‘Atmospheric Rivers’ Aid the West — and Imperil It

When a rainstorm slammed California’s Russian River watershed in December 2012, water rushed into Lake Mendocino, a reservoir north of San Francisco. The cause? An atmospheric river, a ribbon of moisture-laden air that can ferry water thousands of miles across the sky. When the tempest hit, the state was on the brink of an exceptional drought. But instead of storing the surge the storm brought for the dry days to come, the reservoir’s owner, the U.S. Army Corps of Engineers, let it run downstream.