Rapid response of the Changjiang (Yangtze) River and East China Sea source-to-sink conveying system to human induced catchment perturbations

摘要

In order to investigate human impact in the Changjiang East China Sea source to sink conveying system, we analyze the siltation and erosion state of the estuary-shelf deposition systems, and discuss the response intensity and timing of the depositional system of the Changjiang subaqueous delta and the related Changjiang distal mud. Results show that after the Three Gorges Dam was put into operation in 2003, the abrupt reduction of the sediment discharge to the estuary disturbed the equilibrium between the estuary-coast-shelf deposition systems, modifying the siltation-erosion balance and seabed sediment types. From the Changjiang subaqueous delta to its distal mud areas, the degree of response to catchment changes diminishes, with a larger response lag from north to south of the distal muds. Influenced by dam emplacements, the change of the Changjiang - East China Sea source to sink conveying system can be divided into three stages, i.e., pre-1950, 1950-2002 and post-2003. Before 1950, the source to sink system remained relatively stable. Between 1950 and 2002, man-made fluvial impoundments created an important sink for terrigenous sediment of the upstream regions, but without profound system changes. In contrast, after 2003, the morphodynamic patterns for the entire mid-lower reaches of the Changjiang River, the subaqueous delta and the distal muds shifted rapidly from siltation to erosion within a short period of time. Furthermore, the upper reaches of the river became disconnected from the source to sink conveying system, and the mid-lower reaches of the river, together with its subaqueous delta and northern distal mud area were converted into net sediment sources. As such, the sink area has shrunk while the source area expanded for the Changjiang East China Sea source to sink conveying system.