There are two sources of saltwater intrusion in Qingcaosh a waters, one directly from the outer sea and the other from North Branch, with the latter being dominant. During a tidal cycle, if peaks of chlorinity occur ar ound the flood slack and the troughs are near the ebb slack, then vertical distr ibution of the chloride content is relatively unhomogeneous, indicating that sal twater intrusion is from the outer sea; likewise, if the peaks and troughs of th e chloride content occur at the ebb slack and the flood slack then vertical dist ribution of the chloride content is relatively homogeous, indicating saltwater i ntrusion from Noth Branch to South Branch. In the Changjiang River Estuary, duri ng a spring-neap cycle, if high chloride values occures on the spring tide and a low chloride zone occures on the neap tide, then the saltwater comes from the outer sea. If the zone of high chloride values occures on neaps and the low zone occures on springs then the seltwater comes from North Branch into South Branch .rn The patterns and values of the chloride variation at Gaoqiao station are similar to that at CHⅠ and CHⅡ stations mostly compared with other stations in th e waters of Qingcaosha. Based on the field observations from 1974 to 1997 at Gao qiao station, the period with saltwater mass return from North Branch accounts f or about 60%. Direct intrusion from outer sea accounts for about 30%, the remain ing peroid undergoes mixed effects by the above two sources. We suggest that com prehensive regulation of North Branch should be carried out, in order to reduce and even prevent saltwater intrusion from North Branch into South Branch.%自1978年以来，在长江口的几个关键岸段(例青龙港、新建 、高桥、堡镇等)设置盐度观测站；1992-1994年的枯季，在青草沙水源地的南、北两侧各 抛测量船一艘，在一个完整的大、中、小潮期间，连续逐时观测流速、流向、水深、盐度等 ，同时在青龙港等处设置6个岸边观测点同步取样；1995-1996年在船站位置各设置氯离子 自动监测仪一台；1996年3月又进行了一次大规模的长江口水文测验。本文对大量的现场资 料作了分析计算。研究结果表明，青草沙水源地盐水来源主要有北支倒灌咸水团和外海咸水 入侵。前者的特征为，氯度的半月变化是小潮期(或小潮后的寻常潮)的氯度反高于大潮期， 氯度的潮周日变化是日最高值出现在落憩附近，日最低值出现在涨憩附近，氯度的垂向分层 不明显。这与外海盐水入侵引起的氯离子浓度在半月和潮周日内的变化特性正好相反。
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