The State of Montana's Lakes 2026

Cleanest lakes in Montana

Cleanest combined nutrient and algae readings, 2026.

1. 1.Tally Lake — Flathead County15.2 ft clarity · 5 µg/L P · 1 µg/L chl-a
2. 2.Whitefish Lake — Flathead County22.0 ft clarity · 3 µg/L P · 1 µg/L chl-a
3. 3.Swan Lake North End — Lake County3 µg/L P · 1 µg/L chl-a
4. 4.Little Bitterroot Lake Center — Flathead County4 µg/L P · 1 µg/L chl-a
5. 5.Inez Lake — Missoula County7 µg/L P · 2 µg/L chl-a
6. 6.Alva Lake — Missoula County5 µg/L P · 1 µg/L chl-a
7. 7.Cooney Reservoir — Carbon County12.5 ft clarity · 20 µg/L P · 3 µg/L chl-a
8. 8.East Bay Lake — Lake County3 µg/L P
9. 9.Big Arm Bay Lake — Lake County2 µg/L P
10. 10.Dickey Lake — Lincoln County11 µg/L P · 2 µg/L chl-a

Highest measured algae

Highest measured chlorophyll-a (algae) concentration.

1. 1.Boneau Reservoir — Chouteau County39 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.Little Bitterroot Lake Center — Flathead County
2. 2.Alva Lake — Missoula County
3. 3.Lake Alva Mid-Lake Deep — Missoula County
4. 4.Canyon Ferry Lake — Broadwater County
5. 5.Jette Lake — Lake County
6. 6.Elbow Lake — Powell County
7. 7.Lake Koocanusa — Lincoln County
8. 8.Rainy Lake Deep — Missoula County

Declining

1. 1.Loon Lake — Lake County
2. 2.Mary Ronan Lake West — Lake County
3. 3.Foys Lake — Flathead County
4. 4.Rogers Lake — Flathead County
5. 5.Mary Ronan Lake East — Lake County
6. 6.Lake Helena Outlet — Lewis and Clark County
7. 7.Clark Canyon Reservoir — Beaverhead County
8. 8.Dickey Lake — Lincoln County