342 



FIRST YEAR SCIENCE 



year is a mere brook may after a rain become a devastat- 

 ing torrent, bursting its banks and carrying destruction 

 to settlements and farm lands along its course. It may 

 even change its entire lower course. 



160. Accidents in River Development. While a river is 

 developing its drainage area many accidents may happen 



to it. The competition 

 of other rivers in the 

 same region affect it. 

 The river that has the 

 shortest course to the 

 sea or the most easily 

 eroded bed has the ad- 

 vantage. It lowers its 

 valley more rapidly, 

 thus giving its side 

 streams steeper grades 

 and enabling them to 

 wear back faster into the upland, and thus to gain more 

 than their share of the drainage of the region. 



As soon as the un- 

 appropriated drainage 

 area has been channeled, 

 it begins to push back 

 the divides of its neigh- 

 bors, thus appropriat- 

 ing some of the run-off 

 they may have acquired. 

 In Figure 113 a case 

 of this kind is shown. 

 The river A reaches the 

 sea by a long course, Flg ' 



while the streams B, C, D have short courses. These 

 short streams have steeper grades than A and thus are 



Fig. 113. 



