Hi all, I was one of the 15, so I may be able to comment on some of the questions brought up in the post.
First of all, let me say this. I've hiked the canyon over a dozen times, and the friend I had with me has hiked more than that. Each time I've gone, I've studied the flood patterns for the season on the USGS website. We know it floods in the summer, but the danger is pretty easy to avoid. Be off the creek by early afternoon, don't camp on the beach, and don't explore the side canyons but in the morning. Also, keep, a close eye on weather to the east, since it's rain over there that feeds the canyon. On the day we went, rain chance was 20% to the east.
Anyway, we had followed our own guidelines, but around 3 we experienced heavy rain and saw a huge muddy waterfall falling from the top ridge of one of the side canyons, indicating that a wash on the flats above had overtoped its banks. At that point we climbed the cliffs, and we were never in the way of the water. However, even though the water receded at times, it always rose again ultimately we evacuated our positions five times. We spent the night on the cliffs watching forb rockslide above us, concerned about the trees below falling, as they cracked and snapped under the pressure of the water.
There were five of us, my friend, me, and three children. We had permits for for 2 nights, 3 days, but during the night my friend that I decided we were going to get the hell out of there in the morning. When dawn broke, we saw that the river was a huge brown torrent, and there was deep mud on either side where the water had receded. We realize we probably weren't going to be able to hike out for days. We had provisions, and we had back up water treatment tablets since the water was unfilterable, but we were concerned about the kids. No phone service, so we figured we would not see search and rescue until our third night. Escape by overland route was not feasible because of the limited and unpredictable water supplies up above, never mind the navigational challenges..
As it turns out, people we had met in the parking lot camped very close to the entrance. They went on a day hike, and returned to find it all of their supplies, tens, everything else had been washed away by the initial flood, which wasn't that big. They were able to hike to a high spot and get a phone signal. They're the ones who called SAR. As they were evacuated, they mentioned that we were there too, so we got a ride out as well.
Frankly, I was embarrassed. I feel like I should have been able to I get out on my own at some point but when we were dropped off by the helicopter, the ranger told us we would not have been able to hike out for days because of the quicksand along the banks.
So anyway, an interesting experience. It taught the kids a lot about proper shelter, alternative shelters, backup water filtration systems or treatment systems, creative sheltering, blah blah blah.