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Winter/Spring 2007 Vol. 7 No. 1

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Colorado River, ©Photodisc Troubled Waters

New Findings and Regional Trends Are Complicating Colorado River Water Management

For most of the last century, the only information on the Colorado River's streamflow came from a series of gauges that measure flows at various points along the river. Over the years, these gauges provided the data upon which many contentious water-allocation negotiations were based. In fact, measurements from the U.S. Geological Survey's gauging station at Lees Ferry, Arizona, were cited in the 1922 Colorado River Compact, which to this day governs the allocation of water between states in the upper and lower basin.

More recently, scientists have started looking further back in history to get a better idea of the river's average flow. They were able to do so by studying coniferous trees with long life spans across the region. Because moisture availability is reflected in the annual growth rings of trees that grow at low elevations, scientists can use this information to reconstruct past climatic conditions and, in turn, estimate river flows.

What they have learned is that the Colorado River's average flow over the past four to five centuries has fluctuated more than previously assumed, exhibiting periods when average flows were higher and lower than the average measured by gauges during the last century, according to a new report from the National Research Council. In particular, the tree-ring data show that there were several periods when flows were considerably lower than those measured at Lees Ferry since 1921, and that the period just prior to the signing of the compact was exceptionally wet. Equally important, the tree rings indicate that extended droughts, like the one experienced in recent years, are a recurrent feature of the Colorado River basin.

The new data are prompting much discussion among water managers in many arid parts of the western United States where the Colorado River is the main source of surface water. River management decisions rely heavily on forecasts that assume the instrumental record of past water conditions will generally be replicated in the future. But the tree-ring data call these assumptions into question, the report says.

Parker Dam and Powerplant and Lake Havasu, from which the Metropolitan Water District of Southern California pumps water, located in Arizona, photo by Andy Pernick, U.S. Bureau of ReclamationFurther complicating the forecasts is a warming trend in the West that shows no signs of dissipating. The recent drought is not unprecedented, as the tree-ring data show, and could be chalked up to natural climate variability. Droughts in the future, however, are likely to be more severe because of rising temperatures. A preponderance of evidence suggests that warmer temperatures will reduce Colorado River streamflow and water supplies, the report says. Even if precipitation levels remain the same, streamflow could drop because warmer temperatures mean more rain will fall than snow, reducing the snowpack that gradually feeds the river. More water will be lost to evaporation as well.

Higher temperatures will also increase the demand for water from a rapidly growing population across the western United States. Although some of the added stress placed on water supplies by this burgeoning population has been abated through technology and conservation, demand is rising sharply. Water consumption doubled from 1985 to 2000 in Clark County, Nevada, where Las Vegas is located, for example.

Technology and conservation will not provide a panacea for coping with water shortages in the long run, the report warns. It also notes that the practice of transferring agricultural water rights to municipalities -- often a preferred method for meeting urban water demand in the basin -- may have undesirable effects on "third parties," such as downstream farmers or ecosystems. The agricultural water supply is also not unlimited. Cooperation among basin states, informed in part by a comprehensive basinwide study of water practices, will be essential in managing future droughts, as will better communication between scientists and water managers.
  -- Bill Kearney

Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability. Committee on the Scientific Bases of Colorado River Basin Water Management, Water Science and Technology Board, Division on Earth and Life Studies (2007, approx. 218 pp.; ISBN 0-309-10524-2; available from the National Academies Press, tel. 1-800-624-6242; $44.75 plus $4.50 shipping for single copies).

The committee was chaired by Ernest T. Smerdon, emeritus dean of the College of Engineering and Mines, University of Arizona, Tucson. The study was funded by the National Academies, U.S. Bureau of Reclamation, California Department of Water Resources, Metropolitan Water District of Southern California, and the Southern Nevada Water Authority.

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Copyright 2007 by the National Academy of Sciences