Flathead Lake is the largest natural freshwater lake west of the Mississippi River in the contiguous United States.
With a surface area of between 191.5 sq mi and 197 sq mi, Flathead Lake is slightly larger than Lake Tahoe. The lake is a remnant of the ancient inland sea, Lake Missoula of the era of the last interglacial. Flathead Lake is 27.3 mi long and up to 15.5 mi wide. Flathead 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.
Montana Lake Vacation Guide: Learn more about Flathead Lake with this Montana Lake Vacation Guide.