APPLICATION TO NATURAL STREAMS. 



223 



The least symjnetric has a relatively narrow 

 tractional space, but traction is there relatively 

 active. The partition of the load between 

 traction and suspension is not the same for the 

 two sections, the tractional load having the 

 greater range in the symmetric section and the 

 suspended load in the asymmetric. 



There is reason to believe that the sectional 

 area is about the same in different parts of an 

 adjusted channel. At low stages, when form 

 is least adjusted to discharge, the sectional area 

 is much larger for the asymmetric sections. 

 At higher stages the contrast is less, and the 

 greater area may be associated with either type 

 of section. It is also true, if attention be re- 

 stricted to the channel proper and the expan- 

 sions over flood plains be excluded, that the 

 variations in width from point to point of an 

 adjusted channel are not of large amount. If 

 it were strictly true that both sectional areas 

 and widths are equal in different parts of an 

 adjusted channel, it would follow (1) that mean 

 depths are equal, and (2) that form ratios are 

 equal, provided form ratio be defined as the 

 ratio of mean depth to width. Such a generali- 

 zation, while crude and doubtless subject to 

 important qualification, nevertheless warrants 

 the selection of mean depth rather than maxi- 

 mum depth as the quantity to be used in ap- 

 plying the conception of form ratio to rivers. 



Assuming the generalization as an approxi- 

 mation to the actual fact and connecting with 

 it the fact that all sections of an adjusted river 

 are equally efficient for transportation, we are 

 able to make a general application .of the 

 laboratory results on optimum form ratio to 

 rivers. The ratio of mean depth to width in 

 alluvial rivers, as a class, is very much smaller 

 than in the laboratory examples by means of 

 w.hich the optimum ratio was discussed in 

 Chapter IV. It is so much smaller that the 

 range of form ratio for alluvial rivers overlaps 

 but slightly the range observed in the labora- 

 tory. This disparity indicates, though without 

 demonstrating, that the form ratios of the 

 rivers are less than the optimum, and that their 

 tractional capacities would be greater if they 

 were narrower and deeper. As the optimum 

 ratio is the one which enables a stream to 

 transport its load with the least expenditure of 

 head, it is probable that the slopes of most 

 alluvial rivers can be lessened by artificially 

 reducing their widths. 



THE SUSPENDED .LOAD. 



In speaking above of the transfer of load from 

 traction to suspension no consideration was 

 given to capacity for suspension. Certain stu- 

 dents of rivers, comparing discharges or veloci- 

 ties with the percentage of suspended material 

 and finding a rough correspondence, have in- 

 ferred that suspended load is a function of 

 velocity; others, giving attention to conspicu- 

 ous examples of noncorrespondence, have in- 

 ferred that the suspended load depends only 

 on the supply of suitable material. There is a 

 taeasure of truth in both views, and their diver- 

 gence is largely to be explained by considera- 

 tions connected with competence. 



The subject is illustrated by observations on 

 the suspended load of Yuba River. Its water 

 was sampled during flood stages, in 1879 ' at 

 Marysville, a load of 0.35 per cent, by weight, 

 being found when the discharge was estimated 

 at 26,000 ft. 3 /sec., and a load of 0.42 per cent 

 when the discharge was 18,000 ft. 3 /sec. In 

 1906, at a time when the discharge was esti- 

 mated at 33,000 ft. 3 /sec., samples were taken 

 at the same place and the load was found to 

 be only 0.065 per cent. At the time of the 

 earlier samplings hydraulic mining was in full 

 operation in the basin of the Yuba, and the 

 suspended load consisted chiefly of clay com- 

 ponents of the auriferous gravels, artificially fed 

 to the stream. In 1906 there was little hy- 

 draulic mining and the suspended load con- 

 sisted of material washed from the surface of 

 the land by rain. 



At low stages in 1906 the water at Marysville 

 was clear, but in 1879 a sampling when the 

 discharge was 510 ft. 3 /sec. gave a load of O.S<5 

 per cent. The fact that the river's load in 1879 

 constituted a higher percentage at low stage 

 than during flood is explained by the considera- 

 tion that the turbid tributaries from the mines 

 were less diluted by other tributaries at low 

 stage than at high. 



If we assume, first, that 0.065 per cent, ob- 

 served at flood stage in 1906, represents the 

 normal tribute from other sources than mining, 

 and, second, that the mines contributed the 

 same total amounts at low and high stages in 

 1879 (discharges being 510 and 18,000 ft. 3 /sec.), 

 and if we base on these assumptions a compu- 



i Manson, Marsden, Report of determinations of sediment held in sus- 

 pension, etc.: California State Engineer Report, 1880, Appendix B. 



