The new research also confirms that using stream gauge records alone may overestimate the average amount of water in the river because the last 100-year period was wetter than the average for the last five centuries.
"This work updates the original landmark Colorado River reconstruction that was done at The University of Arizona's Laboratory of Tree-Ring Research," said David M. Meko, a UA associate research professor of dendrochronology, the science of tree-ring dating.
"The main points of the 1976 research hold up. Droughts more severe and intense than we've seen in the gauged record occurred in the past, and the long-term mean flow is lower than the gauged mean flow."
Connie A. Woodhouse said, "The updated reconstruction for Lee's Ferry indicates that as many as eight droughts similar in severity, in terms of average flow, to the 5-year 2000-2004 drought have occurred since 1500."
Woodhouse, who led the research team, is a physical scientist at the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center Paleoclimatogy Branch in Boulder, Colo.
Allocations of Colorado River water made in the 1922 Colorado River Compact between the states of Arizona, California, Colorado, Nevada, New Mexico, Wyoming and Utah therefore overestimate the amount of river water available. Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque are among the many cities dependent on Colorado River water.
"The long-term perspective provided by tree-ring reconstructions points to a looming conflict between water demand and supply in the upper Colorado River basin," the researchers wrote in their report.