Rainy Lake

Rainy Lake earns a C — middle-of-the-pack water quality, with one or two parameters dragging the average down from a stronger grade. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

A TSI near 48 places Rainy Lake in the mesotrophic band — moderate productivity, with seasonal swings between clearer spring water and richer late-summer conditions. 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 ranks 1 — in the top quartile locally.

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. 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. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.