Cleanest Lake in Wisconsin: Top 25 Ranked
The single cleanest lake in Wisconsin is Geneva Lake, with the highest combined water quality score in our 2026 dataset (grade A). Cleanliness is measured by Secchi depth (clarity), total phosphorus, and chlorophyll-a — all sourced from EPA + Wisconsin DNR sampling. The full top 25 ranked list appears below.
Filtered to remove lakes with limited data (<3 sampling years). Sorted by overall water quality score.
The "cleanest" ranking on LakeGrade is composite: water clarity (Secchi), total phosphorus, and chlorophyll-a — the three core inputs to the LakeGrade rubric — taken together. The cleanest Wisconsin lake on the current data is Geneva Lake (A).
The top of the cleanest-lakes list is dominated by deep, glacial, cold-water basins — the same basins that produce the A-grade bracket overall. The pattern is not coincidence: depth and watershed land-use are the dominant inputs to water-clarity and nutrient-loading both.
| Rank | Lake | County | Grade | Clarity | Phosphorus | Chlorophyll-a | Area |
|---|---|---|---|---|---|---|---|
| 1 | Geneva Lake | Walworth | A | 20.2 ft | 13 µg/L | — | 5.4K acres |
| 2 | Round Lake | Sawyer | A | 21.1 ft | 14.4 µg/L | 1 µg/L | 3.3K acres |
| 3 | Round Lake | Oconto | A | 16.8 ft | 13.8 µg/L | — | 31 acres |
| 4 | Deep Lake | Adams | A | 15.5 ft | 13.3 µg/L | — | 33 acres |
| 5 | Beasley Lake | Waupaca | A | 18.4 ft | 15.1 µg/L | — | 12 acres |
| 6 | Bass Lake | Washburn | A | 16 ft | 12.3 µg/L | — | 132 acres |
| 7 | Bass Lake | Oconto | A | 16.8 ft | 10.7 µg/L | — | 145 acres |
| 8 | Clear Lake | Sawyer | A | 17 ft | 11.2 µg/L | — | 77 acres |
| 9 | Clear Lake | Polk | A | 15.5 ft | 13.9 µg/L | — | 30 acres |
| 10 | Clear Lake | Lincoln | A | 18 ft | 10.6 µg/L | — | 259 acres |
| 11 | Explosion Lake | Oconto | A | 17 ft | 15.7 µg/L | — | 43 acres |
| 12 | Sugar Camp Lake | Oneida | A | 16.5 ft | 10.3 µg/L | — | - |
| 13 | Little Spider Lake | Vilas | A | 16 ft | 11.5 µg/L | — | 223 acres |
| 14 | Rosalind Lake | Vilas | A | 19.3 ft | 9.9 µg/L | — | - |
| 15 | Lake Lucerne | Forest | A | 18.5 ft | 9.3 µg/L | — | 1.0K acres |
| 16 | Two Island Lake | Chippewa | A | 15.3 ft | 15.1 µg/L | — | 29 acres |
| 17 | Long Lake | Bayfield | A | 16.2 ft | 13.7 µg/L | — | 280 acres |
| 18 | Long Lake | Burnett | A | 21.5 ft | 11.3 µg/L | — | 222 acres |
| 19 | Lake Metonga | Forest | A | 22 ft | 14 µg/L | — | 2.0K acres |
| 20 | Anderson Lake | Vilas | A | 21 ft | 10.4 µg/L | — | 35 acres |
| 21 | Archibald Lake | Oconto | A | 17.6 ft | 10.6 µg/L | — | 392 acres |
| 22 | Carlin Lake | Vilas | A | 21 ft | 7.3 µg/L | — | 155 acres |
| 23 | Wolf Lake | Adams | A | 15 ft | 13.1 µg/L | — | 42 acres |
| 24 | Butternut Lake | Forest | A | 19.5 ft | 11.8 µg/L | — | 1.2K acres |
| 25 | Green Lake | Oconto | A | 18.3 ft | 17.9 µg/L | — | 21 acres |
Frequently Asked Questions
What is the cleanest lake in Wisconsin?
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.
What is the most polluted lake in Wisconsin?
Among monitored Wisconsin lakes, Dexter 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.
Where in Wisconsin has the cleanest water?
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.
Are these lakes safe to swim in?
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.
How are these grades calculated?
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.
What are the top 5 cleanest lakes in the US?
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.
How we rank cleanest lakes
Cleanliness here means three things working together:
- Secchi clarity — how deep you can see into the water (in feet). Greater depth = clearer water = lower suspended sediment and algae.
- Total phosphorus — the nutrient that fuels algae. Lower phosphorus levels mean less algae growth potential.
- Chlorophyll-a — the green pigment in algae. Direct measurement of how much algae is currently in the water.
Each signal is normalized against Wisconsin state norms and combined into a single 0–100 score. The lakes above are sorted by that combined score.Full methodology →
Source: EPA National Aquatic Resource Surveys, 2026.