Level IV: (10c) Umatilla Plateau
The Umatilla Plateau ecoregion, named for the Umatilla who originally inhabited the area, is characterized by a nearly level to rolling, treeless plateau, underlain by basalt and veneered with loess deposits. Elevation varies from 1,000 to 3,200 feet (300 to 980 m). Glacial features such as patterned ground are common.
Mean annual precipitation is 9 to 15 inches (230 to 380 mm) and increases with elevation. In uncultivated areas, moisture levels are generally high enough to support grasslands of bluebunch wheatgrass, Sandberg bluegrass, and Idaho fescue, without associated sagebrush, which is more common in the Pleistocene Lake Basins. Stiff sagebrush may be found on very shallow soils. Alien cheatgrass covers broad areas.