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LakeQuality

Data Report · Updated July 2026

The State of Washington’s Lakes 2026

Of 89 monitored lakes in Washington, 52.8% earn a water-quality grade of A or B and 23.6% score D or F (average grade B). Ross Lake is the cleanest;

89
Monitored lakes graded
B
Average grade
52.8%
Grade A or B
0%
On impaired list

Washington grade distribution

A
B
C
D
A: 28 (31.5%)B: 19 (21.3%)C: 21 (23.6%)D: 17 (19.1%)F: 4 (4.5%)

Cleanest lakes in Washington

Highest combined water-quality score, 2026.

  1. 1.ARoss Lake — Whatcom County18.8 ft clarity · 5 µg/L P
  2. 2.ANla Lake — Skamania County26.3 ft clarity · 11 µg/L P · 1 µg/L chl-a
  3. 3.ANla Lake — King County77.4 ft clarity · 2 µg/L P · 0 µg/L chl-a
  4. 4.ANla Lake — Chelan County42.8 ft clarity · 4 µg/L P · 1 µg/L chl-a
  5. 5.ANla Lake — Snohomish County22.6 ft clarity · 14 µg/L P · 2 µg/L chl-a
  6. 6.ANla Lake — Pierce County20 µg/L P · 0 µg/L chl-a
  7. 7.AFdr Lake — Lincoln County16.2 ft clarity · 10 µg/L P · 2 µg/L chl-a
  8. 8.AFdr Lake — Okanogan County21.5 ft clarity · 11 µg/L P · 1 µg/L chl-a
  9. 9.ABanks Lake — Grant County19.4 ft clarity · 13 µg/L P · 1 µg/L chl-a
  10. 10.ALake: Ward Lake — Thurston County11 µg/L P · 3 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FNla Lake — Clark County3.3 ft clarity · 74000 µg/L P · 36 µg/L chl-a
  2. 2.FNla Lake — Grant County1.1 ft clarity · 175 µg/L P · 136 µg/L chl-a
  3. 3.FLake — Thurston County127 µg/L P · 34 µg/L chl-a
  4. 4.FNla Lake — Lewis County6.3 ft clarity · 26514 µg/L P · 37 µg/L chl-a
  5. 5.DVancouver Lake — Clark County1.2 ft clarity · 60 µg/L P · 28 µg/L chl-a
  6. 6.DNla Lake — Thurston County4.9 ft clarity · 46030 µg/L P · 22 µg/L chl-a
  7. 7.DNla Lake — Grant County2.2 ft clarity · 99000 µg/L P · 15 µg/L chl-a
  8. 8.DNla Lake — Pacific County3.3 ft clarity · 10517 µg/L P · 8 µg/L chl-a
  9. 9.DNla Lake — Stevens County7.7 ft clarity · 93000 µg/L P · 30 µg/L chl-a
  10. 10.DNla Lake — Kittitas County3.1 ft clarity · 90017 µg/L P · 5 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

  1. 1.FNla Lake — Grant County136 µg/L chlorophyll-a
  2. 2.FNla Lake — Lewis County37 µg/L chlorophyll-a
  3. 3.FNla Lake — Clark County36 µg/L chlorophyll-a
  4. 4.FLake — Thurston County34 µg/L chlorophyll-a
  5. 5.DNla Lake — Stevens County30 µg/L chlorophyll-a
  6. 6.DVancouver Lake — Clark County28 µg/L chlorophyll-a
  7. 7.DNla Lake — Thurston County22 µg/L chlorophyll-a
  8. 8.DNla Lake — Spokane County20 µg/L chlorophyll-a

Explore Washington lakes

Lake Fishing Forecast

Species activity outlook by water temperature and season

Grades combine Secchi clarity, total phosphorus, and chlorophyll-a 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,” lakequality.org/report/wa.