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LakeQuality
Pink and orange afterglow over the treeline on Slim Lake after sunset

Updated August 2026

The State of Oregon's Lakes 2026

Of 73 lakes in Oregon, 21.8% sit in EPA's favourable condition classes for nutrients and algae and 63.6% in its poorer ones. Odell Lake is the cleanest; 49 lakes (67.1%) are on an official impaired-waters list.

Data Report

73
Monitored lakes sampled
C
Average readings
21.8%
low nutrient and algae readings
67.1%
On impaired list

Oregon nutrient and algae readings

Cleanest lakes in Oregon

Cleanest combined nutrient and algae readings, 2026.

  1. 1.Odell Lake — Klamath County22.3 ft clarity · 30 µg/L P · 1 µg/L chl-a
  2. 2.Billy Chinook Reservoir — Jefferson County4.4 ft clarity · 70 µg/L P · 2 µg/L chl-a
  3. 3.Bully Creek Lake — Malheur County4.9 ft clarity · 128 µg/L P · 15 µg/L chl-a
  4. 4.Deschutes River Arm Lake — Jefferson County4.6 ft clarity · 60 µg/L P · 4 µg/L chl-a
  5. 5.Round Butte Dam Forebay Lake — Jefferson County5.6 ft clarity · 40 µg/L P · 5 µg/L chl-a
  6. 6.Fremont Bridge Lake — Klamath County200 µg/L P
  7. 7.Crane Prairie Reservoir — Deschutes County40 µg/L P · 6 µg/L chl-a
  8. 8.Williamson River Outlet Lake — Klamath County165 µg/L P · 68 µg/L chl-a
  9. 9.Malheur Lake — Harney County461 µg/L P · 46 µg/L chl-a
  10. 10.Mid-Trench Lake — Klamath County240 µg/L P · 81 µg/L chl-a

Highest measured algae

Highest measured chlorophyll-a (algae) concentration.

  1. 1.Shoalwater Bay Lake — Klamath County144 µg/L chlorophyll-a
  2. 2.Cold Springs Reservoir — Umatilla County126 µg/L chlorophyll-a
  3. 3.Rattlesnake Point Lake — Klamath County125 µg/L chlorophyll-a
  4. 4.Brownlee Reservoir — Baker County108 µg/L chlorophyll-a
  5. 5.Mid-North Lake — Klamath County81 µg/L chlorophyll-a
  6. 6.Mid-Trench Lake — Klamath County81 µg/L chlorophyll-a
  7. 7.Williamson River Outlet Lake — Klamath County68 µg/L chlorophyll-a
  8. 8.Agency Lake South — Klamath County62 µg/L chlorophyll-a
  9. 9.Thief Valley Reservoir — Union County51 µg/L chlorophyll-a
  10. 10.Mckay Lake — Umatilla County49 µg/L chlorophyll-a

Trending lakes

Improving

  1. 1.Crescent Lake — Klamath County
  2. 2.Lake of the Woods — Klamath County
  3. 3.Henry Hagg Lake — Washington County
  4. 4.Green Peter Lake — Linn County
  5. 5.Billy Chinook Reservoir — Jefferson County
  6. 6.Willow Lake — Jackson County
  7. 7.Foster Lake — Linn County
  8. 8.East Lake Hot Springs Boat Ramp — Deschutes County

Declining

  1. 1.Williamson River Outlet Lake — Klamath County
  2. 2.Agency Lake South — Klamath County
  3. 3.Brownlee Reservoir — Baker County
  4. 4.Cold Springs Reservoir — Umatilla County
  5. 5.Diamond Lake — Douglas County
  6. 6.Thief Valley Reservoir — Union County
  7. 7.Bully Creek Lake — Malheur County
  8. 8.Prineville Reservoir — Crook County

Impaired waters in Oregon

49 lakes appear on the EPA/state 303(d) impaired-waters list (48 formally listed, 22 with a cleanup plan/TMDL).

Algal growth · 25Mercury · 17Temperature · 17Oxygen depletion (low DO) · 13pH / acidity · 11Hydrologic alteration · 6Metals (other than mercury) · 4Pathogens (E. coli, fecal) · 3

Explore Oregon lakes

Lake Fishing Forecast

Species activity outlook by water temperature and season

Oregon Lake Trips

Where to stay at Oregon's top lakes, each with its readings

Readings combine total phosphorus, total nitrogen, chlorophyll-a, and turbidity from the EPA Water Quality Portal and state agencies. See the full methodology, the national State of America’s Lakes report, or all Oregon lakes.

Cite this report: LakeQuality, “The State of Oregon's Lakes 2026”.