Flathead Lake is one of the cleanest lakes in the populated world for its size and type. It is the largest natural freshwater lake by surface area that is west of the Mississippi River in the contiguous United States. The lake has a maximum depth of 370.7 ft, and an average of 164.7 ft. This makes Flathead Lake deeper than the average depths of the Yellow Sea or the Persian Gulf. Polson Bay, at the lake's outlet, was raised 10 ft by Kerr Dam.