Sure, most people know not to shit in the river. That's simple. But we do use the lake and river for other things that could contaminate it, like bathing and washing clothes. So the rule is that we only do that stuff below the waterfall, and we only get drinking water above the waterfall. That way they never mix.
I know someone's showed you some stuff already, but just in case there's anything that got left out, there's a post for new arrivals here