Jefferson County Lake Quality

Jefferson County's 29 monitored lakes fall in the moderate-density bracket for Colorado. 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. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. With an average grade of B, Jefferson County sits comfortably above the Colorado median.

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. Chatfield Reservoir (B) is the cleanest in the county.