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<br />G\lZJ8'j <br /> <br />7 <br /> <br />PHYSICAL RESOURCES <br /> <br />The March/April 1996 Test Flow was a scientific and sediment management success; <br />however, it created nearly as many new questions and concerns as it did new sand bars. Many <br />of the new beaches have been partially or substantially eroded by 1996 and 1997 high steady <br />flows. <br /> <br />Flow and Sediment Relationships: The 1996 Test Flow provided an opportUnity to collect <br />important data on streamflow, water chemistry, and sediment transport at discharges well <br />above powerplant capacity. The U.S. Geological Survey (USGS) collected data at five <br />streamflow gaging stations on the mainstream and in four tributaries (Konieczki et. a1., 1997). <br />Reach-averaged flow rates were measured by tracking the movement of a tracer dye, and <br />sediment movement was measured by measuring bed elevation at a network of 120 <br />monumented cross-sections before and after the 1996 Test Flow. <br /> <br />Sand stored in the mainstream below Glen Canyon Dam was mobilized at discharges <br />between 20,000 and 45,000 cfs, and was deposited as separation, reattac~ent and channel <br />margin bars throughout the river corridor (Figure 2). However, deposition varied by <br />geomorphic reach, and with respect to sediment availability (upstream versus downstream of <br />the LCR). Beach-building in many reaches did not meet or exceed that observed following <br />the 'high-flow period that occurred between June, 1983 and October, 1984, largely because <br />the 1983 peak flow (97,000 cfs) was twice the magnitude of the 1996 Test Flow. <br /> <br />The primary hypothesis tested during the 1996 Test Flow was whether artificial <br />flooding could be used for sediment management. Sand was entrained quickly and efficiently <br />by the flood, and aggradation occurred rapidly at most depositional sites throughout the <br />Colorado River ecosystem. In some cases, subaqueous bar surfaces accumulated up to five <br />meters of new sand, far in excess of researcher predictions. Deposition of sand in fan-eddy <br />complexes occurred so quickly that some research instrumentation was buried. Overall, the <br />1996 Test Flow was effective in restoring some of the sediment transport processes. <br /> <br />Suspended sediment concentrations within mainstream flow and within eddies fell off <br />rapidly during the first 48 hours after stage reached 45,000 cfs, and continued to decline at a <br />slower rate throughout the seven-day long peak flow. The particle-size distribution of <br />suspended sediment simultaneously coarsened in the main stem and in eddies as the sediment <br />concentrations declined. <br /> <br />Sediment Transport Process: During the flood, rapid deposition was attributed to <br />"secondary recirculation" within eddies and along shorelines in pools and runs. This process <br />had not been previously recognized by USGS researchers in Grand Canyon, and reduced sand <br />export to Lake Mead. Secondary recirculation was especially effective in system-wide <br />deposition of channel-margin bars; an important point with respect to sediment conservation. <br /> <br />Hysteresis. a change in sediment transport/deposition with respect to discharge over <br />time, was documented by USGS in association with sediment coarsening and reduced <br />sediment concentrations during the 1996 Test Flow. A similar phenomenon was previously <br /> <br />Final Draft - 12/12/97 - For AMWG Review <br />