Long‐Term Morphodynamic Responses to Flood Diversion in the Lower Kano River, Japan

ABSTRACT Fluvial morphodynamics—the interaction between water flow, sediment transport, and channel morphology—is a central concern in sustainable river management. However, the long‐term effects of flood diversion remain underexplored. This study investigated the decadal‐scale morphodynamic responses of the lower Kano River in Japan following the construction of a flood diversion channel in 1965. Using 60 years of cross‐sectional and grain‐size data, we divided the lower Kano River into four reaches with contrasting characteristics of morphological change. In the 5.4 km reach immediately downstream of the diversion point, we observed more than 1 m of bed incision and a gradual reduction in bankfull discharge. Further downstream, in the subsequent 6.2 km reach, no substantial decrease in bankfull discharge was observed. To interpret these responses, we applied a one‐dimensional numerical model and evaluated its ability to reproduce the observed changes. The model reproduced the grain‐size responses when using adjusted sediment‐supply settings; however, it could not reproduce the bed degradation observed in the 5.4 km reach immediately downstream of the diversion point. The best agreement was obtained when the channel width was reduced to 60% of the observed bankfull width, indicating that a narrower hydraulically effective width governs sediment transport under reduced floods. These findings suggested that the relationship between flood discharge and channel morphology is governed by mechanisms that vary with bed material and channel gradient. The study underscored the importance of incorporating sediment connectivity and reach‐specific geomorphic processes into flood management strategies to adapt to unintended morphological changes.