You could own a river if you built a river. You don't have any claim of ownership over something simply because you claim it's yours. That's what governments do. People in this imaginary land don't acknowledge governments. You can only claim ownership over something that you put your labor into, bought with your own money, someone (who had legitimate ownership of it first) gave it to you, or you inherited it. You can't just walk up to a river or a piece of land and say you own it.