Three people survived as the flash flooding washed the vehicles carrying 16 people several hundred yards downstream at about 5pm local time on the border of Utah and Arizona, assistant fire chief Kevin Barlow said.
Search and rescue teams called off the search late on Monday owing to treacherous conditions in the sister towns of Hildale, Utah, and Colorado City, Arizona, he said.
The floods came after heavy rain fell in the canyons just north of the towns, sending waves of water barrelling through the streets of a community of 7,700 people. The towns are about 315 miles south of Salt Lake City.
The women and children were in two different vehicles, Mr Barlow said – a full size van and an SUV.
Search and rescue teams were scheduled to resume efforts to find the missing people at daybreak.
Flash floods in the US state of Utah have killed at least eight people and left five missing.
Hildale once served as a home base for polygamous sect leader Warren Jeffs, and there are concerns flood warnings may not reach members of the sect.
The National Weather Service had issued a flash flood warning earlier, leading nearby Zion National Park to close canyons as a precaution.
But members of Jeffs’ sect, the Fundamentalist Church of Jesus Christ of Latter-Day Saints are discouraged from having contact with the outside world and may have not received the warnings.