Topographic asymmetry and climate controls on landscape evolution

Abstract

Landscapes are expected to evolve differently under the influence of different climate conditions. However, the relationship between landscape evolution and climate is not well understood. I investigate the relationship between landscape evolution and climate by using natural experiments in which climate varies with slope aspect (geographic orientation) and causes differences in landscape form, such as steeper equator- or pole-facing slopes. In order to understand which mechanisms are responsible for the development of this topographic asymmetry, I adapted a numerical landscape evolution model to include different asymmetry-forming mechanisms such as aspect-induced variations in soil creep intensity, regolith strength, and runoff, and also lateral channel migration. Numerical experiments reveal topographic signatures associated with each of these mechanisms that can be compared with field sites that exhibit asymmetry. I used these numerical model results, along with estimates of field-saturated hydraulic conductivity, soil strength, evidence of stream capture and channel beheadings, and erosion rates determined from cosmogenic radionuclides to determine which asymmetry forming mechanisms are likely responsible for the topographic asymmetry at Gabilan Mesa, a landscape in the central California Coast Ranges. I find that aspect-dependent differences in runoff are most likely responsible for the bulk of the asymmetry at Gabilan Mesa, but lateral channel migration has contributed to the asymmetry in some locations. To further investigate climate's influence on landscape evolution, I compiled new and previously published estimates of slope-dependent soil transport efficiency across a range of climates. I find that soil transport efficiency increases with mean annual precipitation and the aridity index, a measure that describes water availability for plants. I also find that soil transport efficiency varies with lithology and that different measurement techniques can bias estimates of the soil transport coefficient.