Linking Fluvial Incision and Landslide Activity in a Tectonically Active Landscape

Abstract Tectonic uplift drives fluvial incision and hillslope adjustment in mountains, and landslides are a major mode of hillslope response that contributes to landscape evolution. However, quantitative constraints on the coupling between fluvial incision and landslide activity remain limited, largely due to difficulties in quantifying incision and isolating the fluvial incision effect from other landslide‐controlling factors. We addressed this problem in the Xiaojin Basin on the eastern Tibetan Plateau, where the main rivers evolved within a Triassic folded belt that allowed structural controls to be evaluated while minimizing lithologic, topographic, and meteorologic variability along the channels. We used normalized channel steepness index ( k sn ) as a morphometric proxy for relative fluvial incision, and combined river profile analysis with a new landslide inventory, slope unit mapping, and InSAR deformation data. Paired comparisons showed that mean k sn was higher along landslide reaches than adjacent non‐landslide reaches. To reduce the influence of local landslide perturbations, we further evaluated slope unit‐averaged k sn , which better represented the longer‐wavelength incisional background. At this scale, the landslide occurrence probability increased with k sn . This relationship was further modulated by the slope structure, with the strongest dependence observed on anti‐dip slopes and transverse slopes. In contrast, InSAR‐derived landslide deformation rates showed weak correlation with k sn . These results indicate that fluvial incision primarily controls the location of landslides, rather than their current deformation rates. Overall, these findings indicate that k sn captures the long‐term incisional boundary conditions that govern landslide occurrence, implying its value for landslide susceptibility assessment in transient mountain landscapes.