Sad day for sure.
We have done a pretty fair amount of canyoneering and are quite aware of the need to watch the weather in the immediate vicinity and upstream for a day or two previous to the excursion.
However you need to be cognizant of where you park or camp regardless of if it is an arroyo, gully, ditch or not. We stayed in a campground this past weekend just outside Buena Vista CO. There was a very serious storm Sat evening about 7pm. While there is a couple of washes in the campground and the roads that went through them got washed out. There was a surprising amount of damage in areas you would have thought would have been ok at first glance. While no one was hurt and no rvs floated away there were certainly a few that weren't going to move away very easily. I saw two travel trailers that their front tongue jack had been washed out so far that the front of the A frame was resting on the ground. A chevy truck with the front bumper almost resting on the ground maybe 2 inches of clearance. Another trailer that the entry steps were resting on the ground. We were in a pull through site and had to back out of it as the road in the front was severely washed out on the far side to the point that there was no way I was going to be able to make the wide turn without dropping off into the washed out portion. The road behind us was fine where we were at but up about 150 feet there was a 2 foot deep gully that had been washed out of it. For a while there were areas where you could not exit until they filled in the washed out areas. The campground owner was out immediately with a tractor filling in those areas so people could at least get out of the campground. He was out until about midnight then at it again first thing in the morning.