Alton Lake

Alton Lake carries a solid B grade — strong on most parameters, with measurements that hold up well across the summer season. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

The lake's low TSI puts it in oligotrophic territory — the cleanest of the four trophic classes, but also the most vulnerable to nutrient-driven shifts. Alton Lake is unusually deep for Minnesota at 72 ft, which buffers the lake against summer warming and helps the deep water stay oxygenated. At 969 acres, Alton Lake fits the Minnesota median for monitored lakes, with 15.4 miles of shoreline. Among the 128 graded lakes in Cook County, Alton Lake sits at rank 57, above the county median.

No invasive species are currently listed at Alton Lake — the lake remains off the Minnesota infested-waters roster. The fishery includes walleye alongside the lake's other 7 documented species — a notable draw for Minnesota anglers. The lake lacks a documented public access point, so visitor use is limited. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.