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

Updated August 2026

The State of Washington's Lakes 2026

Of 49 lakes in Washington, 22.2% sit in EPA's favourable condition classes for nutrients and algae and 55.6% in its poorer ones. Banks Lake is the cleanest; 36 lakes (73.5%) are on an official impaired-waters list.

Data Report

49
Monitored lakes sampled
C
Average readings
22.2%
low nutrient and algae readings
73.5%
On impaired list

Washington nutrient and algae readings

Cleanest lakes in Washington

Cleanest combined nutrient and algae readings, 2026.

  1. 1.Banks Lake — Grant County19.4 ft clarity · 13 µg/L P · 1 µg/L chl-a
  2. 2.Moses Lake — Grant County20 µg/L P
  3. 3.Lake — Thurston County7 µg/L P · 1 µg/L chl-a
  4. 4.Lake — Thurston County9 µg/L P · 3 µg/L chl-a
  5. 5.Lake — Thurston County20 µg/L P · 3 µg/L chl-a
  6. 6.Lake — Thurston County15 µg/L P · 3 µg/L chl-a
  7. 7.Lake — Thurston County28 µg/L P · 8 µg/L chl-a
  8. 8.Lake — Thurston County31 µg/L P · 17 µg/L chl-a
  9. 9.Lake — Thurston County21 µg/L P · 6 µg/L chl-a
  10. 10.Lake — Thurston County25 µg/L P · 14 µg/L chl-a

Highest measured algae

Highest measured chlorophyll-a (algae) concentration.

  1. 1.Lake — Thurston County34 µg/L chlorophyll-a
  2. 2.Vancouver Lake — Clark County29 µg/L chlorophyll-a

Trending lakes

Improving

  1. 1.Lake — Thurston County
  2. 2.Lake — Thurston County
  3. 3.Rimrock Reservoir — Yakima County
  4. 4.Lake — Thurston County
  5. 5.Lake — Thurston County
  6. 6.Lake — Thurston County
  7. 7.Lake — Thurston County
  8. 8.Lake — Thurston County

Declining

  1. 1.Lake — Thurston County
  2. 2.Lake — Thurston County
  3. 3.Lake — Thurston County
  4. 4.Weallup Lake — Snohomish County
  5. 5.Ross Lake — Whatcom County
  6. 6.Moses Lake — Grant County
  7. 7.Banks Lake — Grant County
  8. 8.Lacamas Lake — Clark County

Impaired waters in Washington

36 lakes appear on the EPA/state 303(d) impaired-waters list (29 formally listed, 3 with a cleanup plan/TMDL).

Nutrients (phosphorus, nitrogen) · 18Nuisance exotic species · 14Mercury · 7Pathogens (E. coli, fecal) · 5PCBs · 4Pesticides · 3Dioxins · 2Oxygen depletion (low DO) · 2

Explore Washington lakes

Lake Fishing Forecast

Species activity outlook by water temperature and season

Washington Lake Trips

Where to stay at Washington'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 Washington lakes.

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