Cleanest Lakes in Washington: Top 25 Ranked by Water Quality
The single cleanest lake in Washington is Ross Lake, with the highest combined water quality score in our 2026 dataset (grade A). Cleanliness is measured by Secchi depth (clarity), total phosphorus, and chlorophyll-a — all sourced from EPA Water Quality Portal and state agency sampling. The full top 25 ranked list appears below.
Filtered to remove lakes with limited data (<3 sampling years). Sorted by overall water quality score.
Ranking the cleanest {{stateName}} lakes means picking the basins that score high on all three water-quality dimensions, not just one. Ross Lake leads the list with a grade of A, with Nla Lake close behind.
The top of the cleanest-lakes list is dominated by deep, glacial, cold-water basins — the same basins that produce the A-grade bracket overall. The pattern is not coincidence: depth and watershed land-use are the dominant inputs to water-clarity and nutrient-loading both.
| Rank | Lake | County | Grade | Clarity | Phosphorus | Chlorophyll-a | Area |
|---|---|---|---|---|---|---|---|
| 1 | Ross Lake | Whatcom | A | 18.8 ft | 4.5 µg/L | — | - |
| 2 | Nla Lake | Skamania | A | 26.3 ft | 10.5 µg/L | 1.3 µg/L | - |
| 3 | Nla Lake | King | A | 77.4 ft | 1.9 µg/L | 0.4 µg/L | - |
| 4 | Nla Lake | Chelan | A | 42.8 ft | 3.9 µg/L | 0.5 µg/L | - |
| 5 | Nla Lake | Snohomish | A | 22.6 ft | 13.5 µg/L | 2 µg/L | - |
| 6 | Nla Lake | Pierce | A | — | 19.6 µg/L | 0.3 µg/L | - |
| 7 | Fdr Lake | Lincoln | A | 16.2 ft | 10 µg/L | 2.3 µg/L | - |
| 8 | Fdr Lake | Okanogan | A | 21.5 ft | 11 µg/L | 1.3 µg/L | - |
| 9 | Banks Lake | Grant | A | 19.4 ft | 13 µg/L | 1.3 µg/L | - |
| 10 | Lake: Ward Lake | Thurston | A | — | 10.5 µg/L | 3.2 µg/L | - |
| 11 | Lake: Summit Lake | Thurston | A | — | 6 µg/L | 1.3 µg/L | - |
| 12 | Lake: Deep Lake | Thurston | A | — | 7 µg/L | 3.8 µg/L | - |
| 13 | Nla Upper Lake | Grays Harbor | A | — | 12.4 µg/L | 0.9 µg/L | - |
| 14 | Rimrock Reservoir | Yakima | A | — | 15 µg/L | 1.8 µg/L | - |
| 15 | Nla Lake | Grant | A | 12.8 ft | 15.7 µg/L | 2 µg/L | - |
| 16 | Fdr Lake | Ferry | A | 13.5 ft | 12 µg/L | 2.6 µg/L | - |
| 17 | Fdr Lake | Ferry | A | 19 ft | 20 µg/L | 1.3 µg/L | - |
| 18 | Nla Lake | Mason | A | — | 14.1 µg/L | 8 µg/L | - |
| 19 | Spearhead Lake | Skagit | A | 10.8 ft | 14 µg/L | — | - |
| 20 | Lake: Hicks Lake | Thurston | A | — | 16 µg/L | 8.5 µg/L | - |
| 21 | Nla Lake | Douglas | B | 13 ft | 55.9 µg/L | 4.4 µg/L | - |
| 22 | Nla Lower Lake | Grays Harbor | B | — | 38.9 µg/L | 0.4 µg/L | - |
| 23 | Nla Lake | Kitsap | B | 18.8 ft | 9506.1 µg/L | 1.7 µg/L | - |
| 24 | Nla Lake | Okanogan | B | 22.7 ft | 20518.3 µg/L | 2.7 µg/L | - |
| 25 | Nla Lake | Grant | B | 16.4 ft | 39517.1 µg/L | 1.5 µg/L | - |
Clearest lakes in Washington by water clarity
“Clearest” ranks by Secchi depth — how far you can see into the water — which can differ from the combined cleanliness score above. The clearest water in Washington is Nla Lake at 77.4 ft of Secchi visibility.
- 1. Nla Lake — 77.4 ft, King County
- 2. Nla Lake — 42.8 ft, Chelan County
- 3. Fdr Lake — 28.2 ft, Grant County
- 4. Nla Lake — 26.3 ft, Skamania County
- 5. Gravelly Lake — 25.4 ft, Pierce County
Frequently Asked Questions
What is the cleanest lake in Washington?
Ross Lake has the highest water quality score in our 2026 Washington dataset (grade A), based on combined Secchi clarity, phosphorus, and chlorophyll-a measurements. Cleanliness here means low nutrient load, deep water clarity, and minimal algae — measured directly from EPA and state DNR samples.
What is the most polluted lake in Washington?
Among monitored Washington lakes, Lake ranks among the most polluted by combined phosphorus and chlorophyll-a load. See our full ranking on the most-polluted-lakes trend page for state-by-state details.
Where in Washington has the cleanest water?
Washington's clearest waters are concentrated in deeper, lower-nutrient glacial basins with forested or low-agriculture watersheds. Among the current top-ranked lakes above, counties like Thurston, Grant, and Okanogan produce the highest-scoring lakes.
What is the clearest lake in Washington?
Nla Lake is the clearest lake in Washington by water clarity, with a Secchi depth of 77.4 feet — the deepest transparency in our 2026 dataset. "Clearest" ranks by Secchi depth (how far you can see into the water) specifically, while "cleanest" combines clarity with phosphorus and chlorophyll-a. See our full clearest-lakes ranking for Washington for the top 25 by Secchi clarity.
Are these lakes safe to swim in?
A and B grade lakes are typically safe for swimming under normal conditions. Chlorophyll-a below 10 µg/L means low risk of harmful algal blooms. Always check posted advisories before swimming — water quality changes seasonally, especially in late summer when blue-green algae bloom risk peaks.
How are these grades calculated?
Each lake is scored on three signals: Secchi depth (water clarity in feet), total phosphorus (nutrient load in µg/L), and chlorophyll-a (algae concentration in µg/L). Each gets a 0–100 sub-score relative to Washington state norms. The three sub-scores average into the overall score, which maps to an A–F letter grade. Methodology details live on our methodology page.
What are the top 5 cleanest lakes in the US?
There is no single national ranking of cleanest lakes — each state agency uses different metrics. Crater Lake (Oregon) is widely cited as the clearest US lake, with Secchi depths over 30 m. Lake Tahoe and Lake Superior are also top-tier nationally. Within Washington, the lakes ranked above are the top 25 by our standardized 2026 grading.
How we rank cleanest lakes
Cleanliness here means three things working together:
- Secchi clarity — how deep you can see into the water (in feet). Greater depth = clearer water = lower suspended sediment and algae.
- Total phosphorus — the nutrient that fuels algae. Lower phosphorus levels mean less algae growth potential.
- Chlorophyll-a — the green pigment in algae. Direct measurement of how much algae is currently in the water.
Each signal is normalized against Washington state norms and combined into a single 0–100 score. The lakes above are sorted by that combined score.Full methodology →
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Source: EPA National Aquatic Resource Surveys, 2026.