Every spring in the western United States, snow melts off mountains, feeding rivers with surges of water that can cause disastrous floods. But warm weather isn’t the main culprit, a new study finds. Instead, dusty soil that sticks to snow can darken it and accelerate its melting. The findings could establish a new way of forecasting snowmelt flooding—and suggest that the current prediction system has been getting it all wrong. The work is “groundbreaking,” says Adrian Harpold, a hydrologist at the University of Nevada in Reno who wasn’t involved with the study. “Dust is a really big deal for snowpack.”