Chihuahuan Desert Research Institute North American Deserts North America's arid lands can be divided into four distinct deserts: the Great Basin, the Sonoran Desert, the Mojave Desert, and the Chihuahuan Desert. All are found in the western United States and northern Mexico. Of the four, the Chihuahuan Desert (which occupies 175,000 sq. miles) is the largest. The Great Basin lies farthest north and is highest in elevation, so it is a very cold desert, especially in its northern reaches. Winters are long, and much of the precipitation falls during the winter in the form of snow. Cacti and other succulents are limited in number by the cold winter temperatures. The vegetative cover consists largely of remarkably uniform stands of small shrubs, with sagebrush (Artemisia tridentata, not a true sage), shadscale (Atriplex confertifolia) and sometimes other members of these two genera the most common dominants. The Great Basin Desert has little out ward drainage and is characterized by heavy accumulations of alkaline salts in its lower areas. On a year-round basis, the Sonoran Desert, in contrast, is our warmest desert. The Sonoran supports a great diversity of plant life due to the mild winters and rains that fall during both the summer and winter. Cacti and thorny shrubs dominate the vegetation. This desert is well-known for its large cacti, including the saguaro (Cereus giganteus), a  popular symbol of this desert, the cardón (Pachycerus pringlei), a Mexican counterpart to the saguaro, and the organ pipe cactus (Lemaireocerus thurberi). Factors influencing the presence of the Sonoran Desert include warm, dry, descending air currents, the partial rain shadow of the Sierra Nevadas and other mountains, and the cold California Current. Between the Great Basin and Sonoran deserts lies our smallest and dryest desert—the Mojave. Summers there are very long and hot, but winters are cooler than in the Sonoran and Chihuahuan deserts. Most of the Mojave receives the majority of its scant precipitation during winter. Its arid conditions result from descending global air currents and the rain shadows of bordering mountains. Low, open, uniform stands of creosote (Larrea divaricata) and chamiso (Franseria dumosa) cover much of this desert. The most distinct plant of the region is a very large yucca, the Joshua Tree (Yucca brevifolia), which grows primarily in higher elevations but often serves as the Mojave's trademark. These three deserts, the Mojave, Great Basin, and Sonoran, form a relatively continuous belt, their margins overlapping to create blending zones. Arid grasslands on the lower elevations of the Continental Divide form a transition zone separating the Chihuahuan Desert from the others. To find four deserts such as these, clustered in a group, yet with each remaining distinct, is a quite unusual and interesting situation.

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