Abstract: Study on the change of saltwater intrusion pattern in estuaries can deepen the understanding of its mechanism and better ensure the safe use of freshwater resources. In this paper, based on the measured topography and salinity data, the changes of river regime and saltwater intrusion pattern in the Changjiang Estuary are analyzed, and the causes of the pattern changes are revealed by using a numerical model. From 2007 to 2017, the major reclamation projects in the Changjiang Estuary led to the narrowing of the river channel and significantly changed the local topography. Due to severe siltation in the south side of the lower reaches of the North Branch and the emergence of new sand body at the upper end, the channel volume decreased by -33.33%, and -13.26%, respectively. The salinity observation data in the dry season of 2007 and 2025 showed that the saltwater intrusion and salt water backflow from the North Branch into the South Branch were significantly weakened, and the saltwater intrusion in the North Channel was weakened under the climate wind and intensified under strong northerly wind. The pattern of saltwater intrusion in the Changjiang Estuary has changed. Considered the multi-year monthly mean river discharge and wind, the numerical simulation results show that from 2007 to 2017, the saltwater intrusion in the North Branch has weakened significantly, weakened in the upper reaches and most areas of middle reaches of the North Channel, and intensified significantly in the South Channel, North Passage and South Passage. The saltwater backflow from the North Branch into the South Branch was greatly weakened, resulting in the weakening of the saltwater intrusion in the South Branch, which is conducive to water intakes in the water sources. In the North Branch, the tidal prism decreased by 2.88 and 1.98 m³/s during spring tide and neap tide in February, the net water flux into the South Branch decreased by 423 and 369 m³/s, and the net salt flux transporting into the South Branch decreased by 10.06 and 1.10 kg/s. The variation of net unit width salt flux in the upper reaches of the North Branch also shows that the salt transport into the South Branch is greatly reduced. In 2007, the saltwater backflow from the North Branch into the North Branch was significant, and it almost disappeared in 2017. In the North Channel, during spring tide and neap tide in February, the tidal prism decreased by 1.92 and 1.86 10⁸ m³/s, the net water flux increased by 857 and 1379 m³/s, the water diversion ratio increased by 12.79% and 7.79%, and the net salt flux to the sea decreased by 2.28 and 5.42 kg/s. The changes of these factors explain the weakening of saltwater intrusion in the upper reaches and most areas of the middle reaches of the North Channel. The obvious increase of the diversion ratio in the North Channel (corresponding to the decrease of the diversion ratio in the South Channel) also explains the causes of the intensification of saltwater intrusion in the South Channel, North channel and South Passage. The rise of salinity in a small part of the North Channel during spring tide is caused by the significant increase of net salt flux from the North Passage into the North Channel. This paper reveals the dynamic causes of the change of saltwater intrusion pattern in the Changjiang Estuary from the changes of net water flux, net salt flux, tidal prism and water diversion ratio.

Key words: river regime change, saltwater intrusion, tidal prism, water diversion ratio, net salt flux