Monsoon storms impacted the area last week, dumping more than an inch of rain late Tuesday afternoon, July 15, in the McDowell Mountains and running off toward the Verde River.

McDowell Mountain Road between Fountain Hills and Rio Verde had to be closed about 45 minutes due to flash floods.

A flash flood is a torrent of water rushing through a normally dry or low-flow creek, stream, wash, ravine and culvert.

During a rainstorm, normally dry waterways can quickly become raging rivers due to water running off mountains and higher elevations onto the flat terrain below.

As little as two feet of water has enough force to wash a car away. Six inches can knock a person off his or feet, according to the Maricopa County Flood District.

Town Development Services Director Paul Mood said large rocks and debris might be hidden under the swollen wash runoff and could damage the under-carriage of vehicles.

The road was reopened about 4:30 p.m., after the water receded and a payloader had cleared the debris and rocks. Two Maricopa County Sheriff’s deputies directed traffic in both directions.

Mood said staff does receive alerts from the Flood Control District when there is a potential for flash flooding. Street Department personnel can respond to close roads or place warning signs.

While there was rain falling in the downtown area during the storm the flash flooding was caused by large amounts of rain falling in the higher elevations to the west. Up to an inch of rain fell within a short time.

When this happens, the ground cannot absorb the water fast enough so it accumulates in channels and flows downhill. Flash floods are often preceded by a debris flow that contains rocks, brush, logs and anything else it picks up along the way. This initial debris flow can be anywhere from one foot to 10 or more feet in height.

In the desert Southwest, this situation can become very hazardous to people engaged in outdoor activities such as hiking and off-roading, and to the motoring public in general. Flash floods are very dangerous because they happen quickly and often without warning.

Areas that have a potential for flash flooding should be avoided when thunderstorms are nearby. A vehicle is not a safe refuge during a flash flood. Fifty percent of flash flood fatalities occur when drivers try to cross flooded washes.

Do not try to cross a roadway covered by floodwaters – the flood may conceal damage to the roadway beneath. Just two feet of water can cause even a large truck or SUV to float. Find an alternate route or wait it out. Most flash floods dissipate in an hour or so.

In 2006, the National Weather Service reported 59 deaths in the United States due to flash floods, the third highest in weather-related deaths, surpassed only by heat and tornadoes.