Yeah, look at 1585.
I can see that first northern approach sidespan pier or truss letting go, flexing the cantilever over the north mainspan pier. It cracks over the pier, the stresses snap the other end of the mainspan and it drops straight down.
The truss system remaining over the north pier, overloaded and losing integrity, fails and leans to the west, taking that piece of the sidespan deck with it.
My gut likes it.
Question, what happened to that sidespan pier/truss? It’s accessible from land, and probably on relatively dtable ground, compared to the riverside piers.
The picture from 1,585 is viewing South.
I tend to agree with what you are saying. However, pay attention to the lock and dam on the right side of the image and on the far side of the river.