'Low-head nondestructive hydroelectric power' isn't enough energy to provide a significant amount of power unless you're looking at powering a tiny village.
True, at the current scale. I think what all this points to is that we need better energy storage technology. Chemical storage is currently the most efficient (i.e. petroleum fuel), but in the long run, what's needed is better battery technology - something equivalent in energy density and price to petroleum. It's a tall order, but at least there's some progress.
The downside, of course, is that this isn't likely to happen soon enough to provide enough cheap power to run a global "just-in-time" supply chain. Transportation energy is the major problem, or at least it is if you don't want a lot of people to starve.