Holocene and recent valley-bottom sediment storage decouples natural and anthropogenic hillslope erosion from sediment delivery to streams at time scales of 101–104 yr in a third-order Yangtze River basin, Sichuan, China

To assess the time scales and relative importance of temporal decoupling between hillslope erosion and the introduction of sediment to streams in a Yangtze River headwater basin, we used multiple techniques to date sediments in alluvial fans and terraces in a third-order stream valley draining a 30-km2 catchment in SW Sichuan, China. Poorly sorted angular sediments in tributary-junction alluvial fans ranged in age from 11261 BCE to 1844 CE, and predominantly fine-grained overbank sediments in alluvial terraces date to approximately 1700-1950. Ethnographic observations and field mapping of hillslope soil depths indicate that terrace sediments and upper strata of several fans correspond to a period of hillslope erosion associated with the intensification of hillslope swidden agriculture. Contemporary sediment production is dominated by lateral fluvial erosion of valley-bottom landforms rather than by hillslope erosion. The long-term temporal decoupling by valley storage of hillslope erosion from sediment delivery to streams has relevance to contemporary hillslope erosion and sedimentation control efforts in the Yangtze Basin. It also motivates investigating whether valley-filling anthropogenic legacy sediments may play a role in decoupling hillslope erosion from sediment production in other Yangtze Basin headwater basins.<p></p>