HR: 1340h
AN: H53D-0496    [Abstracts]
TI: Dilute Rivers and low Sediment Yields in the Western Ghats (Sahyadri): Slow Erosion of a Steep Terrain With High Rainfall
AU: * Anderson, S P
EM: spanders@colorado.edu
AF: Univ. of Colorado, INSTAAR and Dept. of Geography, Boulder, CO 80309 United States
AU: Narayana, A
EM: acnarayana@cusat.ac.in
AF: Cochin Univ. of Science and Technology, Dept. of Marine Geology and Geophysics, Cochin, 682 016 India
AU: Anderson, R S
EM: andersrs@colorado.edu
AF: Univ. of Colorado, INSTAAR and Dept. of Geological Sci., Boulder, CO 80309 United States
AU: Molnar, P
EM: molnar@cires.colorado.edu
AF: Univ. of Colorado, CIRES and Dept. of Geological Sci., Boulder, CO 80309 United States
AB: The correlations of erosion with relief, and of chemical weathering with erosion, suggest that total denudation rates of steep terrain should be high. Moreover, chemical weathering rates increase with temperature. Thus, one might expect high solute fluxes in tropical monsoon-fed rivers that originate in highlands 600-1800 m a.s.l. and cross the steep, escarpment along the western margin of southern India, the Western Ghats (Sahyadri). In May 2005, we sampled 15 rivers in the midlands and highlands of Kerala, and found dilute waters, with electrical conductivity ranging from 17-47 μS/cm, remarkably low values given that we visited during low flow in the dry season when rivers typically display concentration maxima. Given the average rainfall rate of 3 m/yr and a relationship of 1 ppm total dissolved solids per μS/cm (probably an over-estimate), the annual solute flux is estimated to be 90 t/km²/yr, corresponding to a chemical denudation rate of approximately 0.04 mm/yr. This rate matches the physical denudation rate of 0.01-0.05 mm/yr [Narayana, 2005, J. Geol. Soc. India]. These preliminary observations indicate that both chemical and physical erosion in the Western Ghats proceed at low rates. Studies of other steep, wet tropical landscapes in tectonically inactive regions also reveal low erosion rates and roughly equal partitioning between physical and chemical rates. Why is it that high relief does not erode rapidly in the absence of tectonic activity, even when temperatures are high and rainfall abundant? Smooth rock slopes and domes are common in the southern Western Ghats, the notable absence of joints and fractures perhaps reflecting the lack of tectonic activity of the range. We hypothesize that the total denudation rate is low due to the lack of fracturing--- the first step in erosion--- by tectonic processes. Massive rock presents few weaknesses for failure processes to exploit and little access for weathering solutions.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005