Desert Features

Streams. Because of the dry conditions, most deserts do not have streams or rivers that run all year long. Streams that flow intermittently as a result of periods of sudden rainfall are called ephemeral streams. Exceptions are the Colorado River in the southwestern United States and the Nile River in Egypt, which originate in mountainous regions and have enough stream flow to cross desert areas.

Deserts often exhibit an interior drainage pattern where streams empty into landlocked basins. The basins become temporary sources of water, and evaporation can precipitate salt beds and other evaporitic minerals.

Flash floods. Most desert rainfall comes from short, violent thunderstorms. The rain is so abundant that it cannot soak into the hard ground or be controlled by the narrow stream channels. Rain (as sheetwash) flows rapidly over the land, creating flash floods in the stream beds that can be very destructive in populated areas. The lack of vegetation allows severe erosion, which carves new scarps and gullies; the water can become so choked with sediment it becomes a mudflow. The rapid downcutting by floodwaters produces narrow gorges with steep walls and gravel bottoms called arroyos or dry washes (wadis in Arabia and North Africa, dongas in South America, and nullahs in India).

Basin and Range topography. Some deserts, such as those in the American southwest, display Basin and Range topography—a series of steep mountain ranges separated by broad valleys. The mountain ranges cause a series of rain shadows that create the desert climate. Most of the rainfall in the mountains carries rock debris and sediment out to the alluvial fan that forms at the mouth of a canyon. If the water runs out farther into the center of the valley, it forms a playa lake. Typically shallow and muddy from clay, it evaporates quickly, leaving a flat, hard, dried clay surface that is broken by desiccation cracks. If the water carries dissolved salts, salt flats will result.

Eventually alluvial fans at the front of a mountain range may join to form a bajada, or rolling surface of sediment and gravel. Between the bajada and the range front is the pediment, a low‐angle erosion surface at the foot of the mountain range that is typically covered by up to 30 meters (100 feet) of sediment. Isolated bedrock remnants of the former mountain front called inselbergs may project abruptly through the pediment cover as rocky hills.

Plateaus, mesas, and buttes. Hills underlain by resistant rock such as sandstone, limestone, or volcanic lava are called plateaus. Plateaus are edged by steep‐sided scarps and gullies. As weathering and erosion cut back a plateau's slopes, remnant flat‐topped towers or columns called mesas may be left behind. The continued erosion of a mesa results in a similar but narrower landform called a butte. (The plateau‐mesa‐butte sequence is an example of parallel retreat.) Although most common in desert climates, these landforms are more a function of rock structure than climate.