Droughts are a natural part of the American West
Although human-caused climate change is causing snow to melt earlier in spring in the American West and rivers to shrink earlier in the summer, natural droughts were already common in this part of the continental US before human-induced climate change began. Among the studies that support this claim are two papers by lead author Edward Cook, of the Lamont-Doherty Earth Observatory the journal Earth-Science Reviews in 2007.[[Cook, E. R., C. A. Woodhouse, C. M. Eakin, D. M. Meko, and D. W. Stahle. “Long-Term Aridity Changes in the Western United States, 2004.” (Abstract) Science. Vol. 306. no. 5698 November 5, 2004. pp. 1015 - 1018.]] [[Cook, E. R., R. Seager, M. A. Cane, and D. W. Stahle. “North American drought: Reconstructions, causes, and consequences.” (PDF) Earth Science Reviews 81, no. 1-2 (2007): 93-134.]]
The former gives a detailed analysis of the drought history in the Western US dating back to medieval times. A website related to the latter study provides more information as well as an animation that shows the regularity of droughts over recent centuries.
Other studies suggest that medieval droughts in the West were much more severe and prolonged than any since the 19th century.[[Stine, S. 1994. “Extreme and persistent drought in California and Patagonia during medieval time.” Nature 369:54]]
While droughts are natural in the US West, it is still possible for climate change to increase their number and intensity.