Many of you may be aware of the flash flooding that hit US89A between Marble Canyon and House Rock Valley on August 9th. The rain total was between 1.3 and 1.7 inches in 15-20 minutes and is an estimated 200 year event. The really large (trailer-size) boulders in some of the photos were washed down to the culverts from House Rock Valley which is about 1.5 miles upstream. Some of these were actually pushed up onto the road surface when they met the culverts.
Source: David Zimmerman, ADOT Historic Preservation