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<br />00)798 <br /> <br />Large diversions on a number of channels on the Great Plains have resulted in narrowing <br />of the channels to one quarter of their width prior to diversion. In some instances, where little <br />flow remains in the channel, the reduction in width is even more pronounced, and post-diversion <br />widths of 1/4 to 1/10 of the pre-diversion widths have been reponed (Williams and Wolman. <br />1984, pp. 42-49). <br /> <br />In most instances channel narrowing is associated with establishment of vegetation. <br />Vegetation impedes flow and traps sediment, enhancing deposition within the channel adjacent <br />to the banks. The combination of a shrunken channel and dense vegetation significantly reduces <br />the conveyance capacity of the channel; that is, its ability to transmit water. This effect was <br />illustrated on the Republican River below Harlan County Dam when it was found that planned <br />flow releases that were formerly transmitted below the bankfull stage overflowed the much <br />reduced channel, flooding adjacent farmland (Williams and Wolman, 1984). Diversions on Rush <br />Creek in eastern Califomia, have reduced streamflow and groundwater levels resulting in the loss <br />of riparian vegetation (Kondolf, 1990). The type of response to water withdrawal is influenced <br />by the local geomorphic conditions, such as width of the floodplain (Harris et al,. 1987). <br /> <br />Additional examples of channel narrowing as a result of diminished flows are widely <br />documented: see e.g., Andrews (1986); Collier et al., (1996). Petts (1984) summarized the <br />effects of flow regulation on a variety of rivers outside of the United States. Additionally, <br />Schumm (1977, p. 159-171) summarized a wide range of examples of "river metamorphosis" <br />where channels have changed over time in response to changes in streamflow and sediment load. <br /> <br />Studies in the intermountain West demonstrate that small streams also become narrower <br />and shallower when large amounts of water are diverted from them. Wesche (1991) compared <br />the characteristics of mountain channels upstream and downstream from diversions in southern <br />Wyoming and northern Colorado. Wesche found that lower gradient mountain stream channels <br />respond to flow depletion by becoming narrower, shallower, and less capable oftransponing <br />former flows. Aggradation and encroachment of riparian vegetation also occurred at most of <br />these sites. These stream channels occurred at an average elevation of 8,600 ft and have an <br />average drainage basin area of about 21 mi2 (Wesche, 1991, Table 2). The streams studied by <br />Wesche are typical of many streams on lands managed by the U. S. Forest Service in the <br />intermountain West. Other measurements made by Wesche (1991) on foothill and basin streams <br />in southern and western Wyoming with similar geomorphic characteristics, also showed that <br />these channels significantly reduce their width (by 26 percent), depth (by 14 percent), area (by <br />32 percent), and conveyance capacity (by 55 percent) when significant streamflow diversions <br />occur. In mountain streams that were of higher gradient, Wesche (1991) did not find similar <br />changes due to diversion. Ryan (1994), who measured channel response on streams in Colorado, <br />also found that lower gradient, alluvial channels in mountain settings narrowed where significant <br />streamflow diversion occurred whereas steep gradient streams did not significantly change in <br />channel width. The sites where the Forest Service has claimed reserved water rights in the Snake <br />River Basin Adjudication are most similar to those for which Wesche (1991) and Ryan (1994) <br />found channel response to flow depletion. <br /> <br />United States' Expert Report Dlsdosing Meltmologies for Quantification of Organic Ad Claims Consolidated Subcase No, 63-25243 <br /> <br />8 <br />