Valley County Lake Quality

Valley County's 26 monitored lakes fall in the moderate-density bracket for Idaho. The county-level average smooths over real differences — a single agricultural watershed near the southwest corner can pull the average down even if the rest of the county runs clean. 3 of those lakes 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. Deadwood Reservoir (A) is the cleanest in the county.