China Is Choking Off Asia’s Most Important River

Upstream dams are destroying the Mekong Basin.

By , a writer and editor. She is the author of Troubling the Water: A Dying Lake and a Vanishing World in Cambodia.
Rangers stop a fishing boat for questioning at a bird sanctuary and protected area by Prek Toal floating village in Battambang province, Cambodia on October 14, 2020.
Rangers stop a fishing boat for questioning at a bird sanctuary and protected area by Prek Toal floating village in Battambang province, Cambodia on October 14, 2020.
Rangers stop a fishing boat for questioning at a bird sanctuary and protected area by Prek Toal floating village in Battambang province, Cambodia on October 14, 2020. Tang Chhin Sothy/AFP via Getty Images

It’s a March afternoon in Kampong Khleang, and the boats keep coming down a muddy river leading from the Tonle Sap lake, docking to unload their haul, stilt houses casting shadows up the bank. The boats are battered square boxes, long and low, filled halfway up to the gunwales with watery masses of fish. Skinny teenagers in broad hats and flip-flops balance on the thwarts to shovel the catch into plastic baskets. When each basket is full, a pair of men slide it down the plank leading to the bank and drag it by its rope handles 20 yards up. Kids lie in wait with small nets, ready to snag any escapees.

The village, a mass of towering homes packed against one another along the lake’s northern rim, sits about 30 miles southeast of the major city of Siem Reap and the temples of Angkor. Each day, thousands of pounds of fish will pass through this one small market alone. Countless more will go through countless markets set along countless inlets across the Tonle Sap. And at this particular spot on this particular day in 2017, there is only one type of fish the buyers are interested in: trey riel, which flicker like silver coins when the nets are drawn up.

The largest freshwater fish family in the world is Cyprinidae, among them carps and minnows, silvery and delicate. Trey riel—“money fish” —is the Khmer name for several of its species. Most are no larger than a thumb, though some can grow to the size of a forearm. Riel is the country’s currency, but think of this fish like a 100 riel note—about $0.02—something even the poor have plenty of. Trey riel is oily and astoundingly nutritious. It is used, most commonly, in the fermented fish paste prahok—a staple so omnipresent only rice is more commonly seen on a Cambodian table. It is not hyperbole to say that in all of Cambodia, no animal is as important as trey riel.

It’s a March afternoon in Kampong Khleang, and the boats keep coming down a muddy river leading from the Tonle Sap lake, docking to unload their haul, stilt houses casting shadows up the bank. The boats are battered square boxes, long and low, filled halfway up to the gunwales with watery masses of fish. Skinny teenagers in broad hats and flip-flops balance on the thwarts to shovel the catch into plastic baskets. When each basket is full, a pair of men slide it down the plank leading to the bank and drag it by its rope handles 20 yards up. Kids lie in wait with small nets, ready to snag any escapees.

The village, a mass of towering homes packed against one another along the lake’s northern rim, sits about 30 miles southeast of the major city of Siem Reap and the temples of Angkor. Each day, thousands of pounds of fish will pass through this one small market alone. Countless more will go through countless markets set along countless inlets across the Tonle Sap. And at this particular spot on this particular day in 2017, there is only one type of fish the buyers are interested in: trey riel, which flicker like silver coins when the nets are drawn up.

Troubling the Water: A Dying Lake and a Vanishing World in Cambodia by Abby Seiff

This article is adapted from Troubling the Water: A Dying Lake and a Vanishing World in Cambodia by Abby Seiff (Potomac Books, 162 pp., $21.95, March 2022).

The largest freshwater fish family in the world is Cyprinidae, among them carps and minnows, silvery and delicate. Trey riel—“money fish” —is the Khmer name for several of its species. Most are no larger than a thumb, though some can grow to the size of a forearm. Riel is the country’s currency, but think of this fish like a 100 riel note—about $0.02—something even the poor have plenty of. Trey riel is oily and astoundingly nutritious. It is used, most commonly, in the fermented fish paste prahok—a staple so omnipresent only rice is more commonly seen on a Cambodian table. It is not hyperbole to say that in all of Cambodia, no animal is as important as trey riel.

Past the trey riel buyers, farther up the bank, the prahok-makers are already at work. Two of the plastic baskets of fish have ended up here, under the shade of a house. This is how prahok is made: rinse the fish, chop off the head, gut them, and slake off the fingernail scales. Do this with a cleaver on a small wooden board, do this squatting, do this in several movements so swift they’re hard to catch. This is a small-scale operation, the women and their daughters hired at a few dollars a day. But the basics apply equally to the industrial processor or the home cook.

Leave the cleaned fish in fresh water overnight. Mix the fish with piles of salt, spread them out to dry for several days, pound them by hand or by machine, add spices, and transfer the mixture to a jar or a bucket, a ceramic basin, a giant metal tub. And then: wait. When it has fermented adequately—for 20 days or several months or more than a year—the prahok is ready to eat. Cambodians eat, on average, 140 pounds of fish each year (while 44 pounds is the global average). Don’t underestimate how large a portion of that comes from prahok—about half of the Tonle Sap catch is trey riel.

The women in Kampong Khleang are working late in the year; the large-scale prahok industry operates in a brief season, from December to February when the cyprinids surge out of the lake toward the Mekong. Along the Tonle Sap river, dais—massive 500-foot-long bag nets slung behind stationary boats and platforms—form a series of Vs leading up to the mouth of the lake. Around 150 years old, these seasonal dai fisheries were introduced to the Tonle Sap by Vietnamese fishers and formalized by the French. They have operated in more or less the same manner since their inception: Workers swing in the dais, spill their contents into waiting boats, and release the nets out once more. At the height of the season, six tons of trey riel can be collected in this manner each hour.

And even still, the dais are catching just a fraction. Of migrating fish, there is none more abundant than the cyprinids. When they surge, they are hunting for new habitats, for cool, dark places to survive as the lake shrinks inward, stealing their homes. Trey riel will travel a dozen miles each day, hundreds of miles across the Mekong Basin—to Laos, and even Thailand. They eddy up the Mekong into its tributaries, reaching fishers in every corner of the basin.

Only now, the route they have followed for eons is gradually being sealed off.


The Mekong River wends some 2,700 miles from its source at the mountain-ringed Tibetan Plateau to its mouth, where Vietnam’s delta spills into the South China Sea. Nearly half of the river lies in China, where it is called the Lancang. From there, the Mekong traces the Golden Triangle linking Myanmar, Thailand, and Laos before meandering along the border of Thailand and Laos, flowing into Cambodia, and arcing toward Vietnam. Along the way, this river feeds into numerous tributaries both large and small, spanning a 300,000-square-mile river basin that provides fish, fresh water, nutrients, and irrigation to millions of people.

The Lower Mekong Basin, which starts near the Laos-China border, is a broad mass of tributaries covering most of Laos and Cambodia and significant parts of Thailand and Vietnam. At its widest, the basin nearly reaches Bangkok in the west and the Gulf of Tonkin in the east. In all, three-quarters of the Mekong Basin lies south of China, and even more of its water does—about 85 percent of the volume that flows out of the river comes from the Lower Mekong Basin. While Southeast Asia’s lower basin is a bulging spread of rivers and wetlands, China’s upper basin is long and thin. The Lancang feeds few tributaries in China. Instead, it rushes through steep, narrow canyons—an ideal landscape for hydropower damming.

Since the 1990s, China has gone on a dam-building spree. Eleven dams are now operational on the mainstem of the Lancang, a 12th is under construction, and a 13th is planned. The dams impact fish migration and block many tons of silt from flowing downstream—necessary for moving soil nutrients, preventing erosion, and keeping saltwater from the delta. Perhaps most crucially, the Lancang dams have reservoirs capable of holding trillions of gallons of water—something that is changing the Lower Mekong Basin in staggering ways.

In recent years, the impact of those upstream dams has come into sharp focus. While the Lancang typically contributes just a small portion of downstream water flow, about 15 percent, the figure rises steeply during the dry season, and even more so in a drought—when as much as half of the lower Mekong’s water comes from China. Possessing full control of the taps, Beijing now holds outsized power. In 2019, while Laos, Thailand, Cambodia, and Vietnam suffered from the worst drought in a century, China held back an unprecedented amount of water, though it had plenty to spare.

The Lancang dams have an enormous impact but are hardly the only ones reshaping the Lower Mekong Basin. Laos has several mainstem dams operational, with plans for more, and Cambodia has explored its own. Across the lower Mekong’s tributaries, more than 100 dams are already up and running in Vietnam, Cambodia, Thailand, and Laos, and hundreds more have been proposed.

Coupled with the changing climate, these dams are causing water levels in the Mekong to plummet. As the Mekong dips, so do its tributaries. In July 2020, the level of the Tonle Sap river was 13 feet below average.

In some ways, it is a wonder the Mekong Basin survived this long. In 1957—with the sponsorship of the United Nations—Cambodia, Vietnam, Laos, and Thailand created the Mekong Committee. The body sought to survey the river and build the region’s first hydroelectric dams in an effort to kick-start economic development. As the foreign aid race among the United States, China, and the Soviet Union approached its apogee, America entered as a major backer—providing technical support along with nearly a fifth of the budget. The U.S. Army Corps of Engineers drafted a plan of action, and hydrologists began charting possible locations for Mekong dams. By the late 1980s, models projected, four megaprojects along with hundreds of smaller dams would be generating enough electricity to power the rapidly growing Mekong nations.

The river’s brief reprieve came at the cost of the region. Even as the American war raged in Vietnam, American bombs fell on Laos and Cambodia, and American troops and money poured into Thailand, the Mekong Committee valiantly pushed forward, drafting new plans and surveying what it could. But after the Khmer Rouge takeover in 1975, when Cambodia went dark, the large projects couldn’t legally proceed. In a joint declaration signed just months before Phnom Penh fell, the committee had flagged the Mekong mainstream as a “resource of common interest.” No dams could be built without unanimous consent.

Two decades later, the Mekong River Commission (MRC) was signed into being in 1995. By then the geopolitical and economic landscape of the region had shifted tremendously, and it took years of intense negotiations to create the committee’s modern iteration. Unlike its predecessor, the MRC is focused far more on management and conservation of the Mekong Basin and has no enforcement powers. Laos has already built two dams on the river’s main stem, with plans for seven more, despite vociferous opposition from its downstream neighbors. In 2020, Cambodia did announce a decadelong postponement on building any mainstem dams, but only at the urging of a Japanese consultant—not in response to the reams of MRC research showing the havoc the dams would wreak on Vietnam and Cambodia. Tributary dams, meanwhile, are still moving forward.


Activism comes naturally to An Socheat, a community leader who grew up in the protected wetlands of Prek Toal, located at the lake’s northern tip.

A row of long wooden homes, their roofs silver and brick-red, make up her riverside village. Some have been expanded out and out again, appendages of new rooms perched on yet more docking. Clothes dry on bamboo railings, potted plants sit beneath the windows; at one house, a length of tarp is rolled tight beneath the eaves—to be pulled down and lashed along the width of the entryway when the rains come.

Socheat points out the houses: “You can see that we have big houses here because in the past there was good fishing.”

Those days are over. There are fewer fish, smaller fish, and everyone has to travel farther for their meager catch. What Socheat watches, these days, is how life is being siphoned from the Tonle Sap.

When I speak to her in March 2017 she has recently returned from Thailand, where she met with fishers from across the Mekong. There, she learned every community that relies on this river is facing similar problems. The droughts and the shallow water, the unceasing illegal fishing, and the dams: All of the Mekong’s fisheries are being unmade in precisely the same manner.

Socheat came home and tried to impart this lesson: “Even though we are far from the Mekong, we are affected by its dams.”

She has been teaching her neighbors how to fight against hydropower, how to join their voices with those across the Mekong Basin. But she is also trying to get them funding to diversify their income: raising chickens, planting vegetables. She pushes for girls to go to school, tries to lessen the burdens that are contributing to a rise in domestic violence and alcoholism. She wants to offer training in vocational skills—tailoring, for instance.

“The poor families don’t have enough food and they face malnutrition. Even when they can catch big fish, they sell that fish to the middleman and keep only the small fish to eat,” she says.

Socheat begins describing what life was like as a child. So much has been lost, it’s hard to know where to start. “Back then, when the water level rose in rainy season, the water was fresh. There was the river, the flooded forests, the birds, the streams. We used to have more than 100 types of fish here.”

Now, there’s just the looming prospect of the Mekong being sealed off by dams, draining water from the Tonle Sap never to return it, leaving the lake without its pulse.

“My main concern is the dams. In the future, I’m not sure there will even be water here.”

Abby Seiff is a journalist and editor who was based in Southeast Asia for nearly a decade, writing for such publications as Time, Al Jazeera, Mekong Review, and Pacific Standard, among others. Her reporting has garnered several awards as well as fellowships from Yaddo and the Logan Nonfiction Program. She is the author of Troubling the Water: A Dying Lake and a Vanishing World in Cambodia. Twitter: @instupor

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