San Juan County Lake Quality

San Juan County has 8 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. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county-wide average lands at B — solid water quality with most lakes carrying clear summer transparency and low-to-moderate phosphorus.

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. Little Eldorado Lake (B) is the cleanest in the county.