A massive crevasse that formed in a spillway at Northern California’s Oroville Dam has spurred mass evacuations, with nearby residents fleeing the worst-case specter of a three-story wall of water rushing downstream. In all, about 188,000 people, mostly in Butte, Sutter and Yuba counties, evacuated from the area, some being given only minutes to gather their things. Water releases have eased some of the pressure but the danger has not passed, and won’t be until a damaged spillway can be repaired.