A series of unusual spring dust storms has left the snowcapped mountains of western Colorado stained brown and red, even a bit pink. The dust is speeding up the runoff to rivers that supply millions of people with water and raising fears of an increasingly arid West.

Twelve dust storms barreled into the southern Rockies from the deserts of Arizona, Utah and New Mexico so far this year. In contrast, four storms hit the mountains all year long in 2003. Eight occurred in each of the last three years.