Wisconsin Lake Water Quality
Water quality grades for 25 monitored lakes across 13 counties.
Only about 8% of Ohio's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 60% 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. Lake ranks highest on average county grade (A). Ashland sits lowest. With only 13 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 10 reservoirs →10 of 25 Wisconsin lakes (40%) are man-made reservoirs in the USACE National Inventory of Dams. 10 carry a High or Significant hazard rating. The oldest matched dam dates to 1936.
EPA-Impaired Lakes in Wisconsin
View all 5 impaired lakes →5 of 25 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 Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity | Max Depth |
|---|---|---|---|---|---|
| 1 | Er32 Lake | Lake | A | 20.5 ft | - |
| 2 | Er78m Lake | Lake | A | 16.4 ft | - |
| 3 | Er36 Lake | Lake | B | 9 ft | - |
| 4 | Er73 Lake | Cuyahoga | B | 7 ft | - |
| 5 | Caesar Creek Lake | Warren | C | 3.8 ft | - |
| 6 | Caesar Creek Lake | Warren | C | 3.5 ft | - |
| 7 | Er60 Lake | Lucas | C | 9.8 ft | - |
| 8 | Er58 Lake | Ottawa | C | 7.4 ft | - |
| 9 | Lake Vesuvius | Lawrence | C | 8.2 ft | - |
| 10 | Er43 Lake | Cuyahoga | C | 6.6 ft | - |
| 11 | Caesar Creek Lake | Warren | D | 3.2 ft | - |
| 12 | Caesar Creek Lake | Warren | D | 3 ft | - |
| 13 | Leesville Lake | Carroll | D | 5 ft | - |
| 14 | Sites Lake | Richland | D | 5.2 ft | - |
| 15 | Er59 Lake | Lucas | D | 6.6 ft | - |
| 16 | Tappan Lake | Harrison | D | - | - |
| 17 | Buck Creek Lake | Clark | D | 2.5 ft | - |
| 18 | Buck Creek Lake | Clark | D | 2.3 ft | - |
| 19 | Buck Creek Lake | Clark | D | 2.1 ft | - |
| 20 | Buck Creek Lake | Clark | D | 2.5 ft | - |
| 21 | Charles Mill Reservoir | Ashland | F | 0.9 ft | - |
| 22 | Seneca Lake | Noble | F | 1.7 ft | - |
| 23 | Tappan Lake | Harrison | F | 1.8 ft | - |
| 24 | Clear Fork Reservoir | Morrow | F | 3.1 ft | - |
| 25 | Pleasant Hill Reservoir | Richland | F | 3 ft | - |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Pleasant Hill Reservoir | Richland | F | 3 ft |
| 2 | Clear Fork Reservoir | Morrow | F | 3.1 ft |
| 3 | Tappan Lake | Harrison | F | 1.8 ft |
| 4 | Seneca Lake | Noble | F | 1.7 ft |
| 5 | Charles Mill Reservoir | Ashland | F | 0.9 ft |
| 6 | Buck Creek Lake | Clark | D | 2.5 ft |
| 7 | Buck Creek Lake | Clark | D | 2.1 ft |
| 8 | Buck Creek Lake | Clark | D | 2.3 ft |
| 9 | Buck Creek Lake | Clark | D | 2.5 ft |
| 10 | Tappan Lake | Harrison | D | - |
| 11 | Er59 Lake | Lucas | D | 6.6 ft |
| 12 | Sites Lake | Richland | D | 5.2 ft |
| 13 | Leesville Lake | Carroll | D | 5 ft |
| 14 | Caesar Creek Lake | Warren | D | 3 ft |
| 15 | Caesar Creek Lake | Warren | D | 3.2 ft |
| 16 | Er43 Lake | Cuyahoga | C | 6.6 ft |
| 17 | Lake Vesuvius | Lawrence | C | 8.2 ft |
| 18 | Er58 Lake | Ottawa | C | 7.4 ft |
| 19 | Er60 Lake | Lucas | C | 9.8 ft |
| 20 | Caesar Creek Lake | Warren | C | 3.5 ft |
| 21 | Caesar Creek Lake | Warren | C | 3.8 ft |
| 22 | Er73 Lake | Cuyahoga | B | 7 ft |
| 23 | Er36 Lake | Lake | B | 9 ft |
| 24 | Er78m Lake | Lake | A | 16.4 ft |
| 25 | Er32 Lake | Lake | A | 20.5 ft |
Source: EPA National Aquatic Resource Surveys, 2026.