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TOPPING ET AL, COLORADO RIVER SEDIMENT TRANSPORT, 1 dam river in Marble and Grand Canyons was annually supply- limited with respect to fine sediment Therefore, to test whether the predam Colorado River between the Lees Ferry and Grand Canyon gages met the integral constraint of annual supply limitation with respect to fine sediment, we constructed a sediment budget for the 141-km-long reach of the Colorado River in Marble Canyon and upper Grand Canyon. 6.1, Annual Sediment Budget Fine sediment is supplied to the Colorado River in Marble Canyon and upper Grand Canyon from (1) the Colorado River in Glen Canyon (this ceased (Q be a major source after closure of the dam), (2) the Paria River (which enters the Colorado River at river mile 0.9 at the head of Marble Canyon), (3) the Little Colorado River (which enters the Colorado River at river mile 61.5 at the lower end of Marble Canyon), and (4) the smaUer ungaged tributaries between river mile 0 and 87.4- During the predam era, large quantities of fine sediment were supplied to the Colorado River in Marble Canyon and upper Grand Canyon during tributary floods that typicaUy occurred in both July-October and January-April, whereas the largest quantities of fine sediment were exported during the annual snowmelt flood. Thus neither calendar years (January- December) nor water years (October-September) were the best time period to use in computing annual sediment budgets because the season of largest sediment export fell between the two seasons of greatest tributary sediment supply. Therefore we constructed a sediment budget for years beginning on July 1 of the preceding calendar year and ending on June 30 of the current calendar year, a year herein defined as a "sediment year." For example, by this definition, sediment year 1957 was the year between July 1, 1956, and June 30, 1957. Constructing a sediment budget for Marble Canyon and the upper Grand Canyon was a relatively straightforward process because of the high spatial density and long duration of sedi- ment-transport data collected by the USGS. For over 2 de- cades, daily suspended-sediment concentrations were mea- sured at two stations on the main stem in this reach and at stations on the two major tributary suppliers of sediment, that is, the Paria and Little Colorado Rivers. On the main stem Colorado River these data were collected at the Lees Ferry and Grand Canyon gages (Figure 1). At the Lees Ferry gage, suspended-sediment concentrations were measured with mod- em USGS sampling equipment from October 1, 1947, through August 13, 1965. After August 13, 1965, suspended-sediment concentrations were measured only occasionally at this site. At the Grand Canyon gage, suspended-sediment concentrations were measured with modern USGS sampling equipment from June 1, 1944, through November 28,1944, from April 25, 1945, through April 27, 1945. and from May 13, 1945, through Sep- tember 30, 1972. Suspended-sediment data collected at the Grand Canyon gage prior to June 1, 1944. from November 29. 1944, through April 24, 1945, and from April 28, 1945, through May 12, 1945, were not used in this analysis because the sus- pended-sediment sampler used during these periods has been shown to oversample suspended sand by as much as a factor of 2-2.5 depending on the method of sampler deployment lFed- eral/mer-Agency River Basin Committee, 1957; Topping et al., 1996]. On the Paria River, suspended-sediment concentralions were measured at the Paria River Lees Ferry gage from Oc- tober I, 1947. through Septcllll'cr 30,1976 (Figure I). On the Litlle Colorado River. susp,:"ded-sec1iment concentrations 531 were measured from October 1, 1947, through September 30, 1970, at the highway 89 bridge at Cameron, Arizona (Figure 1). In this sediment budget the monthly fine-sediment supply was set equal to the combined measured monthly loads of the Colorado River at Lees Ferry, the Paria River, the Little Col- orado River, and an estimation of the monthly loads of the ungaged tributaries (described in Appendix A). The monthly fine-sediment export was set equal to the measured monthly load of the Colorado River atlhe Grand Canyon gage. During periods of missing record, most notably between the measure- ments made after August 1965 at the Colorado River Lees Ferry gage, daily sediment loads were estimated by interpola- lion. Longer periods of missing record at the Grand Canyon gage prevented sediment budgets from being constructed from June 2, 1956, through August 31, 1956, and from August I, 1958, through March 31,1959. Prior to constructing and interpreting any sediment budget, it is essential that the sources and magnitudes of measurement error be understood and quantified so that appropriate uncer- tainties can be assigned. As justified in Appendix B, the fol- lowing uncertainties were assigned to the measured monthly and annual sediment loads, (1) 5% for the Colorado River at the Lees Ferry and Grand Canyon gages and (2) 20% for the Paria and Little Colorado Rivers. As justified in Appendix A, an uncertainty of a factor of 3 was assigned to the estimated monthly and annual sedirnentloads of the ungaged iributaries. These uncertainties in the measured and estimated loads are not unreasonably high and, if anything, may be too low. The annual supplies of fine sediment (from each of the four sources) to the reach between the Lees Ferry and Grand Can- yon gages and the annual export of fine sediment from this reach are shown in Figure 9a. Prior to closure of the dam, the major supplier of fine sedirnentto this reach was tbe Colorado River above the Lees Ferry gage (i.e., the Colorado River in Glen Canyon). During the predam year> with complete sedi- ment-transport data at the Lees Ferry gage (sediment years 1949-1962), the mean annual supply of sediment from the Colorado River in Glen Canyon was 57 :!: 3 million t (t indi- cates metric ton) (of which approximately 40% was sand). During the predam year> with complete sediment-transport data at the Grand Canyon gage (i.e., sediment years 1948- 1955, 1958, and 1960-1962), the mean-annual export of fine sediment from this reach was 83 :::!:: 4 million t (of which ap- proximately 35% was sand). Because the first few yeaTS after closure of Glen Canyon Dam in March 1%3 were not repre- sentative of the fluctuating flows that typify most of the post- dam era [U.S. Departmenl of the In/erior, 1995], these years were not used to determine mean-annual postdam loads of the Colorado River. Dam releases during these years consisted of quasi-steady low flows through much of 1963 and 1964 and sustained high flows during April-June 1965 (Figure 9b). In the initial period of daily fluctuating flows following the high dam releases of 1965 (i.e., sediment yeaTS 1966-1970), the mean-annual supply of sediment from the Colorado River in Glen Canyon was only 0.24 :!: 0.01 million 1. This represented a decrease of 99.5-99.6% in the mean-annual predam supply of fine sediment from this source. Following the high flows in 1965 (i.e., sediment yeaTS 1966-1972), the mean-annual export of fine sediment past the Grand Canyon gage was 14 :!: 1 million 1. This represented a decrease of 81-85% in the mean- annual predam export of fine sediment. The other major predam suppliers of fine sediment, and the only major suppliers of sediment to the reach between the Lees