We don't exactly have a river handy, so I could make a watermill farm with 5 deep (or 6 deep, to account for ice) wells that would house watermills. Just have to hook them up to a voltage amplifier to the main part of the grid.
Of course though, if we were by an ocean, we could very well make a massive watermill farm or maybe it would be better called a tidal farm.
Edit: Or better yet, an array of them in a large tank. Perhaps something like this prototype, except imagine it in an array of many in a much, much larger pool. Maybe I'll put them under the hill behind my lab to avoid the issue of ice forming.