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The reservoirs used for hydro power are very enviro-friendly indeed; they hugely benefit wildlife,

That's really not true. There's a huge amount of information out there on the environmental damage dams cause, like driving salmon and other species to near-extinction. It's been a cause ci©li¨bre in recent years to destroy old and unnecessary dams everywhere that is remotely practical.

"Sierra Club California does not support any proposal that provides funding for the construction of new dams or surface storage in California."

"2014 was a monumental year for dam removal projects. A record-setting 72 dams were removed across 19 states, restoring hundreds of miles of river, and last year's Patagonia-produced film DamNation raised even more awareness about the environmental impact of defunct dams that block salmon and other fish from their upriver breeding grounds." http://sierraclub.org/sierra/2015-2-march-april/green-life/3-dam-detonation-videos-prove-going-green-blowing

Providing big watering-holes for wild life in the Nevada desert is not necessarily an environmental benefit at all. Instead it can bring in more large wild life, which should not naturally be there, which then displaces and kills the native desert species. And destroying dozens or hundreds of square miles of land and habitat by flooding it is never going to be looked on as environmentally friendly. Those kinds of projects tend to be supported by hunters, rather than environmentalists.