Fluviatile and Marine Gravels. 491 



transported and arranged in a manner determined by the 

 resultant effect of all the factors conditioning the work of run- 

 ning water. The method of transportation by suspension need 

 not be considered in the present discussion, for pebbles make 

 their way down stream by saltation, rolling and sliding. For 

 this method of transport, entrainement* of the French en- 

 gineers, the basal portion of a stream obviously mast be utilized, 

 hence the action of water at the bottom of a current and the 

 nature of the bed of a stream are matters of prime significance. 

 Unfortunately these are features with which we are relatively 

 unfamiliar and which are least susceptible to direct observation. 

 Since, under the influence of a current pebbles advance in 

 large part by saltation, the ability of a stream to transport 

 gravel is fundamentally affected by slope and discharge, two 

 factors whose relations to capacity are, as shown by Gilbert, 

 essentially parallel, with the exception that capacity is slightly 

 more sensitive to changes of slope than to fluctuations in 

 volume.f Another significant result of the Berkeley experi- 

 ments is the demonstration of the fact that fragments of a 

 single size are moved by river currents less freely than mixed 

 debris, and that if fine material be added to coarse, not only is 

 the total load increased but a much greater quantity of coarse 

 material may be carried.;); 



For a complete analysis of gravel production and gravel dis- 

 tribution, the factors concerned in stream traction which require 

 evaluation are the amount and classification of the load, the 

 discharge, the stream profile, the character of the bottom, 

 and the valley form. The last-named factor includes form ratio 

 as defined by Gilbert,g and the alignment of the channel. The 

 relative quantitative effects of these various factors as operative 

 in natural streams remains to be determined, but for prelimi- 

 nary physiographic studies only their qualitative influences, con- 

 ditioned by climate and topographic age, need be considered. 



Distribution and Preservation of River Gravels during a 

 single Physiographic Cycle. 



During the course of a physiographic cycle the landscape 

 passes through a brief period of rapidly increasing relief, 

 followed by a period characterized by strong relief and great 

 variety of form. This stage is followed by a period of rela- 

 tively rapid decrease in relief, replaced in turn by a very long 



*The well chosen term " traction " is suggested by Gilbert as an English 

 equivalent of entrainement. All hydraulic transportation may thus be 

 classified as suspension and traction. 



+ Gilbert, U. S. Geol. Survey, Prof. Paper 86, pp. 145, 149, 1914. 



JLoc. cit., p. 11, 184. 



§U. S. Geol. Survey, Prof. Paper No. 86, p. 36, 1914. 



