This question is for a fantasy story loosely based on medieval west Europe.
The story is centered around a small remote village in a mountain that sits by a river. The village uses its river for drinking, bathing in, running its mill(s), etc. It would probably dump some wastes in it as well, but this got me concerned: what about those who would use the water further downstream?
I do have a settlement that sits on a flatland below the village mentioned above. For starters, is it realistic that two settlements would share a river at all, since the settlement downstream may be concerned about what might flow down from upstream? Or would some medieval wastes not matter much as long as the two settlements are far enough apart? Would there be agreements between the two settlements on what can be thrown in the river and what can't?
Also for that matter, what about bathing? We know that, contrary to popular belief, medieval people did try to keep themselves clean. Would people downstream complain about body residues and soapy water?
It'd be great if an answer can address how things happened back then, and also in present day where contamination is a bigger concern and things like spread of diseases by water is better known.
Also note: "the village upstream doesn't need to care if no one has a strong enough political power to have a say about it" is not an answer I can accept, even if that was true for medieval Europe.