FEMA in January withdrew certification for levees designed to keep California's Sacramento River within its banks. The weak levees leave thousands of homeowners at risk of a catastrophic flood. "Levees are spectacularly unreliable partners in flood management," says Jeffrey Mount, a professor of geology and director of the Center for Watershed Sciences at the University of California, Davis. "We designed a flood-control system for Sacramento in the early 20th century when we lived in a kinder, gentler climate. We got the design wrong."