Abstract: The wandering reach of the lower Yellow River are characterized by rapid adjustments and significant morphological changes, particularly evident in the complexity and variability of mid-channel bars. Understanding the response relationship between morphological characteristics of these mid-channel bars and the water-sediment processes is crucial for analyzing the evolution mechanisms in the wandering reach of the lower Yellow River. In this study we examined the morphological features (area and quantity) of mid-channel bars along the wandering reach from Tiexie to Gaocun. Based on water-sediment and scouring-deposition characteristics, we elucidated the relationship between the area and quantity of mid-channel bars and the water-sediment processes to unveil the mechanisms underlying the impact of hydrological changes on mid-channel bar morphology via remote sensing and numerical simulation. Our findings reveal that since the operation of Xiaolangdi Reservoir, the downstream channel has experienced significant erosion, with the wandering reach contributing 68% of the cumulative scouring in the downstream, reaching 1.537 billion m³. Before the operation of Xiaolangdi Reservoir, mid-channel bars in the wandering reach exhibited notable interannual fluctuations in area and quantity, characterized by periods of growth and decline. However, after the operation, the area and quantity of mid-channel bars initially decreased, followed by an increase, and then subsequent decline. Moreover, along with the increase of four-year sliding average flow rate and incoming-sediment coefficient during flood seasons, the area and quantity of mid-channel bars displayed downward and upward trends, respectively. Numerical simulations not only accurately replicate the development and evolution of mid-channel bars but also elucidate the qualitative impact of specific hydrodynamic factors on their morphology. This comprehensive analysis not only advances our understanding of river plane morphology adjustments but also provides theoretical insights for future river regulation projects.

Key words: mid-channel bar evolution, hydrodynamic model, water and sediment process, wandering river reach, morphological characteristics