And it's not limited just to rivers. To this day, there are Shinto shrines that are dedicated directly to Kami that live in natural formations, like mountains or forests. And I mean literally: praying at those shrines actually has you praying right in front of the sacred place where the Kami is said to reside.
Trying to bring this topic back to the OP's topic, this is kind of why it doesn't really matter to me whether there are or aren't any "Gods" in terms of my overall outlook and behavior. While my religion is man-made, it is based on earlier indigenous religions, and so I make the effort to instill that mindset. "Belief in the Gods" is almost meaningless in my religion, like it often seems to be in those precious few indigenous traditions that exist in modernized societies.
That's another aspect of these indigenous religions; they're almost never proselytizing, almost never care about what its adherents "believe", and are almost never fully separated from the cultures of which they're part. (I say "almost" because, as with all things, there are exceptions.)
