Ch. 20] MEASUREMENT OF SEDIMENTATION IN STREAMS 349 



stream reservoir. The downstream reservoirs then receive a certain 

 amount of material from each upstream reservoir and in addition the 

 contributions from the local drainage areas directly tributary to each 

 reservoir. 



MEASUREMENT OF SEDIMENTATION IN STREAMS 



Accurate measurements of sediment loads in natural streams are 

 essential to estimating the useful life of any proposed reservoir. Meas- 

 urement of volume of silt deposited in a reservoir is important to de- 

 termine the rate at which sediment is being deposited and the locations 

 where the deposits are being made. The latter also provides a check on 

 the computations that may have been made on the basis of measure- 

 ments of suspended sediment loads before the construction of the reser- 

 voir. 



The science of accurate measurement of sediment loads in streams 

 is relatively new. Until some ten years ago engineers throughout the 

 country who were concerned with sedimentation problems largely de- 

 vised their own equipment for sampling streams and carried on sam- 

 pling programs, each according to his own ideas. This led to both 

 inaccuracy and non-uniformity of results with little basis for com- 

 parison or dependable utilization. To remedy this situation, govern- 

 ment agencies interested in sediment problems formed a group that 

 sponsored a research project at the Iowa Institute of Hydraulic Re- 

 search. This project notably developed standard sediment samplers 

 for use under different conditions of stream flow. These samplers have 

 since been adopted and are being used by practically all government 

 agencies. The series of publications issued by this project is a monu- 

 mental contribution to the science of sediment measurement (Iowa 

 Institute of Hydraulic Research, Nos. 1-5, 7-9). Figure 1 shows one 

 of these samplers. 



In the measurement of the suspended-sediment load of streams, the 

 program of measurement must be carefully planned and executed. The 

 volume of sediment carried varies with stream flow but does not bear 

 a constant relationship to the quantity of water flowing. In low water, 

 the sediment volume is usually small, but during rises in the stream the 

 amount of sediment increases rapidly. Ordinarily the peak-sediment 

 load will occur prior to the peak stream flow discharge. To determine 

 the volume of sediment carried during rises in stream flow, rather fre- 

 quent sampling must be carried out. The amount of the sediment will 

 depend on the season of the year, the condition of the land in the water- 



