they can hold back some water at the diversion DAM just downstream of the spillway....this is to even the flow out down stream and to release water when the spillway and power plant both have ZERO outflow so the feather river wont dry up
the diversion DAM and the smaller after bay branch is the last part “Oroville complex” ....if the diversion DAM is low it can hold back some of the outflows from just upstream and release them later if or when the outflows from the spillway and power plant slow to stop
so total outflows from the spillway and power plan don’t always equal the releases from the diversion DAM and after bay (aka Oroville complex)
If you look on google Earth you can see this complex on the extreme north side of the city of Oroville
OK, thanks.