The State of Montana's Lakes 2026

Cleanest lakes in Montana

Highest combined water-quality score, 2026.

1. 1.ATally Lake — Flathead County15.2 ft clarity · 5 µg/L P · 1 µg/L chl-a
2. 2.AWhitefish Lake — Flathead County22.0 ft clarity · 3 µg/L P · 1 µg/L chl-a
3. 3.ASwan Lake North End — Lake County3 µg/L P · 1 µg/L chl-a
4. 4.ALittle Bitterroot Lake Center — Flathead County4 µg/L P · 1 µg/L chl-a
5. 5.AInez Lake — Missoula County7 µg/L P · 2 µg/L chl-a
6. 6.AAlva Lake — Missoula County5 µg/L P · 1 µg/L chl-a
7. 7.BCooney Reservoir — Carbon County12.5 ft clarity · 20 µg/L P · 3 µg/L chl-a
8. 8.BEast Bay Lake — Lake County3 µg/L P
9. 9.BBig Arm Bay Lake — Lake County2 µg/L P
10. 10.CDickey Lake — Lincoln County11 µg/L P · 2 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

1. 1.DLoon Lake — Lake County7 µg/L P · 2 µg/L chl-a
2. 2.DMary Ronan Lake West — Lake County34 µg/L P · 5 µg/L chl-a
3. 3.DFoys Lake — Flathead County16 µg/L P · 4 µg/L chl-a
4. 4.DRogers Lake — Flathead County12 µg/L P · 3 µg/L chl-a
5. 5.DJette Lake — Lake County17 µg/L P · 5 µg/L chl-a
6. 6.DMary Ronan Lake East — Lake County16 µg/L P · 4 µg/L chl-a
7. 7.DLake Helena Outlet — Lewis and Clark County116 µg/L P
8. 8.CDickey Lake — Lincoln County11 µg/L P · 2 µg/L chl-a
9. 9.CMcgilvray Lake — Flathead County6 µg/L P · 2 µg/L chl-a
10. 10.CHanson-Doyle Lake — Flathead County7 µg/L P · 2 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

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

Trending lakes

Improving

1. 1.ALittle Bitterroot Lake Center — Flathead County
2. 2.AAlva Lake — Missoula County
3. 3.BLake Alva Mid-Lake Deep — Missoula County
4. 4.CCanyon Ferry Lake — Broadwater County
5. 5.DJette Lake — Lake County
6. 6.DElbow Lake — Powell County
7. 7.NRLake Koocanusa — Lincoln County
8. 8.NRRainy Lake Deep — Missoula County

Declining

1. 1.DLoon Lake — Lake County
2. 2.DMary Ronan Lake West — Lake County
3. 3.DFoys Lake — Flathead County
4. 4.DRogers Lake — Flathead County
5. 5.DMary Ronan Lake East — Lake County
6. 6.DLake Helena Outlet — Lewis and Clark County
7. 7.CClark Canyon Reservoir — Beaverhead County
8. 8.CDickey Lake — Lincoln County