Plumer has a good rundown of the complexity of dam building around the world. The world’s rapidly growing demand for energy means that every way we can turn the natural world into power is going to be considered. Given how many of those methods of energy production also transform the climate in horrible ways, hydropower seems smart. But hydropower also has its own major problems. It forces sometimes hundreds of thousands to move from their homes. It drastically transforms aquatic ecosystems, imperiling fish and other species. It may well create unintended consequences that undermine its clean energy reputation. Building dams also reflects power differentials in society as a whole. Thus you have a nation like Chile seeking to dam rivers over the desires of the people who occupy the land, i.e., the Mapuche. So dam building becomes another round in the 500+ year history of colonialism and racism against indigenous peoples in the Americas.
And mostly, we don’t really know what we are doing when we build dams. In the U.S., this led to a lot of bad dams that provided little power but had significant negative consequences for people and ecosystems. That’s almost certainly happening around the world today.
As with all energy questions, there are no easy answers. But hydroelectric is not a panacea either and should be expanded with the kind of caution one would want to see with gigantic projects that will reshape entire parts of the world. Unfortunately, that rarely happens.