As others have mentioned, the spillway is around a hill (that's, I believe solid rock) from the dam itself. The "emergency spillway" is actually a large flat area that serves normally as the boat launch ramp parking area. There is a concrete berm, looks a lot like a large curb that goes from the dam "end" to the hillside north of the dam. I don't know how big that "curb" is under the ground. If the water ever gets that high, it's job is to keep the water from eroding the hillside. The water actually has to go a short distance before it heads downhill, first crossing the road to the boat launch ramp. The big problem besides not being able to control the outflow (it will be as much as is coming into the lake) is that the water will carry away the fauna and then the soil, which will go downstream to the fish barrier dam and the forebay channel, where it will likely make a mess of things.
Back in '97 they were releasing something like 100,000 Cubic feet, the river was nearing the top of the Oroville levee and we were within 1' of the water going over the emergency spillway, which would have made the river top the levee and flood downtown Oroville, and LOTS of communities south of us, including Marysville and Yuba City. If it goes over right now, the river is high, but only about 1/2 way up the levee, so the town would be ok for a while.
HOWEVER: if the storms come in next week anything like what they did this week, all bets are off.
No, I'm not a meteorologist, nor a flood control person, not even an engineer; just someone who has lived here many years and have observed a lot of things.
One of the problems we faced back in '97 was soft spots in the levee here that were seeping. The City has some pumps designed to pump storm drain water over the levee if needed. The pumps are the ones originally used to pump out the caissons used to build the Golden Gate Bridge piers. However, I don't how long ago the drain shut off valves at the levee have been tested--if at all!