China recently approved the construction of the world’s largest hydropower dam, across the Yarlung Tsangpo river in Tibet. When fully up and running, it will be the world’s largest power plant – by some distance. Yet many are worried the dam will displace local people and cause huge environmental disruption. This is particularly the case in the downstream nations of India and Bangladesh, where that same river is known as the Brahmaputra.

The proposed dam highlights some of the geopolitical issues raised by rivers that cross international borders. Who owns the river itself, and who has the right to use its water? Do countries have obligations not to pollute shared rivers, or to keep their shipping lanes open? And when a drop of rain falls on a mountain, do farmers in a different country thousands of miles downstream have a claim to use it? Ultimately, we still don’t know enough about these questions of river rights and ownership to settle disputes easily.

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The Yarlung Tsangpo begins on the Tibetan Plateau, in a region sometimes referred to as the world’s third pole as its glaciers contain the largest stores of ice outside of the Arctic and Antarctica. A series of huge rivers tumble down from the plateau and spread across south and south-east Asia. Well over a billion people depend on them, from Pakistan to Vietnam. Yet the region is already under immense stress as global warming melts glaciers and changes rainfall patterns. Reduced water flow in........

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