HR: 0800h
AN: H51C-1138    [Abstracts]
TI: Quantifying Hillside Erosion Rates in California Over Varying Timescales
AU: * O'Farrell, C R
EM: telemark@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755
AU: Heimsath, A
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755
AU: Kaste, J
AF: Dartmouth College, Department of Earth Sciences, Hanover, NH 03755
AB: Quantifying erosion rates is important for predictive landscape modeling and helps to evaluate a landscape's response to forcing from climate, tectonic, and human impacts. Determining representative erosion rates is difficult, however, as rates vary spatially and temporally. Large episodic events like landslides and large floods may dominate over long timescales but often occur too infrequently to be recognized by monitoring programs over human time-scales. We present a quantification of erosion rates from a small (33 ha), well-studied basin in northern California using three independent methods operating on different timescales. Analysis of pond sediment volume from a trap at the base of the catchment yielded an average basin wide erosion rate equivalent to a rate of bedrock lowering of 87 +/- 20 m/Ma, over a 50 year period. Cosmogenic nuclide data for this site from Heimsath et al. (1997, 1999) indicates an average basin erosion rate of 102 +/- 25 m/Ma, for a timescale of 10,000 years. The use of the fallout radionuclides 137Cs and 210Pb to trace soil movement on the hillside yields a rough correlation of erosion rates with slope and curvature, supporting previously assumed transport laws. Rates range from 64 +/- 24 m/Ma on convex ridges to 264 m/Ma in hollows. The range of erosion rates determined using fallout nuclides agrees with rates from pond sediment volume and validates the use of our 137Cs and 210Pb to offer a calibration relationship for this site, enabling its application at other sites with similar characteristics. The good agreement between these three independent methods representing erosion rates over 40 to 10,000 year timescales suggests that the rates and processes observed today are dominant over long timescales as well. Such agreement validates assumptions made by past researchers at this site regarding temporal uniformity of process.
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting