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<br />00"1793 <br /> <br />Reductions in stream flow may not only result in encroachment of vegetation and channel <br />narrowing, but reductions in flow may also decrease the capacity of a receiving stream to <br />transpon sediment contributed by tributaries. For example, on the Rio Grande River below <br />Elephant Butte Reservoir, reductions in flow due to irrigation diversions along with continued <br />delivery of sediment from tributaries resulted in diminished channel capacity and diminished <br />ability to transpon sediment. As the river bed aggraded, flows traversed the bottom land <br />damaging valuable agricultural land. The water table was elevated beneath adjacent floodplain <br />lands resulting in poor drainage and water logging of these same lands (Wolman, personal <br />observation, 1963). Collier el af., (1996, p. 33-34) summarizing Everitt (1993) noted that when a <br />small flood in 1942 encountered a valley without a well defined channel below F on Quitrnan. <br />Texas, "water fanned out over the old floodplain and sediment settled out." Bray and Kellerhals <br />(1979) repon a similar process in Canada where reduced flows on the Peace River are unable to <br />transpon gravels that continue to be contributed by the tributaries. <br /> <br />Channel response to altered streamflow also has been well-documented on large rivers <br />that flow in narrow valleys whose course is greatly affected by the delivery of hills lope-derived <br />debris. Schmidt el af.. (1995) showed that large floods depleted of sediment result in large-scale <br />erosion of riverside alluvial deposits in Hells Canyon of the Snake River. Grams (1997) showed <br />that reduced floods in Dinosaur National Monument resulted in channel narrowing, even in very <br />narrow canyons dominated by debris flows from tributary drainage basins. <br /> <br />C. Alluvial River Behavior: Concepts and Illustrations <br /> <br />1. Self-formed Channels <br /> <br />Over a period of time, a natural stream channel at a given location establishes a cross- <br />section and planform that reflect the quantity of water and the quantity and characteristics of <br />sediment delivered to it from the drainage basin as well as the imposed topography and local <br />geologic conditions. Neither the water supplied (discharge) nor the quantity and distribution of <br />sizes of the sediment load are delivered to the channel at a constant rate. All are subject to the <br />variations of weather and climate which dictate the magnitude, timing, and frequency of the <br />range of flows and sediment delivered to a given channel reach. Thus, the channel experiences <br />varying sequences of low and high flows depending on runoff from the drainage basin. Largely <br />due to the varying runoff, the sediment supplied from the landscape and from sediments stored in <br />and adjacent to the channel varies as well. <br /> <br />Alluvial channels, composed of sediments deposited by the river itself, are free to adjust <br />their form, and to a lesser extent, their gradient. Because of this, an alluvial river develops a <br />cross section, and shape, over time, reflecting the quantities of water and sediment and the sizes <br />of sediment brought to it. While this form, in any given period, responds to the variability of <br />flow and sediment, observations of natural alluvial channels demonstrate that the channel, over <br />time, develops a cross-sectional form reflecting an integration of these temporal variations. <br />Thus, despite considerable variability, natural alluvial channels subject to larger flows <br />characteristically have greater widths and depths than those carrying smaller flows. In general, <br /> <br />United Slates' Expert Report Disclosing MeltlOdologies for Quantification of Organic Ad Claims Consolidated Subcase No. 63-25243 <br /> <br />9 <br />