Figure 1。 The Yangtze estuary and the JDS shoals,  JS: Jiangya’nan Shoal, US: Upper Shoal, MS: Middle Shoal, LS: Lower    Shoal。

In the present analysis, topographical data were used to construct a Digital Elevation Model (DEM)。 The objectives were (1) to quantify the morphological changes in JDS at dif- ferent periods since its formation, (2) to discuss the possible natural and anthropogenic causes of the morphological changes, and (3) to predict future trends in morphological evolution resulting from a global rise in sea level and local anthropogenic impacts。

PHYSICAL SETTING

The water discharge and sediment load from the Yangtze River was 893 × 109 m3 y—1 and 407 × 106 t y—1 during the period 1958–2005。 The Yangtze River mouth is presently 120 km long and 90 km wide at its outer limit。 Here, the river is pided first by Chongming Island into the North and South branches, and then the South Branch is split into the North and South channels by Changxing and Hengsha islands。 Fi- nally, the South Channel is pided into the North and South passages by JDS (Figure 1)。 In the 18th century, the major river flow shifted from the North Branch to the South Branch, which has since resulted in the narrowing and silting of the North Branch。 Since the 1950s, more than 98% of the river discharge flowed through the South Branch, and around a quarter passed through the North and the South passages combined (Chen et al。, 1985)。 Nearly half of the riverine sed- iment was deposited in the mouth area (Yang et al。, 2000)。 The Yangtze estuary is mesotidal to macrotidal, with mean and maximum tidal ranges of ~2。7 m and ~4。7 m, respec- tively, around JDS (GSCCI, 1988, 1996)。 Tidal peak velocities in the North and South passages around JDS are typically 2–2。5 m s—1 (GSCCI, 1988), and waves on the seaward side of JDS are, on average, ~1 m in height (GSICI, 1996)。 The suspended sediment concentration (SSC) around JDS is 0。1–

Table 1。 Scales of the bathymetric maps and the Digital Elevation Model (DEM) interpolation Root Mean Square (RMS) error  for  the study   years。

Years Map Scale DEM RMS

1958 1 : 100,000 0。49

1965 1 : 75,000 0。41

1971 1 : 100,000 0。57

1981 1 : 50,000 0。27

1989 1 : 120,000 0。62

1994* 1 : 50,000 0。87

1999* 1 : 50,000 0。86

2001* 1 : 50,000 0。68

2005 1 : 25,000 0。43

* Years that lack sample points above 1 m in    elevation。

0。7 g L—1 in the surface water and 1–8 g L—1 in the nearbed (Shen et al。, 1992)。 Under natural conditions, the tidal wet- land islands in the Yangtze estuary have tended to move northward because of the effect of the Coriolis force。 Over the past two millennia, nine large sediment islands have moved northward to merge into the north bank of the Yangtze es- tuary (Chen et al。, 1979)。 A dual jetty–groyne complex con- structed in 1998 along the North Passage has changed the local hydrodynamics of the area and affected the morpholog- ical evolution of JDS (Du et al。,   2005)。