Evolution of the first mouth bar, distributaries and floodplains of the Pearl River Delta

As one of the most densely populated areas in the world, the Pearl River delta has been well studied in its postglacial sedimentary stratigraphy and the effects of Holocene sea-level rise. However, the effects of fluvial force on its geomorphological evolution are much less understood. This paper analyzed the Holocene sedimentary archives and reconstructed the geomorphologic history of an area where the West and North Rivers join and enter the deltaic basin. The reconstruction is based on the analyses of particle size, total organic carbon, organic carbon isotope ratios and microfossil diatoms of five radiocarbon-dated core sediments. The results show rapid paleo-incised valley infilling since c. 9500 cal. a BP, in response to sea-level rise. During marine regression starting from c. 8000 cal. a BP, a bifurcation occurred, and a mouth bar was developed. While the mouth bar emerged to mean tide level around c. 5500 cal. a BP, the surrounding estuarine subtidal flats were gradually transformed into tidal wetlands by c. 3000 cal. a BP. The immediate adjacent waters to the mouth bar were gradually infilled from c. 3000 cal. a BP onwards which narrowed the distributary channels. This landform evolution in the head area of the Pearl River delta resulted in the gradually strengthening fluvial force and weakening tidal force since the relative sea level approached its stabilization. During the past 3000 years, the continuous strengthening of fluvial force supported the accretion of deltaic floodplains. The discovery of mouth bar formation, channel narrowing and deltaic floodplain accretion in this location brings a new perspective of scientific investigation to river delta geomorphologic evolution in response to natural and anthropogenic factors.