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Characteristics The character I st i cs of a mounts I n torrent or f I ash flood differ from the mainstream low-gradient flooding described in the chapter on flooding. The extremely rapid rise of water, Its very high velocity as it rushes down mountainsides and across open areas and the extreme I y h' gh percentage of sediment and debris carried in the water makes flash floods especially destructive and dangerous. Where the gradient of the stream is steep, tremendous erosive powers act on the stream channel and banks, transporting and destroying almost everyth i ng I n the way. Th Is mater J a I then is deposl ted downstream where the gradient decreases. (See debris fan.) Flash flooding of smal lar basins can occur either with major mainstream flooding or as small, isolated events. There Is seldom much time to warn people or evacuate the areas threatened by mountal n torrents. aecause many streams are periodically dry or contain only intermittent flows, there is a danger of underestimating the potential hazard. Many channels which contain a small, peaceful stream can become raging torrents within minutes after a cloudburst. Consequences The tremendous destructive power of the mountain torrent and flash flood process can destroy essentially all works of man within the flood path. Erosion can undercut buildings that are above the flood, causing them to fal I into the torrent. Dam failures can also cause flash floods. They are often catastroph J c I n loss of II fe and property because of development below them. The levels of flood waters often exceed any that ....0U I d occur natura I I Y had the dam not existed. Aggravating Circumstances Excessive logging, overgrazing and forest fires reduce the land's capacity to absorb water and slow down runoff. ( 10 Mitigation I f property damage and loss of I J fe are to be avoi ded. the only economically reasonable and physically safe mitigatjon Is complete avoidance of areas subject to mountain torrent and flash flood processes. Monitoring of dams can give warning of impending fai lure so rapa i rs can be made or peop I e evacuated. I n some cases Inundation maps prepared In advance can precisely detail the route and depth of a flood in case of dam failure. Th is house was tota II y destroyed by the 1976 a I g Thompson flood. Note debr 15 and sed i ment pi I ed around the trees and the house.