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 km2)and 197 sq mi (510 km2), 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 (43.9 km) long and up to 15.5 mi (24.9 km) wide. Flathead lake has a maximum depth of 370.7 ft (113.0 m), and an average of 164.7 ft (50.2 m). This makes Flathead lake deeper than the average depths of the Yellow Sea or the Persian Gulf. The lake was raised 10 ft (3.0 m) by Kerr Dam.It is one of the cleanest in the populated world for its size and type.