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Speed
Flash flooding is usually caused by very heavy rain falling in a short amount of time over a small area. As a result, flash floods swell and move downstream incredibly fast. Creeks and streams that are normally just a trickle can leave their banks in a matter of minutes. The more rugged the terrain, the more quickly water moves downhill into river networks. Because of its rapid movement, a flash flood ends almost as quickly as it begins.
Surprise
The element of surprise makes flash flooding a deadly disaster. A strong thunderstorm miles away can send a wall of water downstream onto unsuspecting residents. Mountains and canyons are especially vulnerable to this kind of surprise flash flood. In the western United States, many flood deaths have occurred because campers and hikers were swamped by unexpected flood waters. Steep canyon walls funnel the water into a deadly surge, and make escape nearly impossible. A flash flood at night while people are sleeping is even more dangerous.
Power
Nothing can resist the power of flood water moving at high speed. A cubic foot of water weighs a little over 60 pounds. Now imagine that weight multiplied thousands of times and hurling past you at 20 miles per hour. Two feet of moving water is all that's needed to carry most cars downstream. A wall of water 10 or more feet high will destroy buildings, uproot trees and possibly alter river channels.
Multiple Causes
Brief torrential downpours aren't the only cause of flash flooding. Dam failures and levee breeches can release huge amounts of water upon vulnerable communities. The New Orleans flooding after Hurricane Katrina was caused not by rain or wind, but by levee breeches. Ice jams in northern areas are also a common flood threat. Chunks of ice can block a river channel, sending water into places it shouldn't go. Volcanic activity underneath mountain glaciers can instantly melt tons of water, sending a monstrous surge downstream.