Stream gauges have only been in use for a century. To understand earlier conditions on the Colorado River, scientists study ancient tree rings.
Paleoclimatologists at the University of Arizona discovered an epic megadrought that lasted for more than six decades during the mid-1100s. The drought was remarkable because of the absence of very wet years, and might well have created the conditions that led the Anasazi to abandon Mesa Verde.
It took place during the so-called Medieval Climate Anomaly, from about 900 to 1300, a time when much warmer conditions prevailed throughout much of the world.
That suggests a link between today’s extended drought and the warming western climate.
According to the National Academy of Sciences, temperature records for the Colorado River Basin and the western United States show a warming trend during the past century. Higher temperatures result in less upper basin precipitation being stored as snow and increased evaporative losses.
Rapid population growth across the West is driving increases in water demand. From 1990-2000, Arizona’s population increased by about 40 percent, while Colorado’s population increased by about 30 percent.
Water consumption in Clark County, Nev. (which includes Las Vegas), for example, nearly doubled between 1985 and 2000. And since 2000, growth has continued.
In April 2007, Colorado, Utah, Wyoming, New Mexico, Arizona, California and Nevada, the seven Colorado River Compact signatories, agreed to modify the compact.
The revised compact (which must be approved by the federal government) requires that all states share in drought-related shortages and contains provisions allowing for interstate water exchanges.
Also, during a declared drought, the Upper Basin states (Colorado, Utah, Wyoming and New Mexico) would no longer be required to deliver the full compact allotment to the Lower Basin states (Arizona, California and Nevada).
The agreement spells out steps to be taken to prevent Lake Mead from falling below “dead pool” level — at which no hydroelectric power can be generated and no water can be removed from the reservoir.
But even these measures might not be enough.
During a Senate Energy and Natural Resources Committee hearing, Bradley Udall of the University of Colorado’s NOAA-funded Western Water Assessment, warned that Lake Mead, which as of Sept. 19 contained 12.54 million acre-feet of water, could run dry in 10 years or less.
While reluctant to endorse Udall’s assertion, regional water managers at the hearing described the situation as “dire.”
Tim Brick, chairman of the Metropolitan Water District of Southern California, said the strength of the Colorado River, the primary artery that feeds Lake Mead, could decrease by 15 percent.
According to Brick’s written testimony, reservoirs in the Colorado River’s Upper Basin are at their lowest levels since their initial fillings.
The region’s growth might mean that each state will seek to use, or to sequester, its full entitlement.
California, the most populous of the seven signatories, would no longer be able to use more than its compact entitlement, which it has done for many years.
Nevada, originally allocated only 4 percent of the river’s flow, would have to find other ways of supporting the explosive growth of Las Vegas.
Arizona would have to find more water or face a future without population growth.
And politicians in Colorado, who have long argued that the state should take more water from the Colorado, might have to accept the fact that there’s no water to take.