Skip to main content
LakeQuality

Data Report · Updated July 2026

The State of Idaho’s Lakes 2026

Of 96 monitored lakes in Idaho, 47.9% earn a water-quality grade of A or B and 20.8% score D or F (average grade C). Nla Lake is the cleanest;

96
Monitored lakes graded
C
Average grade
47.9%
Grade A or B
0%
On impaired list

Idaho grade distribution

A
B
C
D
A: 23 (24.0%)B: 23 (24.0%)C: 30 (31.3%)D: 15 (15.6%)F: 5 (5.2%)

Cleanest lakes in Idaho

Highest combined water-quality score, 2026.

  1. 1.ANla Lake — Gem County17.6 ft clarity · 20 µg/L P · 1 µg/L chl-a
  2. 2.ANla Lake — Valley County18.6 ft clarity · 10 µg/L P · 1 µg/L chl-a
  3. 3.ANla Lake — Blaine County29.9 ft clarity · 15 µg/L P · 1 µg/L chl-a
  4. 4.ANla Lake — Kootenai County19.2 ft clarity · 16 µg/L P · 2 µg/L chl-a
  5. 5.ANla Lake — Shoshone County10 µg/L P · 1 µg/L chl-a
  6. 6.ANla Lake — Valley County18 µg/L P · 4 µg/L chl-a
  7. 7.ANla Lake — Custer County18 µg/L P · 1 µg/L chl-a
  8. 8.ADeadwood Reservoir — Valley County17.2 ft clarity · 14 µg/L P · 2 µg/L chl-a
  9. 9.ARirie Reservoir — Bonneville County19 µg/L P · 3 µg/L chl-a
  10. 10.ABear Lake Off North Beach — Bear Lake County21.7 ft clarity · 11 µg/L P

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FNla Lake — Camas County0.8 ft clarity · 705000 µg/L P · 428 µg/L chl-a
  2. 2.FNla Lake — Owyhee County0.7 ft clarity · 275000 µg/L P · 65 µg/L chl-a
  3. 3.FNla Lake — Bear Lake County3.3 ft clarity · 175000 µg/L P · 24 µg/L chl-a
  4. 4.FNla Lake — Camas County5.2 ft clarity · 548 µg/L P · 33 µg/L chl-a
  5. 5.DLittle Wood Reservoir — Blaine County65 µg/L P · 31 µg/L chl-a
  6. 6.DNla Lake — Latah County3.4 ft clarity · 85000 µg/L P · 20 µg/L chl-a
  7. 7.DNla Lake — Owyhee County0.7 ft clarity · 201000 µg/L P · 18 µg/L chl-a
  8. 8.DNla Lake — Fremont County3.4 ft clarity · 79000 µg/L P · 28 µg/L chl-a
  9. 9.DNla Lake — Elmore County3.4 ft clarity · 93000 µg/L P · 27 µg/L chl-a
  10. 10.DNla Lake — Franklin County5.2 ft clarity · 69 µg/L P · 23 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

  1. 1.FNla Lake — Camas County428 µg/L chlorophyll-a
  2. 2.DMann Creek Reservoir — Washington County179 µg/L chlorophyll-a
  3. 3.FNla Lake — Owyhee County65 µg/L chlorophyll-a
  4. 4.FNla Lake — Camas County33 µg/L chlorophyll-a
  5. 5.DLittle Wood Reservoir — Blaine County31 µg/L chlorophyll-a
  6. 6.DNla Lake — Fremont County28 µg/L chlorophyll-a
  7. 7.DNla Lake — Elmore County27 µg/L chlorophyll-a
  8. 8.CNla Lake — Caribou County25 µg/L chlorophyll-a
  9. 9.FNla Lake — Bear Lake County24 µg/L chlorophyll-a
  10. 10.CNla Lake — Cassia County24 µg/L chlorophyll-a

Explore Idaho 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 Idaho lakes.

Cite this report: LakeQuality, “The State of Idaho’s Lakes 2026,” lakequality.org/report/id.