Sioux County Lake Quality

Sioux County holds 1 monitored lake. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. Every one of those lake carries an A–F grade, so the county-level summary covers the full set rather than a monitored subset. The county-wide average grade is D — water quality issues are persistent across most of the monitored basins. 0 lakes fall into the F bracket outright.

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. Froelich Dam Lake (D) is the cleanest in the county.