> People are getting pumped storage and river dams mixed up. It seems mschuster91 also mixed them up.
The environmental impact is bad for both.
River dams break fish crossings, the dammed up area gets flooded and wipes out nature as well as archeological artifacts and the dams are at constant risk of damage - especially in a war, see Ukraine for multiple examples, but also due to maintenance neglect, negligence during construction and natural disasters like earthquakes. In the worst cases such as China's Three Gorges dam, millions of people were displaced as well [1].
Pumped storage can come in two variants, either as an associate to ordinary river dams (so they inherit their issues), or as greenfield construction, where they have the same impact on the flooded are, with the additional impact of countless animals dying during pump and empty cycles.
A link to the three gorges dam wikipedia page says exactly nothing about the potential environmental impact of pumped storage but it does confirm that you are confusing the two technologies.
... which is why I linked to the Three Gorges Dam in the paragraph where I described the issues with dammed storage, and made an entirely separate paragraph describing the issues of pumped storage.
Yes, the capex and lead times on one of these things can be huge, but it's comes out ahead of nuclear power on those fronts.