Flathead Lake is the largest natural freshwater lake in the western United States. Studies at the Flathead Biological Station show the water quality in Flathead Lake is among the best in the world.
Its major tributaries are the Flathead and Swan Rivers. Numerous small streams flow directly into the lake at its shoreline, particularly on the wetter East Shore.
Kerr Dam is located at the outlet of Flathead Lake in Polson. Regulations of outflow by the dam maintain the lake between 2,883 and 2,893 feet above sea level.