Most of Nebraska is prairie; more than two-thirds of the state lies within the Great Plains proper. The elevation slopes upward gradually from east to west, from a low of 840 ft (256 m) in the southeast to 5,424 ft (1,654 m) in Kimball County. Rolling alluvial lowlands in the eastern portion of the state give way to the flat, treeless plain of central Nebraska, which in turn rises to a tableland in the west. The Sand Hills of the north-central plain is an unusual region of sand dunes anchored by grasses that cover about 18,000 sq mi (47,000 sq km).
The Sand Hills region is dotted with small natural lakes; in the rest of the state, the main lakes are artificial. The Missouri River—which, with its tributaries, drains the entire state—forms the eastern part of the northern boundary of Nebraska. Three rivers cross the state from west to east: the wide, shallow Platte River flows through the heart of the state for 310 mi (499 km); the Niobrara River traverses the state's northern region; and the Republican River flows through southern Nebraska.