China has begun building a $170 billion hydropower complex on the Yarlung Zangbo river in Tibet, a project that is meant to outsize the Three Gorges Dam but one that downstream India and Bangladesh fear could throttle the Brahmaputra river that sustains tens of millions.

Work started at the weekend on five “run-of-the-river” dams along a steep 50-km gorge where the river drops nearly 2,000 metres, officials said. Beijing says the scheme will generate enough electricity each year to power Britain, creating jobs and fresh stimulus for the slowing economy.

Neighbouring countries worry chiefly about water security. Arunachal Pradesh’s chief minister warned earlier this year that as much as 80% of the Brahmaputra’s flow through the Indian state could dry up, while sediment vital for downstream farming would be trapped.

Michael Steckler, a geophysicist at Columbia University, said the dams would hold back nutrients as well as water, potentially affecting agriculture across India’s Assam plain and low-lying Bangladesh.

China’s foreign ministry, responding on Tuesday, called the project “a matter of China’s sovereign affairs,” adding that it would cut flood risks and that Beijing was sharing hydrological data with neighbours.

Some experts say the impact on flows may be limited because most of the Brahmaputra’s volume comes from monsoon rains south of the Himalayas.

Sayanangshu Modak of the University of Arizona noted that India itself is planning two dams on the same river, including an 11.5-gigawatt plant in Arunachal Pradesh, partly to reinforce its own water rights.

The isolated mountain region is prone to earthquakes, glacial-lake floods and landslides. A series of quakes in Tibet earlier this year revived concerns about safety as dam building gathers pace on the plateau.

India and China fought a brief border war in the area in 1962, and the lack of detailed technical disclosures from Beijing has heightened speculation that water could again become a strategic weapon in any future conflict. Bangladesh, whose deltaic economy depends on the river, has also sought more information.

New Delhi’s foreign and water ministries had not commented by late Tuesday, while Beijing has released no construction timetable beyond saying first power is expected in the early-to-mid-2030s.

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