This also happened in Utah recently:
http://www.weather.com/news/news/utah-flash-floods
It was said that the families in Hildale had gone to the wash to watch the flood, but it was much larger than past ones and overwhelmed them before they could get away. I cannot explain why experienced canyoneers would have entered that slot canyon in Zion's with heavy rain forecast for the area. Maybe it was pressure to use their reservation or lose it?
Please be very careful around desert washes and slot canyons during the monsoon season. The cameraman could just as easily have been overtaken by that debris flow. There are some very good flash flood videos taken from a drone. That is a lot smarter.
On a lighter note, I found the NatlGeo
video to be fascinating. We don't see that kind of growth very often around here because it is so dry.
Wood only rots if it is covered with damp soil. An exposed stump can last for decades (that is why the beetle killed forests are such a fire hazard). I suppose flood borne debris is more easily buried and has better access to moisture, so it would be a great place to grow things -- until the next flood.