Abstract: To comprehensively assess the impact of sediment retention by reservoirs on downstream fluvial processes and flood control, this study analyzed sediment and cross-sectional data collected during the sediment retention periods at the Sanmenxia (SMX) and Xiaolangdi (XLD) reservoirs. The research explored patterns of channel aggradation and degradation, flood scouring efficiency, channel geometric characteristics, and flood discharge capacity in the Lower Yellow River (LYR). The results revealed uneven scouring throughout the LYR, with notable riverbed coarsening—the median grain size of the bed sediment increased by 2.1 to 2.5 times that before scouring. Above Gaocun, enhanced flood discharge capacity at equivalent water levels was observed, and the larger channel storage can temporarily detain sediment from the XLD Reservoir. However, following riverbed coarsening, the depletion of fine and medium silt led to a significant decline in channel scouring efficiency over time. The long-term sediment and water management by the reservoirs, coupled with advanced river engineering in the LYR, shifted the focal area of channel scouring downstream during the XLD Reservoir′s sediment retention period compared to that during the SMX period. This shift was accompanied by an increase in riverbed coarsening and a marked decrease in scouring efficiency. These findings provide a scientific foundation for defining technical parameters for reservoir sediment and water regulation.