Central Valley of California, 19,200 mi2 (47,900 km2)
Land-surface form.--This province lies within the Central Valley of California--a flat alluvial plain between the Sierra Nevada and the Coast Ranges. Elevations range from sea level to 500 ft (150 m). This area has broad, nearly level valleys bordered by sloping alluvial fans, slightly dissected terraces and the lower foothills of the surrounding uplands. Large undrained basins lie in the south.
Climate.--Annual temperatures in this climate average 60 to 67F (16 to 19C), but can fall as low as 55F (13C) in the south. Precipitation is largely limited to winter rainfall, which peaks in December, January, and February. Except near the coast, summers are hot and the winters mild--often foggy, with little or no snow. Annual rainfall ranges from approximately 6 in (150 mm) in the upper San Joaquin Valley to nearly 30 in (760 mm) along the coast. Potential evaporation during the warmest months is often much greater than the precipitation. Low rainfall and small streamflow result in water scarcity in many areas.
Vegetation.--Evidence indicates that the Central Valley of California was once dominated by natural grasses that the plow, fire, and grazing have eliminated except in a few remaining stands. These stands suggest that the dominants were bunch grasses on lands similar in appearance to mixed prairie. Apparently, needlegrass was the principal species except near the coast. Today, introduced annual grasses, including various species of avens, brome, fescue, and barley, occupy most of the remaining grassland areas.
The rivers flow through alkaline flats where greasewood, picklewood, salt grass, and shadscale provide the chief cover. Tule marshes border the lower reaches of the San Joaquin and Sacramento Rivers.
Soils.--The soils of this region are mostly Entisols and Alfisols. The Entisols are usually at the lower elevations and the Alfisols at slightly higher elevations, away from the valley floor. A small area of Aridisols occurs in the more arid southern portions of the San Joaquin Valley.
Fauna.--Intensive agricultural development has changed the fauna of the annual grasslands. Larger species, such as the California grizzly bear, wolf, and pronghorn antelope, have been eliminated or pushed up into the hills. Common mammals include the Beechy ground squirrel, cottontail, blacktail jackrabbit, California mouse, and kangaroo rats. Several subspecies of mule deer live in brushy areas. Other species, such as the coyote and bobcat, live in adjacent woodlands, occasionally entering from them. The San Joaquin kit fox is classified as an endangered species.
Common birds include the mourning dove, horned lark, western meadowlark, western kingbird, mockingbird, loggerhead shrike, house finch, lesser goldfinch, red-shafted flicker, and scrub jay. The roadrunner feeds on reptiles and insects. The California quail is numerous in areas where brush or rock outcrops provide cover. Avian predators include the golden eagle, red-tailed hawk, and Cooper's hawk.
Several species of snakes and lizards are present; rattlesnakes are important predators on rodents.