"That we can now say with confidence," said Robert Webb, lead author of the new fact sheet released Thursday. "Now I'm completely convinced."

The drought has produced the lowest flow in the Colorado River on record, with an annual average flow of only 5.4 million acre-feet at Lees Ferry during the period 2001-2003, adjusted for the effect of Glen Canyon Dam. By comparison, during the Dust Bowl years, between 1930 and 1937, the annual flow averaged about 10.2 million acre-feet, the report said.

Scientists use tree-ring reconstructions of Colorado River flows to estimate what conditions were like before record-keeping began in 1895. Using that method, the lowest five-year average of water flow was 8.84 million acre-feet in the years 1590-1594. From 1999 through last year, water flow has been 7.11 million acre-feet.

"These comparisons suggest that the current drought may be comparable to or more severe than the largest-known drought in 500 years," the report said.

Environmental groups say water managers should take heed.

"The water managers, they just continue to pray for rain," said Owen Lammers, director of Living Rivers and Colorado Riverkeeper. "They just say, well, we hope that things change and we see rain."

Lammers said the report reinforces the need to figure out a better way to manage the Colorado River before reservoirs run dry.

"Once our reserve supply is gone, we have no plan of action for what to do," he said.

The report said the river had its highest flow of the 20th century during 1905 to 1922, the years used to estimate how much water Western states would receive under the 1922 Colorado River Compact.

The compact should now be reconsidered because of the uncertain water flow, said Steve Smith, a regional director for the Wilderness Society.

"We've got to figure out a new way of distributing the water that exists in the Western United States and be a lot more deliberate about our cautious and efficient use of the water," he said.

The report didn't surprise water managers.

"The big lesson is communities cannot afford to put all their eggs in the proverbial basket. You need .. a diverse portfolio of resources," said Adan Ortega, spokesman for the Metropolitan Water District of Southern California, the largest wholesale water supplier in the country.

Ortega said the water district is increasing water storage, buying water from farmers and investing in alternatives to the Colorado River.

Vince Alberta, spokesman for the Southern Nevada Water Authority, said water use in southern Nevada this year is actually down and the authority continues to enforce watering restrictions, impose fines for water waste and promote conservation.

"I think we can be successful managing this drought, but it's going to take a unified effort with everybody making sacrifices," Alberta said.

Herb Guenther, director of the Arizona Department of Water Resources, said the agency continues to plan for a continuing drought.

"It's serious, but the sky is not falling. Of course, we wish it would in the form of rain," he said.

Webb said predicting when the drought will end isn't easy because the Colorado River is difficult to forecast.

"It's sort of a split-personality river. It has headwaters up in Wyoming as well as headwaters in Colorado," Webb said. "Those two regions tend to respond to different things. ... We can't explain it very well."

Droughts seldom persist for longer than a decade, the report noted. But that could mean the current drought is only half over.

"If you're a betting person, you will bet that we will come out of this drought next year," Webb said. "It's a very severe event and these things tend to end fast. There are other indications, though, that suggest that this drought could persist for as long as 30 years.

"We don't really know."

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EDITOR'S NOTE -- Angie Wagner is the AP's Western regional writer, based in Las Vegas.

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On the Net:

http://water.usgs.gov/pubs/fs/2004/3062/

http://www.livingrivers.org

http://www.wildernesssociety.org

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Last Update: October 30, 2007