Engineers move in on the Mekong

New Scientist 13 July 1991

By Larry Lohmann 

Three decades of war in Southeast Asia have stopped exploitation of the Mekong river. Now the waterway may be plugged by a series of large hydroelectric dams. Will dams secure the peace or exacerbate local tensions?

The lands through which the Mekong river flows on its 4200-kilometre journey through Southeast Asia to the South China Sea are bywords for isolation and conflict. Tibet and Burma have been all but cut off from the outside world for decades, while Vietnam, Cambodia and Laos have been engulfed by warfare. Only Thailand, itself now under military rule, is regarded by the West as firmly on the path of economic development. Before the shadow of war fell over the region in the 1960s, American engineers had great plans to develop the region by harnessing the single common thread that ran through the Southeast Asian mainland, the Mekong river. With one of the largest flows of any river on Earth, the Mekong appeared a bounteous natural resource, ripe for exploitation. Now, with peace restored, the Americans, along with the Australians, Japanese and others are back to build dams to tap the river's power.

Over much of the Mekong watershed, hardly any rain falls during the early months of the year. The river level drops allowing salt water from the South China Sea to penetrate 500 kilometres inland; as far as the middle of Cambodia. Once the monsoon rains begin in May, however, the Mekong is transformed. By September, the flow of water into its delta in Vietnam has increased around 30 times. Along some stretches, the river rises as much as 20 metres, or about the equivalent of six storeys of a building. There is more water in the river than can reach the sea and huge amounts back up into a sluggish wetland, Tonle Sap, in northwest Cambodia, turning the region into a lake covering 10 000 square kilometres, the size of Yorkshire. Tributaries also reverse their flows as the broad, swampy flood plains are filled. In the Vietnamese delta region alone, more than 12 000 square kilometres of forests and paddy fields are flooded each year.

The variability of the river can make living on its flood plain a risky proposition for the 50 million residents who depend on the Mekong basin for food. Yet, as a French colonial administrator observed in the early 1900s, 'the rhythmical movement of the waters, regular, like breathing, furnishes much of what is needed to improve the land. High waters deposit on the soil a layer of rich silt and spare the farmer the drudgery of transporting fertiliser to the fields, and when the water recedes he finds the ground all ready to support rich and remunerative crops.' For peasant farmers, this still holds true. Cambodia's low-lying farmlands receive a coating of up to 10 millimetres of silt in a normal flood year, and up to 30 millimetres during peak floods. In neighbouring Vietnam, the Mekong delta, much of which floods each year, contains 34 per cent of the country's farmland and provides 40 per cent of its agricultural output. Large dams upstream could upset the delicate hydrological balances on which this productivity depends.

Flood cycles are also crucial in maintaining fish stocks. V. R. Pantulu is a retired fisheries expert with the Secretariat of the Committee for Coordination of Investigations of the Lower Mekong Basin (Mekong Committee), an international organisation set up in 1957 by the United Nations to manage development of the Mekong basin. Pantulu estimates that 90 per cent of the fish in the Mekong basin spawn not in the rivers themselves, but in the surrounding lakes, submerged fields and flooded forests that fill up during the wet season. Between June and October, fish swim into the 30 or 40 kilometre-wide band of flooded forest and shrubland around the edge of the Tonle Sap, where they can avoid predators and feast on decaying vegetation in the shallow water. As the water recedes, fattened fish, including three species of carp and five of river catfish, return to the lake and into thousands of waiting fishing nets.

Tonle Sap and other freshwater fisheries in Cambodia and Vietnam yield more than 200 000 tonnes of fish per year; the whole Mekong basin yields nearly half a million tonnes. In coming years, however, the Mekong's fertile cycles are likely to be considerably altered by the building of dams to tap the river's hydropower and by deforestation, either for the timber or to introduce new farming methods. The Mekong Committee estimates that forest cover in the basin declined from 50 per cent in 1970 to 27 per cent in 1985. The rate of forest-cutting continues to escalate in Laos and Cambodia.

Commercial logging and modern agriculture are exposing upland soils to sun, wind and rain, resulting in soil compaction, erosion and loss of natural fertility. Soil compaction in particular can alter seasonal flows in upland streams. Compacted soils are less able to store water from monsoon storms, which means that streams flood more often in the wet season and are empty for longer in the dry season because there is no moisture left in the hill soils to feed them. Eroded soil can also clog up streams with silt. Siltation attributed partly to local deforestation has already made Tonle Sap shallower, warmer and less hospitable to fish, reducing yields. In the Thailand Natural Resources Profile, published in 1987, the Thailand Development Research Institute reports that a quarter of Thailand's farmland is now subject to severe or very severe soil erosion, and many small irrigation systems have become choked with silt. On the left bank of the river in Laos and Vietnam, the mountainous upper watersheds of many Mekong tributaries retain their tree cover. However, if these areas are opened up to extensive logging and new methods of farming, the results are likely to be severe.

The biggest development project pending in the Mekong basin is a series of hydroelectric dams that would change the face of the river for most of its length. On the books since 1957, the scheme has been held up by three decades of war. It originated when UN planners, enthusiastic about the hydropower potential of the basin, helped to form the Mekong Committee, with members from Laos, Thailand, Cambodia and Vietnam. Raymond Wheeler, chief engineer at the US Army Corps of Engineers, which has built many of the US's largest dams, arrived to determine research priorities. And Western construction companies began studying potential dam sites. The completed plan emerged in the early 1960s. It called for a total of 100 dams to be built on the mainstream of the Mekong and its tributaries. One dam, the High Pa Mong, which would straddle the Mekong near Vientiane, where it forms the boundary between Laos and Thailand would, if built today, displace a quarter of a million people. Another proposed dam, the 28-kilometre long Stung Treng in Cambodia, would have generated 7200 megawatts of electricity, many times the then energy requirements of all four countries in the lower Mekong region.

It is hardly surprising that such a costly scheme made little progress during or after the Vietnam War, which hotted up in the mid-1960s. By 1970 only a half-dozen small projects on minor tributaries of the Mekong had been built. All of them, including the 150-megawatt Nam Ngum Dam in Laos, serve Thailand, which escaped most of the fighting. Since 1970, although the committee's planners have kept busy with continued support from the UN and over a dozen industrialised countries, they have not completed any large projects.

Now, several big schemes are in the offing, including the first dams to be built across the mainstream of the Mekong. The Mekong Committee is currently recommending that $5 billion be spent in the next few years, mostly on hydropower, with much more to follow. Chuck Lankester, a senior official from the UN Development Programme who has recently taken charge of the secretariat, says that the dams 'are an idea whose time has come'.

As the governments of Southeast Asia turn from warfare to economic development, the region's little-exploited resources, including its great river, will become increasingly attractive. As more roads, industry and commercial agriculture are introduced, hydroelectric dams will power the economic transformation. Already, hydropower is feeding industrialisation in Thailand, whose economy is the fastest-growing in the world. Agencies such as the World Bank and the UN Development Programme are contemplating loans and funding studies for the construction of dams. Ray Oram, the Mekong Committee's information officer, identifies Sweden, Australia and Japan as the countries with the greatest potential role in supporting hydropower in the region.

Thailand's hunger for electricity is a prime reason for undertaking the scheme. National demand for electricity has risen 70-fold in the past 30 years and is still rising at a phenomenal 10 to 15 per cent per year. Feasible sites for hydropower within its own borders are being used up fast. Existing dams, moreover, are often so short of water (possibly because of deforestation) that it is difficult for them to satisfy the needs of irrigation and power. What sites do remain tend to be in forests or other areas that conservation-minded Thais would like preserved.

Many Thai farmers also oppose dams. In recent years, there have been local uprisings against planned dams on at least four Thai rivers by villagers concerned about losing their land, and about forest destruction, water pollution and the risk of catastrophic floods triggered by earthquakes that could crack the dams. A start on the Mekong Committee's own $120 million Pak Mool Dam, designed by the French firm Sogreah for the River Mool, a major tributary of the Mekong in northeast Thailand, has been delayed several times by resistance from villagers whose land it would flood. In June 1990, villagers from around the country gathered in Bangkok to call for a nationwide moratorium on building dams. In March this year, 12 000 villagers signed a petition opposing the Pak Mool Dam and hundreds of them occupied the site in protest when construction work began in May. Late last year, the World Bank's local chief, Philippe Annez, said that dams in Thailand have become 'too political' for the bank to touch. But the bank now seems to have changed its tune and is considering lending money for the project.

Through plans for a coordinated development of the Mekong's hydroelectric potential, the Thai government is trying to place Thai dams across the border in Laos andother countries where population is sparse and hydropower potential high. Complaining that 'you can't build dams in this country any more', Subin Pinkhayan, the former Thai Foreign Minister, last year likened proposed dams inside Laos to 'sleeping beauties' awaiting the touch of their Thai 'Prince Charmings'.

The Laos government, while wary of Thailand's intentions, finds it difficult to turn down the opportunity to trade in hydropower. Selling electricity abroad is one of the few ways that the country can earn foreign exchange. Power and timber already account for over 60 per cent of Laotian exports.

What would be the social and environmental effects of the new dams in the Mekong basin? A look at two of the best-studied projects suggests some disquieting answers. The $3 billion Low Pa Mong Dam (a more modest replacement for the 1960s High Pa Mong scheme on the border between Laos and Thailand) could, at peak flow, deliver 2250 megawatts to Thailand. Combined with other proposed mainstream dams such as Upper Chiang Khan, High Luang Prabang and Sambor, it could also store runoff after the rains for release during the dry season. That would both reduce flooding during the wet season and check the rush of seawater into the delta during the dry season, which can poison crops with salt. Engineers calculate that the lowest flows in the delta during the dry season, at about 2000 cubic metres per second, could be augmented by at least 10 per cent with water from the Pa Mong reservoir, and tripled if the chain of eight planned dams on the mainstream were built.

However, according to William van Liere, former director of the agricultural division of the Mekong Committee, all the dams would need to keep their reservoirs at high levels in order to generate steady amounts of electricity. In effect, this gives urban demands for electricity priority over those of farmers who want greater releases of water and reservoir levels kept low. During the dry season, farmers need the water to serve their crops; during the wet season, they want the reservoirs to be able to absorb incoming flood waters.

Even if Low Pa Mong and other mainstream dams were given over entirely to moderating the Mekong's flow, it could take generations for Vietnamese farmers to adjust their current delicately balanced farming systems to the new quantities of fresh water. As Mekong Committee documents suggest, current crops are 'well adapted to the present pattern of salinity'. Regulating the flow by mainstream dams would also affect coastal ecosystems such as mangrove forests and, by reducing the supply of nutrients that reaches the sea from the river's mouth, even damage marine fisheries.

The Low Pa Mong is also likely to disrupt the spawning and migration habits of the wide variety of species that roam the river. These include the indigenous catfish, Pangasianodon gigas. At its adult length of two metres and weight of up to 300 kilograms, this is the biggest freshwater fish in the world. Tyson Roberts, a specialist on freshwater fish from the University of California, Berkeley, says that the Mekong has the 'richest riverine fauna in all of Asia', with more species than either the Yangtze or the Ganges. He warns that continued dam construction will have a serious effect on fishlife.

According to John Dennis, who has studied natural resource developments for the UN Development Programme, Low Pa Mong would remove from the flood zone about 1840 square kilometres of land around Tonle Sap and a further 2000 square kilometres along the Mekong and Bassac rivers. And, by eliminating peak floods and reducing variation in river levels, it would reduce silt deposition in Cambodia by 90 per cent, which would threaten the fertility of farmland.

Downstream of Low Pa Mong, erosion of the banks of the Mekong would increase because of the scouring effect of water released from behind the dam. The secretariat estimates it would cost $80 million to protect the banks.

A more immediate concern is resettlement. Although plans for the High Pa Mong Dam, which would have displaced 250 000 people, were scrapped in the mid-1980s, the more modest substitute, with a reservoir level rising to 210 metres above sea level instead of 250 metres, will still require the evacuation of more than 42 000 people, three-quarters of them from Thailand.

In Laos the Nam Theun 2 dam, which is designed to generate electricity for export, is expected to cause deforestation. The site is the unspoiled upper reaches of the Theun tributary in the middle of more than 3000 square kilometres of Laos's evergreen forest, an area that harbours an abundance of rare wildlife, including elephants, big cats, such as tigers, leopards and clouded leopards, wild cattle and primates. So far, this remote wilderness has been subject to little human interference, aside from some logging near the area's single road, Highway 8 to Vietnam, and scattered rice farming by a few thousand inhabitants.

Construction of the dam, which could begin in a few years with aid from the World Bank, the Asian Development Bank and the German and Japanese governments, would bring devastating changes to the entire area. Current plans developed by Australia's Snowy Mountains Engineering Corporation call for a reservoir 300 square kilometres or larger feeding a 300 to 600 megawatt power plant. The estimated cost is $500 million.

Ultimately the project could be expanded up to 1200 megawatts. Nearly 4000 people would have to be evacuated to make way for a reservoir of such dimensions.

Water from the dam's reservoir would overwhelm the stream in the neighbouring Kathang river system to which it is scheduled to be diverted. Flow in the Theun river itself would cease entirely for up to 10 months of the year. For the rest of the time, the water would be undrinkable even by animals, according to a study in 1987 by the Motor Columbus engineering firm of Switzerland on behalf of the World Bank. Salt deposits in the valley that are now being exploited by local villagers would be covered by the reservoir and would be likely to find their way into local watersheds, further damaging water quality, says Prinya Nutalai, a geologist at the Asian Institute of Technology. Malaria-carrying mosquitoes would find an ideal breeding ground in the vast mud flats created between high and low-water marks of the shallow reservoir. The risk of liver fluke, lung fluke, schistosomiasis and filiariasis would also increase.

No less damaging would be the effects of dam construction itself. Quarries would be dug, timber taken out, transmission lines laid and over 200 kilometres of road built or upgraded. Forests and streams would be obliterated and new settlers, loggers and wildlife poachers attracted to the edge of the reservoir. The Motor Columbus study concludes that the area's 'undisturbed forests of great variety' would be 'irrevocably affected'.

Because they use no fossil fuels, large-scale hydropower plants are often put forward as the correct response to global warming and the need for 'sustainable' sources of electricity. The case of the Mekong dams suggests, however, that the hydropower 'solution' will create more problems than it solves. Growing environmental degradation and uncertainties over the effects of global warming are only the latest reasons to slow the momentum of these giant projects.

Larry Lohmann is associate editor of The Ecologist magazine. In the late 1980s, he worked in Thailand for the Project for Ecological Recovery, a local environmental pressure group.

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The high risks of knowing too little

Waiting in the wings to ambush any attempts to develop the Mekong basin is the greenhouse effect. Some recent climate models predict heavier Asian monsoon rains in a warmer world.

If this is correct, it would increase runoff during the wet season, and, ultimately, swell the flow of the Mekong. That would flush the delta with larger amounts of fresh water and silt during the wet season, forcing farmers to plant lower-yielding, deep-water rice in fields now reserved for more productive varieties.

At the same time, rising sea levels would push salt water further up the river in the dry season. More of the Mekong's delta would be exposed to salt water. Because rice has a low tolerance to salinity, this influx of salt water would also reduce crop yields.

Even without the effects of global warming, the presence of salt water is one factor keeping rice from being grown closer than 40 to 50 kilometres to the coast in the delta.

However, it may be dangerous to base development programmes on any single analysis of the effects of global warming. 'It will be many years before we can have confidence in model forecasts,' warns Mick Kelly, atmospheric scientist with the Climatic Research Unit at the University of East Anglia. Kelly has been studying the implications of climate change for development planning in the region.

The message for planners, Kelly says, is that while climatic changes are 'near-inevitable' in the region, not enough is known about them to base development projects on model projections. Instead, he suggests, planners should 'try to ensure that long-term projects are climate-proof. It would be undesirable to add further stress or change to the system through human action'.


From issue 1777 of New Scientist magazine, 13 July 1991, page 44