lake in the Flathead National Forest of northwestern Montana, U.S. Flathead Lake marks the southern limit of the Rocky Mountain Trench, a structural depression extending northward to the Liard Plain of British Columbia, Canada. Bordered on the eastern shore by the Mission Range and on the west by the forested foothills of the Salish Mountains, it is 30 miles (48 km) long, 15 miles (24 km) wide, and 220 feet (67 metres) deep. It has an area that ranges from about 197 square miles (510 square km) to about 188 square miles (487 square km), making it the largest U.S. freshwater lake west of the Mississippi River. Named for the Flathead (Salish) Indians (whose reservation adjoins the lake on the south), it was formed by glacial damming of the Flathead River, which enters the lake below Kalispell. Power and irrigation facilities are supplied by the Kerr Dam (completed 1958) near Polson at the south end of the lake. The University of Montana Biological Station is on the eastern shore, and the Swan River National Wildlife Refuge is nearby.