Montana Lake Water Quality

36 of the 865 Montana lakes in this index carry an A–F water-quality grade, spread over 54 counties. Those grades rest on measurements from the Montana 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. Around 19% of the graded lakes pull an A, 7 top-tier waters. The B and C brackets together hold most of the rest, with the F bracket capturing 1 stressed lakes mostly in the agricultural south of the state.

By surface area the largest lake here is Lake Koocanusa, at about 46,500 acres, and 77 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. The dominant trophic class among the graded lakes is oligotrophic: clear, nutrient-poor, slow-growing. These lakes are also the most sensitive to disturbance, because the same volume that buffers them against nutrients also slows recovery once those nutrients arrive in quantity. The cleanest county on average is Missoula (grade B); the most-stressed is Chouteau. The gap between the top and bottom counties is large enough that a single figure for Montana lake water quality hides real local variation.