Updated Jul 2026
Montana Lake Water Quality
Water quality grades for 233 monitored lakes across 31 counties.
{{stateName}} has 233 graded lakes in our dataset, sampled across 31 counties, drawing on monitoring by {{agency}} plus federal and volunteer measurements. Roughly 74% of the state's graded lakes carry an A — 172 lakes that earn the top bracket on the combined Secchi/phosphorus/chlorophyll-a rubric. That share is high by national standards and reflects the depth and natural clarity of glacial lakes in the region.
Trophically, oligotrophic lakes — cold, clear, low-nutrient — dominate the state's graded inventory. These are the deep glacial basins that buffer against nutrient inputs by sheer volume, and they hold most of {{stateName}}'s A-grade waters. Granite ranks highest on average county grade (A). Liberty sits lowest. With only 31 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
How Montana’s 233 graded lakes break down from A (excellent) to F.
Top-Graded Montana Lakes
The highest water-quality grades in Montana — clarity, depth, and A-F grade at a glance.
Big Sky Lake South Hole
Missoula County, Montana
See grade for Big Sky Lake South Hole →Tally Lake
Flathead County, Montana
See grade for Tally Lake →Nla Lake
Granite County, Montana
See grade for Nla Lake →Nla Lake
Missoula County, Montana
See grade for Nla Lake →Nla Lake
Flathead County, Montana
See grade for Nla Lake →Nla Lake
Flathead County, Montana
See grade for Nla Lake →Nla Lake
Lewis and Clark County, Montana
See grade for Nla Lake →Nla Lake
Lincoln County, Montana
See grade for Nla Lake →Nla Lake
Lincoln County, Montana
See grade for Nla Lake →Lake Map
Explore Montana Lakes
Montana reports, fishing & travel
Montana Fishing & Recreation
Licenses, regulations, records, and where to fish across Montana.
Counties
All 31 Montana counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Big Sky Lake South Hole | Missoula | A | 17.7 ft |
| 2 | Tally Lake | Flathead | A | 15.2 ft |
| 3 | Nla Lake | Granite | A | 28.4 ft |
| 4 | Nla Lake | Missoula | A | 41.9 ft |
| 5 | Nla Lake | Flathead | A | 47.6 ft |
| 6 | Nla Lake | Flathead | A | 20 ft |
| 7 | Nla Lake | Lewis and Clark | A | 15.9 ft |
| 8 | Nla Lake | Lincoln | A | 16.9 ft |
| 9 | Nla Lake | Lincoln | A | 25.4 ft |
| 10 | Nla Lake | Lake | A | 32.3 ft |
| 11 | Nla Lake | Ravalli | A | 18 ft |
| 12 | Lake Alva Mid-Lake Deep | Missoula | A | 21.6 ft |
| 13 | Salmon Lake South Basin | Missoula | A | 16.6 ft |
| 14 | Seeley Lake Middle Basin | Missoula | A | 15.4 ft |
| 15 | Whitefish Lake | Flathead | A | 22 ft |
| 16 | Whitefish Lake | Flathead | A | 23.5 ft |
| 17 | Placid Lake North Finger Outlet Hole | Missoula | A | 16.8 ft |
| 18 | Placid Lake Midlake Hole | Missoula | A | 16.5 ft |
| 19 | Lake Inez North Hole | Missoula | A | 17.4 ft |
| 20 | Salmon Lake Mid-South Basin | Missoula | A | 16.4 ft |
| 21 | Lake Inez South Hole | Missoula | A | 17.8 ft |
| 22 | Seeley Lake North Basin | Missoula | A | 16.8 ft |
| 23 | Clearwater Lake Deep | Missoula | A | 28.6 ft |
| 24 | Lake Koocanusa | Lincoln | A | - |
| 25 | Lake Koocanusa | Lincoln | A | - |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Big Sky_deep Lake | Missoula | F | - |
| 2 | Lake Helena Outlet | Lewis and Clark | F | - |
| 3 | Kipp Lake | Glacier | F | - |
| 4 | Nla Lake | Liberty | F | - |
| 5 | Nla Lake | Phillips | F | 0.9 ft |
| 6 | Nla Lake | Blaine | F | 1.2 ft |
| 7 | Nla Lake | Blaine | F | 0.5 ft |
| 8 | Nla Lake | Valley | F | 1 ft |
| 9 | Nla Lake | Hill | F | 0.8 ft |
| 10 | Nla Lake | Chouteau | F | 1.1 ft |
| 11 | Nla Lake | Sheridan | F | 0.6 ft |
| 12 | Nla Lake | Toole | F | 2.6 ft |
| 13 | Nla Lake | Phillips | D | 2.1 ft |
| 14 | Lake Helena Center Deep | Lewis and Clark | D | 5.6 ft |
| 15 | Clark Canyon Reservoir South of Island | Beaverhead | D | 4.9 ft |
| 16 | Nla Lake | Rosebud | D | 4.9 ft |
| 17 | Nla Lake | Sheridan | D | 1.9 ft |
| 18 | Nla Lake | Glacier | D | 1.6 ft |
| 19 | Nla Lake | Phillips | D | 2.3 ft |
| 20 | Nla Lake | Big Horn | D | 4.9 ft |
| 21 | Boneau Reservoir | Chouteau | D | - |
| 22 | Nla Lake | Wheatland | D | 2.2 ft |
| 23 | Nla Lake | Hill | D | 4.2 ft |
| 24 | Nla Lake | Valley | D | 2.1 ft |
| 25 | Williamson Reservoir | Chouteau | C | - |
Where to stay near Montana
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Source: EPA National Aquatic Resource Surveys, 2026.