desert (des’-ert) A region with a mean annual precipitation of 10 inches or less, and so devoid of vegetation as to be incapable of supporting any considerable population.

This definition found in the third edition of the Dictionary of Geological Terms, paints a less than pleasant portrait of the landmass which covers one-third of our planet. Deserts do receive very little rainfall, but they are far from devoid of life, and are quite capable of supporting massive populations (with a little carefully planned water diversion), as cities such as Phoenix, Las Vegas and Los Angeles have proved. Deserts are often depicted in Hollywood films as scorching, barren, wind-blown landscapes with little to offer other than a sunburn and a mouth full of sand! Deserts are hot, and yes, deserts are dry, but they have their own special beauty found nowhere else. They span vast distances, criss-crossed by old mule trails beaten with the footsteps of our ancestors. They hold secret places for those willing to seek them out. Our deserts offer hikers, backpackers, 4WD enthusiast, and of course, gold prospectors thousands of acres of wilderness to explore away from the hustle-bustle of city life. Our deserts, and the creatures that inhabit them, are unique and deserving of our respect and protection. Discover for yourself the wonders of the desert, but please leave no trace of your passing.

The information below has been taken from www.desertusa.com; a website we have found to be an excellent source of information for the desert regions of the southwestern United States – be sure to have a look.

What is a Desert? Deserts in the Southwestern United States are areas of extreme heat and dryness, just as most of us envision them. More scientifically, deserts, also called arid regions, characteristically receive less than 10 inches of precipitation a year. In some deserts, the amount of evaporation is greater than the amount of rainfall. Semiarid regions average 10 to 20 inches of annual precipitation. Typically, desert moisture occurs in brief intervals and is unpredictable from year to year. About one-third of the earth's land mass is arid to semiarid (either desert or semidesert).

Evaporation is also an important factor contributing to aridity. In some deserts, the amount of water evaporating exceeds the amount of rainfall. Rising air cools and can hold less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. Areas with few clouds, bodies of water and little vegetation absorb most of the sun's radiation, thus heating the air at the soil surface. More humid areas deflect heat in clouds, water and vegetation, remaining cooler. High wind in open country also contributes to evaporation.

Locations of deserts have changed throughout geologic time as the result of continental drift and the uplifting of mountain ranges. Modern desert regions are centered in the horse latitudes, typically straddling the Tropic of Cancer and the Tropic of Capricorn, between 15 and 30 degrees north and south of the equator. Some deserts, such as the Kalahari in central Africa, are geologically ancient. The Sahara Desert in northern Africa is 65 million years old, while the Sonoran Desert of North America reached its northern limits only within the last 10,000 years. Because they are poised in such harsh extremes of heat and aridity, deserts are among the most fragile ecosystems on the planet.

Three of the four major deserts of North America are contained within a geological region called the Basin and Range Province, lying between the Rocky Mountains to the east and the Sierra Nevadas to the west. While the distinctiveness of each desert is based on the types of plant life found there (determined both by evolutionary history and climates), the geological structures of these three deserts are rather similar.

Captain John C. Fremont coined the term Great Basin. Actually, the region is a series of many basins, interrupted with mountain ranges produced by tilted and uplifted strata. Each range typically has a steep slope on one side and a gentle slope on the other. The ranges are roughly parallel. The basins or playas have no drainage. During wet cycles they become shallow playa lakes which may last from a few months, a few years or for longer periods.

During the Pleistocene interglacial, much of the Great Basin was flooded producing Lake Lahotan. The lake evaporated during the last 12,000 years, leaving only a few salty lakes between the Sierra Nevadas and the Rocky Mountains.

Undrained basins are also characteristic of the Mojave and Chihuahuan deserts. But the Sonoran Desert usually has hydraulic systems forming streams draining into the Gulf of California or the Pacific. There are also a few playas in the Sonoran Desert. One of these, called the Salton Sea, was filled by Colorado River flood waters in 1906 and remains full.

Alluvial fans are common in the Mojave Desert and the California portions of the Sonoran Desert. These are formed through geologic time where an arroyo or wash drains a mountain, depositing the detritus in a semicircle at the canyon's mouth.

In the Sonoran Desert, the linear ranges, usually formed by volcanic uplift, are often surrounded by a skirt of detritus -- boulders, rocks, gravel, sand, soil -- that has eroded from the mountain over time. Much of this has been washed down during torrential summer downpours. In the Southwest these detritus skirts or pediments are frequently called bajadas. The substrate is coarser, with larger rocks on the upper bajada and finer at the lower elevation.

Deep arroyos may cut through the bajadas. Special plants such as the Desert Ironwood and Canyon Bursage may grow along the arroyos, giving them the appearance of dry creeks.

The areas between the desert ranges have been filled with water-washed alluvium. This alluvium, or fine soil, produces the extensive flat spaces one usually associates with deserts. The water table may be high on the flatlands, and the drainage is often slow. Poorly drained patches and larger playas become alkaline through accumulation of soluble chemicals. Special types of plants called halophytes (salt lovers) can grow here.

Desert streams and rivers are formed where there are grasslands, semiarid woodlands and forested uplands called watersheds. Like giant geological sponges, the upland watersheds collect and hold water throughout the year, releasing it slowly into the desert below. These desert streams with their riparian woodlands of cottonwoods, willows and other hydrophilic (water loving) plants were centers for abundant wildlife, as well as native peoples. However, abuse to the watersheds through overgrazing, timber cutting, mining and other modern activities has dried up many desert rivers. Also, much of the water table, once just below the desert floor, has been pumped lower and lower, and may now be hundreds of feet below the surface.

The Sonoran Desert

The Sonoran Desert is an arid region covering 120,000 square miles in southwestern Arizona and southeastern California, as well as most of Baja California and the western half of the state of Sonora, Mexico. Subdivisions of this hot, dry region include the Colorado and Yuma deserts. Irrigation has produced many fertile agricultural areas, including the Coachella and Imperial valleys of California. Warm winters attract tourists to Sonora Desert resorts in Palm Springs, California, and Tucson and Phoenix, Arizona.

This is the hottest of our North American deserts, but a distinctly bimodal rainfall pattern produces a high biological diversity. Winter storms from the Pacific nourish many West Coast annuals such as poppies and lupines, while well-developed summer monsoons host both annuals and woody plants originating from the south. Freezing conditions can be expected for a few nights in winter.

Trees are usually well developed on the desert ranges and their bajadas. Often abundant on these well-drained soils are Little-leaf Palo Verdes, Desert Ironwoods, Catclaw and Saguaro.

The understory consists of three, four or even five layers of smaller woody shrubs. Tall chollas may occur in an almost bewildering array of species. The alluvial lowlands host communities of Desert Saltbush, wolfberry and bursage. On coarser soils, Creosote Bush and bursage communities may stretch for miles. Where the water table is high, Honey or Velvet Mesquite may form dense bosques or woodlands.

Other species are restricted to alkaline areas. Stream sides may be lined with riparian woodlands composed of Arizona Ash, Arizona Black Walnut, Fremont Cottonwood and various willows, with a dense understory of Arrow-weed, Seepwillow and Carrizo. The Sonora Desert is rich in animal life as well, with many species in all groups derived from tropical and subtropical regions.

The western part of the Sonora Desert (sometimes called the "Colorado Desert") is closer to the source of Pacific storms and is noted for spectacular spring flowering of ephemerals when there is winter-spring rainfall. (This phenomenon is not limited to here.) However, the western portion is relatively depauperate, lacking many of the species such as the Saguaro that depend on good summer rainfall.

Approximate Boundaries: Bordered on the west by Borrego Springs, and San Gorgonio Pass in southern California, on the north by Interstate 10 in California and Interstate 40 in Arizona, on the east by Arizona's U.S. Route 191, south to the tip of Baja California, Mexico.

The Mojave Desert

The transition from the hot Sonoran Desert to the cooler and higher Great Basin is called the Mojave Desert. This arid region of southeastern California and portions of Nevada, Arizona and Utah, occupies more than 25,000 square miles.

On the northwestern boundary it extends from the Sierra Nevada range to the Colorado Plateau in the east; it abuts the San Gabriel-San Bernardino Mountains in the southwest. Near the Great Basin-Mojave border lies Death Valley, the lowest point in North America and a national park.

The Mojave's desert climate is characterized by extreme variation in daily temperature and an average annual precipitation of less than 5 inches. Almost all the precipitation arrives in winter. Freezing temperatures occur in winter, while summers are hot, dry and windy.

The Mojave has a typical mountain-and-basin topography with sparse vegetation. Sand and gravel basins drain to central salt flats from which borax, potash and salt are extracted. Silver, tungsten, gold and iron deposits are worked.

While some do not consider the Mojave a desert in its own right, the Mojave Desert hosts about 200 endemic plant species found in neither of the adjacent deserts. Cactus are usually restricted to the coarse soils of bajadas. Mojave Yucca and, at higher elevations Desert Spanish Bayonet, a narrow-leafed yucca, are prominent. Creosote Bush, Shadscale, Big Sagebrush, Bladder-sage, bursages and Blackbush are common shrubs of the Mojave Desert.

Occasional Catclaws grow along arroyos. But, unlike the Sonoran Desert, trees are few, both in numbers and diversity. The exception is the Joshua-tree. While this unusual tree-like yucca is usually considered the prime indicator of Mojave Desert vegetation, it occurs only at higher elevations in this desert and only in this desert.

Approximate Boundaries: Bordered on the south by Interstate 10 in California, on the west by California's U.S. Route 395, on the North by U.S. Route 50 in Nevada, and on the east by Interstate 15.

The Great Basin Desert

The Great Basin Desert, the largest U. S. desert, covers an arid expanse of about 190,000 square miles and is bordered by the Sierra Nevada Range on the west and the Rocky Mountains on the east, the Columbia Plateau to the north and the Mojave and Sonoran deserts to the south.

This is a cool or "cold desert" due to its more northern latitude, as well as higher elevations (at least 3,000 feet, but more commonly from 4,000 to 6,500 feet). Precipitation, generally 7-12 inches annually, is more evenly distributed throughout the year than in the other three North American deserts. Winter precipitation often falls as snow.

Playas are a conspicuous part of this desert, due to its recent geological activity. In notable contrast to the other three deserts, Great Basin vegetation is low and homogeneous, often with a single dominant species of bush for miles. Typical shrubs are Big Sagebrush, Blackbrush, Shadscale, Mormon-tea and greasewood. There are only occasional yuccas and very few cactus.

The Colorado Plateau, centered in northeastern Arizona, and including the adjacent Four Corners region of Utah, Colorado and New Mexico, is sometimes included in the Great Basin Desert, sometimes considered a separate desert -- the Navajoan -- and sometimes not considered a true desert. The Plateau includes large barren areas, spectacular geological formations, more juniper and pinyon trees and generally higher elevations.

Approximate Boundaries: Bordered on the south by the Mojave and Sonoran deserts at Interstate 40, on the north by Interstate 70, on the west by U.S. Route 395 in Nevada, and on the east by U.S. Route 550 in Colorado and the Continental Divide in New Mexico.

The Chihuahuan Desert

Most of the Chihuahuan Desert -- the largest desert in North America covering more than 200,000 square miles -- lies south of the international border. In the U.S. it extends into parts of New Mexico, Texas and sections of southeastern Arizona. Its minimum elevation is above 1,000 feet, but the vast majority of this desert lies at elevations between 3,500 and 5,000 feet.

Winter temperatures are cool, and summer temperatures are extremely hot. Most of the area receives less than 10 inches of rainfall yearly. While some winter rain falls, most precipitation occurs during the summer months.

This desert covers such a large area that it is difficult to characterize its geology, but limestone and calcareous soils are common.

Like the Great Basin Desert, this is a shrub desert, but the biological diversity of perennial plant life is relatively low. Yuccas and agaves, growing with grasses and often Creosote Bushes, give this desert its characteristic appearance.

Prickly-pears and Mormon Tea are also contribute prevalent. Tarbush is sometimes a dominant shrub. Honey Mesquite grows along washes and playas. White-thorn Acacia, Allthorn and Ocotillo are other large, conspicuous plants of the Chihuahuan Desert.

Approximate Boundaries: Bordered on the west by Arizona's U.S. Route 191, on the north by Interstate 40, on the east by Texas' U.S. Route 385, and south to the Mexican border.