Abstract:

Datong Hydrometric Station and Sediment Monitoring Profile are located in the landward limit of the Yangtze Estuary. The river channel from Datong (tidal limit) to the river mouth is as long as 680 km. The increasingly intensified human activities since the 1980s have significantly changed the sediment and water discharges from the Yangtze River into the estuary, therefore greatly influenced the sediment dynamics and morphodynamics from the lower Yangtze River to the Yangtze Estuary. This paper mainly examined the longterm variations in the grain size of bottom sediments since the 1950s at Datong, the landward limit of the Yangtze Estuary. There is an increasing trend in grain size (〖WTBX〗d〖WTBZ〗50) of bottom sediments from 1977 to 2004 mainly in response to a sustained decrease in suspended sediments from upstream. It is noticed that the riverbed coarsening process has been occurring under the background of local riverbed aggradations. The increasing trend in grain size was mainly caused by an increase of grain size of bottom sediments from the adjacent upstream reaches and by the local sediment exchange between suspended sediments and bottom sediments during the erosion and deposition process. It is expected that the grain size of bottom sediment at the landward limit of the Yangtze Estuary will increase further in the coming decades due to a more dramatic reduction of suspended sediment after the Three Gorges Project and other upstream largescale reservoirs.〖

Key words: bottom sediment, suspended sediment, grain size, Three Gorges Dam, the Yangtze River〖