Fluvial sediment transfer in the Changjiang (Yangtze) river-estuary depositional system

Abstract

Knowledge of the transfer of sediment through river systems is essential for understanding the physical, chemical and biological processes on the Earth’s surface. A holistic analysis of long-term records of water discharge, sediment transport, riverbed morphology and estuarine hydrology is here used to quantify spatial and temporal variations in fluvial sediment fluxes along the Changjiang River. We show that the establishment of the Three Gorges Dam (TGD) has directly changed the fluvial sediment-transport process by annually trapping 1.23 × 108 t of sediment. The upper Changjiang reach has switched from being the main sediment source before 2003 to a depositional sink of fluvial sediment subsequently. The major lakes, such as Dongting Lake and Poyang Lake, have shifted from being local sediment sinks before 2003 to sediment sources thereafter, such that they now provide sediment to the Changjiang River. Since the 2003 closure of the TGD the riverbed of the middle-lower Changjiang has become the major source of sediment being transmitted downstream, now providing almost 50% of the material entering the estuary. Shoals in the estuarine channels and landward sediment transport from the sea have become major sediment sources for the river estuary. We conclude that dams currently in preparation along the upper Changjiang reach and adjacent lakes may cause the cessation of sediment supply to downstream reaches. Rising sea levels and frequent storms may terminate landward sediment transport, increasing estuarine erosion and inducing seaward sediment transport. It can therefore be expected that substantial erosion could occur in the near future in the Changjiang estuary system.