Mirror Lake is one of the most popular locations for viewing scenery in the Yosemite Valley. But it's not really a lake -- at least not any more. In the spring, when water levels in the Yosemite Valley run very high, Tenaya Creek is a torrent and there is indeed a Mirror Lake. As spring fades to summer, the rivers drop and the lake slowly shrinks; by the end of the summer, most of the water is gone, as you see here.

Mirror Lake is actually slowly filling in and turning into a meadow; this is a natural process that was delayed for many years due to NPS dredging in an effort to stop the lake from filling in. (In fact, geologists say that all of Yosemite Valley was once a lake, which was gradually filled in over many hundreds of years by river sediments.)