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26 The State of the Colorado River Ecosystem in Grand Canyon studies reported above, howevCl~ have demonstrated that in the postdam Colorado River the relationship between 110w and sand transport is not stable but instead shifts quickly and substantially relative to the grain size of sand on the bed of the river (which is controlled by tributary inputs and mainstem 110ws). Rubin and Topping (200 I) and Rubin and others (2002) showed that the grain size of the sand in the regulated Colorado River ecosystem depends greatly on the recent histOlY of tributary activ- ity. For example, during low tributary 110w periods the only source of sand to the mainstem Colorado River is that on the channel bed and in eddies, and that sand tends to be much coarser than tributary-delivered sand because of the winnowing of the finer sizes. \Vhen tributaries are 1100ding and delivering large quantities of fine sand (fig. 8), however, the supply is no longer lim- ited to the coarser channel bed sand, resulting in much higher mainstem sand concentrations and, hence, greatly increased suspended-sediment export for any given 110w released from the dam. Figure 8. Looking upstream into Glen Canyon from the Paria River confluence with the main channel Colorado River during a Paria River flood. Tributary inputs of sand, such as the one pictured, now encounter clear Colorado River water because Lake Powell traps incoming fine sediment The Paria River is the primary source of sand to Marble Canyon but is only about 6% of the predam sand supply (photograph by Scott A. Wright, U.S. Geological Survey). Because sand transport cannot be predicted based on discharge alone, sediment budgets for the Colorado River in Grand Canyon can only be constructed based on measurements of sand transport at a frequency great enough to capture changes in concentration and grain size resulting from tributary inputs. Fundamentally, the conclusions drawn by the EIS team, which are not supported by the more recent data, resulted from a lack of continuous data in the postdam era; that is, if daily records had been continued beyond 1972 and into the EIS period, then the fine-sediment budget would have been constructed based on these data rather than on stable rating curves. Recent sediment budgets suggest that under this scenario the conclusions of the EIS would have been different and possibly would have led to a different strates')' for operation of Glen Canyon Dam in 1996. Though it is somewhat costly to collect long-term, high-frequency sediment-transport records, in this case it may have prevented 13 yr of dam operations that have continued to erode sandbars from Grand Canyon. Current Experimental Plan for Fi ne Sedi ment Because recent research has shown that sand does not accumulate on the river bed in Marble and Grand Canyons under normal Record of Decision dam opera- tions, scientists have recently proposed two possible field tests of dam operating options that might more effec- tively conserve limited, downstream sand resources. One approach is to implement floods immediately following large tributary inputs that commonly occur in late sum- mer and early fall. A second approach is to follow tribu- tary sand-input events with low flows, in order to limit export and retain most of the sand input, until flooding can be implemented. This approach would require a change in the pattern of monthly release volumes and associated dam operations because July and August releases of recent drought years still resulted in half of the sand introduced by a tributary flood being exported within days or weeks (Rubin and others, 2002). In September 2002, the U.S. Department of the Interior (2002) approved implementation of the second approach described above. Under this plan, changes in dam operations and restoration floods are linked to triggering thresholds based on sand inputs [rom the Paria River and lesser Marble Canyon tributaries and retention o[ sand in Marble and Grand Canyons. For example, the "autumn sediment input" scenario described in the 2002 environmental assessment (EA) (U.S. Department of