Read here. Often it is heard that recent drought conditions in various western and southwestern areas of the U.S. are unprecedented. The implications are that these drought conditions are a result of human-induced global warming (climate change) and are more extreme than ever. Science findings, however, do not support the label of "unprecedented."
Gray et al. studied streamflow attributes for several Colorado river tributaries and found that earlier extreme drought conditions were likely worse than those of the modern era and persisted multiple times longer.
"...derived millennial-length records of water year (October-September) streamflow for three key Upper Colorado River tributaries -- the White, Yampa and Little Snake Rivers -- based on tree-ring data they obtained from seventy-five preexistent chronologies for a number of sites scattered throughout the region, where each chronology was derived from average annual ring-widths...report that "as in previous studies focused on the Upper Colorado River system as a whole (e.g., Meko et al., 2007)," their sub-basin reconstructions "show severe drought years and extended dry periods well outside the range of observed flows." Although they note that 1902 and 2002 "were among the most severe in the last ~1,000 years," they state that "pre-instrumental dry events often lasted a decade or longer with some extended low-flow regimes persisting for 30 years or more." In addition, they indicate that their research "shows anomalous wetness in the 20th century..." [Stephen T. Gray, Jeffrey J. Lukas, Connie A. Woodhouse 2011: Journal of the American Water Resources Association]
Additional drought-flood and peer-reviewed postings.