I ve generated river mesh and good news is it works. But honestly in way how it is implemented it is not usable at all. To be really useful, you need to generate it as extruded spline with UV mapping in direction of water flow (planar mapping is not usable for this case). It can be (should be) little bit wider than river bed, so no need to copy river shape precisely up to pixel. It should look like this. Each segment is usually 0-1 UV range and should have smiliar length:
Current output is this:
I am not sure, if this is possible to easily implement, on the other side it would be great feature. Doing river meshes for big terrains by hand…