J. Wang1,2*, J.H. Nienhuis1 , Z.J. Dai2,3
1 Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands; 2 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China; 3 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China
* Corresponding author: j.wang7@uu.nl
Introduction
River deltas are among the most dynamic and biologically productive regions on Earth’s surface. However, many deltas face declining river sediment supply. There is a theoretical expectation that this decline will reduce delta land growth, or cause delta land loss, but delta-scale field verification has so far remained incomplete. This may be because of large time delays between river sediment supply fluctuations and delta response.
Objective and Methods
Here, our primary objectives are: (1) to quantify changes in suspended sediment discharge and land area in 94 major river deltas worldwide since 1984; (2) to analyze the gains and losses of deltaic tidal wetlands (mangroves, salt marshes, and bare flats) and their transition patterns; (3) to compare the differences in morphodynamics and time lags between river-, tide-, and wave-dominated deltas. The main methods include: (1) obtaining fluvial sediment supply timeseries to deltas; (2) extracting delta land area and tidal wetland changes from Global Surface Water and Tidal Wetland Change dataset, respectively, in Google Earth Engine platform; (3) using cross-correlation to analyze time lags between suspended sediment discharge and land area changes in different deltas; (4) classifying all deltas based on their ratio of riverine, tidal and wave hydro- and sediment dynamic conditions (Qriver, Qtide, Qwave).
Results
Suspended sediment discharges have decreased in 38 of the global 94 major rivers since 1984, and increased in 37 of them, the rest remained relatively stable. Meanwhile, deltas gained 22,174 km2 and lost 17,572 km2 of wetland. As a result, its net gained area amounted to 4,602 km2 with an average building rate of 121 km2/yr. Besides, the net loss of mangrove and salt marsh lost in 82 river deltas (between 60°N and 60°S), respectively, accounted for 3,487 km2 and 2,471 km2 between 1999-2019; bare flat expanded 295 km2. Hence, the loss rate of deltaic wetlands reached 283 km2/yr in recent two decades. In particular, the Mississippi Delta and Mekong delta suffered the most dramatic wetland retreat (177 km2/yr and 42 km2/yr). While the Yangtze Delta and Ganges Delta showed the greatest increases (32 km2/yr and 23 km2/yr). We find that timeseries cross-correlations between riverine suspended sediment discharge and delta land area change show lagged effect over several in many deltas. But, more, and longer, field observations will be necessary to further constrain these findings.
References
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Acknowledgement
This study was financially supported by the National Key R&D Program of China (Grant No. 2023YFE0121200).
