A flash flood that transformed a series of idyllic, azure waterfalls into a monstrous brown froth was ferocious but not unusual for the summer rainy season on the Havasupai reservation, one of the most remote reaches of the continental U.S. that attracts visitors worldwide. A woman was swept away toward the Colorado River within the Grand Canyon, touching off a dayslong search and rescue effort involving the National Park Service in a unique environment beyond the reach of cellphones, within desert canyons accessible only by foot, mule or helicopters. The day of the flash flood began before dawn for hikers descending into a verdant canyon on an 8-mile (13-kilometer) trek along switchback trails to a village in the heart of the Havasupai reservation.

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