Evaluation of Human Impact on Sediment Dynamics in an Intensively Managed Agricultural Watershed Using Distributed Modeling

By altering hydrological and geomorphological processes at watershed scales, humans have substantially influenced the movement of sediment on Earth's surface. Despite widespread recognition of human impacts on erosion and deposition, few studies have assessed the magnitude of change in watershed-scale sediment fluxes before and after the implementation of industrial agriculture and how agricultural development has altered the spatial distribution of sediment fluxes throughout watersheds. This study uses a modeling approach to explore changes in sediment fluxes before and after agricultural development in the upper Sangamon River Basin—an agricultural watershed in the midwestern United States. Comparison of model predictions with river hydrological and sediment data and with information on soil erosion and floodplain sedimentation shows the model accurately captures contemporary fluxes of water and sediment. To assess human impact, native land-cover conditions are used to estimate the magnitude and spatial distribution of sediment fluxes before the landscape was transformed by farming practices. Results suggest that sediment delivery from hillslopes to streams in this low relief watershed has increased 11-fold and the sediment load in streams has increased eight-fold since European settlement. Floodplain sedimentation has also increased dramatically, a finding consistent with recent estimates of post-settlement alluvium accumulation rates. The proportion of sediment exported from the basin is now slightly greater than it was in the 1800s. Overall, the model results indicate that humans have greatly enhanced the movement and storage of sediment within the upper Sangamon River basin.