Updated August 2026
Most Polluted Lakes: Worst Water Quality
The 50 lakes with the lowest nutrient-and-algae grades. High phosphorus, frequent algae, or very low visibility. Check advisories before swimming — cabins and rentals around these lakes are still listed on each lake page.
| Rank | Lake | County | State | Grade | Clarity | Phosphorus | Algae |
|---|---|---|---|---|---|---|---|
| 1 | Cowley Lake | Hennepin | MN | F | 0.8 ft | 738.6 µg/L | 343.5 µg/L |
| 2 | Alice Lake | Otter Tail | MN | F | 0.8 ft | 494 µg/L | 254 µg/L |
| 3 | Mccarthy Lake | Dakota | MN | F | 1 ft | 195 µg/L | 169.5 µg/L |
| 4 | Goose Lake | Carver | MN | F | 1.2 ft | 132.5 µg/L | 160.5 µg/L |
| 5 | Shoalwater Bay Lake | Klamath | OR | F | - | 215 µg/L | 143.5 µg/L |
| 6 | Goose Lake | Ramsey | MN | F | 0.7 ft | 173.5 µg/L | 139 µg/L |
| 7 | Bois D Arc Lake | Hempstead | AR | F | - | 200 µg/L | 137 µg/L |
| 8 | Hazeltine Lake | Carver | MN | F | 1 ft | 142 µg/L | 136.5 µg/L |
| 9 | Ardmore Lake | Hennepin | MN | F | 1.3 ft | 285 µg/L | 130 µg/L |
| 10 | Augusta Lake | Dakota | MN | F | 0.7 ft | 147.5 µg/L | 130 µg/L |
| 11 | Cedar Lake | Linn | IA | F | 0.9 ft | 256.5 µg/L | 125.4 µg/L |
| 12 | Rattlesnake Point Lake | Klamath | OR | F | - | 185 µg/L | 124.5 µg/L |
| 13 | Mud Lake | Hennepin | MN | F | 0.9 ft | 150 µg/L | 123.5 µg/L |
| 14 | Hannen Lake | Benton | IA | F | 1.2 ft | 141 µg/L | 119.9 µg/L |
| 15 | Rice Lake | Hennepin | MN | F | 2.3 ft | 333.8 µg/L | 117.9 µg/L |
| 16 | Meyers Lake | Black Hawk | IA | F | 1.1 ft | 183.5 µg/L | 116.5 µg/L |
| 17 | Little Rock Lake | Benton | MN | F | 1.8 ft | 156.5 µg/L | 115 µg/L |
| 18 | Big Woods Lake | Carver | MN | F | 1.5 ft | 197 µg/L | 113 µg/L |
| 19 | Wheeler North Lake | Kandiyohi | MN | F | 1 ft | 294.5 µg/L | 112.5 µg/L |
| 20 | Shelbyville Lake | Shelby | MO | F | 1 ft | 252.5 µg/L | 112.4 µg/L |
| 21 | Albion Lake | Wright | MN | F | 1 ft | 105 µg/L | 112 µg/L |
| 22 | Miller Lake | Carver | MN | F | 1.6 ft | 211 µg/L | 108 µg/L |
| 23 | Rock Creek Max Depth Lake | Jasper | IA | F | 1.1 ft | 167.5 µg/L | 107.5 µg/L |
| 24 | Water Works Park Lake | Johnson | KS | F | - | 101.7 µg/L | 107 µg/L |
| 25 | Farmington Bay Lake | Davis | UT | F | - | 405 µg/L | 106 µg/L |
| 26 | Benton Lake | Carver | MN | F | 1 ft | 161 µg/L | 104 µg/L |
| 27 | Littlefield Lake | Audubon | IA | F | 0.9 ft | 225 µg/L | 103.1 µg/L |
| 28 | Schultz Lake | Kandiyohi | MN | F | 1.3 ft | 189.5 µg/L | 103 µg/L |
| 29 | Pearl Lake | Jackson | MN | F | 1 ft | 153 µg/L | 102.2 µg/L |
| 30 | Alice Wyth Lake | Black Hawk | IA | F | 1.6 ft | 183 µg/L | 101.6 µg/L |
| 31 | Spring Lake | Hennepin | MN | F | 3.4 ft | 434 µg/L | 99.1 µg/L |
| 32 | Lake Hendricks | Howard | IA | F | 1.5 ft | 108 µg/L | 99.1 µg/L |
| 33 | Hanska Lake | Brown | MN | F | 1 ft | 219 µg/L | 98.8 µg/L |
| 34 | Knife Lake | Kanabec | MN | F | 2 ft | 161.5 µg/L | 97.2 µg/L |
| 35 | Foster Lake | Wright | MN | F | 1.6 ft | 179 µg/L | 97 µg/L |
| 36 | Goose Lake | Hennepin | MN | F | 0.8 ft | 132.1 µg/L | 96.5 µg/L |
| 37 | Casey Lake | Tama | IA | F | 1.2 ft | 113 µg/L | 95.5 µg/L |
| 38 | Eagle Lake | Carver | MN | F | 2 ft | 169 µg/L | 93.8 µg/L |
| 39 | Desoto Bend Lake | Harrison | IA | F | 1 ft | 218 µg/L | 93.4 µg/L |
| 40 | Clear Lake | Brown | MN | F | 0.8 ft | 196 µg/L | 92.8 µg/L |
| 41 | Lake Luna | Cass | MO | F | 1.2 ft | 192.9 µg/L | 92.7 µg/L |
| 42 | Lake Orient | Adair | IA | F | 1.3 ft | 674 µg/L | 92.2 µg/L |
| 43 | Hemitite Lake | St. Francois | MO | F | 1.1 ft | 186.9 µg/L | 91.9 µg/L |
| 44 | Cameron Lake | Polk | MN | F | 1 ft | 130 µg/L | 90.8 µg/L |
| 45 | Avenue of the Saints Lake | Bremer | IA | F | 1.2 ft | 174.5 µg/L | 89.7 µg/L |
| 46 | Indian Lake | Van Buren | IA | F | 1.2 ft | 158 µg/L | 89.1 µg/L |
| 47 | Union Grove Lake | Tama | IA | F | 1.6 ft | 143.5 µg/L | 88.5 µg/L |
| 48 | Vadnais Lake | Ramsey | MN | F | 1 ft | 90 µg/L | 88.1 µg/L |
| 49 | Ottumwa Lagoon Max Depth Lake | Wapello | IA | F | 0.9 ft | 261 µg/L | 86.9 µg/L |
| 50 | Henshaw Lake | Wright | MN | F | 1 ft | 104 µg/L | 86.6 µg/L |
More on lake water quality
The cleanest and dirtiest U.S. lakes in the monitoring record, ranked on EPA nutrient and algae condition classes across thousands of graded lakes.
The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.
The cleanest U.S. lakes under 500 acres, ranked on measured nutrients, algae, and water clarity — with the size cutoff applied to the lake, not the sample.
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