1876.] 



Windings of Rivers in Alluvial Plains, 



7 



bank and carry the worn substance in a great degree down to the bottom, 

 where, as explained before, there will be a general prevailing tendency 

 towards the inner bank. 



Now, further, it seems that even from the very beginning of the curve 

 forward there will thus be a considerable protection to the inner bank. 

 Because a surface stream-line C D, or one not close to the bottom, flomng 

 along the bank which in the bend becomes the inner bank, will tend to 

 depart from the inner bank at D, the commencement of the bend, and to go 

 forward along D E, or by some such course, leaving the space Gr between it 

 and the bank to be supplied by slower-moving water which has been moving 

 along the bottom of the river perhaps by some such oblique path as the 

 dotted line F G-. 



It is further to be observed that ordinarily or very frequently there 

 will be detritus travelling down stream along the bottom and seeking for 

 resting-places, because the cases here specially under consideration are 

 only such as occur in alluvial plains ; and in regions of that kind there is 

 ordinarily*, on the average, more deposition than erosion. This consi- 

 deration explains that we need not have to seek for the material for de- 

 position on the inner bank in the material worn away from the outer 

 bank of the same bend of the river. The material worn from the outer 

 bank may have to travel a long distance do^;^^l stream before finding an 

 inner bank of a bend on which to deposit itself. And now it seems very 

 clear that in the gravel, sand, and mud carried down stream along the 

 bottom of the river to the place where the bend commences, there is an 

 ample supply of detritus for deposition on the inner bank of the river even 

 at the earliest points in the curve which mil offer any resting-place. It is 

 especially worthy of notice that the oblique flow along the bottom to- 

 wards the inner bank begins even up stream from the bend, as already 

 explained, and as shown by the dotted line F G- in fig. 3. The transverse 

 movement comprised in this oblique flow is instigated by the abatement 

 of pressure, or lowering of free level, in the water along the inner bank 

 produced by centrifugal force in the way already explained. 



It may now be remarked that the considerations which have in the 

 present paper been adduced in respect to the mode of flow of water 

 round a bend of a river, by bringing under notice, conjointly, the lower- 

 ing of free level of the water at and near the inner bank, and the raising 

 of free level of the water at and near the outer bank relatively to the free 

 level of the water at middle of the stream, and the effect of retardation 

 of velocity in the layer flowing along the bed of the channel in diminishing 

 the centrifugal force in the layer retarded, and so causing that retarded 

 water, and also frictionally retarded water, even in a straight channel of 

 approach to the bend, to flow obliquely towards the inner bank, tend very 



* That is to say, except when by geological clianges the causes which have been pro- 

 ducing the alluvial plain have become extinct, and erosion by the river has come to 

 predominate over deposition. 



