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 2 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 and taken to Banner hospital in nearby Payson for treatment for hypothermia.
Crews were walking Sunday along the banks and scoured a five-mile stretch down the East Verde River and will continue south.
Hornung said the treacherously swift waters gushed for about 10 minutes before receding in the narrow canyon. He estimated flood waters reached 6 feet high and 40 feet wide.
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 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 due 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," Sattelmaier said, explaining these are the first fatalities in recent memory.
The flooding came after a severe thunderstorm pounded down on a nearby remote area that had been burned by a recent wildfire, 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."
Hornung said there was no way to notify people of the flash flood warning, as cell service is limited and there are no officials stationed in the area. He said visitors are reminded to be vigilant about the weather.
Seven people were killed in Utah's Zion National Park in 2015 when they were trapped 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, Ariz., after a wall of water from a rainstorm miles upstream boomed through a narrow, twisting series of corkscrew-curved walls on Navajo land, known as Lower Antelope Canyon.