Overview

A large shallow impoundment of the Rock River, Lake Koshkonong exhibits the same water quality problems as other impoundments in southern Wisconsin but on a much larger scale. The lake was originally a shallow and deep-water marsh; a dam constructed in 1850 created a shallow lake. The dam was rebuilt in 1917, raising the lake level an additional two feet. The lake is a major stop-over for migratory waterfowl and was renowned for waterfowl hunting. The drainage area above the lake encompasses 2,640 square miles.

Tremendous sediment and nutrient loads from the Rock River, Koshkonong Creek and other tributaries enter the lake annually. These loads are primarily from agricultural and some urban sources. In addition, shoreline erosion is severe on the lake. Failing septic systems were a problem in the past but the creation of a sanitary district around a portion of the lake has reduced the impact of this source on water quality. Carp are a widespread problem despite a large commercial carp fishery; the carp destroy habitat and increase the lake's existing level of turbidity. As a result, severe nuisance growth of algae and algae blooms are common, often resulting in fish kills.

The lake's shallow depth combined with its long fetch--the distance the wind blows across it uninterrupted-- causes serious shoreline erosion during major storms. All these conditions combined adversely affect water quality and the lake's fisheries.

The Indianford Dam, which controls the water level of the lake, is in need of repair. The allocation of repair costs among the counties and towns that border the lake has become a local political issue. One group of property owners wants a higher water level to improve boating while another group wants lower levels to protect and improve wetland habitat.

Date  2002

Author   Aquatic Biologist

General Condition

Being a large shallow impoundment of the Rock River, Lake Koshkonong exhibits the same water quality problems as other impoundments in southern Wisconsin but on a much larger scale. The drainage area above the lake encompasses 2,640 square miles. According to DNR water condition data, tremendous sediment and nutrient loads from the Rock River, Koshkonong Creek and other tributaries enter the lake annually. These loads are primarily from agricultural and some urban sources. The lake is included on the WDNR's 303(d) Impaired Waters list. Impairments include low dissolved oxygen, degraded habitat, and turbidity from total phosphorus and sediment/total suspended solids. The lake is included in the Rock River TMDL for phosphorus and sediment reductions. Failing septic systems were a problem in the past but the creation of the sanitary district around a portion of the lake has reduced the impact of this source on water quality.

According to DNR, carp are a widespread problem despite a large commercial carp fishery; the carp destroy habitat and increase the lake's existing level of turbidity. As a result, severe nuisance growth of algae and algae blooms are common, often resulting in fish kills. The lake's shallow depth combined with its long fetch (the distance the wind blows across it uninterrupted) causes serious shoreline erosion during major storms. All these conditions combined adversely affect water quality and the lake's fisheries.

Date  2017

Author  Mike Kakuska

Impaired Waters

Lake Koshkonong Lakeland Campground Beach was evaluated for bacteria in the 2022 cycle; E. coli levels were above listing thresholds as outlined in 2022 WisCALM. This beach was added to the 2022 Impaired Waters List.

Date  2022

Author  Ashley Beranek

Impaired Waters

The lake is included on the WDNR's 303(d) Impaired Waters list. Impairments include low dissolved oxygen, degraded habitat, and turbidity from total phosphorus and sediment/total suspended solids. The lake is included in the Rock River TMDL for phosphorus and sediment reductions. Failing septic systems were a problem in the past but the creation of the sanitary district around a portion of the lake has reduced the impact of this source on water quality.

Date  2017

Author  Mike Kakuska

Condition

Wisconsin has over 84,000 miles of streams, 15,000 lakes and milllions of acres of wetlands. Assessing the condition of this vast amount of water is challenging. The state's water monitoring program uses a media-based, cross-program approach to analyze water condition. An updated monitoring strategy (2015-2020) is now available. Compliance with Clean Water Act fishable, swimmable standards are located in the Executive Summary of Water Condition in 2018. See also the 'monitoring and projects' tab.

Reports

Recommendations

Management Goals

Wisconsin's Water Quality Standards provide qualitative and quantitative goals for waters that are protective of Fishable, Swimmable conditions [Learn more]. Waters that do not meet water quality standards are considered impaired and restoration actions are planned and carried out until the water is once again fishable and swimmable

Management goals can include creation or implementation of a Total Maximum Daily Load analysis, a Nine Key Element Plan, or other restoration work, education and outreach and more. If specific recommendations exist for this water, they will be displayed below online.

Monitoring

Monitoring the condition of a river, stream, or lake includes gathering physical, chemical, biological, and habitat data. Comprehensive studies often gather all these parameters in great detail, while lighter assessment events will involve sampling physical, chemical and biological data such as macroinvertebrates. Aquatic macroinvertebrates and fish communities integrate watershed or catchment condition, providing great insight into overall ecosystem health. Chemical and habitat parameters tell researchers more about human induced problems including contaminated runoff, point source dischargers, or habitat issues that foster or limit the potential of aquatic communities to thrive in a given area. Wisconsin's Water Monitoring Strategy was recenty updated.

Grants and Management Projects

Monitoring Projects

Watershed Characteristics

Lake Koshkonong is located in the Rock River - Milton watershed which is 48.76 mi². Land use in the watershed is primarily agricultural (49.70%), forest (16.40%) and a mix of suburban (13.20%) and other uses (20.70%). This watershed has 31.74 stream miles, 123.56 lake acres and 290.20 wetland acres.

Nonpoint Source Characteristics

This watershed is ranked Low for runoff impacts on streams, Not Ranked for runoff impacts on lakes and High for runoff impacts on groundwater and therefore has an overall rank of High. This value can be used in ranking the watershed or individual waterbodies for grant funding under state and county programs.However, all waters are affected by diffuse pollutant sources regardless of initial water quality. Applications for specific runoff projects under state or county grant programs may be pursued. For more information, go to surface water program grants.