Arizona, monsoon season, which goes from July through September, can bring about flash floods, particularly in the mountains and deserts. Steep slopes and hard soil keep the water moving fast through normally dry washes; fast enough to do real damage to anything in its path. A wall of water up to ten feet high and full of debris ranging from boulders to logs to cars can charge down a wash at up to 20 mph.
The National Weather Service declares that flash floods “have the power to move boulders, tear out trees, destroy buildings, and obliterate bridges. Walls of water can reach heights of 10 to 20 feet or more, and generally carry a huge amount of debris with them. Flash floods are the number one thunderstorm-related killer in the United States.”
Don’t be in that wash when this happens – and it happens – that’s how the wash formed in the first place. Don’t be near the wash when this happens.
It doesn’t have to be raining for a flash flood to roar by. It just has to be raining somewhere upstream or uphill. It doesn’t have to rain a whole lot; in some areas, less than an inch will suffice.
What to do
The single best thing to do when facing a flash flood is to get to high ground, and stay there. Typically, an established campsite has been built on the highest or best drained spot available, but if there’s a higher spot, go there.
If caught in a wash or other drainage filling with water – climb out as fast as possible. Do not try to outrun the flood. If hiking in a dry wash, but rain is a possibility, keep an eye out for easy exit points.
What not to do
Flash floods, by the numbers, are far more hazardous to motorists that to those who tough it out in a campsite. In 1970, a record flash flood originating at Workman Creek ended up killing 14 people camping nearly a hundred miles away near Kohl’s Ranch (east of Payson, at the base of the Mogollon Rim). Most of the people who died were trying to flee in motor vehicles.
Do not try to drive across the wash. It only takes six inches of rapid current to render a vehicle uncontrollable, and less than a foot of current will wash most vehicles downstream. Cars and SUV’s make terrible boats; they leak and they they sink. If caught in such a vehicle get on the roof, or better yet, if possible, get to shore.
Arizona has specific laws penalizing “stupid motorists” who try to cross a flooded wash in a vehicle.
Don’t camp in the wash, or very near the wash. It may seem soft and flat and sandy, but its nature’s storm sewer.
For more info:
The National Weather Service has an excellent article.
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