A powerful flash flood tore through Arizona’s Grand Canyon, sending torrents of muddy water, boulders and debris through Bright Angel Canyon and into the Colorado River. The National Park Service (NPS) said at least 20 people may be missing or unaccounted for, prompting a public appeal for information.The flooding happened on August 29, but the park reported aftermath on Sunday (local time) about the damage and the missing people.

Meanwhile, more than 60 people were evacuated from Phantom Ranch and Bright Angel Campground as the flood swept through the canyon. Footbridges were destroyed, the creek bed was dramatically reshaped and debris, including what appeared to be a bunkhouse, was carried downstream while campers seeked safety.

The NPS is asking for information about hikers and backpackers who were in the Bright Angel Creek corridor on August 29, including people who had reservations at affected campgrounds. Phantom Ranch, Bright Angel Campground and the North Kaibab Trail remain closed.The flood also damaged the Grand Canyon’s sole water pipeline, forcing the park to suspend overnight hotel stays from August 31 and ask residents to conserve water. The pipeline is already being rehabilitated as part of a $208 million infrastructure project.The flooding followed roughly 0.5 to 1 inch of rain in about 30 minutes, with much of the water running off immediately through the steep terrain. More rain and thunderstorms are possible, raising the threat of further flooding.