306 



GENERAL SCIENCE 



leaving its little path for another similar stream when the 

 next rain occurs. (Figures 260, 261.) 



When the surface water is concentrated into a perma- 

 nent stream, the work of eroding its bed goes on rapidly. 

 Each flood period makes many changes in the stream bed. 

 Some streams have cut deep gullies or troughs for them- 



U. S. Geological Survey. 



FIG. 261. Bad Lands, Scott's Bluff, Nebraska, Showing Gullies Made by 

 Water Erosion. 



selves. The run-off water sometimes scars the slopes 

 of hillside farms in this way, but usually such gullies are 

 an indication of careless farming. The rapidity of the 

 cutting by a stream depends upon the material of the 

 stream bed, the fall of the stream, and the quantity of 

 water. 



The irregular erosion of stream beds produces the char- 

 acteristic features of streams known as falls, rapids, and 



