Geoscience Reference
In-Depth Information
(about 400 years ago). A period of cool summer temperatures, with a greater
than average number of storms per year—for decades to up to a century—
would have been required to form these lakes in the Mojave.
northern california floods
h e ominous history of ancient megal oods identii ed in Southern California
is not unique. Researchers in the northern half of the state have found depos-
its suggesting l oods of similar magnitude and frequency—from river valleys
to the higher mountain ranges. One of these studies was located on the l ood-
plain of the Sacramento River, the largest river in California and the one
that drains much of the northern half of the state. Geographers Roger Byrne
and Don Sullivan from the University of California, Berkeley, have found
evidence for large l oods in sediments from beneath oxbow (or arc-shaped)
lakes on the Sacramento River l oodplain of California's Central Valley. h ey
were not initially looking for evidence of megal oods; rather, they were evalu-
ating how the burgeoning population in California at er the Gold Rush had
inl uenced the natural vegetation in the region. But as they examined their
sediment cores, they found an unexpected legacy of major l ood events.
To understand what Sullivan and Byrne discovered, it helps to visualize
the setting of their research. Floodplains, by their nature, contain the most
accurate picture of past l ooding events. During a l ood, the waters swell out
of their channels and spread across the surrounding plain, depositing layers
of sediment. h ese layers build on each other, l ood at er l ood, over hun-
dreds to thousands of years. h e thickness of the layers yields information
about the size of the l oods. h e lakes within the l oodplain, though some are
short-lived, contain records that can be interpreted both for past l ood events
and for the climate conditions under which the lakes formed.
h e lake Sullivan and Byrne chose to core, Little Packer, began forming
800 years ago. Once a bend in the Sacramento River, Little Packer was aban-
doned by the meandering main channel and was let behind as an oxbow
lake. h is occasionally occurs when rivers meander in wide and gently slop-
ing l oodplains, as the Sacramento does in the Central Valley. Such aban-
doned lakes become recorders of large l ood events that temporarily reunite
them with their rivers. Sediment-laden l oodwaters spill into them, and the
sediment settles to the bottom, forming a distinct layer within the normal
lake sediments.
