HR: 0800h
AN: H51C-0376    [Abstracts]
TI: The Relation Between Topography and Millennial Erosion Rates in the San Gabriel Mountains, California
AU: * Whipple, K X
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Heimsath, A M
EM: Arjun.Heimsath@Dartmouth.EDU
AF: Department of Earth Sciences, Dartmouth College, Hanover, NH 03755 United States
AU: Ouimet, W B
EM: wouimet@mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Crosby, B T
EM: mountain@mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, MIT, Cambridge, MA 02139 United States
AU: Wobus, C W
EM: cwobus@mit.edu
AF: Department of Earth, Atmospheric and Planetary Science, MIT, Cambridge, MA 02139 United States
AB: Working with the framework provided by previous studies as a guide, we have conducted a detailed DEM analysis, made preliminary field observations, and collected and analyzed a preliminary set of 13 sediment samples for basin-average erosion rate estimation using cosmogenic 10Be in quartz. The basin-average erosion rates we measured vary between 100 and 1000 m/Ma (thus averaging erosion rate over 1-10 ka) and are in generally good agreement in with published estimates of both long-term exhumation rates and modern sediment yields. Our preliminary DEM and CRN analyses yield several interesting results that we report here. First and foremost, our data from catchments within the Big Tujunga and the East Fork San Gabriel River drainages suggest a monotonic relation between channel steepness index determined from slope-area analysis of channel profiles and basin-average erosion rate. Additional data is required to determine if this relation is linear or sub-linear, and detailed field observations are required before we can assess the combination of factors (adjustments in channel slope, width, rock exposure, bed material size, roughness, etc) that lead to the observed relation. For instance, there is some evidence for a contrast between roughly transport-limited conditions in slowly eroding areas and more detachment-limited conditions in rapidly eroding areas. In contrast to the monotonic relation between erosion rate and channel steepness index, mean hillslope gradient increases approximately linearly with erosion rate up to the angle of repose (~33 degrees) and then remains invariant as erosion rates increase. Remarkably, the transition to invariant mean slopes occurs at an erosion rate of ~300 m/Ma, a value that corresponds closely to preliminary estimates of the local maximum surface soil production rate (300-340 m/Ma). Although long anticipated, there is little previous data explicitly supporting this result. Finally, our detailed DEM analysis has identified prominent knickpoints recording a transient channel response to an increase in rock uplift rate that is expressed in channels throughout the San Gabriel Mountains. This interpretation of stream profile form is supported by our preliminary CRN-based estimates of basin-average erosion rates that document a factor of ~3 increase in erosion rate downstream of knickpoints in the Big Tujunga drainage. The amount of uplift recorded in the channel profiles suggests this increase in rate occurred ca. 1.5Ma, approximately coinciding with the initiation of the San Jacinto and Cucamonga faults.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
DE: 1856 River channels (0483, 0744)
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005