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3.2. DETERMINATION OF CRITICAL RISK On Sunday morning, February 12, after more than 24 hours of water flowing over the Emergency Spillway, work in the EOC was quiet. Water continued to spill over the Emergency Spillway at a rate that posed no direct threat to the downstream communities. Experts remained at the EOC and at the top of the dam communicating with each other by traveling back and forth or through text messaging with spotty cell reception. Later that morning, staff monitoring the situation at the dam noticed deep erosion rills cutting back toward the emergency spillway at a rate and size much larger than anticipated. At approximately 2:00 pm, the erosion began to concern engineering geologists about the safety of the Emergency Spillway. If the erosion continued and reached the base of the Emergency Spillway, one of the concrete monoliths could be undermined and collapse. Engineers were concerned that a failure of one monolith could result in a domino effect, causing successive collapse of the adjacent monoliths. Engineering geologists estimated that the erosion would reach the base of the Emergency Spillway in two hours. Those monitoring the situation at the top of the dam discussed if the circumstances constituted an emergency declaration, and who had the power to make that decision. Around the same time, at the Butte County Sheriff’s Office, the Sheriff believed the entire incident was nearly complete and decided to head home for the rest of the day. Being unaware of the developing situation at the Emergency Spillway, the Sheriff stopped by the EOC to thank the people for their hard work. Meanwhile, staff monitoring the Emergency Spillway brought photographs of the advancing erosion to the EOC and showed them to the Incident Commander. The Incident Commander received the photos at approximately 3:00 pm and received a briefing on the developing situation. He asked, “Does the Sheriff know about this?”, unaware that the Sheriff had just arrived at the EOC and was standing behind him. Seeing the elevated concern, the Sheriff asked what would happen if the Emergency Spillway were to fail. DWR staff informed him that failure of the Emergency Spillway could result in a 9-10 m (30 ft) wave of water flowing from the breached spillway until the top 9-10 m (30 ft) of Lake Oroville had drained. 3.3. DECISION TO EVACUATE At 3:30 pm, several dam managers, engineering geologists, and the Butte County Sheriff were discussing how to address the situation developing at the Emergency Spillway. The dam managers decided to allow a flow of 2,830 m3/s (100,000 ft3/s) through the FCO Spillway chute to decrease the water level in the reservoir below the Emergency Spillway crest as quickly as possible. All 30