There is a pressing need to understand how different delta morphologies arise because morphology determines a delta's ecologic structure, resilience to relative sea-level rise, and stratigraphic architecture. We use numerical modeling (Delft3D) to explain how deltaic processes and morphology are controlled by the incoming sediment properties. We conducted 36 experiments of river-dominated delta formation varying the following sediment properties of the incoming grain-size distribution: the median, standard deviation, skewness, and percent cohesive sediment, which is a function of the first three properties. Changing standard deviation and skewness produces minimal morphological variation, whereas an increase in dominant grain size (D 84 ) and decrease in percent cohesive sediment produce a transition from elongate deltas with few channels to semicircular deltas with many channels. This transition occurs because critical shear stresses for erosion and settling velocities of grains set the number of channel mouths and the dominant delta-building process. Together, the number of channel mouths and the dominant process-channel avulsion, mouth bar growth, or levee growth-set the delta morphology. Coarse-grained, noncohesive deltas have many channels dominated by avulsion, creating semicircular planforms with relatively smooth delta fronts. Intermediate-grained deltas have many channels dominated by mouth bar growth, creating semicircular planforms with rugose delta fronts. Fine-grained, cohesive deltas have a few channels, the majority of which are dominated by levee growth, creating elongate planforms with smooth delta fronts. The process-based model presented here provides a previously lacking mechanistic understanding of the effects of sediment properties on delta channel network and planform morphology.

Climate change is intensifying tropical cyclones, accelerating sea-level rise, and increasing coastal flooding. River deltas are especially vulnerable to flooding because of their low elevations and densely populated cities. Yet, we do not know how many people live on deltas and their exposure to flooding. Using a new global dataset, we show that 339 million people lived on river deltas in 2017 and 89% of those people live in the same latitudinal zone as most tropical cyclone activity. We calculate that 41% (31 million) of the global population exposed to tropical cyclone flooding live on deltas, with 92% (28 million) in developing or least developed economies. Furthermore, 80% (25 million) live on sediment-starved deltas, which cannot naturally mitigate flooding through sediment deposition. Given that coastal flooding will only worsen, we must reframe this problem as one that will disproportionately impact people on river deltas, particularly in developing and least-developed economies.

River deltas are sites of sediment accumulation along the coastline that form critical biological habitats, host megacities, and contain significant quantities of hydrocarbons. Despite their importance, we do not know which factors most significantly promote sediment accumulation and dominate delta formation. To investigate this issue, we present a global dataset of 5399 coastal rivers and data on eight environmental variables. Of these rivers, 40 % (n = 2174) have geomorphic deltas defined either by a protrusion from the regional shoreline, a distributary channel network, or both. Globally, coastlines average one delta for every ∼ 300 km of shoreline, but there are hotspots of delta formation, for example in Southeast Asia where there is one delta per 100 km of shoreline. Our analysis shows that the likelihood of a river to form a delta increases with increasing water discharge, sediment discharge, and drainage basin area. On the other hand, delta likelihood decreases with increasing wave height and tidal range. Delta likelihood has a non-monotonic relationship with receiving-basin slope: it decreases with steeper slopes, but for slopes >0.006 delta likelihood increases. This reflects different controls on delta formation on active versus passive margins. Sediment concentration and recent sea level change do not affect delta likelihood. A logistic regression shows that water discharge, sediment discharge, wave height, and tidal range are most important for delta formation. The logistic regression correctly predicts delta formation 74 % of the time. Our global analysis illustrates that delta formation and morphology represent a balance between constructive and destructive forces, and this framework may help predict tipping points at which deltas rapidly shift morphologies.Published by Copernicus Publications on behalf of the European Geosciences Union.