HR:     0830h
AN:     H21A-30

TI:     Response of Bed Mobility to Sediment Supply in Natural Gravel Bed Channels: A 
        Detailed Examination and Evaluation of Mobility Parameters

AU:     *Lisle, T E
EM:     tel7001@axe.humbodlt.edu
AF:     Pacific Southwest Research Station, USFS, 1700 Bayview Dr. Arcata, CA 95521  
        United States

AU:     Nelson, J M
EM:     jmn@brrcr.mail.cr.usgs.gov
AF:     US Geological Survey, Denver Federal Center Denver, CO 80225  United States

AU:     Barkett, B L
EM:     barkettb@ucsu.colorado.edu
AF:     Dept. of Geography, Univ. of Colorado Boulder, CO 80309  United States

AU:     Pitlick, J 
EM:     John.Pitlick@Colorado.EDU
AF:     Dept. of Geography, Univ. of Colorado Boulder, CO 80309  United States

AU:     Madej, M A
EM:     mam2@axe.humboldt.edu
AF:     US Geological Survey, Redwood National Park Arcata, CA 95521  United States

AB:     Recent laboratory experiments have shown that bed mobility in gravel bed channels responds 
        to changes in sediment supply, but detailed examinations of this adjustment in natural channels
        have been lacking, and practical methodologies to measure bed mobility have not been tested.
        We examined six gravel-bed, alternate-bar channels which have a wide range in annual 
        sediment yield (2 reaches of Redwood Cr., Jacoby and Grouse Cr. in northern California, and 
        Gunnison R. and Williams Fork in Colorado). From detailed measurements of channel 
        topography and surface particle size, we modeled local values of boundary shear stress and 
        bedload transport rate at bankfull stage and use these to evaluate two mobility parameters: 
        mean Shields stress (t*) and q* [Dietrich et al, 1989]. In all channels, small areas conveyed 
        large percentages of bedload, especially in channels with low mobility.  This nonuniformity 
        insured significant bedload transport even in channels with low mean t*.  Values of mean t* 
        correlated well with sediment supply: values of mean t* were $>$0.1 in channels with high 
        supplies and $<$0.02 in those with low supplies.  Values of q* also correlated well with supply, 
        but there was one exceptionally high value in a channel with low supply.  Substituting 
        subsurface bed material size for bedload particle size (a variable in q*) may have been 
        inaccurate in this channel. We conclude that both t* and q* are valid measures of the link 
        between sediment supply and bed mobility in natural channels.  However, t* is more practical 
        because it does not rely on a measure of bedload particle size, which can be difficult to obtain. 
        A reasonably accurate value of mean t* can be readily obtained by carefully measuring mean 
        hydraulic variables and segregating grain roughness.

DE:     1815  Erosion and sedimentation
DE:     1860  Runoff and streamflow
SC:     H
MN:     1998 Fall Meeting