Desert Rivers

Deserts are indeed dry, with little to no rainfall. But there can be water - large rivers sometimes flow through deserts, like the Colorado River, which flows across the southwestern United States. Where do they come from? Desert rivers begin farther north, receiving water from wetter regions. The entire time they flow through the desert, water is generally seeping out of the riverbed into the surrounding land.

Importance

To humans, deserts appear dead and lifeless - often described as a wasteland. But they are a valuable natural resource, both as a habitat for certain types of plant and animal life, and for their energy potential. Two-thirds of the world´s crude oil is found by drilling in deserts, and they hold great potential for solar and wind energy, too.

Desert Defined

When we think of deserts we typically imagine dry, hot, sandy places like Arizona, in the United States, or the Sahara, in Africa. Technically a desert is any regions which receives 25 cm (10 inches) of precipitation per year.

Dry Rivers

Rivers or streams that flow through deserts are typically leaking water through the streambed the whole distance. As a result, some streams dry up before they make it out of the desert. These dry streambeds are called washes, or arroyos. Travellers who set up camp in a wash have sometimes when caught by flash floods when heavy rain in distance mountains causes raging torrents to crash down the streambed.

Latitude Effects

Some deserts occur as a result of wind patterns and latitude (distance north or south of the equator). Warm, wet air rises from the equator. As it cools, the water vapor in the humid air falls as precipitation, leaving dry air. The dry air flows north and south away from the equator, cooling as it goes. Cooler air is denser, so it tends to drop down towards the earth´s surface at about 30o North (or South) of the equator. The dry air, now falling down towards earth, once again becomes warmer, which means it can hold more water vapor - so it causes lots of evaporation at about 30o latitude.

The two factors combine - dry air is constantly coming into an area, not moisture-laden air that could bring rain; plus, the dry air is actually "sucking" up any moisture it finds already there. Result? Desert.

Mudslides and Debris Flows

Occasionally even deserts might have wet years. When that happens, the desert soil, which lacks the thick protective cover of vegetation you find in other areas, can become saturated. That can lead to a mudslide (or debris flow), especially on hillsides - which is, coincidentally, where people living in the southwest like to build their houses. In the US, southern California is well-known for twin risks: fires in dry years, and mudslides in wet years. Something to remember when you´re looking at buying a home. . . .

Landforms

Deserts often appear harsh, not just because of the lack of vegetation, but because the landforms there are stark - steep cliffs, deep canyons. That´s because of the type of weathering that occurs there. In human climates, rocks disintegrate (break into smaller pieces) but also "decompose" (called chemical weathering, it´s most often due to chemical actions with rain). In the desert, on the other hand, only mechanical weathering occurs - in other words, rocks just break into smaller rocks. Like being hit with a hammer, mechanical weathering produces angular, sharp breaks.

Desert Lakes

In humid environments, ground water or streams can feed into lakes, supplying water to replace what evaporates out. In the desert, however, streams are often intermittent, and ground water is usually well below the lake bed, so that lake water leaks into the ground. That means that lakes often will dry up, forming a dry lake bed called a playa.

The water evaporates, but the dissolved minerals (the salts) in the water stay behind. Over time, as the lake fills and dries many times, the water can become much saltier than you expect!

Driest of the Dry

Think it´s dry where you live? Some regions of Peru and Chile have not received any rainfall for a decade or more!

Desert Humans

Most of the world´s population lives near the water, especially along coasts. But about 13% of us live in deserts.

Rain Shadow Deserts

Some deserts form because they are close to mountains -these are known as "rain shadow deserts." Air masses have to rise to go over mountains - as the air masses rise, they cool. Because they are cooler, they can´t hold as much water, so it rains. Then the air, now dry, continues over the mountain to the other side. Dry air doesn´t bring rain, so a desert forms on the far side of the mountains.