HOW WELL CAN HILLSLOPE EVOLUTION MODELS EXPLAIN REAL TOPOGRAPHY?

Soil transport processes on hillslopes dictate the pace of landscape evolution as well as control the rate and character of sediment input into channels. The advent of high-resolution topographic data via airborne laser swath mapping (ALSM) allows for the comparison of real and predicted landforms with additional information generated from field-based soil depth measurements. Here, I describe efforts to formulate, calibrate, and test a soil transport model for steep, forested terrain using ALSM data, field-based soil depth data, and previously published erosion rate estimates via cosmogenic radionuclides. In our Oregon Coast Range study site, the depth of soil disturbance processes that drive soil transport depend on biological processes such as tree root growth and turnover. As such, we use the depth distribution of roots and other biological entities to quantify how soil transport rates may vary with soil depth. As soil depth increases, the effective transport rate becomes increasingly constant because disturbance processes are limited in their depth of penetration. In addition, our model indicates that soil flux increases nonlinearly as slope angles approach a critical value, consistent with experimental studies. I coupled the transport model, which is nonlinearly dependent on soil depth and slope angle, with a soil production model calibrated for the Oregon Coast Range, and simulated over 50,000 years of slope evolution using ALSM data as the initial condition. The model maintains the current pattern of slope and curvature better than models for which transport varies linearly with soil depth and slope angle. Specifically, the model preserves highly convex hilltops and ridgelines as well as steep, planar sideslopes. Predicted soil depths are shallow along the ridgelines and thicken on sideslopes, consistent with field observations. Given changes in the rate of baselevel lowering, slope response is most apparent along drainage divides. In contrast, modeled climate-driven changes in fire frequency can thin soils along sideslopes and subdue the amplitude of ridge-hollow sequences. Results from this research enable site-specific predictions of slope response with implications for: 1) the frequency of debris flow initiation, 2) sediment delivery variations driven by climate and vegetation change, and 3) the topographic signature of tectonic forcing.