Topographic and Erosional Asymmetry in the Otoishi River Watershed: Climatic and Tectonic Insights

This study examines the topographic and erosional asymmetry of the Otoishi River watershed in Kyushu, Japan, by integrating high-resolution LiDAR-based terrain analysis with long-term landscape evolution modeling. Hilltop curvature and hillslope length were calculated across eastern and western sub-watersheds. Systematic differences in morphology were identified, reflecting spatial variability in erosion processes and subsurface hydrology. These patterns correspond to variations in silica flux and sediment dynamics. Numerical simulations were performed using the Landlab framework with both real and synthetic DEMs. Uplift rates and erodibility parameters were varied to assess their impact on landscape development. Results show that low D/K ratios—where D is hillslope diffusivity and K is fluvial erodibility—preserve ridge variability, while high D/K ratios promote convergence in hillslope form. These outcomes support a conceptual link between geomorphic uniformity and erosional regime. The results are consistent with chemostatic behavior in the stable western region and with prior observations from sediment disasters and climatic trends. This integrated approach offers insights into the interactions among climate, tectonics, and surface processes, contributing to the understanding of landscape evolution and geomorphic stability in humid mountainous regions.