HR: 1340h
AN: H43A-0363    [Abstracts]
TI: Adjustments of Bed Sediment Texture to Variations in Shear Stress in High Gradient Streams
AU: * Pitlick, J
EM: pitlick@colorado.edu
AF: Geography Department, Box 260, University of Colorado, Boulder, CO 80309-0260 United States
AU: Mueller, E
EM: erich.mueller@colorado.edu
AF: Geography Department, Box 260, University of Colorado, Boulder, CO 80309-0260 United States
AU: Segura-Sossa, C
EM: catalina.segurasossa@colorado.edu
AF: Geography Department, Box 260, University of Colorado, Boulder, CO 80309-0260 United States
AU: Torizzo, M
EM: margaret.torizzo@anr.state.vt.us
AF: Vermont Agency of Natural Resources, 103 South Main St, Bld 10N, Waterbury, VT 05671-0408 United States
AB: The sediment supplied to mountain streams consists of a mix of grain sizes ranging from sand to boulders. In most streams the sediment supplied quickly separates into two modes: A coarse mode that is retained on the bed surface, forming a mobile armor layer, and a fine mode that goes into temporary storage in the bed, forming the substrate. Interactions between the flow, the bed surface, and the substrate help to balance the sediment supply between neighboring stream reaches. This study investigates trends in surface and substrate grain sizes in relation to reach-scale geomorphology using data from gravel-bed streams in Colorado and Utah. The data set includes more than 100 co-located measurements of surface and substrate sediment, bankfull channel geometry and reach-average slope. Slopes at the study sites range from 0.0003 to 0.07; bankfull depths range from 0.2 to 5 m; and bankfull widths range from 2 to 200 m. The data indicate that both the bed surface and the substrate become more poorly sorted as the bankfull shear stress increases. The decrease in sorting with increasing stress reflects a greater separation between surface and substrate grain-size parameters (D84 and D50). The ratio of surface D50 to substrate D50 increases from about 2 in reaches with low shear stress to about 4 in reaches with high shear stress. Similar trends are observed in the surface D84 and substrate D84. Supposing the size distribution of the load is similar to the substrate, as proposed in some models of bed load transport, our results suggest that retention of the coarsest fraction of the sediment supplied to headwater reaches limits the mobility of the finer sizes which constitute the bulk of the load. The high potential for transport of fine grains is therefore offset by the sheltering provided by large grains.
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting