
Clay County Lake Quality
Iowa, 6 lakes, average grade C (Fair)
Clay County holds 6 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county average is C — the middle of the rubric, with one or two lakes pulling the average up or down depending on year.
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. Stolley Pit Lake (C) is the cleanest in the county.
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Quick Answers for Clay County
Planning a trip? Check special fishing regulations for Stolley Pit Lake — or check the official Iowa fishing regulations.
All Lakes in Clay County
6 lakes · 1 graded, 5 not rated1 graded lake · 5 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | CStolley Pit LakeClay | C | 2.7 ft | 48 µg/L | 17.3 µg/L | No recent samples | ✓ Public |
| NRElk LakeClay | NR | No recent samples | No recent samples | No recent samples | No recent samples | ✓ Public | |
| NRDan Green SloughClay | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRTrumbull LakeClay | NR | No recent samples | No recent samples | No recent samples | No recent samples | ✓ Public | |
| NRMud LakeClay | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRRound LakeClay | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded |
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.
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Other Counties in Iowa
See how Clay 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.