Rates and variability of hillslope erosion in steepland catchments in the Oregon Coast Range, Pacific Northwest, USA

Abstract

Hillslope soil residence time (average particle age making up the soil) reflects the rates of soil production (= bedrock erosion) and soil transport, and thus is a useful parameter for quantifying hillslope dynamics. On steady state hillslopes, the distribution of particle ages is determined by soil production rate, soil depth and soil and bedrock density. This distribution, used in combination with chronofunctions describing soil weathering at a particle scale, can be used to derive expressions relating hillslope soil properties to soil residence time. Thus soil properties can be used to estimate soil residence time and quantify hillslope dynamics. We analysed variability of soil residence time at a broad scale and a fine scale in the Oregon Coast Range in the Pacific Northwest, USA. Our results are consistent with an approximate balance of erosion rate and rock uplift rate for a large part of our study area, but at both scales, parts of the landscape with long soil residence time were identified. We attribute the lower erosion rates and slower soil transport (contributing to long soil residence time) in these areas to decoupling from base-level lowering.