The impact of major projects in the Yangtze River Basin on regulating river discharge and salt intrusion

Major projects in watershed can significantly affect estuarine salt intrusion and utilization of freshwater resources.Extensive reservoir construction in the Yangtze River basin has resulted in a total effective storage capacity of 84.85 billion cubic meters. Combined with water diversions from the South-to-North Water Transfer Project, these projects have induced a pronounced seasonal redistribution of river discharge. Observed data at the Datong hydrological station confirms that the decline in river discharge during August to October from 2004 to 2024 is primarily attributable to these projects. The numerical simulation results show that, under climate winds, the reduced river discharge during August to October exacerbates the salt intrusion in the Yangtze River Estuary, but has not yet threats the water intake safety of the estuarine reservoirs. However, under adverse winds, particularly typhoon in October, the river discharge reduction can significantly intensify salt intrusion, posing a threat to freshwater supply security. Conversely, basin reservoirs release water from January to April effectively weaken the salt intrusion. Under climate winds, the regulated river dischargeeliminates salt intrusion impacts on the estuarine reservoirs. Even during adverse winds, the increased river discharge substantially mitigates the salt intrusion, and reduces unsuitable water intake time of Qingcaosha reservoir by over 40%. Collectively, major projects have secured freshwater availability in the estuarine region through seasonal river discharge redistribution, butattention should be paid to the risk of salt intrusion caused by its store water at the end of the flood season.