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LakeQuality

Data Report · Updated July 2026

The State of Montana’s Lakes 2026

Of 233 monitored lakes in Montana, 84.1% earn a water-quality grade of A or B and 10.3% score D or F (average grade B). Big Sky Lake South Hole is the cleanest;

233
Monitored lakes graded
B
Average grade
84.1%
Grade A or B
0%
On impaired list

Montana grade distribution

A
B
A: 172 (73.8%)B: 24 (10.3%)C: 13 (5.6%)D: 12 (5.2%)F: 12 (5.2%)

Cleanest lakes in Montana

Highest combined water-quality score, 2026.

  1. 1.ABig Sky Lake South Hole — Missoula County17.7 ft clarity · 10 µg/L P
  2. 2.ATally Lake — Flathead County15.2 ft clarity · 5 µg/L P · 1 µg/L chl-a
  3. 3.ANla Lake — Granite County28.4 ft clarity · 6 µg/L P · 1 µg/L chl-a
  4. 4.ANla Lake — Missoula County41.9 ft clarity · 4 µg/L P · 0 µg/L chl-a
  5. 5.ANla Lake — Flathead County47.6 ft clarity · 1 µg/L P · 1 µg/L chl-a
  6. 6.ANla Lake — Flathead County20.0 ft clarity · 14 µg/L P · 1 µg/L chl-a
  7. 7.ANla Lake — Lewis and Clark County15.9 ft clarity · 12 µg/L P · 1 µg/L chl-a
  8. 8.ANla Lake — Lincoln County16.9 ft clarity · 14 µg/L P · 3 µg/L chl-a
  9. 9.ANla Lake — Lincoln County25.4 ft clarity · 13 µg/L P · 1 µg/L chl-a
  10. 10.ANla Lake — Lake County32.3 ft clarity · 8 µg/L P · 1 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FNla Lake — Sheridan County0.6 ft clarity · 3436000 µg/L P · 43 µg/L chl-a
  2. 2.FNla Lake — Chouteau County1.1 ft clarity · 343000 µg/L P · 192 µg/L chl-a
  3. 3.FNla Lake — Hill County0.8 ft clarity · 390000 µg/L P · 144 µg/L chl-a
  4. 4.FNla Lake — Valley County1.0 ft clarity · 337 µg/L P · 137 µg/L chl-a
  5. 5.FNla Lake — Blaine County0.5 ft clarity · 485 µg/L P · 367 µg/L chl-a
  6. 6.FNla Lake — Blaine County1.2 ft clarity · 374 µg/L P · 171 µg/L chl-a
  7. 7.FNla Lake — Phillips County0.9 ft clarity · 267 µg/L P · 95 µg/L chl-a
  8. 8.FNla Lake — Liberty County738000 µg/L P · 31 µg/L chl-a
  9. 9.FNla Lake — Toole County2.6 ft clarity · 2170 µg/L P · 28 µg/L chl-a
  10. 10.DNla Lake — Phillips County2.1 ft clarity · 78000 µg/L P · 11 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

  1. 1.FNla Lake — Blaine County367 µg/L chlorophyll-a
  2. 2.FNla Lake — Chouteau County192 µg/L chlorophyll-a
  3. 3.FNla Lake — Blaine County171 µg/L chlorophyll-a
  4. 4.FNla Lake — Hill County144 µg/L chlorophyll-a
  5. 5.FNla Lake — Valley County137 µg/L chlorophyll-a
  6. 6.FNla Lake — Phillips County95 µg/L chlorophyll-a
  7. 7.FNla Lake — Sheridan County43 µg/L chlorophyll-a
  8. 8.DBoneau Reservoir — Chouteau County39 µg/L chlorophyll-a
  9. 9.FNla Lake — Liberty County31 µg/L chlorophyll-a
  10. 10.FNla Lake — Toole County28 µg/L chlorophyll-a

Explore Montana 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 Montana lakes.

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