Sedimentation and morphological changes at Yuantuojiao Point,estuary of the North Branch,Changjiang River

The North Branch,separated by the Chongming Island,was once the main channel in the estuary of the Changjiang River.Reclamation and a decrease in runoff to the North Branch had led to the narrowing and hallowing of the channel.The Yuantuojiao Point is located at the intersecting point connecting the North Branch of the Changjiang River and the Jiangsu coastline.Erosion cliffs are developed between the typical ilty-muddy tidal flat and the salt marsh occupied by Spartina alterniflorea,and this has changed rapidly ver the past few years.The sediment grain size analysis results of the surficial and two core samples indiate that the Yuantuojiao Point tidal flat experienced continuous accretional processes.Based upon ¹³⁷Cs nalysis results of the YT and YY Cores sampled from the tidal flat at the Yuantuojiao Point,the average sedmentation rate of the YT Core was 2.30 cm/a from 1963 to 2007,and 2.38 cm/a from 1954 to 2007 for the YY Core.The sedimentation rates of both core locations have declined since the 1960s corresponding to the eaward reclamation at the Yuantuojiao Point.The average sedimentation rates at the Yuantuojiao Point ere similar to that of the silty-muddy tidal flat at the northern Jiangsu coast,but lower than that of the outh of the Changjiang River Estuary.According to field morphological investigations from 2006 to 2008 n the salt marsh at the Yuantuojiao Point,cliffs retreated markedly by storm surges and disappeared gradully because of the rapid sedimentation on the silty-muddy tidal flat.The maximum annual retreat reached 0 m.The recent sedimentation and morphological changes of the Yuantuojiao Point tidal flat not only isplayed the retreat of the salt marsh and the disappearance of cliffs,but also was accompanied by rapid edimentation of the silty-muddy tidal flat and the salt marsh,indicating the responses to the tidal currents, torm surges,Spartina alterniflorea trapping sediments and large-scale reclamation.The sediment grain ize and their trends,southward coastal flow,and sandspits of the longshore bars suggest that the main ediment source at the Yuantuoijao Point,estuary of the North Branch was possibly from the Changjiang River before 1958,since then,it has been from the south of the submarine radial sand ridges of the southern Huanghai Sea (Yellow Sea) .