If a hand quern needs nothing (obviously because its done by hand) and a windmill quern needs a windmill already, then the standard 'quern' you're referring to would need a watermill. Rather than requiring an actual watermill, requiring a river being enough is actually rather generously allowing imo, given that the windmill quern still requires one. What I MIGHT advocate would be to keep the hand mill from obsoleting until they all do, though its hard to see it managing the needs of a whole city. I sometimes think we need a MAXIMUM city size prereq tag to make certain building designs work properly. Hand querns wouldn't scale well to the needs of a much larger community. Though they would be better than nothing.