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PARSONS <br />• DRAFT TECHNICAL MEMORANDUM <br />• <br />A2 - EVALUATION OF HISTORICAL AND CURRENT SEDIMENT <br />SUPPLIES <br />INTRODUCTION <br />One of the key questions to be addressed in understanding geomorphic and vegetation <br />issues on the Platte River is the extent to which sediment supply affects channel <br />morphology and vegetation. Changes or imbalances in sediment supply can result in <br />aggradation or degradation, which affects channel morphology (shape and changes in <br />shape of the channel) and the response of vegetation. In discussing this issue, Simons & <br />Associates (2000) cited Leopold, et al. (1964), <br />The shape of the cross- section of a river channel at any location is a <br />function of the flow, the quantity and character of the sediment in <br />movement through the section, and the character and composition of the <br />materials making up the bed and banks of the channel. In nature, the last <br />will usually include vegetation. <br />The issue of the effect of sediment supplies on channel morphology and vegetation <br />can be separated into two main questions. First, how has the channel geometry itself <br />changed? Second, how has any channel geometry change subsequently affected <br />vegetation? <br />The evaluation of historical and current sediment supplies focuses first on the <br />availability of data and then turns attention to addressing the fundamental questions <br />discussed above. <br />EVALUATION OF HISTORICAL AND CURRENT SEDIMENT SUPPLIES <br />Several key comments have been compiled regarding the evaluation of historical and <br />current sediment supplies on the Platte River. The initial comments focus on the <br />availability of sediment data and then comments are made on the evaluation of issues. <br />1. Sediment data on the Platte River and principal tributaries were primarily <br />collected in the early 1930s, with another focus of effort in the 1970s- 1980s. <br />Some general sediment data collection also occurred over the decades from the <br />1940s through the present. Most of the data collection was suspended sediment <br />data and some limited bed material data. <br />2. Bed material data collected in 1931 showed that silt and clay sized particles <br />(<0.0625mm) were not found in any appreciable quantities in the bed (i.e., <3%). <br />Recent data show that an even smaller percentage of the bed consists of silt and <br />clay sized particles. <br />S:\ES \WP\ PROJECTS \3- States\A2- revised.doc <br />-1- <br />