Flathead Lake is the largest natural freshwater lake west of the
Mississippi River in the contiguous United States, taking Red Lake (Minnesota)
and Lake of the Woods to be north of the Mississippi River, rather than west of
the river. With a surface area of between 191.5 sq mi (496 km)
and 197 sq mi (510 km), Flathead Lake is slightly larger than
Lake Tahoe. The lake is a remnant of the ancient, massive glacial dammed lake,
Lake Missoula of the era of the last interglacial. Flathead Lake is
27.3 mi (43.9 km) long and up to 15.5 mi (24.9 km) wide.