HR: 0800h
AN: H31A-1267    [Abstracts]
TI: Geologic Influences on Downstream Fining in the Clearwater River Basin, Western Washington State: Implications for Transient Landscapes
AU: * Belmont, P
EM: pwb3@Lehigh.edu
AF: Lehigh University, EES Dept. 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Pazzaglia, F J
EM: fjp3@Lehigh.edu
AF: Lehigh University, EES Dept. 31 Williams Drive, Bethlehem, PA 18015 United States
AB: Grain size exerts a primary control on river longitudinal profile concavity and thus has important implications for bedrock incision and the feedbacks between hillslope and fluvial processes in both steady-state and transient landscapes. In the Clearwater River basin, Olympic Peninsula, western Washington state, a setting where flux steady-state conditions have been argued, we propose that downstream fining and the downstream development of bimodality in the grain size distribution is primarily attributed to variable weathering and hillslope processes throughout the basin in addition to differential grain size reduction in the channel. The grain size data showing the downstream fining trend have been collected from three sites on each of six lateral alluvial bars in the Clearwater trunk channel. Volumetric grain size analyses separated into the three major rock types found in the Clearwater, siltstone, sandstone and conglomerate, indicates that siltstones are preferentially broken down during transport despite equal recruitment of all rock types along the river continuum. The source, although underlain by relatively uniform greywacke sandstone, siltstone, and shale, is found to be delivering texturally different material to the channel as a function of mean relief, which for the Clearwater, is tightly coupled to the rate of rock uplift. Weathering-resistant sandstone cobbles, primarily derived from the upper basin, therefore dominate the coarse grain size fractions at the river mouth. The potential implications of the geologic control of downstream fining on the estimation of basin-wide erosion rates from ¹⁰Be inventories of alluvial sediment are profound. For basins that exhibit a grain size dependency of ¹⁰Be concentrations, differential grain size reduction works to dilute or concentrate quartz grains (in which ¹⁰Be is generated) differently in each grain size fraction. Whereas this problem is minimized in the Clearwater basin, where sandstones are more or less uniformly distributed throughout the catchment, these results stress the importance of understanding geologic context and geomorphic processes in interpreting concentrations of cosmogenic nuclides in alluvial sediment.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005