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<br />0018J2 <br /> <br />Vegetation also plays an imponant role in forming the stream channel. both through the <br />binding effect of root systems on soil in the banks. and by diminishing flow velocity during <br />overbank flows. Changes in streamside vegetation immediately adjacent to the stream channel <br />have the most dramatic impact on channel processes and morphology (US Forest Service. 1997). <br />At the same time then, while vegetation contributes to the structural stability of the channel by <br />reinforcing the bank material (Smith, 1976), encroachment of vegetation can reduce the capacity <br />or conveyance of the channel (Williams and Wolman, 1984). <br /> <br />Simply put, a complex set of processes involving water, sediment and vegetation are <br />responsible for the creation of the natural channel - a three dimensional geometric form, that is <br />characterized by a shape or form and an area of cross section. In an alluvial channel, the shapes <br />and areas of cross section are the result of processes of erosion and deposition as flows of <br />varying magnitude and frequency deposit and remove sediments of varying sizes over time. <br />Deposition may occur within the developing channel or, during higher flows, where sediment <br />laden water over-tops previous deposits. In regions where moisture is available to suppon plant <br />growth, seeds germinate and roots expand to colonize fresh deposits of sediment. Deposition of <br />sediment and development of vegetation are inextricably associated. With time, vegetation such <br />as sedges and tree roots help to bind the sediment, adding strength to the channel bank materials <br />and providing resistance to erosion and contributing to channel stability. This role of vegetation <br />is less significant on very large rivers where the force of the water and depth of flow can more <br />easily overcome the binding strength of tree roots. <br /> <br />3. Features of Alluvial Channels <br /> <br />As stated previously, alluvial channels are free to adjust their form over time in response <br />to quantities of water and sediment. The term adjustment means the mutual interaction of <br />channel features such as width, depth, slope and rugosity (physical roughness or configuration of <br />the channel boundaries) in association with the velocity and stress of the moving water and <br />sediment. A simple manifestation of the process of adjustment is captured in Schumm's (1977, <br />p. 180) observation, <br /> <br />... the largest river in the world today is the Amazon ... At <br />Obidos, where the U.S. Geological Survey has made <br />discharge measurements, the channel is 200 feet deep and <br />about 7500 feet wide. "In contrast, the Mississippi River, <br />the world's fourth largest, is about 2000 feet wide and 60 <br />feet deep at Vicksburg. These channels are enormous, as <br />they need to be to transpon an average flow of about 5.5 <br />million and 0.6 million cfs respectively. <br /> <br />Many studies have generalized this observation that stream channels are larger where <br />larger volumes of flow occur (Leopold, 1994). This relationship has been demonstrated for <br />Idaho stream channels in the Upper Salmon River basin by Emmett (1975). These concepts are <br /> <br />United Slates' Expert Report Disclosing MeItmologies for Quantification of Organic Act Claims ConsoIldatad Subcase No. 63-25243 <br /> <br />12 <br />