From the book “Off the Wall: Death in Yosemite”
“Entering any significant stream which flows over a waterfall-regardless of any sense of security inherent in what might seem like a ‘safe’ distance between one’s entry point and the fall-is a proven killer... The power of moving water, even knee deep, has carried hundreds of people to their doom in the Sierra Nevada... The shorelines of these streams are of finely polished rock which is smooth and very slippery even when dry. When wet the rock becomes extremely slick. When wet and also with algae growing on it the water’s edge becomes unbelievably tricky, worse than a melting sheet of ice. Hence the warning signs and safety railings so far from the actual stream... It is probably safer to jump into the ocean with a great white shark than to tempt fate above a high waterfall in Yosemite.”
Just a brief rain shower over the desert floor can create a wall of water only a few inches deep to race down the dry stream beds at higher speeds than a person can stand up in. It would be like trying to stand up while someone is blasting your ankles with a fire hose connected to a hydrant on full blast.
You lose your footing, and it's curtains for you. You just get carried a few hundred yards along with the water banging your skull on exposed rocks while nature tries to drown you at the same time. Even if you survive it initially, you're likely to be gravely injured out in the middle of the desert in need of emergency medical care that you just won't get.
Until I saw my first flash flood, I had a hard time understanding these warning about how so little water in an arid desert can accumulate fast enough to cause a flash flood. It sure as hell can, and it's frightening to witness. People have died in just a few inches of fast moving water that's had time to build up speed.