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<br />WATER RESOURCES RESEARCH, VOL. 36, NO. 2, PAGES 515-542, FEBRUARY 2000
<br />
<br />Colorado River sediment transport
<br />
<br />1. Natural sediment supply limitation and the influence
<br />
<br />of Glen Canyon Dam
<br />
<br />David J. Topping
<br />u.s. Geological Survey, Reston, Virginia
<br />
<br />David M. Rubin
<br />u.s. Geological Survey. Menlo Park, California
<br />
<br />L. E. Vierra Jf.
<br />Department of Civil and Environmental Engineering, University of Colorado, Boulder
<br />
<br />Abstract. Analyses of flow, sediment-transport, bed-topographic, and sedimentologic data
<br />suggest that before the closure of Glen Canyon Dam in 1963, the Colorado River in
<br />Marble and Grand Canyons was annually supply-limited with respect to fine sediment (Le.,
<br />sand and finer material). Furthemlore, these analyses suggest that the predam river in
<br />Glen Canyon was not supply-limited to the same degree and that the degree of annual
<br />supply limitation increased near the head of Marble Canyon. The predam Colorado River
<br />in Grand Canyon displays evidence of four effects of supply limitation: (1) seasonal
<br />hysteresis in sediment concentration, (2) seasonal hysteresis in sediment grain size coupled
<br />to the seasonal hysteresis in sediment concentration, (3) production of inversely graded
<br />flood deposits, and (4) development or modification of a lag between the time of a flood
<br />peak and the time of either maximum or minimum (depending on reach geometry) bed
<br />elevation. Analyses of sediment budgets provide additional support for the interpretation
<br />that the predam river was annually supply-limited with respect to fine sediment, but it was
<br />not supply-limited with respect to fine sediment during all seasons. In the average predam
<br />year, sand would accumulate and be stored in Marble Canyon and upper Grand Canyon
<br />for 9 months of the year (from July through March) when flows were dominantly below
<br />200-300 m'/s; this stored sand was then eroded during April through June when flows
<br />were typically higher. After closure of Glen Canyon Dam, because of the large magnitudes
<br />of the uncertainties in tbe sediment budget, no season of substantial sand accumulation is
<br />evident. Because most flows in tbe postdam river exceed 200-300 m'ls, substantial sand
<br />accumulation in the postdam river is unlikely.
<br />
<br />1. Introduction
<br />
<br />River channels range in type from classical self-formed al-
<br />luvial channels to bedrock channels. In self-formed alluvial
<br />channels the sediment supply is either in equilibrium or ex-
<br />ceeds the transport capacity of the channel over long time-
<br />scales, whereas in bedrock channels, the transport capacity of
<br />the channel exceeds the supply of sediment over long time-
<br />scales. Most rivers fall between these two channel types and
<br />may be expected to show some evidence of sediment supply
<br />limitation with respect to some portion of the size classes of
<br />sediment in transport. The Colorado River in Glen, Marble,
<br />and Grand Canyons (Figure 1) is one such river. Though in
<br />many places the river bed consists of patches of erodible fine
<br />sediment (i.e., sand and finer material) overlying gravel and
<br />bedrock, in at least 10 reaches in Marble and Grand Canyons the
<br />river flows directly over bedrock (R. Anima, U.S. Geological
<br />SUlVey, pe=nal communication, 1999). Therefore, although the
<br />bed of the river is, in many places, easily erodible, the river
<br />displays some geomorphic evidence of sediment supply limitation.
<br />
<br />Copyright 2000 by the American Geophysical Union.
<br />
<br />Paper number 1999WR900285.
<br />0043-1397/00/1999W R 900285$09.00
<br />
<br />In addition to this geomorphic signature, sediment supplY
<br />limitation produces four effects found in sediment-transport,
<br />sedimentologic, and bed-topographic data: (I) seasonal hyster-
<br />esis in sediment concentration, (2) seasonal hysteresis in sed-
<br />iment grain size coupled to the seasonal hysteresis in sediment
<br />concentration, (3) production of inversely graded flood depos-
<br />its, and (4) development or modification of a lag between the
<br />time of a flood peak and the time of either maximum or
<br />minimum (depending on reach geometry) bed elevation. The
<br />Colorado River is a good natural laboratory for studying these
<br />effects of sediment supply limitation in a fluvial system because
<br />of the wealth of available flow and sediment-transport data
<br />collected both prior to and after the closure of Glen Canyon
<br />Dam in 1963. As in the postdam era [Rubin et al., 1998; Top-
<br />ping et ai., this issue], sediment-transport rates in the predam
<br />era varied independently of discharge and as a function of the
<br />grain sizes of sediment present in the river. Seasonal decreases
<br />in the upstream supply of sediment during the annual snow-
<br />melt flood have been obselVed to produce significant hysteresis
<br />in suspended-sediment concentration at the Grand Canyon
<br />gage [Leopold and Maddock, 1953[. Decreases in the predam
<br />upstream supply of sediment have been interpreted to cause
<br />coarsening of the bed [Colby, 1964; Burlcham. 1986J and scour
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