Until a couple of decades ago you were allowed to explore the canyons on your own. A group of 11 hikers entered the lower canyon on their own on a day when it rained hard upstream. They were caught in a flash flood. Only one person survived. Some of the bodies were never found. Thus, today there are only guided tours that only take place on days when there is no threat of a flash flood.
Here is a selection of the hundreds of photos I took during the tours.