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.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. At 161 ft of maximum depth, Rainy Lake stratifies reliably through the summer — the hypolimnion stays cooler and clearer than the surface layer suggests. 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. Rainy Lake ranks 7 of 9 in Koochiching County — at the lower end of the locally monitored distribution.

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. The lake has a long ice-out record — 106 observations on file, with a median ice-out around May 4. 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.