Abstract: The backwater effect of the connected lakes (Dongting Lake and Poyang Lake) in the middle reaches of the Yangtze River on its main stream is directly related to regional flood control safety. By constructing a tributary backwater intensity index and combining Copula functions, the changes and differences in the backwater effect of the outflows of the two lakes on the Yangtze River before and after the impoundment of the Three Gorges Reservoir were assessed. The contributions of changes in the distribution of main and tributary stream discharges in wet and dry seasons and topographic adjustments in the confluence area to the changes in the backwater effect of the two lakes were also evaluated. The study produced some important results : ① After the impoundment of the Three Gorges Reservoir, during the wet season, the backwater effect of Dongting Lake changed little, whereas the effect of Poyang Lake weakened significantly. During the dry season, the backwater effect of Dongting Lake strengthened slightly, while that of Poyang Lake remained relatively unchanged. ② The variations in the backwater effect following the impoundment were attributable to the cumulative impacts of alterations in flood confluence ratio and modifications in the confluence area topography. Following the impoundment of the Three Gorges Reservoir, alterations in the distribution of main and tributary stream discharges led to reductions of 5.11% and 13.45% in the average backwater effect of Dongting Lake and Poyang Lake on the main stream, respectively. Simultaneously, the increase in the elevation difference between the main stream and tributary riverbeds attributable to riverbed erosion resulted in increases of 6% and 0.9% in the backwater effect of Dongting Lake and Poyang Lake on the main stream, respectively.