But the big difference is that the effects are almost completely local and not anywhere as threatening on a global scale compared to burning stuff and causing everything to heat up. This is actually a decisive point because whoever builds the dam will also lookout for the effects it causes and be more reasonable.
On top of that I think that the extinction of species is really not comparable to global warming, which will probably cause mass extinctions on a scale that has not been seen before in human history.
Actually, if mhurron is referring to what I think they are, then it can be fairly global. Think fish migration and spawning, such as with Salmon. Global Salmon population is very susceptible to being disrupted by blocking rivers. That said, I believe there are modern ways to address this problem.
Poorly built dams can damage the environment. Except you know, its 2016 and we've figured out how to preserve the environment while providing clean energy now.
Salmon, as an example, turn out to be a major component of nutrient transfer mechanisms returning fertilisers from sea environments to littoral watershed regions.
On top of that I think that the extinction of species is really not comparable to global warming, which will probably cause mass extinctions on a scale that has not been seen before in human history.