if a beaver who is part of nature builds a dam for it's purposes, it is ok, but if a man who is part of nature builds a dam for his purposes, it is not ok.
Beavers don't destroy whole valleys the size of Hetch Hetchy..... and all the other animals and plants. In 5-10 years man can destroy Yosemite Valley and turn it into a big lump of cement like LA, but a beaver can never do anything in that scale.
How much more money is needed to restore the vegetation etc....?