Continuous Holocene Streamflow Rise and ENSO Linked Floods in the Upper Reaches of Yarlung Tsangpo

Understanding the long‐term streamflow variability of the Yarlung Tsangpo (called the Brahmaputra River in downstream) is critical for water resources management due to its provision of freshwater for millions of people and its flood risk. However, previous streamflow reconstructions, based on dendrochronology and palaeo‐flood sediments, are either discontinuous or cover only a few hundred years. Here, we provide a continuous palaeolimnological record of the streamflow of the Yarlung Tsangpo upstream. Our results reveal an unexpected streamflow rise after 6,000 years ago, indicating the Westerlies and glacier meltwater replaced Indian Summer Monsoon as the dominant driver of runoff. Furthermore, the relatively frequent palaeo‐flood events during the early and late Holocene may be attributed to El Niño‐Southern Oscillation (ENSO) variance. These findings suggest the future occurrence of unsustainable streamflow and an increased flooding risk, as a result of weakening Westerlies, reduced glaciers, and strengthened ENSO activity under climatic warming. Plain Language Summary Yarlung Tsangpo (the upper reaches of the Brahmaputra River) is one of the most important transboundary rivers in South Asia, providing crucial freshwater resources for over 60 million people, while also experiencing frequent hazardous floods. To improve water resources management with goal of improving the public's lives, it's important to understand long‐term changes in streamflow and to determine underlying mechanisms. Here, we provide a continuous Holocene streamflow record using a river‐connected lake in the upstream of the Yarlung Tsangpo. Our results reveal that the streamflow was primarily controlled by the Indian Summer Monsoon before 6,000 years ago, which is consistent with modern observations. Later, however, the streamflow increased continuously despite the weakened monsoon. This increased streamflow was caused by the enhancement of the Westerlies and increased glacial meltwater. Palaeo‐floods, identified using Rate of Change analysis, show a relatively high frequency during the early and late Holocene, possibly linked to the variability of the El Niño‐Southern Oscillation (ENSO). If correct, our findings suggest that future warming may weaken the Westerlies and accelerate glacier melting, thereby threatening the long‐term sustainability of the streamflow of the Yarlung Tsangpo. Additionally, climatic warming leading to an intensified ENSO would result in an increased flood r