River systems help sustain lives and livelihoods. But, thanks to excessive damming and drastic overuse of their water resources, some rivers are drying up before reaching the sea. Nowhere is this truer than Asia.

Building large dams has increasingly run into grassroots opposition in established democracies but has gained momentum in autocratic states, which often tout their benefits for combating droughts and water shortages. But as the Mekong River (Southeast Asia’s lifeline) illustrates, giant upstream dams can contribute to depletion and intensify parched conditions in downriver regions.

The spate of dam building in Asian autocracies is exacerbating already fraught water security disputes.

India and Japan demonstrate that dams and democracy normally don’t go well together. Whereas China continues to build giant dams, trumpeting them as symbols of its engineering prowess, the public pressures generated by the Japanese and Indian democracies act as a brake on ambitious water projects that displace many people or flood vast areas.

For example, India’s plan to link up its major rivers through man-made canals remains in the realm of fantasy. But China’s similar program, known as the South-North Water Diversion Project, has been transferring water domestically through the central and eastern routes.

In fact, given the power of nongovernmental organizations in many democratic states, it has become difficult to build large dams in most democracies. Cost and time overruns are common problems in democracies when dam projects are pursued.

The cornerstone of India’s controversial Narmada Dam — 15.5 times smaller than China’s Three Gorges Dam, the world’s largest — was laid in 1961. More than 56 years later, the dam was dedicated to the nation by Prime Minister Narendra Modi. The Narmada project, however, is still not fully complete.

Likewise, the Yamba Dam in Gunma Prefecture has been decades in the making. The 116-meter-tall multipurpose dam on the Agatsumagawa River, upstream of the Tonegawa River, is finally to be commissioned next spring. With a storage capacity of 107.5 million cubic meters, the dam will be utilized for power generation and water control and release.

The recent debut of the 1,285-megawatt Xayaburi Dam in Laos illustrates how autocracies steamroll protests and discount environmental concerns to complete projects. The Xayaburi — the first of at least nine Mekong dam projects in communist-ruled Laos — was commissioned despite concerns that it could worsen the drought in the downstream basin in Cambodia and Vietnam.

No nation, however, can match China’s dam-building frenzy. It is increasingly damming international rivers that serve as the lifeblood for the countries of Southeast and South Asia.

Take the Mekong: Just before that river crosses from the Tibetan Plateau into Southeast Asia, China has erected a cascade of mega-dams. Its 11 Mekong dams have a capacity to generate more than 21,300 megawatts of electricity — greater than the installed hydropower capacity of the downriver countries combined. It is working on at least eight more giant dams on the Mekong.

Every since the cascade of Chinese dams came up on the Mekong, droughts have become more frequent and intense in the downriver countries. The Mekong, which is normally at least 3 meters high at this time of the year, is today running at a record low level, with its flow reduced to a trickle in some stretches. This has resulted in seawater intrusion into the Mekong Delta in southern Vietnam, with rice farmers forced to switch to shrimp farming or growing reeds.

China has promised to release more dam water for the drought-stricken countries. The offer, however, highlights the newfound reliance of downriver countries on Chinese goodwill. By diverting river waters to its mega-dams, China has emerged as Asia’s upstream water controller, arming it with powerful leverage.

The downriver countries have also been saddled with long-term environmental costs. For example, Asian rivers’ monsoon-season flooding cycle helps to refertilize farmland naturally by spreading nutrient-rich sediment, which rivers bring from the mountain ranges. The flooding cycle also opens giant fish nurseries.

China’s dams, however, have disrupted the Mekong’s annual flooding cycle and impeded the flow of sediment, affecting even marine life.

The Mekong Delta, for its part, exemplifies how heavy upstream damming, by reducing a river’s discharge of freshwater and sediment into the sea, causes a delta to retreat. Indeed, according to a Mekong River Commission study, the upstream dams’ cumulative effect would likely be the extinction of most migratory fish species in the basin.

What is happening to the Mekong today could happen tomorrow to the other Tibet-originating international rivers that are the target of China’s dam builders.

Simply put, the proliferation of upstream dams is beginning to impose costs across much of Asia. Dams are also set to exacerbate water-sharing disputes, which have already become common between Asian nations and provinces. Add to the picture the security dynamic: The Mekong, like the South China Sea, is emerging as a new flash point, with U.S. Secretary of State Mike Pompeo slamming China’s dam frenzy for reducing the river’s flow to a record low.

Dams help generate electricity and store water for the dry season. But heavy upstream damming, by irreparably damaging a river system and wreaking broader environmental havoc, eventually leaves no winners, only losers.

To avert a parched future, defiant unilateralism must give way to basin-wide institutionalized collaboration, centered on a balance between each county’s rights and obligations.

Brahma Chellaney is a geostrategist and author of “Water: Asia’s New Battleground,” winner of the Bernard Schwartz Award.

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