Fig. 10. A "pothole" being crowded by drifting sand, ten miles south of Moses Lake, Washington, March 23, 1940. (Fish and Wildlife Service photo by Victor B. [Scheffer], No. 925.)
The Columbian Plateau and southeastern Washington present desert conditions. At Odessa, 1590 feet, Lincoln County, the average annual precipitation is only 9.38 inches, and only in the winter may more than one inch of precipitation per month be expected. The average temperature is 48.5°. In the winter the average is below freezing but in July it is 71.3°. The average maximum for July is 90° and an extreme of 111° is recorded. Walla Walla, 991 feet, has a higher annual precipitation (16.66 inches) but higher temperature (yearly average 53.5°, July average 75.0°, average July maximum 88.6°, extreme 113°). Winter temperatures on the Columbian Plateau are low. The January average at Odessa is 25.3° and at Walla Walla 32.4° The average yearly snowfall at Odessa is 19.4 inches and at Walla Walla 23.5 inches. Vegetation of the Columbian Plateau and southeastern Washington is of the desert type. A few pines and junipers grow in favored places. Along streams the cottonwood (Populus hastata) and willow (Salix) of several species are common. Most typical are grasses and shrubs such as the bunch grass (Agropyron inerme, Agropyron spictatum), foxtail (Alopecurus aequalis), cheat grass (Bromus tectorum), saltbrush (Atriplex truncata), greasewood (Sarcobatus vermiculatus) mustard (Arabis sp., Brassica sp.), sagebrush (Artemisia rigida, Artemisia tridentata), rabbit brush (Chrysothamnus nauseosus, Chrysothamnus viscidiflorus) and cactus (Opuntia polyacantha). The arid climate of the Columbian Plateau affects, to some extent, surrounding areas. Thus the Yakima Valley Area, the Columbia Valley, where it borders the Plateau, and the Okanogan Valley possess vegetation typical of the Columbia Plateau.
LIFE-ZONES AND ECOLOGY
The Transition Life-zone is the principal life-zone in Washington. It is divisible into three subdivisions: Humid, Arid-timbered and Arid-grasslands (Fig. 11) subdivisions. The Humid and Arid-timbered subdivisions of the Transition life-zone are closely related in some respects but different in others. They are separated by the Cascade Mountains. All of the Transition Life-zone west of the Cascades belongs to the Humid subdivision and the timbered Transition Life-zone east of the Cascades belongs to the Arid-timbered subdivision.
