The Colorado River is an essential source of water for people, fish, and wildlife in seven western states and Mexico. Major U.S. cities that depend on the river include Denver, Albuquerque, Salt Lake City, Phoenix, Tucson, Las Vegas, Los Angeles, and San Diego. This iconic river, however, is threatened today by drought. It is already well known that precipitation is an influential factor in determining the river's streamflow. However, a recent study by researchers from the University of Arizona, University of Nevada, and USGS uncovered the lesser known role of spring and summer temperatures on streamflow in the upper Colorado River basin. 

In the upper reaches of the Colorado, the majority of streamflow comes from melting snowpack. John Wesley Powell, USGS’s second director and an early explorer of the Colorado River, vividly depicted the role of snowmelt in an account of his expedition down the river: 

“Very little water falls within the basin, but, all winter long, snows fall on its mountain-crested rim, filling the gorges, half burying the forests, and covering the crags and peaks with a mantle woven by the winds from the waves of the sea. When the summer sun comes, these snows melt and tumble down the mountain-sides in millions of cascades. Ten million cascade brooks unite to form ten thousand torrent creeks; ten thousand torrent creeks unite to form a hundred rivers beset with cataracts; a hundred roaring rivers unite to form the Colorado, which rolls a mad turbid stream into the Gulf of California” (Powell, 1875, p.294).

Snow-fed rivers like the Colorado are largely influenced by winter precipitation. However, after examining temperature, cool-season precipitation, and streamflow records over the past 100 years, researchers discovered that temperature plays a larger role than previously recognized. When temperatures are warmer than average during the run-off season (March - July), streamflow decreases, exacerbating drought conditions. This is because warmer temperatures can reduce the amount of snow that accumulates, cause snowpack to melt earlier in the year, and result in an earlier peak runoff, all of which are factors that can contribute to decreased stream flow. Understanding how the river responds to changes in temperature is critical for resource managers in the face of ongoing drought and climate change.

These findings resulted from a project funded by the Southwest Climate Science Center (managed by USGS).