Wisconsin Lake Water Quality
Water quality grades for 189 monitored lakes across 75 counties.
Only about 2% of Iowa's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 84% 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. The cleanest county on average is Floyd (grade B); the most-stressed is Pottawattamie. The gap between the top and bottom counties is large enough that "Iowa lake water quality" hides real local variation.
Lake Map
Reservoirs in Wisconsin
View all 133 reservoirs →133 of 189 Wisconsin lakes (70%) are man-made reservoirs in the USACE National Inventory of Dams. 98 carry a High or Significant hazard rating. The oldest matched dam dates to 1900.
EPA-Impaired Lakes in Wisconsin
View all 136 impaired lakes →136 of 189 Wisconsin lakes (72%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 97 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity | Max Depth |
|---|---|---|---|---|---|
| 1 | West Okoboji Lake | Dickinson | A | 15.7 ft | - |
| 2 | Blue Pit Lake | Cerro Gordo | A | 17.1 ft | - |
| 3 | Nine Eagles Lake | Decatur | A | 11 ft | - |
| 4 | Terra Park Lake | Polk | A | 11.5 ft | - |
| 5 | Dale Maffitt Reservoir | Dallas | B | 9.8 ft | - |
| 6 | Rudd Lake | Floyd | B | 8.1 ft | - |
| 7 | Slipbluff Lake | Decatur | B | 9 ft | - |
| 8 | Lacey Keosauqua Lake | Van Buren | B | 7.1 ft | - |
| 9 | Yenruogis Pond | Keokuk | B | 9.9 ft | - |
| 10 | Ada Hayden Heritage Park Lake | Story | B | 6.8 ft | - |
| 11 | Lambach Lake | Scott | B | 7.9 ft | - |
| 12 | Sand Lake | Marshall | B | 6.4 ft | - |
| 13 | Arrowhead Lake | Sac | C | 5.6 ft | - |
| 14 | Railroad Lake | Scott | C | 5.1 ft | - |
| 15 | Mariposa Lake | Jasper | C | 5.1 ft | - |
| 16 | Mormon Trail Lake | Adair | C | 6.2 ft | - |
| 17 | Willow Lake | Harrison | C | 4.8 ft | - |
| 18 | Lake Wapello | Davis | C | 7.5 ft | - |
| 19 | Oldham Lake | Monona | C | 5.1 ft | - |
| 20 | Little Sioux Park Lake | Woodbury | C | 5.7 ft | - |
| 21 | Green Castle Lake | Marshall | C | 4.6 ft | - |
| 22 | Poll Miller Park Lake | Lee | C | 5.9 ft | - |
| 23 | Kent Park Lake | Johnson | C | 4.8 ft | - |
| 24 | Upper Gar Lake | Dickinson | C | 3.9 ft | - |
| 25 | Big Spirit Lake | Dickinson | C | 5.5 ft | - |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Prairie Rose Station 50 Lake | Shelby | F | 0.7 ft |
| 2 | Prairie Rose Station 40 Lake | Shelby | F | 0.9 ft |
| 3 | Prairie Rose Station 70 Lake | Shelby | F | 3 ft |
| 4 | Prairie Rose Station 60 Lake | Shelby | F | 0.9 ft |
| 5 | Lpinelk 1 Lake | Hardin | F | 1.4 ft |
| 6 | Central Park Lake | Jones | F | 1.6 ft |
| 7 | Iowa Lk4 Lake | Iowa | F | 3.1 ft |
| 8 | Lpinelk 2 Lake | Hardin | F | 1.2 ft |
| 9 | Rkcrk 3 Lake | Jasper | F | 1 ft |
| 10 | Rkcrk 2 Lake | Jasper | F | 1.1 ft |
| 11 | Lpinelk 3 Lake | Hardin | F | 0.9 ft |
| 12 | Rkcrk 1 Lake | Jasper | F | 1 ft |
| 13 | Humeston Reservoir | Wayne | F | 1.6 ft |
| 14 | Rock Creek Beach Lake | Jasper | F | 1.1 ft |
| 15 | Corydon Reservoir | Wayne | F | 1.9 ft |
| 16 | Lake Fisher | Davis | F | 2 ft |
| 17 | Lpinelk T Lake | Hardin | F | 1.3 ft |
| 18 | Rkcrk 5 Lake | Jasper | F | 1.1 ft |
| 19 | Iowa Lk3 Lake | Iowa | F | 3.1 ft |
| 20 | Alice Wyth Lake | Black Hawk | F | 1.6 ft |
| 21 | Pine Lake South Beach | Hardin | F | 1.4 ft |
| 22 | Rkcrk 4 Lake | Jasper | F | 1.1 ft |
| 23 | Terry Trueblood Lake | Johnson | F | 1.1 ft |
| 24 | Volga Lake | Fayette | F | 2.1 ft |
| 25 | Lake Pahoja | Lyon | F | 1.7 ft |
Source: EPA National Aquatic Resource Surveys, 2026.