Geoscience Reference
In-Depth Information
precipitation in the West during the ice age and cooler air temperatures, caus-
ing less evaporation of their surfaces. During the Pleistocene, eight lakes in
the Great Basin covered some 27,800,000 acres, eleven times the surface area
covered by lakes today.
One of the largest of these Pleistocene lakes was Lake Bonneville. Mapped
by G. K . Gilbert in the late nineteenth centur y, Lake Bonneville once reached
a size equal to Lake Michigan, covering the northwest corner of modern-day
Utah and extending into Nevada and Idaho (see i gure 18 in the next chap-
ter). At over a thousand feet deep, it was signii cantly deeper than modern
Lake Michigan. h e weight of this lake was so great that it depressed the
underlying land surface some 240 feet, and, when the lake later went away,
the land surface rebounded. Gilbert mapped l ood features just to the north
of Lake Bonneville and hypothesized that the lake may have burst its dam
catastrophically about 15,000 years ago, creating a megal ood that l owed
into the Snake River Basin in southern Idaho and i nally into the Columbia
River. Today, the only remnant of this enormous ice age lake is the Great Salt
Lake in Utah, so named because it is one of the saltiest lakes on Earth—seven
times saltier than the ocean.
Glacial summers were cooler in the northern hemisphere than they are
today, largely as the result of astronomical factors that conspired to reduce,
ever so slightly, the total amount of incoming solar radiation and to miti-
gate the extremes of the seasons. h ese cooler summers allowed the enor-
mous continental ice sheets, including those that covered Canada and the
northernmost United States, to grow to two miles high—a height that was
sui cient to inl uence atmospheric circulation patterns.
Climate scientists have run computer models showing that the jet
stream, a high-speed wind current that brings winter storms to the Pacii c
Northwest and as far south as Northern California today, may have
been split into two limbs as it encountered the ice sheet. h ese models suggest
that the southern limb of the jet stream during the late Pleistocene would
have brought Pacii c storms into the Great Basin, increasing precipitation
during the winter. Cooler, cloudier conditions during the summer would
have also decreased evaporation in the Great Basin, where today an average of
15 to 30 inches of water evaporates of the lakes every year. h e overall ef ect
would have been an increase in moisture in the region—and the expansion
of lakes.
Paleoclimatologist Larry Benson and his colleagues at the U.S. Geological
Survey have spent their careers studying Great Basin lakes, noting that those
