Droughts of the Northwestern U.S. Snake River Basin
Wise, E.K. 2010. Tree ring record of streamflow and drought in the upper Snake River. Water Resources Research 46: 10.1029/2009WR009282.
To answer this question, the author 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. So what did she find?
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."
"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.
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.