A desert is an area that receives less than 10 inches of annual rain, and we have three distinct deserts in California. The smallest is the Coloradan Desert, which is a subset of the largest desert in North America -- the Sonoran Desert. The Sonoran is low-elevation, from zero to 2,000 feet, has blisteringly hot summers, mild winters, mostly monsoon-type summer rains and a large diversity of trees and columnar cactus.
Our portion of this desert lays in eastern San Diego County and in Imperial County along the Colorado River, hence the name. This desert is exceptional to the rest of the Sonoran in that it does not have the saguaro cactus and most of the rainfall occurs in the winter rather than the summer. Two dominant plants are the ocotillo and creosote. Anza-Borrego Desert State Park is our most protected example of this habitat.
East of the Sierra Nevada beginning in the Owens Valley and stretching all the way to the Oregon border lies the Great Basin Desert. This desert is high, 5,000 to 7,000 feet, has brutal winters, precipitation mostly in the form of snow, few tall trees or cactus, and the dominant shrub is the Great Basin sagebrush. This desert is by far the largest in the U.S., covering nearly all of Nevada and large parts of adjacent states.
But the biggest desert in California is the smallest one in the U.S. -- the Mojave Desert. This desert is transitional between the two previous deserts. With an elevation of 2,000 to 5,000 feet, winters can be cold but not as cold as the Great Basin and the summers hot but not as hot as the Coloradan. The plant that outlines the Mojave Desert is the Joshua tree. Many parts of this desert are actually below 2,000 feet, like Death Valley, and many mountains rise high out of the Mojave and are more like the Great Basin.
All three of these deserts are rain shadow deserts -- high mountain ranges block the moisture bearing clouds from the ocean. California has some of the greatest biodiversity in all of North America. Part of this remarkable assemblage of life is due to the three deserts that exist in this state.