How big is a flash flood?

On September 28, the Albuquerque Journal reported another deadly incident that may provide life-saving lessons. On a June night, a flash flood tore through a camp set up by eight Boy Scouts and their chaperones. Four Scouts were swept away and one died. The flash flood occurred about 4:30 in the morning, in Ponil Canyon on the Philmont Scout Ranch in northeast New Mexico. The group was camped well above the stream but that didn't protect them from the flooding.

 According to the Journal, "The creek is normally 2 to 3 feet wide and less than a foot deep. That morning, at least 2 inches of rain fell in a short amount of time, and the surge of water that swept through the canyon was as high as 20 feet and as wide as a football field."


The story is a reminder that you can't avoid flash floods simply by staying out of the recent flood channels. An unusually bad flash flood will overwhelm those flood channels and sweep across the adjacent terraces. Because of that, while hiking canyon bottoms during rainy seasons I apply what I call "the principle of infinite retreat." In other words, if flood waters keep rising, can I keep retreating up a slope? If the answer is no—if rock walls would cut off my retreat—it's best to stay out of the canyon until dry weather returns. 


That assumes, of course, that you can see the water rising and have time to retreat. If the flash flood arrives in a single wall of water, you may have only seconds to get to safety. And if the flood arrives in the middle of the night (it can, and did for the Boy Scout group), by the time you wake up there may be no time to react. In New Mexico and elsewhere, save slot canyons for dry weather and always camp as far above the nearest stream as you can manage.