Two water scientists have published a paper which argues that about one-third of the decline in the country's two largest reservoirs, Lake Mead and Lake Powell, was likely due to higher temperatures in the Colorado River’s Upper Basin, which result from climate change. Lake Mead on the Arizona/Nevada border and Lake Powell on the Arizona/Utah border, were brim full in the year 2000. Four years later, they had lost enough water to supply California its legally apportioned share of Colorado River water. Now, 17 years later, they still have not recovered. This threatens water supplies in major Western cities, such as Los Angeles, San Diego, Phoenix, Tucson, Denver, Salt Lake City, Albuquerque, as well as some of the most productive agricultural lands anywhere in the world. The scientists attribute the decline to more than one cause - overuse, an ongoing drought, which started in 2000 and has led to substantial reductions in river flows, and higher temperatures in the Colorado River’s Upper Basin, which result from climate change.
The study showed that from 2000 to 2014 annual flows in the Colorado River averaged 19 percent below the 20th-century average. During this period temperatures in the Upper Basin, where most of the runoff that feeds the Colorado River is produced, were 1.6 degrees Fahrenheit higher than the 20th-century average. High temperatures continued in 2015 and 2016, as did less-than-average flows. Runoff in 2017 is expected to be above average, but this will only modestly improve reservoir volumes.
How do high temperatures affect river levels? Coupled with earlier snow melt, they lead to a longer growing season, which means more days of water demand from plants. Higher temperatures also increase daily plant water use and evaporation from water bodies and soils. In sum, as it warms, the atmosphere draws more water, up to 4 percent more per degree Fahrenheit from all available sources, so less water flows into the river. These findings also apply to all semi-arid rivers in the American Southwest, especially the Rio Grande.