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<br />94 GRAIN-SIZE EVOLUTION DURING FLOODS
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<br />Figure 7. Photo of the upper 1.5 m of a S-m.thick deposit of the 1996 flood, The lower left shows finer-grained
<br />climbing-ripple slructures (ripple foresels dip toward (he righI), whereas the top 0.5 m shows coarser cross-bedded strata.
<br />
<br />270-470 m)/s (fluctuating daily for powerplant needs),
<br />Deposits of this 1993 combined flow commonly exhibit a
<br />vertical sequence of 3 sedimentary structures produced by
<br />the daily flow fluctuations (Figure 8a): re-deposition of
<br />clasts of older sand (formed as waves undercut banks and
<br />clasts of older cohesive sand fen off the bank), deposition
<br />of flat-sand beds (within the wave. swash zone adjacent to
<br />the cut bank), and deposition of climbing ripples (produced
<br />by waves and currents in slightly deeper water farther from
<br />the bank), This sequence can be thought of as a trans-
<br />gressive sequence consisting of basal conglomerate, beach,
<br />and offshore deposits, The vertical succession arises
<br />because the 3 environments (cut bank, swash zone, and
<br />offshore ripples) occur adjacent to each other in succes-
<br />sively deeper water (Figure 8b), As the river stage
<br />fluctuates, as during the 1993 example, the depositional
<br />environments shift lalerally. producing a vertical succession
<br />of these deposits at anyone point. Because a single ~ood
<br />may have multiple fluctuations in discharge (and therefore
<br />multiple local inundations), identifying the deposit of an
<br />individual flood can be difficult. Moreover, grain.size
<br />trends may be influenced by differences in sorting between
<br />bank undercutting, beach swash, and offshore ripples,
<br />In some beds produced during pre-dam floods, however,
<br />the base and lOp of the deposit are clearly identifiable (e,g"
<br />by thin soil horizons with concentralions of rootlets), and
<br />the entire deposit consists of a single depositional facies.
<br />We locmed 4 such pre-dam !lood beds, and sampled them
<br />
<br />vertically for grain size, The results (Figure 6) show an
<br />upward coarsening from 0,039-0,061 mm 10 0,057-0,12
<br />mm,
<br />Although these pre-dam deposits are finer than the 1996
<br />deposits, the relative upward coarsening is the same (a
<br />factor of 1,6 from base to top), lbese similar rates of coars-
<br />ening are probably fortuitous; a longer or shorterflood may
<br />have achieved a greater or lesser degree of upward coars.
<br />ening, respectively, The 1996 flood might have been
<br />expected to produce a more rapid grain-size coarsening,
<br />because the 1996 flood supplied clear water released from
<br />Glen Canyon Dam, rather than the sediment-laden water in
<br />the pre.dam regime, However, the 1996 flood was of
<br />shorter duration and lower peak discharge relative 10 pre-
<br />dam floods, and these factors would lend to reduce the rale
<br />of coarsening, Evidently these opposing changes from pre-
<br />dam to 1996 balanced out in such a way as to produce a
<br />similar degree of upward coarsening between the 1996 and
<br />pre-dam flood deposits,
<br />The fact that the individual flood deposits coarsen
<br />upward might seem to con~ict with the fining-upward
<br />structure that is characteristic of Olher fluvial deposits. The
<br />apparent conflict is due 10 a difference in scale between
<br />individual ~ood deposils and lhe cverall depositional
<br />sequence. which encompasses many Aood deposits. Despite
<br />the upward coarsening of individual flood beds, the gener-
<br />alized fluvial sequence In Grand Canyon still fines upward
<br />on a larger scale, from boulders, gravel. and sand on the
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