Wisconsin Lake Water Quality
Water quality grades for 152 monitored lakes across 45 counties.
Only about 5% of Illinois's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 72% of lakes fall in the D or F brackets.
The state's lakes lean eutrophic: enough nutrient input to support an active algal community, with the trade-offs in clarity and dissolved oxygen that come with it. Winnebago ranks highest on average county grade (A). Sangamon sits lowest. With only 45 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Lake Map
Reservoirs in Wisconsin
View all 74 reservoirs →74 of 152 Wisconsin lakes (49%) are man-made reservoirs in the USACE National Inventory of Dams. 55 carry a High or Significant hazard rating. The oldest matched dam dates to 1885.
EPA-Impaired Lakes in Wisconsin
View all 118 impaired lakes →118 of 152 Wisconsin lakes (78%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 57 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity | Max Depth |
|---|---|---|---|---|---|
| 1 | Diversey Harbor Lake | Cook | A | 25.6 ft | - |
| 2 | Loch Lomond Lake | Lake | A | 16.5 ft | - |
| 3 | Csmi 2015 Station Eg Lake | Lake | A | - | - |
| 4 | Csmi 2015 Station S Lake | Cook | A | - | - |
| 5 | Axehead Lake | Cook | A | 11.9 ft | - |
| 6 | Powderhorn Lake | Cook | A | 10.1 ft | - |
| 7 | Baumann Park Lake | Winnebago | A | 10.2 ft | - |
| 8 | Silver Lake | McHenry | B | 7 ft | - |
| 9 | Silver Lake | McHenry | B | 10 ft | - |
| 10 | Timber Lake | Lake | B | 10.6 ft | - |
| 11 | Boulder South Lake | Perry | B | 8.8 ft | - |
| 12 | Wampum Lake | Cook | B | 6.9 ft | - |
| 13 | Timber Lake | Carroll | B | 11.5 ft | - |
| 14 | Big Rock Lake | Kane | B | 8.1 ft | - |
| 15 | Round Lake | Lake | B | 7.4 ft | - |
| 16 | Kinkaid Lake | Jackson | B | - | - |
| 17 | Lake Calumet | Cook | B | - | - |
| 18 | One Horse Gap Lake | Pope | B | 4 ft | - |
| 19 | Flatfoot Lake | Cook | B | 6.3 ft | - |
| 20 | Opeka Lake | Cook | B | 3.9 ft | - |
| 21 | Glendale Lake | Pope | B | 4.8 ft | - |
| 22 | Tecumseh Lake | Hardin | B | 3.6 ft | - |
| 23 | Grays Lake | Lake | B | 3.5 ft | - |
| 24 | Calumet Lake | Cook | B | 4.3 ft | - |
| 25 | Lily Lake | McHenry | B | 3.3 ft | - |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Boujan Swale Lake | Cass | F | - |
| 2 | Quiver Lake | Mason | F | - |
| 3 | Mauvaise Terre Lake | Morgan | F | - |
| 4 | Vermont City Lake | McDonough | F | - |
| 5 | Paradise Lake | Coles | F | - |
| 6 | Paradise Lake | Pike | F | - |
| 7 | Coulterville Lake | Randolph | F | 2.7 ft |
| 8 | Kinmundy Old Lake | Marion | F | 1.5 ft |
| 9 | Borah Lake | Richland | F | 2.2 ft |
| 10 | Highland Silver Lake | Madison | F | 1.3 ft |
| 11 | Nashville City Lake | Washington | F | 1.3 ft |
| 12 | Governor Bond Lake | Bond | F | 1.2 ft |
| 13 | Lou Yaeger Lake | Montgomery | F | 1.8 ft |
| 14 | Decatur Lake | Macon | F | 1.3 ft |
| 15 | Mattoon Lake | Cumberland | F | 1.3 ft |
| 16 | Glenn Shoals Lake | Montgomery | F | 1 ft |
| 17 | Fairfield Lake | Wayne | F | 1.3 ft |
| 18 | Slm Side Channel Lake | St. Clair | F | 0.8 ft |
| 19 | Carlyle Lake | Clinton | F | 1.8 ft |
| 20 | Raccoon Lake | Marion | F | 1.2 ft |
| 21 | Sam Dale Lake | Wayne | F | 1.2 ft |
| 22 | Skokie Lagoons Lake | Cook | F | 1.6 ft |
| 23 | Holiday Shores Lake | Madison | F | 1.7 ft |
| 24 | Vandalia Lake | Fayette | F | 1.3 ft |
| 25 | Saganashkee Slough Lake | Cook | F | 0.8 ft |
Source: EPA National Aquatic Resource Surveys, 2026.