An epic winter of rain and snow has refilled California’s reservoirs and pressed into service a spillway at the nation’s tallest dam Tuesday, a $1 billion structure that drained excess water for the first time since it crumbled two years ago and drove hundreds of thousands to flee the threat of catastrophic flooding. Water flowed down the spillway and into the Feather River as storms this week and melting snowpack are expected to swell the lake behind Oroville Dam in the foothills of the Sierra Nevada, said Molly White, principal engineer with the California Department of Water Resources.