A ephemeral lake in an arid or semiarid region, most of whose
water comes from groundwater percolating through the soil
of higher surrounding areas.
Many playa lakes form in intermontane valleys in mountainous desert
regions, such as North America's Basin and Range country. Others form in deflation hollows or pans which have eroded down to the water table.
In a desert, the rocky debris of erosion only travels as far as there
is water above ground to push it along, or as far as there is a steep slope
to roll down. Debris builds up into alluvial fans along the bases
of the mountains. Alluvial fans are much larger than they first appear:
The steep, visible part of the fan is made up of large rock fragments.
The bases of the fans, however, are made up of finer particles such as silt,
sand, and clay. The fans' lower slopes are very gentle, so gentle
that a hiker might not think that he or she is hiking on a hill slope.
In certain intermontane valleys, fans merge to block all drainage
out of the valley. Any blockage of drainage traps water, even in
a desert. Sometimes, enough water collects in the lowest portion
of the valley for a lake to exist year-round. Periodic floods distribute
the finest sediment to flatten the valley floor even further, resulting
in vast lakes miles across and a quarter inch deep.
During the last Ice Age, many intermontane environments were not as
dry as they are now. There was more water, sometimes flowing
directly from melting glaciers. Consequently, the lakes were much
larger. Although these lakes have receded since the Ice Age, they
have not lost all of the water from that time. Playa lakes
that have had assistance from historic periods of increased rainfall are
known as pluvial lakes. The best-known pluvial lakes are the Great
Salt Lake and Yellowstone Lake.
In other places, such as the southern Great Plains, playa lakes are
low spots in the soil where groundwater collects. The formation
of these playa lakes is the subject of debate; there are tens of thousands
of them and no alluvial fans to block drainage. One theory
holds that they lie above weak spots in an impermeable layer of stone,
where water percolates through into the underlying deep Ogallala Aquifier.
Still other desert lakes are fed from continent-sized regions of internal
drainage. Lakes such as Lake Eyre in Australia, Lake Chad in
Africa, and the Aral Sea and Lake Balkhash in Central Asia are
more complex than their smaller cousins, being fed from both groundwater
and surface drainage. Such lakes are not properly called playa lakes,
however, they sometimes act like their smaller counterparts, going through
"playa phases", When the Mediterranean Sea dried up during the Miocene epoch, the largest playa lakes
in Earth's history were formed!
Salts dissolved from rock, or leached from the soil during percolation
have nowhere to go but the lake, and the lakes frequently build up high
salinity. Quite often, large sections of playa lakes dry up completely,
leaving vast salt pans. such as the Bonneville Salt Flats.
However, even in a dry lake bed, groundwater is closer to the surface
here than in other places. In places where salt has not built
up to toxic levels, the lakes and lakebeds provide vital water for vegetation.
These are wetland oases for wildlife and humans.
Many playa lakes are home to salt-resistant crustaceans that crawl through
the lake bed; flocks of millions of flamingoes feed upon them.
Cheyenne Bottoms in central Kansas is a temperate playa wetland, a
haven for migrating birds. Lake Texcoco's wetlands caused the nomadic
Aztec to settle down, build their city Tenochtitlán, and eventually