Clearest Lake in Minnesota: Top 25 by Secchi Depth

St James Pit Lake is the clearest lake in Minnesota by Secchi depth, with water transparency of 47.6 feet. Secchi depth is measured by lowering a black-and-white disk until it disappears from view — deeper readings mean clearer water. Located in St. Louis County, St James Pit Lake holds an overall water quality grade of A.

Water clarity is measured by Secchi depth, named after the 19th-century Italian astronomer Pietro Angelo Secchi who developed the method. A standardized black-and-white disk is lowered into the water on a rope; the depth (in feet or meters) at which the disk disappears from view is recorded. Greater Secchi depth means clearer water — usually a sign of low sediment, low phosphorus, and limited algae growth.

Lake clarity is driven by three factors: low suspended sediment (clay, silt), low nutrient load (especially phosphorus, which fuels algae), and low chlorophyll-a (active algae biomass). Deep glacial lakes with rocky basins and forested watersheds — common in northern Minnesota — tend to be clearest because they have little soil runoff and limited nutrient input. Shallow agricultural-watershed lakes are typically less clear because they receive more sediment and fertilizer runoff.

Not always. Clarity (Secchi depth) is one of three indicators we use; the others are total phosphorus and chlorophyll-a. A lake can be very clear but still have elevated phosphorus that just hasn't bloomed yet, or chlorophyll-a from a recent bloom that's still settling. The most reliable measure of overall lake health is the combined grade, which weights all three signals. See our companion [cleanest lakes in Minnesota](/best/cleanest/mn) page for the combined-score ranking.

Minnesota's clearest waters compete with the deepest, most-pristine lakes in the Upper Midwest. Across Minnesota and Wisconsin combined, top-tier lakes typically register Secchi depths of 15–25 feet. Nationally, the gold standard is Crater Lake in Oregon (~30 m / ~100 ft) and Lake Tahoe (~20 m / ~65 ft). Within the Great Lakes region, Minnesota's top lakes hold their own among the best.