<p>The sediment delivery ratio is greatly affected by the water-sediment and the riverbed boundary, which represent the river’s capacity to transport sediment under specified conditions. This study examines the response of the sediment delivery ratio to water-sediment and riverbed boundary conditions in the Lower Yellow River (LYR) since the operation of the Xiaolangdi Reservoir began. It evaluates the spatial-temporal variations of water-sediment and riverbed boundaries based on hydrological data and topographic data from 2000 to 2023. Based on the sediment transport rate equation, a theoretical equation for the sediment delivery ratio during flood events has been developed, thoroughly considering the effects of riverbed boundary conditions, including median particle size of bed sediment, river gradient, and width-to-depth ratio. The results show that the sediment delivery ratio negatively correlates with the incoming sediment coefficient, the median particle size of bed sediment, and the width-to-depth ratio. In contrast, it positively correlates with the water load variation coefficient and river gradient. In comparison to solely accounting for water and sediment conditions, incorporating the riverbed boundary into the theoretical equation results in a more precise alignment with the measured sediment delivery ratio data. This indicates that the riverbed boundary is a crucial factor influencing the sediment delivery ratio. Under the current boundary conditions, the Aishan to Lijin reach has the highest sediment transport capacity. To improve the sediment transport capacity of the Tiexie to Lijin reach, it is recommended to narrow the river width upstream of Gaocun and increase the width-to-depth ratio. The findings of this study provide essential scientific insights into the sediment transport capacity of alluvial rivers subjected to variations in water-sediment and riverbed boundary conditions, thereby offering important references for hydrological engineering and river management practices.</p>