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24 The State of the Colorado River Ecosystem in Grand Canyon contradicts the approach of the EIS, where it was assumed that sand transport was regulated only by the discharge of water. . Topping and others (2000b) showed through their analysis of the 1965-70 daily sediment- transport data collected by USGS that, under normal powerplant flows, newly input tributary sand is exported past the Grand Canyon gaging station within several months. Their analysis of predam data indicated that, prior to closure of Glen Canyon Dam, sand would accumulate in Marble and upper Grand Canyons only during the 9 mo of the year when discharges were typically lower than about 9,000 cfs. . Measurements of the channel bed indicate that tributary sand, which is typically much finer than the sand on the bed of the Colorado River, accu- mulates on the bed for only a short time before being eroded and transported out of the canyon under normal MLFF dam operations (Topping and others, 2000a). . Since August 1999, detailed suspended-sediment transport measurements have been collected at the Paria and Little Colorado Rivers to document 331 E B 328 co -0 CI:l > 0 .0 co 325 ... CI:l ~ C 0 Completion of .." 322 ----+ co > Glen Canyon Dam CI:l w 318 1900 1925 1950 1975 2000 Figure 5. A decrease in elevation of the sandbar surface is seen at Jackass Creek camp located along the left shore of the Colorado River, 23 mi (37 km) downstream of Glen Canyon Dam. Elevations were determined by examining oblique and aerial photographs of the site and by field survey of the elevation and the former sand surface at its contact with large talus blocks. This graph shows the elevations near one prominent talus block that was inundated by predam mean annual floods, but since the dam was completed, the talus block has been inundated infrequently (modified from Rubin and others, 2002). 700,000 600,000 U ID '" ro '" > ID ~ ID ID ~ ID = ro ID '" ro = u '0 > ~ <= ro ~ 500,000 ~ 400,000 ro ro ~ ~ 300,000 '" 200,000 100,000 1992 1994 2002 1996 1998 2000 Figure 6. Changes in sandbar size (total surface area) are shown for 14 long-term sandbar study sites between the Lees Ferry and Grand Canyon gages (RM 0 to RM 87). Area of bars exposed above water discharges of 8,000 cfs decreased by 22% from 1991 to 2004. The 1996 beach/habitat-building flow resulted in a net transfer of sand from mid elevations to high elevations (modified from Rubin and others, 2002). inputs and at the USGS gaging stations above the mouth of the Little Colorado River and near Grand Canyon to document export. Initially, these quasi-daily measurements were made by using only conventional USGS methodologies to obtain cross-sectionally integrated samples of suspended-sediment concentration and grain size (methods described in Edwards and Glysson, 1999). Because substantial and rapid (within a day) changes that are due to tributary inputs can occur in suspended-sediment concentration and grain size, emerging technologies for continuous monitoring of suspended-sediment concentra- tion and grain size were tested and implemented beginning in 200 I. These technologies include acoustic backscatter and laser-diffraction methods and are described in detail in Melis and oth- ers (2004) and Topping and others (2004). The detailed sediment-transport measurements allow for the ability to construct sediment budgets based on continuous data instead of on rating curves, a very important distinction from the EIS approach of using a limited data set. These data show that the overall mass balance of sand (input minus export) continues to be negative (fig. 7), as originally predicted by Laursen and