Montana Lake Water Quality

83 of the 865 Montana lakes in this index carry a recent clarity, phosphorus or algae reading, spread over 54 counties. Those readings come from the Montana Department of Environmental Quality, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, with no recent nutrient or algae sampling behind them. The measured lakes spread across the range: low phosphorus and algae readings in the deeper basins, higher readings 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 measured 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 county with the lowest nutrient and algae readings on average is Missoula. The gap between counties is large enough that a single figure for Montana lake water quality hides real local variation.