178 THE WORK OF RUNNING WATER 



hold them up in falls and rapids. These they at once 

 begin to smooth down. Rivers of this kind may well be 

 called young, as their life work is just beginning. The 

 Red River of the North, with its shallow, narrow valley 

 and tortuous course, and the Niagara River, with its lakes 

 and falls, are examples of young rivers. 



STREAM WORKING BACK INTO AN UNDISSECTED AREA 



Where the slope of the newly exposed surface is consid- 

 erable, the streams flow much more rapidly and develop 

 their courses more quickly. The small irregularities are 

 sooner straightened and the trough deepened, thus form- 

 ing side slopes down which run little rivulets which in 

 time form side streams. The heads of these, like the heads 

 of the larger streams, are constantly working back into the 

 undissected area. Gradually the side streams develop side 



