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001577 . Grand Canyon, Arizona, and referred to here- after as the Grand Canyon gage). We present evidence from gaging station measurements, channel cross-section surveys, comprehensive hydrographic surveys, sand-bar surveys, historical and modem matched ground-level oblique photographs, and analysis of aerial photographs within a geographic information system (GIS). 1.1 The Importance of a Comprehensive History of Geomorphic Change . Numerous studies have described aspects of the environmental history of the Colorado River in Glen, Marble, and Grand Canyons. These studies generally described the decrease in fine sediment in and along the channel and the increase in riparian vegetation. Each of these studies emphasized analysis of a specific type of data. Burkham (1986) analyzed dis- charge measurement data at gaging stations. Howard and Dolan (1981), Beus et al. (1985), Schmidt and Graf (1990), and the Sand Bar Studies Group of Northern Arizona University (NAU) reported on topographic surveys of sand bars. Turner and Karpiscak (1980), Stephens and Shoemaker (1987), and Webb (1996) matched ground level photographs spanning a century. Brian and Thomas (1984) and Kearsley et al. (1994) inventoried camp- sites in the field and on aerial photographs, respectively. Although each of these studies generally described the same style of environmental change, each analytical technique had limita- tions of temporal resolution or spatial robust- ness. These limitations blocked the attempt to assign a magnitude to the decrease in fine- sediment storage in the channel and alluvial valley, and there is no consensus as to the nature of longitudinal trends in channel change. Comprehensive understanding of the timing, magnitude, style, and spatial extent of channel change in Glen, Marble, and Grand Canyons is essential as a benchmark against . which to understand changes in the aquatic and riparian ecosystem and to inform public policy debate about environmental management of Glen Canyon Dam. The attempt to reverse environmental conditions that are determined to be undesirable will partly be founded on a clear understanding of the magnitude of the geomor- phic transformation that has occurred down- stream from the dam. 2.0 WATER AND FINE-SEDIMENT FLUXES The fluxes of water and of sand, silt, and clay were highly variable before completion of Glen Canyon Dam (Topping et al., 2000b). The median discharge of the Colorado River at USGS gaging station 09380000 (Colorado River at Lees Ferry, Arizona, and referred to hereafter as the Lees Ferry gage) was 227 m3/s for the pre-dam period between May 8, 1921, and March 12, 1963, and the range of flows during the year was large (Fig. 2A). The 10% exceedence flow was 1359 m3/s, and the 90% exceedence flow of 127 m3/s (Fig. 3) was more than an order of magnitude less (Topping et al., 2003). Prior to completion of the dam, the total annual load of fine sediment was 57 :t 3 million metric tons at the Lees Ferry gage and 83 :t 4 million metric tons at the Grand Canyon gage, based on measurements made after 1944 (Top- ping et al., 2000b). Between 1949 and 1962, the difference between the annual load of the year of largest transport and the year of smallest transport was one order of magnitude (Topping et al., 2000b). Approximately 40% and 35% of the annual fine sediment load passing the Lees Ferry and Grand Canyon gages, respectively, was sand. The concentration of sand in suspension and the discharge of water were reasonably well correlated at the Lees Ferry gage, but this correlation was poor at the Grand Canyon gage (Topping et al. 2000a; Rubin and Topping, 2001). At the Grand Canyon gage, concentration was much greater during the rising limb of the spring snowmelt flood than during the flood's recession. The variation in suspended-sand Introduction 3