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In-Lake Management Improves McMabon Lake Water Quality
Waterbody Improved McMa'"lon Lake is used for recreation (canoeing and fishing) and
wildlife habitat. In 2002 the lake was listed on the Clean Water
Act (CWA) section 303(d) list as impaired for exceeding Minnesota's water eutrophication criteria.
To address this impairment, a total maximum daily load (TMDL) study was completed in 2012. This
study called for an 81 percent reduction in phosphorus loads, predominantly from sources internal
to the lake (decaying curlyleaf pondweed and sediment release). Water quality improved, thanks to
the harvesting of curlyleaf pondweed and completion of multiple restoration projects. As a result,
the lake has been proposed for removal from the impaired waters list in 2018.
Problem
McMahon Lake (MN70-0050), also known locally
as Carl's Lake, is a 130-acre lake in Scott County,
Minnesota, just southwest of Minneapolis (Figure 1).
The lake is in the North Central Hardwood Forests
(NCHF) ecoregion and is defined as a shallow lake, with
a mean depth of 8.5 feet and a maximum depth of 14
feet. McMahon Lake is used primarily for canoeing and
fishing, but also provides modest wildlife habitat. The
552-acre McMahon Lake watershed (including the lake
surface area) has varied land uses, including forest,
agriculture and rural residential areas.
The lake was listed as impaired in 2002 for exceeding
eutrophication criteria. The shallow lake standards for
the NCHF are growing season averages of 1-meter (m)
Secchi disk depth and a phosphorus level of no more
than 60 micrograms per liter (|ig/L). A 2012 TMDL
study was conducted using data from a 10-year period
(1999-2008). The TMDL confirmed impairment. Data
showed averages of 85 ug/L total phosphorus, 70 |ig/L
chlorophyll a and 0.9 m Secchi disk depth during the
growing season.
The TMDL called for an overall phosphorus loading
reduction of 81 percent and identified internal loading
and watershed runoff to be the major source of excess
phosphorus. The internal ioading was determined to
be primarily from decaying curlyeaf pondweed and
sediment release. External loading (i.e., runoff) was
identified as another source of phosphorus, albeit to a
much lesser extent.
Figure 1. Shoreline stabilization projects helped
improve water quality in McMahon Lake.
Story Highlights
Over the past 28 years, the land usage around this lake
has changed from row crop agriculture to large-lot
rural residential. Approximate 80 to 90 percent of the
watershed is now covered with perennial vegetation
(Figure 2). Several homes were added, wetlands were
restored, and areas that were likely pastured in the
past have been allowed to become forested.
The increased rainfall this area has experienced has
fallen onto a stable, perennially vegetated watershed

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Figure 2. A comparison of aerial views of McMahon Lake in 1980 (left) and 2018 (right) show an
increase in perennial vegetation land cover.
with cleaner surface runoff combined with some
dilution by the increased direct precipitation.
This story is an example of collaboration between
private landowners and local units of government.
Actions that contributed to declines in phosphorus
levels include both in-lake management and external
loading reduction by the Scott Water Management
Organization (WMO) and Scott County Soil and Water
Conservation District (SWCD).
The story of the restoration of McMahon Lake centers
on working with landowners, networking, and the
power of peers. During the development of the TMDI
Scott County encountered difficulty in bringing the
landowners to the table to address the problems of
McMahon Lake. A local group had taken a proactive
stance. The local group offered to host a meeting of
the lakeshore owners, leading to a successful work-
ing relationship. The first meeting, held at around a
card table in the back of a local establishment, led
to the first landowner making the connection to the
Scott SWCD. The landowner's support and zeal for the
project led to another 17 acres of cropland conversion.
The power of the local connections and leadership
from the community has led to powerful actions.
Restoration efforts included the harvesting of curlyleaf
pondweed, a 20-acre native grass planting project
near the north shore adjacent to the lakeshore, two
shoreline stabilization projects, and one shoreline
restoration project (see Figure 1).
Results
Review of recent data (2010-2013) indicates grow-
ing season averages of 55 [ig/L total phosphorus, 28
|ig/L chlorophyll a and 1.2 m Secchi disk depth. With
phosphorus levels meeting the standard of no more
than 60 |_ig/L and at least one of the response variables
(chlorophyll a or Secchi disk) meeting its criteria, the
lake is proposed for delisting from the draft 2018 CWA
list of impaired waters.
In-lake water quality monitoring will continue to
evaluate trends over time and to inform the need for
modifying in-lake management. Also, opportunities for
additional watershed runoff control will continue to be
explored.
Partners and Funding
This restoration was achieved through the network-
ing and support of the Scott WMO, the Scott County
SWCD, the New Market Sportsman's Club, and the
landowners around McMahon Lake. Funding included
a portion of a $2.2 million state Clean Water Fund
grant provided to Scott WMO for the Sand Creek
watershed. The work done shows how efforts between
both public and private organizations can result in
actions to restore water quality in an impaired lake
and the importance of building these relationships.
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©
U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA 841-F-19-Q01A
January 2019
For additional information contact:
Chris Zadak
Minnesota Pollution Control Agency
651-757-2837 • chris.zadak@state.mn.us

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