Columbia Plateau

Major section of the Intermontane Region of the northwestern USA, taking in parts of eastern Washington State, southern and western Idaho, and eastern Oregon. The plateau covers a total area of some 260,000 sq km/100,000 sq mi, between the Cascade Range in the west and the Rocky Mountains in the east.

Geologically, the Columbia Plateau is covered with lava flows, through which older mountains (for example the Blues and the Wallowas) protrude in places. The terrain is almost uniformly harsh and arid, with coulees, scablands, and other phenomena characteristic of a rush of glacial waters across the land surface, together with isolated volcanic features. Sedimentary material is mixed with the lava, and the loess is deep enough on some surfaces to provide fertile land for farming and ranching; irrigation by the Columbia Basin Project has also helped make much of the region agriculturally productive. The plateau encompasses lands within the Big Bend of the Columbia River in Washington (the Walla Walla Plateau), the Blue Mountains of eastern Oregon, the Snake River Plain of southern Idaho, and the Harney Desert of southern Oregon.