LAUSR

A large portion of freshwater and sediment is exported to the ocean by a small number 22 of major rivers. Many of these mega-rivers are subject to substantial anthropogenic pres23 sures, which are having a major impact on water and sediment delivery to deltaic ecosys24 tems. Due to hydrodynamic sorting, sediment grain size and composition varies strongly 25 with depth and across the channel in large rivers, complicating flux quantification. To 26 account for this, we modified a semi-empirical Rouse model, synoptically predicting sed27 iment concentration, grain-size distribution, and organic carbon (%OC) composition with 28 depth and across the river channel. Using suspended sediment depth samples and flow 29 velocity data, we applied this model to calculate sediment fluxes of the Irrawaddy (Ayeyarwady) 30 and the Salween (Thanlwin), the last two free-flowing mega-rivers in Southeast Asia. Deriving sediment-discharge rating curves, we calculated an annual sediment flux of 326+91 −70 31 Mt/yr for the Irrawaddy and 159+78 32 −51 Mt/yr for the Salween, together exporting 46% as 33 much sediment as the Ganges-Brahmaputra system. The mean flux-weighted sediment 34 exported by the Irrawaddy is significantly coarser (D84 = 193±13 µm) and OC-poorer 35 (0.29±0.08 wt%) compared to the Salween (112±27 µm and 0.59±0.16 wt%, respec36 tively). Both rivers export similar amounts of particulate organic carbon, with a total of 1.9 +1.4 37 −0.9 Mt C/yr, 53% as much as the Ganges-Brahmaputra. These results underline 38 the global significance of the Irrawaddy and Salween rivers and warrant continued mon39 itoring of their sediment flux, given the increasing anthropogenic pressures on these river 40 basins.