Floods in Yosemite Valley aren't uncommon during spring runoff, when the Merced River swells with melted snow and overflows into meadows. The largest, least common floods occur during winter--most recently in January 1997. See what Yosemite Valley looked like during the floods of May 1996 and January 1997. (6 minutes)
You can also download a Quicktime version of this movie: [24 MB].