It’s a gentle one-mile stroll on a paved path to Mirror Lake, at the head of the valley made by Tenaya Creek behind Half Dome (which stands to the right of this photo, you are looking east and north).
The Park Service is gradually letting Mirror Lake return to its natural state, which means that it is at best a seasonal lake. In the summer time you can walk across the sandy lake bottom. Essentially, Mirror Lake was artificially constructed as part of the start of the tourist trade in Yosemite—replete with elegant Edwardian structures now gone, and ice skating in winter months. Left to its own devices, the “lake” silts up over time.
Here are some more photographs of Mirror Lake.
This photo was double-processed from the RAW original, once for the sky and once for the reflections in the water. The two versions were then put together using a layer mask and a gradient. This is a subject worthy of longer comment, but I’ll say it now briefly: the resulting image partially created in Photoshop is closer to what I saw when I was there than it would have been if I had left the photo as a single exposure.