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PITLICK AND CRESS: DOWNSTREAM CHANGES IN CHANNEL GEOMETRY 34 - ii
<br />channel, such that the shear stress is sufficient to carry coarse
<br />material across a lower slope with little additional discharge.
<br />[31] The physical mechanisms of channel adjustment in
<br />the Colorado River are strongly tied to bed load transport
<br />processes, as shown by the consistency in the bank-full
<br />Shields stress, Tb. Our measurements indicate that Tb does
<br />not vary with distance or discharge, averaging 0.049 for the
<br />reach as a whole. The consistency in T b suggests that in
<br />spite of differences in slope and grain size, the Colorado
<br />River has adjusted its bank-full width and depth to a Shields
<br />stress that is 20-50% higher than typical thresholds for
<br />motion (0.03-0.04). The equilibrium width and depth
<br />therefore appear to be maintained by flows slightly in
<br />excess of the threshold for motion.
<br />[32] Acknowledgments. Financial support for this study was pro-
<br />vided by the US Fish and Wildlife Service (USFWS); we thank Doug
<br />Osmundson and Chuck McAda of the USFWS for their assistance. Mark
<br />Van Steeter supplied cross section data for the 15- and 18-mile reaches;
<br />Brent Barkett, Cheri Cornell, Ingrid Corson, Margaret Franseen, Lex Ivey,
<br />David Lewis, Jennifer Nissenbaum, and Rebecca Thomas assisted in the
<br />collection of field data. Discussions with Gary Parker helped clarify our
<br />interpretation of the data, and written comments from Michael Church, Rob
<br />Ferguson, Tom Lisle, Chris Paola, Jack Schmidt, and Peter Wilcock helped
<br />sharpen the focus of the paper.
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<br />R. Cress and J. Pitlick, Department of Geography, Box 260, University
<br />of Colorado, Boulder, CO 80309, USA. (pitlick@spotcolorado.edu)
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