<p>Water–sediment transport in large reservoirs has received increasing attention because of its implications for flood control, channel evolution, and ecological conservation. This study aimed to characterize monthly water–sediment transport patterns in the Three Gorges Reservoir (TGR) in the context of upstream dam construction and reservoir operation adjustments. On the basis of daily observations from 8 hydrological stations during 2003–2022, monthly inflow and outflow water–sediment patterns were characterized under different impoundment stages and operation periods. The water discharge ratio (WDR) and sediment discharge ratio (SDR) were quantified for different reservoir reaches, and reach-specific empirical relationships for estimating the flood-season SDR were developed using a genetic algorithm. The results indicate that the dams on the lower Jinsha River and the Wu River substantially altered the inflowing water–sediment regime of the TGR, and these effects extended through the fluctuating backwater zone. However, the dam on the Jialing River had a relatively minor impact. Reservoir operation reshaped the monthly water level–discharge relationship and regulated the seasonal and spatial patterns of sediment retention. Sediment deposition was concentrated during the flood season, whereas the lowest SDR occurred during the impoundment period, reflecting the combined effects of sediment supply and reservoir regulation. With increasing impoundment level, the main sedimentation zone shifted upstream, indicating that the backwater effect and the associated reduction in sediment transport capacity extended farther upstream. The developed SDR formulas provide a practical tool for reach-scale sediment transport assessment, and the findings offer reference for sediment management in large reservoirs worldwide.</p>