Lake Koshkonong

On the scoring rubric Lake Koshkonong grades a D: clarity at 5.7 ft and 287 µg/L of phosphorus keep it in the lower bracket for Jefferson 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. Lake Koshkonong is a shallow lake at 7 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. With 10,595 acres of surface and partial shoreline records, Lake Koshkonong is a large water by Wisconsin standards — recreational pressure is spread across a long shoreline. Within the 8 graded lakes of Jefferson County, Lake Koshkonong sits at rank 8, near the bottom of the county list.

Lake Koshkonong is on the Wisconsin infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. The fishery includes walleye alongside the lake's other 7 documented species — a notable draw for Wisconsin anglers. 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.