I might define a desert as a land area characterized by sparse and infrequent rainfall. I wonder what images pop into your mind when I mention deserts: vast expanses of dry, rocky, rugged land? enormous shifting sand dunes? Both of these images are good representations of many deserts, but I think it’s important to point out that
- most deserts have at least some vegetation, and
- sand dunes constitute only a small percentages of the area of most deserts.
It was once generally believed that the land surface of deserts is shaped mainly by the wind. But it’s clear now that except for areas of sand dunes, the desert landscape is shaped almost entirely by running water rather than by wind. It rains, albeit infrequently, even in the driest of deserts. And in very dry deserts the vegetation is so sparse as not to provide much stability to the surficial layer of regolith, so the running water can readily mobilize the regolith and shape the landscape.
In the Atacama Desert in Chile the average annual rainfall is between one and two millimeters, and no rain at all falls for years at a time. But when it does rain in such a desert, a lot of rain—as much as several centimeters—can fall in a short time; that’s when almost all of the sediment movement takes place.
Another misconception we Americans tend to have about deserts is that they are mountainous areas with characteristic relief made up of mountain ranges with intervening basins. In fact most of the desert areas of the world are not mountainous; they are broad plains of relatively low relief, except for large sand dunes in certain places.