Paper No. 130-12
Presentation Time: 4:45 PM
THE LASTING LEGACY OF MEGAFLOOD BOULDER DEPOSITION IN MOUNTAIN RIVERS
SHOBE, Charles M., U.S. Forest Service, Rocky Mountain Research Station, Fort Collins, CO 80526; Department of Geology & Geography, West Virginia University, PO Box 6300, Morgantown, WV 26506-6300, MOREY, Susannah, Department of Earth and Space Sciences, University of Washington, 4000 15th Avenue NE, Seattle, WA 98195, HUNTINGTON, Katharine W., Earth and Space Sciences, University of Washington, Seattle, WA 98195, LANG, Karl, Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Dr., Atlanta, GA 30318, JOHNSON, Graham, Earth and Space Science Department, University of Washington, Johnson Hall Rm-070, Box 351310, 4000 15th Avenue NE, Seattle, WA 98195 and DUVALL, Alison R., Department of Earth and Space Sciences, University of Washington, 4000 15th Avenue NE, Seattle, WA 98195-1310
Current conceptual and numerical models of net-erosional mountain landscapes cast rivers as efficient agents of sediment detachment, entrainment, and transport. While this must be true on timescales over which river incision keeps pace with baselevel lowering, field evidence shows that some of Earth’s most erosive rivers experience intense episodes of sediment deposition driven by glacier- and landslide-lake outburst megafloods. The influence of megaflood-derived sediment—which is typically poorly sorted and can contain meter to decameter scale boulders—on post-flood river evolution is poorly understood. Here we investigate the fluvial response to megaflood boulder deposition. We use a numerical model of river longitudinal profile evolution that accounts for the erosion-inhibiting effects of boulders to ask 1) how megaflood boulder deposition changes channel morphology and 2) how long those changes last. The model uses the size distributions of boulders measured from field observations and satellite imagery of the Yarlung-Siang River, eastern Himalaya, and interpreted to be megaflood-derived.
Megaflood boulder deposition generates knickpoints as boulder bars preferentially shield underlying portions of the channel bed from erosion. When the river has insufficient power to abrade and mobilize the boulders, those knickpoints grow in amplitude and remain at the location at which they were initiated. When, with the aid of steepening at boulder-induced knickpoints, the river is able to move the boulders, boulder-induced knickpoints migrate upstream and diffuse. Results suggest that river profiles may host geomorphic signatures of boulder deposition from a single megaflood over timescales longer than known megaflood recurrence intervals (103-104 years) and possibly longer than glacial-interglacial cycles. Fluvial deposition may therefore exert a critical control on river incision, landscape form, and source-to-sink sediment dynamics even in Earth’s most rapidly eroding landscapes.
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