The reservoirs we're using as an example are in the hundreds of acres of surface area. That's not "huge parts of the Sierras". If you look at Bath County Pump

The reservoirs we're using as an example are in the hundreds of acres of surface area. That's not "huge parts of the Sierras". If you look at Bath County Pumped Storage Station on Google maps, the bigger reservoir is a skinny lake about a mile long. Perhaps it makes sense to use much larger reservoirs, but even then I have trouble equating the creation of a few artificial lakes with severe environmental destruction. Really, it all depends on where they put it and whether they do reasonable due diligence to not impact existing habitat too severely. The environmental destruction that results from burning petroleum for ground transportation is far worse.