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<br />NA U Sand Bar Studies
<br />
<br />Final Report
<br />
<br />Controlled Flood, The estimated sand export from Marble Canyon during the 2-day test flow was
<br />70,000 ml (0,19 :t 0.04 million Mg), approximately 9% of the total Paria River sand inputs in August-
<br />September, 1997. This suggests that a substantial portion of the 1997 Paria River sand input was still
<br />retained in Marble Canyon when the 1997 Test Flow was released, Much of this sediment, however,
<br />may have accumulated in lower Marble Canyon as the sand inputs moved downstream, This
<br />interpretation is supported by the fact that the net effect of the 1997 Test Flow within the first 3-km
<br />reach of Marble Canyon was erosion, Less than 10% of the 1997 August-September sand inputs
<br />remained in that reach after implementation of the 2-day test, More accurate estimates of sand
<br />transport rates following tributary inputs require longer intervals of suspended sediment sampling at
<br />the gages. In addition, we could not determine if eddies were a source of low elevation sand (similar
<br />to the 1996 Controlled Flood response) during the 1997 Test Flow because the required hydrographic
<br />data could not be collected,
<br />Despite occurring within 2 months of the Paria River sand inputs, the 1997 Test Flow was not
<br />effective at long-term conservation of the tributary supplied sediment at elevations above the 566 ml/s
<br />(20,000 fe/s) stage, The topographic measurements at the study sites in Marble Canyon show that the
<br />1997 Test Flow did not result in significant and persistent high elevation deposition, The new deposits
<br />were completely eroded by April 1998, Aggradation of at high elevation was limited, even though
<br />sites for potential deposition (accommodation space) were available and suspended sand
<br />concentrations were high, We conclude that the geomorphic effects of the 1997 Test Flow were
<br />largely stage-limited rather than controlled by fine-sediment supply limitations in Marble Canyon,
<br />Although the magnitude and duration of the test did not result in widespread deposition of high-
<br />elevation bars, it did duplicate processes at lower elevations which were observed during the 1996
<br />Controlled Flood: high suspended sediment concentrations that decreased with time, suspended and
<br />bed material grain size increases, inversely graded deposits, and at least one bar failure,
<br />The results of this study should provide some guidance for those developing physically based
<br />models of the transport and deposition of tributary sand inputs through the Colorado River ecosystem,
<br />For example, a one-dimensional sand transport model, coupled with an unsteady flow model that
<br />incorporates reach-averaged hydraulic geometry (Wiele and Smith, 1996), is being developed to
<br />predict the rate at which different grain size fractions are transported downstream under a range of dam
<br />operations (Wiele and Franseen, 1999), Continued monitoring and research of the physical processes
<br />that control sediment transport and deposition in eddies and main channel pools are needed for the
<br />formulation and application of models so that fluvial processes are accurately represented.
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