Geoscience Reference
In-Depth Information
For the more distant past, drought is dei ned in relative terms, such as
when precipitation, runof , and lake levels fall below the long-term average
by at least one standard deviation, or when the growth rings of trees become
narrower than average by a certain amount. In drought-prone regions, native
plants and animals usually employ adaptations to survive through periods
of lower than average precipitation; such adaptations have evolved over very
long periods of time. Humans in the West today, in contrast, have l ourished
only by manipulating the natural hydrology using modern engineering and
technology.
h
e Driest Single Year in the West: 1976-1977
h e worst single year of drought in recorded history over much of the American
West occurred during the winter of 1976-77. h e drought was widespread,
extending across the entire state of California and to the midwestern states
and north into the Canadian prairie region north of Montana. Even the Pacii c
Northwest experienced its worst drought on record, leading to shortfalls in
crop yields and region-wide water rationing. Record low l ows were measured
on the Columbia River, leading to restrictions on power consumption from
hydroelectric plants. h e snowpack in mountains from California to Colorado
reached historic lows, and ski resorts from the Sierra Nevada to the Rockies
remained closed for much of the season.
h e 1976-77 drought was particularly devastating in California.
Precipitation levels that year dropped to less than half of the average level
throughout the state (see i gure 7). Northern California was especially hard-
hit, with rainfall reaching only 15 percent of the average in some regions.
Runof and river l ow in California fell to a quarter of their normal levels,
and storage reservoirs saw dramatic declines, falling to about a third of their
normal levels. In the Sierra Nevada, Lake Tahoe dropped below its outlet
at the Truckee River (near Tahoe City), causing the Truckee River to cease
l owing, which in turn caused Pyramid Lake in northwestern Nevada to
drop as well. Aquatic habitats were severely af ected, with the worst impacts
in the lower to mid-elevations of the Coast Ranges and the western slope of
the Sierra Nevada. Low river l ows, combined with higher water tempera-
tures, severely disrupted the migration and spawning of migratory i sh spe-
cies, including all species of salmon and trout.
h e shortage of surface water in the state prompted an increased use of
groundwater for agriculture and cities. h
e proportion of groundwater used
