Is it legal for stock (horses) to feed while standing in the river and eat folage from banks and islands?

I have been observing horses feeding _IN the portneuf river a lot in the last two weeks. They walk right up and down in the river and rip up moss in the shallow parts and grass and cover from the banks and small islands. I think they do this because the field they are locked in is pretty much eaten down and there is little feed. I have called the owner and he has not ever called me back.