The one thing I don't understand is how the original posts were driven into the river bed. You have to deal with the current, deal with the fact that wood floats, deal with a long post, have to have a means of driving it down far enough, etc. I imagine that nowadays they'd use a pile driver on a huge barge with engines that can automatically compensate for the current, but I can't imagine what the equivalent was back…
You could pre-soak the wood so it's less buoyant, and hold a barge in place with ropes to the riverbanks. I'm not saying that's how they did it, but that's what I would try if I were them. You'd also want to pick the right season when the water flow was the least. You might even be able to dredge a channel so that the water is directed away from the position of the supports. I doubt they had a means of controlling the river flow back then, so I doubt they could have redirected the river into a temporary side channel, but that could also help if possible.