As a first step of studying the variations of water discharge and sediment load entering the East China Sea by the Changjiang, we select the Poyang Lake catchment as study area, and aim to simulate its large contribution of fluvial fluxes towards the East China Sea Shelf, as well as how these fluxes might have changed over the last 1000 years by humans and climatic influences. In addition, to investigate the impact of the changes in the Changjiang catchment on the estuarine coast-shelf sedimentary system, we selected Pb-210 as our tracer, and analyzed the variations in the transport, distribution and budget of Pb-210 in the sediment of the Changjiang subaqueous delta (CSD) and the Zhejiang-Fujian coastal mud belt (ZFCMB) system before and after the impounding of the Three Gorges Dam (TGD).
Simulation results of the model Hydrotrend indicate that sediment load discharging into Poyang Lake by the five rivers was only 9.21± 0.43Mt y-1 between AD 1000 and 1700, when the anthropogenic impact on the landscape was still low. However, due to intensified soil loss, the total sediment load entering Poyang Lake during AD 1800-1950 increased by 58.7%, compared to AD 1000-1700. After AD 1950 the sediment load variation is mainly reflected by the combined impact of dam emplacement and soil erosion. As sediment interception by dams continuously increased over time, the total sediment load entering Poyang Lake during AD 1990-2000 is only 60.9% of that of the highest riverine sediment flux during AD 1951-1980, which almost equal to that of the lowest level during 1000-1700. In addition, due to the trapping effect of TGD, 75.51% of the Pb-210 supply from the Changjiang upstream areas was retained in the TGD, and the Pb-210 flux into the sea was greatly decreased.
Our findings also demonstrate that the cross-shelf plume is the main channel for the transport of Pb-210, from the inner shelf towards the outer shelf of the East China Sea. The variation of Pb-210 budget in the sediment of the CSD–ZFCMB also indicated that, the rate of Pb-210 burial in the sediments of the ZFCMB decreased 35.75% after 2003, and the retention index of 210Pb and the amount of Pb-210 retained in the CSD increased significantly, suggesting that after 2003, the function of the CSD of transferring the sedimentary materials from the river into the open sea was weakened, in response to the overall reduction in suspended sediment discharge.
