Sully County Lake Quality
South Dakota, 9 lakes — no lake here has enough monitoring data for a grade
Sully County has 9 monitored 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. No lake in Sully County has enough sampling behind it for an A–F grade yet. The 9 monitored lakes below carry whatever clarity, phosphorus, and chlorophyll-a measurements exist.
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.
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Top-rated places to stay near Sully County
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Quick Answers for Sully County
Planning a trip? Check special fishing regulations for Fuller Lake — or check the official South Dakota fishing regulations.
All Lakes in Sully County
9 lakes · 0 graded, 9 not rated0 graded lakes · 9 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| NRFuller LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRLake OkobojoSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRPost LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRWalker LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRDurrstein LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRWarnes SloughSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRSully LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRStone LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRCottonwood LakeSully | NR | No recent samples | No recent samples | No recent samples | No recent samples | ✓ Public |
NR = not rated: there is not enough recent sampling to support a letter. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
Lake Match
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Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
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 United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs..
Source: EPA National Aquatic Resource Surveys, 2026.