Rainy Lake

On the scoring rubric Rainy Lake grades a D: clarity at 5.9 ft and phosphorus readings still being added keep it in the lower bracket for St. Louis 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. Within the 187 graded lakes of St. Louis County, Rainy Lake sits at rank 146, near the bottom of the county list.

Rainy Lake is on the Minnesota infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. 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. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.