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<br />NAU Sand Bar Studies
<br />
<br />Final Report
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<br />velocity pools caused by drops that occur at rapids or riffles (Leopold, 1969), Rapids and riffles are
<br />associated with river constricting debris fans fanned by debris flows and floods from steep, side
<br />canyon tributaries (Howard and Dolan, 198 I; Webb et ai" 1989; Melis et ai" 1994), Pool length is
<br />determined by the spacing between debris fans, whose locations are controlled by local jointing,
<br />faulting and bedrock structure (Dolan et ai" 1978), Schmidt and Rubin (I995) tenned this basic,
<br />repeating channel unit as the "fan-eddy complex", which is composed of the channel constricting
<br />debris fan, an upstream pool created by the backwater effect of the constriction, a channel expansion
<br />and plunge pool immediately downstream from the fan, and a gravel bar further downstream, The
<br />accelerated flow through the rapids results in downstream scour holes in the main channel bed and
<br />flow separation leading to the formation of eddies in the channel expansion, Scour holes can be as
<br />much as nine times deeper than the depth in the upstream rapid (Schmidt and Graf, 1990) and
<br />potentially have the capacity to accumulate significant amounts of sand during tributary flooding
<br />(Wiele et ai" 1996), The low velocities in eddies promote deposition from the suspended load
<br />(Leopold, 1969),
<br />Sand deposits in eddies have been described and classified by Schmidt (I990) as separation bars,
<br />which fonn near the upstream part of the eddy; and reattachment bars, which form where flow
<br />reattaches to the bank, Eddy sand bars in some reaches contain up to 75 percent of the total sand
<br />stored along the banks of the Colorado River (Schmidt and Rubin, 1995), Eddies have the potential to
<br />completely fill with sediment (Schmidt et ai" I 999b) and contain bars more than 10-m thick (Rubin et
<br />ai"1994), In contrast, the extent of sand distribution in the main channel varies from complete
<br />coverage of dune fields I to 2 m thick to patchy coverage over an immobile bed of bedrock and gravel
<br />(Howard and Dolan, 1981; Wilson, 1986; Anima et ai" 1998), A greater percentage of the bed in
<br />narrow reaches is composed of bedrock and gravel because the channel is generally deeper and has a
<br />steeper water slope (Wilson, 1986; Schmidt and Graf, 1990),
<br />Sand supplied to the river by tributary floods is temporarily stored in pools and in eddies; however,
<br />the relative proportion of sand stored in these two environments has not been detennined (Schmidt,
<br />1999). An important finding now in question, in the 1996 Record of Decision (ROD) for the Glen
<br />Canyon Dam Final Environmental Impact Statement, is that sand accumulates on the bed in Marble
<br />Canyon at most dam releases because of tributary inputs from the Paria River and smaller, ungaged
<br />tributaries (V,S, Department of Interior, 1995), Sand mass balance models using stable sediment
<br />rating curves were developed that predicted aggradation of sand between the Paria and Little Colorado
<br />Rivers if peak discharges were less than power plant capacity, tributary inflows were at least average,
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