It's not just about release of CO2 in construction. Dams also cause GHG emissions by creating anoxic reservoirs. In an anoxic environment, organic matter decays into methane rather than CO2. Methane is a far more potent greenhouse gas than CO2. In some cases, calculations have been done showing more GHG emissions from hydro than from coal.

While hydro may be "renewable", it isn't cleantech by any stretch. It's not great from a global warming perspective, it ruins river and riparian ecology, destroys canyons and chasms, and leads to increased water evaporation.

Yes, some people will complain about anything, such as the proportionally tiny number of birds killed by wind turbines or the shading of a small patch of some massive desert with solar. But in the case of hydro, these really are significant problems.