Interdecadal variation in sediment yield from a forested mountain basin: The role of hydroclimatic variability, anthropogenic disturbances, and geomorphic connectivity

Variation in sediment yield may reflect a signal of disturbances in the upstream landscape, modified by sediment routing. This study, conducted in a forested drainage basin in the inland Pacific Northwest, USA, sought to generate a better insight into the interdecadal variability of sediment yield in mountain landscapes in response to environmental change during the last century. To this end, we examined: (1) sediment yield fluctuations; and (2) their association with streamflow and land use changes; as well as (3) streamflow links to climate variability modes; and (4) the influence of sediment delivery from hillslope sources to streams (lateral connectivity) and its downstream routing through the stream network (longitudinal connectivity) on land use signal at the basin's outlet. Sediment yield between 1910 and 2017, estimated based on reconstructed fluvial delta growth, displayed an order of magnitude variability, which indicates a substantial geomorphic sensitivity. The interpretation of temporal patterns and an exploratory statistical analysis pointed to land use-related sediment supply changes as the primary driver of these fluctuations, dominating system behavior before changes in environmental regulations and practices in the mid-1970s. Hydroclimatically controlled streamflow variability appeared to be more prominent in the subsequent period. Our connectivity analysis suggested that a considerable portion of coarse sediment mobilized by harvest and road construction may still reside within the channel network. In light of previous research in this landscape system, we speculate that, despite limited anthropogenic pressures in the recent decades, its characteristics and behavior continue to be conditioned by land use legacies. Overall, this study contributes to the growing understanding of profound anthropogenic transformation of the earth surface. Specifically, it demonstrates that historical resource extraction may have left a lasting imprint even in relatively remote mountain landscapes. Given the ongoing rapid environmental change, such understanding is crucial for watershed management, conservation, and restoration.