Farmers, hydroelectric power producers,
shippers and wildlife managers remember the Columbia River Basin
drought of 1992-1993 as a year of misery.
Now researchers using
tree-ring data have determined six multiyear droughts between
1750 and 1950 that were much more severe than
anything in recent memory because they persisted for years, including
one that stretched for 12 years.
"Imagine what a drought lasting that long would do to the
resources and economy of the region today," says Dave Peterson
of the U.S. Agriculture Department's Forest Service Pacific Northwest
Research Station and the University of Washington's College of
The study, recently published in the Journal of
the American Water Resources Association, is the first to establish
flow estimates back 250 years, says lead author Ze'ev Gedalof
of the University of Guelph, Ontario. Reliable natural-runoff estimates
extend back only about 75 years, he says.
Of the six major multiyear
droughts researchers detected in the Columbia River Basin, the
most severe and persistent started in
the 1840s and lasted 12 years in a row. Flows were 20 percent
below long-term averages, Gedalof says, and could have been even
the extreme low flows being something the model used in the project
cannot pinpoint as precisely as it can the years the droughts
The second worst corresponds to the 1930s dry period
that, together with poor farming practices, caused the Dust Bowl.
River Basin experienced multiple years of low flows, punctuated
with some average years, during that time.
In addition to the
extremes of the 1840s and 1930s, other periods of low flows around
1775, 1805, 1890 and 1925 were notable, but
shorter, lasting some three to five years each.
"This is a wake up call for the importance of drought planning,
and seeking ways to restore some flexibility in Western water supply
systems that have a limited ability to respond to multiyear droughts," says
co-author Nate Mantua of the Climate Impacts Group based at the
UW. "The drought like the one indicated in 1840s, for instance,
simply hasn't been part of the modern water systems experiences."
used tree rings, which indicate how much a tree grows each year,
to determine when forests in the Columbia River Basin
experienced drought. Tree growth is particularly sensitive to
winter snow pack, which is also the main driver of stream flow
Columbia. The scientists first related tree-ring data to stream-flow
records since 1931, then considered the implications for flows
of tree-ring data back to 1750, much of the time when record
keeping was anecdotal, inconsistent or nonexistent.
reconstructions have been done previously by other research groups
for rivers such as the Sacramento and Colorado.
The work was funded by the Joint Institute for the Study of the
Atmosphere and Ocean based at the UW, Natural Sciences and Engineering
Research Council of Canada and USDA Forest Service.
"The big lesson is that prolonged low flow years are a normal
part of the Columbia Basin's history," Mantua says. "The
problem is that water in most sub-basins is fully allocated. These
demands have gone a long way to eliminating flexibility and buffers
needed in the face of drought caused water-supply shortages."