Wow, Ciderhelm, I couldn't stop staring at this map. It provokes images of mountainside settlements and foggy lakeside fisheries. Here's some rep!

I would just point out that with so many tributaries feeding into it, the southernmost river would either be wider or running with such force that it will cut a deep valley in the lower hills. If you ask me, both sound fun, but either way it shouldn't look identical to its sister river to the west.