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<br />O\)2~91 <br /> <br />9 <br /> <br />documented in the 1950s on the Colorado River. but has never before been documented <br />below Glen Canyon Dam under controlled flood conditions. The shifting particle-size <br />response over a relatively short time period (48 hours) during the flood, suggests that the <br />system may be perpetually sediment limited; especially upstream of the mainstream and LCR <br />confluence. Finer sand inputs from the Paria and Little Colorado Rivers may not remain in <br />storage for more than a few days to weeks under high-constant ROD dam operations. <br /> <br />Sand Bar Rejuvenation: The 1996 Test Flow caused a net increase in the area and volume <br />of high-elevation sand deposits throughout Grand Canyon, regardless oflocation or <br />geomorphic setting of the deposits (Figure 2). The volume of sand deposits at elevations <br />greater than the 15,000 cfs stage increased by more than 90 percent among 34 survey sites <br />(Parnell et al. 1996). These sites were 197 percent larger than they had been immediately <br />prior to the flood. Abundant new high-elevation sandbars were mapped along 16 miles of <br />river in the Point Hansbrough-Saddle Canyon and Little Colorado River-Urikar Rapids <br />study reaches (Schmidt 1997). <br /> <br />There was a consistent form of bar response throughout Grand Canyon. Low <br />elevation parts of reattachment bars in the centers of eddies, as well as the adjacent <br />mainstream channel, were scoured, while the reattachment zones were filled (Schmidt 1997; <br />Figure 3). <br /> <br />The 1996 Test Flow impacts also varied by reach (Figure 2). The flood caused a net <br />increase in the area and volume of sand at sites upstream from the Little Colorado River. <br />Sites upstream from the LCR measured by Northern Arizona University proportionally <br />increased more than sites located elsewhere (parnell et aI. 1996). Although there was a reach- <br />average response for bars, there was also substantial variability in net aggradation from site- <br />to-site. Specific sites eroded more or deposited more than the average condition. The <br />distribution of responses seems to be normally distributed (parnell et al. 1996, Schmidt 1997). <br /> <br />Sandbars were 97% larger .after the Test Flow than immediately prior to the Test <br />Flow. High rates of erosion occurred at all sites during the 6 months immediately following <br />the flood (Figure 2). Nevertheless, sandbars were 97 percent larger than they had been <br />immediately prior the flood. Typically, high elevation sand deposits have eroded, low <br />elevation parts of eddy bars have increased in area, and channel pools remain in a scoured <br />condition (parnell et al. 1996; Kaplinski et al. 1997). <br /> <br />Until the summer monsoons in August 1997. the bed of the Colorado River below <br />Glen Canyon Dam existed in a coarsened state. Because of below average inputs from the <br />Paria and Little Colorado rivers, and because of the constant 18,000 cfs of 1995, the 1996 <br />Test Flow of 45,000 cfs, and the long-term sustained 20,000 to 27,000 cfs high releases, the <br />sand stored in the channel appeared to be as coarsened as it had been following the 1983-1986 <br />high flow period. Under such conditions, a flood identical to the one that occurred in 1996 <br />would likely have a much lesser depositional impact on sandbars, as well as downstream <br />sediment transport. . <br /> <br />Final Draft - 12112/97 - For AMWG Review <br />