St. Marys Lake

St. Marys Lake sits at the top of the Minnesota PCA grading rubric with an A, a mark reached by the cleanest lakes in Minnesota. Clarity, phosphorus, and chlorophyll-a all rate similarly, so there is no obvious single lever to pull on watershed management.

A TSI value of 35 puts St. Marys Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. The lake bottoms out at 44 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. St. Marys Lake covers 234 acres alongside 4.1 miles of shoreline — a mid-sized water that supports a working fishery without being so large that conditions diverge between basins. St. Marys Lake ranks 18 of 187 in St. Louis County, putting it in the upper tier of locally monitored lakes.

No invasive species are currently listed at St. Marys Lake — the lake remains off the Minnesota infested-waters roster. Walleye are documented at St. Marys Lake, one of 14 fish species on record for the lake. The lake lacks a documented public access point, so visitor use is limited. Ice-out has been recorded 9 times at St. Marys Lake, with a median around Apr 24. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.