The National Weather Service estimated up to 1.5 inches of rain fell over the area in an hour.
The thunderstorm hit about eight miles (12 kilometres) upstream along Ellison Creek, which quickly flooded the narrow canyon where the swimmers were.
Mr Hornung noted that the National Weather Service had issued a flash flood warning about 90 minutes before, "but unless they had a weather radio out there, they wouldn't have known about it. There is no cellphone service out here."
Source : http://www.independent.ie/world-news/north-america/five-children-among-nine-family-members-killed-in-arizona-flash-flood-35941079.html