Mike, you could make a lift out section, and have any type of bridge that you would like on it. The riverbed could be like a tray underneath the bridge, and the whole 30" length could lift out and be placed in a receiving area, if you have the space, or put back in immediately. Of course you would want to make it as light in weight as possible. Connections can be a pain, but there are forum members who have done some ingenious things regarding getting the juice from the bench work to the lift out sections, and back to the benchwork.
And then of course, there is the hinged versions, and counter weighted assemblies, and there are forum members here that have done some remarkable things for those. I suggest doing a search up above for hinged aisleway bridges (or the like), and you should find some topics and photos of what others have done, they are ingenious.