222 



TBANSPOKTATION OF DEBRIS BY RUNNING WATEE. 



ally finer and at last ceases to move, while the 

 graduated deposit caused by its arrest receives 

 a final contribution from the suspended load. 

 The tractional load on the shoals changes less 

 in mean fineness and may cease to change 

 altogether when the supply from the deep is 

 cut off. It is then derived wholly from the 

 subjacent bed and is greatly reduced in quan- 

 tity. Soon the derivation becomes selective, 

 the finer part being carried on while the coarser 

 remains, with the result that the shallow chan- 

 nels on the bars come to be paved with par- 

 ticles which the enfeebled currents can not 

 move. 



If the section of the alluvium underlying a 

 shoal be afterwards exposed, it is seen to be in 

 the main heterogeneous but veneered at the 

 top by a layer of its coarser particles. The 

 typical section of a deposit in a deep shows the 

 coarsest de'bris below and the finest at top, 

 with a gradual change. 



With the return of large discharge the model- 

 ing work of smaller discharges is rapidly ob- 

 literated, and the debris deposited in the pools 

 rejoins the tractional and suspended loads. 



SECTIONS OF CHANNEL. 



Rock-walled channels result from the aggra- 

 dation of corraded channels. Often they are 

 recurrent temporary conditions of corraded 

 channels. Their widths have been developed 

 in connection with the work of conasion and 

 are less than the widths of alluvial streams. In 

 the fact that their sides are immobile they 

 lesemble the laboratory channels, and their 

 types of cross section are illustrated by the 

 experiments with crooked channels. The 

 channels of all alluvial streams are strongly 

 asymmetric at the bends, and in the meander- 

 ing streams the bends constitute the greater 

 part of the course. Departure from symmetry- 

 is less pronounced in the reaches of direct 

 alluvial streams, but even there a close approxi- 

 mation to symmetry is exceptionr.l. 



Alluvial streams tend to broaden their 

 channels by eroding one or both banks. The 

 influence of vegetation opposes this tendency. 

 Often, the erosion of the bank exposes roots, 

 and some trees extend rootlets into the water. 

 At low stages the bared parts of the flood 

 channel are occupied by young plants. In 

 these ways vegetation creates obstacles which 

 retard the current at its contact with the bank 



and thus oppose erosion. If the current is 

 strong erosion is merely retarded, not pre- 

 vented; if the current is weak deposition may 

 be induced. As a meandering stream en- 

 croaches on its concave bank, the convex bank 

 encroaches on the stream, and channel width 

 is maintained. A large stream is less affected 

 than a small stream by the opposition of 

 vegetation and maintains a channel of rela- 

 tively small form ratio. 



Some streams aggrade so rapidly that vegeta- 

 tion does not secure a foothold. By erosion 

 of its banks such a stream broadens its channel 

 and reduces its depth until the slackened 

 current clogs itself by deposition of its load. 

 The built-up bed becomes higher than the 

 adjacent alluvial plain, and the stream takes a 

 new course. Before the assumption of the 

 new course the banks are overtopped by 

 shallow distributaries which deposit their loads 

 on the banks, thus building them up, until the 

 stream is made to flow on a sort of elevated 

 conduit; and when the main body of water at 

 last leaves this pathway, it is apt to start its 

 new course with a steepened slope and scour 

 for itself a relatively narrow channel. 



The building up of the bank by deposition 

 from overflow is more pronounced in the pres- 

 ence of vegetation. The ridge thus created is 

 called a natural levee. Its crest separates the 

 channel from the flood plain and delimits at 

 flood stage two provinces in which the condi- 

 tions affecting transportation are strongly con- 

 trasted. In both provinces the general slope 

 of the water surface is the same, but the broad 

 sheet covering the plain has so little depth that 

 its currents are sluggish. Between the banks 

 are the normal channel depths and currents, 

 and transportation is active, alike by traction 

 and suspension. Beyond them transportation 

 is effected almost wholly by suspension, and 

 the coarser particles of the suspended load are 

 deposited. As the flood subsides the lateral 

 sheets of water are returned to the main chan- 

 nel by a draining process which involves the 

 making and maintenance of small channels 

 within the plain. 



When the channel of a river is fully adjusted 

 to the discharge the same load is transported 

 through each section. All sections are then 

 equally adapted to transportation, though in 

 different ways. The most symmetric has a 

 wide space at the bottom devoted to traction. 



