Question 1

What is the cleanest lake in Wisconsin?

Accepted Answer

Geneva Lake has the highest water quality score in our 2026 Wisconsin dataset (grade A), based on combined Secchi clarity, phosphorus, and chlorophyll-a measurements. Cleanliness here means low nutrient load, deep water clarity, and minimal algae — measured directly from EPA and state DNR samples.

Question 2

What is the most polluted lake in Wisconsin?

Accepted Answer

Among monitored Wisconsin lakes, Tilleda Off Bridge Lake ranks among the most polluted by combined phosphorus and chlorophyll-a load. See our full ranking on the most-polluted-lakes trend page for state-by-state details.

Question 3

Where in Wisconsin has the cleanest water?

Accepted Answer

Wisconsin's clearest waters are concentrated in the deeper, lower-nutrient lakes of the state's northeast — typically older glacial lakes with rocky basins and forested watersheds. The top 25 above shows the highest-scoring lakes statewide; counties like Cook, Lake, and Itasca (MN) or Vilas, Oneida, and Iron (WI) consistently produce A-grade waters.

Question 4

Are these lakes safe to swim in?

Accepted Answer

A and B grade lakes are typically safe for swimming under normal conditions. Chlorophyll-a below 10 µg/L means low risk of harmful algal blooms. Always check posted advisories before swimming — water quality changes seasonally, especially in late summer when blue-green algae bloom risk peaks.

Question 5

How are these grades calculated?

Accepted Answer

Each lake is scored on three signals: Secchi depth (water clarity in feet), total phosphorus (nutrient load in µg/L), and chlorophyll-a (algae concentration in µg/L). Each gets a 0–100 sub-score relative to Wisconsin state norms. The three sub-scores average into the overall score, which maps to an A–F letter grade. Methodology details live on our methodology page.

Question 6

What are the top 5 cleanest lakes in the US?

Accepted Answer

There is no single national ranking of cleanest lakes — each state agency uses different metrics. Crater Lake (Oregon) is widely cited as the clearest US lake, with Secchi depths over 30 m. Lake Tahoe and Lake Superior are also top-tier nationally. Within Wisconsin, the lakes ranked above are the top 25 by our standardized 2026 grading.

Rank	Lake	County	Grade	Clarity	Phosphorus	Chlorophyll-a	Area
1	Geneva Lake	Walworth	A	19 ft	13 µg/L	—	5.4K acres
2	Lac Courte Oreilles Lake	Sawyer	A	38.7 ft	16 µg/L	1.4 µg/L	5.1K acres
3	Sunset Lake	Portage	A	41 ft	12 µg/L	—	63 acres
4	Sunset Lake	Vilas	A	56.5 ft	11 µg/L	—	207 acres
5	Grindstone Lake	Sawyer	A	57.7 ft	14.4 µg/L	1.7 µg/L	3.2K acres
6	Middle Sugarbush Lake	Vilas	A	50 ft	13 µg/L	—	254 acres
7	Irving Lake	Vilas	A	36.1 ft	18.6 µg/L	—	419 acres
8	Windigo Lake	Sawyer	A	36.1 ft	15.6 µg/L	—	503 acres
9	Towanda Lake	Vilas	A	34.5 ft	12.6 µg/L	—	139 acres
10	Parker Lake	Adams	A	55.8 ft	14.8 µg/L	—	57 acres
11	West Bay Lake	Vilas	A	42.7 ft	16.4 µg/L	—	417 acres
12	Long Lake	Washburn	A	29.5 ft	17.5 µg/L	—	-
13	Long Lake	Waushara	A	45.9 ft	11.6 µg/L	—	254 acres
14	Long Lake	Waupaca	A	44.3 ft	12.4 µg/L	—	112 acres
15	Long Lake	Burnett	A	68.9 ft	11.3 µg/L	—	222 acres
16	Long Lake	Lincoln	A	47.7 ft	12.4 µg/L	—	119 acres
17	Long Lake	Chippewa	A	32.8 ft	12.4 µg/L	—	936 acres
18	Long Lake	Fond du Lac	A	32.8 ft	19.3 µg/L	—	423 acres
19	Long Lake	Bayfield	A	52.5 ft	13.7 µg/L	—	280 acres
20	Long Lake	Oconto	A	47.6 ft	10.2 µg/L	—	-
21	Chain Lake	Oconto	A	44.3 ft	9.5 µg/L	—	76 acres
22	Upper Eau Claire Lake	Bayfield	A	61.5 ft	11.5 µg/L	—	1.0K acres
23	Spring Lake	Vilas	A	27.9 ft	16.3 µg/L	—	236 acres
24	Spring Lake	Manitowoc	A	26.2 ft	19.6 µg/L	—	9 acres
25	Black Hawk Lake	Iowa	A	17.2 ft	19.9 µg/L	—	212 acres

Cleanest Lakes in Wisconsin

Frequently Asked Questions

How we rank cleanest lakes