This report contains results of observations that were made from the time when a natural constriction was formed in the Niobrara River near Valentine, Nebr., to the time when it was widened by high flows. After the constriction was widened, insufficient turbulence was developed to suspend most of the sediment. The report gives information on the amount and characteristics of the sediment that moves as unmeasured load at normal sections of the stream near the constriction and at the measuring sections at the Sparks gaging station.
Lateral distributions of depth, velocity, and concentration are shown for the sections where sediment discharge was measured. In addition, vertical distributions of concentration, velocity, and percentage of particles coarser than 0.25 mm are shown for the contracted section. The vertical distributions seem to indicate that the total sediment load of the stream was usually in suspension at the contracted section. Exclusive of the period during the winter when upstream reservoirs. were not flushed, the measured discharge of suspended sediment at the normal sections near the contraction averaged 47 percent and at the Sparks gaging station 41 percent of the measured sediment discharge at the contracted section.
Particle sizes of suspended sediment and bed material are tabulated, and some size distributions are plotted. In conformity with bedload theory, the computed particle sizes of the unmeasured load were slightly smaller than the average particle sizes in the samples of bed material.