You will want to check, but as far I I understand, it is perfectly legal to stack rock, concrete, trees, old telephone poles and other stuff next to the bank of a river and if the river happens to take the ground below the pile of “stuff” then so be it
I know that this type of thing happens along the Loup here in central NE and as far as I know, it is leagal but you will want to check on that to be 100% sure. Good Luck!!