Hey, thanks for the feedback.
Yeah, the provinces not straddling rivers can look a bit weird in some cases but I think it's not too bad of a price.
A bunch of the oddities around isolated mountains, and mountains getting grouped with rivers have been sorted out in 3.1.2. Similarly the known tribes window has been redone, and shouldn't look so odd before you meet anyone.
The idea of controlling a river if you have the majority of the surrounding terrain is interesting, I'll look into it.