The Colorado River may shrink in this century to its lowest level in at least 500 years because of global warming, threatening water supplies to Utah, California and five other states, researchers at the U.S. Geological Survey say.

But Utah Division of Water Resources director Dennis Strong isn't jumping to any conclusions just yet.

"Right now we're projecting over 120 percent inflow" this year into Lake Powell, one of the river's primary reservoirs, Strong says.

However, a "modest" 0.86 degree Celsius (1.5 degree Fahrenheit) increase in the 21st century could trim the average flow of the river — the primary water supply for residents in much of the U.S. Southwest — to the low end of a range marked between 1490 and 1998, USGS scientist Gregory McCabe said.

The Earth is likely to warm by more than twice that amount in the period, Rajendra Pachauri, chairman of the U.N. Intergovernmental Panel on Climate Change, said last month. McCabe will brief Congress on the findings in June, when legislators expect to debate plans for the world's largest producer of greenhouse gases to begin capping its emissions.

"A 2-degree Celsius warming pushes the risk so high that it's beyond anything that has happened in the last 500 years," McCabe said on a conference call. "The average flow in the Colorado drops to lower than anything we've seen."

While the long-term prognosis may be dire according to the USGS study, Strong said the Colorado's flow will be ample this year. The lake is expected to rise by 50 feet from above-average upstream runoff, mostly from snowfall in Colorado.

The river is fed by the nation's seventh-largest drainage area. Less precipitation from periodic droughts or climate change leaves reduced snow to feed the 1,450-mile waterway.

In a report presented this past week in Boston and co-written by USGS research hydrologist David Wolock, McCabe used data from an earlier study that reconstructed annual stream flows from measurements of tree rings. The technique provides a way to show how temperatures will affect flows in the future, Wolock said.

"It allows us to place the 20th century conditions that were used in developing plans for managing water resources in the basin in the context of a much longer record of flow," Wolock said in an interview. "We can estimate flow during periods when we were never able to measure."

About 40 percent of Southern California's water supply is likely to be vulnerable within the next two decades as rising temperatures lead to reductions in snow pack in the Sierra Nevada and the Colorado River basin.