Fordyce will have 5 days of releases in mid August, it is a great run. One of my favorites. You will also be able to catch some releases on the NF Feather, Tobin section. I believe there is also a release on the Pit in September as well. August, Sept, and Oct are probably 3 of the slowest months as far as whitewater goes. SF American will always have some class 3 action, and Cherry should run as well as already stated.
Bottom line...boating options very limited that time of year, you might want to switch to canyoneering and go see some AMAZING canyons at low water. Also, are you set on Yuba City? Chico is a great town with a significantly better scene 45 minutes north. Sacramento is bigger but alright too. Alright that is enough from me.