Spatial and temporal variations in coastline morphology along Ganjiang-Poyang Lake: sediment supply as a cause of variability

Coastline changes over some 24 years along the Ganjiang-Poyang Lake have allowed us to monitor the coastal morphological evolution, with its attendant social consequences. This study focuses on the coastal morphology changes controlled by the north and middle branches of the Ganjiang River as the lake delta prograded into the Poyang Lake. The north branch resulted in an elongated coastal headland with one sinuous channel complex, whereas the middle branch produced a prograding shoal-water river-dominated delta system with a distributary channel network. The main units of the north branch consist of a single channel and associated point bar and levee, whereas those of the middle branch consist of distributary channels and mouth bar. The evolution of the north branch was dominated by changes of the channel, while the middle branch became dominated by distributary channel bifurcation and mouth-bar formation. Changing morphology of both the branches of Ganjiang shows a decreased progradation rate from 1992 to 2015, which can be evidenced by a decreasing prograding rate from 0.875 to 0.21 km/year of the middle branch. The lakeward outbuilding process was characterized by an initial stage of strong progradation, whereas the subsequent stage was dominantly aggradation associated with weak progradation. Human activity such as dam building and forestation generated decrease of sediment input, resulting in the decrease in the prograding rate of the Ganjiang delta. Sediment grain-size variations control the geometry of the prograding body of the two branches. The finer sediment delivery of the north branch resulted in the elongated coastal headland, whereas the relatively coarser sediment of the middle branch resulted in the development of the lobate-shaped delta lobe. This study highlights the significant role of upstream factors, which can be evidenced by that the grain size and magnitude of supply that controls the construction and shape of the coastal morphology.