I'm sure you could do it with a closed system, e.g. pump water up a cliff, and when you need power, bring it back down. I'm guessing using dams on existing rive

I'm sure you could do it with a closed system, e.g. pump water up a cliff, and when you need power, bring it back down. I'm guessing using dams on existing rivers is just more convenient than building a closed system from scratch since it can double dip on hydro infrastructure.