HR: 13:55h
AN: H33F-02    [Abstracts]
TI: Testing Model Predictions of the Evolution of Valley Spacing
AU: * Perron, J T
EM: perron@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Kirchner, J W
EM: kirchner@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AB: Landscapes often exhibit a characteristic spatial scale of dissection, which is most apparent as the periodic spacing of adjacent valleys. We use a numerical landscape evolution model to show that valley spacing is controlled primarily by the relative rates of advective (e.g., stream incision) and diffusive (e.g., soil creep) erosion processes. The time-dependent model behavior reveals that a steady-state valley spacing is approached more rapidly than the overall topographic steady state. This suggests that some aspects of the steady-state model solutions may be applicable to a landscape that has not yet reached a topographic equilibrium. The model solutions show that valley spacing depends linearly on the ratio of the magnitudes of the advective and diffusive erosion terms, which is analogous to a Péclet number. As the Péclet number increases, the spacing between adjacent valleys narrows. We conduct a test of this scaling relationship by measuring process rates in the Gabilan Mesa, a landscape in central California with strongly periodic valley spacing of ~200 m. The topography of the Gabilan Mesa has developed through the erosional dissection of a formerly planar alluvial surface, of which some portions are preserved. We use surface exposure and burial ages derived from concentrations of cosmogenic ¹⁰Be and ²⁶Al in quartz to bracket the time when the erosional dissection began. Dividing this timescale into the elevation of the initial surface above the present ground surface gives a time-averaged erosion rate at any point in the landscape. Using these erosion rates and high resolution topographic profiles of hillslopes and stream channels, we invert for the parameters in the diffusive and advective erosion terms. We then calculate the Péclet number for the Gabilan Mesa and compare the valley spacing predicted by the scaling relationship with the observed valley spacing. We discuss the result of this test in light of two timescales: the measured time during which the landscape has developed from a planar initial condition, and the modeled time required to develop a valley spacing that is close to the steady-state value.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1824 Geomorphology: general (1625)
DE: 4460 Pattern formation
DE: 4485 Self-organization
SC: Hydrology [H]
MN: Fall Meeting 2005