Based on the observed data analysis and mathematical modeling,the effect of runoff,tide current and river regime change on the salt-water intrusion into the South Branch of the Yangtze Estuary has been studied.Meanwhile,the effect of the three salt-water intrusion sources of the North Branch of the estuary,South and North Channel at the downstream of the South Branch has been emphatically studied.The results show that there is little brine influence on the Reach of Xuliujing;the upstream reachfrom Liuhekou is mainly controlled by the brine intruded from the North Branch:the water area near the Baoshan Steel and Iron Complex is influenced by all of the three intrusion sources.The peak values of chlorinity usually appear in medium or small tidal period,depending on the brine from the North Branch.Af-ter considering the various factors,this paper suggests that the reach from Baoshan Steel and Iron Complex to Xuliujing in the South Branch be taken as the main reach for development and utilization of fresh water in the estuary.For the short term objective,it is suggested that the Chenhang bay reservoir and its inland reser-voir should be built to meet the fresh water requirment of Shanghai City in 2000.