I tried making a river stamp and I've not yet found a satisfying combination. The trouble is they don't look like rivers, but like a lake. The S shaped one that comes with the game looks like an oxbow lake, which is cool, but those are found near rivers.
I tried making a very large river system stamp with the idea that it might overlay the entire map. The troubles were (1) it got placed more than once, (2) it got scaled up/down, (3) and other stamps overlayed it making a river with weird connecting lakes and very weird paths through mountains.
While I'm entirely cool with using stamps for terrain and the speed of map generation they represent, but I'm wondering if rivers need to be generated to look good.