Well, this is about pumped storage hydroelectric plants, which are not dams. These plants aren’t generally built on rivers because they require much larger water reservoirs.
As with any land development, they require land, of course. But all power grid elements do.
They don't turn Alpinesque ecologies into tidal pools if you don't build them in Alpinesque ecologies.
The truth is that we are developing a lot of land every year for different purposes, from industrial to residential. The question is whether we will develop it for everyone's benefit, or for some yet another tax exempt monopoly's warehouse.
We could even build these plants in place of abandoned industrially exploited land, such as quarries, which conveniently already form quite large water reservoirs. We could reclaim a completely dead parcel of land and once we stop using the reservoir, it could blossom into a set of new natural ecologies.
Don't take the worst case scenario and present it as the only one when there is a world of possibilities.
As with any land development, they require land, of course. But all power grid elements do.