Montana, as mountainous in parts as its name implies, has an approximate mean elevation of 3,400 ft (1,037 m). The Rocky Mountains cover the western two-fifths of the state, with the Bitterroot Range along the Idaho border; the high, gently rolling Great Plains occupy most of central and eastern Montana. The highest point in the state is Granite Peak, at an elevation of 12,799 ft (3,904 m), located in south-central Montana, near the Wyoming border. The lowest point, at 1,800 ft (549 m), is in the northwest, where the Kootenai River leaves the state at the Idaho border. The Continental Divide passes in a jagged pattern through the western part of the state, from the Lewis to the Bitterroot ranges.
Ft. Peck Reservoir is Montana's largest body of inland water, covering 375 sq mi (971 sq km); Flathead Lake is the largest natural lake. The state's most important rivers are the Missouri, rising in southwest Montana and flowing north and then east across the state, and the Yellowstone, which crosses southeastern Montana to join the Missouri in North Dakota near the Montana border. Located in Glacier National Park is the Triple Divide, from which Montana waters begin their journey to the Arctic and Pacific oceans and the Gulf of Mexico.