Mason County Lake Quality

Mason County's 21 monitored lakes fall in the moderate-density bracket for Illinois. 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. No lake in Mason County has enough sampling behind it for an A–F grade yet. The 21 monitored lakes below carry whatever clarity, phosphorus, and chlorophyll-a measurements exist.

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.