In the slick rock canyons of Utah and Arizona, a brief thundershower can create a dangerous flash flood.
The hard soil and exposed rock prevent the rain from soaking into the ground as it would in other natural areas, and so it runs off with a ferocity that can instantly turn a dry canyon into a dangerous river. That’s the way it has been for thousands of years.
In a short time our urbanized environment has created something similar, though much safer. Concrete sidewalks and asphalt stree...
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