Running Water

Date: 2015-10-07; view: 360.

Running water is the most powerful agent of erosion. Wind, glaciers, and ocean waves are all confined to relatively limited land areas, but running water acts almost everywhere, even in deserts. One fourth of the 35,000 cubic miles of water falling on the continents each year runs off into rivers, carrying away rock fragments with it. In the United States, this erosion of the land surface takes place at an average rate of about one inch in 750 years. Running water breaks down the crust by the impact of rock debris it carries.

A single shower may involve the downpour of more than a billion tons of water. Each raindrop in the shower becomes important in erosion, especially in areas with sparse vegetation and unconsolidated sediments. Runoff water rarely travels far as a continuous sheet, for it is spoken up into rivulets and streams by surface irregularities in rock type and relief.

Running water carries its load of rock debris partly in suspension, partly by rolling and bouncing it along the Bottom, and partly in solution. The carrying power of a stream is proportional to the square of its velocity, and so is enormously increased in time of flood.