478 JOHN LYON RICH 



the stream would have lowered its bed so that each succeeding 

 meander in its sweep would be opposed by rock. 



CONTROLLING FACTORS DETERMINING THE DOMINANT PROCESS 



Down-cutting will dominate whenever a stream finds its bed 

 well above its local profile of equilibrium. After a stream reaches 

 grade it may still lower its bed as the general level of the country is 

 lowered, but the process must be gradual — -too gradual for the 

 continued dominance of down-cutting over lateral cutting or sweep. 



Lateral cutting. — The tendency toward lateral cutting, which, 

 it appears, is dependent upon a balance between two opposing 

 forces^ — (i) that due to inertia, tending to displace the thread of 

 fastest current tangentially from one bank to the other, thereby 

 favoring lateral cutting, and (2) the down-stream component of 

 gravity, tending to make the current follow parallel to the walls of 

 the channel — is controlled largely by the gradient of the stream, 

 which determines the latter of these two forces. The lower the 

 gradient the greater the proportion of the stream's energy used in 

 lateral as distinguished from down- valley ward corrasion. 



Volume also must play a part, for increased volume, by decreas- 

 ing friction and thereby increasing velocity, would augment the 

 value of the force due to inertia, and would therefore favor lateral 

 cutting. 



Sweep, it appears, particularly the ratio of the rate of sweep to 

 that of down-cutting, plays a most fundamental part in determining 

 the form of the valley. It deserves, therefore, a more detailed 

 study. 



Three factors suggest themselves as determining the rate of 

 down-valley migration of meanders, or sweep. These are: the 

 gradient of the stream, its volume, and the character of the material 

 against which it impinges at the bends. 



The importance of gradient and volume will be apparent from a 

 study of Fig. 5, which illustrates the forces acting in a meandering 

 stream. In a bend such as that shown in the diagram, inertia, 

 if acting alone, would throw the current from A diagonal^ across 

 the channel to B in the direction of the tangent to the curve 



'See Fig. 5. 



