Lewis and Clark County Lake Quality
Montana, 12 lakes, average grade B (Good)
Only 12 lakes in Lewis and Clark County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. The county-wide average lands at B — solid water quality with most lakes carrying clear summer transparency and low-to-moderate phosphorus.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Nla Lake (A) is the cleanest in the county.
Quick Answers for Lewis and Clark County
Planning a trip? Check special fishing regulations for Nla Lake — or check the official Montana fishing regulations.
All Lakes in Lewis and Clark County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Nla Lake | A | 15.9 ft | 12.3 µg/L |
| 2 | Hauser Lake | A | 15.6 ft | - |
| 3 | Hauser Lake D | A | 21.8 ft | - |
| 4 | Holter Reservoir | A | 19 ft | - |
| 5 | Holter Reservoir | A | 15.4 ft | - |
| 6 | Holter Reservoir | A | 26.2 ft | - |
| 7 | Hauser Lake | B | 12.7 ft | - |
| 8 | Canyon Ferry Lake | B | - | 21 µg/L |
| 9 | Canyon Ferry Lake | B | 11.5 ft | 39 µg/L |
| 10 | Hauser Lake | C | 8.7 ft | - |
| 11 | Lake Helena Center Deep | D | 5.6 ft | - |
| 12 | Lake Helena Outlet | F | - | 115.5 µg/L |
Other Counties in Montana
See how Lewis and Clark 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 Lewis and Clark County
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Source: EPA National Aquatic Resource Surveys, 2026.