North Dakota Lake Water Quality

87 of the 826 North Dakota lakes in this index carry an A–F water-quality grade, spread over 51 counties. Those grades rest on measurements from the North Dakota Department of Environmental Quality, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, without enough recent nutrient or algae sampling behind them to support a letter. A-grade lakes are scarce among the North Dakota lakes that carry a letter: about 3% of them. Most sit in C or D, productive, often shallow, frequently algal in late summer.

By surface area the largest lake here is Lake Audubon, at about 19,095 acres, and 132 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. Eutrophic conditions dominate the graded set: nutrient-loaded, productive, with algal pressure that shows up most years in late summer. These lakes tend to be shallow, in agricultural watersheds, and over-warmed by the time July ends. The cleanest county on average is Rolette (grade B); the most-stressed is Pierce. The gap between the top and bottom counties is large enough that a single figure for North Dakota lake water quality hides real local variation.