Different fates of the Yangtze and Mississippi deltaic wetlands under similar riverine sediment decline and sea-level rise

The Yangtze River has experienced a sharp sediment decrease as a result of extensive dam constructions since the 1980s, similar to the sediment decline in the Mississippi River since the 1950s. Historical land losses in the Mississippi Delta have been reported extensively, but this issue in the Yangtze Delta is still under debate. Here we presented the morphological responses of deltaic wetlands to the sediment decline with the comparison of hydrodynamic forces in the Yangtze and Mississippi deltas. We found that the wetlands of the Yangtze Delta continued their expansion even after the sediment decline since 2003 when the Three Gorges Dam began operation, in contrast to the historical scenario in the Mississippi. Based on observations at water depth around 10 m in both of the open subaqueous deltas, the bed shear stresses caused by waves and currents (τcw) during neap and spring tides were 0.18–0.30 N/m² in the Yangtze Delta and 0.09–0.10 N/m² in the Mississippi Delta under fair weather conditions. The periods of τcw > τcr (the critical bed shear stress for erosion) occupied on average 80% of a tidal cycle in the Yangtze Delta but only 47% of a cycle in the Mississippi Delta. The rates of relative sea-level rise and riverine sediment decrease in these two deltas are similar, but the sediment-supply capacity to the deltaic wetlands from the Yangtze's subaqueous delta is much larger than that of the Mississippi Delta. Thus, we infer that in contrast to the historical land loss after sediment decline in the Mississippi Delta, the Yangtze deltaic wetlands stand still with a good opportunity to survive for years to decades with the aid of abundant sediment from the subaqueous delta due to higher capacity of seabed erodibility by stronger coastal hydrodynamics.