Natural and reservoir-induced channel changes in the Yangtze River Tidal Reach

The Yangtze River Tidal Reach (YRTR) is one of the most important land-sea transition zones in China. The YRTR is 640 km long and the adjacent land area of 220,000 km(2) has a population of ca. 150 million. Even though the effect of Three Gorges Dam (TGD) on its downstream has been extensively studied, a comprehensive understanding of natural and dam-induced changes in the YRTR is still a challenging task. Here we use a digital elevation model and a numerical model to identify the causes of channel changes in the YRTR. Our analysis highlighted three main points.1) Changes in the tidal range caused an increase in the ebb maximum discharge and led to an increase in channel width, depth, and cross-section area. However, the channel depth abruptly starts to dwindle at the transition zone where unidirectional ebb flow changes to bidirectional ebb-flood flows. 2) Dam interception has reduced the sediment supply into the YRTR causing a change from deposition to erosion in the river bed. An increase of one billion m3 in reservoir capacity cut down annual suspended sediment supply by 2.26 million tons and scoured 1.54 million tons of riverbed sediment annually from the YRTR into the estuary. 3) Dam storage caused an increase of ca. 3800 m(3)/s runoff into the YRTR in the spring season during 2001-2013 compared with the same period in 1959-1970. This caused an average increase of ca.10% in the water diversion rate into the branch and an increase in flow velocity in flood plains and branch channels of 2.9 and 2.5 times that in the trunk channels. Such hydrodynamic increases have accelerated erosion of the branches and flood plains in recent decades especially post TGD. The study presents original insights into the tidal channel longitudinal abrupt changes induced by shift of natural flow regime, and riverbed changes from deposition to erosion induced by reservoirs, and indicates the likely response of the YRTR to further decline in sediment load in the future.