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. High phosphorus is the leading drag on the score — likely from upland agriculture, lawn runoff, or in-lake sediment release.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. 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. Delavan Lake sits at rank 25 of 34 in Walworth County, in the lower half of the local distribution.

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. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.