http://www.youtube.com/watch?v=rhGgnr44SVI

From the description:

"Havasupai July 2012 flash flood. My Friends and I were hiking out of the havasupai falls area on saturday evening july 14th around 4:30pm when we witnessed 2 seperate flash floods. I got the second one on video. we were stranded on a hill side of this canyon for 4 hours until the waters died down enough for us to continue our hike out. This usually 3 hour hike took us 10 hours!"

In the video one of the people says, "It hasn't even been raining!"