Southwest Adventure, Living & Travel

What Is A Desert?

Desert Biome

Desert Sand Dunes

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 holds less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. Areas with few clouds, few 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.

Painted Desert
Painted Desert in Arizona

Geomorphology of the Deserts

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.

Death Valley

Undrained basins are also characteristic of the Mojave and Chihuahuan deserts. 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 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.

North American Deserts and Deserts of the World

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