On the second day of my visit to Yosemite NP I went hiking in Hetch Hetchy Valley.
In 1932 a dam was built on the Tuolumne River flowing through the valley, creating a reservoir that until today supplies water for the San Francisco Bay Area. Already then building of the dam was highly controversial, and one of its biggest opponents was John Muir. Also today the Sierra Club continues fight to remove the dam and restore Hetch Hetchy Valley. As much as I would like to see it happen, I doubt that it will. First, an alternative water supply source has to be offered to the Bay Area inhabitants and so far I've not heard about any. Second, after hundred years under water it probably will take at least twice as much time for the valley to recover. (This, of course, is not an argument against doing it. I'm just saying that I'm not likely to see restored Hetch Hetchy Valley within my lifetime).
In Hetch Hetchy I went for a 5.4 mile hike to Wapama Falls. It is an easy hike, with not much elevation change (800ft), that takes around 2.5 hours to complete. The trail goes parallel to the shore of the reservoir, offering great views throughout, and culminates at spectacular Wapama Falls. At the base of the falls there are several wooden footbridges that can be dangerous to cross when the water is high. I was a bit worried that that part of the trail might be closed, but luckily it wasn't and I managed to get close to the falls and take a few cool shots of them.
I would strongly recommend making this small detour and visiting Hetch Hetchy during your next visit to Yosemite. It is equally beautiful as Yosemite Valley, but less crowded and more serene.
Wapama Falls, Hetch Hetchy Reservoir and a piece of O'Shaughnessy Dam: