Yeah well, not only do OSM mistakes need to be fixed (by users? or some other system?) but the terrain isn't 100% accurate to real life, and can't really be, so there's gonna be weirdness with rivers, especially if the OSM data isn't accurate.
I'd think putting in a system to try to correct river weirdness would be rather challenging, and if individual segments were to be set as always flowing downhill, then you'd have rivers flowing in 2 different directions, changing by segment... unless.... something were implemented to delete upwards waypoints (and probably cut through the terrain), or average the point out so it's actually still there, just not upwards. I guess that's about the best way to do it, Wouldn't know what other weirdness would happen beyond that.
But actually, I haven't been around rivers very much in Outerra yet still, as I only have 1 large flat one miles away from me (from the data).