Study: Colorado River Could Fail States

May 30, 2006 —  Estimates are way off on how much water the Colorado River can reliably supply to 30 million people and 3.5 million acres of farmland in surrounding states, say researchers.

By using tree ring data to examine the Colorado River's history back to 1490 A.D., the researchers have confirmed that the last 100 years were unusually wet, and therefore not representative of the river's history.

That's a problem because managers have used direct measurements of the river’s flow in the 20th century as a basis for divvying up the water. And now there’s no water to spare.

"The system is fully used," said Eric Kuhn, general manager of the Colorado River Water Conservation District. "Even a five percent change is a huge issue."

The tree rings show the Colorado's volume has been more variable over the centuries than previously thought. Since 1500, it appears there have been at least eight droughts at least as severe as the one experienced by the region from 2000 through 2004.

"We were interested in getting the best estimate of the water supply," said tree ring researcher David Meko of the University of Arizona. Meko co-authored a report on the new 508-year river flow record, published in the May issue of Water Resources Research.

To reach that estimate, Meko and his colleagues at the National Climatic Data Center, U.S. Geological Survey and the University of Colorado used ring data collected from 1,200 old growth trees in 60 locations around the Colorado River watershed.

In particular, the researchers looked for trees of certain species living in steep spots where humans haven’t cut timber and water is scarce.

"We try to get moisture-stressed trees," Meko told Discovery News. Trees that "feel" droughts easily also show clear signs in their rings. "You don’t want them fat and happy."

Finally, the researchers compared ring data from the 20th century to existing measurements— taken with instruments that monitor water levels and speed— to help calibrate the entire five-century period.

In addition to discovering more periods of severe drought, the researchers learned the Upper Colorado’s average annual flows vary from decade to decade by as much as a million acre-feet. That’s about equivalent to the annual water use of two million, four-person households.

The new study comes at a critical time for managing the river, says Kuhn. Right now the U.S. Bureau of Reclamation is preparing what are called "shortage criteria" for the river. These will determine who gets what water in drought years, when there isn’t enough to go around.

"You need to know what the probability of a shortage is," said Kuhn. "It’s a very intense and important discussion."

Besides providing water for vast farming tracts, the Colorado River is a major source of water for the cities of Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque.