The American West was colonized by Anglos during some of the wettest periods in the last 1,000 years, according to the February 2008 National Geographic. These abnormally wet years are depicted as mountains surrounded by valleys of drought in a graph of Colorado River Streamflow reconstructed using University of Arizona Lab of Tree Ring Research. The highlight denotes a 60-year drought that was consistently as severe or worse than the current (1998-2008) drought. Judging from the jagged oscillations of this graph, such droughts may be a regular and normal occurrence, although not for our modern water-spoiled civilization.
See also National Geographic News, "U.S. Drought Could Be Start of New Dust Bowl"