Koronis Lake

Koronis Lake sits at a C on the grading scale — not impaired, not pristine, with summer measurements that vary year to year. The lake's weakest score is on Secchi depth — the water looks more turbid than its phosphorus number alone would predict.

Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. Koronis Lake is unusually deep for Minnesota at 132 ft, which buffers the lake against summer warming and helps the deep water stay oxygenated. With 2,968 acres of surface and 15.8 miles of shoreline, Koronis Lake is a large water by Minnesota standards — recreational pressure is spread across a long shoreline. Among the 55 graded lakes in Stearns County, Koronis Lake sits at rank 26, above the county median.

Zebra mussels have been documented at Koronis Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. The fishery includes walleye alongside the lake's other 22 documented species — a notable draw for Minnesota anglers. Public access is available — the lake is on the Minnesota PCA public-access list. Ice-out has been recorded 7 times at Koronis Lake, with a median around Apr 14. 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.