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NONPOINT SOURCE SUCCESS STORY
Ore-aon
Managing Stormwater and Restoring Streams and Riparian Areas
Improved Columbia Slough
U/qtprhnHv lmnrn\/prl ln t^ie mic'-1990s, water samples showed that the Columbia
Slough exceeded the applicable bacteria water quality standard,
prompting the Oregon Department of Environmental Quality (DEQ) to add the slough to the 1998
Clean Water Act (CWA) section 303(d) list of impaired waters for failing to support its recreational
designated use. Stakeholders implemented riparian restoration and revegetation projects to slow
and filter water, added green infrastructure practices to reduce the volume of stormwater, educated
residents to increase acceptance of environmental practices, and installed a "Big Pipe" project to
reduce combined sewer overflows (CSOs). Bacteria levels dropped and now meet water quality
standards, prompting DEQ to remove an 8-mile segment of the Columbia Slough from the impaired
waters list for bacteria in the 2018/2020 Integrated Report.
Problem
The Columbia Slough is a 19-mile-long complex of
narrow, shallow channels located on the southern
floodplain of the Columbia River between Fairview Lake
and the Willamette River (Figure 1). The slough was
originally a series of wetlands and marshes; it is now
a highly managed system with dikes and pumps that
provide drainage and flood control for the lowlands
surrounding It. The slough's 32,700-acre watershed
is 54% impervious and is home to Industries, trans-
portation corridors, residential areas, historic Native
American settlement sites, the Portland International
Airport, and many golf courses and vegetable farms.
In the mid-1990s, the Columbia Slough routinely
exceeded the applicable bacteria water quality
standard, prompting DEQ to add it to the 1998 CWA
section 303(d) list of impaired waters for failing to
support its recreational designated use. DEQ devel-
oped a total maximum daily load (TMDL) for bacteria
and other pollutants in 1998. Columbia Slough was
originally listed as impaired for fecal coliform bacteria,
but DEQdeveloped the TMDL for the Columbia Slough
for Escherichia coli to meet the updated contact
recreation standard adopted in 1996. TMDL modeling
efforts indicated that the primary sources of bacteria
were combined sewer overflows (CSOs) in the lower
slough, illicit connections and failing septic systems in
the middle slough, and stormwater in the upper slough.
Figure 1. Columbia Slough is iri northwest Oregon.
Story Highlights
In 1994, the City of Portland Bureau of Environmental
Services (BES) began restoring and revegetatirig the
canopy and understory on 15- to 150-foot riparian
areas along the banks of the slough. BES partnered
with several stakeholders on the effort, including the
Multnomah County Drainage District (MCDD), which
changed its operating procedures to allow stream-
banks to be planted rather than mowed. BES devel-
oped partnership agreements with landowners along
slough waterways to allow restoration projects to
take place. BES has also purchased and restored some
Legend
	Columbia Slough delisted reaches '
I I Columbia Slough Watershed

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larger parcels that support significant wetland areas or
long slough frontage. Over the years, BES has added
1.4 million native trees and shrubs across 1,100 acres
of natural areas in the slough watershed. In addition,
the City has applied environmental conservation zones
on sensitive ecological resources throughout the city
to protect wetlands and riparian buffers.
For the past two decades, the City's downspout
disconnection program has removed stormwater from
the combined sewer system, thereby reducing CSOs
into local waters. This program also helped to educate
community members. Portland has also added many
green infrastructure practices (e.g., swales, planters,
rain gardens) to infiltrate stormwater onsite. In 2018,
the Columbia Slough Watershed Council worked with
willing homeowners to install eight rain gardens in
the Cully Neighborhood in northeast Portland. Youth
workforce crews helped install the gardens, which
captured runoff from 5,950 square feet of impervious
surface (approximately 133,527 gallons annually).
These projects reduce the amount of stormwater
runoff flowing into the city's combined sewer system;
these work in combination with the City's Big Pipe
project, which was completed in 2011. In the Columbia
Slough portion of the Big Pipe project, a 3.5-mile-long
underground pipe collects excess volume from the
combined sewers and stores it until it can be trans-
ported to the wastewater treatment plant, thereby
reducing the volume of CSOs reaching the slough's
waters by 99 percent.
The Port of Portland, which manages three airports,
four marine terminals and five business parks,
supports watershed protection efforts through
multiple programs. The Airport Futures Natural
Resource Enhancement (AFNRE) Program and the
Port Mitigation Management Program both support
watershed enhancement projects (e.g., healthy habi-
tat, riparian areas, wetlands). In 2000-2004, the Port
used resources from its Port Revegetation Program
to revegetate 73.5 acres along the slough, which it
continues to monitor and maintain. Numerous river-
bank vegetation and stabilization projects, conducted
through the Port's Riverbank Management Program,
are coordinated and implemented by the Port's natural
resources staff, Port landscape maintenance staff and
volunteers.
E. Coli Pooled Sample Results in Delisted Segment of the Columbia Slough
(2008 - 2017)
Single Sample 	Single Sample Max Criterion (406)	Geomean	Geomean Criterion (126)
Figure 2. Data show that bacteria levels remained
below both the single sample criterion (dotted gray
line) and the geomean criterion (dotted green line).
Results
The riparian restoration, stormwater control and CSO
reduction measures have led to reduced bacteria
levels in Columbia Slough. The applicable freshwater
contact recreation standard requires that samples
not exceed a 90-day geometric mean of 126 E. coli
organisms per 100 milliliters (mL), based on a mini-
mum of five samples. No single sample may exceed
406 E. coli organisms per 100 mL. Data collected in
2008-2017 indicate that the Columbia Slough has
consistently met both parts of the water quality
standard (Figure 2). As a result, DEQ removed the
bacteria impairment from an 8-mile segment (OR_
WS_170900120201_02_104554.1) on the Columbia
Slough in DEQ's 2018/2020 Integrated Report.
Partners and Funding
Partners include the City of Portland's planning
division and BES; Verde, a local nonprofit group
supporting low-income communities; MCDD, Port of
Portland; local homeowners and youth groups; and the
Columbia Slough Watershed Council (a diverse group
of neighbors, property owners, businesses, environ-
mental groups, recreation advocates and government
agencies). The Council received a CWA section 319
grant in 2018 ($10,000 plus a $7,978 nonfederal
match) to implement the Cully Neighborhood rain
garden project. The City of Portland has invested
$1.4 billion into the Big Pipe project over the past 20
years and approximately $500,000 in State Revolving
Loans for revegetation work. The Port of Portland's
AFNRE Program dedicates $50,000 every year to fund
Columbia Slough watershed projects.
^£DSrX
PRO^°
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U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-21-001Q
September 2021
For additional information contact:
Andrea Matzke
Oregon Department of Environmental Quality
503-229-5350 • andrea.matzke@deq.state.or.us

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