Encroachment of riparian vegetation in a braided river due to catchment management in Japan

Over the past 70 years, riparian vegetation has progressively encroached onto braided gravel bars in the Sagae River, Japan, indicating a planform shift from a braided to a more meandering pattern. This study clarifies the mechanisms driving riparian vegetation encroachment by disentangling the combined effects of multiple catchment-scale management interventions. The construction and operation of the Sagae Dam upstream reduced high flows, especially in the recurring 2–5-year period. This coincides with the onset of vegetation expansion, with the riparian vegetation cover in the channel increasing from 5% pre-dam to 56% post-dam. To assess the combined impacts of the hydrological and hydraulic regulations on the mobility of bed material, dimensionless shear stress during high flows was estimated. Riverbed stabilisation works resulted in the post-dam conditions exhibiting lower dimensionless shear stresses, thereby reducing the frequency and spatial area of sediment mobilisation. Additionally, sediment from upstream is largely trapped in the dam reservoir and in a series of check dams in the catchment. In combination, these interventions stabilised gravel bars by allowing the establishment and succession of pioneer vegetation, which then further reinforced bar stability, thereby enabling the development of woody vegetation within the channel and promoting the progression of a meandering planform that accelerates flow accumulation. These results indicate how multiple anthropogenic interventions can interact to cumulatively alter fluvial conditions. The results also offer an important example of biogeomorphological feedback at a large scale, with important implications for river processes and the management of hydrological hazards in braided systems. • Aerial images over 70 years reveal substantial expansion of in-channel vegetation. • The channel planform shifted from a braided to a meandering pattern. • Regulated floods influenced vegetation survival and stabilised the riverbed. • Check dams and the Sagae dam disconnected sediment, reshaping vegetation patterns. • The braiding-to-meandering shift further accelerated vegetation expansion.