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LakeQuality
Pink and orange afterglow over the treeline on Slim Lake after sunset

Updated August 2026

The State of Idaho's Lakes 2026

Of 29 lakes in Idaho, 20% earn a water-quality grade of A or B and 53.3% score D or F (average grade C). Black Cnyn Reservoir Ns of is the cleanest; 6 lakes (20.7%) are on an official impaired-waters list.

Data Report

29
Monitored lakes graded
C
Average grade
20%
Grade A or B
20.7%
On impaired list

Idaho grade distribution

A
C
D
A: 3 (20.0%)B: 0 (0.0%)C: 4 (26.7%)D: 8 (53.3%)F: 0 (0.0%)

Cleanest lakes in Idaho

Highest combined water-quality score, 2026.

  1. 1.ABlack Cnyn Reservoir Ns of — Gem County7.5 ft clarity · 30 µg/L P · 3 µg/L chl-a
  2. 2.CPalisades Lake — Bonneville County13.1 ft clarity · 21 µg/L P · 2 µg/L chl-a
  3. 3.DCas Reservoir Inlet Between Lf And — Valley County4.3 ft clarity · 40 µg/L P · 12 µg/L chl-a
  4. 4.DLake Walcott — Cassia County4.3 ft clarity · 51 µg/L P · 15 µg/L chl-a
  5. 5.DCascade R W of Sugarloaf Island Lake — Valley County8.2 ft clarity · 29 µg/L P · 8 µg/L chl-a
  6. 6.DLittlewood Lake — Blaine County12.5 ft clarity · 28 µg/L P · 14 µg/L chl-a
  7. 7.DMountain View Lake — Owyhee County39 µg/L P · 14 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.DCas Reservoir Inlet Between Lf And — Valley County4.3 ft clarity · 40 µg/L P · 12 µg/L chl-a
  2. 2.DLake Walcott — Cassia County4.3 ft clarity · 51 µg/L P · 15 µg/L chl-a
  3. 3.DCascade R W of Sugarloaf Island Lake — Valley County8.2 ft clarity · 29 µg/L P · 8 µg/L chl-a
  4. 4.DLittlewood Lake — Blaine County12.5 ft clarity · 28 µg/L P · 14 µg/L chl-a
  5. 5.DMountain View Lake — Owyhee County39 µg/L P · 14 µg/L chl-a
  6. 6.CPalisades Lake — Bonneville County13.1 ft clarity · 21 µg/L P · 2 µg/L chl-a
  7. 7.ABlack Cnyn Reservoir Ns of — Gem County7.5 ft clarity · 30 µg/L P · 3 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

  1. 1.NRMann Creek Reservoir — Washington County179 µg/L chlorophyll-a
  2. 2.NRLittle Wood Reservoir — Blaine County30 µg/L chlorophyll-a
  3. 3.NRSoldier Meadow Reservoir East Arm — Nez Perce County29 µg/L chlorophyll-a
  4. 4.NRSoldiers Meadow Reservoir West Arm — Nez Perce County29 µg/L chlorophyll-a

Trending lakes

Improving

  1. 1.CMud Lake — Bear Lake County
  2. 2.DCas Reservoir Inlet Between Lf And — Valley County
  3. 3.DLake Walcott — Cassia County
  4. 4.DIsland Park Lake — Fremont County
  5. 5.DCascade R W of Sugarloaf Island Lake — Valley County
  6. 6.DIsland Pk Reservoir — Fremont County
  7. 7.NRWaha Reservoir Middle — Nez Perce County
  8. 8.NRSoldier Meadow Reservoir East Arm — Nez Perce County

Declining

  1. 1.DLittlewood Lake — Blaine County
  2. 2.CPalisades Reservoir — Bonneville County
  3. 3.ALake Lowell Across From Upper Embankment — Canyon County
  4. 4.NRLittle Wood Reservoir — Blaine County

Impaired waters in Idaho

6 lakes appear on the EPA/state 303(d) impaired-waters list (3 formally listed, 5 with a cleanup plan/TMDL).

Nutrients (phosphorus, nitrogen) · 2Hydrologic alteration · 2Mercury · 2pH / acidity · 1Metals (other than mercury) · 1Algal growth · 1Oxygen depletion (low DO) · 1Sediment / turbidity · 1

Explore Idaho lakes

Lake Fishing Forecast

Species activity outlook by water temperature and season

Idaho Lake Trips

Where to stay at Idaho'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 Idaho lakes.

Cite this report: LakeQuality, “The State of Idaho's Lakes 2026”.