The Irrawaddy and Salween rivers in Myanmar deliver water fluxes to the ocean equal to~70% of the GangesBrahmaputra river system. Together these systems are thought to deliver about half the dissolved load from the tectonically active Himalayan-Tibetan orogen. Previously very little data was available on the dissolved load and isotopic compositions of these major rivers. Here we present time series data of 171 samples collected fortnightly at intervals throughout 2004 to 2007 from the Irrawaddy and Salween at locations near both the river mouths, the up-stream Irrawaddy at Myitkyina, the Chindwin, a major tributary of the Irrawaddy and a set of 28 small tributaries which rise in the flood plain of the Irrawaddy between Yangon and Mandalay. The samples have been analysed for major cation, anion and 87 Sr/ 86 Sr ratios. The new data indicates that the Irrawaddy has an annual average Na concentration only a third of the widely quoted single previously published analysis. The Irrawaddy and Salween drain about 0.5% of global continental area and deliver about 3.3% of the global silicate-derived dissolved Ca + Mg fluxes and 2.6% of the global Sr riverine fluxes to the oceans. This compares with Ganges and Brahmaputra which deliver about 3.4% of the global silicate-derived dissolved Ca + Mg fluxes and 3.2% of the global Sr riverine fluxes to the oceans from about 1.1% of global continental area. The dischargeweighted mean 87 Sr/ 86 Sr ratio of the Irrawaddy is 0.71024 and the Salween 0.71466. The chemistry of the Salween and the Irrawaddy waters reflects their different bedrock geology. The catchment of the Salween extends across the Shan Plateau in Myanmar through the Eastern syntaxis of the Himalayas and into Tibet. The Irrawaddy flows over the Cretaceous and Tertiary magmatic and metamorphic rocks exposed along the western margin of the Shan Plateau and the Cretaceous to Neogene Indo-Burma ranges. The 87 Sr/ 86 Sr compositions of the Salween and Upper Irrawaddy (between 0.7128 and 0.7176) are significantly higher than the downstream Irrawaddy (0.7095 to 0.7108) and the Chindwin (0.7082 to 0.7095). The Irrawaddy and the Chindwin exhibit lower 87 Sr/ 86 Sr and Na/Ca ratios during and immediately post-monsoon, interpreted to reflect higher weathering of carbonate at high flow. The Salween exhibits higher 87 Sr/ 86 Sr ratios but lower Na/Ca ratios during the monsoon, interpreted to reflect higher inputs from the upper parts of the catchment in the Himalayas.

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