The Irrawaddy River sediment flux to the Indian Ocean: the original nineteenth-century data revisited

The Irrawaddy (Ayeyarwady) River of Myanmar is ranked as having the fifth-largest suspended load and the fourth-highest total dissolved load of the world's rivers, and the combined Irrawaddy and Salween ( Thanlwin) system is regarded as contributing 20% of the total flux of material from the Himalayan-Tibetan orogen. The estimates for the Irrawaddy are taken from published quotations of a nineteenth-century data set, and there are no available published data for the Myanmar reaches of the Salween. Apart from our own field studies in 2005 and 2006, no recent research documenting the sediment load of these important large rivers has been conducted, although their contribution to biogeochemical cycles and ocean geochemistry is clearly significant. We present a reanalysis of the Irrawaddy data from the original 550-page report of Gordon covering 10 yr of discharge ( 1869-1879) and 1 yr of sediment concentration measurements ( 1877-1878). We describe Gordon's methodologies, evaluate his measurements and calculations and the adjustments he made to his data set, and present our revised interpretation of nineteenth-century discharge and sediment load with an estimate of uncertainty. The 10-yr average of annual suspended sediment load currently cited in the literature is assessed as being underestimated by 27% on the basis of our sediment rating curve of the nineteenth-century data. On the basis of our sampling of suspended load, the nineteenth-century concentrations are interpreted to be missing about 18% of their total mass, which is the proportion of sediment recovered by a 0.45-mm filter. The new annual Irrawaddy suspended sediment load is MT. Our revised estimate of the annual sediment load 364 +/- 60 from the Irrawaddy-Salween system for the nineteenth century ( 600 MT) represents more than half the present-day Ganges-Brahmaputra flux to the Indian Ocean. Since major Chinese rivers have reduced their load due to damming, the Irrawaddy is likely the third-largest contributor of sediment load in the world.