Pumee Boontom lives in northern Thailand, but he tunes his television to the Chinese weather forecast. A big storm in southern China means a big release of water from the Chinese dams upstream—and, in turn, a good chance his village will be flooded. The Chinese government is supposed to warn downstream countries. In Boontom’s experience, that warning tends to arrive too late or not at all.

“Before the dams, the water would go up and down gradually, with the seasons,” he says. “Now the water goes up and down drastically, and we don’t know when it’s going to change—unless we watch the storms.”

Boontom is the leader of Ban Pak Ing, a scattering of cinder block houses and unpaved streets that reach from the precipitous west bank of the Mekong toward a quiet, well-cared-for Buddhist temple. Twenty years ago, like many of his neighbors, Boontom caught fish for a living. But as China completed one, then two, and then seven dams upstream, the few hundred residents of Ban Pak Ing saw the Mekong change. The sudden fluctuations in water levels interfere with fish migration and spawning. Though the village has protected local spawning grounds, there are no longer enough fish to go around.

In recent years Boontom and many others here have sold their fishing boats and switched to farming corn, tobacco, and beans. It’s a chancy living, and not the one they know best—and it’s made even more challenging by the frequent flooding. In 2008 some homes were flooded to the second floor. The temple was inundated too.

Ban Pak Ing may be a vision of the future for many Mekong villages. Five more dams are under construction in China. Downstream, in Laos and Cambodia, 11 major dams—the first on the main stem of the lower Mekong—are either proposed or already being built. By disrupting fish migration and spawning, the new dams are expected to threaten the food supply of an estimated 60 million people—most of whom live in villages much like Ban Pak Ing. The electric power generated by the lower Mekong dams is destined largely for booming urban centers in Thailand and Vietnam. Kraisak Choonhavan, a Thai activist and former senator, calls the lower Mekong dams “a disaster of epic proportions.”

One of the proposed dams in Laos is just 40 miles downstream from Ban Pak Ing. Its construction would squeeze the village between flooding from the north and a rising reservoir in the south. Boontom, now in his 50s, says he’s worried not for himself but for the next generation. “Just close your eyes and imagine,” he says. “Imagine what will happen to us.” He slams his hands together.

The Mekong River (pictured above) has its headwaters in Dzado county in the Kham region of Tibet

The Mekong begins on the Tibetan Plateau and runs for more than 2,600 miles through China, Myanmar, Thailand, Laos, Cambodia, and Vietnam before emptying into the South China Sea. It’s the longest river in Southeast Asia, the seventh longest in Asia, and—most important for the people who live along it—the world’s most productive inland fishery. Cambodians and Laotians catch more freshwater fish per capita than anyone else on the planet; in many places along the river, fish is a synonym for food. Grilled, fried, or boiled; wrapped in palm leaves; garnished with ant eggs; or simply mixed with rice in a wooden bowl, the more than 500 known species of Mekong fish have sustained millions of people through droughts, deluges, and even the genocidal Cambodian regime of Pol Pot.

Yet the Mekong’s narrow gorges and roaring waterfalls, which frustrated 19th-century European explorers in search of a trade route from the South China Sea to western China, have long tempted dambuilders. In the 1960s the United States advocated the construction of a series of hydropower dams on the lower Mekong, hoping to develop the region’s economy and head off the rise of communism in Vietnam. The plans languished, the region descended into war, and in the 1990s China, not Southeast Asia, became the first to dam the main stem of the river.

Today Southeast Asia is relatively peaceful, and for the most part, its economies are humming. But only about a third of Cambodians and just over two-thirds of Laotians have access to electricity, and that power is often painfully expensive. Economic and population growth will further strain electricity supplies: A 2013 analysis by the International Energy Agency predicts that the region’s demand for power will increase by 80 percent in the next 20 years. Clearly the region needs more energy—and if the worst effects of global warming are to be avoided, the world needs that energy to produce as little carbon as possible. The hydropower potential of the Mekong is more tempting than ever.

Dam construction on the lower Mekong is overseen, nominally, by the Mekong River Commission (MRC). Funded by international development agencies and by its four member nations—Vietnam, Cambodia, Thailand, and Laos—the commission is held together not by a legally binding treaty but by a shared interest in the river and in regional peace.

China is not a full member of the commission; it has no explicit obligation to consult with its downstream neighbors about its activities on the upper Mekong. The consequences of this arrangement became all too clear in 1995, when member countries planned to celebrate the signing of a major agreement with a ceremonial boat trip on the Mekong: The river was filling the reservoir behind a newly constructed Chinese dam, and the water downstream was too shallow for boats. The trip had to be scrapped.

More recently the 11 main-stem dams proposed in Laos and Cambodia have undercut the fragile power of the commission. In 2010 an environmental assessment sponsored by the MRC called for a ten-year moratorium on construction of main-stem dams, citing their potentially devastating effects on regional food supplies and the likelihood of “irreversible environmental damage.” But Laos, a poor and long-isolated country that is now courting foreign investment, aims to become the “battery of Southeast Asia” by selling hydroelectricity to Thailand and other neighbors—and it was not deterred by opposition from the MRC or even from Vietnam, its traditional ally. In late 2012, after years of denials, Laotian officials admitted that construction of the Thai-financed Xayaburi dam, on a remote stretch of the Mekong in northern Laos, was under way.

The Xayaburi dam will be more than a hundred feet high and a half mile long when it’s completed, perhaps as soon as this year. When I visited the site in 2013, the riverbanks upstream were already punctuated with mines supplying sand and gravel for dam and road construction. At the site itself, cranes dangled over the river, and knots of workers in hard hats were using explosives to carve the steep banks into smooth terraces, ready to be filled with cement.

In a small village directly across the river, residents said they’d been enduring regular blasting for the past three years. They were preparing to move to a newly constructed village upstream, and some sounded optimistic. They were looking forward to new houses and to escaping the lengthening shadow of the dam. Many hoped to continue fishing.

Until 2012 another village lay immediately downstream of the dam site. In 2013 its residents were settling into a grid of new cinder-block-and-wood houses well out of the river canyon. There, optimism was scarce. Residents said the money and land promised by the dam company as compensation for relocating were inadequate and slow in coming. Many were feeling the unfamiliar bite of the cash economy. “In the old village you didn’t make much money, but you could eat the rice you grew,” said a young woman with two children. “Here you can make money every day, but every day you have to spend more than you make.”

Even as the Xayaburi dam upsets the lives of the people around it, its biggest impact may lie in the example it has set. By defying the recommendations of the MRC-sponsored assessment and building the Xayaburi, Laos has paved the way for the rest of the proposed cascade of dams—some of which pose much more fearsome threats to the Mekong.

The heart of the Mekong fishery is in Cambodia, where a large lake called the Tonle Sap is bound to the main stem of the Mekong like a lung to a windpipe. The Tonle Sap expands during the wet season and shrinks during the dry, and at its peak is so large that from the center it seems as vast as the ocean.

The muddy water and shifting currents of the Tonle Sap form a natural fish factory, nurturing finger-length silverfish, 650-pound catfish, and hundreds of species in between. The bounty supports a small nation of “floating villages,” clusters of houseboats anchored along the lake’s edge. Despite pollution and heavy fishing pressure, fish are still so numerous in the Tonle Sap that in the winter, when the water is low, fishermen and women can scoop up their catches in large bamboo baskets.

More than a hundred species of fish that hatch in the Tonle Sap migrate long distances upstream, some as far north as Laos. The Xayaburi dam, approximately 550 miles upriver, may be too distant to have much of a direct effect on them, but other projects are much closer. Just north of Cambodia’s border with Laos, another main-stem dam called the Don Sahong will soon be under construction. Though it will close off only one channel of the braided river, it will certainly interfere with fish migration, and it will further threaten the habitat of Irrawaddy dolphins, of which fewer than one hundred remain in the Mekong.

An even greater danger to the fishery looms in northern Cambodia itself, on a tributary of the Mekong called the Tonle San, or Sesan River. The Sesan originates in Vietnam and meets the Mekong about 30 miles downstream from the Don Sahong. It’s known to be a key migration route for dozens of fish species, including many that local people depend on. A dam called the Lower Sesan 2, which would sever the Sesan’s connection with the Mekong, is now being built 16 miles east of the confluence.

The village of Vern Houy lies just upstream from that dam site. It’s accessible only by boat, and most of its residents grew up there. Many speak Lao as a first language. When I asked a group of women what the dam will mean to them, they said: “We’re going to die.” I asked my translator, a young reporter from the national capital of Phnom Penh, if their words were meant literally. “It’s a real fear,” he said. “They really think they’re going to die.” This is the only life they know; they cannot imagine another. The reservoir will flood the village so frequently as to make it all but uninhabitable.

In the house of the village leader, a single room built on stilts and enclosed with grass mats, a group of men gathered for a meal of freshly killed duck and hot sauce. The deputy chief of the village, In Pong, said that with the help of a regional advocacy group, the 3S Rivers Protection Network, residents of Vern Houy had joined with nearby villages to protest the dam, writing letters to the Cambodian Parliament and traveling to the capital to press their case—fruitlessly, so far. “I wouldn’t move anywhere, especially not to the city,” Pong said. “I have no idea what to do.”

Loek Soleang, a local teacher and one of the few residents not born in the village, countered Pong. “I’m not worried,” he said. “We can use the electricity. We need development. If they flood here, we’ll just move to higher land.”

The men around him did not argue; they stared quietly at their laps. Pong pulled on his cigarette and puffed the smoke contemplatively out the open window.

Loek Soleang was right about one thing: People in the Mekong Basin need more electricity. Vern Houy has none at all. In the village of O Svay, downstream from the Don Sahong dam site, there are dirty, noisy diesel generators. Only the most prosperous families can afford them. In O Svay, as in much of rural Southeast Asia, only the most fortunate kids can finish their homework by lamplight or use a small fan to help them sleep in the sweltering summer heat. Life without reliable power isn’t easy or romantic.

Both Cambodian and Laotian officials say that dams can benefit their countries’ poor people by making electricity cheaper and more available. While Cambodia opposes the main-stem dams upstream in Laos, officials praise the Lower Sesan 2 and other tributary dam projects. “The livelihoods possible with energy are better than those possible with the fishery,” says Touch Seang Tana, chairman of a Cambodian commission for Mekong dolphin conservation and economic development. “Dams are a way for people to move beyond subsistence.”

The dams planned for Cambodia and Laos would produce power far in excess of the countries’ domestic demand, but they would not make electricity universally available in those two countries. Ninety percent of the power the main-stem dams produce would be sold to Thailand and Vietnam, and most of the cash they brought in would go to the companies that built them, not the poor people who live along the river. The 2010 analysis commissioned by the MRC predicted that the fishery losses caused by the projects would actually worsen poverty.

Some officials argue that aquaculture and rice cultivation can make up for any decline in the food supply, but fisheries experts vehemently disagree. The effects of the main-stem dams on the Mekong fishery, they say, would be cumulative, inexorable, and devastating. On other rivers in the region, fish catches dropped between 30 and 90 percent after dams were built. And though aquaculture is already widely practiced along the Mekong—in many houseboats a trapdoor leads to a watery pen of farmed fish—those fish are fed with smaller wild fish from the river. Replacing those feed fish with factory food would be prohibitively expensive for most producers. Like the Laotian villagers forced to move by the Xayaburi dam, many who depend on fishing and small-scale aquaculture are likely to be shoved into the cash economy without the capital or knowledge they need to survive in it.

The Mekong is not the region’s sole source of low-carbon power. The 11 dams proposed for the main stem of the lower Mekong are projected to meet roughly 6 to 8 percent of Southeast Asia’s electricity demands by 2025, and analyses show that efficiency measures and investments in solar and other cleaner-energy technologies such as cogeneration—the use of waste heat from power plants—could yield as much or more power at less cost. But in Southeast Asia, such alternatives are in their infancy. To the Cambodian and Laotian governments, hydropower is both more familiar and accessible, and more valuable as an export commodity.

Is it possible to harness the Mekong’s power while protecting its abundance? A 2012 study by Princeton ecologist Guy Ziv and his colleagues analyzed 27 dams proposed for the river’s tributaries, comparing the projected power from each with its likely damage to fisheries. They found vast differences in the ecological costs of the projects. The Lower Sesan 2 was by far the worst; it alone would reduce fish biomass in the lower basin by more than 9 percent. Conversely, a few dams carefully placed elsewhere in the watershed could produce significant power with minimal damage to food supplies.

Such planning, however, would require the Mekong nations and their investors to coordinate with one another, and coordination is exactly what’s lacking in the haphazard, secretive push to dam the Mekong Basin. “To really do water development well, you have to work at the basin scale,” says Brian Richter, a water expert with the Nature Conservancy. “You have to in some sense look at the Mekong as a game board, one where you can decide to put a dam here and not there and by doing so maintain the ecological functioning of the whole river basin. That’s been extremely difficult to do on the Mekong.”

More than a thousand miles downstream from the Chinese dams, the Mekong Delta’s seemingly endless network of marshes, canals, and polders—tracts of reclaimed land—stretches to the South China Sea. The delta has long been a literal and metaphorical quagmire, especially for the Vietnamese, French, and American forces who spent decades fighting and dying there.

Near its center, in the market town of Can Tho, wetlands ecologist Nguyen Huu Thien stands on the waterfront and gestures at the phalanxes of passing motorcycles, most ridden by young Vietnamese. “How many of them know about the dams?” he asks. “Very, very few of them have any idea what’s going to happen.”

Nguyen grew up in the delta in the 1970s, and like many other kids, he routinely swam in the canals and flooded fields, catching fish with his hands. Unlike his older siblings, whose schooling was constantly interrupted by war, Nguyen was able to attend college and, eventually, to study conservation biology at the University of Wisconsin. Today he speaks colloquial English and professes a soft spot for Mark Twain and the education Twain got on the Mississippi River. “I learned theory in Wisconsin, but the Mekong Delta is unique,” Nguyen says. “I had to learn about it here, in the middle of it.”

The mix of salt water and freshwater in the delta, and the centuries of human efforts to direct it, have resulted in a complex engineered landscape, one that is too often treated as separate from the rest of the Mekong. In 2009 Nguyen was working on wetlands restoration when he was asked to contribute to the MRC’s assessment of the proposed main-stem dams in Laos and Cambodia. He soon realized the dams would doom all his earnest efforts in the delta.

Its balance of river and sea is already shifting. Recent droughts have weakened the river and allowed seawater to intrude farther upstream, causing serious problems for farmers. The upstream dams would convert more than half the lower Mekong into reservoirs, completely altering its flow. They’d trap much of the nutrient-rich sediment that now fertilizes delta fields and feeds fish throughout the Mekong system—which extends beyond the river itself. The boats that fish its enormously productive plume in the South China Sea can catch more than half a million tons a year.

In the delta Nguyen sees the limits of human ingenuity: Though its canals and polders have boosted rice production, they’re ultimately no match for the sea. Likewise, he says, engineering could never fix the damage done by the dams. “As the climate changes, whatever God makes is going to be more resilient than what we make,” he says, as we ply the canals around Can Tho. “The natural system is always more resilient.”

Nguyen is working on other assessments of the dams, but he doesn’t expect them to have any more effect than their predecessors. Sometimes he talks about the dams to his older brothers, all of whom have returned to the family land to farm. They just shrug their shoulders. “Nothing we can do about it,” they say.

These days Nguyen feels much the same way. “We just have to wait and see,” he says. “We have to wait and see what the future is like.”

On a chilly evening in late January 2013, a group of several dozen local activists gathered near the edge of the Mekong in Ban Huay Luek, a village in northern Thailand. Many were bundled in blankets near impromptu campfires. They’d just finished a 77-mile walk along the river, a protest aimed at drawing public attention to the proposed dams downstream. Led by a cadre of Buddhist monks and joined by a rotating cast of farmers, local politicians, and foreign backpackers, many of these marchers had spent nearly two weeks on the road, camping in the courtyards of schools and temples. “We’ve done everything we can imagine,” said organizer and high school teacher Somkiat Khuenchiangsa. “We’ve researched these dams, we’ve sent letters, we’ve walked, we’ve protested again and again.”

That night as the marchers rested their blistered feet, they listened to speeches from visiting members of parliament. They quieted as Kraisak Choonhavan, the progressive activist, took the makeshift stage. Thailand, unlike neighboring countries, has a tradition of grassroots organizing and popular protest. Speaking through a scratchy loudspeaker, Choonhavan reminded the audience that years earlier, when the Chinese government was blowing up river rapids in order to clear a section of the Mekong for boat traffic, protesters in northern Thailand had kept them from finishing the job. Some of the veterans of that fight were in the audience. “Without you, they would have blown up everything,” Choonhavan said. “So now you have to stand on your feet and use that power again.”

The words weren’t hollow: Thailand does indeed have influence over the main-stem dams. Thai utilities are the intended market for much of the electricity the dams would produce, and such deals require the approval of the Thai government. Public opposition could persuade it to call for redesigns or even cancellations of dam projects. After the march, a group of 37 villagers, including some of the marchers, pressed forward with a lawsuit against the government. Last summer a national court agreed to hear the case. It’s probably too late, though, to stop the Xayaburi dam. Within the next few months, it is expected to reach from bank to bank, closing off the main stem of the lower Mekong for the first time.

By Michelle Nijhuis