Hypereutrophic Lakes in Oregon
14 hypereutrophic lakes in Oregon. Very high nutrients, dense algae, poor clarity.
Hypereutrophic lakes (TSI above 70) are at the upper end of the productivity scale: visible algal blooms most weeks of summer, clarity often under 3 feet, and meaningful risk of harmful-algal-bloom toxin events. 14 hypereutrophic lakes are on the Oregon books. Almost none of them are deep — 0% break 50 feet. Shallow basin plus heavy nutrient input is the consistent recipe.
Hypereutrophic lakes carry the highest phosphorus and chlorophyll-a readings on record; the trophic class and the readings are measuring closely related dimensions of the same underlying nutrient regime.
| # | Lake | County | Clarity | Depth | TSI |
|---|---|---|---|---|---|
| 1 | Fremont Bridge Lake | Klamath | - | - | 81 |
| 2 | Williamson River Outlet Lake | Klamath | - | - | 75 |
| 3 | Malheur Lake | Harney | - | - | 80 |
| 4 | Mid-Trench Lake | Klamath | - | - | 78 |
| 5 | Agency Lake South | Klamath | - | - | 71 |
| 6 | Shoalwater Bay Lake | Klamath | - | - | 80 |
| 7 | Rattlesnake Point Lake | Klamath | - | - | 79 |
| 8 | Mid-North Lake | Klamath | - | - | 76 |
| 9 | Cullaby Lake | Clatsop | - | - | 66 |
| 10 | Tenmile Lake | Coos | - | - | 64 |
| 11 | Brownlee Reservoir | Baker | - | - | 77 |
| 12 | Cold Springs Reservoir | Umatilla | - | - | 78 |
| 13 | Diamond Lake | Douglas | - | - | 68 |
| 14 | Thief Valley Reservoir | Union | - | - | 69 |
All hypereutrophic lakes →Minnesota version →Wisconsin version →Illinois version →Michigan version →Iowa version →Ohio version →Pennsylvania version →New York version →Missouri version →Indiana version →North Dakota version →South Dakota version →Florida version →Colorado version →North Carolina version →Vermont version →Texas version →Arkansas version →Oklahoma version →Kansas version →Nebraska version →Nevada version →Arizona version →Idaho version →Montana version →New Mexico version →Tennessee version →Alabama version →Virginia version →Kentucky version →Utah version →Washington version →Wyoming version →Georgia version →Massachusetts version →Ontario version →Québec version →Nova Scotia version →British Columbia version →Newfoundland and Labrador version →All Oregon lakes →
Source: EPA National Aquatic Resource Surveys, 2026.