When I thought about the small bumbs, I realized there is an easy fix to get rid of them on the areas you don't want them. Just "render" two images, one with the bumbs (so the rivers get placed right) and one without any rivers & bumbs. Then use photoshop, align the two pictures, place on top the one without the bumbs, set opacity to ~50%, and use eraser on the areas where rivers etc are shown, to expose them from the layer underneath. This way you will have the rivers exactly where you want them, and you can decide wether you want any bumbs or not
EDIT: And afterwards, just set the top layer opacity back to 100%