Two-dimensional paleotidal simulations were carried out in order to investigate the influence of large morphological changes that occurred during the postglacial period in the region around the Changjiang estuary on tides and tidal currents of the Yellow/East China Sea. Calculations were made for 6 kyr B.P., which corresponds to period of the maximum postglacial transgression in eastern China. Special consideration was taken to reconstruct the paleotopography around the Changjiang estuary (from lat 31-33°N and long 119 -123°E), which was accomplished by using available borehole datasets. The aim was to remove the effect of the massive sedimentation that obscures the original topographic features in this area. We also simulated the coastline and bathymetry along the Jiangsu coast and Hanzhou Bay for the same reason. Sea level was assumed to be the same with the present level.

At 6 kyr B.P., the M2 tidal currents converge (diverge) from (to) the mouth of the estuary, forming a radial tidal-flow pattern originating from the estuary. Tidal amplitude reached a maximum value in the middle of the estuary and attenuated toward its head. These features agree with the numerical results of Zhu (1999), who estimated paleotides at 7 kyr BP using a different depth elevation model. Tides and tidal currents in the estuary at 6 kyr BP seem to have been stronger than those at present. From the analysis of the tidal-current phase pattern and from some supplemental experiments with changing sea levels, it appears that the emergence of the radial flow pattern of the tidal currents might have been caused by the southward shift of the southern tip of the Yellow Sea amphidromic system that accompanied the postglacial sea-level rise.

Tides and tidal currents within the estuary were dominated by the semi-diurnal components. Closer investigation revealed that their relative contribution varied slightly though systematically among the periods. Ratio of the diurnal (K1 and O1) tides to the semi-diurnal (M2 and S2) ones was systematically lower during 6 kyr B.P., when compared to the modern situation. These changes occurred mostly by the amplitude change of the semi-diurnal tides in the estuarine region.

In the Jiangsu coastal area between lat 32.5°N and 33.5°N, the major direction of the M2 tidal current shifted from a SW-NE to E-W direction during the last 6000 years. This directional change is consistent with a paleocurrent direction estimated for the North Jiangsu region using a geomagnetic analysis (Zhang et al., 1998). The SW-NE flow pattern also appeared when an experiment was carried out using the present-day configuration, except that the sand-ridge system off the Jiangsu coast was removed. This result might indicate a relationship between sand-ridge formation and the directional shift of the tidal current.