In the past, environmental groups have often opposed hydroelectric projects, both in the form of large dams and smaller run-of-river projects. Now, I think the seriousness of climate change overrides past objections about destroying habitat and disrupting ecosystems in rivers. While we should definitely take cost-effective measures to reduce the harmful impacts of dams (for instance, removing trees from the area that is to be flooded), I think we need to accept more dam construction as a necessarily part of moving to a sustainable low-carbon economy.
Dams have virtues as a consistent source of energy for electrical generation. Their variable output also means they can be used to balance out production from sources like wind farms and solar facilities. With pumped hydroelectric storage, dams can also save energy at times when production exceeds demand, and do so in a way that is reasonably efficient.
Climate change impacts also support the call for more dams. The loss of glaciers and snowpack mean that natural water flows are going to become more variable. More and larger dams could help to smooth that out, as well as compensate for how our current hydroelectric infrastructure will face challenges as a result of decreased summer water flow.
Climate change is making many people re-think nuclear power, a source of electricity with a lot more black marks against it than hydroelectricity has. As such, I think we should be glad that many past attempts by environmentalists to block dams have failed, and we should strive to support their further development where suitable sites exist.