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LakeQuality
Late afternoon sun over Slim Lake, scattering a bright path across wind-textured water
C

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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All Lakes in Clay County

6 lakes · 1 graded, 5 not rated1 graded lake · 5 not rated, hidden

#LakeGradeClarityPhosphorusAlgaeTrophicAccess
1CStolley Pit LakeClayC2.7 ft48 µg/L17.3 µg/LNo recent samples✓ Public
NRElk LakeClayNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRDan Green SloughClayNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRTrumbull LakeClayNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRMud LakeClayNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRRound LakeClayNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot 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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Source: EPA National Aquatic Resource Surveys, 2026.