Where does this spillway take the excess water to? Somewhere where it's not a problem, or just out into the forest or something?
It just dumps it back into the Feather River down stream. The thinking was, if the emergency spillway is in use, the dam is in danger. The Feather river empties into to the Sacramento River and from there to San Francisco Bay.
The Oroville Dame went in back in the late '60s. The Feather River hasn't flooded in a big way since Christmas of '64. I opened my Christmas presents in a tent up in the Sutter Buttes that year. My dad wasn't taking any chances and we were not alone in the hills.