Abstract: To comprehensively realize the effects of reservoir sediment flushing on fluvial processes and downstream flood control, this study examines flood routing and sediment transport, as well as changes in erosion and sediment patterns within the bottle-neck reach of flood conveyance and the widened reach in the Lower Yellow River (LYR). This analysis employs measured data from the reservoir and river course collected from 2018 to 2020 and allows for assessing changes in the water—sediment relationship before and after flood events. The obtained results reveal that the total sediment load of 6 floods discharged by Xiaolangdi Reservoir (XLD) amounted to 1.09 billion tons. The ratios of the deposition to incoming sediment load in the LYR ranged from 58.0% to 84.2%, significantly higher than those observed in flood events with similar incoming sediment ratios from 1965 to 1999. Coarse sediments accounted for the largest share with 44.4%, followed by medium sediments at 29.0%, and fine sediments at 26.6%. Most of the deposition, 79.1%, occurred upstream of Huayuankou (HYK), with HYK—Gaocun and Gaocun—Aisan accounting for 15.9% and 3.5% of the deposition respectively. The channel storage upstream of HYK played a role in temporarily detaining the sediment. The deposition that occurs during flood periods can be washed away within 1 to 2 years after the flood, without adversely affecting the flood-carrying capacity of the channel. The exchange of suspended sediment with bed materials led to the refining of suspended particles along the river course. After the flood, significant bed material refinement was observed at the HYK station. Sediment flushing of the reservoir during flood period improves the water—sediment relationship post-flood and increases the efficiency of water resource utilization. The findings of this study advance the scientific understanding of the impacts of reservoirs on sediment-laden rivers and their downstream fluvial processes also provides technical support results for channel flood control and water-sediment regulation in reservoirs.