State water officials say they are releasing 50,000 cubic feet per second of water down the damaged concrete structure to reduce lake levels.
VIDEO: Oroville Dam emergency spillway at risk of failing
VIDEO: Oroville Dam emergency spillway at risk of failing
The releases began at 11 a.m. to reduce reservoir levels.
In the meantime workers have removed more than 1-million cubic feet of debris that clogged the river channel.
The water releases down the fragile spillway will be closely monitored.
"Towards the end of this run which will be five to six days depending on the in-flow into the reservoir, we'll ramp down to 40, and continuously evaluate the flood control spillway," Bill Croyle, Acting Director of the California Department of Water Resources said.
Friday's water release is expected to cause the Feather River to rise 10 to 15 feet by the afternoon.
The Butte County Sheriff's Office has warned residents along the river to be ready to evacuate if problems arise.
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