J. Jotautas Baronas , Emily I. Stevenson, Chris Hackney, Stephen Darby, Michael J. Bickle, Robert G Hilton, Christina Sheila Larkin, Dan Parsons, Aung Myo Khaing, Edward T. Tipper

A large portion of freshwater and sediment is exported to the ocean by a small number of major rivers. Many of these mega-rivers are subject to substantial anthropogenic pressures, which are having a major impact on water and sediment delivery to deltaic ecosystems. Due to hydrodynamic sorting, sediment grain size and composition varies strongly with depth and across the channel in large rivers, complicating flux quantification. To account for this, we modified a semi-empirical Rouse model, synoptically predicting sediment concentration, grain size distribution, and organic carbon (%OC) concentration with depth and across the river channel. Using suspended sediment depth samples and flow velocity data, we applied this model to calculate sediment fluxes of the Irrawaddy (Ayeyarwady) 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 Mt/yr for the Irrawaddy and 159 (+78, -51) Mt/yr for the Salween, together exporting 46% as much sediment as the Ganges-Brahmaputra system. The mean flux-weighted sediment exported by the Irrawaddy is significantly coarser (D84 = 193 ± 13 µm) and OC-poorer (0.29 ± 0.08 wt%) compared to the Salween (112 ± 27 µm and 0.59 ± 0.16 wt%, respectively). Both rivers export similar amounts of particulate organic carbon, with a total of 1.9 (+1.4, -0.9) Mt C/yr, 53% as much as the Ganges-Brahmaputra. These results underline the global significance of the Irrawaddy and Salween rivers and warrant continued monitoring of their sediment flux, given the increasing anthropogenic pressures on these river basins.