234 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



experiments with sieve-separated grades of 

 debris fineness and competence were constants 

 with reference to discharge; but in a natural 

 stream, where the tractional load may have 

 great range in fineness, the mean fineness of the 

 load varies with discharge, and the reason of 

 its variation is that competence varies with 

 discharge. The two competences which limit 

 the range in fineness move up and down as 

 discharge changes, and the mean fineness 

 moves with them. Therefore the response of 

 capacity to discharge can not be considered by 

 itself. 



For convenience in analyzing the conditions, 

 let us assume first a discharge which is adjusted 

 to the details of channel form, to the deeps and 

 shoals. If, now, the discharge be increased, and 

 with it the whole system of velocities, trans- 

 portation will be everywhere stimulated, part 

 of the tractional load will join the suspended, 

 and the scouring of the deeps will bring into 

 the tractional load a greater proportion of the 

 coarser elements of the load. The mean fine- 

 ness of that load will be reduced, and the ca- 

 pacity, while enlarged by increase of discharge, 

 will be somewhat reduced by loss of fineness. 

 The increase with discharge will be less than if 

 the fineness were constant. 



If, on the other hand, the discharge be re- 

 duced, some of the coarser material comes to 

 rest, while finer debris is added from the 

 suspended load. So the reduction of capacity 

 from diminished discharge is qualified by the 

 effect of increased fineness. But before the 

 change in discharge has gone far the deeps 

 become pockets for the reception of deposits, 

 and traction is restricted to the intervening 

 shoals, where it causes erosion. The erosion 

 is selective, leaving an ever-increasing assem- 

 blage of residuary coarse material, which tends 

 to protect the finer. The current on the shoals 

 no longer obtains a full supply of the material 

 for which it is competent, and the load and 

 capacity part company. Or we may say that 

 as the erosion of the shoals progresses the mean 

 fineness of the accessible debris is reduced until 

 a grade is reached for which the current is not 

 competent. In either case the decadence of 

 traction follows a law which is not well repre- 

 sented by the discharge term of the laboratory 

 formula. . 



The above analysis postulates a wide range 

 and somewhat equable distribution of fineness 



in the debris of the stream bed, a condition not 

 always found. It might not apply, for ex- 

 ample, to a stream which drains a district of 

 friable sandstone and is therefore supplied with 

 nothing coarser than sand. Nor would it apply 

 well to a stream supplied with very coarse and 

 very fine debris but not well supplied with 

 intermediate grades. 



In most alluvial streams, and probably in all 

 meandering streams, the work of traction which 

 is accomplished on the shoals at low stage and 

 midstage is almost negligible in comparison 

 with the high-stage traction. Not only is the 

 rate of traction slow, but the field of traction 

 is restricted. If a single formula will not fit 

 both low and high stages, the one adjusted to 

 high-stage variations will have the greater 

 practical value. 



Yet another consideration enters here, and 

 one of peculiar importance. When discharge is 

 reduced, and the competence of the current for 

 traction is thereby changed, the coarse material 

 eliminated from the tractional range ceases to 

 be transported; but when discharge is increased, 

 and the competence of the current for suspen- 

 sion is thereby changed, the fine material 

 eliminated from the tractional range continues 

 to be transported. It is, in fact, transported 

 more rapidly, so that a greater amount passes 

 a given section each second. For most or all 

 practical purposes the change in mode of trans- 

 portation is of no moment, and those purposes 

 would be served by a formula which should 

 include the material shifted and ignore the 

 change in mode. In the system of reactions 

 set up by change of discharge the two modes 

 of transportation are so interwoven, in fact, 

 that the practical discrimination of the sus- 

 pended and tractional loads is impossible. 

 Even in the laboratory experiments devised 

 specially for the study of traction a certain 

 amount of interplay was tolerated, for tem- 

 porary suspension appeared over the crests of 

 some of the antidunes and also in the bends of 

 the crooked channels. 



If the purely tractional point of view is to 

 be exchanged for another, what shall be sub- 

 stituted ? One natural suggestion is to include- 

 in a single view the entire load, suspended and 

 tractional; another to include along with the 

 tractional only that part of the suspended load 

 which for part of the time is tractional also. 

 That which would be included in one view and 



