Running water is the most important agent of erosion in humid areas, but arid lands, in which water is in short supply, have their own distinctive landforms. Surface features in arid areas are stark and sharp. Much bare rock is exposed. The alternation of blistering daytime heat and chilly nights shatters the rocks, and the lack of moisture limits the softening effects of chemical weathering. Few plants cloak the contours of the land. The wind blows constantly, unchecked by plants or by human structures. It carries away the finer particles of weathered material and leaves the surface littered with a desert pavement of stones too large for it to move.

Rain is rare in arid areas, but once or twice a year violent thunderstorms pummel the earth with great torrents of water. The water cannot soak into the sunbaked ground, and it is carried away by sudden flash floods. The floodwater is so choked with sediment that the streams cannot erode their valleys deeper, but they undercut the valley sides to create box canyons with steep sides and flat bottoms. The floodwater pours into shallow lakes and soon evaporates. The soils of arid areas contain large amounts of soluble alkaline salts that would be removed by solution in a more humid area. Some of these salts are dissolved by the water that runs off in flash floods. When the water evaporates the salts are deposited on the dry lake beds as glistening salt flats caked with white alkaline salts.

Desert pavement, box canyons, and salt flats all are distinctive features of arid lands. Alluvial fans are not unique to arid lands, but they are sharper and clearer than in more humid areas. An alluvial fan is a gently sloping fan-shaped deposit that is formed at a sharp break in stream gradient, say where it flows from a mountain range onto a plain. The stream dumps its load of sediment and dams its own valley when its speed is reduced at the change in gradient. It repeatedly seeks and dams new channels, and gradually builds up a cone-shaped deposit of alluvium with its apex at the point where the stream leaves the mountains.