Rain rarely falls in desert. When it comes, it comes in the form of thunderstorm.

   In sandy desert, the rain usually drains away promptly and only change the landscapecomparatively slightly. In contrast, the torrential downpour in rocky deserts drains into wadis (rocky watercourses that is dry except after heavy rain). This deepen the dry valleys. Heavy downpour can build up into flash flood, carrying sand, gravel and then large rocks and boulders. Thus, at the end of most wadis, there is an enormous bank of sand and stone( known as  "alluvial fan" ). The surplus sediment from the flash flood forms muddy lakes of different size and duration.  

The Wei River in the Loess Plateau, China (photo. by I-Ming Chou) From USGS

     These lakes are particular seen in Australian desert. They lasts long enough to breed creatures like shrimps, frogs and wildfowl. Some of the lakes formed have high salt content, which is thought to be derived from salt in the atmosphere, brought from oceanic spray.

   Shallow, low-bottom-gradient lakes can be moved by wind stress over many square kilometers. When they dry up, an area of clay, silt, or sand encrusted with salt is found, known as playa.

Left : Running water created this canyon in arid Big Bend National Park, southwest Texas. From USGS

Right : California (photograph courtesy of Kerr-McGee, Inc.) From USGS

Wind dehydrates soils and living things. Sand and dust particles are moved by desert winds. Desert winds also remove organic debris that makes the soil fertile. Since plants are scarce in deserts, wind erosion occurs more easily. Take Prairie States of North America as an example, a productive area was reduced in the 1930s to desert by desert wind (devastating tornado winds from desert), over-cropping and over-cultivating. The fine dust can be carried to kilometers away and thousands of meters up. Large amount of the fine duct rest in more temperate or moister regions and from the basis of loess (a fertile soil).

Want to know more about wind action and deserts? Click here!

ripples on a dunes in Eureka Valley, California (photo. by Terrance Moore) From USGS

From USGS

     In cold desert, rainfall is frozen at night in winter. When water goes into the tiny cervices and expands, the rocks are forced to split up.

     In hot deserts, the fragmenting force of temperature is slower. Rock surfaces reach 70^oC or 80^oC at midday and cool down to freezing point at midnight. Expansion under the sun and contraction at night weaken the surface layers and cause flaking.

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