That's some great worldbuilding, and the maps are looking good so far
I have a quick comment on your background: you say that certain people are born with abilities, but that inheritance is not genetic. You'll need some sort of magical mechanism if that's the case, because without magic anything that you are born with is in fact genetic.
As far as your rivers go, I don't think they look huge, and they look good to my eye ... you just have one spot I would tweak. There's a loop (see my cropped version of your pic) where two spots are so close that actual water probably would just cut a path straight through instead of going the long way around.
Edit: Rereading that, my second paragraph sounded really harsh, but it's so early in the morning I'm not sure how to reword it. Please know I didn't mean it harshly