The eastern third of Wyoming forms part of the Great Plains; the remainder belongs to the Rocky Mountains. Much of western Wyoming constitutes a special geomorphic province known as the Wyoming Basin. It represents a westward extension of the Great Plains into the Rocky Mountains, separating the Middle and Southern Rockies. Extending diagonally across the state from northwest to south is the Continental Divide, which separates the generally eastward-flowing drainage system of North America from the westward-flowing drainage of the Pacific states.
Wyoming's mean elevation is 6,700 ft (2,044 m), 2nd only to Colorado's among the 50 states. Gannett Peak, in western Wyoming, at 13,804 ft (4,210 m), is the highest point in the state. With the notable exception of the Black Hills in the northeast, the eastern portion of Wyoming is generally much lower. The lowest point in the state—3,099 ft (945 m)—occurs in the northeast, on the Belle Fourche River.
Wyoming's largest lake—Yellowstone—lies in the heart of Yellowstone National Park. In Grand Teton National Park to the south are two smaller lakes, Jackson and Jenny. All but one of Wyoming's major rivers originate within its boundaries and flow into neighboring states. The Green River flows into Utah; the Yellowstone, Big Horn, and Powder rivers flow into Montana; the Snake River, into Idaho; the Belle Fourche and Cheyenne rivers, into South Dakota; and the Niobrara and Bear rivers, into Nebraska. The lone exception, the North Platte River, enters Wyoming from Colorado and eventually exits into Nebraska.