It happened to Joe on Gold Bar Rim. Carry one or two more candy bars than you need. Too little water is a serious problem, but too water kills more tourists in canyon country than any other event. The lack of water comes in a close second but some would argue that ignorance is the real culprit. We at Dreamride have experienced numerous flash floods while on tours in the desert.
Thunderstorms can pop up in short order even on a day that started with clear blue skies.
The power of flowing water is deceptive, so do not venture into rushing waters in an arroyo or rushing desert wash. Get to high ground and do not hang out under loose rock or sandstone arches that may crumble with moisture. If you find shelter make sure it is stable and away from water courses. Always carry rain gear even if the sky is blue and the forecast calls for the driest of days. Space blankets are a real life saver.