The Three Gorges Region(TGR),located at the lower reach of the Upper Yangtze River Basin(UYRB) in China,suffers from heavy rainstorm frequently.The runoff generated from TGR composes an important part of the total flood at the famous Three Gorges Reservoir(TGRe).During the severe flood period in 1954,for example,the water from TGR accounted for up to 13.2% of the 30-days maximum flood volume of the UYRB.Considering the short and steep tributaries with rapid concentration,the regional lateral inflow(RLI) may induce more serious effect on the flood peak of the TGRe than the volume.However,hydrological data of the sparse gauge stations is too insufficient to evaluate the effect of RLI.This paper studied the impact by analyzing 880 flood events during 1956-2000.By comparing the observed hydrograph and simulated hydrograph with HEC-RAS software regarding no RLI,the effect of RLI on flood peak value and timing properties was identified and quantified.The variability of this effect among floods of different magnitudes was also analyzed.To evaluate the analysis uncertainty associated with the parameter of roughness coefficient,four sets of roughness coefficients from different research groups were employed in this study.The results showed that RLI contributes discharge of 3524 m3/s to flood peaks of the TGRe on average,with the contribution ratio of 15.9%.RLI contributes 12000 m3/s to the flood peaks larger than 50000 m3/s on average,with 25000 m3/s as its upper bound,while the contribution ratio can reach up to 50%,with an average of 20%.The variability of this effect is great among different events.Statistical analysis showed that to larger flood peak of the TGRe,RLI contributes more discharge with higher variability,and the contribution ratio and its variability are slightly larger,and events with higher contribution ratio occur more frequently.RLI can reshape the hydrograph,leading to earlier appearance of flood peak.This effect and its variability increase with the contribution ratio.This study has revealed that RLI plays an important role in large flood peak of the TGRe,which calls for more reliable flood forecasting methods to prolong the forecast lead time and improve the accuracy for the safety of the Three Gorges Dam and the protection of its lower reaches during severe flood disaster period. The Three Gorges Region (TGR), located at the lower reach of the Upper Yangtze River Basin (UYRB) in China, suffers from heavy rainstorm frequently. The runoff generated from TGR composes an important part of the total flood at the famous Three Gorges Reservoir (TGRe). During the severe flood period in 1954, for example, the water from TGR accounted for up to 13.2% of the 30-day maximum flood volume of the UYRB.Considering the short and steep tributaries with rapid concentration, the regional lateral Inflow (RLI) may induce more serious effect on the flood peak of the TGRe than the volume. However, hydrological data of the sparse gauge stations is too insufficient to evaluate the effect of RLI. This paper studied the impact by analyzing 880 flood events during 1956-2000.By comparing the observed hydrograph and simulated hydrograph with HEC-RAS software regarding RLI, the effect of RLI on flood peak value and timing properties was identified and quantified.The variability of this effect among floo ds of different magnitudes was also analyzed. To evaluate the analysis uncertainty associated with the parameter of roughness coefficient, four sets of roughness coefficients from different research groups were employed in this study. The results showed that RLI contributes discharge of 3524 m3 / s to flood peaks of the TGRe on average, with the contribution ratio of 15.9%. RLI contributes 12000 m3 / s to the flood peaks larger than 50000 m3 / s on average, with 25000 m3 / s as its upper bound, while the contribution ratio can reach up to 50% with an average of 20%. variability of this effect is great among different events. Statistical analysis showed that to larger flood peak of the TGRe, RLI contributes more discharge with higher variability, and the contribution ratio and its variability are slightly larger, and events with higher contribution than occur more frequently.RLI can reshape the hydrograph, leading to earlier appearance of flood peak. This effect and its variability increase with the contribution ratio. This study has revealed that RLI plays an important role in large flood peak of the TGRe, which calls for more reliable flood forecasting methods to prolong the forecast lead time and improve the accuracy for the safety of the Three Gorges Dam and the protection of its lower reaches during severe flood disaster period.