Flash floods are rapid overflows of low-lying areas that occur within six hours of heavy rain, melting snow, or dam failures, such as the historic Johnstown Flood of 1889. They are the deadliest weather hazard in the United States, averaging 127 annual fatalities—more than lightning, tornadoes, and hurricanes combined—and are especially dangerous to drivers, since just 2 feet (0.61 m) of water can sweep away an SUV, prompting the National Weather Service's advice to "Turn Around, Don't Drown." Desert regions face an elevated risk because infrequent but intense storms hit poorly absorbent clay-like soil and lack drainage infrastructure, while the suddenly rushing water is often loaded with dangerous debris like rocks and logs. Deep slot canyons are particularly treacherous for hikers, as a storm miles away can funnel a sudden wall of water through narrow walls, as tragically demonstrated by the 20 deaths caused by a cloudburst in southern Utah on 14 September 2015. Beyond human casualties, flash floods deposit massive quantities of sediment on floodplains and destroy vegetation not adapted to frequent flooding, making them a severe and reshaping natural hazard.