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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% earn a water-quality grade of A or B and 55.6% score D or F (average grade C). Banks Lake is the cleanest; 36 lakes (73.5%) are on an official impaired-waters list.

Data Report

49
Monitored lakes graded
C
Average grade
22.2%
Grade A or B
73.5%
On impaired list

Washington grade distribution

A
B
C
D
A: 2 (11.1%)B: 2 (11.1%)C: 4 (22.2%)D: 9 (50.0%)F: 1 (5.6%)

Cleanest lakes in Washington

Highest combined water-quality score, 2026.

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

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FLake — Thurston County127 µg/L P · 34 µg/L chl-a
  2. 2.DLake — Thurston County20 µg/L P · 3 µg/L chl-a
  3. 3.DLake — Thurston County15 µg/L P · 3 µg/L chl-a
  4. 4.DLake — Thurston County28 µg/L P · 8 µg/L chl-a
  5. 5.DLake — Thurston County31 µg/L P · 17 µg/L chl-a
  6. 6.DLake — Thurston County21 µg/L P · 6 µg/L chl-a
  7. 7.DLake — Thurston County25 µg/L P · 14 µg/L chl-a
  8. 8.DLake — Thurston County26 µg/L P · 6 µg/L chl-a
  9. 9.DLake — Thurston County30 µg/L P · 9 µg/L chl-a
  10. 10.DLake — Thurston County26 µg/L P · 7 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

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

Trending lakes

Improving

  1. 1.CLake — Thurston County
  2. 2.CLake — Thurston County
  3. 3.CRimrock Reservoir — Yakima County
  4. 4.DLake — Thurston County
  5. 5.DLake — Thurston County
  6. 6.DLake — Thurston County
  7. 7.DLake — Thurston County
  8. 8.DLake — Thurston County

Declining

  1. 1.FLake — Thurston County
  2. 2.DLake — Thurston County
  3. 3.DLake — Thurston County
  4. 4.CWeallup Lake — Snohomish County
  5. 5.BRoss Lake — Whatcom County
  6. 6.BMoses Lake — Grant County
  7. 7.ABanks Lake — Grant County
  8. 8.NRLacamas 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 grade

Grades 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”.