Rain does fall occasionally in deserts, and desert storms are often
violent. A record 44 millimeters of rain once fell within 3 hours in the
Sahara. Large Saharan storms may deliver up to 1 millimeter per minute.
Normally dry stream channels, called arroyos or wadis, can quickly fill
after heavy rains, and flash floods make these channels dangerous. More
people drown in deserts than die of thirst.
Though little rain falls in deserts, deserts receive runoff from
ephemeral, or short-lived, streams fed by rain and snow from adjacent
highlands. These streams fill the channel with a slurry of mud and
commonly transport considerable quantities of sediment for a day or two.
Although most deserts are in basins with closed, or interior drainage, a
few deserts are crossed by ‘exotic’ rivers that derive their water from
outside the desert. Such rivers infiltrate soils and evaporate large
amounts of water on their journeys through the deserts, but their
volumes are such that they maintain their continuity. The Nile, the
Colorado, and the Yellow are exotic rivers that flow through deserts to
deliver their sediments to the sea.
[Illustration: The Wei River in the Loess Plateau, China (photograph by
I-Ming Chou).]
[Illustration: Running water created this canyon in arid Big Bend
National Park, southwest Texas.]
Lakes form where rainfall or meltwater in interior drainage basins is
sufficient. Desert lakes are generally shallow, temporary, and salty.
Because these lakes are shallow and have a low bottom gradient, wind
stress may cause the lake waters to move over many square kilometers.
When small lakes dry up, they leave a salt crust or hardpan. The flat
area of clay, silt, or sand encrusted with salt that forms is known as a
_playa_. There are more than a hundred playas in North American deserts.
Most are relics of large lakes that existed during the last Ice Age
about 12,000 years ago. Lake Bonneville was a 52,000-square-kilometer
lake almost 300 meters deep in Utah, Nevada, and Idaho during the Ice
Age. Today the remnants of Lake Bonneville include Utah’s Great Salt
Lake, Utah Lake, and Sevier Lake. Because playas are arid land forms
from a wetter past, they contain useful clues to climatic change.
The flat terrains of hardpans and playas make them excellent race tracks
and natural runways for airplanes and spacecraft. Ground-vehicle speed
records are commonly established on Bonneville Speedway, a race track on
the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at
Edwards Air Force Base, California.
[Illustration: The Qaidam Depression in China is the highest desert in
the world. This Landsat image illustrates a salt lake and evaporite
basins in the depression.]