This is a no brainer. It's one of Earth's most abundant resources. Water. Hydroelectric power was not at all a far cry from innovation when it was once simply a paddle wheel in a fast flowing stream harnessed for grinding grain. When power was connected with the ability to produce electricity there was literally 'light.'
The delicate aspect of hydroelectric is the very aspect that makes it a great form of energy. The water. If we wait too long, the water on this planet will not be as available to us, although that seems a fantasy now, with noted and considerable flooding hardship from flooding.
Rain is abundant in some areas of the world and therefore there is little to worry about with this energy source, but, with continued heating rains may stop at some time and the headwaters to great rivers will begin to dwindle.
