John Denker makes the common error of considering hydroelectric
power to be a renewable resource. It is renewable only in principal.
To obtain hydroelectric power economically one must build dams at
special locations on rivers. Once these locations are gone no more
hydroelectric power can be generated. Pondage behind dams silts up
at a rate which typically has a horizon in time that is nearer than
that for coal! Once all the good dam sites have silted up...