Most streams in deserts evaporate before they reach the sea. Most are dry for long periods of time and subject to flash floods during brief but intense rains. These flash floods transport most of the sediment in deserts and form fan-shaped deposits of sand, gravel, and boulders found at the bases of many mountains in desert regions. These flash floods also erode deep, steep-walled canyons through the upstream mountain regions, which is the source of the boulders and cobbles found on the mountain fronts. Intermountain areas in deserts typically have finer-grained material, deposited by slower-moving currents that represent the waning stages of floods as they expand into open areas between mountains after they escape out of mountain canyons.
Flash floods can be particularly hazardous in desert environments, especially when the floods are the result of distant rains. more people die in deserts from drowning in flash floods than die from thirst or dehydration. In many cases rain in faraway mountains occurs while people in downstream areas are not aware it is raining upstream. Rain in deserts is typically a brief but intense thunderstorm, which can drop a couple of inches (> 5 cm) of rain in a short time. The water may then quickly move downstream as a wall of water in mountain canyons, sweeping away all loose material in its path. People or vehicles caught in such a flood are likely to be swept away by the swiftly moving torrent.
Dry lake beds in low-lying flat areas, which may contain water only once every few years, characterize many deserts. These playas, or hardpans, typically have deposits of white salts that formed when water from storms evaporated, leaving the lakes dry. There are more than 100 playas in the American southwest, including Lake Bonneville, which formed during the last ice age and now covers parts of Utah, Nevada, and Idaho. When there is water in these basins, they are known as playa lakes. Playas are flat surfaces that make excellent racetracks and runways. The U.S. space shuttles commonly land on Rogers Lake playa at Edwards Air Force Base in California.
Alluvial fans are coarse-grained deposits of alluvium that accumulate at the fronts of mountain canyons. Alluvial fans are very common in deserts, where they are composed of both alluvium and debris-flow deposits. Alluvial fans are quite important for people in deserts, because they are porous and permeable and they contain large deposits of groundwater. In many places alluvial fans so dominate the land surface that they form a bajada, or slope, along the base of the mountain range, formed by fans that have coalesced to form a continuous broad alluvial apron.
Pediments represent different kinds of desert surfaces. They are surfaces sloping away from the base of a highland and are covered by a thin or discontinuous layer of alluvium and rock fragments. These ero-sional features are formed by running water and are typically cut by shallow channels. Pediments grow as mountains are eroded.
Inselbergs are steep-sided mountains or ridges that rise abruptly out of adjacent, monotonously flat plains in deserts. Ayres Rock in central Australia is perhaps the world's best-known inselberg. These are produced by differential erosion, leaving behind as a mountain rocks that for some reason are more resistant to erosion.
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Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.