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<br />. ..~~.~1:- ,---~Q <br />,:~'7": -; ....~ <br />'~~' '>>>.- <br />t'~.. ...;..~. ~ '1;.: <br />" :. '~'o' , <br />. .~~i':,:"~!, <br />- ".llf~. ",.:, ......."~ <br />Figure 9. Photographs of Badger Creek Rapids at River Mile 8. View is upstream. The rapid is at the center of the <br />photo and the glassy water surface upstream from the rapid is the ponded backwater. Eddies occur on both banks <br />downstream from the rapids and separation bars mantle the downstream parts of both debris fans. Thl3 EDZ on river <br />left, including the separation bar, is monitored by NAU as RM8. A. Photograph by Franklin Nims taken on December <br />28, 1889. B. Photograph by Dave Edwards taken on January 30, 1991. Both photographs are from Webb (1996, fig. <br />10.2) and are reprinted with permission. <br /> <br />001582 <br /> <br />. <br /> <br />Marble Canyon was "at least a few tens of <br />centimeters and not more than a few meters." <br />Subsequent monitoring using side-scan imag- <br />ing and underwater cameras showed that the <br />proportion of the bed covered by sand is less <br />than 30% and typically occurs upstream from <br />rapids and adjacent to some eddies (Anima et <br />aI., 1998). Schmidt (1999) used a sediment <br />budget to argue that the bed was not a signifi- <br />cant source of sand during the 1996 Controlled <br />Flood, and Hazel et aI. (J. E. Hazel, Jr., D. J. <br />Topping, J. C. Schmidt, M. Kaplinski, and T. <br />S. Melis, "Downstream effects of a dam on <br />sediment storage in a bedrock canyon: the <br />relative roles of eddy and channel storage in <br />the sediment budget for the Colorado River in <br />Marble Canyon, AZ," unpubI. manuscript) <br />argued that most sand stored in the study area <br />occurs in eddies and not on the channel bed. <br />Data presented in subsequent sections of this <br />report support that conclusion. <br /> <br />. <br /> <br />3.2 The Fan-Eddy Complex <br /> <br />The distribution of alluvial deposits along <br />the channel edge is determined by the hydrau- <br />lic patterns created by each fan, and this <br />pattern is similar at each fan (Fig. 9). Schmidt <br />and Rubin (1995) defined the fan-eddy com- <br /> <br /> <br />. <br /> <br />plex as the sequence of hydraulic features that <br />occurs wherever a tributary debris fan partially <br />blocks the flow. The most upstream part of <br />each fan-eddy complex is the ponded backwa- <br />ter upstream from the fan (Fig. 10). The <br />narrow channel and elevated bed of each rapid <br />act as a hydraulic control on flow upstream <br />from the fan, and ponding may extl~nd up- <br />stream between a few channel widths to a few <br />kilometers (Kieffer, 1985). The upstream <br />extent of ponding varies with discharge; <br />differences in channel geometry cause some <br />rapids to be drowned at high flow, whereas the <br />degree of hydraulic control at other sites may <br />become increasingly severe (Kiem~r, 1988). <br />Flow separation occurs immediately <br />downstream from most rapids, whe:re the bank <br />angle diverges abruptly from the orientation of <br />the main flow (Schmidt, 1990). A zone of <br />recirculating flow exists along the bank down- <br />stream from the point of flow separation. This <br />zone is typically organized into a single recir- <br />culating cell with upstream flow along the <br />bank; smaller, secondary cells of n~circulation <br />may exist at some sites at some discharges. <br />The length of the recirculating flow zone <br />changes with discharge, and the zone <br />typically gets longer and thinner at high dis- <br />charge. <br /> <br />A. <br /> <br />B. <br /> <br /> <br />3.0 The Valley of the Colorado River 13 <br />