>
> > Besides, in a few hundred years it looks like
> the American Falls will fall apart completely --
> there's nothing that can prevent the natural
> decay caused by the tremendously strong
> torrent -- and eventually they'll become a set of
> nice, craggly rapids.

Looking further into the future, Niagara Falls will slowly receed up the river until it drains much of Lake Erie. It might be possible to slow it slightly, but it will happen. Waterfalls everywhere tend to do that. The gorge below a waterfall is a product of that recession. Waterfalls usually have a layer of hard, resistant rock at the brink with softer rocks below. Often a plunge basin develops below the falls, sometimes undercutting the brink.
Recently, news reports on TV indicate that reporters don't know the difference between a dam and a waterfall.
Howard