Hydroelectric has the problem that:
1) You're making tens of square kilometers of land uninhabitable by flooding.
2) Such flooding promotes the formation of anaerobic algae which generate thousands of tons of C02
3) Dams collapse too.
Wind cannot provide a stable energy source. The advances required in battery technology and so-called "smart grids" are still too far off. You can't power a steel mill on wind power.
Solar is extremely dependent on location and, once again, requires a power infrastructure overhaul.
Geothermal and tidal are still untested.
All of the mentioned experimental technologies will require at the very least 20 years to mature and will bring their own problems as well (geothermal has the tiny little problem that it causes earthquakes).
Most people don't understand that undertaking an energy infrastructure update is something that takes decades and has to be planned thinking about its effects 30 and 40 years from now. And that also means that while you wait for technologies to mature you have to use stopgaps. Right now there is no alternative for nuclear and there likely won't be for 20 years unless nations are willing to invest hundreds of billions in research and upgrades.
Fortunately people tend to like waterfalls for their aesthetic value so much that they would never allow industrial processing or power plants near them. I would like to see calculations on how much theoretical power could be obtained from all sources if there were no limits, i.e. every river dammed, every waterfall harnessed, every atom of uranium split, every atom of silicon and necessary rare earths turned into solar panels, every acre of desert used in molten salt solar collectors, etc.
Doesn't seem to be a problem and Niagara (2.5GW) or Snoqualmie (54MW). Both are still popular with tourists (obviously, Snoqualmie is not on Niagara's scale in either power or tourism, but I include it because I imagine there are a lot of Snoqualmie-sized waterfalls yet untapped, and probably no Niagara-sized).
I strongly doubt that you can generate more than 0.1% of our power requirements from that source. The world uses 17 trillion kilowatt hours per year, that's 1900 Gigawatts.
