112 A MANUAL OP TOPOGEAPHIC METHODS. 



storm the entire surface is a network of water courses, from the most minute 

 rills to the main streams, and in studying transportation and corrasion the 

 action of these minute rills, which cover the entire terrain, must be considered 

 as fully as that of the main stream and its primary branches. 



Corrasion is effected by the detritus which running water holds in 

 suspension. Soft rocks are corraded rapidly, hard rocks slowly. The rate 

 of corrasion is increased by an increase in the volume of the stream, an 

 increase in its velocity, an increase in the amount of detritus borne by it, 

 and by increased coarseness of that detritus. Hence it is that the tinv rain- 

 water rivulets have very feeble corrasive powers; but as they combine into 

 larger and larger streams, and as they wash into their channels a larger and 

 larger amount of detritus, and as the slope of their beds becomes greater, 

 their power for corrading their beds increases, and hence it is that the cor- 

 rading power of the main stream is greater than that of any of its branches, 

 and in the main stream, if the slope were uniform, the corrasive power 

 would be greatest near its mouth. 



Suppose a. stream to have initially a uniform slope from its source to 

 its mouth — then its volume, its velocity, and the amount of detritus borne 

 by it will be greatest near its mouth; and corrasion, although going on all 

 along its course, will be most rapid there. The slope of the stream will 

 therefore be reduced most rapidly in the lower part of its course, and thence 

 progressively up stream. It results from this that the normal profile of a 

 stream bed is a curve, concave upward. 



While the slope of the stream bed remains considerable and the velocity 

 consequently great, the stream flows in a comparatively straight channel, 

 and devotes its energies to deepening its bed, and thus reducing its slope. 

 As the slope becomes thus reduced the course of the stream changes to a 

 crooked, winding one, and its corrasive energies are diverted from its bottom 

 to the sides of its bed. It is then said to approach "base level." 



Swift streams commonly flow in straight channels; sluggish streams, 

 in crooked channels. 



While lowering its bed by corrasion the main stream lowers, necessarily, 

 the mouths of its immediate affluents, and these affluents are, therefore, in 

 addition to their own proper work, obliged to cut their lower courses down 



