Anthropogenic impacts on fluvial sediment supply: a case study of the lower Yoshino River, Japan

Human activities such as in-channel gravel mining and dam construction have substantially altered sediment dynamics and channel morphology in many rivers, yet their relative and combined long-term impacts remain difficult to distinguish. This study quantifies the multi-decadal effects of gravel mining and sediment trapping by a major dam on sediment dynamics in the lower Yoshino River, Japan, from 1965 to 1999. A one-dimensional bed-deformation model was applied within a scenario-based framework to isolate the impacts of each disturbance by comparing conditions with and without gravel mining and dam trapping. Model–data comparisons suggest that historical gravel extraction prior to regulation likely exceeded permitted volumes by a factor of two to three. Within the tested scenario framework, gravel mining appears to have been the dominant driver of long-term riverbed degradation and changes in bedload transport. The largest geomorphic changes occurred during the period of intensive extraction, whereas the magnitude of subsequent riverbed change became smaller after regulatory restrictions were introduced in the mid-1970s. Nevertheless, recovery toward pre-mining conditions remained limited, suggesting persistent adjustment after the main disturbance period. In contrast, sediment trapping by the dam was primarily associated with a more spatially coherent reach-scale reduction in suspended sediment load and slight coarsening of riverbed material, while exerting comparatively limited influence on riverbed elevation. The combined impacts of gravel mining and dam trapping were not simply additive, with reduced upstream sediment supply likely prolonging recovery from mining-induced degradation. These findings underscore the need to distinguish reach-scale sediment removal from catchment-scale sediment retention when interpreting long-term channel adjustment and developing sustainable river-management strategies.