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0022~_2 . SECTION VI - PRESENT CHANNEL CONDITION 17. FACTORS CONSIDERED During the investigation, the problem of channel deterioration was presented to the Missouri River Division Board of Sediment Con- sultants to inquire if similar problems had been encountered else- where and to solicit suggestions on various phases of the investigation, especially the correlation with the sediment transport characteristics of the stream. Although the Republican River may be considered a typical, sand-bottom meandering stream with a moderately heavy sediment load, a comparable channel deterioration situation does not appear to have been encountered and rectified elsewhere. Therefore the changes which have occurred during the past decade, as compared with the natural stream regimen, were studied as a basis for project formulation. The investigation covered streamflow variations, sediment loads, the nature of the bed and bank materials, the effects of reservoir operation, channel configuration and hydraulic efficiency, and the probably efficacy of an improvement project. A detailed discussion of the study is given in Appendix IV: Hydrology and Hydraulics. 18. STREAMFLOW AND SEDIMENT TRANSPORT ~. Streamflow. As stated previously, the Republican River has had a history.of extremes in the rate and amount of streamflow. With these fluctuations, there have been changes in the channel shape and capacity. In past years, the shifting channel has devel- oped sandbars and islands, and during low-flow periods these have supported a growth of willows and other vegetation. Then, subsequent flood flows have denuded the bars and islands and even eroded them in the meandering process. One result of these cycles has been a fluctuation in the stage-discharge relationship, as the bankfull capacity of the river increased and decreased. However, during the past decade, flood flows were practically eliminated by reservoir operation; the total runoff was diminished by the drought; willows, cottonwoods and other vegetation became established within the chan- nel, and the obstructions reduced the bankfull capacity of the channel. Reservoir regulation will not only control floodflows at the damsites and prevent all but residual flood damage, but also will eliminate the flushing effect of major floodflows. As a result of the curtailment of flushing, the deteriorated channel may be expected to persist unless steps are taken to reestablish all or part of the former capacity. k. Sediment load and bed material. Streamflow and sediment. load are closely related. A program of suspended sediment sampling has been maintained on the Republican River since before the reser- voir construction. On a few occasions in the past, samples have . 23