Flathead Lake is a large natural lake in northwest Montana.
The lake is a remnant of the ancient, massive glacial dammed lake, Lake Missoula of the era of the last interglacial. Flathead Lake is a natural lake along the mainline of the Flathead River. It was dammed in 1930 by Kerr Dam at its outlet on Polson Bay, and the lake level was raised by 10 feet. It is one of the cleanest lakes in the populated world for its size and type.
Wikipedia https://en.wikipedia.org/wiki/Flathead_Lake
Address 59931, United States
Coordinates 47°53'12.355" N -114°7'54.625" E