| Sediment flux into the sea has been a topic of global concern in recent years, and about95%of the global terrestrial sediments transport through rivers. However, drainage basin has been undergoing rapid changes due to the combined effect of natural factors and human activities, which lead to the change of estuary and adjacent geomorphology, environment and ecology. Previous studies seldom combine the sedimentary structures with the sediment fluxes. This article focuses on exploring the deposition process response to the sharp reduction in sediment flux.10cores sediment and34surface sediments were collected in the west bank tidal flat of Yalu River during2010and2011, as well as the cross-section elevation measurement. And, test sediments with grain size,210Pb specific activity and organic matter (TOC, TN,δ13C,δ15N). Then, using remote sensing tool, combine hydrological data and catchment events, analysis estuary tidal flat sedimentary characteristics and its evolution mechanism.Influenced by human activities, especially dam construction, the water and sediment flux of Yalu River displayed stepwise reduction. According to the inter-annual distribution of water and sediment flux during1958-2008, together with the relationship between water and sediment discharge, the variation of sediment flux can be divided into three stages, i.e.1958-1970,1971-1995and1995-2008, Due to the reservoir regulation effect dramatically changed the seasonal distribution of water discharge, the inner-annual distribution of runoff of Yalu River was also greatly changed. Although the annual runoff of main stream is8times than the tributary Ai River, their sediment loads are almost similar. One plausible explanation is that there wasn’t dam construction in Ai River.In addition, some information including variations of different stages of water and sediment discharge and depositional environment change also were observed in the tidal flat sediment record. According to water and sediment fluxes during1958-2008and historical hydrological data, tidal flat sediments variations were divided into4stages by1940,1975and1995. After2000, sediment yield from catchment is insufficient to sustain the growth of the tidal flat. Subtidal material began to suffer from erosion, and gradually accumulated in the intertidal and supratidal, resulting in coarsening trends of shallow tidal flat sediments.Furthermore, catchment changes information were also reflected by the tidal flat sediments variations. With the development of agriculture, damage of forest resources in the1960s, and the followed adjustment of agricultural structure and ecological protection in the late1970s, suspended sediment and tidal flat sediment showed coarser trends and then finer trends.Sediment organic matter(TOC, TN,δ13C, δ15N)can be used to trace tidal flat sediment transport, and depositional environment changes. In addition, distribution of organic matter are highly related to water and sediment load, as well as catchment changes. From the east (estuary) to the west, marine organic matters content rise with the distance increasing; whereas, the total organic matter content showed decreasing trends, attributing to the terrestrial organic matter decomposing. The organic matters origin in the shallow core sediment trend to be marine source, as the subtidal materials were eroded and transported to the intertidal zone after2000. While the organic matters outside the West Channel still tend to be terrigenous origin, as the sands derived from Yalu River were trapped by spur dike.δ15N in the shallow sediment core increases rapidly due to the growing emissions of industrial wastewater and sewage, as well as ecological deterioration of environment. |
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