscape and the watershed functions to which they contribute. Thus, each wetland mitigation
plan should address the target hydrologic (and soil) characteristics of that particular site and
propose methods to restore those characteristics, if necessary.
Ideally, wetland mitigation plans should quantify the wetland water budget in terms of inputs (i.e.,
precipitation, surface, and groundwater inflows), outputs (i.e., evapotranspiration, surface runoff,
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and groundwater outflows) and change in storage. The water balance equation written below can
be used in predicting wetland hydrology (Williams, 1998):
AS — P — E — T -Rs-Rg
Where AS is the change in soil storage of water, which equates to soil saturation; P is precipitation;
E is evaporation; T is transpiration; Rs is surface runoff; and Rg is subsurface runoff.
If AS is positive and larger than the volume of unsaturated soil porosity, then the soil is saturated.
Generally, P is always considered an inflow; E and T are considered water loss terms and are
considered outflows. Water can also be lost from a site via surface runoff (Rs) and/or subsurface
runoff (Rg) in which case these terms would also be outflows. Wetlands represent areas on the
landscape where surface runoff (Rs) or groundwater (Rg) can accumulate and, in these situations,
Rs and Rg can be net inflows (in which case the sign in the storage equation above is positive (+)).
Thus, wetland site hydrology can be grouped based on the four loss terms in the above equation: E,
T, Rs, and Rg. Where all four terms are negative, representing outflows, but precipitation is greater
than these terms, then the wetland is considered rainwater driven since rain is the predominant
water source. An example of this wetland type would be mineral and organic soil flats. Sites where
Rg is positive, representing a net inflow, will have groundwater hydrology like slope wetlands and
wetlands in the depression class. In these areas, subsurface flows augment rainfall. Finally, where
Rs is positive, or a net inflow, sites are flooded by surface water, as in riverine and fringe wetlands.
Given the discussion of hydrologic stressors and monitoring in Section 4.4, wetland water budgets
provide the context for understanding the effects of the stressors and interpreting the data from
hydrologic baseline monitoring (albeit short-term). Wetland mitigation plans should attempt to
assess wetland water budgets and determine the appropriate water sources and hydrodynamics for
the site and place them in one of the seven HGM classes. To date, quantitative water budgets are
rarely required by agencies and/or submitted by providers as part of mitigation plans. However,
without water budgets the degree of knowledge uncertainty and risk increases (see Section 2.4)
since an understanding of hydrologic sources and hydroperiod are essential for an effective plan.
Some potential methods to re-establish water inputs, outputs and storage, based on HGM class are
provided in Table 16 as examples of what reviewers might encounter during their review. HGM
wetland water sources and hydrodynamics are described in Table 3. Reviewers should consider
whether methods proposed to ameliorate hydrologic stresses are appropriate given project
objectives, rationale, effectiveness, assumptions, and constraints. For instance, plugging small
wetland ditches is often done using a non-porous soil (i.e., clay) or with a length of non-pervious
tile. However, large ditches need to be filled by re-grading so that water does not concentrate
behind the ditch plug, (which can lead to flanking of the plug) and instead spreads across the
wetland (Galatowistch and Zedler, 2014).
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Table 16. Examples of potential methods to improve wetland water components by HGM class. (ET=
Evapotranspiration: a combination of the evaporation and transpiration terms of the water budget
equation representing water losses to the atmosphere).
Potential Methods to Improve Impaired Wetland Hydrology
HGM class
and water
source(s)
Inputs
Outputs
Storage
Riverine -
(overbank
flooding,
groundwater,
and
precipitation)
Breach levees, remove
upstream dams,
eliminate toe slope
ditches and/or
groundwater pumping
Fill ditches
Restore micro- and macro-topography relative
to reference wetland, and floodplain
elevations and soil profile relative to onsite
water levels (i.e., no fill and no excavation).
Slope -
(groundwater)
Eliminate ditches and/or
groundwater pumping,
reduce upslope
impervious surfaces
Fill ditches, address
incision in receiving
streams
Restore surface elevations relative to
reference wetland and onsite water levels
(i.e., remove fill and no excavation), soil
profile/ porosity.
Mineral soil
flat-
(precipitation)
Eliminate ditches and/or
groundwater pumping
Fill ditches, remove
impediments to outflow,
restore native vegetation
(ET)
Restore surface elevations, micro- and
macro-topography relative to reference
wetland and onsite water levels (i.e., remove
fill and no excavation). Restore soil confining
layers.
Organic soil
flat -
(precipitation)
Eliminate ditches and/or
groundwater pumping
Fill ditches, remove
impediments to outflow,
restore native vegetation
(ET)
Restore surface elevations, micro and macro-
topography relative to reference wetland and
onsite water levels (i.e., remove fill and no
excavation), soil porosity/ organic matter.
Depression -
(groundwater,
precipitation,
and
floodwater)
Eliminate ditches and/or
groundwater pumping,
reduce upslope
impervious surfaces
Fill ditches, re-establish
natural outlet elevations
and cross-sections,
restore native vegetation
(ET)
Restore surface elevations, contours and
depths (concave shape) relative to reference
wetland and onsite water levels (i.e., remove
fill and no excavation). Restore soil profile.
Estuarine
fringe - (tidal
flooding)
Eliminate levees, toe
slope ditches, and/or
groundwater pumping
Fill ditches
Restore surface elevations and slopes relative
to reference wetland and onsite water levels
(i.e., remove fill and no excavation).
Lacustrine
fringe (wind
seiche
flooding)
Eliminate toe slope
ditches and/or
groundwater pumping
Fill ditches
Restore surface elevations and slopes relative
to reference wetland and onsite water levels
(i.e., remove fill and no excavation).
6.2.2 Soils
Soils are often overlooked as a factor that may need
restoration (Bruland and Richardson, 2005a; 2005b;
2005c; Richardson et al., 2016). The baseline
assessment (Section 4.4) should have included a site
verified soils map, soil profile descriptions, notation
of hydric soil indicators, and impacts to soil integrity
(e.g., tilling, plowing, bedding, land clearing, mining,
fill, contaminants). Based on these findings, the
mitigation workplan should address soil structure
andTertility, in addition to the soil impacts noted.
For instance, if the proposed site's soils have been disturbed by land uses that have either removed
more than the top 12 inches of the soil or have disturbed and/or compacted the top 12 inches of
the soil, then amelioration will have to be undertaken. Reference standard sites, with similar soil
types, should be used to compare the proposed site's soils and address the impairment of the
Restoring wetland soils:
• Ensure spatial variability of soils
onsite
• Incorporate microtopography
• Amend soils with organic matter
• Restore soil texture and bulk
density
• Improve soil fertility
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mitigation site soils in terms of bringing them back towards reference standard condition. Some
important aspects of soil restoration to be considered are:
• Incorporate spatial variability in soil characteristics in order to promote diversity of
ecosystem processes and vegetation patterns;
• Incorporate microtopography (similar in height and area to reference standard sites);
• Incorporate soil organic matter and organic amendments to site to improve plant survival,
biogeochemical functions, and soil characteristics;
• Soil texture, bulk density, and compaction should be documented in relation to vegetation
responses; and
• Soil nutrient concentrations and their availability in soils should be compared to reference
standard sites.
These factors should be addressed along with the site's growing season hydrologic regime (Bruland
and Richardson, 2005a; Bledsoe and Shear, 2000). Sites that have compacted soils will likely need to
have the soils deep-ripped to allow water infiltration and provide a suitable rooting medium. In
situations where soils have been excavated or lost due to generations of poor agricultural soil
conservation practices, soils may require organic amendment (Bruland and Richardson, 2005c;
Richardson et al., 2016).
One other aspect of wetland site design for any HGM class is incorporating topography to allow for
hydrologic, soil, and vegetative variability. Particularly in riverine orfloodplain wetlands,
microtopographic complexity can aid in the wetland's ability to accommodate the varying flood
flows and provide a diversity of hydrologic conditions for germination and establishment of plant
species with variable flood and soil saturation tolerances. Microtopography, including shallow
depressions and mounds caused by flood flow, tree tip-ups, sediment deposits, etc., occurs
naturally in wetlands. Macro-topography also occurs naturally and refers to larger surficial features
with deeper, more permanent water (e.g., oxbows, meander cut-offs, features that can be seen
from aerial photography) (NRC, 2001). Vegetation requiring drier conditions can be planted on
micro-highs, and those that can tolerate wetter conditions should be planted in lower elevations on
the floodplain (Mitsch and Jorgensen, 2004). Floodplains are typically smoothed during major
stream channel restoration projects to decrease roughness and the potential for flood forces to
erode the floodplain. However, where wetland restoration is a goal, topographic relief is
advantageous and should be incorporated into the plan. Efforts should be made to match the
spatial pattern of microtopography (e.g., hummocks and hollows, tussocks) of reference standard
wetlands in similar HGM subclasses in the region where the restoration occurs. This is especially
important at former agricultural sites or other sites where human land-use has created artificially
flat microtopographic conditions (Richardson et al., 2016; Bruland and Richardson, 2005a).
Long-term agricultural activity homogenizes the topsoil, which is most evident for attributes such as
organic matter, nitrogen, and cation exchange capacity (Whisenant et al., 1995; Robertson et al.,
1997; Gonzales et al., 2000) and often decreases, or eliminates, microtopography. Comparing the
spatial distribution of soils in the proposed site against a reference standard site(s) is important
since wetland physical environmental variability (which includes soils, soil organic matter,
topography, and microclimate, etc.) and plant species richness are positively correlated (Jeltsch et
al., 1998; Ettema and Wardle, 2002). More homogeneous soils in the proposed site than those of
the reference standard site could be expected to also have lower diversity of soil biota in the
vertical profile, which could hinder restoration. Efforts should be made to match the soil organic
matter content of reference standard wetlands in similar HGM subclasses of the region where the
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restoration occurs. If no such information is available, then soil amendments should be considered
if soil organic matter concentrations at the restoration site are <2.5% (Richardson et al., 2016;
Bruland and Richardson, 2005c).
Methods to restore soil characteristics, heterogeneity, and microtopography should be evaluated
based on their purpose, expected effect on the site, past effectiveness, and assumptions required in
implementation. Thus, if soils on the proposed mitigation site have been scraped, excavated, filled,
compacted, drained, subsided, or otherwise modified, the reviewer should look for evidence that
the workplan includes measures to restore the soils to pre-disturbed conditions to foster
appropriate biogeochemical processes and vegetation growth. If the soil on the proposed site
appears intact, then additional work may not be needed.
6.2.3 Vegetation
After the hydrologic and soil conditions for a proposed site have been addressed to restore abiotic
processes, the vegetation plan can be implemented. The reviewer should look for the sequencing of
activities in the mitigation plan to ensure that the appropriate wetland hydrologic and soil regime
are in place prior to planting.
Vegetation plans should be informed by baseline assessments (Section 4.4), the anticipated
hydrology on site, and appropriate reference comparisons. A quantitative estimate of growing
season hydroperiod in a normal, wet, and dry year should be used to show how the anticipated
hydrology of the mitigation site will be appropriate for the plant assemblage proposed and should
be compared to reference. The planting plan should be based on reference standard conditions and
not just jurisdictional criteria. Most workplans will include vegetation re-establishment plans and
plant lists, and most USACE Districts have specific criteria for species composition, density, and
survival. In addition to what each District may require in vegetation planting plans, vegetation
should also be planted, or allowed to volunteer, to reflect communities inventoried at reference
sites of the same HGM class, hydrologic regime, and soil type, but at various successional stages. If
an HGM regional guidebook is utilized to assess baseline conditions, and the proposed site is within
the reference domain of the regional guidebook, the reference vegetation list can be used as a
guide to determine if the workplan's plant list is appropriate for the target condition. If an HGM
regional guidebook does not cover the area, or the wetland type being proposed for restoration,
then an appropriate reference site(s) should be evaluated to determine the appropriate species mix
and the edaphic conditions under which they occur.
Site vegetation plans should account for planting locations of different plant species along the
moisture gradient of the site, which often controls successful species reestablishment, including
invasive species reintroductions (Ho and Richardson, 2013). These gradients typically correspond to
the elevational gradient of the site (i.e., drier species planted at higher, and wetter species on lower
elevations) (Galatowisch and Zedler, 2014). Plans should also identify the source of plant material,
as plant nurseries closer to the mitigation site will likely have stock acclimated to local conditions or
be able to propagate and grow appropriate species under contract. If there is evidence of natural
regeneration of desired species, then replanting at a high density may not be necessary. Site plans
should also account for either the presence of, or the potential for, invasive plant species. Invasive
species can cause substantial problems (e.g., Connell and Slayter, 1977; Klotzli and Grootjans, 2001;
Suding et al., 2004). Most effective controls are to remove new populations while small and
manageable and before they spread. If invasive plants are present on the site, they will need to be
controlled before planting desired vegetation can occur. Any potential method (listed below) for
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revegetation should be evaluated as above (i.e., purpose, rationale, effectiveness, and
assumptions).
Site vegetation plans should also identify proactive measures necessary to facilitate survival and
growth of planted vegetation. Both providers and reviewers often assume that simply planting
desired vegetation sets up deterministic succession of the plant community that follow trajectories
over time leading to conditions representative of a reference ecosystem. However, this assumption
of simple and predictable restoration trajectories is unrealistic (Matthews et al., 2009) and
unsupported by numerous successional case studies (e.g., Matthews, 1979; Christensen & Peet,
1984; del Moral, 2007). Careful oversight and proactive maintenance (e.g., mowing, herbicide
application, invasive species control, re-planting) may be critically important to ensure that the
target vegetative community has a reasonable chance of becoming reality. Reviewers should
ensure that appropriate site preparation and post-planting maintenance is part of the mitigation
plan (see Section 8.2).
Potential methods to restore wetland vegetation include:
• Seeding - collecting seeds from multiple wetlands for genetic diversity;
• Direct planting - availability and location of planting stock; species selection based on
presence in reference wetlands and probability that they will establish in the restored site;
• Controlling invasives (depends on biology of invasive)- herbicides, annual removal of
individuals, mowing, prescribed burning, and sometimes flooding (e.g., flooding cattails
after they have been cut down to stubble may reduce populations); and
• Natural regeneration.
6.3 Stream Design and Implementation to Achieve Target Conditions
A plan for stream mitigation should be based on the results of the watershed and baseline
assessments that identify the stressors which affect stream processes (Sections 4.2 and 4.5) and the
goals and objectives of the project (Section 5.2). It is important for the provider to incorporate, and
the reviewer to understand, how the proposed methods and/or design adequately address the
problems with the stream. Hydrology (surface and groundwater flows), hydraulics (energy, shear
stresses, sediment), and geomorphology (channel conditions, habitat) are the fundamental stream
processes that must be considered in a stream design. Where project objectives relate to
improvement of physicochemical and biological functions, reviewers should ensure that workplan
elements address degradation of the supporting stream processes.
There are different methods and techniques which can be used to restore impaired stream
processes, for instance allowing the stream to self-heal (Kondolf, 2011) or stabilizing stream banks
and channel bottoms to prevent bank erosion (Yochum, 2016; Bledsoe et al., 2016; Skidmore et al.,
2011; Saldi-Caromile et al., 2004). Stream restoration may be implemented at any scale, ranging
from removal of a single passage barrier (e.g., a culvert) to alteration of watershed-wide land use
practices (e.g., planting cover crops on agricultural land, stormwater retention in urban areas). As
with wetland mitigation plans, stream mitigation providers should provide a discussion/explanation
of the purpose, rationale, effectiveness, and assumptions and constraints of any and all proposed
method(s). This is particularly important in stream restoration since numerous criticisms have been
levied against many aspects of stream restoration and mitigation, including specific stream
restoration methods themselves (e.g., Simon et al., 2007), project documentation and record
keeping (e.g., Bernhardt et al., 2005; Roni et al., 2008; Miller et al., 2010; Palmer and Hondula,
2014) and project efficacy in terms of both physical stability (e.g., Miller and Kochel, 2010; Palmer
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et al., 2014) and biological community restoration (e.g., Roni et al., 2008; Palmer et al., 2009; Ernst
et al., 2012; Palmer et al., 2014). For example, NCD (Rosgen, 1996), has been criticized as over-
simplifying the complexity of fluvial systems (Doyle et al., 1999; Kondolf, 2006; Simon et al., 2007),
and there are considerable questions regarding the efficacy of NCD as an appropriate stream
restoration technique (Juracek and Fitzpatrick, 2007; Lave, 2009). Although these criticisms have
validity, mitigation to offset authorized stream impacts is a fundamental component of the CWA
Section 404 program, and regulatory agencies must make the most informed decisions possible to
ensure that proposed mitigation projects not only comply with the Mitigation Rule and remain
consistent with its fundamental tenets, but also produce ecologically sustainable projects.
Reviewers should ask appropriate and pertinent questions of any stream mitigation proposal to
determine its ability (if implemented appropriately) to rectify stream impairment and provide
adequate mitigation.
Mitigation strategies and design approaches may consist of one, or many, restoration actions that
rely on multiple lines of evidence (reference, empirical equations, analytical methods, etc.) to
address the problems identified in the baseline condition assessment Ideally, individual actions will
address root causes of identified problems at the appropriate scale. Site-specific disturbances may
be remedied on small scales; systemic disequilibrium and watershed-scale causes of stream
degradation generally require watershed-wide restoration activities to yield measurable benefits.
When mitigation actions include land disturbance as part of construction (e.g., mechanized
grading), each action (i.e., each distinct project component) needs to be well documented,
rationalized, informed by objective reference approaches, and carefully designed.
Typically, stream mitigation plans are submitted with relatively complete stream designs (e.g., 60%
designs), accompanied by geomorphic data relating to anticipated stream pattern, profile and
channel cross-sectional geometry. Mitigation workplans may involve methods to restore hydrologic
connectivity (i.e., longitudinal, lateral, vertical and temporal connectivity); address channel stability
(e.g., through various bank stabilization techniques); reconnect channels and floodplains; remove
barriers; adjust planform and channel geometry; and add, modify or remove instream structures,
among others.
Documentation of the reference approach, restoration techniques, and/or associated design
elements should be presented in the mitigation plan to allow the full review of how the mitigation
plan addresses impairments and achieves the objectives of the project (Box H). Stream mitigation
plans will often be comprised of a variety of project elements which, in total, represent the
provider's plan for addressing the stressors acting on the project site and restoring ecological
functions (e.g., through the establishment of longitudinal, lateral, vertical, and temporal
connectivity of the stream to its watershed). To evaluate a proposed plan, it may be useful for
reviewers to break the plan down into its component parts to determine how the project elements
contribute to addressing the stressors and objectives of the project. The following discussion
elucidates some aspects of stream hydrologic, hydraulic, geomorphic, physicochemical, and
biological restoration for the reviewer to consider. It is not inclusive of all possible techniques but
should provide the reviewer with some basic information to discuss with the provider and as
context for review of the appropriateness and completeness of a stream mitigation plan. For more
detailed information about stream restoration project review, see Skidmore et al. (2011), Bledsoe
et al. (2016), Yochum (2016), Cramer et al. (2012), Roni and Beechie (2013), USDA NRCS (2007) and
FISWRG (2001).
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Box H. Recommended information to address appropriateness of stream mitigation plan/design.
Addressing the following questions should be done in the context of the mitigation site's classification
(Section 3), baseline watershed and site assessment (Section 4), and the goals and objectives for the
project (Section 5):
Hvdroloqic
• From what type of hydrologic data were design flows derived (e.g., gauge, ungauged, models, equations)?
• Are impairments to hydrologic connectivity of the stream reach addressed in the mitigation plan? Including:
o Longitudinal connectivity (e.g., dams, weirs, road crossings, and culverts)
o Lateral connectivity (e.g., levees, floodplain development)
o Vertical connectivity (e.g., hyporheic exchange)
• Is the project reach experiencing hydrologic flashiness (based on assessment of contributing watershed
imperviousness)?
• Do the methods proposed address the current condition of the stream's low, bankfull and flood flows?
• Will the proposed methods restore the timing, magnitude, frequency, and duration of surface and/or
groundwater flows typical for this stream type where these conditions are impaired?
Hydraulic
• What methods were used to address the hydraulics of the reference reach? Were the same methods used for
the designed reach?
• Has the provider accounted for expected sediment flows into and out of the project reach under the full range of
flow conditions (i.e., low, bankfull, and flood flows)?
• If the provider set a goal of equilibrium conditions, how have they accounted for the hydraulic and sediment
capacity and supply conditions upon which the design to achieve equilibrium is based (e.g., modelling)?
• If the provider has NOT set a goal of equilibrium conditions in the project reach, how have they accounted for
sediment capacity and supply through the reach (i.e., sediment storage on the floodplain)?
• If the project reach is incised how, and to what extent, will the provider reconnect the floodplain?
Geomorphic
• What methods are proposed to address degradation of the stream's vertical and lateral stability?
• Will channel reconfiguration (planform, slope, and cross-sectional geometry) be required? Has sufficient
justification for it been included by the provider?
o If so, has the provider supplied adequate rationale for using this method?
o Have the risks associated with this method been evaluated?
• Will bed materials be replaced? If so:
o With what will they be replaced?
o What is the rationale for, and the risks associated with, using this method?
• Will streambanks require protection? If so:
o How will they be protected?
o What is the rationale for the methods that will be used?
o Have the risks associated with the methods chosen been considered?
• Will instream structures be used?
o For what purpose?
o Have the risks associated with how the structures will affect stream processes been considered?
• Were other less physically disturbing methods to the ecosystem evaluated and justifiably determined
inappropriate for the project?
Phvsicochemical
• Will the methods chosen for other aspects of the project (e.g., hydrology, hydraulics, geomorphology)
potentially affect the water quality of the project reach and/or upstream or downstream reaches?
• Are there specific actions proposed to ameliorate on-site or off-site sources of adverse water quality? If so, are
the actions/methods rational and likely to succeed?
Biological
• Riparian Vegetation
o Will planting riparian vegetation be necessary? If so:
¦ Is species composition and spatial placement (with regards to hydroregime) appropriate
when compared to appropriate reference?
¦ Are planting methods and timing appropriate?
¦ Are the appropriate species planted in appropriate zones?
• Instream Habitat:
o Will large wood or instream structures be required for habitat?
o Have the risks and effects on stream processes associated with creating habitat with structures, been
evaluated?
o Have sufficient intact populations of biota been identified near the project area?
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6.3.1 Addressing Hydrologic Connectivity
Connectivity within watersheds is critical for the transport for water, sediment, organic matter,
nutrients, and organisms. Longitudinal, lateral, and vertical hydrologic connectivity (i.e., the ability
of water to flow freely from upstream to downstream, flow into and out of a floodplain during
flood events, and groundwater to flow to the channel during low flows) is critical to ecologic
functioning of streams (EPA 2015). In Section 4.5.2.1, aspects of a baseline hydrologic assessment
were outlined including data sources (gauged and ungauged sites); flow estimation; important
flows needing assessment; and common watershed and reach scale stressors which can affect the
magnitude, frequency, timing and duration of flood, and low and bankfull flows (Table 11).
Hydrologic information for the workplan and the stream design, if one is planned, should be
consistent with that gathered for the baseline assessment.
Longitudinal connectivity restoration often entails the removal of barriers. This could involve
removal or modification of dams, weirs, pipeline crossings, bridges, road crossings, culverts, or
other infrastructure. If removal of a barrier is proposed, the mitigation plan should also address
anticipated changes to channel morphology, stream hydrology and hydraulics, sediment properties
and transport, groundwater quality and quantity, instream habitat, floodplain vegetation, and
instream biota. When evaluating a draft workplan, reviewers should focus on the stream
impairment, its remedy, how and why any proposed method will be used, and the anticipated
effects up- and downstream. Reviewers should also be aware of unintended consequences with
some restoration methods, such as the propensity for exposed sediments behind former dams to
be rapidly colonized by non-native plant species in the absence of diligent management, or the
change in the channel bed slope and stream power causing the stream to quickly erode sediments,
evolve to an incised stage, and eventually lose contact with its floodplain.
Lateral connectivity, or floodplain reconnection (discussed in Section 6.3.2), is the restoration of
the lateral hydrologic connection between the channel and floodplain, allowing for overbank
inundation and infiltration to support riparian groundwater tables, among other processes. Natural
floodplains can be important nutrient and sediment sinks, a function which is often lost as
floodplain-channel connections are severed via channelization, incision, or levee construction.
Hydrologic reconnection of floodplains and streams can be achieved by lowering stream banks,
removing or breaching levees, raising the channel bed, increasing wood recruitment and log-jam
formation, removing infrastructure from floodplains, and/or complete channel reconstruction (e.g.,
Priority 1 restoration under NCD) or valley restoration.
Vertical connectivity, or the interactions between hyporheic and groundwater flows and surface
water, should be considered for habitat design and baseflow maintenance. Sub-surface flow
conditions can be characterized through measurement and/or modeling. In some cases, simply
identifying the variation in groundwater stage throughout the year may be enough to inform
project design. Basic groundwater/hyporheic information can be obtained through installation and
monitoring of piezometers or through pump tests (Cramer et al., 2012).
Hydrologic investigations are used to establish design flows (e.g., baseflow, bankfull discharge,
flood flows), which serve as the basis for design of channel, habitat, and floodplain components.
Hydrologic analyses are foundational in stream restoration design, as virtually every other design
analysis requires the input of design discharge(s). Design criteria for various aspects of any project
may be defined relative to specified flows extracted from the hydrologic analyses, such as channel-
forming flow for channel dimensions, base flow for instream habitat, and flood flow for channel
stabilization structures (Skidmore et al., 2011). Flow statistics such as mean low flow (e.g., to
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determine extent of the low-flow habitat and habitat design), the 100-year flow (e.g., to determine
flood inundation risk and channel stability), maximum peak flows for any given month (e.g., to
determine risk of inundation during construction months), or minimum flows for any given month
(e.g., for passage design), can be statistically derived from existing gauge data or by modeling
streamflows from precipitation records to develop a synthetic flow record (Fischenich and McKay,
2011).
The baseline assessment (Section 4.5.2.1) should document hydrologic stressors that affected the
movement of streamflows downstream and/or interacting with the floodplain as surface and
ground water; and characterize the magnitude, frequency, duration, and timing of current flow(s)
that inundates the floodplain of the mitigation project site, along with the shear stress on the
channel bed and banks that correspond to that flow. Based on those results, the reviewer should
evaluate whether the workplan and/or design accommodates the range of flows delivered from the
watershed and the ability of those flows to connect and/or maintain up- and downstream,
floodplain and hyporheic habitats.
6.3.2 Addressing Hydraulics and Sediment Transport
For many stream mitigation projects, hydraulics analysis is a critical component of the design phase,
and it is necessary to understand existing conditions and the potential effects of proposed designs.
The purpose of hydraulic design analysis is to assess flow velocity and channel boundary shear
stress, the degree to which the channel is connected hydraulically to the floodplain (water surface
profile), and the sediment transport capacity (stream power) through the project reach relative to
upstream and downstream reaches.
In watersheds with increasing amounts of impervious surface, the magnitude and velocities of
stormflow runoff increases and the timing (delivery) of that runoff decreases. Termed hydrologic
flashiness, these flows can occur more frequently and exert greater shear stress which affect
sediment flows, channel stability, biological communities, and other stream functions. Often,
providers will only assess the bankfull discharge as the flow that moves the most sediment over
time. However, in flashy watersheds, frequent stormflow runoff of higher velocities and shear
stresses can move more sediment, both in the channel and from source areas, than what might be
indicated by only an analysis of bankfull flows (Bledsoe et al., 2017). Providers and reviewers will
need to ascertain whether the effects of watershed land use on water and sediment flows have
been accounted for in the mitigation workplan, lest these flashy flows undo the work of the
provider.
The baseline assessment (Section 4.5.2.2) is meant to identify potential hydraulic and sediment
transport problems needing to be addressed, and provides basic information on sediment sources,
stressors affecting sediment transport, and the potential for a project reach to experience
increased sediment supply via channel evolution. It bears reiteration that the provider will need to
identify potential sources of sediment upstream of the site and in connected portions of the
watershed (i.e., areas which have conduits for the transport of sediment to the project reach), as
well as the characteristics of the sediment likely to be generated at the site, as each of these
sources has potential to affect the project design.
Reviewers should consider the methods used to estimate the hydraulics of the channel, floodplain,
and up- and downstream reaches; the effects of channel evolution on future water surface
elevations, floods, and sediment transport; and the effect of the project on the channel hydraulics
up- and downstream of the site. Any models (e.g., WinXSPro, HEC-RAS, HEC-RAS2-D, or Flow 3D)
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used should be justified and explained in the context of the proposed project. For any methods
used to evaluate sediment capacity and supply, the reviewer should have sufficient information to:
• Understand the range of sediment sizes expected to move into the project reach;
• Understand the range of flow conditions necessary to move the expected sediment load
(i.e., pulsed flow conditions) through the reach or onto the floodplain;
• Identify sources of the sediment from the upstream watershed;
• Understand the implications of the proposed design on the sediment capacity of
downstream channel reaches and its potential effects on downstream infrastructure and/or
property owners; and
• Understand how the workplan and/or design account for these conditions.
It is important for the provider to identify channel and floodplain dimensions that will
accommodate anticipated streamflows without mobilizing channel bed or floodplain substrates to
such an extent that channel incision, bank erosion, and avulsion, etc., ensues. Likewise, it is
important for the provider to ensure that the sediment capacity in the project reach is sufficient to
move the anticipated sediment supply. In other words, project design should maintain stream
equilibrium. Velocities and shear stresses that exceed the ability of the bed and bank material to
withstand erosion (i.e., competence) will likely result in instability. The range of flow velocities,
shear stresses, and/or stream powers for the 2-year to 100-year recurrence interval peak flow
events (i.e., 2, 5,10, 25, 50,100-year) should be estimated for the channel and floodplain to
understand the effects of higher shear stress events on the channel bed, banks, floodplain, and
connected sediment sources.
Sediment analyses are used to determine whether sediment transport capacity is balanced with
sediment supply. Sediment transport models (e.g., HEC-RAS, BAGS, and POWERSED) can be useful
for examining the gross magnitude of sediment that may pass through, be eroded from, and/or
stored in a stream reach. They may also be helpful in identifying causes of channel instability and
for evaluating the performance of varying design scenarios. However, a high level of knowledge
and experience is typically required to effectively collect appropriate sediment transport data,
understand the modeling processes, effectively apply sediment transport models and interpret the
results in a useful way (Wilcock et al., 2009). A provider may use a sediment transport model to
provide evidence of the ability of a reach to move sediment loads. As a reviewer, if sediment
transport models are used, be sure that analysis and interpretation of the results are adequately
explained in terms of how the results affect the objectives of the proposed design, streamflows,
channel conditions, and sediment transport.
Floodplain connection and incised channel restoration
Channel incision and projects proposing to remedy incision are common in the southeastern U.S.
Channel incision reduces interaction between the stream and its floodplain, reduces spatial habitat
heterogeneity, increases temporal instability, reduces hydraulic retention, degrades water quality,
enlarges stream channels, and shifts the fish community structure (Doll et al., 2016). Floodplain
reconnection and restoration of incised channels are restoration techniques that affect hydraulic
and sediment transport functions. Floodplain reconnection is the restoration of the hydrologic
connection between the channel and floodplain, allowing for overbank inundation and elevated
riparian groundwater tables (i.e., lateral connectivity). Channel and floodplain processes are
integrated and connected floodplains store flood waters and sediment carried by the stream during
out-of-bank events, as well as allowing for raised riparian groundwater tables by recharging local
shallow groundwater.
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Generally, three restoration approaches are used to address incised channels and re-establish
hydrologic and hydraulic connectivity through floodplain reconnection (Roni and Beechie, 2013;
Skidmore et al., 2011). Site conditions, project goals and objectives, and physical constraints will
determine which of these approaches is most feasible. For all approaches, additional space for the
channel and riparian zone should be incorporated into design and ongoing land management.
These approaches include:
• Lowering the abandoned floodplain (i.e., terrace) to an elevation that allows hydraulic and
sediment coupling with streamflow in the existing incised channel.
• Raising the streambed to its pre-incision elevation to restore hydraulic and sediment
coupling with the existing floodplain.
• Relocating the channel within the floodplain at the desired bed elevation to allow hydraulic
and sediment coupling with the floodplain.
Because of its importance to hydrologic and hydraulic processes, where possible, floodplain
reconnection should be incorporated into all stream designs unless justifiable rationale is provided
to explain why it cannot be accomplished (e.g., valley confinement, property ownership
constraints, adjacent infrastructure). Whatever the approach, floodplain reconnection tends to
raise the riparian groundwater table and allow for more frequent overbank flows. This increases
sediment and nutrient retention and processing while providing ancillary benefits such as enhanced
aquatic and riparian habitat, reduced in-channel erosive power and downstream flooding, and
increased aquifer recharge.
From a hydraulic and sediment standpoint, workplans proposing floodplain lowering or
reconnection should contain provisions addressing the risk of stream avulsion (i.e., the formation of
a new (and unexpected) stream channel across a floodplain due to erosive flows during a flood
event). Reviewers should be aware that risk of avulsion can be minimized by establishing floodplain
plantings 1-2 years prior to reconnecting the channel to the floodplain, using erosion control
measures to ensure floodwaters do not cause avulsion, and/or use of grade control at streambed
grade to protect against incision should avulsion occur (i.e., vertical control across the floodplain
valley) (Skidmore et al., 2011; Cramer et al., 2012).
If a stream has incised because its base level has been permanently lowered (such as in a tributary
where the downstream main river system has lowered its bed) or due to a fundamental change in
the flow and/or sediment regimes (common in urbanized and agricultural watersheds), restoring
the bed elevation to its former or pre-disturbed condition may be impractical. In such cases,
excavating the abandoned floodplain (terrace) to a lower elevation that allows recoupling with the
incised stream channel may be effective. This essentially accelerates the channel evolution process,
whereby channels first incise, then migrate laterally, eroding into the abandoned floodplain, and
ultimately create new floodplain surfaces at elevations adjusted to the new channel elevation.
Baseline data on groundwater levels within the floodplain should be considered in the workplan
and design so the new floodplain is constructed at an elevation consistent with the groundwater
table, based upon targets established in the project objectives. In some instances, reference
wetland and/or anastomosed stream Stage 1 or 8 (SEM) streams may inform the new floodplain
and groundwater level proposed in the project design.
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6.3.3 Addressing Channel Geomorphic Processes
Most stream mitigation plans contain designs and construction elements to address geomorphic
stressors or to address the channel condition caused by stressors (e.g., increased flow velocities
from impervious surfaces or decreased sediment loads that lead to channel incision). Baseline
assessments (Section 4.5.2.3) characterize existing channel morphology and planform, bedform
diversity, aggradational or degradational status, and channel stability; and identify the stressors
leading to vertical and lateral instability in the context of appropriate reference standard condition.
Stream geomorphic designs are often based on the idea that natural alluvial channels tend to
evolve toward a condition where channel form and dimensions are adjusted to flow and sediment
regimes. Identifying the channel form that is appropriate and stable for the valley setting and the
bed and bank materials, while maintaining equilibrium, is challenging (Skidmore et al., 2011),
especially in watersheds with changing land use patterns. As such, it is important that appropriate
reference comparisons (Section 5.3.1) are used to inform the design of channels.
To briefly reiterate from Section 5.3.1., analog design approaches can be applied at any scale (e.g.,
from multiple reaches down to site-specific features), since fundamentally they represent a known
condition. However, the assumptions inherent in the analog approach, acknowledged or not, are
that the reference reach itself is in equilibrium and is representative of flow, sediment, and
boundary conditions at the project site (Bledsoe et al., 2017; Rosgen, 2006; Skidmore et al., 2011).
Empirical equations require similar assumptions as the analog approach; and analytical models, if
used, can simulate past, current, future, or synthetic (i.e., scenario-based) conditions to augment
the design of target stream geometry during the design process (Miller et al., 2012). A combination
of these reference approaches may be prudent to support a particular design.
Efforts to address channel geomorphic processes often involve channel or streambank construction
or reconstruction, including bed and bank stabilization, instream enhancement (e.g., habitat,
increased hyporheic exchange and water retention time), floodplain reconnection, channel
reconfiguration, and barrier removal. Typically, a project design will include one or more of these
project elements. When evaluating project elements in a workplan, reviewers should consider
whether each element has the capacity to meet project objectives, alleviate stressors and allow or
promote stream processes that support and maintain the stream system. Reviewers should ensure
that workplans explain details of the design approach and included project elements in terms of
how the approach addresses the problems affecting the stream (i.e., watershed stressors vs. local
stressors). This discussion should also describe the technical basis for the approach, discuss how
assumptions of equilibrium are met in analog reference, discuss confidence intervals associated
with empirical equations, and explain any assumptions and levels of uncertainty in the chosen
approach(es). To adequately address the complexity of stream restoration projects, it is prudent for
providers to include people with engineering, geomorphology, hydrology, hydraulic, stream ecology
and biology expertise on their design teams.
While a comprehensive discussion of design is beyond the scope of this document, design elements
will need to be carefully considered to anticipate the level of interaction of the reconstructed
channel with the flows of water, sediments, nutrients, and organic matter coming from the project
reach as well as the upstream watershed. Project elements are the distinct project components
(e.g., stream pattern, slope, cross-sectional geometry, and instream structures) that can be
designed independently, but together comprise a complete project design. The information below
provides a brief discussion of some common project elements which may be included in a workplan
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and suggestions on how to evaluate those elements (after Skidmore et al., 2011; Roni and Beechie,
2013; Cramer et al., 2012; USDA NRCS, 2007; Bledsoe et al., 2016).
Channel Reconfiguration
Channel reconfiguration is generally defined as significant realignment or complete channel
reconstruction to provide a more appropriate channel geometry and planform (Bledsoe et al.,
2016). Channel reconfiguration may entail reconnection of a historically abandoned channel, partial
channel realignment or complete construction of a new channel (e.g., NCD Priority 1 or 2
restoration). This method is especially valuable in areas with a significantly altered channel
geometry and planform (e.g., from channelization) where more passive approaches aimed at
restoring natural erosion and depositional processes may not be viable at required time scales.
Channel reconfiguration can decrease velocities by reducing slope and increasing sinuosity and is
typically accompanied by other stream restoration strategies such as erosion protection and
installation of in-stream structures. However, channel reconfiguration may not always be
necessary, and in some cases less intensive restoration strategies may provide acceptable results
with fewer risks.
Depending on project objectives and the geomorphic context of the reach, modification of the
channel may be an appropriate method to accelerate recovery of a sustainable natural channel and
floodplain. This is typically accomplished by relocating the existing channel or by modifying channel:
• Planform (the shape/pattern of a channel in map view; planform is defined by sinuosity and
meander characteristics),
• Cross-section (the shape, width, and depth of a channel from bank to bank and across the
floodplain), and
• Longitudinal profile (the elevation, slope, and shape/pattern along the channel bed).
Planform, cross-section, and profile are integrated and mutually adjusting features. Thus, altering
one will affect the others, and alteration of any of these typically results in a change in the hydraulic
and sediment transport characteristics of the channel. As such, reviewers should carefully consider
proposals to reconfigure channels, as hydraulic responses can be complex. Design considerations
for, and risks associated with, each of these elements in the context of channel reconfiguration are
discussed below.
Planform: Planform design can be approached using analog, empirical, or analytical methods, or
any combination of these reference approaches. Planform is directly related to valley slope and
channel slope, as changes to planform affect channel length and channel slope is a function of
channel length. As such, planform must be designed in conjunction with channel slope (see
discussion of longitudinal profile below).
A critical planform parameter in meandering channels is the ratio between the bend radius of
curvature (Rc) and the channel width (W) (USDA NRCS, 2007). In general, when the meander ratio
(Rc/W) is between 2.0 and 3.0, meanders operate most efficiently (in terms of energy loss) and
tend to migrate rapidly downstream without changing their shape substantially (Skidmore et al.,
2011; Harman, pers comm; Jennings, pers comm). Naturally, this meander ratio incorporates the
effects of riparian and bank stabilizing vegetation. However, for newly restored stream channels
that have relatively nonresistant, unvegetated streambanks immediately following construction,
designing and constructing meander bends within this ratio range will likely be riskier and increase
the probability that rates of bend migration may be unnaturally high before vegetation has fully
colonized the banks and riparian corridor.
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Risk of accelerated bend migration often prompts providers to artificially harden the outer banks in
constructed bends. Hardening the outer margins of meander bends, whether with rock or
rootwads, may be helpful in maintaining a desirable initial cross-section and planform, but such
reinforcement can constrain natural bend migration processes by preventing all bank erosion. If the
goal of a stream mitigation project is to restore dynamic alluvial processes, then hardening the
banks may run counter to that goal. In circumstances where the risk of bend migration immediately
following construction and prior to the establishment of a well vegetated riparian corridor is
unacceptable, biodegradable materials and deformable bank construction techniques (e.g., coir
fabric encapsulated soil lifts) can be used to protect channel bends for a limited number of years
following project completion while riparian vegetation takes root. After the riparian vegetation
takes hold and begins to mature, it can provide natural stability to the banks (Miller and Skidmore,
1998). Regardless, planform design elements and long-term land management plans should
incorporate additional space for the channel and riparian zone to allow the channel to eventually
resume meandering across its floodplain.
Channel cross-sectional geometry: Changing a channel's cross-section involves altering its width,
depth, or shape across the channel, and can include modification of channel banks and bars. Cross-
section modifications are most commonly applied to the main channel but could also include
modifications of floodplain elevations or features. Channel slope is an integral component of cross-
section design, as the target flow capacity of a channel is determined by the relationship of cross-
sectional area and channel slope.
Design criteria for channel geometry parameters generally relate to the channel-forming discharge
and express the allowable or desirable range of variation in dimensions, so that the constructed
channel is not unnaturally uniform. Allowing for variation of width and depth throughout the reach
while still accommodating the channel forming flow, will provide habitat variability as well.
Within an alluvial channel, the shape of the cross-section varies with position in the planform. For
example, the cross-section at the apex of a meander bend is typically triangular and asymmetrical,
with a deep pool adjacent to the outer bank, while the cross-section at riffles is more trapezoidal
and symmetrical. Hence in designing an alluvial channel, providers commonly provide a minimum
of three cross-sectional templates corresponding to pools, riffles, and transitional reaches. While
the width, mean depth, maximum depth, and asymmetry vary between templates, all should be
sized to contain the channel-forming discharge.
Risks associated with designing channel cross-sections are related to the channel's capacity to
maintain equilibrium conditions and the ability of the channel to carry target flows and associated
sediment loads without causing channel aggradation or degradation. This requires careful
adjustment of design channel widths and depths. Channels with similar cross-sectional areas may
transport water and sediment flows differently depending on their width and depth ratios. For
instance, channels that are wide and shallow may experience aggradation as these channels tend to
allow water velocities, and subsequent sediment capacity, to decrease. On the other hand,
channels that have narrow widths and greater depths may allow larger flows to remain in the
channel thus increasing shear stresses on stream bed and banks and inducing degradation. Both of
these channel types may have the same cross-sectional area, therefore reviewers should carefully
consider designed channel widths and depths in relation to channel slope to confirm the channel
cross-sectional geometry will achieve the target flow conditions.
Longitudinal profile/channel slope: Longitudinal profiles characterize the slope of the channel bed
and the variation of that slope through a reach. The slope of a channel is the ratio of change in
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elevation to distance along the channel, often but not always represented as a percentage. Channel
slope will change as a result of any activity that changes the bed elevation at a point or changes the
length of channel between two constant elevation points. Channel slope varies among specific bed
form features (e.g., riffles vs. pools), and thus characterizes bedform diversity, including the pattern
and lengths of important habitat elements (e.g., steps, riffles, runs and pools).
Channel slope is highly integrated with project elements concerned with planform pattern and
cross-section dimensions. These three elements together define the three-dimensional geometry of
the channel. Slope, cross-section, and planform project elements should be iteratively manipulated
to achieve the desired balance between the sediment supply from upstream and the sediment
transport capacity of the project reach. Slope is a driving parameter in this regard and must be
carefully adjusted to achieve the desired balance between transport capacity and upstream supply
over a range of design discharges.
Risks related to modification of channel slope determines whether grade control or stabilization of
the channel slope is necessary. Grade control is commonly incorporated in project design to ensure
that the design slope is maintained during the period following construction when the new channel
is vulnerable to floods and to guard against possible future channel instability associated with post-
project slope adjustments. In situations where the risk of channel slope change is unacceptably
high, regularly spaced grade controls are commonly featured. However, a provider may be able to
avoid using grade control if the alluvial project channel is adequately designed to be in dynamic
equilibrium where the channel neither aggrades nor degrades.
Where grade control is unavoidable, design should focus on ensuring the stability of structures by
installing their foundation materials at or below the maximum depth of scour and beyond the
extent of possible lateral channel migration that might flank the structure. The possibility of a
channel avulsing and bypassing the structure should also be assessed. In many circumstances, it
may be impracticable to constrain the channel laterally so that it cannot flank the grade control
structure, because it would be necessary to install grade control across a significant proportion of
the width of the floodplain. Grade control structures can limit future adjustments of the channel
and act as barriers to the passage of fish or other wildlife; these risks must also be considered and
reduced to an acceptable level. Reviewers need to be aware of the balance between planform,
cross-sectional geometry and channel slope to the extent that modifying one will impact the others.
Channel reconfiguration raises risks to the stream system within, above and below the project
reach, which should also be discussed with the provider.
Bed materials: In stable (equilibrium) alluvial systems, bed material is mobilized during flows
approximating bankfull flow. In most stream restoration projects, the sizes of existing sediments
making up bed surface and substrate materials should be compared to the sizes that will be
mobilized under design flows (see Sections 6.3.1 and 6.3.2 on Hydrology and Hydraulics design
considerations). If the provider's incipient motion calculations indicate that a different size of bed
material is appropriate (i.e., under design flows), it may be necessary to either use appropriately
sized bed material or reconfigure the design channel geometry to achieve the incipient motion
conditions desired.
Risks of having inappropriately sized bed material for the project's flow regime may result in an
imbalance between sediment supply and transport capacity which in turn may result in aggradation
or degradation, or excessive rates of lateral shifting. Either circumstance will result in channel
instability, and likely degraded habitat. In situations where gravel may have to be imported to the
site, providers and reviewers need to be aware of the source of the imported bed material. In some
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cases, imported sediment may contain seeds of invasive or undesirable plants. Imported bed
material from non-fluvial sources is also likely to be more mobile at project design flows than the
same gradation of gravel that has been transported, sorted, and deposited by flowing water.
Streambanks and Lateral stability
Streambank stabilization refers to actions taken to prevent mass failure or further erosion of
streambanks by slowing the bank erosional processes or entraining bank materials. Bank
stabilization is commonly proposed to reduce sedimentation in the channel resulting from
apparently excessive bank erosion rates or to protect further loss of property adjacent to the
stream. Long-term bank stabilization may be desirable to protect infrastructure that cannot be
relocated out of the zone of lateral migration. In these cases, reviewers should ensure that an
appropriate design flow and maintenance schedule is specified within the design criteria to reduce
the erosion risk to infrastructure to an acceptable level.
Risks associated with streambank stabilization are similar to those discussed in planform design. In
situations where adjacent floodplain area is available, the ideal design objective would be to allow
for project stream banks to be free to migrate in the context of a channel in equilibrium (e.g., bank
migration at moderate rates under channel-forming flows). However, newly constructed stream
banks formed in unvegetated alluvium are prone to erosion, so bank design criteria often include
temporary bank protection to stabilize banks immediately after construction and specify the period
of time necessary to allow riparian vegetation to mature and provide natural bank protection.
Many restoration designs involve discontinuous bank protection using structures installed at
intervals along the bank line that extend into the channel to deflect flow away from the bank (e.g.,
j-hooks, barbs, and bendway weirs). Various designs for these structures may create and maintain
pool habitat through scour processes and accumulate sediment along the protected bank. Some
designs may incorporate large wood or other porous features that provide added value of refuge
and may trap additional wood and organic debris. While the use of discontinuous structures for
bank protection can be advantageous for habitat and sediment processes, these structures may
constrain lateral channel movement and natural planform adjustments if overdesigned or built
from nonbiodegradable materials.
Reviewers need to be aware that, in general, over-stabilization of streambanks and the planform
pattern limits the functional capacity within a stream, including limiting the ability to sustain and
renew habitat, and limiting downstream sediment input and wood recruitment from retreating
banks. On the other hand, failure to provide adequate temporary protection may expose a newly
constructed channel to severe erosion by even moderate flows with unacceptable risks to the
project. Reviewers should encourage the use of native materials when bank protection is
warranted. For instance, large rock structures should not be used for streams in the Coastal Plain,
but rather wood structures might be more appropriate.
Instream Structures
Instream structures are modifications within the stream channel that increase geomorphic
complexity and/or vertical channel stability, encourage hyporheic exchange and/or enhance
aquatic habitat. Structures vary widely and can include j-hooks, cross-vanes, constructed riffles, and
log jams which are similar to those installed for bed and bank stabilization. A common symptom of
stream degradation is simplification of the channel and associated loss of aquatic habitat. Many
stream restoration projects attempt to increase geomorphic complexity via installation of
structures. Structures are placed in a stream or floodplain to improve habitat where it is deficient
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or for stabilization of the streambed or banks. Placed structures are often intended to serve as
analogs to otherwise naturally occurring features. Structures are most appropriate in streams with
significant human alteration where natural functions are impaired and are unable to create
adequate habitats on their own.
Generally, instream structures will influence stream processes and habitat by controlling or limiting
vertical erosion or incision (i.e., grade control) by stabilizing the channel profile, controlling
sediment supply or inducing aggradation; controlling or limiting lateral erosion by offering bank
protection (discussed previously above); creating or modifying habitat by inducing local scour or
promoting hydraulic diversity; and/or modifying flow conditions to direct or converge flow to
create desired habitat conditions or protect property or infrastructure (Cramer et al., 2012).
Reviewers should be aware of the risks associated with any in-stream structure, which may also
affect the hydraulics and geomorphology of the channel. Miller and Kochel (2009) found that
approximately 30% of the in-stream structures assessed in 26 reconfigured stream reaches in North
Carolina exhibited damage within six years of their installation that partially or completely affected
their intended function. Structures placed in a stream channel will affect physical processes in the
immediate vicinity of the structure as well as in upstream and downstream locations. Potential
effects of instream structures for which the reviewer needs to be aware are:
• Hydraulic effects on water velocity, depth, and shear stresses which may in turn affect other
channel processes like erosion and deposition, and habitat conditions;
• Sediment transport effects in the vicinity of the structure or upstream of the structure;
• Bed scour caused by flow acceleration around or over a structure which may be either a
desired habitat enhancement objective, or an unwanted consequence;
• Channel planform and profile effects, including decreased erosion which prevents
alterations to channel planform or profile, or conversely, alterations to hydraulics and
sediment transport that affect the channel planform and bed elevation;
• Effects on wood recruitment where structures increase bank erosion, or where structures
entrain wood and alter transport; and/or
• Effects associated with construction and installation of structures, including disturbance of
the streambed, banks, or both, and resulting risks to species on-site and downstream.
Construction in dry, dewatered conditions can greatly alleviate these risks, though the
construction of flow diversions introduces further risks that must also be evaluated and
found acceptable.
For any given project, these considerations may not be applicable, but the reviewer should be
aware that when structures are proposed as part of a mitigation plan, these aspects of design need
to be considered by the provider. A summary of common methods to address stream process
alterations at the reach scale is presented in Table 17.
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Table 17. Potential stream process restoration methods and design considerations to address stream
problems (after Roni and Beechie, 2013).
Process
alteration
Potential Methods
Design Considerations
Channel vertical
instability
Modify inflowing water
and/or sediment; bed and
bank stabilization
Design basis for substrate gradations, allowable range of bed scour
and fill, specify whether grade control is allowable or required;
vertical stability criteria may specify sediment continuity (balance
sediment supply, and transport capacity) objectives.
Channel lateral
instability and
bank instability
Remove encroachment on
floodplain/ provide right of
way for stream; grade
control for mass wasting of
banks; bank vegetation/
floodplain roughness
Allowable range of channel shifting, discharge criteria and shear
stresses for bank erosion and criteria for geotechnical bank stability,
duration for which artificial bank protection and stabilization
measures are required.
Unstable
channel form
and geometry
Channel reconfiguration
Specify the design discharge that the channel is intended to contain
and how that discharge was derived (i.e., gauge data; synthetic data;
and regional regression data); define reach-averaged values and
local variability within reach (i.e., pool, riffle, meander bend) in width,
depth, and width/depth ratio; specify a range of values for planform
characteristics (pattern, sinuosity, meander wavelength, braiding
index, etc.).
Lack of
floodplain
connectivity /
inundation
Floodplain reconnection;
re-establish high flows
(e.g., reservoir operations)
Flood flow frequencies and durations reaching the floodplain and the
extent and location of floodplain inundation; allowable fluvial
processes on the floodplain (overbank scour and sedimentation).
Riparian
vegetation
alteration
Seeding; direct planting;
invasive control
Collected seeds from multiple riparian sites; get planting stock from
nearby nurseries; herbicides, annual removal of individuals, mowing,
prescribed burning, and flooding.
Instream habitat
alteration
Large wood placement;
instream structures
Large wood to increase channel bed and bank complexity; allow
incorporation of beaver; instream structures use to increase bed and
bank complexity.
6.3.4 Restoring Physicochemical Attributes
Where a project's goals and objectives include restoration or improvement of physicochemical
attributes, reviewers should evaluate the workplan to ensure project elements are appropriate to
satisfy these objectives. A combination of enhancement, restoration and/or establishment
strategies that reduce nutrient inputs to streams, re-establish riparian functions, provide balanced
water and sediment regimes, and increase in-stream nutrient processing and retention will likely be
most effective for improving water quality. Some potential restoration methods which could be
beneficial for stream water quality include (Bledsoe et al., 2016; Lammers and Bledsoe, 2017):
• Bank stabilization to reduce phosphorus loading in highly unstable streams.
• Floodplain reconnection to increase inundation frequency, provide more opportunity for
nutrient uptake, and sediment and nutrient deposition.
• Instream structures (e.g., bendway weirs, drop structures, toe wood, and root wads) to
improve dissolved oxygen and encourage hyporheic exchange and nutrient processing
through enhanced geomorphic complexity and bedforms, especially during low flow.
• Riparian buffers to remove groundwater nitrate, protect streambanks, supply instream
wood and organic carbon to increase in-stream processing, and reduce stream
temperatures by shading; and
• Watershed controls to manage flow of water and sediment and reduce nutrient loading.
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Much like the considerations for restoring hydrology and hydraulics, a stream's water quality is
influenced largely by watershed land uses upstream of that stream reach, and the presence of any
stressors should be appropriately considered during compilation of the workplan. The baseline
assessment (Section 4.5.2.4) includes an evaluation of these watershed and site-scale stressors, as
well as collection of temperature, pH, DO, specific conductance, nutrients, and sediment data at
the upstream and downstream ends of the project.
Sediment is a natural constituent of stream ecosystems, as are nitrogen and phosphorus, however
abnormally high levels of any of these constituents as a result of anthropogenic disturbances needs
to be recognized and addressed in the stream mitigation plan. Some work has been done to assess
the effectiveness of certain stream restoration methods to ameliorate the effects of nitrogen and
phosphorus nutrients in streams (Lammers and Bledsoe, 2017; Bledsoe et al., 2016). In general,
ameliorating phosphorus levels in streams involves control of phosphorus adsorbing sediments
along banks and in the watershed. Phosphorus can also be temporarily removed from the system
through biological uptake and burial on floodplains. Nitrogen cycles between organic and inorganic
forms through plant uptake, organic material decomposition, mineralization, and microbially
mediated nitrification (formation of nitrate from ammonia) and denitrification. Denitrification rates
are highest in saturated soils and streambeds where water provides a source of dissolved nitrate
and anoxic conditions, and when organic carbon is available to serve as an energy source.
Results of studies evaluating the benefits of various stream restoration techniques for amelioration
of nutrient issues currently indicate a substantial degree of variability. This variability stems from
natural variability in the streams and their watersheds, as well as the practices utilized to restore
the stream. Regardless of this variability, potential exists for stream restoration practices to provide
water quality benefits.
6.3.5 Restoring Biological Attributes
As stated in Section 4.5.2.5, a stream's biological community is largely influenced by proximity to
refugial habitats, as well as hydrologic, hydraulic, geomorphic, and physiochemical conditions which
operate at watershed, riparian, and reach scales. The complex interactions of these ecosystem
processes contribute to the faunal diversity found in natural streams. From a mitigation planning
standpoint, project elements to address these supporting functions have already been described in
earlier sections. This section includes project elements related to riparian functions, considered
here as a biological attribute, which contribute to the geomorphic stability of, and the organic
carbon/detrital inputs to, the channel. Instream structures are also discussed below, specifically in
terms of their habitat value for biological communities.
Riparian buffers are protected or replanted vegetated areas adjacent to stream channels that can
intercept pollutants in surface and subsurface flow (Bledsoe et al., 2016). The benefits of riparian
buffers include increased bank stability, stream shading to reduce water temperatures, supply of in-
stream wood and organic carbon to streams, and water quality benefits derived from sequestering
and/or transforming elements and compounds that enter the riparian zone from adjacent areas.
Restoration of riparian buffers and vegetation plantings are common elements of most
compensatory mitigation plans.
Generally, it is beneficial to plant forested buffers that consist of mixed herbaceous, shrub, and tree
species typical of a reference standard riparian zone. The reviewer should ensure that riparian
buffers are as wide as possible, have as diverse a planted assemblage as possible and be in contact
with overland flows and shallow subsurface flows to gain the full effect and benefit from the
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buffers. If these conditions are not met, the riparian zone/buffer is likely not to achieve its full
function. For example, planting riparian buffers along incised streams, with no channel work to
address the cause of the incision, is not recommended. The benefits of riparian vegetation roots
holding banks together and being in contact with shallow subsurface water flows to uptake
nutrients are minimized along incised reaches.
Vegetation - Like other components of the workplan, planting plans for revegetation or
enhancement of riparian zones should be guided by the vegetative community speciation and
structure demonstrated by applicable reference standard sites. There should not be a one size fits
all approach to riparian zone plantings because the soil type(s), valley topography, and hydrologic
conditions to which riparian zones are subject differ across ecoregions and position along the
hydrographic continuum from headwaters to main stem rivers. Important elements for reviewers
to ensure are included in each planting zone of a revegetation plan include:
• Selecting suitable species and assemblages: Species selection will typically be based on
reference standard riparian systems, which may or may not correspond to the reference
streams used for instream biota or geomorphic features. Nevertheless, the provider should
rely on reference comparisons to select species and their assemblages. In selecting plants
and assemblages, the role of vegetation in providing natural bank protection should also be
considered. In this context, the vegetal and rooting characteristics determine the resistance
of plants to shear stresses at high flows. Assemblages should be comprised of native
species, either planted or volunteers, and invasive species must be controlled.
• Specifying vegetation growth stage (seed, seedling, rooted stock, etc.): The planting plan
should consider natural successional processes. Many shrubby species can be effectively
propagated using cuttings from local sources. In such cases, soil characteristics and depth to
groundwater during all seasons must be considered, as cuttings typically need access to
saturated soil in order to establish roots, and roots need access to soil moisture. In many
cases, ecological function may be best advanced by planting a mix of riparian species that
includes seral-stage trees that optimize site potential, such as oak and/or hickory species.
• Setting the timing for planting: Many revegetation efforts take place in early spring in an
attempt to get the plants established before summer heat and potential drought conditions
pose a threat to the plantings.
• Delineating the planting zones: The planting zones, which define appropriate locations for
varying species and growth types, should extend beyond the channel and riparian corridor
to encompass as much of the floodplain as possible. Species selection and the building of
assemblages are best based on local reference standard riparian systems. Seeds and rooted
stock should be derived from locally adapted native species whenever possible. Plans should
specify planting densities, soil amendments, and specifications developed for each zone.
Revegetation plans must consider inundation frequencies and durations, the possibility of
drought conditions, the lower elevation limit of vegetation on a bank, and the magnitude
and duration of shear stresses to which the plants may be exposed during the higher design
flows.
Design criteria for vegetation elements of a workplan should factor in potential risks to the growth
and survival of riparian vegetation, including risks associated with drought and excessive browsing
or grazing. Because riparian vegetative growth and success is so dependent on local soil moisture
and streamflow conditions, seasonal and interannual variation in precipitation and discharge can
make or break a revegetation effort.
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Instream habitat: Stream restoration project goals typically include replacement of, or
improvement to, poor physical stream habitat quality using idealized design elements. It is
commonly then assumed that aquatic biota will recolonize on their own; an expectation that has
been criticized as a "Field of Dreams" approach (Palmer et al., 1997; Hilderbrand et al., 2005). As
discussed in Section 4.2, studies indicate that restoring biological condition requires selection of a
project site with appropriate water quality and within proximity of potential colonizing species.
Without these conditions, efforts to restore instream biological communities may not achieve
results. However, assuming the provider has selected an appropriate site, constructing suitable
instream habitat may enhance the recovery of reference biological conditions.
Instream habitat elements generally include channel bed features or installed structures intended
to provide habitat complexity, forage, refuge, or cover for aquatic organisms. Habitat design criteria
should consider the species, life stage, and biologic functions specific habitat elements will address.
Design criteria considerations include the selection of materials used to create habitat, the
deformability/rigidity of constructed habitat elements, the cost effectiveness (e.g., functional life of
constructed elements), and the habitat complexity at multiple scales. Complexity is an important
attribute of natural habitat at all scales: planform complexity at the reach scale; variability in width,
depth and velocity at the site scale; bed material variability; bedform diversity; and habitat patches,
including their lines and edges at a micro-habitat scale. Design criteria for the habitat elements in a
restoration project should, therefore, specify the nature and degree of complexity at all scales.
Ideally, materials used to construct habitat elements should be locally derived and consistent with
natural materials already in the project or adjacent reaches. For example, the introduction of large
boulders would be inappropriate in an alluvial gravel bed stream, while the use of large wood might
be out of place in a small stream flowing in an open, semiarid grassland setting. Locally derived
materials are less prone to introduction of seeds or spores of non-native and undesirable plant or
animal species.
Another form of instream enhancement are beaver dams, which can increase habitat heterogeneity
and may increase nutrient retention and cycling by increasing hydraulic retention time and
encouraging hyporheic exchange. Use of beaver dam analogues as well as reintroducing beaver to
inhabit degraded streams are rapidly expanding restoration practices (Pollock et al., 2007; Pollock
et al., 2014; Pollock et al., 2017). Beavers change the hydraulic, geomorphic, physicochemical, and
biological aspects of the stream. Incorporating them into a project design would require a great
deal of planning from the beginning of the project, as well as management considerations prior to
and after restoration work has been completed. More discussion on the effects and
accommodation of beaver can be found in Section 8.3.2.
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7.0 How will progress toward the target condition be assessed?
The intent of this question is to outline what is needed to
determine the ecological success of a mitigation project,
including ecological performance standards and
monitoring plans. At this point, information has been
gathered to classify the site, assess its baseline condition,
set the target conditions, and develop a plan to address
stressors that are currently impacting the proposed
mitigation site. Project goals and objectives should
provide a vision for the project and set out the tasks
needed to achieve that vision. Performance standards
are based on the specific objectives of the mitigation
project, and monitoring is used to determine if the
project is meeting its performance standards. If
monitoring indicates that the site is not responding as
anticipated, then it will be necessary to implement
adaptive management measures to redirect the site's
trajectory (see Section 8.0).
A list of performance standards and detailed monitoring plans are generally provided as part of a
completed draft mitigation plan, banking instrument or ILF instrument or addendum. Because each
site's mitigation plan is unique, reviewers will want to ensure performance standards are consistent
with the specific goals and objectives of the project and that monitoring elements are appropriate
to inform ecological success. The main opportunity to provide comments on performance
standards and monitoring plans will be during the Public Notice comment period for individual
permits or during review of the draft mitigation bank instrument or ILF project addendums.
Following completion of the project, regular monitoring reports will be available for review
according to USACE District practices for disseminating information to IRT and/or commenting
agencies. Credit releases and closeout of a mitigation site will occur once a project has met specific
ecological success criteria. As such, review of monitoring reports, credit release requests and
information related to project closeout are essential to determine whether performance standards
have been met.
Performance standards must represent a range of functions (e.g., wetland hydrology,
biogeochemical, and plant and animal habitat; stream hydrologic connection, hydraulic
maintenance, geomorphologic form) and each performance standard must tie directly to a project
objective. Reviewers should consider whether each performance standard is informed by an
appropriate comparison to a reference standard, described using clear and concise wording,
scientifically defensible, and supported by objective data collection that is appropriately timed,
repeatable and using methods with clearly defined error rates.
Important elements of monitoring plans include the data collection methods and protocols
necessary to inform performance standards, and should outline the locations, timing, intensity and
scale of sampling. Generally, monitoring plans should clearly and concisely describe the parameters
to be assessed; where and how often each will be measured; the specific methods and/or protocols
for measuring each parameter; the resources needed (e.g., time, money, equipment, and
expertise); how data will be analyzed and interpreted with respect to objectives and performance
standards; the level of statistical rigor necessary to determine treatment effectiveness; and how
Key points associated with
performance and monitoring:
• Performance standards are
directly linked to the project's
objectives to improve
ecological functions.
• Performance standards
should be reference-based,
measurable and verifiable.
• Performance standards
should have clear thresholds
to meet in a given time period.
• Monitoring should determine
that performance standards
are met within an agreed
upon level of confidence.
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data will be made accessible and understandable for decision-making regarding achievement of
performance and/or adaptive management. Monitoring plans should be clearly written and
organized to promote accurate interpretation at any point in the life of the project site. When
reviewing monitoring plans, reviewers should ensure the same parameters and methods have been
used to collect baseline data and post-project monitoring data, and that the timing of monitoring
data is appropriate given credit release schedules and project closeout timeframes for the project.
This section provides the reviewer with background information on important elements to be
included in performance standards and mitigation plans. It also provides additional considerations
for reviewers during the post-project monitoring phase. Technical information and specific
suggestions are also provided for performance standards and mitigation plans and organized by
functional category for wetlands and streams (Box I).
Box I. Recommended information to address performance standards and monitoring.
Performance Standards
• Does each performance standard tie to a specific project objective?
• Are performance standards provided for the suite of functional indicators (see below) tied to project objectives?
• Is each performance standard informed by appropriate reference standard comparisons?
• Is each performance standard described using clear and concise wording?
• Is each performance standard scientifically defensible (e.g., sensitive to changes in function)?
• Is each performance standard supported by data collection methods that are appropriately timed, repeatable and
with clearly defined error rates?
Monitoring Plan
• Are data collection methods for each performance standard outlined in the monitoring plan?
• Have pre-project baseline data been collected for all measures outlined in the monitoring plan?
• Has the timing of data collection (e.g., frequency, duration, and seasonality) been determined?
• Has the location and scale of data collection (e.g., reach, riparian, and watershed) been determined?
• Is the sampling intensity (number and arrangement of locations/plots) appropriate for the chosen parameter?
• Is the sampling of sufficient intensity and precision to detect deviations from performance standards and indicate
the need for adaptive management?
• Does the plan outline how data will be analyzed and reported?
Review of post-project monitoring
• Compare the as-builts to the design criteria outlined in the mitigation plan. Was the project built to specifications
in the plan? If not, why not?
• Do the results of monitoring show the site is on track to meet performance standards? If not, is adaptive
management needed?
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Box I (cont'd). Recommended information to address performance standards and monitoring.
Performance standards and monitoring expectations by functional category
Wetland Hydrology:
• Measures that demonstrate timing, duration and frequency of saturation, ponding, and/or flooding, during at least
3 normal rainfall years, similar to hydrologic monitoring data in the same HGM subclass of reference standard
wetlands on similar soils.
• Methods should be sufficient to monitorthe site's hydrologic sources and gradient (e.g., rain gauges, stream
gauges and/or an array of groundwater wells).
Wetland Soils:
• If soils have been disturbed or drained, measures that demonstrate they meet hydric soil criteria for saturation
(minimum 14 consecutive days) and anaerobiosis as per National Hydric Soil Technical Standard.
• Measures that demonstrate the soil is accumulating organic material; developing appropriate bulk density,
texture, and nutrient concentrations; and soil microtopographic features (shallow depressions and minor
hummocks) similar to reference standard.
Wetland Vegetation:
• Measures that demonstrate plant survival to sapling stage; species composition and density (e.g., establishment
of native species in all strata); response of herbaceous components of vegetative community to hydrologic
changes (i.e., Weighted Prevalence Index) and trending towards a hydrophyte dominated community; relative
abundance or cover of various age classes, including evidence of recruitment; and an expectation for maximum
allowable invasive plant species cover.
• All performance standards should be similar to, or along a trajectory towards, reference standard.
Stream Hydrologic Functions:
• Measures that demonstrate the timing, rate of change (flashiness), duration and frequency of low, high and
design flows comparable to reference standard condition.
Stream Hydraulic Functions:
• Measures that demonstrate sediment transport in the channel and across the floodplain are as designed and in
an equilibrium condition.
Stream Geomorphic Functions:
• Measures that demonstrate that boundary conditions in the restored reach are as designed and comparable to
reference standard condition.
Stream Physicochemical Functions:
• Measures that demonstrate water quality (pH, DO, temperature, and nutrients) has not declined after project from
baseline condition.
• Measures that demonstrate water quality parameters are improving towards reference standard condition.
Stream Biological Functions:
• Measures that demonstrate benthic macroinvertebrates and/or fish and riparian vegetation are improving towards
reference standard condition.
7.1 Regulatory Context
The Mitigation Rule requires the mitigation provider to identify ecological performance standards
that will be used to assess whether the mitigation project is achieving its stated objectives, and also
to clearly describe the monitoring methods to document its performance. Performance standards
are defined as "observable or measurable physical (including hydrological), chemical and/or
biological attributes that are used to determine if a compensatory mitigation project meets its
objectives" and are described in the Mitigation Rule at 40 CFR §230.95(a) as needing to be clearly
tied to the project objectives, the desired resource class or type, and the functions expected of that
resource type. Further, the Mitigation Rule stipulates in 40 CFR §230.95(b) that:
(b) Performance standards must be based on attributes that are objective and verifiable.
Ecological performance standards must be based on the best available science that can be
measured or assessed in a practicable manner. Performance standards may be based on
variables or measures of functional capacity described in functional assessment methodologies,
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measurements of hydrology or other aquatic resource characteristics, and/or comparisons to
reference aquatic resources of similar type and landscape position. The use of reference aquatic
resources to establish performance standards will help ensure that those performance
standards are reasonably achievable, by reflecting the range of variability exhibited by the
regional class of aquatic resources as a result of natural processes and anthropogenic
disturbances. Performance standards based on measurements of hydrology should take into
consideration the hydrologic variability exhibited by reference aquatic resources, especially
wetlands. Where pmcticqble, performqnce stqndqrds should tqke into qccount the expected
stqges of the qquqtic resource development process, in order to qllow eqrly identificqtion of
potentiql problems qnd qppropriqte qdqptive mqnqgement. [emphasis added]
The Mitigation Rule also recognizes that monitoring is necessary to adequately capture the
trajectory towards performance standards and determine if adaptive management is needed. The
Mitigation Rule states that monitoring plans should include a description of parameters to be
monitored, a schedule for reporting those results, the length of the monitoring period, the party
responsible for conducting monitoring and submitting reports and the frequency of those reports
to the USACE District Engineer (40 CFR §230.96). The length of the monitoring period must be
sufficient to demonstrate that the site has met performance standards but should not be less than
five years. The Mitigation Rule further specifies that a longer monitoring period must be required
for aquatic resources with slow development rates (e.g., forested wetlands, bogs) (40 CFR
§230.96(b)).
7.2 Performance Standards
Performance standards allow evaluation of progress towards project objectives (e.g., full
achievement, partial achievement, or failure). Each performance standard should identify: (1) the
attribute being measured (i.e., whqt is meqsured?); (2) the condition or parameter that defines
success (e.g., quantity, coverage, composition, growth rate) (i.e., how is it meqsured?); and (3) the
period of time within which success must be reached and the period of time over which success
must be sustained (i.e., when is success qchieved?).
While efforts have been made in some regions to create broadly applicable performance standards
(e.g., USACE, 2012), they need to be considered on a site-specific basis to accommodate the variety
of aquatic resource types, landscapes, stressors and treatments implemented as part of mitigation
efforts. Therefore, each mitigation project will potentially have a unique set of performance
standards specific to the class(es) of aquatic resource on site and the mitigation project goals and
objectives.
Performance standards should be compared to either reference data from a reference network
(Brooks et al., 2016), or reference sites of similar wetland and/or stream class in similar landscape
positions (Section 5.3). Using or assembling a network of multiple reference sites establishes a
range of natural conditions within which performance can be assessed. If reference data are not
available from a reference network and need to be collected from project-specific reference sites,
careful review of those sites for their appropriateness and similarities to the proposed mitigation
site must be undertaken. Poor selection of reference sites could lead to inconclusive comparisons
of performance standards with reference data. For example, if hydrologic restoration for a
proposed riverine low gradient forested wetland was being attempted, and the objective was to re-
establish the natural hydroregime, the reference site(s) would need to have similar landscape
position, mature vegetation, and hydrologic sources (particularly flood regime) as the proposed
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target mitigation site. Choosing a reference site adjacent to an incised channel, regardless of the
maturity of the vegetation, would lead to different flood frequency, durations, and extents, and
potential erroneous or misleading hydrologic performance standards when compared to the target
condition (see Section 5.3).
To help determine the effectiveness of a provider's performance standards, reviewers should
evaluate if the performance standards address:
(1) A single measure or aspect of condition/function that is sensitive to change due to the
management proposed - Each performance standard should represent one element of function
that is responsive to mitigation activities, such as recruitment of native plant communities,
appropriate hydrology during the growing season, or suitable soils/substrate. Although
compensatory mitigation strives to restore fully integrated functional ecosystems, measuring
specific elements is often more practical and enforceable, and can be more easily tied to
adaptive management.
(2) Objective and repeatable measurement - To the extent possible, standard acceptable
protocols should be used so that providers and IRT members understand and agree on how to
measure to produce data in a consistent, repeatable manner.
(3) Clear targets or benchmarks anchored to reference - Performance should be assessed relative
to a defined target and should include an expected timeframe to meet that target (e.g., at year
five following construction, three years after the first five-year flow event). The target can be
based on conditions at reference standard sites or relative to regional or ambient condition
(e.g., comparable to the 75th percentile of the ambient condition range). In the case of re-
establishing forested systems, performance standards may need to be set based on early
successional stages of the particular wetland class, subclass, or riparian zone.
(4) Quantifiable targets with known certainty - Standard protocols are typically associated with
specific error rates that provide known levels of confidence. These error rates may result from
the variability during data collection or analysis (e.g., inherent errors in species identification or
instrument measurements) and should be accounted for during data interpretation and
performance standard development.
(5) Clear and concise wording - The period of monitoring for performance often exceeds the
tenure of individual reviewers, IRT members, or even some providers. Therefore, the language
of each standard should be written so that an uninitiated staff person can readily interpret the
intent of the standard and reach a clear determination of compliance. For example:
At the end of year three, at least 80% of Area A shall have a natural ground cover vegetation
score within 10 percent of the median reference population score.
• If this standard is not met, the site will be re-evaluated within 120 days of the original
field assessment; and
• If the standard is still not met, metric (i.e., parameter) level analysis and/or causal
assessment shall be conducted to identify likely reasons for failure.
(6) Scientifically defensible - Standards should be grounded in sound scientific principles and
preferably informed by peer-reviewed studies. If links to peer-reviewed literature are not
available, analysis of data from past mitigation projects and/or reference comparisons can also
provide scientific rationale and support for development and ongoing refinement of
performance standards.
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(7) Appropriate timing - Performance standards can be phased over time. For instance, it is
advisable that performance of structural aspects of the stream (e.g., planform, cross-section,
and instream structures) or wetland (e.g., ditch plugs, levee breaches, and constructed
microtopography) be evaluated earlier in the restoration process to establish that hydrologic
and hydrodynamic processes are in place. Physicochemical and biological aspects of the project
can be evaluated later, once the physical and hydrologic elements are established. Such a
phased approach may be more conducive to earlier identification of problems that require
remedial action or adaptive management (Stein et al., 2022).
7.3 Monitoring
Monitoring, in one form or another, occurs throughout the life of the project. There are essentially
three stages of monitoring that are pertinent to the compensatory mitigation program: baseline
monitoring; implementation monitoring; and effectiveness monitoring.
Baseline monitoring is accomplished early in the project as the baseline condition assessment
(Section 4.0). The existing condition of the proposed restoration wetland or stream is described in
terms of functional capacity and the stressors acting on the functions of the project site. The
baseline condition assessment sets the benchmark from which changes to the wetland and/or
stream, as a result of the mitigation project, will be measured.
Implementation monitoring provides verification that the project was actually constructed as
designed and approved by applicable permitting authorities. It is accomplished by comparing the
as-built plans with the 100% drawings and the design criteria discussed in the mitigation plan, as
well as a possible site visit. The mitigation plan should include implementation monitoring
requirements, including scaled, as-built plans from a professional engineer (PE) or Professional Land
Surveyor (PLS), 100% design plans, and a field confirmation of the design criteria in the approved
mitigation plan for both stream and wetland mitigation projects. The design criteria (e.g., amount
of fill in ditches, size, shape and spacing of levee breaches, and number and species of tree
seedlings planted) are important to monitor to determine if the plan was appropriately
implemented and/or constructed and should be documented in as-built plans compiled following
implementation monitoring.
Effectiveness monitoring is used to evaluate whether the project has improved aquatic resources
(e.g., habitat conditions or stream processes) and whether the project has, or is on target to,
achieve its goals and objectives, performance standards, and target condition. Reviewers should
evaluate monitoring plans carefully to determine whether monitoring measures link mitigation
actions (and their anticipated functional improvements) to objectives and performance standards.
Just as the methods and/or treatments chosen for a mitigation plan are dependent on site and
watershed impairments, the choice of monitoring strategies is dependent on the objectives of the
project, the actions taken on the site, and the performance standards selected to represent the
level of ecological lift expected (i.e., the target condition). Similar to consideration of performance
standards, standardized monitoring plans are difficult to prescribe due to the variability in aquatic
resource types, effects of perturbations from a spectrum of sources, and the variety of methods
available to assess performance.
Despite this variability, all monitoring plans should include the data collection methods and
protocols necessary to inform performance standards, and should outline the locations, timing,
intensity and scale of sampling. Monitoring plans should be clearly written and organized to
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promote interpretation at any point in the life of the project site and accommodate personnel
changes in both the provider's team as well as among the IRT. Generally, monitoring plans should
clearly and concisely describe:
• The parameters to be assessed, including where and how often each will be measured;
• How data will be analyzed, interpreted and reported with respect to objectives and
performance standards;
• The resources (e.g., time, money, equipment, and expertise) needed;
• How data will be analyzed and reported to determine the need for adaptive management;
and
• The level of statistical rigor to determine treatment effectiveness.
Specific consideration should be given for how a monitoring plan may inform the need for adaptive
management. If adaptive management measures are triggered when a site is not achieving, or on
track to achieve, performance standards, then the monitoring plan needs to have adequate
monitoring intensity across a site to determine site conditions compared to performance standards,
and analysis of the monitoring data to indicate statistical difference. Some authors have advocated
that adaptive management be predicated on designing and monitoring ecological restoration using
principles of experimental design, so that adequate data are gathered and statistically analyzed to
identify effective alterations to a mitigation project (Galatowitsch and Zedler, 2014; Roni and
Beechie, 2013). For aquatic resource restoration and rehabilitation, this means testing the
hypotheses the project design is based on, demonstrating a good understanding of watershed
processes, and appropriately addressing adverse changes in these processes and ecological
functions. Monitoring for performance, and subsequently using that data for adaptive
management, if necessary, is something for which every project should strive, and for which every
reviewer should be aware.
Reviewers should also consider the time frame proposed for post-project monitoring. The
Mitigation Rule requires a minimum of 5 years of monitoring. However, since many of the aquatic
ecosystems in the southeastern U.S. are forested (either as wetlands or riparian zones), a 10-year
monitoring period may be prudent to determine appropriate establishment and growth of trees. A
10-year monitoring period is also important to capture a degree of hydrologic variability. Resilient
aquatic ecosystems adjust to dry periods, extremely wet periods, low flows and high flows. This
range of climatic and hydrologic conditions is important to monitor effectively to assess the long-
term sustainability and resiliency of the proposed project.
The monitoring plan should outline reporting requirements. Monitoring reports must be submitted
to USACE, but the content and level of detail for those reports will be commensurate with the scale
and scope of the mitigation project, as well as the mitigation project type. They may include plans
(such as as-built plans), maps and photographs to illustrate site conditions. Monitoring reports may
also include the results of functional, condition or other assessments used to provide quantitative
or qualitative measures of the functions provided by the mitigation project. It is important for the
reviewer to collect and evaluate these monitoring reports, not only for their adequacy, but also as a
record of progress for the site. When reviewing monitoring reports, reviewers should consider
whether the results of monitoring show the site is on track to meet performance standards, and if
not, whether the triggers for adaptive management have been met.
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7.4 Wetland Performance Standards and Monitoring
As laid out in Section 4.4.2, hydrology, soils, and vegetation data form the basis of comparison for
monitoring after the mitigation plan has been implemented. Specific hydrologic, soils and
vegetation parameters should be identified to inform performance standards, and monitoring of
these parameters should be used to document the effectiveness of the mitigation treatment at
achieving performance standards as the site trends towards the target condition. Selected
parameters serve as indicators for hydrologic (i.e., flooding, ponding, and/or saturation), soils (e.g.,
saturation, anaerobiosis, and structure), and vegetation (e.g., growth, species composition, and
structure) processes, and the assumption is that as these parameters trend toward a target
condition the underlying functions also improve.
Ideally, performance standards would include direct measures of function (e.g., the quantity of one
or more imported elements and compounds removed or sequestered per unit area during a
specified time period (e.g., g/m2/yr) or sediment retention per unit area per unit time (e.g.,
g/m2/yr) (Ainslie et al., 1999)). However, direct measures of functions are often costly, time
consuming, and likely to require monitoring that exceeds the proposed monitoring period. Thus,
direct functional measures may not be practical in many instances, and indirect measures, or
indicators, associated with the performance of a function are used as proxies for direct measures of
wetland processes.
Attributes of a site's hydrology, vegetation, and soils are monitored for performance because it is
assumed that if these abiotic and biotic components are in good condition, relative to reference,
appropriate functions will develop in time. Existing HGM regional guidebooks contain lists of
indicator parameters and a discussion of each indicator's relationship to wetland function. These
indicators could be used to inform performance standards if they meet the criteria above (i.e.,
objectively measured, quantifiable and reference-based targets, defensible, and concise). HGM
functional capacity indices are scaled to reference standard conditions, so the published threshold
values can be used to inform performance standards. For example, optimal flooding, vegetation,
and habitat structural characteristics can be expressed as thresholds (e.g., flood frequency should
occur every other year, tree density should be 200 stems/acre, and ground cover should not exceed
20%). However, if additional indicators need to be measured to adequately assess performance
(e.g., flood duration, early successional ground cover, shrub/sapling density) and meet the criteria
above, they should be included in the mitigation plan, measured during the baseline assessment,
and represented by an appropriate reference site.
Other assessment methods, like the California Rapid Assessment Method (CRAM; CWMW, 2013),
Ohio Rapid Assessment Method (ORAM; Mack, 2001), and Habitat Evaluation Procedures (HEP;
USFWS, 1980) for wildlife habitat, among others, also include indicator parameters which might be
useful in assessing performance because they are associated with targets and or threshold values
scaled to reference. Many wetland assessment methods have strong reference datasets for
wetland vegetation, and many include a number of vegetation-related indicators. However, they
often lack detailed hydrologic and soil standards. Thus, establishing abiotic performance standards
for a given site or regional subclass will require more detailed monitoring, site specific information
and comparison to reference data, literature, and previous mitigation projects. Scientific literature
and/or experience with similar mitigation sites should be considered if available. However, in some
situations this information may not be available for any given site. Thus, nearby reference sites may
be needed to develop hydrologic, vegetative and soil performance standards that account for
natural variability among aquatic resource types. Any comparisons between reference and
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mitigation site condition should be conducted using a statistical analysis approach (e.g., t-test of soil
redox values, or ANOVA with a multiple comparison test when more than one site is assessed)
(Stein et al., 2022).
More specific criteria for wetland hydrologic, soils, and vegetation performance standards are
provided below.
Hydrology Performance Standards
Hydrologic performance standards should include concisely worded statement(s) that tie the
measured frequency, duration, extent and timing of flooding, ponding, and/or saturation to the
target hydroperiod exhibited by a reference standard site or dataset. At a minimum, hydrologic
criteria should be based on appropriate water sources and hydrodynamics for the particular
class/subclass and characteristics of a given mitigation site. Because many wetland classes do not
have reliable quantitative hydrologic indicators, direct measures of hydrologic processes are
necessary to document the appropriate hydrologic regime for the site has been established. This is
particularly important for groundwater dependent wetlands which require connection with
subsurface water (often of specific chemical composition) during key portions of the growing
season.
In addition to the hydrologic criteria above, the Water Features Table in the Web Soil Survey can
provide insightful information on the hydrologic regime under which the mapped soils formed
(USDA NRCS, 2011). Although the information in the Water Features Table may not be resolute
enough to use directly as wetland hydrologic performance standards, water sources and
hydrodynamics can be fine-tuned for each site by comparing the mitigation site's hydrology with a
reference standard site that has similar landscape position, soils, and water features.
Monitoring considerations: The hydrology at a proposed wetland compensatory mitigation site is
perhaps the most important parameter to be measured and assessed. As discussed previously in
Section 4.4.2, continuous surface and groundwater monitoring devices, as well as rain gauges,
should be installed to assess baseline condition, compare to reference data and establish normal
rainfall (Sumner et al., 2009; Sprecher and Warne, 2000). Monitoring plans should include these
elements to monitor changes in hydrologic criteria throughout the life of the compensatory
mitigation project.
Monitoring schemes should be established to capture direct measures (e.g., flood and/or
groundwater frequency and duration) of hydrologic processes to establish the expected hydrologic
regime for the site and provide the necessary data to inform performance standards. Depending on
a wetland's conceptual water budget (see Section 6.2.1), monitoring should encompass measures
of rainfall, groundwater, ponding and flooding. Monitoring wells are typically arrayed perpendicular
to the predominant water source to determine gradients. For instance, wells in a riverine wetland
are often placed perpendicular to the channel to detect extent of flood flows as well as the
contribution of upslope water sources moving down gradient towards the channel. Flats, on the
other hand, may require fewer wells to indicate that vertical movement of water through the soil
profile and demonstrate that soil saturation is occurring. For more detail on wetland hydrologic
monitoring, see the Engineering Handbook No. 650, Hydrology Tools for Wetland Identification and
Analysis (USDA NRCS 2021).
The Technical Standard for Water Table Monitoring of Potential Wetland Sites (USACE, 2005)
indicates that short-term (i.e., <10 years) evaluation of wetland hydrology requires consideration of
the normality of precipitation that falls during the monitoring period. USACE (2005) further asserts
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that short-term water table monitoring data must be interpreted in relation not only to the normal
precipitation range based on long-term records, but also on the amount of precipitation that fell
during and for at least three months prior to the requisite monitoring period each year. EPA Region
4 considers year-round groundwater monitoring to be most appropriate in the southeastern U.S.
Thus, evaluation of precipitation in previous years prior to monitoring is recommended.
The USACE (2005) outlines three methods for evaluating precipitation normality within any given
year and cites Sprecher and Warne (2000) for detailed descriptions. One of those methods, which
considers the three-month period prior to water table rise, is referred to as the Direct Antecedent
Rainfall Evaluation Method (DAREM) and was recommended by the authors for wetland hydrology
assessment based on their evaluation of wetlands in North Carolina and Minnesota that had long-
term (40-45 years) records of both water table data and precipitation data. The USDA also
recommends using DAREM for analysis of short-term precipitation normality in the NRCS Hydric
Soils Technical Standard (NTCHS, 2015). Thus, normality of rainfall data should be interpreted using
either one of the techniques outlined in Sprecher and Warne (2000), including the DAREM Method.
Note however, that USACE has recently developed an automated tool for evaluating precipitation
normalcy based on the combined method referenced by Sprecher and Warne (2000). The ease of
use and general endorsement of this tool make it arguably the preferred method to assess
precipitation normality. The USACE Antecedent Precipitation Tool (APT) was developed in 2021 to
facilitate comparison of antecedent precipitation conditions at a given location to the range of
normal precipitation conditions that occurred during the preceding 30 years (Gutensen and Deters,
2022). The APT automates a method of evaluating 30-day rolling totals of precipitation combined
with weighting factors in the NRCS Engineering Field Handbook (i.e., the combined method in USDA
NRCS (2021), which is described by Sprecher and Warne (2000)).
Soils Performance Standards
Soil performance should be based on site verified soil series correlated with the appropriate soil
series information in the Web Soil Survey. In cases with disturbed soils, performance standards
should document the restoration of soil structure (e.g., bulk density and soil texture), fertility (e.g.,
nutrients), and organic matter composition. At a minimum, soil series on both the mitigation site
and the reference site should be verified by a certified soil scientist. It is assumed that if a site is
appropriately inundated, saturated and/or ponded in at least three of five normal rainfall years,
and the site compares to reference data or similar sites, then the site will continue to have
functioning hydric soils.
Section 6.2.2 outlined a list of appropriate soil characteristics to include in the mitigation plan,
including soil variability; microtopography; soil organic matter, with soil amendments if necessary;
techniques to restore soil texture, bulk density to compacted soils; and soil nutrient concentrations.
Potential soil performance standards that could accompany these aspects of soil restoration are:
• Matching the soil description for soils typical of reference wetlands in the same landscape
position and HGM class/subclass;
• Microtopographic relief similar in height, spatial pattern and area to reference standard
sites;
• Soil nutrient concentrations and their availability in soils similar to reference standard sites;
• Soil organic matter contents similar to reference standard;
• Meet hydric soil criteria according to the National Hydric Soils Technical Standard; and/or
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• Indicators of recovering soil structure (e.g., bulk density), soil nutrients (e.g., soil nitrogen
and phosphorous), and organic matter content similar to soil descriptions in SSURGO.
Monitoring considerations: Soil monitoring will need to be implemented to ascertain if the
performance standards for the site's soils have been met, and the monitoring plan should outline
the parameters that will be used to inform performance standards (e.g., related to soil variability;
microtopography; soil organic matter with soil amendments if necessary; techniques to restore soil
texture, bulk density to compacted soils; and soil nutrient concentrations).
If the wetland mitigation site has been hydrologically and/or edaphically altered or has soils with
only hydric inclusions or marginal soils, then the methods for assessing the level of saturation and
anaerobiosis outlined in the latest version of the NRCS Hydric Soils Technical Standard should be
utilized to establish that soils on-site are meeting the present definition of hydric soils. As noted in
NTCHS (2015), "the Hydric Soils Technical Standard requires proof of (1) anaerobic conditions and
(2) soil saturation for at least 14 consecutive days for most soils, (3) during normal rainfall periods
when soil microbes are active." Further, it describes three methods to document anaerobic
conditions in soil: IRIS tubes, oxidation-reduction potential measurements using platinum tipped
redox electrodes, and alpha-alpha-dipyridyl dye.
Microtopographic depressions and hummocks are important to establish vegetation and promote
biogeochemical cycling by providing shallow (<0.03 m) areas of prolonged inundation or ponding
(i.e., hollows) and hummocks (<1.5 m tall) (Richardson et al., 2016). For monitoring performance
standards related to microtopography, the areal coverage of hollows and hummocks should be
compared to reference standard sites of similar wetland class on a per area basis (e.g., a visual
comparison of the number of depressions and hummocks per 0.04 ha plot or 10m X 10m plot to the
reference sites). Plots should correspond to the vegetation plots used to monitor vegetation.
Soil structure (i.e., texture and bulk density) should be monitored by digging a soil pit in each
vegetation plot and/or mapped soil series on the site. In addition, a soil sample within the top 25
cm (10 in) should be sent for analysis of soil organic matter, nutrients, and bulk density (Bledsoe
and Shear, 2000). Monitoring of SOM will likely only need to be done after implementation of the
mitigation treatment and, assuming soil amendments were added, at the end of the monitoring to
show progress in these soil parameters.
Vegetation Performance Standards
Specific vegetative performance standards may include parameters of plant community structure
(e.g., strata, density, and composition), sustainability (e.g., saplings composition and densities), and
invasive species control; and will vary based on the target condition and reference data. Vegetation
performance standards should reflect values observed in reference standard sites, or sites on a
trajectory to achieve reference standard vegetation, of similar wetland class and subclass.
Preferably, reference standard vegetation data from existing sources (e.g., HGM regional
guidebooks, National Wetland Condition Assessment data, published literature, university studies)
should be used as a basis of comparison, but providers may need to use vegetation data from
reference sites to inform targets. Vegetation data from at least three reference sites with similar
hydrologic regime and soils and in close proximity to the proposed mitigation site is recommended
to inform performance standards. For woody vegetation, performance standards may need to
reflect a trajectory towards target condition, since the time required for forested sites to fully
mature often exceeds the monitoring period for a project. Vegetation trajectory curves have been
compiled from reference wetland sites of various age classes in Arkansas and the Lower Mississippi
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Valley (Klimas et al., 2004; 2008; Smith and Klimas, 2002) by substituting space (many sites of
different ages) for time.
Overall, vegetative performance standards should reflect a long-term goal of a self-maintaining
plant community. Typically, vegetation species composition, density, and survival (of planted
species) are used to demonstrate the ability of a mitigation site to develop into a characteristic
plant community (assuming the planted species are similar to reference sites). Some studies have
validated the shrub-sapling density and ground vegetation coverage as responsive performance
criteria (Berkowitz, 2013). Relative abundance or percent cover are common indicators of plant
community structure. Plant community sustainability, another important vegetation parameter,
can be measured by indicators of community self-recruitment (e.g., seedlings and saplings of
dominant mature canopy species in the understory). Often, invasive species control may be
necessary, and performance standards can include parameters limiting invasive species coverage,
and/or abundance(e.g., maximum allowance of invasive species coverage). The weighted
prevalence index (PI) could also be used as a performance measure for both vegetative community
and hydrology, as it incorporates herbaceous species present and their wetland plant indicator
status (i.e., obligate wetland, facultative wet, facultative, and upland) to produce an index that uses
the plant community as an indirect indicator of site wetness (i.e., the more obligate and facultative
plants sampled, the lower the index indicating increased site wetness).
Vegetative performance standards should be tied to specific project objectives, informed by the
baseline assessment, workplan and data from reference sites. For example, baseline assessments
may reveal that a site is completely cleared of native vegetation, as is the case with agricultural
fields, and a project objective is to restore the native vegetation community, as compared with
reference. Performance standards should focus on survival, composition, and sustainability of
native species. In another example, a proposed site has been selectively timbered and still has
forested community structure minus some important species, and the project objective focuses on
the enhancement or re-establishment of select species to complete the community composition
and sustainability. Performance measures may focus on the survival or within-stand recruitment of
select planted species instead of the vegetation community as a whole. Each project will be
different and vegetative performance, as those above, need to be incorporated based on objectives
and appropriateness.
Monitoring Considerations: Most USACE Districts have vegetative monitoring schemes that
address establishment, survival and sometimes growth of planted vegetation. However, vegetation
composition on any given proposed compensatory mitigation site will vary along with soil and
hydrologic conditions. In fact, Bledsoe and Shear (2000) determined that plant species composition
in riverine wetlands was based on growing season hydrology and microtopography on a scale of
10cm. This emphasizes the need to pair hydrology, soils, and vegetation data in the same plot(s). In
addition to, or in lieu of, USACE District vegetative monitoring protocols, reviewers should look for
and/or recommend the following vegetation sampling and monitoring criteria:
• Each vegetative stratum should be monitored and sampled individually. It is acceptable for
trees and woody vines to be sampled from larger plot sizes than the herbaceous stratum
(e.g., 100m2 vs. lm2);
• Each unique and separate planting area or cover type/polygon (e.g., monitoring unit) should
be monitored and reported on individually. Do not rely on only average values tabulated
across the entire mitigation area;
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• The level of sampling per area should be determined by using a species-area curve, rather
than an arbitrary pre-determined number of plots (see Kent and Coker, 1992);
• If species-area curves are not used, at least two percent of each cover type within the
mitigation area should be sampled;
• Sampling should occur in homogeneous areas with consistent mitigation treatments and
should not overlap transition areas;
• Herbaceous layer sampling should be included to provide not only insight into vegetative
community development in general (e.g., diversity, evenness), but also to allow calculation
of weighted prevalence indices for each community type on the proposed mitigation site.
Weighted prevalence indices provide early information about the evolution of those
communities toward ones dominated by hydrophytes, and in turn, the degree of wetness
in the sampled area. The herbaceous community responds rapidly to changes in site
wetness; thus, it can be used in league with hydrologic monitoring data to assess
hydrologic development of the site.
• Sampling must be sufficient to recognize areas of invasive plant coverage;
• Ground and shrub layer sampling must be sufficient to determine level of recruitment; and
• If the provider is comparing vegetation monitoring data to analog reference sites, the
monitoring plan should outline the sampling and analysis plan for these sites as well.
7.5 Stream Performance Standards and Monitoring
Given the inherent variability and highly dynamic nature of streams, it is difficult to establish a
static set of performance standards for a given point in time (e.g., achieve reference planform by
year five). The choice of performance standards depends on the results of the classification,
baseline assessment, objectives of the project, and the mitigation activities proposed in the
workplan. The ultimate goal of any stream mitigation project is to re-establish hydrologic, hydraulic,
geomorphic, physicochemical, and biological functions. As indicated in Section 6.3, there are a wide
variety of methods which can be implemented to improve degraded stream conditions. Therefore,
measuring the ecosystem response to any method requires careful consideration of the baseline
condition, appropriate indicator parameters, and the availability of reference stream information
for each function. Performance standards need to be informed by objective, verifiable indicators of
stream function, and tied to reference standards or reference conditions. To this end, the following
discussion lays out considerations for reviewers when determining if a particular project has
appropriate parameters to ascertain the performance of a project.
Similar to wetlands, direct measures of stream functions would be advantageous, but are often
complex, time consuming and costly to obtain. Measuring point-in-time indicators of function is
often more practical for compensatory mitigation sites. These indicators typically represent one
element of function (e.g., recruitment of native riparian vegetation, bedform diversity), and this link
should be described as part of the documentation, so the reviewer and provider are clear about
what is being measured and why. For instance, bank height ratio (BHR) can be used as an indicator
of floodplain/ lateral connection, with the explanation that low BHR (e.g., 1.0-1.2) is an indicator of
the stream's ability to access its floodplain. Similarly, density, composition, and areal coverage of
riparian vegetation can be used as indicators of riparian processes, including floodplain roughness,
availability of floodplain habitat for fish species during floods, or large wood recruitment. These
indicators can be measured objectively and consistently to document trends in recovery of the
stream project. Often, multiple indicators will be used to provide a more complete picture of
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function (e.g., stream pattern, longitudinal profile, and cross-sectional geometry all contribute to
assessment of geomorphology functions).
Specific benchmarks or threshold values to inform performance may be available (e.g., through
university and/or scientific studies; local, state, or federal agency reports; peer-reviewed literature
sources), but reviewers should ensure they are applicable. Stream Quantification Tools (SQTs) have
been developed for a number of states in EPA Region 4, including Tennessee, Georgia, North
Carolina and South Carolina, using various field indicators to generate index values and condition
scores based on reference curves that represent disturbance gradients (TDEC, 2018; Somerville et
al, 2021; Harman and Jones; 2016; South Carolina Steering Committee, 2021). Several other
references include lists of potential or recommended indicators which represent aspects of stream
functions that could be used for monitoring or as performance standards (e.g., Roni and Beechie,
2013; Fischenich, 2003; Fischenich, 2006; Harman et al. 2012). Ideally, performance standards
should be based on locally applicable (i.e., regional reference) values determined from resources
that have developed under similar climatic regimes, geological conditions/soil types, and biological
communities, etc., and that are located in similar positions within the stream network.
In the absence of a specific stream assessment protocol with reference data, the following
indicators could be used, with site-specific performance standards informed by appropriate
reference analogs within the same stream class:
• Bedform diversity (sequences of riffle, run, pool, and glide) appropriate for flow and stream
type;
• Channel planform appropriate for valley type, stream type, and substrate type;
• Bank stability (height, bank angle, evidence of mass wasting and/or toe erosion, and
consolidation of bank materials) and lateral migration rates appropriate for flow type and
stream type;
• Substrate composition, embeddedness;
• Evidence of aggradation/degradation;
• Full annual hydrograph appropriate for the stream type and watershed position, including
peak stormflows, rate of change (flashiness), recessional flows, and baseflows. Specific
parameters may include floodplain connectivity/inundation, frequency and duration of
saturation or depth of inundation, attenuation, and flow parameters relative to dynamic
equilibrium, and species life history needs (e.g., timing);
• Plant community condition using indicators such as prevalence index, age-stand
distribution, evidence of recruitment, and all applicable strata present;
• Indices of biotic community condition (e.g., IBI/MMI, O/E) for bugs, algae, fish (Stein et al.,
2022).
The purpose of restoration is to address the stressors affecting the project reach, and there are
numerous stressors that affect various stream functions (see Section 4.5). Table 18 lists examples of
common restoration actions used to address certain stressors and the potential physical and
biological measures which could be used as indicators to develop performance standards. These
actions provide a means for reviewers to compare the proposed objectives and restoration actions
with appropriate performance and monitoring parameters. While providers will likely propose
monitoring parameters other than, or in addition to, those in Table 18, reviewers should evaluate
whether there are performance standards and monitoring tied to each project objective. The
reviewer should keep in mind that the results of the baseline assessment, or the identification of
stressors, will affect not only the actions needed to achieve the target condition, but also how the
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effects are demonstrated. The potential performance measures in Table 18 are listed as either
physical or biological and represent various aspects of stream functions which may be sensitive to
change as a result of a stream mitigation action. Specific thresholds or numeric values to define
performance standards for these indicators will have to be based on comparison to reference
standard conditions.
Table 18. Restoration actions (Roni and Beechie, 2013) or practices (Bledsoe et al.» 2016) typically used to
alleviate common stream impairments along with examples of indicators which can be used to assess the
progress of restoring physical (hydrologic, hydraulic, geomorphic, water quality) and/or biological
processes and functions.
Restoration
Actions/Practice
Potential Performance Measures
Removal of barriers,
dams, culverts and
road crossings
Physical: change in channel morphology and elevation, sediment storage and composition.
Biological: presence/absence of migratory fish species, seasonal species abundance and
diversity; riparian vegetation composition and age structure.
Reconnect floodplain/
Floodplain connection
Physical: flow connection with main channel, channel morphology and elevation, habitat,
sediment storage, wood and organic retention.
Biological: fish abundance and diversity, fish passage and migration, macroinvertebrate
and periphyton communities
Remove and/or
setback levee/
Floodplain connection/
Riparian buffer
Physical: channel and floodplain morphology, topography and habitat; sediment and wood
delivery and storage, duration of floodplain inundation
Biological: Composition and age structure of riparian vegetation and vegetation diversity;
and abundance offish, macroinvertebrates and periphyton
Aggrade incised
channels
Physical: Channel geometry, elevation and morphology, connectivity of floodplain habitats
and their area and density, wood, water and sediment retention.
Biological: Fish abundance and diversity, fish passage and migration, macroinvertebrate
and periphyton communities
Road removal,
stabilization,
resurfacing
Physical: pool depth, scour, fine sediment, turbidity and water quality, streamflow and
bankfull width
Biological: fish survival, macroinvertebrate diversity and abundance.
Riparian buffer
planting/ Riparian
buffer
Riparian area: tree or plant survival, species composition, density and biomass; tree
growth, height and diameter
In channel: shade, temperature, organic inputs (leaf litter and woody debris), bankfull
width, bank stability, channel migration, pool depth
In-stream structure
placement/ In-stream
enhancement
Physical: channel morphology, habitat area and composition (pool and riffle depth, area
and number), wood abundance.
Biological: fish abundance, diversity, growth and survival.
Reconstruction of
channel/ Bank and bed
stabilization/ Riparian
buffer/ Floodplain
reconnection/ In-stream
enhancement
Physical: Channel bed (vertical) and bank (lateral) stability; channel geometry, slope
change, sinuosity, and morphology; flow velocities, sediment and wood storage;
connectivity of floodplain habitats and their area and density; wood, water and sediment
retention.
Biological: Fish abundance and diversity, fish passage and migration, macroinvertebrate
and periphyton communities
Stream mitigation performance should be evaluated based on the trajectory of indicators relative
to appropriate reference standards conditions over extended time periods (e.g., 10 years or more)
that preferably includes at least one 10-year flow event or larger. Streams in watersheds with
highly variable (flashy) flows in unconfined valleys and/or with rapidly changing land use and
hydrology may need to be monitored longer than streams with more consistent flow patterns that
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have coarse substrates and/or occur in confined valleys. This is due to the effect flashy flows may
have on sediment and geomorphic functions (e.g., flashy flows increase water velocities and
sediment transport capacities which in turn could affect stream geomorphic stability and habitats).
Monitoring considerations: Implementation monitoring can typically be addressed by reviewing
the "as-built" plans, as compared to the approved design plans, to evaluate whether the project
was implemented as it was designed and approved. Typically, 60% design plans are used to develop
the mitigation plan, which is an appropriate level of detail for planning. However, before a stream
design is implemented, a 100% design is completed and used in the field for construction purposes.
Any changes to the 100% design due to unexpected issues encountered in the field (e.g., trees to be
kept or removed, bedrock outcrops, and unexpected infrastructure) are included in the "as-built"
plan which represents what was actually implemented in the field; this represents the "final"
project. The as-built plans should be considered carefully in the context of how any design changes
may affect the objectives and performance standards detailed in the mitigation plan.
The primary reason to monitor the site after construction is to assess the effectiveness of the
management action and progress toward project goals and objectives. In addition, if monitoring
reveals that performance standards are not being attained within the timeframe expected, then
steps may be required to fix a system or a component of the system that is not successful (i.e.,
adaptive management). Because each project is likely to have aspects that differ from other
projects, a one-size fits all monitoring scheme cannot be prescribed, just as with performance
standards. Various authors have offered guidelines for monitoring stream restoration projects (Roni
and Beechie, 2013; Skidmore et al., 2011; Bledsoe et al., 2016; Saldi-Caromile et al., 2004; Cramer
et al., 2012; FISRWG 2001). Reviewers are encouraged to refer to these references as more
complete treatments of stream restoration monitoring. A brief list derived from the previous
references is listed below. Generally, an effective monitoring plan should include:
(1) Selection of a monitoring design that will be robust to represent the achievement of
performance standards for the entire site;
(2) Specification of the number of sites and duration of monitoring (e.g., collecting at least 10
years of hydrologic data will provide more confidence that a range of flow events have
occurred before project closeout);
(3) Sufficient monitoring duration for riparian vegetation of up to, or exceeding, 10 years of
monitoring to ensure desirable (i.e., target) tree, shrub and herbaceous species become
established;
(4) Selection of a sampling scheme that occurs at multiple locations within a mitigation site and
uses statistical principles to achieve an appropriate ability to detect change (i.e., statistical
power);
(5) Parameters measured at the channel cross-section and longitudinal scale; and
(6) Graphical analysis and interpretation of results to show trends in data.
Since streams are intimately linked to the conditions in upstream and downstream reaches,
mitigation actions in a specific reach may affect, and be affected by, upstream or downstream areas
positively or negatively. For example, trapping sediment in one reach may result in downstream
incision due to sediment starvation. As such, reviewers should consider whether providers have
proposed an appropriate domain of analysis, or the extent, that should be assessed when
evaluating mitigation performance. Upstream and downstream areas should be evaluated based on
their hydraulic/geomorphic condition, channel evolution class, and degree of sediment continuity
with the reach of interest (i.e., the mitigation or impact reach) (Stein et al., 2022). Ideally, the
upstream domain should be defined as a distance equal to 20 channel widths or to the next
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upstream natural or engineered grade control, whichever comes first. Downstream domain should
be defined as the closest of the following: at least one reach downstream of the first grade-control
point (but preferably the second downstream grade control location); the nearest downstream tidal
backwater/lentic waterbody; the nearest downstream equal-order tributary; or a two-fold increase
in drainage area.
Hydrologic monitoring should begin prior to implementation of the workplan and should continue
throughout the effectiveness monitoring period to document the delivery and variability of low,
bankfull and flood flows to the channel and the floodplain. Similar to the monitoring approach for
riverine wetlands, this monitoring should be conducted using stage or water level recorders, as well
as shallow groundwater wells on the floodplain to document frequency and duration of inundation.
Other monitoring approaches may be considered, such as wildlife cameras to capture flood events
or modelling to simulate flows, however the method should be accompanied by the rationale for
how it is representative of the site and informs the hydrologic performance standards.
Geomorphic monitoring can be completed using established cross-sections along a stream reach
that are monitored with sufficient frequency to observe changes in channel morphology and
geometry. Longitudinal surveys can be used to measure bed complexity and aggradation or
degradation through time. These surveys should be of sufficient length (e.g., 20 times bankfull
width or two meander wavelengths) to capture bedform morphology and thalweg variability (e.g.,
pool-riffle sequences) throughout the mitigation project reach. Where there are multiple stream
classes or differences in mitigation actions, cross-sections and longitudinal surveys should be
collected within each. Finally, bank heights and angles can be measured or obtained from cross-
sections to assess bank stability and erosion through time. All surveys, whether cross-section
surveys or longitudinal surveys, should be tied to fixed benchmark elevations.
Physicochemical monitoring should collect temperature, DO, flow rate, depth data, and any
additional water quality constituents needed to monitor the specific objectives and performance
standards of the project. Physicochemical, riparian vegetation and instream biology monitoring
should be consistent with the data collected during the baseline assessment and should be
sufficient to inform performance standards for the project. The response of biological communities
to stream restoration efforts varies widely (Dyste and Valett, 2019; Griffith and McManus, 2020a;
2020b), thus monitoring timeframes for biological parameters may not occur at the same
frequency, or as early in the monitoring period, as physical parameters. For additional information
related to riparian vegetation monitoring, see the vegetation sub-section in Section 7.4 above.
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8.0 How will the site be managed to sustain the target condition?
Restoration of aquatic resources is not an exact science.
Aquatic ecosystems are complex and heavily influenced
by landscape scale processes and stressors (e.g.,
hydrologic flows, sediment supplies, and forest patch
sizes and corridors). Due to this complexity, many
uncertainties exist in siting, planning, implementing, and
monitoring any given compensatory mitigation project,
including the effects of site and landscape-scale stressors
(see Section 2.4). Monitoring and adaptive management
are extremely important to achieving the goal of
ecologically sustainable and resilient aquatic ecosystems
to compensate for losses due to permitted impacts.
This question outlines administrative components of a mitigation plan, including site protection
mechanisms and financial assurances, as well as maintenance plans, long-term management plans
and adaptive management plans. Although not specifically ecological, these management
considerations are critical to the ability of the restored aquatic resource to persist on the landscape.
If these components are addressed appropriately, responsibilities related to maintenance, funding,
site protection and management will be clearly articulated, and mitigation projects are more likely
to be self-sustaining once performance standards have been achieved. These components are
generally provided as part of a completed draft mitigation plan, banking instrument or ILF
instrument or addendum. The main opportunity to provide comments on these components will be
during the Public Notice comment period for individual permits or during review of the draft
mitigation bank instrument or ILF project addendums.
Review considerations for these components are similar for both wetland and stream
compensatory mitigation projects (Box J). Reviewers will want to ensure that maintenance plans,
adaptive management plans and financial assurances provide the funding and management
necessary to achieve ecological success at project closeout. In the event a project is not meeting its
performance standards, it is critically important that the information provided in these components
is sufficient to diagnose and address problems. For example, reviewers will want to ensure the
adaptive management plan outlines specific courses of action to rectify problems with the
mitigation site and any additional monitoring that may be required to assure that the remedies are
sufficient. Financial assurances should adequately cover costs involved with planning, constructing,
monitoring and achieving the agreed upon performance standards. Reviewers will want to ensure
site protection mechanisms and long-term management plans are appropriate to protect, fund, and
manage the site in the long-term and ensure a sustainable and resilient aquatic ecosystem is
maintained on the landscape.
In a best-case scenario,
required site protections,
financial assurances,
maintenance and long-term
management plans will facilitate
the sustainability of the site.
However, adaptive management
measures may also be needed
given the level of uncertainty
inherent in mitigation projects.
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Bom J. Information to assess adequacy of wetland and stream mitigation project management.
Maintenance Plan
• What are the aspects of the project that will need maintenance?
• Does the plan outline the types of maintenance activities, their timing and costs?
• Do these activities contribute to goal of site sustainability?
Adaptive Management Plan
• Does the plan outline how monitoring data will be analyzed to identify potential challenges?
• Does the plan outline how and when adaptive management measures will be planned and
implemented?
• If appropriate, have beaver been accounted for and incorporated into the site?
Financial Assurances
• Are specific mechanisms and dollar amounts specified?
• Are these sufficient to ensure successful completion of the project?
Long Term Management Plan
• Does the plan contribute to goal of site sustainability?
• Does the plan include any land uses that would be counterproductive to the goal of restoring a
self-sustaining ecosystem?
• Does the plan outline the long-term management needs and financing mechanisms?
Site Protection Mechanism
• What type of mechanism is proposed (e.g., conservation easement, restrictive covenant, other)?
• Will the mechanism protect the site in perpetuity?
8.1 Regulatory Context
The Mitigation Rule requires site protection, maintenance, long-term management, adaptive
management, and financial assurances be included in a mitigation plan or instrument. Additional
regulatory context is provided for each element below.
8.2 Maintenance Plan
Maintenance plans describe and outline a schedule of maintenance requirements to ensure the
continued viability of the resource once initial construction is completed (40 CFR §230.94(c)(8)).
The maintenance plan should account for activities that will need to take place on the project site
for the duration of the monitoring period, and should include the types of maintenance activities,
their timing, and estimated costs. Maintenance activities should support the goal of making the
compensatory mitigation site ecologically sustainable and resilient, and there should be nothing in
the maintenance plan that allows land uses or management techniques that may damage the
restored stream or wetland's structure or function. Routine maintenance activities could include
maintaining or replacing monitoring equipment, signage and fencing, replanting dead vegetation,
management of invasive species, routine burns and/or vegetation management in fire-adapted
sites, maintaining access to monitoring stations and for field inspections, etc. Maintenance may
also be needed for structures, if any are installed as part of the mitigation plan. Unexpected or
repeated needs for maintenance during the monitoring period may indicate problems that require
resolution (e.g., erosion around the sides of ditch plugs, erosion around or underneath stream
structures, and repeated die-off of planted vegetation), and these non-routine maintenance
activities should be addressed under an adaptive management plan (see Section 8.3).
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8.3 Adaptive Management Plan
According to the Mitigation Rule, adaptive management means the development of a management
strategy that anticipates likely challenges associated with compensatory mitigation projects and
provides for the implementation of actions to address those challenges, as well as unforeseen
changes to those projects (40 CFR §230.94(c)(12) and §230.97(c)). Adaptive management requires
consideration of the risk, uncertainty and dynamic nature of compensatory mitigation projects (see
Section 2.4), and guides the process by which project modifications are implemented to optimize
performance and reduce project uncertainty (i.e., will mitigation actions be effective?).
An adaptive management plan should identify specific measures that ensure aquatic resource
functions are maintained, outline how monitoring results will be analyzed to identify potential
problems, and identify implementation measures to rectify those problems. Plans should be
informed by both a provider's and IRT reviewers' experiences with similar projects in which
performance was not attained without modifying the plan, and monitoring results which indicate
performance standards are not being achieved. For instance, if soil conditions of a riparian or
wetland area are not properly prepared and amended after site construction, experience has
shown that vegetative plantings will likely fail. This can be anticipated and a plan put in place to
rectify the problem should it happen. In another example, hydrologic monitoring of a riverine
wetland site may indicate the target flood regime is not occurring with the frequency, duration, and
magnitude expected. For this example, the results of the monitoring (see Section 7.3) are crucial to
diagnosing the change and providing information on when adaptive management measures may be
needed. If not achieving performance standards, providers will need a plan to address the
deficiencies and achieve performance.
8.3.1 Examples of Conditions Requiring Adaptive Management
Most importantly, adaptive management will be needed if monitoring results indicate performance
standards and target conditions not being met. Some additional examples of challenges to wetland
and/or stream projects that may require adaptive management might include:
• Large flood events that may destroy floodplain/riparian vegetation, avulse into a new or old
channel, or erode unprotected streambanks;
• Establishment of invasive species which threaten to dominate and/or alter the target
condition of the site;
• Droughts, which may delay or prevent the establishment of appropriate channel forming
flows and biological communities (e.g., by slowing their development or restricting
movement, particularly of fish) or lead to drying conditions which will carry fire. In some
wetland types, like mineral soil flats, fire is a natural ecosystem process. However, in other
wetland types, like cypress domes or wetlands with organic soils, fire may diminish the
functions of the wetland.
• Upstream or surrounding watershed disturbances occurring after construction that may
result in changes to sediment or hydrologic processes (e.g., the delivery of large sediment
loads or flashier flows to the site);
• Damage due to trespass (e.g., off-road vehicle damage to stream channels or wetlands,
unwanted cattle access to streams)
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8.3.2 Beaver
Although not an anthropogenic stressor, and in fact an ecological enhancement to many aquatic
resource functions (Naiman et al., 1988; Gurnell, 1998; Meetenmeyer and Butler, 1999; Pollock et
al., 2007; Burchsted et al., 2010; Polvi and Wohl, 2013; Pollock et al., 2014), beaver (Castor
canadensis) can present adaptive management challenges for aquatic resource mitigation sites.
Although beaver may affect wetlands via alterations to planted vegetation, hydroperiods and
nutrient quality (Bason et al., 2017), they primarily affect stream mitigation sites in the
southeastern U.S., where the construction of beaver dams is perceived to disrupt the design and
function of the restored stream reach and associated riparian zone.
Because of their potential benefits to stream stability and biology, reviewers should ensure
adaptive management measures first consider a careful examination of both present and
anticipated effects of beaver-constructed structures on the mitigation site, adjacent lands and
existing infrastructure before destroying the structures. Ideally, this examination should be
coordinated with the USACE and the other IRT member agencies. Beavers are a natural part of the
historic ecosystem, and their presence on a site should not automatically be considered an
impairment. Not only do beaver provide woody debris and habitat diversity for the larger system,
but they also provide stream stability and reduce erosion by reducing water velocities and shear
stress on stream banks. Unless there is a compelling reason to remove beaver dams from the
system, reviewers may want to recommend other actions, such as re-evaluating and adjusting
mitigation credits allocated for the project to reflect ecosystem conditions affected, or caused, by
the beaver activity. In general, reviewers should consider a three-step process for addressing issues
with beaver present on either proposed or implemented compensatory mitigation sites:
(1) If beaver are present on a site being proposed for a mitigation project, reviewers should
consider how workplans, maintenance plans, long-term and adaptive management plans could
accommodate their presence. Ideally, beavers should be allowed to remain, and reviewers
should work with the IRT and the mitigation provider to incorporate the beaver-modified
landscape into the mitigation plan.
(2) If construction of a stream channel is undertaken as a mitigation activity on a mitigation site
and beaver construct a dam on a portion of the constructed reach, reviewers should work with
the IRT and the mitigation provider to see if modifications could be made to accommodate the
beaver, while also endeavoring to allow the mitigation provider equitable mitigation credit. If
the potential exists for beaver to colonize a project site, reviewers should consider how long-
term and adaptive management plans accommodate their presence.
(3) If beaver move into a mitigation site and construct a dam that floods adjacent property or
endangers critical infrastructure, then reviewers should work with the mitigation provider and
IRT to determine if the beaver need to be managed and/or removed from the site. Beaver can
be difficult to successfully eradicate, which presents long-term management challenges from
both a mitigation site performance and financial standpoint. Reviewers should work with the
mitigation provider and IRT to modify mitigation plans, as necessary.
8.4 Financial Assurances Plan
The financial assurances plan describes the mechanism and dollar amounts that will be provided by
the sponsor and demonstrates how they will ensure a high level of confidence that the
compensatory mitigation project can be completed in accordance with approved plans and
performance standards (40 CFR §230.94(c)(13)). Financial assurances are an important mechanism
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for managing the risk of project failure, including failure to complete the project, to meet
performance standards or to maintain the mitigation project. Holding all forms of compensatory
mitigation to equivalent standards, including financial assurances helps to reduce uncertainty,
including risk of project failure (USACE, 2016a). These plans are typically worked out between the
mitigation provider and USACE Districts and cannot be modified without prior notice to USACE. For
information on financial assurances for compensatory mitigation see Implementing Financial
Assurances for Mitigation Project Success (USACE 2016a).
8.5 Long-Term Management Plan
The long-term management plan outlines how the compensatory mitigation project site will be
managed after performance standards have been achieved to ensure the long-term sustainability of
the resource (40 CFR §230.94(c)(ll)). This is a very important aspect of the compensatory
mitigation program since many ecosystem functions will likely not fully develop within the typical
monitoring period. Therefore, the long-term management plan should account for the continued
maturation of the mitigation site and identify any long-term management or maintenance needs on
site. There should be nothing in the long-term management plan that allows land uses or
management techniques that may damage the site's structure or function. This restriction on
potentially damaging management should also be incorporated into the conservation easement
and/or deed restriction to provide some legal enforceability to the long-term management of a site.
If stressors are likely to persist or potentially arise during the long-term management phase of the
project, the long-term management plan should include a discussion of how the site will be
adaptively managed to ameliorate any adverse effects.
The long-term management plan must also include long-term financing mechanisms (e.g.,
endowments or trusts) and identify the party responsible for site ownership and long-term
management of the mitigation project (40 CFR §230.97(d)). In fact, the responsible party must be
identified in the permit conditions authorizing the project, bank or ILF instrument. The permittee or
sponsor may transfer the long-term management responsibilities to a public or private land
stewardship entity approved by the USACE District Engineer after final performance standards are
met. Reviewers should work with USACE to ensure that adequate financial and legal (e.g., land use
restrictions on project site) resources have been put in place to ensure site sustainability.
8.6 Site Protection Mechanism
The site protection mechanism is a description of the legal arrangements and instrument, including
site ownership, that will be used to ensure the long-term protection of the mitigation project site
(40 CFR §230.97(a)). The long-term protection of mitigation sites should be arranged through
appropriate real estate instruments, such as conservation easements or transfer of title to public or
private land managers. The real estate instrument should restrict or prohibit incompatible uses
(e.g., clear cutting, all-terrain vehicle access). Typically, these mechanisms will be conservation
easements or restrictive covenants. These plans are typically worked out between the mitigation
provider and USACE Districts and cannot be modified without prior notice to USACE. Key aspects of
site protection mechanisms and securing them for a site are outlined in the Compensatory
Mitigation Site Protection Instrument Handbook (USACE, 2016b).
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Appendix A, EPA Region 4: Information to Consider for Wetland Mitigation
Projects
Section 3.2 (Box A): Recommended information to inform HGM wetland classification
Hvdroqeomorphic classification:
• What is the HGM classification of the existing wetlands on site?
o Geomorphic position. Data source?
o Water sources. Data source?
o Hydrodynamics. Data source?
o Applicable HGM regional guidebook? Rationale?
• Does the site classification suggest that a different wetland type would be appropriate?
Soils information to support classification:
• Geomorphic position
• Flooding (frequency, timing and duration)
• Ponding (timing, frequency and duration)
• Seasonal high-water table (depth and timing)
Section 4.2 (Box C): Recommended information to evaluate watershed-scale stressors
Watershed scale issues that must be addressed include:
Sediment and water quality functions
• Are land uses impacting sediment supply to the stream reach or wetland?
• Are land uses potentially impacting water quality from urban or agricultural runoff?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting sediment/organic matter transport and/or timing?
• How much of the proposed site is contiguous to natural land cover, indicating adequate buffers?
• Are any water quality impairments occurring in the contributing catchment or watershed?
Hvdroloaic potential
• What is the propensity for site hydrology and wetness?
• Are land uses altering runoff processes (e.g., flashy hydrographs) to the stream reach or wetland?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting flow characteristics (i.e., magnitude, duration, frequency, timing, rate of change)?
• Does soil data support wetland conditions?
Landscape connectivity
• Are land uses impacting the connectivity of stream and wetlands to adjacent habitats?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
limiting biota movement (i.e., longitudinal connectivity)?
• Are any impediments limiting flow access to floodplain (lateral connectivity) during flood events?
• Is the site located within an ecologically connected hub or corridor that identifies both aquatic and
terrestrial connectivity?
Can the mitigation provider ameliorate adverse landscape-scale conditions with onsite management to
reduce or remove the effects of those conditions on restoration site potential?
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Section 4.4 (Box D): Recommended information to evaluate wetland baseline condition.
Information below is in addition to, and complementary of, the results of the watershed analysis:
• Is the site in the reference domain of an existing HGM regional guidebook?
• Is the wetland of similar class and subclass to that in the HGM guidebook? List the wetland functions
of the site and how they have been impaired.
• If HGM guidebook is used, what are the results?
• Was a rapid assessment method used? If so, which one and what were the results?
• What stressors are present on the site (e.g., clear-cut, ditching, fill)?
• Has plot-level vegetation data been appropriately stratified and sampled at a sufficient intensity? If so,
how was the information interpreted? What effect have stressors had on vegetation?
• Has site-specific soil mapping and soil verification been completed? What effect have stressors had
on soils?
• Have soil profile descriptions, notation of hydric soil indicators, and impacts to soil integrity (e.g.,
tilling, plowing, bedding, land clearing, mining, and fill) been provided?
• Is there available hydrologic monitoring data, depending on HGM class and soils (e.g., surface water
ponding, shallow groundwater/water table data)? For what duration?
o Are there active stream stage recorders? For what period of record is data available?
o Are there rain gauges in proximity to the site? For what period of record is data available?
o Are there active ground water wells? How long have they been active and at what frequency/
interval is the data available?
• Have results from hydrologic monitoring been graphically represented with rainfall data?
• What effect have the stressors had on ground and/or surface water hydrology?
Section 5.0 (Box F): Recommended information to identify appropriate target condition.
• What are the goal statements outlining the expectations for the project considering the aquatic
resource class(es), baseline conditions and the identified stressors and impairments affecting the
site?
• Given the conditions in and around the site (from baseline assessment), will the proposed site support
the target functions/condition(s)?
• Are goals and objectives tied to specific functions?
o For wetlands, consider hydrologic, biogeochemical and habitat functions
o For streams, consider hydrologic, hydraulic, geomorphic, physicochemical and biological
functions
• Are the objectives:
o Specific to the aquatic resource class(es) on site?
o Measurable (as indicators of functions outlined above)?
o Achievable given current and projected site-specific and watershed stressors?
o Relevant and supportive of the goals of the project?
o Time bound to the monitoring and performance period of the project?
• What reference approach(es) was (were) used to establish target conditions)? Are there reference
aquatic resources, including reference standard sites or regional references (e.g., HGM regional
guidebooks), to which the target condition can be compared?
• Is (are) the reference site(s) the same class and representing the least-altered condition to which the
biology can be compared?
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Section 6.2 (Box G): Recommended information to address appropriateness of wetland mitigation
plan/design.
The following questions should be addressed in context with the site classification (Section 3),
baseline watershed and site assessment (Section 4), and the goals and objectives for the project
(Section 5):
Hydrology
• Do the methods proposed address degradation of the wetland's water inputs, outputs, and storage
capacity as compared to reference wetlands of the same class?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore the hydroperiod of surface and/or groundwater typical for this wetland
type?
• What is the growing season hydroperiod during normal, wet, and dry years AND will this support a
vegetation community similar to reference standard?
Soils
• Do the methods proposed address degradation of the wetland's soils as compared to mapped soils
and reference?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore the soil characteristics (e.g., permeability, structure, hydric characteristics,
and bulk density) indicative of this wetland type?
Vegetation
• Do the methods proposed address degradation of the wetland's vegetation community as compared
to reference standard?
• Are the methods justified (i.e., purpose, rationale, effectiveness, and assumptions)?
• Will these methods restore a plant community (all strata) typical of the reference wetland class?
• Does the proposed method include a vegetative planting plan that accounts for variations in soil
types, site elevations, planting depths, and timing for planting?
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Section 7.0 (Box I): Recommended information to address performance standards and monitoring.
Performance Standards
• Does each performance standard tie to a specific project objective?
• Are performance standards provided for the suite of functional indicators (see below) tied to project
objectives?
• Is each performance standard informed by appropriate reference standard comparisons?
• Is each performance standard described using clear and concise wording?
• Is each performance standard scientifically defensible (e.g., sensitive to changes in function)?
• Is each performance standard supported by data collection methods that are appropriately timed,
repeatable and with clearly defined error rates?
Monitoring Plan
• Are data collection methods for each performance standard outlined in the monitoring plan?
• Have pre-project baseline data been collected for all measures outlined in the monitoring plan?
• Has the timing of data collection (e.g., frequency, duration, and seasonality) been determined?
• Has the location and scale of data collection (e.g., reach, riparian, and watershed) been determined?
• Is the sampling intensity (number and arrangement of locations/plots) appropriate for the chosen
parameter?
• Is the sampling of sufficient intensity and precision to detect deviations from performance standards
and indicate the need for adaptive management?
• Does the plan outline how data will be analyzed and reported?
Review of post-project monitoring
• Compare the as-builts to the design criteria outlined in the mitigation plan. Was the project built to
specifications in the plan? If not, why not?
• Do the results of monitoring show the site is on track to meet performance standards? If not, is
adaptive management needed?
Performance standards and monitoring expectations by functional category
Wetland Hydrology:
• Measures that demonstrate timing, duration and frequency of saturation, ponding, and/or flooding,
during at least 3 normal rainfall years, similar to hydrologic monitoring data in the same HGM
subclass of reference standard wetlands on similar soils.
• Methods should be sufficient to monitor the site's hydrologic sources and gradient (e.g., rain gauges,
stream gauges and/or an array of groundwater wells).
Wetland Soils:
• If soils have been disturbed or drained, measures that demonstrate they meet hydric soil criteria for
saturation (minimum 14 consecutive days) and anaerobiosis as per National Hydric Soil Technical
Standard.
• Measures that demonstrate the soil is accumulating organic material; developing appropriate bulk
density, texture, and nutrient concentrations; and soil microtopographic features (shallow depressions
and minor hummocks) similar to reference standard.
Wetland Vegetation:
• Measures that demonstrate plant survival to sapling stage; species composition and density (e.g.,
establishment of native species in all strata); response of herbaceous components of vegetative
community to hydrologic changes (i.e., Weighted Prevalence Index) and trending towards a
hydrophyte dominated community; relative abundance or cover of various age classes, including
evidence of recruitment; and an expectation for maximum allowable invasive plant species cover.
• All performance standards should be similar to, or along a trajectory towards, reference standard.
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Section 8.0 (Box J): Information to assess adequacy of wetland and stream mitigation project
management.
Maintenance Plan
• What are the aspects of the project that will need maintenance?
• Does the plan outline the types of maintenance activities, their timing and costs?
• Do these activities contribute to goal of site sustainability?
Adaptive Management Plan
• Does the plan outline how monitoring data will be analyzed to identify potential challenges?
• Does the plan outline how and when adaptive management measures will be planned and
implemented?
• If appropriate, have beaver been accounted for and incorporated into the site?
Financial Assurances
• Are specific mechanisms and dollar amounts specified?
• Are these sufficient to ensure successful completion of the project?
Long Term Management Plan
• Does the plan contribute to goal of site sustainability?
• Does the plan include any land uses that would be counterproductive to the goal of restoring a self-
sustaining ecosystem?
• Does the plan outline the long-term management needs and financing mechanisms?
Site Protection Mechanism
• What type of mechanism is proposed (e.g., conservation easement, restrictive covenant, other)?
• Will the mechanism protect the site in perpetuity?
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Appendix B, EPA Region 4: Information to Consider for Stream Mitigation
Projects
Section 3.3 (Box B): Recommended information to address stream characterization.
What is the stream's watershed/landscape context?
• Where does the stream sit in the watershed?
o e.g., determine Strahler stream order (ensure appropriate mapping resolution) and
watershed size
• What is the current valley type (e.g., very confined, confined, partially confined, unconfined)?
o Data sources? How was valley type determined?
• In what ecoregion does the site occur?
What is the flow permanence?
• Is the stream reach perennial, intermittent, or ephemeral? Data sources?
What is the geomorphic context?
• What classification approach(es) was/were used?
• Were all relevant data provided to support classification? (e.g., entrenchment ratio, width/depth
ratio, sinuosity, bed materials, slope, number of channels)
• What are the stable stream types for the given valley type? How does that compare to the
existing stream type?
• What is the Stream/Channel Evolution stage?
o Model used?
o Justification for stage?
o Field indicators noted?
Section 4.2 (Box C): Recommended information to evaluate watershed-scale stressors
Watershed scale issues that must be addressed include:
Sediment and water quality functions
• Are land uses impacting sediment supply to the stream reach or wetland?
• Are land uses potentially impacting water quality from urban or agricultural runoff?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting sediment/organic matter transport and/or timing?
• How much of the proposed site is contiguous to natural land cover, indicating adequate buffers?
• Are any water quality impairments occurring in the contributing catchment or watershed?
Hvdroloaic potential
• What is the propensity for site hydrology and wetness?
• Are land uses altering runoff processes (e.g., flashy hydrographs) to the stream reach or wetland?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
affecting flow characteristics (i.e., magnitude, duration, frequency, timing, rate of change)?
• Does soil data support wetland conditions?
Landscape connectivity
• Are land uses impacting the connectivity of stream and wetlands to adjacent habitats?
• Are any upstream or downstream impediments (e.g., dams, weirs, perched culverts, road crossings)
limiting biota movement (i.e., longitudinal connectivity)?
• Are any impediments limiting flow access to floodplain (lateral connectivity) during flood events?
• Is the site located within an ecologically connected hub or corridor that identifies both aquatic and
terrestrial connectivity?
Can the mitigation provider ameliorate adverse landscape-scale conditions with onsite management to
reduce or remove the effects of those conditions on restoration site potential?
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Section 4.5 (Box E): Recommended information to evaluate stream baseline condition.
Information below is in addition to, and complementary of, the results of the watershed analysis.
Hvdroloqic (Summary of hydrologic analyses conducted, data sources and period of record)
• How were flood flows, low flows, and bankfull flows calculated? Was a stream gauge used to evaluate
the period of record for the site? If not, how were streamflow estimates made (e.g., Manning's
equation, regional regression equations, StreamStats, hydraulic models, runoff and streamflow
models)?
• Was floodplain groundwater elevation established? Is the groundwater flowing across the floodplain
towards the channel? At what time of year?
• What watershed and site scale stressors affect the hydrology of the site?
Hydraulic
• Were hydraulic models used? If so which ones? Was the confidence in the model results explained?
• Were sediment transport models used? If so, which ones? Was the confidence in the model results
explained?
• What watershed factors contribute to the sediment supply?
o Severe bank erosion in the channel network? Mass wasting?
o Channel incision upstream?
o High availability of coarse sediment in bed or banks?
o What flow and sediment retention areas exist in the upstream watershed? (e.g., reservoirs,
ponds, low head dams)
• How will the evolution of the upstream channels effect sediment supply? What is the CEM / SEM
stage above the reach? Below the reach?
Geomorphic
• Methods used to complete channel stability assessment? Results?
• What is the current planform; longitudinal profile; and dimension of the project reach?
• What are the causes of channel instability? How were they determined?
• How do channel impairments effect stream and floodplain habitat? Is the channel laterally
constrained?
• Extent of channel alteration (e.g., enlargement, floodplain encroachment, or channelization) of the
project reach?
Phvsicochemical
• Are state surface water quality standards being met? If not, how often and under what conditions are
they out of compliance?
• What are the sources of nutrients and other pollutants in the watershed and are they effecting the
site?
• Is this reach on the 303(d) list or other TMDL for any of the following impairments: sediment, nutrient,
metals & toxics, temperature, or flow modification?
• Can the components of existing water quality that are limiting biological productivity be addressed at
the mitigation site (e.g., mass wasting, flooding, streamflow, shade, vegetation, and soils)?
Biological
• Biological community parameters sampled? Protocol? Results?
• Are habitat features altered? Cause of those alterations?
• Are there high-quality sites upstream and downstream of the proposed site? Would this project
connect the upstream and downstream sites?
• Are there sources of beneficial organisms that can migrate from the upstream or downstream portions
of the watershed to repopulate the site? Proximity of those source areas to the mitigation site?
• Are there invasive species (animals, plants, and algae) that will migrate from the upstream or
downstream portion of the site that would affect long term characteristics of the site communities?
• What is the percentage and average width of the intact riparian area contiguous to the site?
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Section 5.0 (Box F): Recommended information to identify appropriate target condition.
• What are the goal statements outlining the expectations for the project considering the aquatic
resource class(es), baseline conditions and the identified stressors and impairments affecting the
site?
• Given the conditions in and around the site (from baseline assessment), will the proposed site
support the target functions/condition(s)?
• Are goals and objectives tied to specific functions?
o For wetlands, consider hydrologic, biogeochemical and habitat functions
o For streams, consider hydrologic, hydraulic, geomorphic, physicochemical and biological
functions
• Are the objectives:
o Specific to the aquatic resource class(es) on site?
o Measurable (as indicators of functions outlined above)?
o Achievable given current and projected site-specific and watershed stressors?
o Relevant and supportive of the goals of the project?
o Time bound to the monitoring and performance period of the project?
• What reference approach(es) was (were) used to establish target conditions)? Are there reference
aquatic resources, including reference standard sites or regional references (e.g., HGM regional
guidebooks), to which the target condition can be compared?
• Is (are) the reference site(s) the same class and representing the least-altered condition to which the
biology can be compared?
Section 6.3 (Box H): Recommended information to address appropriateness of stream mitigation
plan/design.
Addressing the following questions should be done in the context of the mitigation site's
classification (Section 3), baseline watershed and site assessment (Section 4), and the goals and
objectives for the project (Section 5):
Hydrologic
• From what type of hydrologic data were design flows derived (e.g., gauge, ungauged, models,
equations)?
• Are impairments to hydrologic connectivity of the stream reach addressed in the mitigation plan?
Including:
o Longitudinal connectivity (e.g., dams, weirs, road crossings, and culverts)
o Lateral connectivity (e.g., levees, floodplain development)
o Vertical connectivity (e.g., hyporheic exchange)
• Is the project reach experiencing hydrologic flashiness (based on assessment of contributing
watershed imperviousness)?
• Do the methods proposed address the current condition of the stream's low, bankfull and flood
flows?
• Will the proposed methods restore the timing, magnitude, frequency, and duration of surface and/or
groundwater flows typical for this stream type where these conditions are impaired?
Hydraulic
• What methods were used to address the hydraulics of the reference reach? Were the same
methods used for the designed reach?
• Has the provider accounted for expected sediment flows into and out of the project reach under the
full range of flow conditions (i.e., low, bankfull, and flood flows)?
• If the provider set a goal of equilibrium conditions, how have they accounted for the hydraulic and
sediment capacity and supply conditions upon which the design to achieve equilibrium is based
(e.g., modelling)?
• If the provider has NOT set a goal of equilibrium conditions in the project reach, how have they
accounted for sediment capacity and supply through the reach (i.e., sediment storage on the
floodplain)?
• If the project reach is incised how, and to what extent, will the provider reconnect the floodplain?
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Section 6.3 (Box H cont'd): Recommended information to address appropriateness of stream
mitigation plan/design.
Addressing the following questions should be done in the context of the mitigation site's
classification (Section 3), baseline watershed and site assessment (Section 4), and the goals and
objectives for the project (Section 5):
Geomorphic
• What methods are proposed to address degradation of the stream's vertical and lateral stability?
• Will channel reconfiguration (planform, slope, and cross-sectional geometry) be required? Has
sufficient justification for it been included by the provider?
o If so, has the provider supplied adequate rationale for using this method?
o Have the risks associated with this method been evaluated?
• Will bed materials be replaced? If so:
o With what will they be replaced?
o What is the rationale for, and the risks associated with, using this method?
• Will streambanks require protection? If so:
o How will they be protected?
o What is the rationale for the methods that will be used?
o Have the risks associated with the methods chosen been considered?
• Will instream structures be used?
o For what purpose?
o Have the risks associated with how the structures will affect stream processes been
considered?
• Were other less physically disturbing methods to the ecosystem evaluated and justifiably determined
inappropriate for the project?
Phvsicochemical
• Will the methods chosen for other aspects of the project (e.g., hydrology, hydraulics, geomorphology)
potentially affect the water quality of the project reach and/or upstream or downstream reaches?
• Are there specific actions proposed to ameliorate on-site or off-site sources of adverse water quality?
If so, are the actions/methods rational and likely to succeed?
Biological
• Riparian Vegetation
o Will planting riparian vegetation be necessary? If so:
¦ Is species composition and spatial placement (with regards to hydroregime)
appropriate when compared to appropriate reference?
¦ Are planting methods and timing appropriate?
¦ Are the appropriate species planted in appropriate zones?
• Instream Habitat:
o Will large wood or instream structures be required for habitat?
o Have the risks and effects on stream processes associated with creating habitat with
structures, been evaluated?
o Have sufficient intact populations of biota been identified near the project area?
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Section 7.0 (Box I): Recommended information to address performance standards and monitoring.
Performance Standards
• Does each performance standard tie to a specific project objective?
• Are performance standards provided for the suite of functional indicators (see below) tied to project
objectives?
• Is each performance standard informed by appropriate reference standard comparisons?
• Is each performance standard described using clear and concise wording?
• Is each performance standard scientifically defensible (e.g., sensitive to changes in function)?
• Is each performance standard supported by data collection methods that are appropriately timed,
repeatable and with clearly defined error rates?
Monitoring Plan
• Are data collection methods for each performance standard outlined in the monitoring plan?
• Have pre-project baseline data been collected for all measures outlined in the monitoring plan?
• Has the timing of data collection (e.g., frequency, duration, and seasonality) been determined?
• Has the location and scale of data collection (e.g., reach, riparian, and watershed) been determined?
• Is the sampling intensity (number and arrangement of locations/plots) appropriate for the chosen
parameter?
• Is the sampling of sufficient intensity and precision to detect deviations from performance standards
and indicate the need for adaptive management?
• Does the plan outline how data will be analyzed and reported?
Review of post-project monitoring
• Compare the as-builts to the design criteria outlined in the mitigation plan. Was the project built to
specifications in the plan? If not, why not?
• Do the results of monitoring show the site is on track to meet performance standards? If not, is
adaptive management needed?
Performance standards and monitoring expectations by functional category
Stream Hydrologic Functions:
• Measures that demonstrate the timing, rate of change (flashiness), duration and frequency of low,
high and design flows comparable to reference standard condition.
Stream Hydraulic Functions:
• Measures that demonstrate sediment transport in the channel and across the floodplain are as
designed and in an equilibrium condition.
Stream Geomorphic Functions:
• Measures that demonstrate that boundary conditions in the restored reach are as designed and
comparable to reference standard condition.
Stream Physicochemical Functions:
• Measures that demonstrate water quality (pH, DO, temperature, and nutrients) has not declined after
project from baseline condition.
• Measures that demonstrate water quality parameters are improving towards reference standard
condition.
Stream Biological Functions:
• Measures that demonstrate benthic macroinvertebrates and/or fish and riparian vegetation are
improving towards reference standard condition.
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Section 8.0 (Box J): Information to assess adequacy of wetland and stream mitigation project
management.
Maintenance Plan
• What are the aspects of the project that will need maintenance?
• Does the plan outline the types of maintenance activities, their timing and costs?
• Do these activities contribute to goal of site sustainability?
Adaptive Management Plan
• Does the plan outline how monitoring data will be analyzed to identify potential challenges?
• Does the plan outline how and when adaptive management measures will be planned and
implemented?
• If appropriate, have beaver been accounted for and incorporated into the site?
Financial Assurances
• Are specific mechanisms and dollar amounts specified?
• Are these sufficient to ensure successful completion of the project?
Long Term Management Plan
• Does the plan contribute to goal of site sustainability?
• Does the plan include any land uses that would be counterproductive to the goal of restoring a self-
sustaining ecosystem?
• Does the plan outline the long-term management needs and financing mechanisms?
Site Protection Mechanism
• What type of mechanism is proposed (e.g., conservation easement, restrictive covenant, other)?
• Will the mechanism protect the site in perpetuity?
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