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<br />Figur.6 (continued)
<br />
<br />flood [Schmidl el a/., this volume]. At each site, we
<br />collected samples at multiple elevations between the base
<br />and top of the deposit. Just as the suspended sediment and
<br />bed sediment coarsened during the flood, the sediment
<br />deposited during lhe flood also coarsened, Inverse grading
<br />of flood deposits has been described by OSlerkamp and
<br />Cosla [1987] and studied by Iseya [1989], who attributed
<br />formation of inversely graded flood deposits in Japanese
<br />rivers 10 changing sediment supply during floods, In the
<br />1996 flood deposits that we studied at the 5 eddy bars. the
<br />median grain size increased from 0,073-0,11 mm at the
<br />base to 0,11-0,21 mm at the top (Figure 6); on average, Ihe
<br />median grain size coarsened upward by a factor of 1.6, As
<br />with the suspended load (Figure 4), the increase in median
<br />grain size generally occurred not merely by the removal of
<br />fines, but also by an increase in the modal size and an
<br />increase in size of lhe coarsest fraclion (Figure 6),
<br />The most common sedimentary structure in the 1996
<br />flood deposits was climbing-ripple cross-stratification, at
<br />some sites overlain by trough cross-stratification deposited
<br />by migrating fluvial dunes (Figure 7), This locally observed
<br />change from ripples to dunes was not caused by a change in
<br />flow conditions. bUI by the increase in grain size of
<br />sediment supplied to the eddies. The transition from ripples
<br />(0 dunes. driven only by a change in grain size, with no
<br />change In flow condilions. has been empirically
<br />documenled through bedform phase diagrams [Sourhord.
<br />1971; Rubin and McCulloch. 1980; SOllThard and
<br />Boguch....ol. 19901, In Ihe rock record, observations of
<br />
<br />upward coarsening and change in bed configuration from
<br />ripples to dunes are typically interpreted to indicate
<br />stronger flows, but this is nO( necessarily the case. In (he
<br />1996 flood, peak discharge was held conSlam; winnowing
<br />caused the bed to coarsen, and coarsening of the bed caused
<br />the change in bed configuration, This was reflecled in the
<br />sedimenlology of the newly deposiled bars throughout the
<br />river below Glen Canyon Dam,
<br />
<br />6,2, Deposits of Pre-Dam Floods and Other Unsteady High
<br />Flo....s
<br />
<br />The hydrograph of the 1996 controlled flood was unusual
<br />with respect to both pre-dam snowmelt and post-dam
<br />tributary-driven floods, not because of lhe magnitude or
<br />duration of high flow, but because peak discharge was
<br />conslant. Natural floods are typically more unsteady.
<br />making it more difficult to interpret vertical trends in grain
<br />size, The difficulty arises not only because individual flood
<br />deposits become more difficult to recognize, but also
<br />because fluctuations in stage can cause changes in deposi-
<br />tional processes at anyone site (and such changes mighl
<br />influence local grain-size sorting).
<br />Changes in depositional processes are particularly
<br />pronounced at higher-elevation sites on (he bank, where a
<br />small change in stage can produce a large change in the
<br />local flow regime. Such a situation occurred in January
<br />1993, for example, when a flood of 510 m)/s on the Lillie
<br />Colorado River combined with a rnainstern discharge of
<br />
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