ABSTRACT GAO, A。; YANG, S。L。; LI, G。; LI, P。, and CHEN, S。L。, 2010。 Long-term morphological evolution of a tidal island as affected by natural factors and human activities, the Yangtze estuary。 Journal of Coastal Research, 26(1),  123–131。 West Palm Beach (Florida), ISSN  0749-0208。79355

Bathymetric data based on sounding surveys and intertidal elevations measured using Real-Time Kinematic Global Positioning System methodology were analyzed using Geographical Information System techniques to examine the morphological evolution of the tidal island Jiu Duan Sha (JDS) in the Yangtze estuary。 It was found that, from 1958 to 2005, the area and sediment volume of JDS increased by 96% and 156% above the 5 m isobath and by 331% and 504% above the Lowest Astronomic Tide (LAT)。 Specifically, the maximum elevation of the island increased from 0。3 to 4。9 m above the LAT, and the salt marsh increased from 0% to 40% of the intertidal area。 The dominant factor controlling the growth of this wetland was found to have varied with time。 The growth was mainly due to the depo- sition of riverine sediment before the 1970s but was primarily the result of the merging of a shoal into JDS between 1971 and 1994 and, later, the local engineering projects between 1994 and 2005。 It was concluded that human activ- ities are playing an increasingly important role in the morphological evolution of tidal wetlands in the Yangtze estuary。

ADDITIONAL INDEX WORDS:    Morphological evolution, tidal island, anthropogenic impacts, Yangtze estuary。

INTRODUCTION

Long-term (>1 y)  morphological  changes in estuaries are of great importance to ecosystem, economics, and environ- mental management and, therefore, receive considerable at- tention (Blott et al。, 2006; Pari et al。, 2008; Van der Wal, Pye, and Neal, 2002; Yang, Ding, and Chen, 2001)。 These changes are increasingly influenced by human activities (Bale et al。, 2007; Syvitski and Saito, 2007; Yang et al。, 2006) and are becoming more complicated than ever。 One of the important anthropogenic impacts is the human-induced decline in riv- erine sediment supply to the estuary and subsequent delta degeneration (Fanos, 1995; Yang et al。, 2006)。 For tidal wet- lands, another important aspect is whether or not they can survive the threat of a global rise in sea level (Morris et al。, 2002)。

The Yangtze is one of the world’s largest rivers (Walling, 2006), and its delta is of great importance to the Chinese economy。 For example, two-thirds of the land in Shanghai was derived from the tidal wetlands of the Yangtze delta over the past two millennia (Yang et al。, 2006)。 However, human activities, in particular dam construction, have resulted in  a

DOI: 10。2112/08-1052。1 received 1 April 2008; accepted in revision 4 September 2008。

* Corresponding author。

drastic decrease in sediment supply from the Yangtze River in recent years (Yang et al。, 2006)。 Because of land subsi- dence, the local sea level in the Yangtze delta is rising at a faster rate than the global average (Gong, Li, and Yang, 2008)。 In addition, the Yangtze estuary has been greatly af- fected by human activities, such as reclamation, harbor con- struction, sand extraction, and marsh planting。 These natu- ral and anthropogenic impacts mean that the Yangtze estu- ary provides a good example of the modern morphological evolution of estuaries。

Jiu Duan Sha (JDS), defined as the sedimentary body above the 5 m isobath, is representative of the third gener- ation of isolated shoals in the modern Yangtze estuary (Yun, 2004)。 In 2005, JDS was declared a national natural wetland reserve by the Chinese government。 The intertidal area of the JDS is composed of four inpidual parts, termed the Upper Shoal (US), the Middle Shoal (MS), the Lower Shoal (LS), and the Jiangya’nan Shoal (JS) respectively (Figure 1)。 Since the 1950s, bathymetric surveys have been performed frequently around JDS to aid navigation because the island is located between two major waterways in the estuary (Figure 1)。 These records make it possible to examine the detailed evo- lutionary history of the island。 Studying JDS provides an in- sight into the laws governing the evolution of bars at the mouth of bifurcated estuaries。