HR: 0800h
AN: H31A-1284    [Abstracts]
TI: What Controls Deposition on Debris-Flow Fans Along the Eastern Sierra Nevada, California?
AU: * Duehnforth, M
EM: duehnforth@erdw.ethz.ch
AF: ETH Zurich, Institute of Geology, Zurich, 8092 Switzerland
AU: Densmore, A L
EM: densmore@erdw.ethz.ch
AF: ETH Zurich, Institute of Geology, Zurich, 8092 Switzerland
AU: Ivy-Ochs, S
EM: ivy@phys.ethz.ch
AF: University of Zurich, Institute of Geography, Zurich, 8057 Switzerland
AU: Ivy-Ochs, S
EM: ivy@phys.ethz.ch
AF: ETH Zurich, Institute of Particle Physics, Zurich, 8093 Switzerland
AU: Allen, P A
EM: philip.allen@erdw.ethz.ch
AF: Imperial College London, Department of Earth Science and Engineering, London, SW7 2AZ United Kingdom
AU: Kubik, P W
EM: kubik@phys.ethz.ch
AF: Paul Scherrer Institute, c/o ETH Zurich, Zurich, 8093 Switzerland
AB: Many studies have addressed the controls on debris-flow fan segmentation and fan slopes, but we still lack a general quantitative model for large-scale fan geomorphology, including such basic observables as the number of depositional fan lobes and the degree of incision at fan heads. Debris-flow deposition and incision on fans is mostly attributed to either climatic, tectonic, or internal controls. It is often assumed that a climatic control produces a regionally similar geomorphic response in the landscape. But does this mean that we can exclude climatic influence in cases where adjoining landforms have a different geomorphology? If so, does this imply a tectonically-driven landscape or does it indicate intrinsic forcing? Here, we attempt to elucidate the forcing parameters for the large-scale geomorphology of two debris-flow fans, Shepherd and Symmes Creek fans, in Owens Valley, California. We characterize the fan surfaces and depositional history using topographic analysis of 1-m resolution Airborne Laser Swath Mapping (ALSM) data and an absolute fan chronology derived from measurements of cosmogenic ¹⁰Be of debris-flow boulders. Our first results indicate that Shepherd and Symmes Creek fans exhibit variations in the number and ages of depositional lobes, the degree of incision at the fan head and the shape of the catchment stream profile. The existence of overdeepened, sediment-filled subbasins in the trunk valleys, which act as sediment traps, has profound implications for the dynamics of debris-flow processes and large-scale fan geomorphology. Trapping of sediment leads to undercapacity water flows that are able to incise the fan head, causing abandonment and switching of depositional lobes. Thus, fan dynamics and large-scale fan geomorphology are strongly coupled to catchment properties. Even though the basic conditions of sediment delivery and mobilization are set by climatic, tectonic, and lithologic controls, these controls may be modified by intrinsic triggering of debris flows. Therefore, we expect that systematic information on the timing and controls of debris-flow deposition may be derived from fans where sediment deposition is a direct function of sediment availability, mobilization, and transport. In contrast, in cases where sediment trapping occurs, the fan chronology may be decoupled from variations in the external forcing.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1810 Debris flow and landslides
DE: 1824 Geomorphology: general (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005