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18 The State of the Colorado River Ecosystem in Grand Canyon 1996. The MLFF was the preferred alternative identi- fied in the 1995 Operation of Glen Canyon Dam Final Environmental Impact Statement (EIS) and was selected in the Record of Decision (U.S. Department of the Interior, 1996). The U.S. Geological Survey's (USGS) Grand Canyon Monitoring and Research Center and its cooperators have conducted extensive monitoring and research on fine-sediment transport and sandbar evolu- tion in Grand Canyon. This chapter presents a sum- mary of the results of studies since the 1970s, as well as conclusions derived from recent syntheses of streamflow, sediment transport, and geomorphic data from 1921 to 2004, including recent sediment budgets. The effects of the ~vlLFF operating alternative at Glen Canyon Dam (1996-2004) on fine-sediment transport and sandbars are examined in the context of these historical data. Finally, options identified by sediment scientists for testing alter- native operations aimed at more effective conservation of fine-sediment resources are discussed. Background Predam Sediment- transport Processes As described by Rubin and others (2002), sandbars below Glen Canyon Dam in Marble and Grand Canyons are maintained by fine sediment that is transported by the Colorado River through the ecosystem. As sand is car- ried through these bedrock canyons by the river, some of it is deposited along channel margins and along shore- lines within hundreds of eddies, thus building sandbars. The eddy areas, which are typically located immediately downstream from channel constrictions created by tribu- tary debris fans, are susceptible to fine-sediment deposi- tion because the flow tends to recirculate and be of lower velocity than the flow in the main channel. Using histori- cal sediment-transport records from the Lees Ferry (RM 0) and Grand Canyon (RM 87) gages, Laursen and others (1976) and later Topping and others (2000b) identified that before closure of Glen Canyon Dam, sand would accumulate in the Colorado River channel during late summer, fall, and winter. Annual accumulation of sand in the channel during predam years apparently resulted from large sediment inputs from tributaries that occurred during periods of seasonal low flows in the main channel of the Colorado River. Following these periods of sand enrichment in the main channel, spring snowmelt floods would erode the accumulated sand from the channel and transport it out of the canyon, along the way depositing some of the sand in the low-energy eddy areas and thus leading to the building of the high-elevation sandbars. Following the spring replenishment of sandbars, some of this sand would in turn be redistributed to even higher elevations by winds (Topping and others, 2000b). On an annual basis, the inputs of sand to the system would approximately balance the export, maintaining equilib- rium in background sand storage in the eddies. Effects of Lake Powell on Sand Transport Before the closure of Glen Canyon Dam in 1963, approximately 25 million tons (23 million Mg) of sand passed the Lees Ferry stream gage annually. With the addition of 1. 7 million tons (1.5 million Mg) of sand from the Paria River, which joins the Colorado River just downstream from Lees Ferry, the total predam annual sand supply to Marble Canyon reached about 27 million tons (24 million Mg). At the end of Marble Canyon, the Little Colorado River joins the Colorado River and con- tributed, on average, about 1.9 million tons (1. 7 million Mg) to the annual sand supply. Thus, the total predam sand supply to Grand Canyon, from the Colorado River upstream from Lees Ferry and with the Paria and Little Colorado Rivers combined, was approximately 29 million tons (26 million Mg). Today, because Lake Powell traps all of the sediment upstream from Glen Canyon Dam, the Paria River is the primary source of sand to Marble Canyon, supplying approximately 6% of predam sand levels. In the case of Grand Canyon, Glen Canyon Dam has reduced its sand supply to primarily the contributions of the Paria and Little Colorado Rivers. Other lesser tributaries also contribute a small amount of sand to Grand Canyon, with an estimated cumulative supply that is approxi- mately 10% to 20% of the mean annual load provided by the Paria River. Taken together, the contributions of sand from various sources provide Grand Canyon with approximately 16% of its predam sand levels. The find- ings presented here are drawn from Topping and others (2000b) and Webb and others (2000); readers interested in more details on the predam and postdam sediment budgets for Marble and Grand Canyons should consult these reports.