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.