Nine people died and a 13-year-old boy was still missing on Sunday after a furious flash flood tore through a group of family and friends cooling off in a creek in the Tonto National Forest in Arizona.

Gila County Sheriff’s Detective David Hornung told The Associated Press that the group from the Phoenix and Flagstaff areas had met up for a daytrip along the popular Cold Springs swimming hole near Payson in central Arizona and were playing in the water Saturday afternoon when muddy flood waters came roaring down the canyon.

The group had set out chairs to lounge on a warm summer day when miles upstream an intense thunderstorm dumped heavy rainfall on the mountain.

Search and rescue crews, including 40 people on foot and others in a helicopter, recovered the bodies of five children and four adults, some as far as two miles down the river. The victims ranged in age from a 60-year-old woman to a 2-year-old girl. Authorities did not identify them. Four others were rescued on Saturday and taken to Banner hospital in nearby Payson for treatment for hypothermia.

Rescuers got to the four victims quickly after the crew heard their cries while they were nearby helping an injured hiker.

Crews were walking on Sunday along the banks poking through debris, including tree trunks. They’ve scoured a five-mile stretch down the East Verde River and will continue south.

The flash-flooding hit on Saturday afternoon at Cold Springs canyon, about 100 miles northeast of Phoenix, a popular recreation area easily reached by relatively easy hiking paths. Some know it was as Ellison Creek or Water Wheel swimming holes.

Disa Alexander was hiking to the swimming area where Ellison Creek and East Verde River converge when the water suddenly surged. She was still about 2 miles away when she spotted a man holding a baby and clinging to a tree. His wife was nearby, also in a tree. Had they been swept farther downstream, they would have been sent over a 20-foot waterfall, Ms. Alexander said.

Ms. Alexander and others tried to reach them but couldn’t. Rescuers arrived a short time later.

“We were kinda looking at the water; it was really brown,” she said. “Literally 20 seconds later you just see, like hundreds of gallons of water smacking down and debris and trees getting pulled in. It looked like a really big mudslide.”

Video she posted to social media showed torrents of muddy water surging through jagged canyons carved in Arizona’s signature red rock.

“I could have just died!” she exclaims in the video, before showing images of the man in a tree and his wife.

The National Weather Service, which had issued a flash flood warning, estimated up to 1.5 inches of rain fell over the area in an hour. The thunderstorm hit about 8 miles upstream along Ellison Creek, which quickly flooded the narrow canyon where the swimmers were.

“They had no warning. They heard a roar, and it was on top of them,” Water Wheel Fire and Medical District Fire Chief Ron Sattelmaier said. There were no notices or warnings at the trailhead, Ms. Alexander said.

There had been thunderstorms throughout the area, but it wasn’t raining where the swimmers were at the time. But it happened during monsoon season, when strong storms suddenly appear thanks to the mix of heat and moisture in the summer months.

“I wish there was a way from keeping people from getting in there during monsoon season. It happens every year. We’ve just been lucky something like this hasn’t been this tragic,” Mr. Sattelmaier said, explaining these are the first fatalities in recent memory.

Crowds looking to beat the Phoenix metro area’s heat headed to the small creeks that flow out of the mountains forming swimming holes and a series of small waterfalls. Some barbecue along the water’s edge, while others cliff jump into the deeper pools. Farther up the canyon narrows and becomes rockier, its walls steeper.

The flooding came after a severe thunderstorm pounded down on a nearby remote area that had been burned by a recent wildfire, Mr. Sattelmaier said. The “burn scar” was one of the reasons the weather service issued the flash-flood warning.

“If it’s an intense burn, it creates a glaze on the surface that just repels water,” said Darren McCollum, a meteorologist. “We had some concerns. We got a lot worse news.”

Seven people were killed in Utah’s Zion National Park in 2015 when they were tapped during a sudden flash flood while hiking. The group was trapped by floodwaters in a popular “slot” canyon that was as narrow as a window in some spots and several hundred feet deep.

In 1997, 11 hikers were killed near Page, Arizona, after a wall of water from a rainstorm miles upstream boomed through a narrow, twisting series of corkscrew-curved walls located on Navajo land, known as Lower Antelope Canyon.