There is a finite amount of hydro in the world. They will run out of viable dam locations pretty quickly at this rate.
The ideal big dam is Hoover Dam. Large, deep canyon in a desert. Narrow, deep canyon dam site. Hard rock geology. No major towns or agricultural areas in the area to be flooded above the dam. That's the best case.
Most later dams are at worse sites.
Cadillac Desert is a great history of American dam building and the Bureau of Reclamation