Illinois Lake Water Quality

118 of the 594 Illinois lakes in this index carry a recent clarity, phosphorus or algae reading, spread over 96 counties. Those readings come from the Illinois Environmental Protection Agency, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, with no recent nutrient or algae sampling behind them. Most measured Illinois lakes are productive, often shallow, and frequently algal in late summer; each lake page shows its own phosphorus and chlorophyll-a readings.

By surface area the largest lake here is Carlyle Lake, at about 24,580 acres, and 66 of the lakes in this index cover more than 500 acres. Size is worth knowing before you go: a large deep basin buffers nutrient inputs by sheer volume, while a shallow one warms quickly and can turn over its whole water column in a single summer storm. A meaningful share of the state's measured lakes register as hypereutrophic: heavily nutrient-loaded, frequent blooms, low summer transparency. These are usually shallow, surrounded by row-crop agriculture, with little volume to buffer the inputs. The county with the lowest nutrient and algae readings on average is Winnebago. The gap between counties is large enough that a single figure for Illinois lake water quality hides real local variation.