Flash floods can occur wherever land is unable to absorb rainfall or any torrent of water (such as a dam break) quickly enough to prevent water rising. They happen, as their name suggests, in a flash.
In urban areas, the presence of a lot of nonporous or less porous material—asphalt, cement, tar, and stone, for instance—inhibits large amounts of water from seeping into the ground. Low-lying areas, such as bridge underpasses, are especially susceptible to instant flooding as water from the environment gathers there.
In less populated areas, flash floods are as terrifying, because there are no sewer systems or other intentionally designed outlets for water runoff. Gorges, canyons, deserts, and ravines with relatively little porous material can be hit with flash floods during a violent storm, or even hours later and miles away from the torrent.
Water from a storm has to go somewhere. Even if the storm occurs 20 miles (32 kilometers) away and there is nowhere for the water to run, it will build up like a river, racing over the terrain until it reaches absorbent land or some water outlet.
Water seeks lower levels, and deep ravines or gorges provide ideal channels for rushing water.