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Flash Floods

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Image from Joseph City flash flood.
Joseph City flash flood, July 1998
(Photo: Navajo County)

What is a Flash Flood?

Floods are the most common and widespread of all weather-related natural disasters. Most communities in the United States have experienced flooding after spring rains, heavy thunderstorms, or winter snow thaws. Flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed and unpredictability.

Flash floods occur when excessive water fills normally dry creeks or riverbeds along with currently flowing creeks and rivers, causing rapid rises of water in a short amount of time. In mountains, where terrain channels the flow of water, rocky soil or bedrock keeps precipitation from percolating into the ground. Thunderstorm precipitation rates can be high as well over mountainous terrain, so that the combination can lead to flash floods with rainfall of only an inch or two. Flash flooding can become especially likely when a large storm can tap continuous, low-level moisture inflow and is anchored in place for several hours by the topography or weak upper-level winds. Heavy rains from tropical storms and hurricanes can also cause flash flooding. Dam failures can create the worst flash flood events. When a dam or levee breaks, a gigantic quantity of water is suddenly let loose downstream, destroying everything in its path.

Flash flood waters can move at a very fast speed. They have the power to move boulders, tear out trees, destroy buildings, and obliterate bridges. Walls of water can reach heights of 10 to 20 feet or more, and generally carry a huge amount of debris with them. Flash floods are the number one thunderstorm-related killer in the United States. Nearly half of all flash flood deaths are auto-related.

What are some examples of flash floods that have occurred over northern Arizona?

The Labor Day Flash Flood of September 5-6, 1970

The loss of 23 lives and the devastation which occurred during the Labor Day weekend of September 5-6, 1970 make this event the greatest natural disaster in the history of the state. During the 24 hour period from 10 pm September 4th to 10 pm September 5th, 11.4 inches of rain were measured in the official recording rain gauge at Workman Creek, about 60 miles east- northeast of Phoenix at an elevation of 7000 feet. These heavy rains were spawned by an influx of moisture from a dying Pacific tropical storm named Norma. Of the 23 lives lost in the flooding, 14 were in the Kohl's Ranch and Christopher Creek Campground areas at the headwaters of the Tonto Creek drainage. Most of the deaths resulted when people attempted to leave campground areas for home. Some rivers experienced water rises of 5 to 10 feet per hour, with Sycamore Creek, near Sunflower, experiencing flood waters that reached 36 feet above the creek bed.

The Supai Flash Flood of September 3, 1990
Image of Antelope Canyon showing flash flood potential.
Antelope Canyon
Notice the tall, narrow canyon in relation to the people. (Photo: NWS-Flagstaff)

During the evening of September 3, 1990, a flash flood occurred in the village of Supai, near the Grand Canyon. No observations were available for how much rain fell, but a persistent area of thunderstorms moved over the drainage areas of Cataract Creek, which then flowed downstream into the village of Supai. It is estimated that a wall of water up to 14 feet swept through the village causing over one million dollars of damage along with the destruction of three houses and damage to 13 others. Only one hiker was injured, but 50 hikers needed to be evacuated by helicopter during the evening. Only 0.30 inches of rainfall was recorded at the South Rim of the Grand Canyon.

The Antelope Canyon Flash Flood of August 12, 1997

On the afternoon of August 12, 1997, 12 people were caught in a narrow canyon near Page, Arizona during a flash flood event. Eleven of these people died when a wall of water, estimated to be well over 10 feet in height, swept through the canyon with little notice. The thunderstorm that caused this flash flood occurred miles from the slot canyon, with only a trace of rain reported at the canyon itself. However, about 10 miles upstream from the canyon 1.5 inches of rainfall fell, with 0.75 inches falling in approximately 15 minutes. The water made it into the canyon approximately 30 to 45 minutes later, with sudden and deadly results.

Most flash flood deaths and injuries are preventable if proper precautions are taken!

Flash Flood Safety Tips
  • Never, ever, drive into a flooded area if the depth of the water is unknown. As little as 12 inches of running water can wash away most vehicles, including sport utility vehicles.
  • Heed warning signs. If a road is barricaded, turn around and take another route.
  • When hiking in a canyon or near a streambed, climb up from the flash flood. Do not try to outrun it. A flash flood moves much faster than most people.
  • Do not camp or park along streams or washes, particularly during threatening conditions.
  • Be especially cautious at night. Flood dangers are much more difficult to see in the dark.
  • Do not let children play near storm drains or washes after a heavy rain.
  • Street and urban flooding can even be hazardous. Driving too fast through standing water can cause you to hydroplane and lose control of your vehicle. The best defense is to slow down during heavy rains.

Here are terms you will hear from the National Weather Service and/or your favorite media source to alert you of flash floods:

Flash Flood Watch: Conditions are favorable for flash flooding to develop. Stay tuned for updates.

Flash Flood Warning: A serious, life-threatening flash flood is underway or is about to occur. Move to higher ground immediately!

Urban and Small Stream Flood Advisory: Flooding of washes, streets, and low water crossings is underway. Use caution.

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