Sully County Lake Quality
South Dakota, 1 lakes, average grade B (Good)
Sully County has 1 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. With an average grade of B, Sully County sits comfortably above the South Dakota median.
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 a leading lake (—); the most-stressed is a stressed lake (—).
Quick Answers for Sully County
All Lakes in Sully County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|
Other Counties in South Dakota
See how Sully County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 15 states.
Where to stay near Sully County
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Source: EPA National Aquatic Resource Surveys, 2026.