Updated Aug 2026
Idaho Lake Water Quality
Water quality grades for 15 of the 301 lakes we list here, across 41 counties.
Grade Distribution
All 301 Idaho lakes in the index: 15 (5.0%) carry an A–F grade and 286 (95.0%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
Top-Graded Idaho Lakes
The highest water-quality grades in Idaho — clarity, depth, and A-F grade at a glance.
Lake Map
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New grades, clarity readings, and algae advisories for Idaho lakes as samples come in. Unsubscribe anytime.
Where to stay near Idaho's top-graded lakes
Cabins, resorts, and vacation rentals around Black Cnyn Reservoir Ns of and the other top-graded Idaho lakes pinned on the map. Live availability and prices.
Top-rated places to stay near Black Cnyn Reservoir Ns of
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Explore Idaho Lakes
Idaho reports, fishing & travel
Reservoirs in Idaho
View all 12 reservoirs →12 of 301 Idaho lakes (4%) are man-made reservoirs in the USACE National Inventory of Dams. 12 carry a High or Significant hazard rating. The oldest matched dam dates to 1906.
EPA-Impaired Lakes in Idaho
View all 6 impaired lakes →6 of 301 Idaho lakes (2%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 5 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Idaho Fishing & Recreation
Licenses, regulations, records, and where to fish across Idaho.
Counties
All 41 Idaho 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.
Cleanest Idaho Lakes
15 lakes15 graded lakes
| # | Lake | County | Grade | Clarity | Trophic | Access |
|---|---|---|---|---|---|---|
| 1 | ABlack Cnyn Reservoir Ns of | Gem | A | 7.5 ft | Mesotrophic | ✓ Public |
| 2 | ADeadwood Reservoir | Valley | A | 17.2 ft | Oligotrophic | Not recorded |
| 3 | ALake Lowell Across From Upper Embankment | Canyon | A | 7.2 ft | Mesotrophic | ✓ Public |
| 4 | CPasisades Reservoir | Bonneville | C | 10.5 ft | Mesotrophic | ✓ Public |
| 5 | CPalisades Lake | Bonneville | C | 13.1 ft | Mesotrophic | Not recorded |
| 6 | CPalisades Reservoir | Bonneville | C | 12.5 ft | Oligotrophic | ✓ Public |
| 7 | CMud Lake | Bear Lake | C | No recent samples | Eutrophic | Not recorded |
| 8 | DCas Reservoir Inlet Between Lf And | Valley | D | 4.3 ft | Eutrophic | ✓ Public |
| 9 | DLake Walcott | Cassia | D | 4.3 ft | Eutrophic | ✓ Public |
| 10 | DIsland Park Lake | Fremont | D | 9.8 ft | Eutrophic | ✓ Public |
| 11 | DCascade R W of Sugarloaf Island Lake | Valley | D | 8.2 ft | Eutrophic | ✓ Public |
| 12 | DIsland Pk Reservoir | Fremont | D | 2.3 ft | Eutrophic | Not recorded |
| 13 | DLittlewood Lake | Blaine | D | 12.5 ft | Eutrophic | ✓ Public |
| 14 | DMountain View Lake | Owyhee | D | No recent samples | Eutrophic | ✓ Public |
| 15 | DAmerican Falls R-Little Hole Draw Lake | Power | D | No recent samples | Eutrophic | Not recorded |
Idaho Lakes Needing Attention
15 lakes15 graded lakes
| # | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | DAmerican Falls R-Little Hole Draw Lake | Power | D | No recent samples |
| 2 | DMountain View Lake | Owyhee | D | No recent samples |
| 3 | DLittlewood Lake | Blaine | D | 12.5 ft |
| 4 | DIsland Pk Reservoir | Fremont | D | 2.3 ft |
| 5 | DCascade R W of Sugarloaf Island Lake | Valley | D | 8.2 ft |
| 6 | DIsland Park Lake | Fremont | D | 9.8 ft |
| 7 | DLake Walcott | Cassia | D | 4.3 ft |
| 8 | DCas Reservoir Inlet Between Lf And | Valley | D | 4.3 ft |
| 9 | CMud Lake | Bear Lake | C | No recent samples |
| 10 | CPalisades Reservoir | Bonneville | C | 12.5 ft |
| 11 | CPalisades Lake | Bonneville | C | 13.1 ft |
| 12 | CPasisades Reservoir | Bonneville | C | 10.5 ft |
| 13 | ALake Lowell Across From Upper Embankment | Canyon | A | 7.2 ft |
| 14 | ADeadwood Reservoir | Valley | A | 17.2 ft |
| 15 | ABlack Cnyn Reservoir Ns of | Gem | A | 7.5 ft |
Idaho lists 301 lakes across 41 counties, 15 of which carry a grade. Most of the input behind those grades flows through monitoring run by the Idaho Department of Environmental Quality, which feeds state, federal, and volunteer measurements into the EPA Water Quality Portal. The remainder are here because they are real named lakes, not because anyone has sampled them recently.
Lake Pend Oreille is the largest lake in this index at about 92,764 acres, one of 46 here that clear 500 acres. The rest run smaller, and small and shallow is where nutrient loading shows up first, because there is less water to dilute it and the bottom sediment stays within reach of the sun. Gem ranks highest on average county grade (A), and Blaine sits lowest. With only 41 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
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.
Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.
More on lake water quality
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.
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.
The cleanest and dirtiest U.S. lakes in the monitoring record, ranked on EPA nutrient and algae condition classes across thousands of graded lakes.
The clearest lakes in America, ranked by measured Secchi depth in feet from EPA monitoring data — and why the famously clear lakes are missing.