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Droughts of the Northwestern U.S. Snake River Basin
Reference
Wise, E.K. 2010. Tree ring record of streamflow and drought in the upper Snake River. Water Resources Research 46: 10.1029/2009WR009282.

Background
The author writes that "the 1667 km Snake River is one of the largest rivers in the United States, draining a semiarid region that covers 283,000 km2 [that] includes most of Idaho, as well as portions of Wyoming, Utah, Nevada, Oregon, and Washington," and she says that the river's water has been "historically allocated almost entirely for agricultural irrigation." She also notes that the Snake River is "the largest tributary of the Columbia River (based on both discharge and watershed size)," which makes it "also important for users further downstream." In addition, Wise reports that "the 20th century was an abnormally wet period in this region (Gray and McCabe, 2010)," but she says that an early 21st century drought "has raised questions about whether these dry conditions should be considered an extreme event or if this drought is within the range of natural variability."

What was done
To answer this question, Wise used tree ring samples she collected near the headwaters of the Snake River that were augmented with pre-existing tree ring chronologies to extend the short (1911-2006) instrumental water supply record of the region, thereby providing the first multi-century (1591-2005) record of the river's water supply variability.

What was learned
Wise reports that "individual low-flow years in 1977 and 2001 and the longer-term 1930s Dust Bowl drought meet or exceed the magnitude of dry periods in the extended reconstructed period." However, she writes that in terms of overall severity, "the instrumental record does not contain a drought of the extent seen in the mid-1600s."

Enlarging on this finding, Wise further writes that "twenty-four of 34 years in the 1626-1659 time period had below-average flow, including periods of six and seven consecutive below-mean years (1626-1632 and 1642-1647)," and that "during the most severe period from 1626 to 1647, 17 of 22 years (77%) were below-normal flow." Hence, she concludes that "this type of event could represent a new 'worst-case scenario' for water planning in the upper Snake River."

What it means
"In terms of overall magnitude," Wise concludes that "droughts of the recent past are eclipsed by a sustained low-flow period lasting for over 30 years in the early to mid-1600s," once again demonstrating that there has been nothing unusual, unnatural or unprecedented about the early 21st-century drought experienced throughout the Snake River Basin.

Reference
Gray, S.T. and McCabe, G.J. 2010. A combined water balance and tree ring approach to understanding the potential hydrologic effects of climate change in the central Rocky Mountain region. Water Resources Research 46: 10.1029/2008WR007650.

Reviewed 16 February 2011