Comparison of Suspended Sediment and Salinity Vertical Distributions Across the Turbidity Maximum Zoneinthe Yangtze Estuaryin Dry Seasons of 1982 and 2012

Ten ships, three in North Channel, three in North Passage and four in South Passage of the Yangtze Estuary, were employed to collect water samples in January 2012, in a pattern of three longitudinal profiles in the large estuary. The sampling sites are rather similar with those in 1982. Based on the comparison of suspended sediment concentrations (SSC) and salinity values across the turbidity maximum zone (TMZ) of the Yangtze Estuary acquired in 1982 and 2012 respectively, the change in TMZ in the vertical profiles was discussed in this paper. Results suggest that during the past 30 years from 1982 to 2012, even the sediment load coming from the river basin has been continuously reducing, the SSC of the TMZ decreased about 50%; and the average vertical SSC values in the North Channel, in the North Passage and in the South Passage decreased about 43%, 60%, and 40%, respectively. The 2012 SSC maximum of the turbidity zone in the North Channel was similar to that in 1982; the 2012 SSC maximum position in the North Passage migrated inward, and the coincidence of the SSC maximum and salinity maximum position in the turbidity zone may lead the increase in silt in the North Passage and the deposition in the middle section, while the highest SSC position in the South Passage moved outward. The change in runoff, tidal current and the seasonal distribution of runoff dominated the movement of core TMZ of the Estuary. During the dry season, the salinity gradient is obvious. On the section of the core of TMZ, the upper stream transported to the sea, and the lower stream transported to the estuary. The area with the highest SSC value is also the area with the highest salinity gradient.