To be honest, I think they will be fine and you shouldn't have to fence it. But you should definitely coop train them first so they always want to come back home when it starts to get dark. They might want to even sleep on the water though because its safe from predators. Gooooooood luck!!
They might not go past the little waterfall to the left.. But I would be more concerned about them going on the opposite bank. depending on how far your property extends.