I can only imagine how troublesome that is. A while ago I did some simple 2D graphics (nothing compared to your art!) for auto-tiles myself and spent a lot of time on re-drawing the parts until it worked. It’s pretty unnerving, right? :D
But as far as I can remember the rivers turned out to look excellent in the old lets play, so I have faith in you guys, even if things turn out to be more difficult in the end.