I think I read once that the main issue with expanding hydro-power is that they built pretty much all the best dams that can be built in north america by 1960. They require some pretty specific geography. This may be part of that construction cost evaluation. An ideal dam site gets a big high reservoir behind a very narrow solid choke point. The less ideal the more you have to build or the less power you can generate.
We can get some more power out of them by killing the rivers entirely, by trading off agricultural water supply vs power, etc, but we are already on the flattening out part of the curve for investment in hydro-power vs hydro-power output.
The really great thing about hydro-power in a renewable energy system is that it is more flexible than most. One can turn up and down dam generation quickly, and - at least during some seasons - its mostly up to humans exactly when we get the power from them.
