Geoscience Reference
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f igu r e 2 . Donner Lake in northeastern California, the eastern Sierra Nevada, eleven
miles northwest of Lake Tahoe. (Photo by B. Lynn Ingram.)
of brown, tan, and yellow. h e color contrasts are due, of course, to moisture
dif erences. h e humid East supports more forests and, moving westward,
wide prairies. Western North America is more constrained by water avail-
ability, especially in the southwestern deserts.
If we zoom in a bit closer and l y across this western region from west
to east, we begin to see a relationship between the lay of the land and the
colors of the land—a relationship that is largely based on water. Passing
above the Pacii c coast, we observe mountains, especially along the north-
ern coastline, with lush rainforests that give way quickly to grasslands and
then to deserts. Within this region, it is clear where the water falls: the
Pacii c Northwest and Northern California appear deep green, as does
the central and southern Sierra Nevada mountain range (see i gure 2).
Much of Southern California and the states of Nevada, Utah, Arizona,
and New Mexico appear in varying shades of brown, tan, and red, rel ect-
ing the vegetation and the composition of the soils—both related in large
part to the lower amounts of rain and snow that fall there each year (see
i g u r e 3 ) .
Precipitation over the American West is spatially irregular because it
depends on the complex behavior of the Pacii c Ocean and its exchange of
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