Impaired Lakes in Wisconsin
5 of 25 graded Wisconsin lakes (20%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 4 have an approved TMDL restoration plan.
Top Causes of Impairment in Wisconsin
All 5 EPA-Listed Impaired Lakes in Wisconsin
| Lake | County | Grade | Causes | TMDL |
|---|---|---|---|---|
| Lake Vesuvius | Lawrence | C | Metals (other than mercury), pH / acidity, Sediment / turbidity | — |
| Caesar Creek Lake | Warren | D | Algal growth, Other cause | ✓ |
| Tappan Lake | Harrison | D | Algal growth | ✓ |
| Buck Creek Lake | Clark | D | Habitat alterations, Hydrologic alteration | ✓ |
| Tappan Lake | Harrison | F | Algal growth | ✓ |
Frequently Asked Questions
How many lakes are impaired in Wisconsin?
5 of 25 graded Wisconsin lakes (20%) are listed as impaired in the most recent EPA ATTAINS reporting cycle. Of those, 4 are on the federal Clean Water Act §303(d) list and 4 have an approved Total Maximum Daily Load (TMDL) restoration plan.
What does Clean Water Act §303(d) mean?
Section 303(d) of the federal Clean Water Act requires every state to identify waters that don't meet water quality standards even after pollution control measures. Each listed water must then have a Total Maximum Daily Load (TMDL) developed — a plan that caps how much of each pollutant can enter the waterbody. The "303(d) list" is the impaired-waters list. The EPA reviews each state's list every two years.
What are the top causes of impairment in Wisconsin?
The most common causes flagged across Wisconsin's impaired lakes are: Algal growth (3 lakes); Other cause (1 lakes); Habitat alterations (1 lakes); Hydrologic alteration (1 lakes); Metals (other than mercury) (1 lakes); pH / acidity (1 lakes). Mercury is widespread in fish tissue across the upper Midwest from atmospheric deposition. Nutrient impairment (phosphorus, nitrogen) typically reflects agricultural runoff and urban stormwater.
Can I swim in an impaired lake?
It depends on the cause of impairment. A lake impaired for "mercury in fish tissue" is generally safe to swim in — the warning is about long-term consumption of fish. A lake impaired for "pathogens" (E. coli) or "algal growth" may pose a swimming risk and often has a separate beach advisory. Check the cause categories listed for each lake below, and consult your state health department's beach monitoring program for current advisories.
Where does this impairment data come from?
Every impairment record on this page comes from the EPA's ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset, the federal repository of state-reported water quality assessments. EPA ATTAINS is published as GeoParquet files on the EPA Office of Water S3 bucket and refreshed several times per year. Causes and use-support codes follow EPA's standard taxonomy.
Data source
Impairment data from the EPA ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset. Assessment unit IDs are matched to LakeGrade waterbodies by name and proximity; some lakes may not appear here if their ATTAINS name doesn't match our naming convention. For an official query, consult Wisconsin's state water-quality agency.