The kinds of dams involved in these projects aren't doing anything useful. They're mostly small and obsolete and/or at the end of their lifespan. They were generally built before we had a power grid.

You can't just leave them, they'll eventually fail and it costs way too much to bring them up to code and maintain them.

This is one of the very rare "everyone wins" stories. Environmentalists, cities, water districts and wildlife all come away better.