Oh good grief, you're talking about hundreds of billions of gallons, constantly purified through evaporation and rain...Man has so little effect on the environment that it's barely worth talking about. We have been using the same water for millions of years and it works just fine.
Depends on the size of the river and how many towns are on it.
And who said anything about "the environment" I'm talking about the folks in towns further down the same river. I drink nice pure water up here in the woods. City people drink bottled water....I wouldn't drink their tap water either.
Do you?