Laserfiche WebLink
PARSONS <br />reduced the sediment supply to the Platte River downstream. ... The reductions in <br />annual peak floods, annual river flow, and sediment supply resulted in a significantly <br />narrower river channel, and vegetation colonized areas of the formerly active river <br />channel. In general, river channel widths have [been] reduced to about one -fifth or <br />less of the former historic channel. The reductions in annual flood peaks alone would <br />account for a large portion of the channel narrowing; however, the reduction in <br />sediment supply and the growth of dense riparian vegetation have also played a <br />significant role. A simple reduction in annual flood peaks would result in a narrower <br />river channel, but still leave the channel in a braided condition with a bed of fine <br />sand. However, the large decrease in the sediment supply resulted in a few feet of <br />river -bed erosion across separate subchannels. Through the selective erosion of finer <br />sediment particles, the erosion process also resulted in a coarser sediment size on the <br />eroded river bed. The vertical incision, over portions of the formerly active river <br />channel, also aided in the abandonment of the remaining wide, river channel. Once <br />portions of the former river bed degraded, river flow were more frequently contained <br />within the narrower, but deeper, channel. Thus, river flows were not as frequently <br />available to mobilize sediments of the formerly wide and higher river channel. With <br />fewer frequent flows to mobilize these sediments, the remaining portions of the <br />formerly wide river channel were ideal for colonization by riparian vegetation ... " <br />(Murphy and Randle, 2001b, p.2) <br />Purpose of "A"- Series Investigations <br />The five investigation tasks (Tasks Al through AS) included in Issue Category "A" — <br />Investigations to Evaluate Channel Narrowing or Deepening -- were intended to address <br />questions associated with the nature and causes of the evolution of the morphology of the <br />Platte River, from its supposed "pre- development" planform to the channel conditions of <br />the present. The specific issues that were examined in Tasks Al through A5 included <br />possible existence of geomorphic controls on the morphology of the Platte River; the <br />nature of historical and current sediment supplies to the Platte River system; compilation <br />and evaluation of a comprehensive database of Platte River cross - sections; and <br />development and application of a predictive tool to determine what active width of river <br />channel could be maintained with any set of prescribed flows. <br />Purpose and Scope of Task Al <br />Narrowing of the Platte River channel has been attributed by the authors of the draft <br />report entitled "Platte River Channel: History and Restoration" (Murphy and Randle, <br />2001a) to conversion from a braided system to a transitional form, or more specifically, to <br />an anabranched pattern. This conversion is alleged to have been a direct consequence of <br />reductions in peak flows and in sediment transported by the river, due to construction of <br />retention and diversion structures for water development (e.g., Murphy and Randle, 200a, <br />p. 4ff). Classical literature on braided streams (e.g., Thornbury, 1954; Ruhe, 1975; Ritter, <br />1978), and specific research such as work by Schumm. and Lichty (1963) and Schumm <br />(1981), indicates that braided streams are unstable, that they are transitional, that they <br />require a perpetual oversupply of sediment, that some reaches can be braided while others <br />are not, and that they respond to minor shifts in geomorphic thresholds that can be <br />induced by factors other than a coarsening of sediment. Some researchers (e.g., Lane <br />[1955]) suggest that sediment gradation may not be relevant at all. <br />-3- <br />SAES \WP\PR0JECTS\3- States\A1 Final Tech Memo.doc <br />