China’s plans to build a massive hydro project in Tibet have sparked fears about the environmental impacts on the world’s longest and deepest canyon. It has also alarmed neighboring India, which fears that China could hold back or even weaponize river water it depends on.

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A hydroelectric project at a remote river gorge in eastern Tibet, an ecological treasure trove close to a disputed border with India. Indian politicians have reacted angrily, saying it gives China the ability to release destructive “water bombs” across the border in any future conflict. They are planning a retaliatory dam on their side of the border that experts say could be at least as environmentally destructive.

Two Chinese dams will barricade the Yarlung Tsangpo, the Tibetan name for the Brahmaputra River, as it is about to flow through the world’s longest and deepest river canyon — think the Grand Canyon on the Colorado River, only three times as deep. Projected to cost $137 billion, the scheme will be the world’s biggest single infrastructure project, with almost three times the generating capacity of the world’s current largest hydroelectric dam, China’s Three Gorges on the Yangtze River.

Chinese ecologists say the canyon is one of the most precious biodiversity hotspots on the planet, containing some of Asia’s tallest and most ancient trees as well as the world’s richest assemblage of large carnivores, especially big cats. But India’s anger is geopolitical. Pema Khandu, the chief minister of Arunachal Pradesh, the Indian state immediately downstream, called the project “a big threat” that could dry up the river through his state during routine operation and potentially be weaponized to unleash a flood in which, he said, hundreds of thousands could lose their lives.

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The stakes are high, with tensions over scarce water resources in the region rising. India last month suspended its adherence to a treaty in operation for 65 years to share with Pakistan the waters of another great South Asian river, the Indus. Meanwhile the 30-year-old Ganges Water Treaty between India and Bangladesh is set to expire next year, with India widely accused of violating its terms.

“Weaponizing water is a perilous strategy that may backfire,” says Mehebub Sahana, an environmental geographer at the University of Manchester. “The weakening of water diplomacy in South Asia is not just a regional threat; it endangers global climate security.”

Tibet, part of China since 1951, is the water tower of Asia. Its vast glaciers sustain major rivers on which more than 1.3 billion people in 10 countries depend for drinking, irrigating crops, and hydropower. China, already the world’s leading producer of hydroelectricity, sees more dams on these rivers as a key to reducing its carbon emissions.

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Technical details about the project have yet to be published. But Chinese government media say it will have a generating capacity of 60,000 megawatts, almost 30 times that of the Hoover Dam. But the two proposed dams don’t need to be even as high as the Hoover Dam, says Gamble. “This is more a mega-project than a mega-dam.” The site’s unique geography will do the work, as the water rushes downward for thousands of feet through 12-mile-long tunnels to deliver unprecedented power to turbines at the bottom of the canyon, before discharging the flow back into the river close to the border with India. “Indian soldiers will overlook the project from their bunkers,” says Gamble.

Indian scientists believe that operating the dams to meet China’s electricity needs will change the river’s strongly seasonal flow. “Reduced water flow in the dry season, coupled with sudden releases of water during monsoons, could intensify both water scarcity and flooding, endangering millions,” says Sahana.

The project could also impact sediment flows in the river. Erosion in the canyon currently supplies 45 percent of the total volume of sediment that flows downstream on the Brahmaputra, says Robert Wasson, a geomorphologist at James Cook University, in Australia. Bypassing the canyon could reduce sediment supply to the lower reaches and damage the river’s vast delta, says Sahana. “Any disruption to the balance of sediment could accelerate coastal erosion and make the already low-lying [delta] area more vulnerable to sea-level rise.” But this outcome is far from clear, says Wasson, as too little is known about sediment movement on the river.

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The geopolitics of international rivers in South Asia has long been fraught. India itself has often been accused of being an upstream bully — notably on the Indus River, which flows out of the Himalayas and through India to Pakistan.

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Last month, tensions soared again when India unilaterally suspended its adherence to the treaty, as part of its retaliation for a terrorist attack. Pakistan’s prime minister Shehbaz Sharif responded by warning that if India tried to block the river’s flow it would be met with “full force and might.”

The parallel between this standoff on the Indus and the threat posed to India by the Chinese project on the Brahmaputra is compelling, but not exact. There is no treaty governing the management of the Brahmaputra, for instance. But the power of upstream countries over their downstream neighbors is central to both disputes. In each case, the hydrological and political stakes are high in a region with a troubling history of belligerent rhetoric, unilateral actions on shared rivers, and taking up arms over disputed waters.