St. Croix Lake

St. Croix Lake earns a D — measurements through 2019 show persistent nutrient pressure that limits clarity through the summer. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. The lake's 60 ft maximum depth is in the upper tier for Wisconsin — deep enough for cold-water fish to find refuge in summer. St. Croix Lake covers 7,696 acres alongside partial shoreline records, large enough that conditions can vary meaningfully between bays. Within the 7 graded lakes of St. Croix County, St. Croix Lake sits at rank 6, near the bottom of the county list.

Zebra mussel presence at St. Croix Lake means the lake's clarity numbers carry an asterisk — filtered water is not the same as nutrient-poor water. 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.