7601

• UCREFRP Catalog Number: 7601
• Author: Minckley, W. L.
• Title: Native Fishes of Arid Lands
• USFW Year: 1991.
• USFW - Doc Type: A Dwindling Resource of the Desert Southwest.
• Copyright Material: NO

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Figure 2. Left: The Willcox Playa, Arizona, was flooded to form this broad, shallow
lake following extensive rainfall in 1983 (photo by R. Todd). In even wetter times, the
closed Willcox basin was occupied by pluvial Lake Cochise. Right: In periods of
drought, extensive closed-basin lakes quickly dry into a sea of salt and sand, such as
in this playa in Death Valley, California, 1972.

interactions among plates in the earth's crust.
Opening of the Gulf of California was part of this
tectonic pattern. Inland, east-to-west stretching of the
land surface in Miocene (1'\.120 to 5.0 million years
ago) fractured an ancient almost-level terrain into
northwest-southeast-trending mountains and parallel
valleys.
Rivers with large enough watersheds cut deep
canyons through rising plateaus and mountain
blocks. Others were diverted by uplifting mountain
ranges. Huge lakes, represented today by dry playas
and a few remnants such as Great Salt Lake, were
formed in closed basins. Their waters rose and fell,
filling followed by episodic drying, refilling, and
spilling over their rims to connect with other lakes,
rivers, or the sea. In Pliocene and Pleistocene (5.0 to
1'\.12.0 and after 1'\.12.0 million years ago, respectively),
mountain glaciers came and went on the highest
peaks, reflecting colder, wetter conditions than today
and corresponding with alternating pulses of
continental glaciation in the north.
Plio-Pleistocene uplift of the Sierra Nevada and
other mountain ranges of California resulted in
permanent climatic change when they rose high
enough to intercept the eastward flow of Pacific
moisture. Water falling on their western slopes ran
back to the sea, and only dry air passed downslope
to the inland basins and plateaus. Persistent aridity
was thus assured. Even drier conditions were
induced in the Sonoran Desert by formation of
permanent cells of high atmospheric pressure, caused
by descent of high-altitude air to the earth's surface.

The descending air lacks humidity because of its
origin at cold, polar latitudes, so little water is
available to condense to precipitation. In addition, a~
happens to air flowing down a mountain, descent
from high altitude results in compressional heating,
and the hot dry air removes even more water from
land surfaces.

Water, Aquatic Habitats,
and the Impacts of Humans

Essentially the only water available to this vast,
arid region originates on mountains high above the
desert floors. The highlands intercept movement of
air masses, forcing them upward to cool and
condense their sparse humidities into precipitation.
Winter snowpacks accumulate, melt in spring, and
seep into shallow soils or form overland rivulets.
Seeps and rivulets combine to form brooks which
emerge from the mountain fronts. Water in such
channels mostly percolates into the coarse-grained
valley floors, where protected from evaporation it is
stored or continues on a subsurface path to the sea.
Only channels with substantial montane
watersheds support perennial streams. The Colorado
River itself originates in the Rocky Mountains to
flow through Great Basin and Mohave deserts to
enter the Gulf of California surrounded by Sonoran
Desert. The Gila River, most southern major

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