HR: 0830h
AN: H51E-1125    [PDF]
TI: Quantifying Erosion in the Nepal High Himalaya
AU: * McGlynn, R S
EM: Robert.McGlynn@dartmouth.edu
AF: Department of Earth Sciences Dartmouth College, Hinman 6105, Hanover, NH 03755 United States
AU: Heimsath, A
EM: Arjun.Heimsath@dartmouth.edu
AF: Department of Earth Sciences Dartmouth College, Hinman 6105, Hanover, NH 03755 United States
AB: Quantifying erosion rates in high mountain environments is challenging due to the stochastic and often catastrophic nature of the processes. Better constraints on these rates are critical to further our understanding of the couplings between tectonic and climatic forces and surface processes. In the great range of the Himalaya, quantifying erosion rates is especially important to help untangle the role that geomorphic processes played in helping to set Quaternary climates. In this study we focus on determining the erosion rates from a region where erosion by blockfall is observed to be a dominant process. Using a new method to determine cliff retreat rates, we utilized a physically well-constrained, debris-covered glacier in the Nepal Himalaya to quantify an average headwall retreat rate. By measuring supraglacial debris volumes and down-valley transport rates from a glaciated basin on the north slope of the Annapurna Range we calculate a yearly sediment flux of 730$\pm$40 m$^{3}$/yr. We observe all debris to be originating from the steep, rocky headwall of the valley and use the calculated flux with a measured headwall area to determine a headwall retreat rate of 540$\pm$30 m/Ma. This rate is over twice as fast as steady-state erosion rates that we determine for the valley sidewalls and ridge crests using concentrations of the in situ produced cosmogenic radionuclide, $^{10}$Be. Additionally, point specific erosion rates, deduced from nuclide concentrations in samples from ridge crests and valley sidewalls, vary by at least a factor of two, from 82$\pm$9 m/Ma to 204$\pm$16.6 m/Ma. We suggest that the difference in erosion rates across the landscape is the result of incision and removal of material at the base of the headwall by the glacier and that the inability to remove material and incise at the base of the sidewalls results in lower erosion rates. Finally, irrespective of the differences in local erosion rates, they are significantly lower than rates inferred for the incision of major rivers draining the Himalaya, leading us to conclude that even over-steepened, rocky headwalls, where conventional wisdom would suggest erosion rates are the highest, are not keeping up with fluvial down cutting rates.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
SC: Hydrology [H]
MN: 2003 Fall Meeting