For the first time since Oroville Dam was completed in 1968, water from its storm-swollen reservoir overtopped the emergency spillway Saturday, sending sheets of water down a forested hillside and adding to the murk and debris churning in the Feather River below. State officials said they did not expect the flows to cause flooding in Oroville or other communities downstream, but the emergency releases underscored the perilous situation confronting the operators of California’s second-largest reservoir for the rest of this extraordinarily rainy winter.