Depositions as well as erosion, both are important in creating landforms in the desert. There are many areas in deserts where aggradations carried on by water are equally important as erosion.
When ephemeral streams laden with sediments flow out of mountains into the deserts, they deposit huge quantity of debris on the plains which creates alluvial fans. These alluvial fans form in the grabens.
Various reasons account for the development of alluvial fans in arid lands. First, highland areas bordering a desert are subject to a great deal of erosion because of the lack of vegetation and the torrential convectional downpours over the mountains.
These streams carry much more debris than those in more humid regions. As the streams pass from the narrow canyons in the highlands into the open desert, they deposit bulk of coarser materials near the mouth of canyons.
As streams flow out into the desert, their depth and volume both decrease. Ultimately the streams are lost into the desert.
At the bases of mountains in deserts alluvial fans are so large that by their joining together an undulating, ramp like surface is formed. It is called a bajada which is composed of coarse debris and which is characterized by relatively steep and sloping fans.
The larger streams produce alluvial fans much larger in area. Where various fans join together, a piedmont alluvial plain is formed.
Such plains are very fertile, and if sufficient water is available these plains become good agricultural lands. Because of rich soils and accessible water supplies, piedmont alluvial plains have become productive agricultural lands.
Bolsons are desert basins flanked by mountains. They ate formed by faulting. The lowest part of a bolson is occupied by a dry lake bed known as a playa. Flash floods occasionally convert the playa into a lake just within a few hours.
These lakes lose their water through seepage and evaporation in the dry desert air. When the playa lakes dry out, their bed is covered with salt deposits.
It is known as a salt flat or salina. Playas are useful to man because many valuable chemicals like potash, salt, borates and sodium nitrate are mined from them.