More to the point, how many could sustain a turbine that would generate more power in their lifetimes than that required to deliver the turbine to the point of use?
Well, perhaps you google
River power plants are very common in germany, many are over 100 years old. They have regenerated their "construction costs" in energy plentifull.
Our country, and our rivers, are not very big!
You don't need a big river.
My answer was aimed to the US anyway.