ACCIDENTS IN RIVER DEVELOPMENT 



343 



able to gnaw back and cut down their valleys faster. 

 Thus they push the divide EF farther and farther 

 toward A. 



In Figure 114 a stage is shown in which the divide has 

 been pushed back toward A and at one point has ap- 

 proached very near to the upper part of the branch G-. 

 In Figure 115 it has been pushed across this branch and 

 the stream B has tapped 6r and appropriated its head- 

 waters. This is a case 

 of what is called behead- 

 ing or piracy. 



As A has lost some 

 of its water it erodes 

 its valley even more 

 slowly than before, and 

 a branch of the stream 

 D may take ;away the 

 headwaters of its branch 

 H. If time enough is 

 allowed, a branch of the 



Fig. 115. 



stream C may completely behead A, leaving only its lower 

 trunk as an independent river. Cases of river piracy 

 are most interesting phenomena. 



A river may by some accident have its supply of sedi- 

 ment greatly increased, causing it for a time to build up 

 its valley floor instead of eroding it, thus forming a filled 

 river valley. When the supply of sediment fails, the river 

 begins cutting down the filled valley, leaving terraces 

 along the sides to mark the successive levels at which it 

 flowed. 



River terraces are often very prominent along our 

 northern rivers, since by the melting of the ice at the close 

 of the last Glacial Period these rivers were supplied with a 

 vast amount of sediment which they were unable to carry 



