Can you share the height map you created with the rivers? Currently for river generation I am basically picking a coordinate at random around the centre and go downwards until it gets to a sea or a lake.

I should probably edit the post and mention this but didn't want to make it too long.

Love the fact your project is for the Rift and Hydra. I've set up a VR station with the Hydra at the office, maybe I can give it a go next week :)