|
<br />TOPPING ET AL: COWRADO RIVER SEDIMENT TRANSPORT, I
<br />
<br />that a river be erosional; net dCJXlsition of coarser sizes may
<br />occur even if a river is supply-limited with respect to finer slzes.
<br />
<br />3. Hypothesized Effects of Sediment Supply
<br />Limitation
<br />
<br />[n a reach that is supply-limited with respect to a specified
<br />size class of sediment over a given timescale, four coupled
<br />effects are hypothesized to result. First. within the timescale
<br />over which the reach is supply-limited, hysteresis in transport
<br />rates of the specified size class will result [e,g" Nanson, 1974;
<br />Dunne and Leopold, 1978; Moog and Whiting, 1998]. Following
<br />sediment-supplying events to the reach, transport rates of sed-
<br />iment in the supply-limited size class will initially increase
<br />independently of discharge, then subsequently decrease as that
<br />size class becomes depleted. As a result of the same process,
<br />transport rates of the supply-limited size class will be higher on
<br />the rising limb than on the receding limb of a flood.
<br />The interplay between different size classes of sediment in a
<br />supply-limited reach leads to the second effect of sediment
<br />supply limitation. Sediment input to a supply-limited reach will
<br />travel downstream as an elongating "sediment wave," with the
<br />finest sizes (because of their lower settling velocities) traveling
<br />the fastest. This sediment wave will bave a component in the
<br />bed, the bed load, and the suspended load. AI; the front of a
<br />sediment wave passes a given location, the sediment-transport
<br />rate will first increase as the grain size in the reach fines, then
<br />subsequently decrease as the grain size in the reach coarsens
<br />[Topping el aI., this issue). Thus, associated with the hysteresis
<br />in sediment-transport rates described above, hysteresis will
<br />also exist in sediment grain size during a flood passing through
<br />a supply-limited reach. The grain size of sediment in transport
<br />on the rising limb will be finer than that on the receding limb
<br />of the flood. Furthermore, by virtue of the physical linkage
<br />between particle settling velocity and suspended-sediment-
<br />transport rate [e.g., Rouse. 1937; Hunl. 1969; Smith. 1977;
<br />McLean, 1992), in the same flow conditions the transport rate
<br />of finer sediment will be greater than that of coarser sediment.
<br />Therefore, following a discrete sediment-supplying event, the
<br />timescale over which a reach becomes supply-limited with re-
<br />spect to a finite amount of finer sediment is shorter than that
<br />over which it becomes supply-limited with respect to an equiv-
<br />alent amount of coarser sediment.
<br />The third hypothesized effect of sediment supply limitation
<br />follows directly from the second effect. Because the grain size
<br />of sediment in suspension will coarsen over time during floods
<br />passing through a supply-limited reach, the sediment available
<br />for deposition on floodplains, on channel margins, or in eddies
<br />will coarsen through time. Thus deposits produced during
<br />floods passing through a supply-limited reach will coarsen up-
<br />ward [/stya, 1989; Rubin et aI., 1998J.
<br />The fourth hypothesized effect of sediment supply limitation
<br />has to do with the temporal patterns of scour and fill of the bed
<br />during floods passing through a supply-limited reach [Topping
<br />et al., this issue]. If fhe upstream supply of sediment decreases
<br />during a flood, a lag may develop or be modified between the
<br />time of the flood peak and the time of either maximum or
<br />minimum bed elevation. At a cross section where convergence
<br />occurs in the boundary shear stress field with increasing flow,
<br />the time of maximum bed elevation in a supply-limited case
<br />will occur prior to that in a non-supply-limited case. Thus, at
<br />this type of cross section, an observation of maximum bed
<br />elevation leading a flood peak, with scour beginning prior to
<br />
<br />517
<br />
<br />the Rood peak, indicates the presence of sediment supply lim-
<br />italion. At a cross section where divergence occurs in the
<br />boundary shear stress field with increasing flow, the time of
<br />minimum bed elevation in a supply-limited case will OCCur after
<br />that in a non-supply-limited case. However, because, at this
<br />second type of cross section, minimum bed elevation lags the
<br />flood peak in both the supply-limited and non-supply-limited
<br />cases, an observation of minimum bed elevation lagging a flood
<br />peak may suggest, but does not require, the presence of sedi-
<br />ment supply limitation. Therefore, as with the previously de-
<br />scribed hysteresis in sediment-transport rate and grain size,
<br />hysteresis may also exist in bed elevation at a cross section
<br />during floods, and, depending on reach geometry, the presence
<br />of this hysteresis can be used to deduce the presence of sedi-
<br />ment supply limitation.
<br />
<br />4. Systematic Seasonal Changes in Sediment
<br />Concentmtion, Gmin Size, and Bed Elevation:
<br />Evidence for Predam Annual Fine-Sediment
<br />Supply Limitation in Gmnd Canyon
<br />4.1. Coupled Hysteresis in Suspended-Sediment
<br />Concentration, Grain Size, and Bed Elevation
<br />
<br />Calendar year 1954 provides a good example of the behavior
<br />of suspended-sediment concentration, suspended.sediment
<br />grain size, and bed elevation during an annual predam flood
<br />cycle. The peak discharge of the 1954 snowmelt flood was
<br />below average, and the duration was shorter than average,
<br />extending only from mid-April 10 mid-June (Figure 23). Be-
<br />cause of this shorter than normal duration, little overlap ex-
<br />isted between the snowmelt flood and the onset of rributary
<br />sediment-supplying floods during the summer thunderstorm
<br />season. Thus 1954 provides a clear example of the response of
<br />the river to changes in the upstream supply of sediment during
<br />the annual snowmelt flood, without any of the complications
<br />due to resupply of sediment to the river during the subsequent
<br />summer thunderstorm season. AI; the upstream supply of sed-
<br />iment decreased during the 1954 snowmelt flood (Figure 2b),
<br />the suspended sand coarsened (Figure 2<:) and the bed began
<br />to scour (Figure 2d). Also, note that in 1954 the maximum bed
<br />elevation led the flood peak by about a week.
<br />
<br />4.2. Vertical Grain-Size Trends in Predam Flood Deposits
<br />in Marble and Grand Canyons
<br />Sediment deposited in eddies in the Colorado River pro-
<br />vides an accurate record of changes in suspended.sediment
<br />grain size during floods [Rubin et al., 1998; Topping el al., 1999,
<br />this issue]. To determine trends in grain size recorded in pre-
<br />dam flood deposits, we sampled predam flood deposits verti.
<br />cally for grain size at six sites in Marble and Grand Canyons
<br />during 1997 and 1998 (Figure 1). AI; we [ound in deposits of
<br />the 1996 flood experiment [Rubin et al., 1998; Topping et al.,
<br />1999] and the 1997 test flow [Topping el 01., this issue], the
<br />predam flood deposits in the majority of Marble Canyon (i.e.,
<br />below river mile 2) and Grand Canyon coarsened upward. This
<br />coarsening occurred both by a decrease in the percentage of
<br />silt and clay and also by coarsening of the sand (Figure 3). The
<br />deposit at river mile 1 (at the head of Marble Canyon) did not
<br />coarsen upward, however. In this deposit the silt and clay
<br />content increased, and the sand fined upward.
<br />
<br />4.3. Discussion
<br />All [our of the hypothesized effects of sediment supply lim-
<br />itation occurred in the predam Colorado River in Grand Can-
<br />
|