Rainy Lake

Rainy Lake earns a D — measurements through 2023 show persistent nutrient pressure that limits clarity through the summer. 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. Within Koochiching County's 7 graded lakes, Rainy Lake ranks 4 — below the local median, though not at the bottom.

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. The fishery includes walleye alongside the lake's other 17 documented species — a notable draw for Minnesota anglers. Public access is available — the lake is on the Minnesota PCA public-access list. 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.