When rains fall 20, 30, even 40 miles from certain areas of the southwest, these kinds of flash floods full of debris that can be really, really dangerous!
This video shows the result of some flash flooding. Here's what the man who filmed this (who sort of tracks these kind of floods) had to say about it:
"August 30th brought with it great atmospheric conditions for some large long lasting storms over Southern Utah. A massive thunderstorm stalled up this basin and dumped billions of gallons of water in around and hour and a half. This impressive flash flood / debris flow is what resulted from all of the rain. "