In early February, a massive crack opened in a concrete chute that carries water from America’s tallest dam. For nearly a week, Oroville Dam managers assured the public there was no imminent danger as they slowed releases of water down the main spillway to assess the damage. Then, with the lake behind the dam swelling from rain and runoff, water began running down a never-before-used backup spillway. But that, too, began breaking apart, and nearly 200,000 people were suddenly ordered to evacuate.