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.

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. With 210,200 acres of surface and 2176.3 miles of shoreline, Rainy Lake is a large water by Minnesota standards — recreational pressure is spread across a long shoreline. Among the 7 graded lakes in Koochiching County, Rainy Lake sits at rank 3, above the county median.

Zebra mussel presence at Rainy Lake means the lake's clarity numbers carry an asterisk — filtered water is not the same as nutrient-poor water. The fishery includes walleye among the lake's 17 documented species — a notable draw for Minnesota anglers. 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. 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.