Herein you'll find information on dams, water diversions, impoundments, and hydroelectric projects and their impacts, and related links. Primarily because the word you hear on the street is an incomplete one - that of promoters rather than researchers into impacts. It makes little sense to go to a promoter for the known downsides of a project. But for a haircut, I say go to a barber, not a scissors manufacturer, or a butcher.
Real projects have real effects.
Dams in the Northern Hemisphere have even been shown by a NASA geophysicist to be speeding the earth's rate of rotation, as well as altering Earth's magnetic field. Now new research is making it quite evident that profligate damming and redirection of rivers is having some surprising and ugly global effects. It also details the sort of time and spatial scale impacts these projects are having. You can contact the lead author about the paper. That's the joint meeting of the American Society of Limnology and Oceanography and the Ecological Society of America.