Except that in many cases there's people living downstream doing agriculture using that water for irrigation. There's just this tiny dispute about that in the nile delta between Egypt and Ethiopia

https://en.wikipedia.org/wiki/Grand_Ethiopian_Renaissance_Da...

Except for very short term peaks (less than 15 minutes-ish) it doesn't make any sense at all to use hydro to charge batteries. You've got a dam, you might as well let water through later than incur the losses of a round trip to batteries and back to the grid.