Ch. 3] WASH LOAD 63 



TYPES OF LOAD TRANSPORTED BY FLOWING WATER 



In attempting to describe the various relationships between the 

 sediment rate and the flow of the fluid which produces the transport 

 of the material, it is always important to remember that every par- 

 ticle moving through a reach of a stream must satisfy two conditions: 

 (1) It must have been eroded or otherwise have been made available 

 in the watershed upstream from the reach. (2) It must have been 

 moved down to the reach and through it by the flow. The rate at 

 which each kind and size of particle is moving, therefore, is limited 

 to its actual rate by either the first or the second condition. Needless 

 to say, the laws derived from the two conditions restricting the rate 

 of flow are inherently different. It has also proved to be helpful to 

 introduce different terms for the two parts of the sediment load of a 

 stream; thus that part of the load the rate of which is governed by its 

 availability in the watershed is termed "wash load," and that part 

 which is governed mostly by its ability to be moved in the stream 

 channel is termed "bed-material load." 



Wash Load 



The rate at which the various sizes of the wash load of a stream are 

 moving through a reach depends, according to definition, only on the 

 rate with which these particles become available in the watershed and 

 not on the ability of the flow to transport them. This may be inter- 

 preted to mean that, if the rate of flow is greater, more particles of 

 given size can be transported if they are available. From this, again, 

 one may conclude that wash-load particles are not deposited in the 

 stream channels on their way from the place of erosion down to a point 

 of measurement. They travel with the same velocity as the flow. The 

 rate of transport of the wash load may be found to depend upon the 

 different factors determining erosion, such as intensity of precipitation, 

 rain-drop size, soil type, vegetal cover, and previous soil conditions, 

 but not upon the discharge at the reference section, as has been shown 

 by Vetter (1937). As these different parameters effecting erosion are 

 very complex, and as the value of each parameter is not known for 

 most of the larger watersheds, the rates of wash-load transport usually 

 have not been analyzed in detail, but are given as an average annual 

 load as determined from either lake surveys (Eakin, 1936) or sus- 

 pended-load sampling programs. 



As the supply of the wash load never reaches the capacity of the 

 channel to transport it, it is never deposited in the main flow channel, 



