Last summer I went there several times. When you go be sure to go to the rockgarden just below the spillway inside the chained off area. Lots of boulders and rocks for some play time. Nothing fast just technical, just slow and fun. Most of the riding in the Reservior area that I found were mostly singletrack trails, some wide, mostly dirt, but nothing technical stuff. Be sure to ck out the area near the Nature Center downstream of the dam. Hope you find what you like.
Good Luck and look forward to an after hours report of what you found.