HR: 11:15h
AN: B42A-04    [Abstracts]
TI: The Influence of Modern and Holocene Tributary Sedimentation Events on Main-Stem Channel Heterogeneity, South Fork Payette River, Idaho
AU: * Pierce, J L
EM: pierjenn@isu.edu
AF: Idaho State University, Department of Geosciences, Pocatello, ID 83209-8072 United States
AU: Meyer, G A
EM: gmeyer@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131 United States
AB: Sediment-charged floods and debris flows from tributaries are a major source of sediment to the main channel of the South Fork Payette River. A single debris-flow event in 1997 deposited ~ 11,000 m³ of sediment in the main channel, but a lack of cobble-to-boulder sized clasts resulted in little change in mainstem channel morphology. Conversely, large boulders in other tributary debris flows produced persistent major channel rearrangement during the same storm. Analysis and radiocarbon dating of alluvial fan stratigraphy from 32 tributary fans reveals a record of sedimentation events spanning the last ca. 8000 years. Estimated event magnitudes based on deposit thickness indicate that magnitudes vary greatly over time, likely with Holocene climate changes. Over the last ca. 4000 years, small sedimentation events are almost twice as common as large events, but large events account for 66% of the total dated fan thickness. Only 9 large fire-induced debris flows emplaced ~25% of the total dated fan thickness during the Medieval Climatic Anomaly (ca. 1000-800 cal yr BP), a time of widespread and severe western US droughts. Since rare large sedimentation events strongly affect sediment yields and channel heterogeneity, climatic control of these events has major implications for riverine habitat and populations.
DE: 1812 Drought
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
DE: 3344 Paleoclimatology
SC: Biogeosciences [B]
MN: 2005 Joint Assembly