Thompson Lake

Thompson Lake earns an F: phosphorus readings still being added and clarity at 3.0 ft signal heavy nutrient loading from the surrounding watershed.

Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. Thompson Lake is a shallow lake at 9 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. At 207 acres, Thompson Lake fits the Minnesota median for monitored lakes, with 3.9 miles of shoreline. Within the 187 graded lakes of St. Louis County, Thompson Lake sits at rank 180, near the bottom of the county list.

Thompson Lake has no invasive species recorded in Minnesota state databases as of 2023, though prevention practices still apply at all access points. Bass fishing is part of the appeal: the species list runs to 8, anchored by largemouth and/or smallmouth bass. The lake lacks a documented public access point, so visitor use is limited. Ice-out has been recorded 11 times at Thompson Lake, with a median around Apr 23. 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.