Hetch Hetchy used to be a place more beautiful than Yosemite. Now it's just a reservoir.
The most famous example in the world is China's Three Gorges Dam, which turned one of the most stunning and bio-diverse place on Earth into a giant lake with a dam so large that if it bursts, it will flood half of China.
I am not saying not to build them, but it's not black and white at all.