TWENTY NUCLEAR POWER PLANTS - Fifty Percent of the nation's energy needs.

[jeff47]

43. We can already "turn off" Niagra Falls.

The plants near Niagra Falls can already suck in so much water that the falls would stop. To maintain the tourism, they leave just enough water to keep the Falls looking good. And that isn't an awful lot of power lost.

Small rivers don't have enough energy. Sure, you can run a water wheel, but that's not a lot of power. Back in the day, the alternative to a water wheel was a horse or two on a treadmill. That should give you an idea of the amount of energy that could be recovered.

The low head dams you talk about simply don't have enough energy - low head means low pressure, so they'd have to go high volume to produce any significant energy. They just don't have the volume to sustain that, even ignoring their role as drinking water supplies.