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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, I of the bed at the upper Lees Ferry cabIeway during snowmelt floods was also controlled by reach geomelty. Our analyses, in combination with those presented by Top- ping .1 al. [this issueJ, suggest that Leopold and Maddock (1953), Colby [1964], Howard and Dolan [1981J, and Burklwm [1986) were all partially correct and that both reach geometry and a reduction in the upstream sand supply played important roles in the responses of the bed at the Grand Canyon and upper Lees Ferry cableways. The response of the bed at both sites was initially controlled by a reach-geometry-driven redis- tribution of the boundary shear stress field (as the flow in- creased), but the response of tbe bed at the Grand Canyon cableway much more strongly reflected a seasonal depletion of the upstream supply of sand. The bed at the Grand Canyon cableway aggraded with the initial increase in water-surface stage during the snowmelt flood (from March 1 through May 10). Then, from about May 10 through July 20, the bed de- graded back to its presnowmelt-flood elevation. Thus maxi- mum bed elevation at the Grand Canyon cableway led the peak of the snowmelt flood by about 4 weeks (Figure 6a). This is the style of bed response predicted at this type of cross section by Topping ., aI. [Ihis issue] for the case when the upstream supply of sand becomes depleted during a flood. While the bed was scouring at the Grand Canyon cableway, the suspended-sand data at the Grand Canyon gage were domi- nated by sand-depletion events (Figure 60). In the average predam year the bed at the upper Lees Ferry cableway scoured with the increase in water-surface stage during the snowmelt flood and began to aggrade about 2 weeks after the peak of the snowmelt flood (Figure 6b). As shown by Topping et al. [this issue], this style of bed-topography response may be due to, but does not require, depletion of the upstream supply of sand. In any case the lag between the maximum or minimum bed ele- vation and the flood peak was much less at the upper Lees Ferry cableway than at the Grand Canyon cableway. This sug- gests that bed elevation changes at the upper Lees Ferry cable- way were driven more by changes in the spatial distribution of boundary shear stress (caused by changes in discharge) than by depletion of the upstream supply of sediment. Furthermore, unlike at the Grand Canyon cableway, sand-depletion events were never dominant at the upper Lees Ferry cableway (Figure 6b). Thus the Colorado River in Glen Canyon showed the bed-topographic effects of sediment supply limitation to a far lesser degree than it did in Grand Canyon. 5.5. Long-Term T"'nds in Bed Stage allhe Grand Canyon and Upper Lees Feny Cableways If a river is annually supply-limited with respect to fine sediment, then over multiyear timescales the amount of fine sediment present in the channel should either decrease or be zero. However, for a decrease in the amount of fine sediment in a cross section over multiyear timescales to be attributable to sediment supply limitation, this decrease must occur in the absence of changes in the local hydraulics that might cause changes in the local patterns of scour and fill. Burkham [1986] indicated that the channel bed at the upper Lees Ferry cable- way was scouring o....er multiyear timescales prior to the closure of Glen Canyon Dam in 1963 and suggested that this scour was due to a decrease in the upstream supply of sediment. To detennine whether multiyear changes in cross-section geome- try were related to changes in either the sediment supply or the local hydraulics, F -test trend analyses were conducted on the measurements of mean bed stage (Figures 7a and 7b). Because 52S the channel width and stage-discharge relationship were both stable (Figures 7c and 7d) at the Grand Canyon cableway, the 1922-1962 bed-stage data at this site could be analyzed as a single time series (Figure 7a). Howe....er. because of three ma- jor changes in channel geometry (two related to changes at the mouth of the Paria River and one caused by a large rockslide (Figures 7c and 7d)) in the Lees Ferry reach that could have influenced the response of the bed at the upper cableway, the 1921-1962 bed-stage data at this site had to be analyzed in four segments (Figure 7b). 5,6, Discussion or the Long-Term T"'nds in Bed Slage allhe Grand Canyon and Upper Lees Feny Cableways At both the Grand Canyon and upper Lees Ferry cableways, trends in bed stage during the predam era were slightly, but significantly, negative (Figures 7a and 7b). Though these trends are consistent with the interpretation that both Grand and Glen Canyons were, to some degree, supply-limited with respect to fine sediment, other factors (e.g., changes in flow and enema1ly forced changes in channel geomelty) may have affected the response of the bed at these siIes. However, be- cause the bed at both sites (sites at which bed elevation re- sponded in opposing manners during the annual flood) scoured over multiyear timescales, these other factors can probably be ruled out (though they are still discussed below). Therefore the long-Ierm SCOUT at both sites was probably due to the transport capacity of the river in both canyons exceeding the long-term upstream supply of sediment. In other words, the river in both canyons was probably supply-limited over multi- year timescales with respect to some fraction of the sizes of sediment in transport. Because at the Grand Canyon cableway, no major change occurred from 1922 through 1962 in either the channel width (because the channel margins are bedrock) or in the stage- discharge relationship (Figures 7c and 7d), the entire predam period could be analyzed as a single time series. From Novem- ber 12, 1922, through December 31,1962, the bed allhe Grand Canyon cableway SCOUTed at a rate of 1.6 cmIyr; this trend is significant at less than the 1.0 X 10-'. level. Thus, given the approximate 9O-m width of the channel, about 1.4 m2 more sediment was eroded from this cross section than was supplied to it each year. Because the channel width and stage-discharge relationship at the Grand Canyon cableway were effectively constant, this long-tenn erosion of sediment can be interpreted to be either due to a long-term depletion in the upstream sediment supply or due to a long-term change in either the discharge of water or the water-surface stage. Analysis of the USGS discharge records from the Grand Canyon gage indi- cates that from 1922 through 1962 the mean-daily discharge of water decreased by about 25%, corresponding to a decrease in water-surface stage of about 15%. Therefore, though sediment supply limitation may have caused the 1922-1962 decrease in bed elevation, because the bed at the Grand Canyon cableway scours slightly with decreasing water-surface stage [Topping et aI., this issue], the 1922-1962 decrease in water-surface stage cannot be ruled out as a contributing factor. At Lees Ferry, interpretation of long-tenn trends in bed stage are complicated by three externally forced changes in channel geometry that may have affected the response of the bed at the upper cableway. First, during the first week of April 1923, as flows first increased above 500 mJjs during the annual snowmelt flood, a large downward shift in the stage-discharge relationship occurred (Figure 7d). This was likely the result of