BOULDER CITY, Nev. -- Straddling the border between Nevada and Arizona, the Hoover Dam is a symbol of human engineering might. For more than 70 years, its massive walls have tamed the flows of the Colorado River, fueling the growth of cities like Las Vegas that depend on it to supply water and power from its generating station. But these days, what's most striking is the lack of water stored behind the dam's concrete arch.

Climate change is likely to intensify tensions over Colorado River flows, according to a new study that predicts intermittent shortages by the middle of the century.

The river supplies water to 27 million users in seven western states and Mexico, under a system of allotments negotiated decades ago. But the warming climate is expected to reduce the runoff that feeds the river. By 2050, that could leave the Colorado unable to meet its scheduled water deliveries 60 to 90 percent of the time, concludes the study by researchers at the Scripps Institution of Oceanography.

"The current water supply in the Colorado is not sustainable under climate change of the magnitude we expect," said lead author Tim Barnett, a Scripps climatologist. And that will leave water managers with difficult choices, if the predictions by Barnett and colleague David Pierce prove correct.

ISTANBUL -- Don't expect the future to look much like the past, at least when it comes to the Earth's fresh water supplies. That's the message emerging from a major international meeting being held here this week.

More than 27,000 people -- including government ministers from more than 120 countries -- have gathered for the 5th World Water Forum. But in this ancient city, where the thin ribbon of the Bosporus divides Europe and Asia and massive Roman waterworks still dot the landscape, it's the Earth's shifting climate that is on delegates' minds.