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<br />been taken which were of abnormally high concentration and which <br />reflected the transverse transport of sediment from a caving bank <br />on one side of the river to an accreting bar on the other side. <br />In addition to the normal suspended sediment transport, there has <br />been appreciable bedload movement as indicated by the changes in <br />channel configuration. Since closure of the dams, there has been <br />a large reduction in the sediment transport, the load being made <br />up of tributary contributions and material picked up from the bed <br />and cbannel walls. Below the reservoirs, surveyed ranges have <br />.been established to observe the degradation effect of the clarified <br />reservoir effluent. In connection with this investigation, samples <br />for size analysis were taken of the streambed, the islands and <br />high bars, and the channel banks. These samples showed the presence <br />of very little coarse material, which would serve to armor the <br />degraded streambed. The valley alluvium is primarily fine sand <br />with horizons of silt and silty soil in the islands, bars, and <br />banks. Practically no clay was encountered, which might serve as <br />a binder to consolidate the sandy alluvium against erosion. This <br />is particularly significant in considering the possible erosion <br />of denuded bars and islands, once the wilQows are removed, and in <br />designing pilot channel cutoffs. <br /> <br />. <br /> <br />19. THE EXISTING CHANNEL <br /> <br />!. Channel configuration. Changes in the size and shape of <br />the river channel since closure of the dams have been observed at <br />the degradation ranges. In 1949, twenty-six ranges were laid out <br />downstream from Trenton Dam, and in 1951, fourteen ranges were <br />surveyed below Harlan County Dam. Subsequently, these ranges have <br />been resurveyed and the changes in the shape and cross-sectional <br />area have been noted. The observed changes have not been consistent <br />from one range to the next, indicating that influences other than <br />streambed degradation have been operating to produce the existing <br />channel. These include a shifting in the low-water channel and the <br />development of bars and islands. Below Trenton Dam, there has been <br />a slight net loss in cross-sectional area. Below Harlan County <br />Dam, most of the sections showed an increase between 1951 and 1957; <br />but a resurvey of the ranges in 1962 showed little overall change <br />during the last few years. In neither reach, however, could the <br />loss of bankfull capacity be explained by the change in shape or <br />reduction in size of. the channel. To investigate the existing <br />channel in more detail, a test reach was established and studied <br />below Harlan County Dam. <br /> <br />~. Test reach investigation. <br /> <br />(1) Description of reach. <br />analysis was a typical reach of the <br />Bloomington and Franklin, Nebraska. <br /> <br />The area selected for detailed <br />river, 5_1/2 miles long, between <br />See plate 11 of Appendix I: <br /> <br />. <br /> <br />24 <br />