Chouteau County Lake Quality
Montana, 3 lakes, average grade D (Poor)
Chouteau County has 3 graded lakes — a thin enough sample that one or two stressed lakes can move the county average noticeably. Use the per-lake pages below for what actually matters. On average, Chouteau County lakes grade D, with phosphorus loading and reduced clarity showing up across most of the dataset.
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. Williamson Reservoir (C) is the cleanest in the county.
Quick Answers for Chouteau County
Planning a trip? Check special fishing regulations for Williamson Reservoir — or check the official Montana fishing regulations.
All Lakes in Chouteau County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Williamson Reservoir | C | - | 410 µg/L |
| 2 | Boneau Reservoir | D | - | 50 µg/L |
| 3 | Nla Lake | F | 1.1 ft | 343000 µg/L |
Other Counties in Montana
See how Chouteau 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 Chouteau County
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Source: EPA National Aquatic Resource Surveys, 2026.