All the victims are mothers and small children, Assistant Fire Chief Kevin Barlow said. The youngest child was about 4 years old.
The flooding ravaged a community that straddles the border from Hildale, Utah, to Colorado City, Arizona, CNN reported on Tuesday.
Most of the people who live in the community are members of the Warren Jeffs polygamist sect known as the Fundamentalist Church of Jesus Christ of Latter-day Saints (FLDS).
The flash flood started with heavy rains Monday evening in the canyons above Hildale, Barlow said. The rush of water was so intense it washed two vehicles several hundred yards downstream.
The two vehicles were carrying 16 people, Barlow said. Among them, seven died, three were found alive and six remain missing.
"Most of the people were thrown from the vehicles," Barlow said.
Witness Chris Wyler said rain and hail walloped Hildale within minutes -- then quickly subsided.
"It happened within like a half-hour, 45 minutes," he said. "(Then) it was just gone. And then the sun was shining again."
By late Monday night, he said, the water had mostly receded. Nearby residents were trying to organize volunteers on social media to help search for those missing.