The Riverton Bridge is supposed to be open on Saturday the 4th of October in time for the Pittsburgh 250 Legacy ride. Construction will not be complete but it should be open.
However, once you get to the Duquesne side of the bridge you'll have nowhere to go. They will be building a flyover to get the trail across the road and connect to the rest of the trail but that won't be ready until at least January.
I haven't seen the layout yet to see if there will be a way to jump a barrier and get on the road from there or not. I was thinking I might go out on Sunday to see.