If you built on a floodplain then there may be an argument that you are trespassing on land that rightly belongs to the river. The river’s wanderings built that land and I doubt you or anyone who previously owned that property ever negotiated with the river for it.

And I am saying this as someone in the middle of moving to New Orleans, a place humanity has been fighting with a river over for three centuries now. Someday the Mississippi will assert its claim forcefully. And New Orleanians will probably just shrug and either build up houses that sit next to canals instead of streets, or learn to grow gills. New Venice or New Atlantis.