Anatomy of a flash flood

Last week 20 people died in a wave of flash floods in southern Utah, eerily similar to a summer in Arizona 18 years ago. Of those who died, seven were in a narrow canyon in Zion National Park and another 13 were lost when their cars were swept away from around the town of Hilldale. The seven in Zion were geared up with helmets and ropes, not the most trained group, but certainly capable. The 13 from around Hilldale were drivers and passengers who found themselves unexpectedly swallowed by a flood that dammed itself with debris and then burst through Short Creek. It was the desert announcing itself yet again.

Most people don’t think of the desert as flood prone. But most people don’t live in the desert. Yet the strange, Roadrunner-cartoon topography is directly and indirectly caused by flooding. Storms break over ground that holds little vegetation. Rainwater flies across the land looking for any downhill passage. Arroyos and washes funnel together, as the contents of thunderheads arrive in tight, narrow spaces: a canyon where you can touch both walls, or a storm drain dry almost every day of the year, until suddenly it is not.

This is where the word flash comes from in flash flood. A canyon can be dry for months or even years. A storm lands far away. The water comes all at once.

Samples taken from the floods that August of ’97 were sometimes only 10 or 20 percent water. The rest was mud. The earth was being reduced and transported. Where a flood hit the town of Kanab, Utah, not far from Hilldale, a scientist waded into one of its red-brown eddies with specimen bottles. Viscous mud draped down his legs. He was wearing the earth.

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1 Comment

  1. Mitch Stevens

    phenomenal article about flash flooding in the desert. I’ve descended a couple of those narrow slots in Zion. It would be very difficult to escape from a flash flood.

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