Water pollution incidents can cause rapid deterioration of water quality, potentially leading to the illness and death of surrounding residents. Therefore, it is imperative to assess the impact of water pollution incidents, and to understand the mechanisms of pollutant transport and transformation. To address this problem, this paper constructed a hydraulic water quality model for the lower reaches of the Xiangjiang River, China, using the hydrodynamic module and convective diffusion module of MIKE21. Six pollution incident scenarios were simulated to investigate the transport process of pollutants, as affected by an upstream dam structure, the Changsha Comprehensive Control Project dam (CCCP). Analysis of the results suggests that the CCCP plays an essential role in controlling the transport and transformation of pollutants. With the CCCP, the process of transport is weakened, and the dispersion effect is strengthened. In particular, after the construction of the CCCP, the same amount of upstream discharge leads to lower peak pollutant concentrations and longer pollutant arrival times to each waterworks' intake, thereby alleviating the impact of water pollution incidents. Further, comparative analysis suggests that the role of CCCP is much more significant with lower discharges (e. g., during the dry season), largely due to the higher amount of water quantity within the reach.