HR: 1340h
AN: H53D-0514    [Abstracts]
TI: Bedrock Channel Width Along an Orographic Precipitation Gradient in the Upper Marsyandi River in Central Nepal
AU: * Craddock, W H
EM: bill@crustal.ucsb.edu
AF: University of California, Department of Earth Science 552 University Road - Bldg 526, Santa Barbara, CA 93106 United States
AU: Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: University of California, Department of Earth Science 552 University Road - Bldg 526, Santa Barbara, CA 93106 United States
AU: Gabet, E
EM: manny.gabet@mso.umt.edu
AF: University of Montana, Dept. of Geology, Missoula, MT 59801 United States
AB: Recent research suggests that climatically driven fluvial erosion of bedrock drives landscape evolution in rapidly denuding, non-glaciated mountain landscapes. As a result, an increasing number of theoretical studies investigate the linkage between climate and fluvial erosion, but few empirical data sets exist with which to evaluate the predictions. Pronounced and well constrained spatial variations in climate combined with uniform exhumation history at Quaternary time scales and uniform rock strength make the upper Marsyandi River valley in Central Nepal an ideal laboratory in which to test bedrock channel width variations in small catchments (2 - 18 km2) along a > 10-fold gradient in monsoon precipitation. At 81 sites located on tributaries to the Marsyandi, 3 - 5 measurements of longitudinal river profiles and of bedrock channel width were made at indicators of high scour. The drainage area above each site was measured using 90-m digital topographic data and combined with 6 years of half hourly precipitation data measured by a dense weather network in order to model discharge. The discharge model was evaluated with river gauging station data. 40 Schmidt hammer measurements were made at each of 64 sites in order to quantify rock strength. Bedrock channel width scales as a power law function of discharge, with a scaling exponent of 0.37. Whereas a weak correlation (R2 = 0.16) exists between catchment area and channel width, a much stronger correlation exists between discharge and channel width (R2 = 0.56). Drainage area-discharge relationships implicit in many DEM-based river incision models may not be applicable in regions with orographic precipitation gradients. Weakly inverse correlations between channel width and channel gradient are observed. Various estimations of storm precipitation exhibit a < 5-fold gradient over the same distance that monsoon precipitation changes by a factor of > 10, indicating that large storms penetrate far into the study area. Because storms most likely drive fluvial erosion, monsoon precipitation totals may lead to an overestimation of the magnitude of the effective discharge gradient. An analysis of discharge and suspended sediment data suggests sub-annual return times for channel forming discharges in wet catchments, whereas in dry catchments, effective discharges occur 1-2 yrs. Furthermore, the magnitude of large storms is more variable in dry catchments. Precipitation differences between catchments, which are potentially overestimated based on seasonal totals, play a key role in establishing bedrock channel width in this landscape.
DE: 1807 Climate impacts
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1854 Precipitation (3354)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005