The Butte County Sheriff's Office says the emergency spillway could fail within an hour unleashing uncontrolled flood waters from Lake Oroville.
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The department says people in downstream areas need to leave the area immediately.
It says residents of Oroville, a town of 16,000 people, should head north toward Chico and that other cities should follow orders from their local law enforcement agencies.
This evening's order to evacuate comes just hours after state officials assured the public that everything was safe. "At the moment we don't anticipate a flood risk," U.S. Department of Water Resources Director Bill Croyle said.
The Yuba County Office of Emergency Services says people should take routes to the east, south, or west and avoid traveling north toward Oroville.
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The California Department of Water Resources says it is releasing as much as 100,000 cubic feet per second from the main, heavily damaged spillway to try to drain the lake.
An evacuation center has been set up at 2357 Fair Street in Chico, northwest of Oroville.
Water began flowing over the emergency spillway at the dam on Saturday after for the first time in its nearly 50-year history after heavy rainfall.
ABC7 News contributed to this story.