| Study of tidal limit and tidal current limit is one of the important components of global Land-Ocean interaction. Systematic study of them is of significant meaning both in practice and theory. This paper takes the part of Yangtze River affected by tides as study area, using the methods of statistics and numerical simulation and focusing on the situation of the river's tidal limit and tidal current limit, their response to some major engineering projects and the sea level change.Based on analysis of the data collected from 9 stations along Yangtze estuary, this article finds out the rule of tidal wave propagation up the river, from the estuary to the zone where it becomes extinct. With the consideration of the period of flood tide, tidal range and the propagate speed of tide, there exists a node point. The items mentioned above act in the opposite way in each side of the point. This point is not fixed, which oscillates according to the discharge of the river. When the discharge increases, the point will move downstream.Using a 2-D numerical model and designing a series of numerical experiments in an ideal estuary, the dynamic processes of the tide propagation up the river and the tidal limit and tidal current limit are studied. The results is as follows: The tidal limit and the tidal current limit move upstream with the decrease of the discharge or the increase of tidal range in the river mouth. But this process is non-linear, which means when the river discharge or the tidal range is smaller, the influence of their change to the tidal limit and tidal current limit is greater. The two limits will go upstream with the rise of sea level. But the intrusion of tidal current limit is not as linear as the rise of sea level. Pumping can also influence the station of the two limits. If the pumping point is located in the range of the tidal limit, the location of pumping point will also influence the tidal limit, and so does the tidal current limit.Based on the study of ideal estuary, this article has studied the tidal limit and tidal current limit of Yangtze estuary by using the 2-D numerical model. The influence of South-to-North Water Diversion Project and the Three Gorges Project to tidal limit and tidal current limit have been simulated as well as the sea level rise. The characters are similar with the experiment in the ideal estuary. Using the numerical simulation we can draw these conclusions: 1) The mean location of tidal limit in flood season is about 50km downstream from Datong, and the mean locationof tidal limit in dry season is about 67km upstream from Datong, the mean location of tidal limit in a year is about 20km downstream from Datong. In January the Three Gorges Project will make the tidal limit move about 5km downstream, but in October it will make the tidal limit move about 28km upstream. South-to-North Water Diversion Project have little effect to tidal limit. Sea level rise will make tidal limit move upstream, and the effect of sea level rise to tidal limit is larger in flood season than in dry season. 2) The mean location of tidal current limit in dry season is about 342km upstream from the river mouth, in flood season it can only reach at 220km upstream from the river mouth, the mean location of tidal current limit in a year is about 260km upstream from the river mouth. In January the Three Gorges Project will make the tidal current limit move about 13-18km downstream, but in October the project will make the tidal current limit move about 35-48km upstream. South-to-North Water Diversion Project will make neap tidal current limit move upstream about 10km,but affects spring tidal current limit little. Sea level rise will make tidal current limit move upstream, and the effect of sea level rise to tidal current limit is larger in flood season than in dry season. But the intrusion of tidal current limit is not as linear as the rise of sea level. The tidal limit intrusion enhances when the sea level rise from 0.25m to 0.5m.
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