Geoscience Reference
In-Depth Information
Floods
h e paleoclimate records suggest that the West has experienced far worse
l oods, at regular intervals, than the region has seen over the past 150 years.
h e 1861-62 l ood, for instance, was the result of a series of large storms that
l owed one at er another across the Pacii c in what climatologists describe
as an atmospheric river. h ese storms slammed the West Coast—from the
Mexican border to Canada—for 43 days, bringing triple the average amount
of rainfall to the region and leading to unprecedented l ooding. Based on the
relatively short historic record, water planners have estimated that a l ood of
this magnitude may recur perhaps every thousand years. However, paleocli-
mate records spanning the past two thousand years indicate that l oods of
this magnitude, or perhaps larger, have recurred every one to two centuries.
h e largest of these l oods occurred during the Little Ice Age, four hun-
dred years ago. h is event predated the history of modern California, yet it
is interesting to note that Native American residents in 1861-62 appeared to
recognize the early warning signs of these storms and the subsequent l oods,
and they quickly moved out of the l oodplains and up to higher ground.
h eir long history in the region had given them a deeper understanding that,
though the l oodplains could provide abundant rich soils and resources, they
could only be safely occupied seasonally. h e Native Americans knew better
than to build permanent settlements in California's Central Valley or delta.
future flood risks
California's Central Valley
Many regions in the western states are vulnerable to l ooding today, despite
decades of construction of l ood control infrastructure. Modern society has
promoted growth, with cities sprawling out onto the l oodplains, deltas, and
other low-lying areas. Many of these regions are protected by aging levees that
have been shown to fail under larger l ood events.
Other human activities have added to the l ood risk. Take, for example,
California's Central Valley: viewed from above, it looks like a giant bath-
tub—up to 400 miles long and 70 miles wide—with the San Francisco
Bay estuary and delta as the drain. Most of California's agriculture, and
much of its future population growth, will occur there. Early accounts from
California history describe winter l oods, like the 1861-62 event, that turned
