Section 319
NUNPUINT SUURCE PROGRAM SUCCESS STORY
Removing Carp Restored the Big Wall Lake Ecosystem
Waterbody Improved
Common carp, a nuisance species, entered Iowa's Big Wall Lake
during the early 1990s. The fishes' feeding behavior continuously
stirred up bottom sediment and eliminated much of the lake's beneficial aquatic vegetation. The
resulting water turbidity blocked light and prevented the growth of new aquatic plants. As a result,
the Iowa Department of Natural Resources (DNR) added Big Wall Lake to the 2002 Clean Water Act
(CWA) section 303(d) list due to degraded aquatic habitat caused by the presence of common carp. To
address the problem, local and state partners installed a new outlet structure to draw down the lake's
water level, and then treated the lake with rotenone. This eliminated the carp population and allowed
preferred types of submersed and emergent aquatic vegetation to become re-established, which led to
an overall improvement in water quality. After the renovation, DNR re-assessed the lake and found that
it fully supported its overall designated use. As a result, DNR removed Big Wall Lake from the state's
list of impaired waters in 2012.
Problem
Big Wall Lake is a 978-acre shallow glacial lake
in southeastern Wright County, in north-central
Iowa (Figure 1). At its deepest, the lake is only
about 6 feet deep, with an average depth of less
than 2 feet. Surrounding the lake is a nearly level,
1,205-acre agricultural watershed.
In 1974 the lake level was raised 6 inches by an
agreement between the DNR, the Wright County
Board of Supervisors and private landowners. The
goal was to maintain an artificially high water level
that would provide additional surface area and a
deeper pool for boaters. However, the agreement
hindered the ability of the DNR to lower the water
level as part of proper shallow lake management.
In 1993 a large population of common carp moved
into the lake during high-water events. When feed-
ing, the carp uprooted and eliminated the lake's
existing submerged aquatic vegetation (SAV) and
continuously stirred up bottom sediment, which
caused turbidity that blocked light and prevented
the growth of new plants on the lake bed. Because
the loss of SAV can degrade habitat and allow
undesirable aquatic species to dominate the
ecosystem, the suppression of SAV constitutes a
violation of Iowa's narrative water quality criteria
protecting against undesirable or nuisance aquatic
life. A site assessment in October 2000 by staff
from DNR and U.S. Environmental Protection
Big Wall Lake
IA
Figure 1. Big Wall Lake is in north-central Iowa.
Agency (EPA) Region VII showed that the lake was
impaired bythedamage caused by the common
carp. As a result, DNR added the lake (segment IA
02-IOW-00860-L _ 0) to the state's list of impaired
waters in 2002.
Once introduced into Big Wall Lake, the carp popu-
lation thrived because the lake level was maintained
at a higher and more stable water level, based on
the 1974 agreement. The local DNR wildlife biolo-
gist recommended that, to restore the lake's water
quality, the lake's outlet would have to be rebuilt in
a fashion that would enable DNR to manipulate the
water level to dewater the lake, thus killing the carp
population and allowing re-establishment of the
beneficial in-lake vegetation.
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Project Highlights
Partners collaborated with the local drainage district
and agreed upon a lake renovation plan that replaced
the 1974 lake management agreement. Using an EPA
CWA section 319 grant and state funds, the DNR
and the Wright County Soil and Water Conservation
District (SWCD) worked together to redesign the
existing Big Wall Lake dam to include a new outlet
in 2006. A stoplog structure was installed to enable
the DNR to manipulate the lake's water level. Next,
the DNR conducted a complete drawdown of the
lake from the fall of 2006 through the spring of 2007.
In June and August 2007 DNR applied rotenone to
eradicate the remaining carp in the lake.
Results
A wetland assessment guidelines document
released in 2003 by the Upper Mississippi River
Conservation Committee's (UMRCC's) Water
Quality Technical Section notes that an average
total suspended solids (TSS) concentration during
the growing season of less than 30 milligrams per
liter (mg/L) is necessary to provide sufficient water
clarity to support the growth of beneficial SAV in
wetlands and shallow lakes.
The results of water quality monitoring at Big Wall
Lake during the post-project period (from 2008
to present) show that the average levels of TSS
during the growing season have been 10 mg/L or
less—well belowthe UMRCC guideline of 30 mg/L
necessary to protect growth of beneficial SAV. The
removal of common carp and the return of SAV have
allowed the water clarity to improve; as a result, the
shallow lake system now meets the narrative water
quality criteria and fully supports its designated
aquatic life use. DNR removed Big Wall Lake from
the state's list of impaired waters in 2012.
Big Wall Lake is now populated with beneficial wet-
land vegetation, including SAV as well as emergent
vegetation such as softstem bulrush, smartweed,
arrowhead, pond lily and rice cutgrass (Figure 2).
In addition to improving water quality, the renova-
tion improved waterfowl habitat. During the spring
migration of 2008, roughly 10,000 waterfowl used
the lake during the northward spring migration. To
ensure that invasive species such as common carp
do not degrade water quality in the future, the DNR
will continue to manipulate the lake's water levels
as needed.
Figure 2. Before the project, rooting invasive carp had
eliminated the lake's vegetation and continuously
stirred up bottom sediment (top). After carp removal,
the lake's natural aquatic vegetation returned
(bottom), restoring Big Wall Lake's ecosystem.
Partners and Funding
The Wright County SWCD received a $10,000
Watershed Development Grant from the Iowa
Department of Agriculture and Land Stewardship
to evaluate all potential causes of watershed
degradation of Big Wall Lake. The SWCD then
collaborated with local landowners and lake users,
the local DNR wildlife biologist, and the Wright
County Supervisors (representing the local drain-
age district), to agree upon a lake renovation plan.
In 2005 the SWCD was awarded an EPA CWA
section 319 grant through the DNR Watershed
Improvement Section to fund 75 percent of project
costs ($120,070). The DNR Wildlife Bureau, the
entity responsible managing the lake, contributed
the additional funding to cover the remaining
25 percent of the project's costs ($38,508).
U.S. Environmental Protection Agency
Office of Water
Washington, DC
EPA841-F-14-001LLL
November 2014
For additional information contact:
Doug Janke, Iowa Department of Natural Resources
641-330-0241 • Doug.janke@dnr.iowa.gov
Kyle Ament, Iowa Department of Natural Resources
515-242-6196 • Kyle.Ament@dnr.iowa.gov
Connie Roys, Wright County Soil and Water
Conservation District
515-532-2165 • connie.roys@ia.usda.gov
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