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The Yellow River (YR) is famous for its 'hanging river' due to the high sedimentation rate in the lower reaches. Reservoir sediment retention (RSR) and water-sediment regulation (WSR) of the Xiaolangdi Reservoir (XLDR) contribute to mitigating the siltation and adjusting the bankfull channel geometry in the lower Yellow River (LYR). However, few studies have been conducted to quantify the contribution of RSR and WSR to medium-sized channel shaping. In this paper, we analyzed the temporal and spatial variations of bankfull discharge (BFD) in the LYR, established empirical expressions between the BFD and the previous five-year average discharge above 2,000 m³/s and the incoming sediment coefficient during flood seasons, and then constructed three different water and sediment scenarios to quantify the contribution of the WSR of the XLDR to medium-sized channel shaping in the LYR. Calculated results indicated that WSR accounted alone for 68%-94% of the medium-sized channel shaping, with RSR contributing 6%-32%. The contribution of WSR to the recovery of medium-sized channels increased with time and became stabilized eventually. Along the LYR, such contribution also increased from 68% to 97% and eventually became stabilized in narrow reaches, reaching over 90%. The results will lay a solid technical foundation for the WSR of the XLDR, river regulation downstream of the reservoir, and sediment disposal of the YR. [ABSTRACT FROM AUTHOR]