Integrating Suspended Sediment Flux in Large Alluvial River Channels: Application of a Synoptic Rouse‐Based Model to the Irrawaddy and Salween Rivers

Aung Myo Khaing,Michael J Bickle,Stephen E Darby,Emily I Stevenson,Edward T Tipper,Christina S Larkin,J Jotautas Baronas,Daniel R Parsons,Robert G Hilton,Christopher R Hackney

doi:10.1029/2020jf005554

Abstract

AbstractA large portion of freshwater and sediment is exported to the ocean by a small number of major rivers. Many of these megarivers 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 vary 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 megarivers in Southeast Asia. Deriving sediment‐discharge rating curves, we calculated an annual sediment flux of Mt/year for the Irrawaddy and Mt/year 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 Mt C/year, 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.

Highlights

Rivers are the main conduits of dissolved and particulate matter from the continents to the oceans
The Irrawaddy discharge and sediment flux are about twice that of the Salween, due to the higher %OC of Salween sediments, the particulate organic carbon (POC) fluxes were similar in both rivers, ranging from 0.3 to 12 · 109 g C/day
The bedload POC flux is ignored, given that coarse sand contains low %OC (Figure 4) and that the majority of sediment is carried as suspended load; this approximation should result in a negligible underestimation of the total POC flux

Summary

Introduction

Rivers are the main conduits of dissolved and particulate matter from the continents to the oceans. The world's 30 largest rivers by discharge are estimated to account for ∼50% of all freshwater and ∼25% of all particulate matter export to the ocean (Milliman & Farnsworth, 2011). Southeast Asian rivers in particular dominate the global sediment flux, delivering about 2/3 of the supply to the ocean, due to a combination of active tectonics and monsoonal climate (Milliman & Farnsworth, 2011). Constraining the sediment and particulate organic carbon flux of large Southeast Asian rivers can significantly reduce uncertainties in the global exogenic carbon cycle, helping determine both the importance of natural feedback processes, as well as the scale of human perturbation in these river basins

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Discussion

Conclusion