Minnesota Lake Water Quality

Minnesota lists 6,760 lakes across 85 counties, 2,278 of them with a recent water-quality reading. Most of that sampling flows through monitoring run by the Minnesota Pollution Control Agency and the Minnesota DNR, which feeds state, federal, and volunteer measurements into the EPA Water Quality Portal. The remainder are here because they are real named lakes, not because anyone has sampled them recently. Much of Minnesota's Secchi-disk work is done by the Citizen Lake Monitoring Network, whose volunteers sample through the open-water season alongside the Minnesota Pollution Control Agency and the Minnesota DNR. Coverage is uneven by county: the northeast, on the Iron Range and along the Boundary Waters, tends to be best monitored, while parts of the southwest still have real data gaps. A large share of the state's measured lakes carry low phosphorus and chlorophyll-a readings, which reflects the depth and natural nutrient poverty of glacial lakes in the region.

By surface area the largest lake here is Rainy Lake, at about 210,200 acres, and 713 of the lakes in this index cover more than 500 acres. Size is worth knowing before you go: a large deep basin buffers nutrient inputs by sheer volume, while a shallow one warms quickly and can turn over its whole water column in a single summer storm. Most measured Minnesota lakes sit in the mesotrophic band: moderate productivity, decent clarity, capable of supporting both warm-water and cool-water fisheries. This is the band where well-managed working lakes tend to land. The county with the lowest nutrient and algae readings on average is Clay. The gap between counties is large enough that a single figure for Minnesota lake water quality hides real local variation.