Rainy Lake

On the scoring rubric Rainy Lake grades a D: clarity at 3.6 ft and phosphorus readings still being added keep it in the lower bracket for Koochiching County. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

At a TSI of 59, Rainy Lake reads as eutrophic — nutrient-rich enough that summer algal growth and reduced clarity are expected, not unusual. Rainy Lake is unusually deep for Minnesota at 161 ft, which buffers the lake against summer warming and helps the deep water stay oxygenated. Rainy Lake covers 210,200 acres alongside 2176.3 miles of shoreline, large enough that conditions can vary meaningfully between bays. Among the 7 graded lakes in Koochiching County, Rainy Lake sits at rank 3, above the county median.

Zebra mussels have been documented at Rainy Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. Walleye are documented at Rainy Lake, one of 17 fish species on record for the lake. A documented public access point at Rainy Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. Ice-out has been logged at Rainy Lake more than two dozen times, putting the median around May 4 — a useful baseline for tracking climate trends. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.