South Twin Lake

South Twin Lake earns an A — water clarity, phosphorus, and chlorophyll-a all rate among the best monitored waters in Portage County. Sub-grades cluster within a single letter of each other, which usually means the lake is in stable trophic balance rather than fighting one specific stressor.

A TSI value of 37 puts South Twin Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. South Twin Lake is a shallow lake at 9 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. The lake is compact at 54 acres, with partial shoreline records and little volume to buffer nutrient inputs. Among the 29 graded lakes in Portage County, South Twin Lake ranks 7 — in the top quartile locally.

South Twin Lake has no invasive species recorded in Wisconsin state databases as of 2021, though prevention practices still apply at all access points. 1 fish species are documented at South Twin Lake, a mixed warm-water community typical for Wisconsin lakes of this size. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.