More than 20 may be missing after ‘significant flash flood' at Grand Canyon

More than 20 people may be missing or unaccounted for after a flash flood ripped through Grand Canyon National Park's Bright Angel Canyon on Saturday.

Strong thunderstorms dropped more than an inch of rain in an hour atop recently burned areas around 2:30 p.m. Saturday, sending a slurry of mud and debris rushing through the canyon. Floodwaters rose more than 5 feet in 10 minutes and destroyed bridges across Bright Angel Creek and forced the closure of one of the park's most difficult trails.

A flash flood watch remained in effect in the Grand Canyon through Monday as forecasters warned of dangerous downpours in Arizona and Utah.

The National Park Service on Sunday morning said 62 campers and hikers were airlifted out of the area. An update released Sunday afternoon indicated more than 20 people could be missing or unaccounted for:

"Grand Canyon National Park staff, in coordination with the Arizona Department of Public Safety, are continuing evacuation and response efforts and working to account for individuals who may have been in the affected area.

"The National Park Service is asking anyone who knows of hikers or backpackers who were in the inner canyon along the Bright Angel Creek corridor on August 29, as well as anyone who had a campground reservation in the affected corridor, to provide information to the National Park Service Investigative Services Branch (ISB).

"This includes individuals who may have been traveling along the North Kaibab Trail from the North Kaibab Trailhead to Phantom Ranch, staying at Bright Angel Campground, or traveling or staying in the area known as 'the Box' north of Phantom Ranch.

"If you know of anyone who may have been hiking or backpacking in these areas, please contact the NPS Investigative Services Branch Tip Line: 888-653-0009," the park service said.

Park officials closed Phantom Ranch, Bright Angel campground, the Phantom Ranch canteen and cabins along with the North Kaibab Trail. The Colorado River was shut down to traffic after metal structures and other debris entered the waterway. The Black Bridge and Silver Bridge crossing the Colorado River was also closed.

Staff warned they may need to make additional inner canyon closures as the chance of additional thunderstorms and heavy rainfall Sunday and Monday threatened to unleash more flash flooding, park officials said.

Cell phone service throughout much of Northern Arizona is difficult to maintain, the park service warned.

"It is nearly impossible to get and keep a signal at Grand Canyon, Marble Canyon, Arizona Strip, and area tribal lands," it said. "Satellite phones are being tested with some success in places outside of the narrowest portions of the canyon."

The Colorado River rose 8 feet in one hour as floodwaters rushed by, according to a U.S. Geological Survey gauge. Waters on Bright Angel Creek increased by more than 5 feet in 10 minutes before the sensor was damaged by flooding.

Saturday's flood occurred a year after the Dragon Bravo Fire ripped through the Grand Canyon's North Rim and destroyed a historic lodge .

Bursts of heavy rain on recently burned landscapes can trigger debris flows, a slurry mix of mud, trees and rocks. A moderate to high-severity fire can alter the physical and chemical makeup of the soil and may make it completely repellent to water.

A park service assessment of the Dragon Bravo burn scar last summer found that "debris flow risk is localized to steep, confined drainages, like Dragon Creek and upper Bright Angel Creek."

Short-duration rain bursts, such as thunderstorm downpours, are more likely to trigger debris flows than long-duration moderate rainfall.

A weather station along Bright Angel Creek measured 1.17 inches of rain between 2 and 3 p.m. Saturday, which far exceeded the National Weather Service's flash flood guidance for the Grand Canyon. That prompted the agency to issue a flash flood warning.

A slight (level 2 of 4) risk of flash flooding remained across Arizona on Sunday and Monday. A flood watch - indicating favorable conditions for flooding - was in effect for a majority of the state through Monday. A flash flood warning - indicating an imminent or occurring flood - was issued just before noon along the Colorado River in northern Arizona, upstream of Grand Canyon National Park.

The weather service said overall coverage of downpours Sunday "should be a bit less" than Saturday, but flooding was more likely in areas that are hit by heavy rainfall multiple hours or days in a row due to "saturated grounds."

A moderate (level 3 of 4) risk of flash flooding was warned in south-central Utah on Monday. That included many recently burned areas in the state, which experienced an extreme burst of fire activity in June.