The Behavior of Rivers 



49 



yons resist weathering so that the bottoms of the channels are 

 cut down faster than the side walls weather. Young tributary 

 streams, gullies, push their heads back after the fashion of con- 

 sequent streams, that have been described. The river as a 

 whole, however, can hardly be regarded as a consequent stream. 

 It is rather a subsequent stream, that is, it tears along where it 

 can find the path of least resistance between obstructing masses 

 of hard rocks. The principal source of water supply is high in 



Courtesy Los Angeles Flood Control Commission 



FIG. 1 5. Rubble wall, in Sierra Madre conduit, Los Angeles County, show- 

 ing effects of flood. 



the mountains and the water simply has to go somewhere. It 

 therefore goes down hill by the easiest course it can find. The 

 course of the river from the high mountains across a broad plain 

 and then across a mountain range and finally meandering across 

 the flat coastal plain to the sea suggests that the course of the 

 river was determined during an earlier geologic period, and that 

 the stream has been let down, as it were, from an earlier land- 

 scape which has been eroded away. Thus the stream is called 

 a superimposed stream. 



