Desert: a region with so little vegetation that no significant population can be supported on that land.  This implies that it is dry, but not necessarily hot.

There are several kinds of deserts, but they all have one thing in common - they each receive less than 25 cm (~10") of precipitation per year.

Desert Processes

Aeolian processes - caused by the wind (remember, the wind is a low density, low viscosity fluid)

Depositional features - sand dunes
In a desert, material moves primarily by the wind through a process called saltation.  This is the same type of jumping processes in which some material is carried as bedload in a stream.  However, like in a stream, the most erosive agent is water.  Even though rainfall is scarce, it usually comes in a few sudden, intense thunderstorm events.  The water is not able to soak into the ground in such a short amount of time, so it flows across the ground as sheetflow, coalescing into gullies and rivers that can erode arroyos into the landscape very quickly.  An arroyo is a term for a gully in an arid environment that has almost vertical sides and a flat channel floor.

Saltation may not move the larger particles; they may be left and form a desert pavement.  Any vegetation, or other item, may also block the wind.  That would allow small dunes to begin to form in the object's windshadow.  In the images below, the wind is blowing from left to right and a windshadow is being created downwind of the object - allowing the finer material to accumulate.

There are many different types of sand dunes found in the desert.  Below are some drawings from the US Geological Survey that show some of the major types of sand dunes. and how they relate to the various wind directions.

           barchan dune: crescent shaped dune with "horns" pointing downwind

Above is an isolated barchan dune that has some small transverse dunes nearby.

Above is a barchan dune that has various features labeled.  (Yes, I know transverse is spelled wrong)

longitudinal dune: linear dune that moves ~parallel to the wind

transverse dune: linear dune that moves ~perpendicular to the wind

star dune: dune that forms when there are a number of dominant wind directions

The airphoto above is a good example of star dunes

coppice dune: a small dune that has been stabilized by vegetation

Parabolic dunes are similarly shaped as barchan dunes (crescent shaped) except that in this case, the wind is blowing so that the "horns" are pointing upwind since they have been stabilized.

When the winds are primarily from one direction, but occasionally reverse directions a feature known as a Chinese Wall can develop on the crest of dunes.  The photograph below shows a good example of Chinese Walls on dunes in a dune field.  If the dune field is very large, it is called a sand sea or erg.

If the wind erodes all of the fine material from an area it may start to erode into the local bedrock.  This is called a blowout.  If the erosion continues streamlined bedrock hills may form - they are called yardangs (see the airphoto below)

Processes associated with water (a relatively high density, high viscosity fluid)

Ephemeral streams in steep sided valleys (arroyos and canyons) are very common in arid and semiarid environments.

Alluvial fans - debris that accumulates at the foot of a mountain range in an arid environment.  As a stream's gradient is abruptly decreased, the bedload and suspended load drops out.  In the airphoto below, the number "1" points out the edge of a single alluvial fan that has been cut by an arroyo.

Bajadas - coalescing alluvial fans.  In the airphoto below, the bajada is the region between the dashed blue lines.

Playas - ephemeral lake; in the airphoto below, the a playa can be seen as the bright patch southwest of the number "2".

In the photo above, another feature that is common in some deserts can be seen.  Immediately to the east (right) of the number "1" is an inselberg - the top of a peak in which the rest of the mountain has been covered by sediment.  As you can imagine, this process has taken millions of years to happen.

Terms for desert surfaces