64 einstein-johnson. SEDIMENT TRANSPORT [Ch. 3 



and its rate, therefore, has no significance in determining the stability 

 of the stream channel itself. As it represents usually a large part of 

 the total load (90 percent in the Rhine River above Lake Constance) , 

 and as it is usually deposited at a lower volume-weight than the bed- 

 material load (Iowa University, 1943), it is the most important part 

 of any lake or reservoir deposit. 



Several government agencies which are interested in the manage- 

 ment of reservoirs, or in the rates of erosion, are today engaged in a very 

 active program of sediment sampling. These agencies use suspended- 

 load samplers for this purpose and thus sample the total load; thus 

 some of the bed-material load may be included in estimates of the 

 sediment load. All information so obtained is assembled by the Inter- 

 Agency Committee on Sedimentation, which meets regularly in Wash- 

 ington, D. C, and which is responsible for the publication of the data. 

 With this committee serving as a clearing house, duplication of effort 

 within the government service is eliminated. 



Some attempts have been made to map the sediment productivity 

 of different areas in the country (Brown, 1945), but it is well known 

 today that even the rate of wash load (measured in tons per year per 

 square mile of drainage area) varies inversely with increasing size 

 of the watershed area. The reason for this inverse relationship may 

 be found in the deposition of wash-load material in overbank and dead- 

 water areas. 



Bed-Material Load 



Basically the movement of bed-material load behaves differently 

 from that of the wash load of a stream. In the past it has been fur- 

 nished, and usually it still is being furnished, by the watershed at a 

 rate that is higher than the capacity of the channel to transport the 

 material. A certain percentage of this load has been and is still being- 

 deposited along the channel, reducing the actual transport down to 

 the capacity load. If for any reason the sediment inflow into a given 

 reach of a channel is smaller than the capacity load, the flow imme- 

 diately scours some of the formerly deposited sediment from the bed 

 and keeps the load constantly at capacity level. Bed-material load is 

 thus always transported to capacity. For a given channel reach, the 

 flow conditions usually can be described in terms of the stage or of 

 the discharge. It must be expected, therefore, that the capacity to 

 transport bed material can be given also in terms of the discharge. In 

 this respect the relationship which gives the capacity of the stream to 

 transport the various sediment sizes of the bed material at different 

 flows is termed the "sediment function" (Fig. 1). 



