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<br />NA U Sand Bar Studies
<br />
<br />Final Report
<br />
<br />INTRODUCTION
<br />
<br />After 1963, deposition of fine-grained sediment in the Colorado River ecosystem below Glen
<br />Canyon Dam (Glen, Marble and Grand Canyons) depended on annual operations combined with
<br />limited sediments supplied by tributaries (V,S, Department ofInterior, 1995), The dam eliminated
<br />upper basin sediment supply, making downstream tributaries the only source for replenishing sand
<br />deposits on the bed and banks of the Colorado River. Channel margin sand deposits are important
<br />environmental resources, providing substrate for some habitats of endangered and native fish species
<br />(Valdez and Ryel, 1995), riparian vegetation, marsh and wetlands (Stevens et aI., 1995), and are also
<br />used as recreational campsites (Kearsley et a!., 1994), However, sand delivery from tributaries is
<br />highly variable and the dam has dramatically reduced the frequency and magnitude of both low and
<br />high flows, This flattening of the annual hydrograph has also reduced the net storage potential of
<br />sand at higher shoreline elevations, Additionally, operations have elevated seasonal minimal flows
<br />to the point that most dam releases now have considerable sediment-transport capacity, In the
<br />pre-dam era, prolonged low flows (less than 200 to 250 m3/s) allowed fine sediment to accumulate
<br />in the channel during summer through winter seasons (Topping et aI., 2000a).
<br />The first opportunity to study physical processes during controlled flooding of the Colorado
<br />River ecosystem occurred in spring 1996, with the re.lease of a seven day, controlled flood of 1,274
<br />m3/s (45,000 ft3/s) from Glen Canyon Dam (Collier et aI., 1997; Webb et aI., 1999), A major
<br />objective of the 1996 Controlled Flood was to detennine if high releases from Glen Canyon Dam
<br />could effectively redistribute sand from the river bed to the channel margins (Schmidt et aI., 1999a),
<br />Results from investigations conducted during the experiment indicate that this objective was
<br />achieved (Andrews et aI., 1999; Hazel et aI., 1999; Schmidt, 1999; Wiele et al., 1999), Rates of sand
<br />bar deposition were more rapid than expected and the majority of sand bar deposition occurred
<br />within the first two days (Andrews et aI., 1999; Rubin et aI., 1998; Schmidt, 1999), Suspended-
<br />sediment progressively coarsened over time (Topping et aI., 1999; 2000b) and the grain size of post-
<br />flood deposits vertically-coarsened (Rubin et aI., 1998; Topping et aI., 1999; 2000b), presumably
<br />because of rapid depletion of the supply of fine-grained sediment. Modeling of depositional
<br />processes indicates that rates of bar deposition were proportional to the supply of sediment (Wiele et
<br />aI., 1999), These physical process studies demonstrated that controlled flooding could be used in
<br />river management but that the design of future high releases need take into account the effect of
<br />decreasing main-channel sand concentrations on transport and deposition of sediment (Rubin et ai"
<br />1998; Schmidt, 1999),
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