The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , 17 with a significant fraction being sequestered in large deltas, home to over 500 million 18 people. Most (>70%) large deltas are under threat from a combination of rising sea 19 levels, ground surface subsidence and anthropogenic sediment trapping 2,3 , and a 20 sustainable supply of fluvial sediment is therefore critical in preventing deltas being 21 'drowned' by rising relative sea levels 2,3,4 . Here, we combine suspended sediment 22 load data from the Mekong River with hydrological model simulations to isolate the 23 role of tropical cyclones (TCs) in transmitting suspended sediment to one of the 24 world's great deltas. We demonstrate that spatial variations in the Mekong's 25 suspended sediment load are correlated (r = 0.765, p < 0.1) with observed variations 26 in TC climatology, and that a significant portion (32%) of the suspended sediment 27 load reaching the delta is delivered by runoff generated by TC-associated rainfall. 28Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 29 explaining past 5,6,7 , and anticipating future 8,9 , declines in suspended sediment loads 34 reaching the world's major deltas. However, our study shows that changes in TC 35 climatology affect trends in fluvial suspended sediment loads and thus are also key to 36 fully assessing the risk posed to vulnerable coastal systems. 37 Mt over recent years (1981-2005The world's largest rivers contribute a disproportionately large fraction (Extended 38Data Table 1) of the terrestrial sediment flux, which has both created, and is critical in 39 sustaining, their great deltas. Moreover, river borne sediments are a key vector for carbon 40 and nutrients, thereby playing a vital role in global biogeochemical cycles 10,11 . However, a 41 significant majority (>70%) of large deltas are now recognized as being under severe 42 threat from rising relative sea levels 2,3 , in part due to reported anthropogenically-driven 43 reductions in sediment loads 5,6,7 . Many large rivers are located in tropical regions 44 (Extended Data Figure 1) that exhibit highly seasonal flow regimes affected by tropical 45 cyclones (TCs). The potential destructive or constructive impacts of tropical cyclones that 46 directly strike deltas are well established 12,13 . However, when they strike further upstream 47TCs deliver much higher than normal levels of rainfall, effectively triggering landslides 48 and mobilizing sediments into the river network, thereby generating very high 49 instantaneous sediment loads 14,15,16 . Such high sediment loads could compensate for the 50 potential destructive effects of TCs striking deltas proper but, notwithstanding some prior 51 studies in smaller drainage basins 17,18 , the role of TCs in driving sediment delivery to the 52 lowlands and coast remains unclear. As noted, this is particularly the case for large rivers 53 that carry much of the terrestrial sediment flux because these rivers are, in t...

Recent growth of the construction industry has fuelled demand for sand, with considerable volumes being extracted from the world's large rivers. Sediment transport from upstream naturally replenishes sediment stored in river beds, but the absence of sand flux data from large rivers inhibits assessment of the sustainability of ongoing sand mining. Here, we demonstrate that bedload (0.18 Mt yr-1 ± 0.07 Mt yr-1) is a small (1%) fraction of the total annual sediment load of the lower Mekong River. Even when considering suspended sand (6 Mt yr-1 ± 2 Mt), the total sand flux entering the Mekong delta (6.18 Mt yr-1 ± 2.01 Mt yr-1) is far less than current sand extraction rates (50 Mt yr-1). We show that at

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The three‐dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion. Although submerged slump blocks and associated large‐scale bank roughness elements have both previously been proposed to divert flow away from the bank, direct observations of the interaction between eroded bank material and the 3‐D flow field are lacking. Here we use observations from multibeam echo sounding, terrestrial laser scanning, and acoustic Doppler current profiling to quantify, for the first time, the influence of submerged slump blocks on the near‐bank flow field. In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may also deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion. We use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

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