Yes, a concrete bottom. All the dirt you see them scooping out is what has washed in over the years.
CB, thanks for the pictures but I still can't see the guide wheel. Based on the design of the guide rail my guess is that the guide wheel would look a lot like what you see under the monorail. That being a wheel or wheels on top in the channel on the top side of the rail to set the elevation of the guide arm with wheels on the outside of the guide rail that guide the boat. The "guide wheel assembly" would then be on the end of a hinged arm connecting it to the boat which would allow for variations in both water level the draft of the boat depending on passenger, fuel, and boiler water load.
Not that it matters, but my engineer's curiosity has been piqued on just what it looks like.