The influence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China)

Deep landscape dissection by the Dadu and Yalong rivers on the eastern margin of the Tibetan plateau has produced high-relief, narrow river gorges and threshold hillslopes that frequently experience large landslides. Large landslides inundate river valleys and overwhelm channels with large volumes (>10⁵ m³) of coarse material, commonly forming stable landslide dams that trigger extensive and prolonged aggradation upstream. These observations suggest that strong feedbacks among hillslope processes, channel morphology, and incision rate are prevalent throughout this landscape and are likely characteristic of transient landscapes experiencing large increases in local relief, in general. Landslide effects are a by-product of rapid incision initiated by regional uplift. However, over timescales relevant to landscape evolution (>10⁴ yr), large landslides can also act as a primary control on channel morphology and longitudinal river profiles, inhibiting incision and further preventing the complete adjustment of rivers to regional tectonic, climatic, and lithologic forcing. We explore a probabilistic, numerical model to provide a quantitative framework for evaluating how landslides influence bedrock river incision and landscape evolution. The time-average number of landslide dams along a river course, and thus the magnitude of the landslide influence, is set by two fundamental timescales - the time it takes to erode landslide deposits and erase individual dams and the recurrence interval of large landslides that lead to stable dams. Stable, gradually eroding landslide dams create mixed bedrock-alluvial channels with spatial and temporal variations in incision, ultimately slowing long-term rates of river incision, thereby reducing the total amount of incision occurring over a given length of river. A stronger landslide effect implies that a higher percentage of channel length is buried by landslide-related sediment, leading to reduced river incision efficiency. The longer it takes a river channel to incise into a landslide dam and remove all landslide-related sediment, the more control these events have on the evolution of the river profile and landscape evolution. This can be the result of slow erosion of stable dams, or a higher frequency of large events.