Modeling the Formation of Topographic Asymmetry by Aspect‐Dependent Erosional Processes and Lateral Channel Migration

Some landscapes exhibit the intriguing characteristic that the steepness of hillslopes varies systematically with the direction they face, even where there is no bias introduced by bedrock structure. This topographic asymmetry has inspired numerous explanations. In the simplest scenario, insolation‐driven microclimatic differences lead to different erosion rates on opposing slopes and topographic asymmetry develops. Alternatively, lateral channel migration and the corresponding steepening of undercut slopes have also been suggested as a dominant cause of topographic asymmetry. To examine these proposed origins of asymmetric topography, we adapted a numerical landscape evolution model to include lateral channel migration as well as aspect‐induced variations in soil creep, regolith strength, and runoff. We compared the resulting topography and erosional response produced by each mechanism and found that the model with lateral channel migration produces unique signatures in erosion rates and the ridgetop Laplacian of elevation. To further investigate lateral channel migration, we developed a model of hillslope profile evolution in which sediment fluxes from opposing slopes control lateral channel migration rate and direction. We find that topographic asymmetry in this system can be self‐sustaining. All else being equal, we find that the aspect‐dependent models with variable regolith strength or runoff predict that less topographic asymmetry should develop as slope length decreases. These two models also predict that weaker asymmetry should develop as rock strength increases, whereas the lateral channel migration model predicts that greater rock strength should lead to weaker asymmetry only if lateral channel migration efficiency increases faster than fluvial incision efficiency. Key Points Numerical modeling results predict that asymmetry‐forming mechanisms have unique topographic and erosion rate signatures Most of the models predict that landscapes with stronger bedrock or shorter hillslopes should develop less asymmetry for some conditions Lateral channel migration can be self‐sustaining and may not require microclimatic differences in order for asymmetry to develop