Spatial, Temporal, and Human-Induced Variations in Suspended Sediment Concentration in the Surface Waters of the Yangtze Estuary and Adjacent Coastal Areas

To delineate temporal and spatial variations in suspended sediment concentration (SSC) in the Yangtze (Changjiang) Estuary and adjacent coastal waters, surface-water samples were taken twice daily from 10 stations over periods ranging from 2 to 12 years (total number of samples >28,000). Synoptic measurements in 2009 showed an increase in surface SSC from 0.058 g/l in the upper sections of the estuary to ∼0.6 g/l at the Yangtze River turbidity maximum at the river mouth, decreasing seaward to 0.057 g/l. Annual periodicities reflect variations in the Yangtze discharge, which affect the horizontal distribution and transport of SSC, and seasonal winds, which result in vertical resuspension and mixing. Over the past 10–20 years, annual surface SSC in the lower Yangtze River and the upper estuary has decreased by 55%, due mainly to dam construction in the upper and middle reaches of the river. The 20–30% decrease in mean surface SSC in the lower estuary and adjacent coastal waters over the same period presumably reflects sediment resuspension, in part due to erosion of the subaqueous Yangtze Delta. SSCs in the estuary and adjacent coastal waters are expected to continue to decline as new dams are constructed in the Yangtze basin and as erosion of the subaqueous delta slows in coming decades.