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002372 flows are known to have occurred since 1963 with those where such events are unknown. Resurvey of the bed at 6 cross-sections at the 2 alternative sites of the proposed, but never constructed, Marble Canyon Dam site (Fig. 23) show that the Colorado River bed degraded between the 1950s and 2000. Be- tween 1950 and 2000, there was little change in bed sediment at 2 cross-sections at River Mile 32.8 but between 0.4 and 1.0 m of bed sediment were removed from each of 4 cross- sections at River Mile 39.5 (Table 7). These measured changes probably reflect long-term adjustment to the reduced fine-sediment supply to Marble Canyon instead of short-term bed adjustments, because the changes measured between 1950 and 2000 greatly exceeded the measured changes that occurred at the same cross-sections between 1998 and 2000. 5.3.2 Changes at the Grand Canyon Gage between 1963 and 2000 and at the Lower Marble Canyon Gage between 1983 and 2000 Discharge measurements at the Grand Canyon and at U. S. Geological Survey gaging station 093831 00 (Colorado River near Desert View, Arizona, and referred to hereafter as the Lower Marble Canyon gage) demonstrate the relative roles played by mainstem sediment mass balance and by nearby tributaries in determining bed elevation in ponded backwa- ters. These measurements indicate that post- dam multi-year fine-sediment accumulation did not occur except when the channel control immediately downstream had aggraded, the downstream tributary was flooding and thereby increased mainstem stage, or the discharge of the Colorado River was very low for more than 1 year. Most ofthe significant increases in mean bed elevation at the Grand Canyon gage occurred immediately following debris flows or flash floods in Bright Angel Creek that presumably aggraded Bright Angel Rapid (Fig. 17). The most significant of these events was the December 1966 debris flow that delivered large boulders to the rapid (Cooley et aI., 1977). The bed at the gage aggraded about 1.5 m during the next 9 months. Aggradation of about 0.5 m occurred immediately after a flash flood in July 1971, and an additional 0.5 m of aggradation between November 1972 and January 1973 may be related to this change at Bright Angel Rapid. Short episodes of aggra- dation occurred in response to 3 flash floods in Bright Angel Creek between 1995 and 1997. The first of these was a +0.35-m shift in the stage-discharge relation at the gage that oc- curred when the riffle was aggraded by a -85 m3/s flood that occurred on March 5, 1995, on Bright Angel Creek (Rihs, 1995). The rapid was aggraded again by coarse sediment from a flash flood on Bright Angel Creek that prob- ably occurred on September 26, 1997, and the stage-discharge relation at the gage shifted +0.2 m. Additional aggradation occurred due to a tributary flood that probably occurred on April 24, 1999, when the stage-discharge relation shifted +0.15 m. Bed aggradation of about 2.5 m occurred between August 1963 and March 1965 and was unrelated to changes at Bright Angel Rapid, because no floods in Bright Angel Creek were reported. Discharge was typically less than 40 m3/s during much of this period and only exceeded 500 m3/s for about 2 wks in spring 1964. There were several floods on the Paria and Little Colorado Rivers during this period and Topping et aI. (2000b) estimated that about 4 million tons of fine sediment accumulated in the study area. Bed aggradation at the gage was probably due to accumulation on the bed, because there were no high flows that could have transferred this sediment into eddies. Significant degradation of the bed oc- curred in a few weeks in May 1965 and June 1983 during large post-dam floods. Degrada- tion at the gage caused by the former flood was probably a widespread phenomena, because Rubin and Topping (2001) estimated that 17.6 x 106 tons were eroded from Marble and upper Grand Canyons in May and June 1965. Much of this sediment had accumulated during the 34 System-wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor ...