The Snake River Plain forms a 6 million ha arc-shaped depression across southern Idaho. Basalt flows, fresh water sediments, loess and volcanic deposits cover its surface. Elevation increases eastward from 650 to 2,150 m altitude. Climate is semi-arid with annual precipitation ranging from 150 to 400 mm, arriving primarily in winter and spring. Native shrub steppe vegetation is dominated by Artemisia tridentata (big sagebrush) and bunchgrasses, e.g. Pseudoregneria spicatum (bluebunch wheatgrass), Elymus (wildrye) and Poa (bluegrass) spp., with interspersed Purshia tridentata (antelope bitterbrush) and Pseudoregneria spicatum communities. Salt desert shrub communities, e.g. Atriplex spp. (saltbush), Ceratoides lanata (winterfat), occupy drier areas.