Accelerating sea-level rise and decreasing riverine sediment supply are widely considered to lead to global losses of deltaic marshes and their valuable ecosystem services. However, little is known about the degree to which the related erosion of the seaward delta front can provide sediments to sustain salt marshes. Here, we present data from the mesomacrotidal Yangtze Delta demonstrating that marshes have continued to accrete vertically and laterally, despite rapid relative sea-level rise ($10 mm yr −1) and a > 70% decrease in the Yangtze River sediment supply. Marsh progradation has decelerated at a lower rate than fluvial sediment reduction, suggesting an additional source of sediment. We find that under favorable conditions (e.g., a mesomacrotidal range, strong tidal flow, flood dominance, sedimentary settling lag/scour lag effects, and increasing high-tide level), delta-front erosion can actually supply sediment to marshes, thereby maintaining marsh accretion rates in balance with relative sea-level rise. Comparison of global deltas illustrates that the ability of sediment remobilization to sustain marshes depends on coastal processes and varies by more than an order of magnitude among the world's major deltas. Salt marshes are among the world's most valuable ecosystems (Costanza et al. 1997). They sequester carbon, protect shorelines from storm impacts, transform nutrients, contribute to fisheries production, and maintain biodiversity (Barbier et al. 2011; Kirwan and Megonigal 2013; Temmerman et al. 2013; Möller et al. 2014). Unfortunately, many salt marshes have disappeared due to reclamation and waste disposal during the past century (Gedan et al. 2009; Ma et al. 2014). Deltaic marshes are one of the most dynamic landscapes on Earth's surface (Wagner et al. 2017) and are threatened by accelerating sea-level rise and decreases in fluvial sediment supply. Decreasing fluvial sediment supply reduces the ability of salt marshes to accumulate sediments and to build up their soil elevation in balance with the rising sea level (Kirwan et al. 2010; Weston 2014). Although the morphology and evolution of deltas are influenced by various factors (Paola et al. 2011), such as riverine water and sediment discharges (Besset et al. 2019), sediment properties (Caldwell and Edmonds 2014), flow patterns (Shaw et al. 2016), vegetation height and density (Nardin et al. 2016), marine hydrodynamics (waves, tides, and longshore currents) (Caldwell and Edmonds 2014; Besset et al. 2017), land subsidence and sea-level changes (Jerolmack 2009; Syvitski et al. 2009), changes in the fluvial sediment supply and relative sea level are usually the most important for the long-term morphological evolution of deltaic marshes. The rate of global mean sea-level rise increased from 1.

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Human society and ecosystems worldwide are increasinAagly threatened by water shortages. Despite numerous studies of climatic impacts on water availability, little is known about the influences of socioeconomic development on streamflow and water sustainability. Here, we show that the streamflow from the Yellow River to the sea has decreased by more than 80% in total over the last 60 years due to increased water consumption by agricultural, industrial and urban developments (76% of the streamflow decrease, similarly hereinafter), decreased precipitation (13%), reservoir construction (6%) and revegetation (5%). We predict that if the past trends in streamflow will continue, year-round dry-up in the lower Yellow River will commence in the late 2020s or early 2030s, unless effective countermeasures such as water diversion from the Yangtze River are taken. These results suggest that streamflow in semiarid basins is highly vulnerable to human impacts and that streamflow decline would in turn hinder further socioeconomic development and endanger river-sea ecosystems.