Wisconsin Lake Water Quality
Water quality grades for 1,405 monitored lakes across 69 counties.
Wisconsin has 1,405 graded lakes in our dataset, sampled across 69 counties. The Wisconsin DNR runs a Citizen Lake Monitoring Network that contributes a large share of the actual measurements. About 32% of Wisconsin's lakes pull an A — 446 top-bracket waters. That high A-share comes mostly from the cold, deep, kettle-formed lakes in the north of the state, which buffer themselves against nutrient inputs.
Mesotrophic is the dominant trophic class — a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. The cleanest county on average is Outagamie (grade A); the most-stressed is Calumet. The gap between the top and bottom counties is large enough that "Wisconsin lake water quality" hides real local variation.
Wisconsin Lake Water Quality Dataset (1,405 lakes)
Every graded Wisconsin lake in one spreadsheet — overall grade and 0–4 score, water clarity, phosphorus, chlorophyll-a, trophic state index, surface area, and max depth, drawn from the EPA Water Quality Portal and state DNR surveys. Enter your email for the CSV.
Lake Map
Explore Wisconsin Lakes
Reservoirs in Wisconsin
View all 204 reservoirs →204 of 1,405 Wisconsin lakes (15%) are man-made reservoirs in the USACE National Inventory of Dams. 89 carry a High or Significant hazard rating. The oldest matched dam dates to 1836.
EPA-Impaired Lakes in Wisconsin
View all 511 impaired lakes →511 of 1,405 Wisconsin lakes (36%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 75 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity | Max Depth |
|---|---|---|---|---|---|
| 1 | Geneva Lake | Walworth | A | 20.2 ft | 135 ft |
| 2 | Round Lake | Sawyer | A | 21.1 ft | 74 ft |
| 3 | Round Lake | Waupaca | A | 17.5 ft | 67 ft |
| 4 | Round Lake | Oconto | A | 16.8 ft | 31 ft |
| 5 | Big Bass Lake | Washburn | A | 17.9 ft | 27 ft |
| 6 | Deep Lake | Adams | A | 15.5 ft | 49 ft |
| 7 | Beasley Lake | Waupaca | A | 18.4 ft | 47 ft |
| 8 | Bass Lake | Washburn | A | 16 ft | 31 ft |
| 9 | Bass Lake | Oconto | A | 16.8 ft | 40 ft |
| 10 | Bass Lake | Oconto | A | - | 25 ft |
| 11 | Clear Lake | Sawyer | A | 17 ft | 32 ft |
| 12 | Clear Lake | Polk | A | 15.5 ft | 26 ft |
| 13 | Clear Lake | Lincoln | A | 18 ft | 45 ft |
| 14 | Clear Lake | Oneida | A | - | 95 ft |
| 15 | Explosion Lake | Oconto | A | 17 ft | 10 ft |
| 16 | Razorback Lake | Vilas | A | 15.2 ft | 35 ft |
| 17 | White Sand Lake Deep | Vilas | A | 18 ft | 63 ft |
| 18 | Sugar Camp Lake | Oneida | A | 16.5 ft | - |
| 19 | Little Spider Lake | Vilas | A | 16 ft | 23 ft |
| 20 | Rosalind Lake | Vilas | A | 19.3 ft | - |
| 21 | Lake Lucerne | Forest | A | 18.5 ft | 73 ft |
| 22 | Two Island Lake | Chippewa | A | 15.3 ft | 38 ft |
| 23 | Long Lake | Bayfield | A | 16.2 ft | 28 ft |
| 24 | Long Lake | Burnett | A | 21.5 ft | 41 ft |
| 25 | Lake Metonga | Forest | A | 22 ft | 79 ft |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Lower Kelly Lake | Waukesha | F | - |
| 2 | Grafton Millpond | Ozaukee | F | - |
| 3 | Giltners Lake | Fond du Lac | F | - |
| 4 | Thiensville Millpond | Ozaukee | F | - |
| 5 | Graber Pond | Dane | F | - |
| 6 | County K Marsh Lake | Green Lake | F | - |
| 7 | Silver Creek Estuary Lake | Green Lake | F | - |
| 8 | Little Yellow Lake | Burnett | F | - |
| 9 | Hawthorne Hills South Pond | Dane | F | - |
| 10 | Lake Tomah | Monroe | F | - |
| 11 | Decorah Lake | Juneau | F | - |
| 12 | Cathedral Point Pond | Dane | F | - |
| 13 | Prairie Oaks Pond | Dane | F | - |
| 14 | Gateway Pond | Dane | F | - |
| 15 | Outlet To Lake Geneva | Walworth | F | - |
| 16 | Silent Street Pond | Dane | F | - |
| 17 | Investment Court Pond | Dane | F | - |
| 18 | Little St. Germain Lake-West Bay-at St. Germain | Vilas | F | - |
| 19 | Spruce Lake | Oneida | F | 2.5 ft |
| 20 | Dexter Lake | Wood | F | 1.5 ft |
| 21 | Radigan Flowage Lake | Douglas | F | 2 ft |
| 22 | Mead Lake Nb Control Site | Clark | F | 2 ft |
| 23 | Mead Lake Nb Test Site | Clark | F | 2 ft |
| 24 | Jennie Weber Lake | Oneida | F | 1.5 ft |
| 25 | Lake Creek | Oneida | F | 2.5 ft |
Source: EPA National Aquatic Resource Surveys, 2026.