146 



Professor W. M. Davis — River Meanders. 



The most important process in the development of river meanders 

 is the displacement of the line of fastest current by inertia from mid- 

 channel toward the outside of every curve. As a result erosion 

 tends to take place on the outside and deposition on the inside of the 

 curve. This process is self-perpetuating. However slight the 

 initial bends, they will be increased ; and as the valley floor is 

 broadened the curves will be developed into systematic meanders 

 of increasing radius and breadth, as in Fig. 1. The only conditions 

 under which the river course will tend to straighten itself are : 

 strong tilting in the direction of river flow, and downward erosion 

 upon a weak stratum between two resistant strata in an inclined 

 structure. Both these conditions are only temporary ; for as grade 

 is reached in either case and the valley floor is widened, the residual 

 departures from a perfectly rectilinear course will be exaggerated 

 again, and in advanced maturity the river must always be curved. 

 The smaller the stream, the greater the effect of accidental causes, 

 such as falling sods and trees, tributary deltas, etc., in forming new 

 bends. The larger the river, the greater the effect of inertia in 

 exaggerating pre-existant bends and in overcoming local or accidental 

 irregularities. 



Fig. 1. 



Fig. 2. 



A river not only tends to increase its meanders; it also tends to 

 push the whole meander system down the valley. This is because 

 the line of fastest current, displaced toward the outside of each curve, 

 enters the succeeding curve (or stretch between two curves) near 

 the down- valley bank, which is therefore worn away, while the 

 opposite up-valley bank is built out. As a result, Fig. 1 should 

 be modified by a persistent down-valley migration of every bend, 

 as in Fig. 2. Cut-offs occur now and then, here and there, butjthe 

 shortened course at a cut-oiF is not straight, and its faint curves are 



