Delavan Lake

Delavan Lake earns a C — middle-of-the-pack water quality, with one or two parameters dragging the average down from a stronger grade. Of the three parameters, phosphorus is the worst — at 73 µg/L of phosphorus, it keeps the lake susceptible to algal blooms in warm years.

At a TSI of 55, Delavan Lake reads as eutrophic — nutrient-rich enough that summer algal growth and reduced clarity are expected, not unusual. Delavan Lake is unusually deep for Wisconsin at 52 ft, which buffers the lake against summer warming and helps the deep water stay oxygenated. With 1,906 acres of surface and partial shoreline records, Delavan Lake is a large water by Wisconsin standards — recreational pressure is spread across a long shoreline. Within Walworth County's 35 graded lakes, Delavan Lake ranks 18 — below the local median, though not at the bottom.

Zebra mussel presence at Delavan Lake means the lake's clarity numbers carry an asterisk — filtered water is not the same as nutrient-poor water. Walleye are documented at Delavan Lake, one of 6 fish species on record for the lake. Public access is available — the lake is on the Wisconsin DNR public-access list. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.