The San Francisco Public Utilities Commission, which operates Hetch Hetchy reservoir and O'Shaughnessy Dam, hosted us for a beautiful afternoon.

It's located in a glacial valley in the northern end of Yosemite National Park. The damming and flooding of the valley in the 1920s was, and still is, a major environmentalist controversy. But the system provides very reliable, very clean water supplies for the city of San Francisco and many other Bay Area communities. 

We had lunch at Cottage 1, formerly known as O'Shaughnessy's Chalet.  Perched above the water, it was where the chief engineer of the original project lived during the construction years.  It's a beautifully restored wood and stone building with great vistas of the facilities and the wilderness beyond. There's a sleeping porch which reminds me of our cabin. 

No swimming or recreational boating is allowed in the reservoir. San Francisco is able to deliver this water to its customers with no additional water treatment because it is so pure. Strict rules about human contact with the water and extreme measures are taken to keep runoff and other pollution out of the waters.

There are hiking trails throughout the wilderness area (it is within Yosemite national park). Visitors are allowed on and around the dam. But the park gets only a fraction of the visitors of the Yosemite Valley to the south. I got the impression they'd like to keep it that way.