DEVELOrMENT OF DRAINAGE AND ITS LESSON 267 



from its smaller neighbors. This process, with minor variations in the 

 method of work, goes on until the divides of the valley are forced back 

 to a point where they encounter those of another stream which is doing 

 similarly effective work. 



DRAINAGE DEVELOPMENT AND ITS APPLICATION TO STUDY OF RIVER SPACING 



In the advancing stages of drainage development in the natural model 

 we are considering we find the process of lateral appropriation of the 

 smaller channels is brought about in at least three ways : (a) As the 

 channel is deepened the side slopes of the valley keep their original angle 

 of declivity, and thus invade the basins on either side ; these, by reason 

 of the reduction of the drainage thereby brought about, work downward 

 with less rapidity, (b) The lateral shifting of the streams as they are 

 blockaded by the less movable waste also causes them here and there to 

 invade the drainage of their neighbors, (c) Lastly, the peculiar process 

 termed " piracy " occurs --that is, the small branches of the larger main 

 stream work back through the divides until they tap and divert the ad- 

 jacent higher lying water-courses. In a word, all the methods by which 

 an ordinary river extends its field of drainage are fairly well represented 

 in the miniature stream system. 



If with these observations in mind the student proceeds to inquire 

 concerning the difficult question as to the equal spacing of river chan- 

 nels of like size, he will find the explanation he seeks. He will note 

 that the slopes of the drainage basins are, as regards their angles of de- 

 clivity, quite as uniform as are their spacings ; all of them meet in sharp 

 crests, the divides between several valleys. These crests, as well as the 

 lowlands, are wasting ; but while the streams cut down freely, the crest 

 lines descend at a slower rate. As long as the streams on either side of 

 the divide work their beds downward with equal rapidity the ridge 

 maintains its position ; but if one descends more slowly than the other, 

 then, as the slopes of their valleys are uniform, the deeper lying river 

 pushes the crest back, thereby capturing a part of the watershed of its 

 neighbor, with the result that the diminished valley will be less rapidly 

 worn down than before, and therefore the more easily destroyed by the 

 vigorous contestant. In this way it comes about that the lesser velleys 

 are apt to be ruined by the development of the greater adjacent excava- 

 tions. There is, however, as before noted, a limit to this action, which 

 is set by the fixed angle of the side slopes of the valleys. This is de- 

 termined by the character of the material which is wearing away. In 

 the miniature example this wearing is purely erosive, and is commonly 

 rather even in rate, for the reason that the soil is likely to be of a uniform 

 composition. It is different ia the case of ordinary river valleys, but 



