230 



TRANSPORTATION OF DEBRIS BY RUNNING WATER. 



per cent average reduction of velocity corre- cubic foot of discharge, or 0.52 per cent, by 

 sponds to an average load of 148.5 gm./sec. per weight, of tractional load. 



TABLE 80. Comparison of mean velocities of streams with and without tractional load. 



[Width, 1.00 foot.] 



THE TWO LOADS. 



The reaction of tractional load on velocity 

 affects primarily and chiefly the zone of salta- 

 tion, but there is also a general retardation of 

 the stream. The reaction of suspended load 

 reduces all velocities, including those of the 

 tractional zone. Thus the magnitude of each 

 load affects capacity for the other load, and it 

 also affects the conditions of competence for 

 suspension and competence for traction. 



As all parts of the load influence velocity, so 

 all parts influence the general slope of an allu- 

 vial stream, which is automatically adjusted 

 so as to give to the discharge the ability to 

 transport all the material, coarse and fine, 

 which is supplied. The adjustment is actually 

 made through the coarser material, for the per- 

 manent deposits of the stream bed are from 

 the tractional load. 



Along with the adjustment of slope goes an 

 automatic partition of the varied load into sus- 

 pensional and tractional. If the lower Missis- 

 sippi, for example, were to be supplied for the 

 future with only that part of its load which is 

 npw carried in suspension, it would so reduce 

 its slope that the slackened current would drop 

 a portion of that load and thereafter move it 

 by traction. If, on the other hand, the river 

 were to be deprived of the fine debris now car- 

 ried in suspension, it would so quicken its cur- 

 rent as to lift into suspension a portion of the 

 de'bris now carried by traction and would 

 adjust its slope in such way as to maintain the 

 partition of load. 



An exception to the general law of automatic 

 partition is found when the load has only small 

 range in fineness, and this was illustrated by 

 the artificial conditions of the laboratory; but 

 it is not known that natural streams illustrate 

 the exceptional case. 



This phase of river adjustment is well illus- 

 trated by Yuba River, which I studied in 

 1904-1908. Where it issues from the moun- 

 tains it carried a heavy load of coarse debris 

 with which it was building up its bed at the 

 edge of the Sacramento Valley. A dam thrown 

 across it in the region of deposition arrested, 

 the tractional load for a time and gave an 

 opportunity, by the aid of measurements, to 

 estimate its amount. There were also samp- 

 lings of the water and measurements of sus- 

 pended load. The tractional load consisted 

 mainly of gravel, with coarse sand and many 

 bowlders, and the suspended load during flood 

 included sand and finer debris. Fifteen miles 

 below, near the mouth of the river, the trac- 

 tional load consisted of sand, with rare small 

 pebbles, and only clay and silt were in suspen- 

 sion. Here, too, the load suspended at flood 

 stage' was estimated from a sampling of the 

 water. A computation based on the various 

 data indicated that at flood stages the sus- 

 pended load was approximately equal to the 

 tractional load at each of the two localities. 1 

 While the data for this estimate were imperfect 

 in many ways, they were nevertheless better 

 than any other with which I am acquainted. 



i A somewhat fuller statement may be found in Geol. Soc. America 

 Bull., vol. 18, pp. 657-658, 1898. 



