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LakeQuality

Updated Aug 2026

Washington Lake Water Quality

Water quality grades for 56 monitored lakes across 18 counties.

Washington has 56 graded lakes in our dataset, sampled across 18 counties, drawing on monitoring by the Washington State Department of Ecology plus federal and volunteer measurements. Only about 7% of Washington's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 20% of lakes fall in the D or F brackets.

Mesotrophic is the dominant trophic class — a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. Lincoln ranks highest on average county grade (A). Thurston sits lowest. With only 18 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.

Grade Distribution

All 56 Washington lakes in the index: 22 (39.3%) carry an A–F grade and 34 (60.7%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.

A
4
Excellent
B
2
Good
C
5
Fair
D
10
Nutrient-rich
F
1
Algae-prone
34
Not rated

Where to stay near Washington

Top-rated places to stay near Washington

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Top-Graded Washington Lakes

The highest water-quality grades in Washington — clarity, depth, and A-F grade at a glance.

A
Fdr Lake
Lincoln County, Washington · 16.2 ft clear
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A
Fdr Lake
Okanogan County, Washington · 21.5 ft clear
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A
Banks Lake
Grant County, Washington · 19.4 ft clear
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A
Fdr Lake
Ferry County, Washington · 19 ft clear
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B
Ross Lake
Whatcom County, Washington
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B
Moses Lake
Grant County, Washington
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C
Fdr Lake
Ferry County, Washington · 13.6 ft clear
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C
Lake: Summit Lake
Thurston County, Washington
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C
Weallup Lake
Snohomish County, Washington
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Lake Map

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Explore Washington Lakes

Clearest lakes in Washington →Deepest lakes in Washington →Largest lakes in Washington →Cleanest lakes in Washington →Best swimming lakes →

Washington reports, fishing & travel

The State of Washington's Lakes

Washington's cleanest and most-polluted lakes, ranked A–F

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

Reservoirs in Washington

View all 11 reservoirs →

11 of 56 Washington lakes (20%) are man-made reservoirs in the USACE National Inventory of Dams. 7 carry a High or Significant hazard rating. The oldest matched dam dates to 1911.

Recreation (5)Flood Risk Reduction (4)Fish and Wildlife Pond (1)Irrigation (1)

EPA-Impaired Lakes in Washington

View all 42 impaired lakes →

42 of 56 Washington lakes (75%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 6 have an approved Total Maximum Daily Load (TMDL) restoration plan.

Nutrients (phosphorus, nitrogen) (19)Nuisance exotic species (14)Mercury (7)Pathogens (E. coli, fecal) (5)PCBs (4)Pesticides (3)Total toxics (3)Temperature (2)

Washington Fishing & Recreation

Licenses, regulations, records, and where to fish across Washington.

Best Fishing Times
Solunar calendar & sun/moon timing per lake

Counties

All 18 Washington counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.

A
Lincoln County
2 lakes
A
Grant County
5 lakes
A
Okanogan County
1 lakes
NR
Pierce County
8 lakes
B
Whatcom County
1 lakes
B
Ferry County
2 lakes
C
Yakima County
2 lakes
NR
Pend Oreille County
1 lakes
NR
King County
2 lakes
C
Snohomish County
6 lakes
NR
Skagit County
1 lakes
D
Thurston County
16 lakes
NR
Grays Harbor County
2 lakes
NR
Clark County
2 lakes
NR
Stevens County
2 lakes
NR
Spokane County
1 lakes
NR
Franklin County
1 lakes
NR
Clallam County
1 lakes

Cleanest Washington Lakes

22 lakes22 graded lakes

#LakeCountyGradeClarityTrophicAccess
1AFdr LakeLincolnA16.2 ftMesotrophic✓ Public
2AFdr LakeOkanoganA21.5 ftOligotrophic✓ Public
3ABanks LakeGrantA19.4 ftOligotrophic✓ Public
4AFdr LakeFerryA19 ftOligotrophic✓ Public
5BRoss LakeWhatcomBNo recent samplesOligotrophic✓ Public
6BMoses LakeGrantBNo recent samplesMesotrophic✓ Public
7CFdr LakeFerryC13.6 ftMesotrophic✓ Public
8CLake: Summit LakeThurstonCNo recent samplesOligotrophic✓ Public
9CWeallup LakeSnohomishCNo recent samplesEutrophicNot recorded
10CLake: Deep LakeThurstonCNo recent samplesMesotrophic✓ Public
11CRimrock ReservoirYakimaCNo recent samplesMesotrophic✓ Public
12DLake: Lake St. Clair @ East BasinThurstonDNo recent samplesMesotrophicNot recorded
13DLake: Ward LakeThurstonDNo recent samplesMesotrophic✓ Public
14DLake: Lawrence Lake @ Big BasinThurstonDNo recent samplesEutrophicNot recorded
15DLake: Black Lake @ South BasinThurstonDNo recent samplesEutrophicNot recorded
16DLake: Hicks LakeThurstonDNo recent samplesMesotrophic✓ Public
17DLake: Long Lake @ LoThurstonDNo recent samplesEutrophicNot recorded
18DLake: Lake St. Clair @ Sw BasinThurstonDNo recent samplesMesotrophicNot recorded
19DLake: Lawrence Lake @ West BasinThurstonDNo recent samplesEutrophicNot recorded
20DLake: Pattison LakeThurstonDNo recent samplesEutrophic✓ Public
21DLake: Offutt LakeThurstonDNo recent samplesEutrophic✓ Public
22FLakeThurstonFNo recent samplesHypereutrophic✓ Public

Washington Lakes Needing Attention

22 lakes22 graded lakes

#LakeCountyGradeClarity
1FLakeThurstonFNo recent samples
2DLake: Offutt LakeThurstonDNo recent samples
3DLake: Pattison LakeThurstonDNo recent samples
4DLake: Lawrence Lake @ West BasinThurstonDNo recent samples
5DLake: Lake St. Clair @ Sw BasinThurstonDNo recent samples
6DLake: Long Lake @ LoThurstonDNo recent samples
7DLake: Hicks LakeThurstonDNo recent samples
8DLake: Black Lake @ South BasinThurstonDNo recent samples
9DLake: Lawrence Lake @ Big BasinThurstonDNo recent samples
10DLake: Ward LakeThurstonDNo recent samples
11DLake: Lake St. Clair @ East BasinThurstonDNo recent samples
12CRimrock ReservoirYakimaCNo recent samples
13CLake: Deep LakeThurstonCNo recent samples
14CWeallup LakeSnohomishCNo recent samples
15CLake: Summit LakeThurstonCNo recent samples
16CFdr LakeFerryC13.6 ft
17BMoses LakeGrantBNo recent samples
18BRoss LakeWhatcomBNo recent samples
19AFdr LakeFerryA19 ft
20ABanks LakeGrantA19.4 ft
21AFdr LakeOkanoganA21.5 ft
22AFdr LakeLincolnA16.2 ft

Lake Match

Which lake suits how you actually use one?

Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.

Take the quiz →

Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.

More on lake water quality

The Cleanest Lakes in Every State We Track

The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.

The Clearest Lake in Every State We Grade (2026)

State-by-state Secchi champions: the clearest measured lake in each U.S. state we grade, which is not the same as the cleanest nutrient grade.

The State of America's Lakes 2026

A data snapshot of the lakes we grade: how the A–F distribution breaks down, the clearest and the greenest states, and where the water is improving.

Lake Water Quality Grades Explained: What A–F Means

How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.