Ok, thanks for that. But I'm still left wondering, if the river permanently moved away from your property a good distance, let's say 100ft, and let's say you have (had) ~200ft of waterfront, there is now about half an acre of uninhabited land between your property line and the river's new location. Who owns it? Is it forbidden/protected land in perpetuity because it was once a navigable waterway?We do love our home and the riverside location. If I understand your question, the answer is that I have a property line that extends slightly into the river. That doesn't shift. If the river rose, I could build (a larger) deck to reclaim usable property there. If the river left our bank dry, I would just have dry land. The property doesn't respect the river's edge, per se. It is a line that stays fixed relative to the survey of the dry land.
On the other hand, the river does have implications. In most states I know about, if there is a navigable river, it is not-quite-owned by the property owner. It is a state thing, and it means anyone can use it for transit, so long as they are in the water and not on the land. If the river moved away, they couldn't walk on the dry land because it was once navigable.
There are different regulations and laws in different states, so you have to look to that. But I think it is largely similar.