Abstract: Salt-water intrusion in estuaries depends mainly upon runoff,tide and riverbed configuration.These factors,however,will be changed under human activities,e.g.,low water discharge will increase due to the erection of a large-scale reservoir,and the tide range will be changed after training/reclamation in the estuary.Using the field chlorinity data recorded before and after those human activit ies and a 1-D mathemat ical model of salt-water intrusion,this paper analyses the impact of human activities on the saltwater intrusion in Qiantang River Estuary.The results from those two methods,the maximum,minimum and average chlorinity and the longest period of the days exceeding the chlorinity permissible standards,are basically identical.