From a rain that would be nothing special most places.
It’s remarkable. Even on large mesa tops flooding can be severe. There is no “soil” to begin with as we understand it. Just sand, and sandstone eroded into weird spires and alcoves, bone dry most of the time. But when it does rain, there is little absorption at all. It builds up into a raging torrent with astonishing speed.
In the slot canyons it is particularly dangerous because there might not be any rain or even clouds in the immediate area.