South Twin Lake

On the scoring rubric South Twin Lake grades a D: clarity at 5.0 ft and phosphorus readings still being added keep it in the lower bracket for 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.

Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. 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. South Twin Lake ranks 25 of 28 in Portage County — at the lower end of the locally monitored distribution.

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.