Updated Aug 2026
Montana Lake Water Quality
Water quality grades for 188 monitored lakes across 13 counties.
Montana has 188 graded lakes in our dataset, sampled across 13 counties, drawing on monitoring by the Montana Department of Environmental Quality plus federal and volunteer measurements. Only about 14% of Montana's lakes pull an A grade — the smaller, agricultural-watershed lakes that dominate the dataset tend to carry persistent nutrient loads. 11% of lakes fall in the D or F brackets.
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 Montana's A-grade waters. Carbon ranks highest on average county grade (B). Chouteau sits lowest. With only 13 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
All 188 Montana lakes in the index: 106 (56.4%) carry an A–F grade and 82 (43.6%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
Where to stay near Montana
Top-rated places to stay near Montana
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Top-Graded Montana Lakes
The highest water-quality grades in Montana — clarity, depth, and A-F grade at a glance.
Lake Map
Explore Montana Lakes
Montana reports, fishing & travel
Reservoirs in Montana
View all 32 reservoirs →32 of 188 Montana lakes (17%) are man-made reservoirs in the USACE National Inventory of Dams. 28 carry a High or Significant hazard rating. The oldest matched dam dates to 1911.
EPA-Impaired Lakes in Montana
View all 24 impaired lakes →24 of 188 Montana lakes (13%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 6 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Montana Fishing & Recreation
Licenses, regulations, records, and where to fish across Montana.
Counties
All 13 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
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity | Trophic | Access |
|---|---|---|---|---|---|---|
| 1 | ATally Lake | Flathead | A | 15.2 ft | Oligotrophic | Not recorded |
| 2 | AWhitefish Lake | Flathead | A | 22 ft | Oligotrophic | ✓ Public |
| 3 | AWhitefish Lake | Flathead | A | 23.5 ft | Oligotrophic | Not recorded |
| 4 | ALake Koocanusa | Lincoln | A | No recent samples | Oligotrophic | Not recorded |
| 5 | ASwan Lake South End | Lake | A | No recent samples | Oligotrophic | ✓ Public |
| 6 | ASwan Lake North End | Lake | A | No recent samples | Oligotrophic | Not recorded |
| 7 | AFlathead Lake | Flathead | A | No recent samples | Oligotrophic | ✓ Public |
| 8 | AFlathead Lake | Lake | A | No recent samples | Oligotrophic | ✓ Public |
| 9 | AFlathead Lake | Lake | A | No recent samples | Oligotrophic | ✓ Public |
| 10 | AUpper Stillwater Lake | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 11 | AHolland Lake | Missoula | A | No recent samples | Oligotrophic | Not recorded |
| 12 | AFlathead Lake | Lake | A | No recent samples | Oligotrophic | ✓ Public |
| 13 | ASmith Lake | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 14 | AUpper Whitefish Lake | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 15 | AFlathead Lake | Flathead | A | No recent samples | Oligotrophic | ✓ Public |
| 16 | AGlen Lake | Lincoln | A | No recent samples | Oligotrophic | Not recorded |
| 17 | ALindbergh Lake | Missoula | A | No recent samples | Oligotrophic | Not recorded |
| 18 | ALower Stillwater Lake | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 19 | ALittle Bitterroot Lake | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 20 | ABig Therriault Lake | Lincoln | A | No recent samples | Oligotrophic | Not recorded |
| 21 | AFlathead Lake | Lake | A | No recent samples | Oligotrophic | Not recorded |
| 22 | ALake Mcdonald | Flathead | A | No recent samples | Oligotrophic | ✓ Public |
| 23 | ALake Mcdonald | Flathead | A | No recent samples | Oligotrophic | Not recorded |
| 24 | AFlathead Lake | Lake | A | No recent samples | Oligotrophic | ✓ Public |
| 25 | ALittle Bitterroot Lake Center | Flathead | A | No recent samples | Oligotrophic | Not recorded |
25 Lakes Needing Attention
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | FBoneau Reservoir | Chouteau | F | No recent samples |
| 2 | DElbow Lake | Powell | D | No recent samples |
| 3 | DBig Sky_deep Lake | Missoula | D | No recent samples |
| 4 | DLake Helena Outlet | Lewis and Clark | D | No recent samples |
| 5 | DSpencer Lake | Flathead | D | No recent samples |
| 6 | DBootjack Lake | Flathead | D | No recent samples |
| 7 | DLake Five | Flathead | D | No recent samples |
| 8 | DDollar Lake | Flathead | D | No recent samples |
| 9 | DMary Ronan Lake East | Lake | D | No recent samples |
| 10 | DJette Lake | Lake | D | No recent samples |
| 11 | DTetrault Lake | Lincoln | D | No recent samples |
| 12 | DBeaver Lake | Flathead | D | No recent samples |
| 13 | DRogers Lake | Flathead | D | No recent samples |
| 14 | DFoys Lake | Flathead | D | No recent samples |
| 15 | DMary Ronan Lake West | Lake | D | No recent samples |
| 16 | DSkyles Lake | Flathead | D | No recent samples |
| 17 | DLoon Lake | Lake | D | No recent samples |
| 18 | DLake Mary Ronan East Location | Lake | D | 15.1 ft |
| 19 | DLake Mary Ronan West Location | Lake | D | 14.7 ft |
| 20 | DBig Sky Lake South Hole | Missoula | D | 17.6 ft |
| 21 | CBig Sky Lake | Missoula | C | No recent samples |
| 22 | CBlanchard Lake | Flathead | C | No recent samples |
| 23 | CEcho Lake | Flathead | C | No recent samples |
| 24 | CLake Blaine | Flathead | C | No recent samples |
| 25 | CBailey Lake | Flathead | C | No recent samples |
Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.
More on lake water quality
The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.
State-by-state Secchi champions: the clearest measured lake in each U.S. state we grade, which is not the same as the cleanest nutrient grade.
A data snapshot of the lakes we grade: how the A–F distribution breaks down, the clearest and the greenest states, and where the water is improving.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.