[Nashe]

The ironic thing about flowing waters is that once you stop interfering with them, you have to keep interfering with them because they'll do "what they want".

Just because the river looked a certain way in 1960, doesn't mean removing the causeway will make it return to exactly that... channels move and rivers carve and recarve themselves. Nothing to say it wouldn't naturally reroute itself into a place it wasn't before it was dammed up.

Hence the need for riprap and stones...