Lake County Lake Quality
Montana, 27 lakes, average grade A (Excellent)
There are 27 graded lakes in Lake County. The mix is varied enough that the county-level average tells you the broad strokes, not the choose-this-cabin specifics. On average, Lake County lakes grade an A. That puts it near the top of the {{stateName}} county ranking.
The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Nla Lake (A); the most-stressed is Flathead Lake Unknown Culvert (C).
Quick Answers for Lake County
Planning a trip? Check special fishing regulations for Nla Lake — or check the official Montana fishing regulations.
All Lakes in Lake County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Nla Lake | A | 32.3 ft | 7.8 µg/L |
| 2 | Lake Mary Ronan East Location | A | 15.1 ft | 9 µg/L |
| 3 | Swan Lake | A | 23.3 ft | - |
| 4 | Lake Mary Ronan | A | 16.8 ft | - |
| 5 | Jette Lake | A | 15.8 ft | - |
| 6 | Swan Lake South End | A | - | 3.7 µg/L |
| 7 | Loon Lake | A | - | 6.7 µg/L |
| 8 | Swan Lake North End | A | - | 3.6 µg/L |
| 9 | Jette Lake | A | - | 16.6 µg/L |
| 10 | Mary Ronan Lake East | A | - | 15.8 µg/L |
| 11 | Flathead Lake | A | - | 3.6 µg/L |
| 12 | Flathead Lake | A | - | 2.6 µg/L |
| 13 | Flathead Lake | A | - | 3 µg/L |
| 14 | Flathead Lake | A | - | 2.4 µg/L |
| 15 | Flathead Lake | A | - | 2.4 µg/L |
| 16 | Lake Mary Ronan-Cormorants | A | - | 13 µg/L |
| 17 | East Bay Lake | A | - | 3 µg/L |
| 18 | Flathead Lake Polson Bay South | A | - | 3.5 µg/L |
| 19 | Flathead Lake Polson Bay North | A | - | 3 µg/L |
| 20 | Big Arm Bay Lake | A | - | 2 µg/L |
| 21 | Flathead Lake Skidoo Bay | A | - | 3.5 µg/L |
| 22 | Swan Lake South Basin | A | - | - |
| 23 | Swan Lake North Basin | A | - | - |
| 24 | Lake Mary Ronan West Location | A | 14.7 ft | 13.5 µg/L |
| 25 | Flathead Lake | B | 11.8 ft | - |
| 26 | Mary Ronan Lake West | B | - | 33.6 µg/L |
| 27 | Flathead Lake Unknown Culvert | C | - | 43 µg/L |
Other Counties in Montana
See how Lake County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 34 states.
Where to stay near Lake County
Top-rated places to stay near Lake County
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a trip to Lake County?
Get lake water-quality report cards plus the best cabin & lodging deals near Lake County — straight to your inbox. No spam, unsubscribe anytime.
Source: EPA National Aquatic Resource Surveys, 2026.