A comprehensive understanding of water-quality spatiotemporal variations is essential for the long-term management of aquatic environments. However, the absence of indicators that fully capture the extent of eutrophication, the lack of long-term water-quality monitoring data, and the complexity of water pollutants sources have limited research on pollution characteristics and eutrophication assessments in plain river network areas. In this study, based on the monitoring data of water-quality indicators in the Beijing–Hangzhou Grand Canal (Changzhou section), the temporal and spatial distribution characteristics of nutrient salts, as well as the eutrophication status of the water body, were revealed by using the comprehensive trophic level index (TLI) method. Meanwhile, the main sources of water pollutants were defined, and targeted control measures were proposed. The results showed that water-quality deterioration is more pronounced during the non-flood season, with significantly higher concentrations of ammonia nitrogen (NH3-N) and total phosphorus (TP) compared to the flood season. Additionally, the analysis of the nitrogen-to-phosphorus (N:P) ratio suggested that some sampling sites exhibited phosphorus limitation. The eutrophication assessment indicated that most sections are eutrophic, with S8 and S2 being the most heavily polluted and at risk of cyanobacterial blooms. The primary sources of pollutants were identified as agricultural runoff, domestic sewage, and industrial discharges. To address these issues, it was recommended to reduce external pollution sources while focusing on internal control (1. Enhance the management of livestock and poultry farming; 2. Upgrade wastewater purification facilities; 3. Establish ecological protection zones along the riverbanks) and enhance aquatic ecosystem restoration. A coordinated and watershed-wide approach is crucial to improving water quality in this region. The findings of this study provide a scientific basis for the protection of the water environment and pollution control in plain river network areas.
