There are no natural barriers to stop smallies from entering the middle provo, but you won't find them in large numbers either. The middle provo is a tailwater fishery below a dam which keeps the temperature from warming to the optimal level that smallies prefer in the summer. Deer Creek warms quickly in the shallows in the spring and summer and provides more forage for smallies than the river does, so they have no driving reason to leave the reservoir in favor of the river. I helped with an electroshocking last fall on the lower provo and we shocked up a healthy smallie that had apparently taken a ride down the DC spillway (we were sampling a strech just below the dam). The smallie was healthy but I'm sure it wished it could get back into the lake somehow. From what I gather, smallies prefer rivers that are slower moving and warmer, like those found in the midwest.