HR: 16:45h
AN: H42L-04    [PDF]
TI: Episodic Mass Transfer During Extreme Floods from a 70,000 km$^{2}$ Andean River Basin to Amazonian Floodplains
AU: Maurice-Bourgoin, L
EM: lmaurice@unb.br
AF: Institut de Recherche pour le D‚veloppement, UMR Laboratoire des M‚canismes de Transfert en G‚ologie, CP 7091 Lago Sul, CEP, Brasilia, DF 71619-970 Brazil
AU: * Aalto, R
EM: aalto@u.washington.edu
AF: Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310 United States
AU: Dunne, T
EM: tdunne@esm.ucsb.edu
AF: Donald Bren School of Environmental Science and Management and Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Montgomery, D R
EM: dave@geology.washington.edu
AF: Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-1310 United States
AU: Nittrouer, C A
EM: nittroue@ocean.washington.edu
AF: School of Oceanography, University of Washington, Seattle, WA 98195-7940 United States
AU: Guyot, J
EM: guyot@cict.fr
AF: Institut de Recherche pour le D‚veloppement, UMR LMTG, CNRS/IRD Paul Sabatier University, Toulouse, F-31400 France
AB: Continental-scale, sand-bedded rivers sequester a significant proportion of their sediment load in lowland basins, but the hydraulic and hydrologic conditions under which sediment efflux from mountainous sources and downstream floodplain storage occur are poorly understood. A new geochronological method makes it possible to identify discrete events and measure the accumulation from such processes with near-annual resolution, and thereby resolve the spatial and temporal distribution of individual accumulation events across large, dynamic fluvial dispersal systems. Our measurements along two rivers at a variety of distances from the channel reveal previously undocumented spatial and temporal patterns that reflect the hydraulics of sedimentation and the occurrence of flooding and sediment delivery forced by global atmospheric processes. Specifically, we quantify century-scale floodplain sedimentation rates across a pristine 720,000 km$^{2}$ basin covering the principal sediment and water sources for the Madeira River, the largest sediment source for the Amazon. Accumulation is dominated by crevasse splays emplaced hundreds to thousands of meters from the channel as lenticular, lobate deposits, with system-wide and local recurrence intervals of ~8 years and multiple decades, respectively. Ocean temperature and stream flow records link these episodic events to cold-phase ENSO rapid-rise floods that also debouch extraordinary volumes of sediment from the Andes; we explore the timing and mechanics of these events, both in terms of the Andean denudation (from a 70,000 km$^{2}$ mountainous basin) and the downstream transfer and emplacement of sediment across lowland foreland basins. Transient processes control the formation of floodplains and sedimentary strata, and modulate the efflux, transport, floodplain storage, exchange, and downstream delivery of sediment and associated carbon, nutrients, and heavy metals to the Amazon River.
DE: 1035 Geochronology
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2003 Fall Meeting