Wisconsin Lake Water Quality
Water quality grades for 158 monitored lakes across 50 counties.
Only about 8% of North Dakota's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 33% 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. Benson ranks highest on average county grade (B). Ransom sits lowest. With only 50 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 93 reservoirs →93 of 158 Wisconsin lakes (59%) 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 1890.
EPA-Impaired Lakes in Wisconsin
View all 41 impaired lakes →41 of 158 Wisconsin lakes (26%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 20 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity | Max Depth |
|---|---|---|---|---|---|
| 1 | Wood Lake | Benson | A | - | - |
| 2 | Free Peoples Lake | Benson | A | - | - |
| 3 | Fort Totten Bay Lake | Benson | A | - | - |
| 4 | Arnegard Dam Lake | McKenzie | A | - | - |
| 5 | Epping-Springbrook Dam-Littoral Lake | Williams | A | - | - |
| 6 | Jarvis Beach Lake | Rolette | A | - | - |
| 7 | Garlk1390a Lake | Mercer | A | 13 ft | - |
| 8 | Spiritwood Lake | Stutsman | A | 31 ft | - |
| 9 | Lake Upsilon | Rolette | A | 11.2 ft | - |
| 10 | Twin Lakes | LaMoure | A | 14.1 ft | - |
| 11 | Round Lake | McHenry | A | 12.1 ft | - |
| 12 | Qacmainstfbk Lake | Mercer | A | - | - |
| 13 | Garlk1412dw Lake | Mercer | B | 10.4 ft | - |
| 14 | Garlk1445dw Lake | McLean | B | 11.7 ft | - |
| 15 | Velva Sportsmans Dam Lake | Ward | B | 13.6 ft | - |
| 16 | Davis Dam Lake | Slope | B | 7.5 ft | - |
| 17 | Arroda Lake | Oliver | B | 6.7 ft | - |
| 18 | Kota Ray Dam Lake | Williams | B | 7.7 ft | - |
| 19 | Skjermo Lake | Divide | B | 7.7 ft | - |
| 20 | Hooker Lake | Rolette | B | 12.2 ft | - |
| 21 | Whitman Dam Lake | Nelson | B | 12 ft | - |
| 22 | Qacmainstdup Lake | Mercer | B | - | - |
| 23 | Garnflmor1 Lake | Dunn | B | - | - |
| 24 | Patterson Lake | Stark | B | - | - |
| 25 | Garlk1481dw Lake | McKenzie | B | 5.9 ft | - |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Harmon Lake | Morton | F | - |
| 2 | Niagara Dam Lake | Grand Forks | F | 1 ft |
| 3 | Kolding Dam Lake | Grand Forks | F | 1.7 ft |
| 4 | Mirror Lake | Adams | F | 2.1 ft |
| 5 | Cedar Lake | Slope | F | 1.3 ft |
| 6 | Belcourt Lake Beach | Rolette | F | 1 ft |
| 7 | Sweet Briar Dam-Mid Reservoir | Morton | F | 3 ft |
| 8 | Sweetbriar Dam Lake | Morton | F | 3 ft |
| 9 | Gordon Beach Lake | Rolette | F | 1 ft |
| 10 | Kalmbach Lake | LaMoure | F | 0.7 ft |
| 11 | Juanita Lake | Foster | F | 1 ft |
| 12 | Dead Colt Creek Dam Lake | Ransom | F | 3.3 ft |
| 13 | Sprague Lake | Sargent | F | 1.6 ft |
| 14 | Buffalo Lake | Pierce | F | 0.3 ft |
| 15 | Mchugh Slough Lake | Nelson | F | 1.6 ft |
| 16 | Morrison Lake | Ramsey | F | 1.9 ft |
| 17 | Eastern Stump Lake | Nelson | F | 4.7 ft |
| 18 | Western Stump Lake | Nelson | F | 4.3 ft |
| 19 | J. Clark Salyer Pool 357 | Bottineau | F | 2.2 ft |
| 20 | Tioga Dam Lake | Williams | D | 2.2 ft |
| 21 | Odland Dam Lake | Golden Valley | D | 1.5 ft |
| 22 | Froelich Dam Lake | Sioux | D | 3.9 ft |
| 23 | Nelson Lake | Oliver | D | 1.3 ft |
| 24 | Nygren Dam Lake | Morton | D | 2.6 ft |
| 25 | Warsing Dam Lake | Eddy | D | 2.3 ft |
Source: EPA National Aquatic Resource Surveys, 2026.