An epic drought during the mid-1100s dwarfs any drought previously documented for a region that includes areas of Arizona, Colorado, New Mexico, Utah and Wyoming.
The six-decade-long drought was remarkable for the absence of very wet years. At the core of the drought was a period of 25 years in which Colorado River flow averaged 15 percent below normal.
The new tree-ring-based reconstruction documents the year-by-year natural variability of streamflows in the upper Colorado River basin back to A. D. 762, said the tree-ring scientists from The University of Arizona in Tucson who led the research team.
The work extends the continuous tree-ring record of upper Colorado streamflows back seven centuries earlier than previous reconstructions.
"The biggest drought we find in the entire record was in the mid-1100s," said team leader David M. Meko, an associate research professor at UA's Laboratory of Tree-Ring Research. "I was surprised that the drought was as deep and as long as it was.
Colorado River flow was below normal for 13 consecutive years in one interval of the megadrought, which spanned 1118 to 1179.
Meko contrasted that with the last 100 years, during which tree-ring reconstructed flows for the upper basin show a maximum of five consecutive years of below-normal flows.
The Colorado supplies water for cities and agriculture in seven western states in the U.S. and two states in northwestern Mexico. Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque are among the many cities dependent on Colorado River water.
The Intergovernmental Panel on Climate Change predicted in a recent report that the southwestern U.S. will become hotter and drier as the climate warms.
Co-author Connie A. Woodhouse said, "We have natural variability that includes this time in the 1100s. If we have warming it will exacerbate these kinds of droughts."