The Colorado River’s average annual flow has declined by nearly 20 percent compared to the last century, and now a new study has identified one of the main culprits: Climate change is causing mountain snowpack to disappear, leading to increased evaporation.

Four recent studies have found that up to half of the drop in the Colorado’s average annual flow since 2000 has been driven by warmer temperatures. Now, two U.S. Geological Survey researchers have concluded that much of this climate-induced decline — amounting to 1.5 billion tons of missing water, equal to the annual water consumption of 10 million Americans — comes from the fact that the region’s snowpack is shrinking and melting earlier. Having less snow to reflect heat from the sun, known as the albedo effect, creates a feedback loop, they say.