How big would the river in the city of ember be to power everything in the city of ember?

Its not really about the size, its more about the force its providing. The City of Ember seemed to run on some mysterious machine that created electricity from water. In reality, its simply a hydroelectric power station. The river must have been about 3,000 gallons of water per minute. The book doesnt specify how wide the river was or anything like that but it does give a map of Ember. If im guessing correctly that's about how big the river has to be.