Delta response to decline in sediment supply from the Yangtze River: evidence of the recent four decades and expectations for the next half-century

Abstract

In order to examine the delta response to decline in sediment supply from the Yangtze River during the recent decades, bathymetric maps of the Yangtze subaqueous delta surveyed from 1958 to 1997 were analyzed together with the river sediment dataset of the same period. The net accretion rate over the study area decreased from 38 mm/yr in 1958–1978 to 8 mm/yr in 1978–1997, while the river sediment discharge decreased from 466×10 6 to 394×10 6 t/yr and the suspended sediment concentration decreased from 0.543 to 0.448 kg/m 3 during the same periods, respectively. The outer subaqueous delta is more sensitive to decline in river sediment load in contrast with the inner subaqueous delta. From 1958–1978 to 1978–1997, the net accretion rate decreased from 51 to 2 mm/yr in the outer subaqueous delta and from 25 to 14 mm/yr in the inner subaqueous delta. The accretion rate decreased more rapidly than the river sediment discharge because the amount of sediment transported out of the delta has not decreased. A concept of the critical threshold of river sediment discharge that separates delta progradation from recession is proposed. Although this critical value was found to change temporally and spatially, it was instrumental in predicting the future delta response. Under the impacts of human activities, particularly, the Three Gorges Dam Project, the river sediment load would be reduced below the critical value during the next five decades. This would result in a general delta recession replacing delta progradation; the recession would occur first in the outer subaqueous delta.