The eastern two-fifths of South Dakota is prairie, belonging to the Central Lowlands while the western three-fifths fall within the Missouri Plateau, part of the Great Plains region. The High Plains extend into the southern fringes of the state. The Black Hills, an extension of the Rocky Mountains, occupy the southern half of the state's western border. The highest point in the state is Harney Peak at a height of 7,242 ft (2,209 m) while the state’s lowest point is at Big Stone Lake at 966ft. Tributaries of the Missouri include the Grand, Cheyenne, Bad, Moreau, and White rivers in the west and the James, Vermillion, and Big Sioux in the east. Four dams control the Misouri River. They include Gavins Point, Ft. Randall, Big Bend, and Oahe—which provide water for irrigation, flood control, and hydroelectric power. Major lakes in the state include Traverse, Big Stone, Lewis and Clark, Francis Case, and Oahe.