Some hydroelectric power plants actually have greater emissions than even coal power plants, and that's ignoring that dams are much worse for the extended local ecosystems.

The dams in the Pacific Northwest have decimated the wild salmon population. Salmon are big, meat eating fish that sustain several other animals, such as the Southern Resident orcas. This population of orcas is literally being starved by us destroying the salmon population., amongst the other challenges they face due to humans, such as poisoning and toxicity.