Even as the number of Americans relying on the Colorado River for household water swells to about 40 million, global warming appears to be taking a chunk out of the flows that feed their reservoirs.

Winter storms over the Rocky Mountains provide much of the water that courses down the heavily tapped waterway, which spills through deep gorges of the Southwest and into Mexico.

ow water levels in late 2014 at Lake Powell, which is a Colorado River water reservoir built along the border of Utah and Arizona.
Low water levels in late 2014 at Lake Powell, which is a Colorado River water reservoir built along the border of Utah and Arizona.Jessica Mercer

But flows in recent decades have been lighter than would have been expected given annual rain and snowfall rates — and a new study has pinpointed rising temperatures as the likely culprit.

“For a given precipitation over the cool season, from October through April, we’re seeing less flow than we’ve seen in the past,” said James Prairie, a researcher with the U.S. Bureau of Reclamation’s Upper Colorado Regional Office. He was not involved with the new study.