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I I I I I I I I I I I I I I I I I I I reaches unaffected by planform-induced backwater at higher flows and must have been transported through the upstream pool in 1995. These sediment supply reaches are located upstream of the deeply I incised meandering reach that characterizes the Yampa River downstream of Harding Hole (RM 20). These result~ therefore, that a rather moderate discharge of relatively long duration may be sufficient to maintain the spawning bar at Mathers Hole. 2.2. Cleopatras Couch (RM 16.5) At Cleopatras Couch, the five original cross sections surveyed in 1983,1984,1991, and 1993 were resurveyed (XS1-XS5) and three new cross sections were surveyed and monumented upstream of the spawning bar (XS6, XS7, XS8) (Figure 2.5). Cross Sections 6 and 7 are located in the pool upstream of the bar, and XS8 is located at the head of the next upstream riffle. The water's edge at a main channel discharge of 516 cfs for all of the chute channels and the main channel were also surveyed, to better define the planform characteristics of the spawning bar. Discharge measurements were made in the left and right chute channels (Figure 2.5). Six Wolman Counts were made at locations where the sediments had been previously sampled (Figure 2.5). The gradations from the 1995 Wolman Counts are shown on Figure 2.6. Comparison of the 1995 cross section plots with those from 1993 indicates that there may have been some lowering of the thalweg at each of the cross sections (Appendix B). However, the most significant changes since the initial survey in 1983 occurred at XS1, XS2, and XS3 where the chute channel widened and, to some extent, the thalweg lowered. It appears, therefore, that channel widening has occurred following the extreme runoff events of 1983 and 1984 when significant deposition occurred on the bar and in the chute channels. The widening trend indicated by the cross section surveys supports previous qualitative assessments of the changes in channel width that were made on the basis of comparisons of pre- and post-1983 photographs of the spawning bar. The changes in channel geometry have caused changes in the proportions of the total flows in the two chute channels. In 1991 at a total flow of 1208 cfs, (52 percent of the flow) was in the left channel. In 1993 at a total flow of 301 cfs, (61 percent) was in the left channel, and in 1995 at a total flow of 516 cfs, (57 percent) was located in the left channel. Comparison of the Wolman Count data for the Cleopatras Couch spawning bar (Table 2.2) indicate that there has been a fining trend at all of the sampled locations between 1991 and 1995. On average, the 050 at each of the locations has been reduced by about 20 mm. There has been only limited channel adjustment at the spawning bar since 1991 when the first sediment sampling was done, and therefore, it is reasonable to conclude that sediment supply from upstream has been responsible for the I I change in sediment caliber because there is no local source of gravel-sized sediment. t ' t? (Q @oJ CJ1 '~J I 2.7 Mussstter Engineering. Inc.