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<br />OJ17S7 <br /> <br />described below. The term channel maintenance appears in a number of places in the literature <br />on river channels (Carling, 1988; Dunne and Leopold, 1978; Newbury, 1993; and Petts. 1994). . <br /> <br />Generally, the terms "maintenance" or "maintain" imply persistence over time. And. <br />with respect to stream channels, persistence is defined with regard to the concepts of equilibrium. <br />grade, and steady state, that characterize the size and shape of natural alluvial river channels. By <br />definition, a stream channel is maintained if it retains its capacity, over time, to carry the water <br />and sediment delivered to it A set of processes responsible for transpon of sediment and water <br />are, therefore, required to maintain the channel. Thus, as will be described below. channel <br />maintenance must account for fluctuations in flow and in the quantity and characteristics of <br />sediment and debris delivered to the channel. While sediment may accumulate in the channel <br />during one period of time, accumulated sediments are removed in subsequent periods. The <br />hydrologic regimen at most places consists ofa range of flows of varying magnitude. Because <br />low flows in gravel bed rivers are unable to move the gravel sizes which constitute the primary <br />determinants of the channel size and form in the Upper Snake River system, higher flows that <br />transpon these gravels are essential to maintaining the channel. <br /> <br />2. Adverse Consequences <br /> <br />The fact that alluvial river channels adjust to the imposed water and sediment regime <br />means that as water is abstracted or divened from a channel, the sediment transport capacity and <br />competence is often reduced, as is the ability of the river to resist encroachment or establishment <br />of vegetation within the channel. The impact on stream channels of depletion of flows by <br />irrigation diversions has been recognized for hundreds, perhaps thousands, of years. More <br />recently, such effects have been documented under many circumstances, some of which are <br />discussed below. The magnitude of the channel response reflects the interaction of water <br />discharge and sediment discharge, the composition of bed and bank materials, and the <br />longitudinal gradient (slope) of the river channel. <br /> <br />Since antiquity, large and small reservoirs have been constructed throughout arid and <br />semi-arid regions to provide water for irrigation. Though modem structures may serve multiple <br />purposes including flood control, hydroelectric power generation and navigation, storage for <br />irrigation diversions remains a primary purpose. Diversion of water results in significant <br />alteration of river channels downstream from these storage and diversion structures. The nature <br />of these alterations varies depending upon the ways in which the regimens of water and sediment <br />are altered, the panicle size distribution of bed materials, river gradients, and the topography and <br />distribution of bedrock in the valley. Emphasis is given here to those conditions in which <br />abstraction or diversion of flow, primarily for irrigation, has resulted in channel narrowing. <br />Dams operated for hydropower, flood control, or navigation coupled with the diversity of <br />sediments and geology encountered in alluvial channel environments can, of course, produce a <br />range of channel responses. <br /> <br />7 A full bibliography of literature cited is included beginning at page 107. Additional references to <br />documents relied upon to support these claims are contained in Appendix 24. <br />United States' Expert Report Disclosing Meltmologies for Quantilica1ion of Organic Ad Claims Consolidated Subcase No. 63.25243 <br /> <br />7 <br />