The State of Oregon's Lakes 2026

Cleanest lakes in Oregon

Highest combined water-quality score, 2026.

1. 1.COdell Lake — Klamath County22.3 ft clarity · 30 µg/L P · 1 µg/L chl-a
2. 2.CBilly Chinook Reservoir — Jefferson County4.4 ft clarity · 70 µg/L P · 2 µg/L chl-a
3. 3.DBully Creek Lake — Malheur County4.9 ft clarity · 128 µg/L P · 15 µg/L chl-a
4. 4.DDeschutes River Arm Lake — Jefferson County4.6 ft clarity · 60 µg/L P · 4 µg/L chl-a
5. 5.DRound Butte Dam Forebay Lake — Jefferson County5.6 ft clarity · 40 µg/L P · 5 µg/L chl-a
6. 6.DFremont Bridge Lake — Klamath County200 µg/L P
7. 7.DCrane Prairie Reservoir — Deschutes County40 µg/L P · 6 µg/L chl-a
8. 8.FWilliamson River Outlet Lake — Klamath County165 µg/L P · 68 µg/L chl-a
9. 9.FMalheur Lake — Harney County461 µg/L P · 46 µg/L chl-a
10. 10.FMid-Trench Lake — Klamath County240 µg/L P · 81 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

1. 1.FWilliamson River Outlet Lake — Klamath County165 µg/L P · 68 µg/L chl-a
2. 2.FMalheur Lake — Harney County461 µg/L P · 46 µg/L chl-a
3. 3.FMid-Trench Lake — Klamath County240 µg/L P · 81 µg/L chl-a
4. 4.FAgency Lake South — Klamath County110 µg/L P · 62 µg/L chl-a
5. 5.FShoalwater Bay Lake — Klamath County215 µg/L P · 144 µg/L chl-a
6. 6.FRattlesnake Point Lake — Klamath County185 µg/L P · 125 µg/L chl-a
7. 7.FMid-North Lake — Klamath County180 µg/L P · 81 µg/L chl-a
8. 8.DBully Creek Lake — Malheur County4.9 ft clarity · 128 µg/L P · 15 µg/L chl-a
9. 9.DDeschutes River Arm Lake — Jefferson County4.6 ft clarity · 60 µg/L P · 4 µg/L chl-a
10. 10.DRound Butte Dam Forebay Lake — Jefferson County5.6 ft clarity · 40 µg/L P · 5 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

1. 1.FShoalwater Bay Lake — Klamath County144 µg/L chlorophyll-a
2. 2.FCold Springs Reservoir — Umatilla County126 µg/L chlorophyll-a
3. 3.FRattlesnake Point Lake — Klamath County125 µg/L chlorophyll-a
4. 4.FBrownlee Reservoir — Baker County108 µg/L chlorophyll-a
5. 5.FMid-North Lake — Klamath County81 µg/L chlorophyll-a
6. 6.FMid-Trench Lake — Klamath County81 µg/L chlorophyll-a
7. 7.FWilliamson River Outlet Lake — Klamath County68 µg/L chlorophyll-a
8. 8.FAgency Lake South — Klamath County62 µg/L chlorophyll-a
9. 9.FThief Valley Reservoir — Union County51 µg/L chlorophyll-a
10. 10.NRMckay Lake — Umatilla County49 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.ACrescent Lake — Klamath County
2. 2.ALake of the Woods — Klamath County
3. 3.AHenry Hagg Lake — Washington County
4. 4.AGreen Peter Lake — Linn County
5. 5.CBilly Chinook Reservoir — Jefferson County
6. 6.CWillow Lake — Jackson County
7. 7.CFoster Lake — Linn County
8. 8.CEast Lake Hot Springs Boat Ramp — Deschutes County

Declining

1. 1.FWilliamson River Outlet Lake — Klamath County
2. 2.FAgency Lake South — Klamath County
3. 3.FBrownlee Reservoir — Baker County
4. 4.FCold Springs Reservoir — Umatilla County
5. 5.FDiamond Lake — Douglas County
6. 6.FThief Valley Reservoir — Union County
7. 7.DBully Creek Lake — Malheur County
8. 8.DPrineville Reservoir — Crook County