The Mekong between Thailand and Laos: a border drawn on a shared river

For hundreds of kilometres, the Mekong separates Thailand from Laos. Yet this line of water is no immemorial natural barrier. It was born of a colonial show of force in 1893, and it cuts in two a Lao-speaking world whose inhabitants share, from one bank to the other, the same fish, the same seasons and the same beliefs. The river feeds both banks, it carries the imagery of the Naga, and today it is under pressure from dams. To understand this border is to follow how a political boundary was laid over a living environment, and why that environment refuses to be divided.

One river, two states, a single world

From the quaysides of Nong Khai or the riverbanks of Chiang Khan, Laos lies within sight. Between the two banks flow the silt-laden waters of the Mekong, the Mae Nam Khong of the Thais. The term mae nam, literally ‘mother of waters’, is the everyday word for a river. An AFP dispatch put its width at around one kilometre in Loei province. That is enough to mark a separation, but too little to prevent glances, exchanges and crossings.

The Mekong ranks among the great rivers of Asia. Radio Free Asia credits it with 4,350 kilometres across six countries. Published lengths vary, however, according to the source and the method of measurement: the order of magnitude matters more than the exact figure. The same outlet estimates that more than 60 million people depend on the river and its tributaries for food, transport and water.

Between Thailand and Laos, the Mekong forms a large part of the shared border. An audio guide in Vientiane suggests it accounts for more than half of it, out of a total length of a little over 1,800 kilometres. These figures come from a tourist source and await official confirmation. More than the measurement, it is the nature of this boundary that commands attention.

On either side of the water live populations who share much of their language and culture. In Isan, Thailand’s north-east, the everyday language is very close to the Lao spoken across the river. The distinction that took hold between ‘Lao’ on one side and ‘Isan’ on the other is often traced back to the partition of 1893: a single linguistic space found itself divided between two states.

Understanding the Mekong as a border therefore means holding three realities together: a recent political line, a natural environment that feeds both banks, and a shared religious imagination. All three are being put to the test today.

1893: when the river became a line

An axis rather than a boundary

Before it became a border, the Mekong ran through a political space. The weakened Lao kingdoms had come under Siamese suzerainty in the late 18th century. The date of 1779 is often cited, but it has yet to be confirmed by an academic source. In the early 1890s, Bangkok still administered towns on the eastern bank of the river. The Mekong was not a dividing line: it served as the axis of a realm that stretched across both its banks.

The Franco-Siamese crisis

Everything changed in 1893. On 13 July, French gunboats forced their way into the mouth of the Chao Phraya, Bangkok’s river, under fire from Siamese batteries. This was the Paknam incident. In August the ships left the capital, but France occupied Chanthaburi, on the eastern coast of the Gulf of Thailand, to ensure that Siam honoured its commitments.

The Franco-Siamese treaty of 3 October 1893 granted France the left bank of the Mekong, that is to say its eastern bank. It also created a 25-kilometre demilitarised zone, on the Siamese side alone. British parliamentarians, debating the affair in the House of Commons on 14 November that year, referred to a strip of ‘16 miles’ on the right bank, a comparable order of magnitude. According to the study published by New Mandala, Siam was given just one month to withdraw its forces from the towns it administered on the far bank.

An article in the Journal of the Siam Society highlights an often-overlooked point. The diplomatic crisis is well known, but its land-based military dimension far less so. The history of this border thus remains partly unwritten.

The reasons for the line

Why the river? The study Borders in motion on the Upper Mekong identifies three French interests:

• opening the markets and resources of the left bank to French entrepreneurs;
• curbing British expansion, already established in the Shan States further west;
• opening a river route to China.

The study sees in this the birth of a new geography, at once military, economic and political. The river did not become a border because it already separated two peoples. It became one because a colonial power found in it a line that was legible, defensible and useful to its commercial ambitions.

A line still to be defined

Assigning a bank does not settle everything. It remains to be decided who owns the riverbed, the main channel and the islands. According to the material gathered for this article, treaties of 1904 and 1907, followed by a convention in 1926, are said to have completed the arrangement. Their exact provisions on these questions could not be verified, nor could the current state of demarcation. This uncertainty illustrates a fundamental difficulty: a river border is a legal construct laid over something that moves.

The river that feeds

Preserving a season’s abundance

For those who live along its banks, the Mekong is first and foremost a source of food. Fishing follows the rhythm of the seasons, with marked peaks in the catch. A technical paper from the Mekong River Commission (MRC), the intergovernmental body for the lower basin, shows that these seasonal peaks gave rise to a genuine processing industry: dried and fermented fish, fish sauce and fish paste. The logic is simple: preserve one season’s abundance for the months of scarcity. It remains at the heart of the cuisines of both banks.

The orders of magnitude are considerable:

• around 2002, the MRC estimated the wild catch of the lower Mekong at about 1.5 million tonnes, plus some 240,000 tonnes from reservoirs, worth around $1.2 billion;
• for 2009, a regional data platform cites 2.2 million tonnes, with a retail value of $4.3 to $7.8 billion. It also reports an estimate that fish supply around 75% of the protein consumed by riverside communities;
• in 2015, the MRC’s magazine, Catch and Culture, valued the fisheries of the lower Mekong at around $17 billion a year.

A river of migratory fish

One fact sheds light on everything else. According to a scientific review published in 2024, drawing on figures released by the MRC in 2023, between 40 and 70% of the lower Mekong catch depends on migratory species. These fish travel up or down the river to breed and feed. Their life cycle pays no heed to the border: a fish caught at Chiang Khan may have been born upstream in Laos and grown up in shared waters.

The same review reports that catches exceeded 2.3 million tonnes in 2010, worth up to $11.5 billion. By 2020 they had fallen to between 1.51 and 1.71 million tonnes, worth $7.1 to $8.4 billion. The authors place this trend within an analysis of the threats facing fish migration. The comparison calls for the methodological caution noted above, but the trend is worrying.

The pla buek, a giant on borrowed time

No species embodies this shared river better than the Mekong giant catfish (Pangasianodon gigas), the pla buek of the Thais. Its traditional fishing ground lies at Chiang Khong, in Thailand, opposite Houay Xai, in Laos. Fishermen from both banks exploited the same resource there and supplied a cross-border market.

According to Zeb Hogan and his co-authors, in an article in American Scientist published in 2004, the species began to disappear in the 1930s and 1940s. It vanished first from the border stretch, then from sections further upstream in northern Laos. At Chiang Khong, the records compiled by Naruepon Sukumasavin for SEAFDEC, the regional fisheries body for South-East Asia, speak for themselves: 69 fish caught in 1990, 7 in 1997, 20 in 1999, then none between 2001 and 2003. The author puts forward a possible cause, the blasting of rapids in Chiang Saen district, upstream. This is a hypothesis, not a proven cause.

The species has been listed in Appendix I of CITES, the convention governing international trade in endangered species, since 1975. In 2003, the International Union for Conservation of Nature classified it as ‘critically endangered’ on its Red List. The decline of the pla buek shows that the river’s fish depend on decisions taken on either bank.

The sacred river: the Naga and its fireballs

A belief rooted in the Buddhist calendar

The Mekong is not merely a resource. It is also inhabited. According to local belief, the phaya nak, a giant serpent of supernatural nature, dwells in its depths. Each year, on the night of Ok Phansa, which marks the end of the monks’ three-month retreat during the rainy season, reddish lights are said to rise from the river. These are the bang fai phaya nak, the ‘Naga fireballs’.

The legend recounted by the sociologist Erik Cohen explains their meaning. According to this tradition, the Nagas launch these fireballs to celebrate the Buddha’s return to earth after three months spent in heaven with his mother. The event is dated to the full-moon night of the 11th lunar month. This is a traditional account, not an established fact: it expresses what the river represents for those who live along it.

The phenomenon is celebrated above all in Nong Khai province, where a festival draws a considerable crowd every year. Its dates follow the lunar calendar. For 2025, the Thailand NOW website announced it for 4 to 8 October. One claim, circulated without a verifiable source, even holds that the current name dates back only a few decades, whereas the luminous phenomenon itself is said to be older. It remains to be documented.

A revealing controversy

In 2002, the Thai channel iTV broadcast a documentary showing Lao soldiers firing tracer rounds from the opposite bank. The same year saw the release of Mekong Full Moon Party, by Jira Malikul. The film features a journalist determined to prove the hoax. Erik Cohen links the influx of visitors that followed to its success. According to secondary accounts whose original source has yet to be traced, the Thai biologist Jessada Denduangboripant is also said to have concluded that the lights were flares fired from the opposite bank.

The Lao authorities responded to these suspicions. In 2021, an investigation carried out during a Covid-19 curfew judged it ‘extremely unlikely’ that anyone could have fired without their knowledge. Explanations involving natural gases have also been proposed. To date, no single explanation accounts for all the observations.

The controversy is instructive in itself. Suspicion falls on the other bank, and the debate implicitly pits two states against each other. The border thus intrudes into a religious narrative that is nonetheless common to both banks. Erik Cohen has spoken of the ‘postmodernisation’ of this mythical event. The title of his study suggests that, for part of the public, the question of authenticity matters less than the experience shared at the water’s edge.

The river under pressure: dams and low water

Xayaburi, a turning point

At the end of October 2019, the Xayaburi dam in Laos came into operation after construction begun in 2012. It is the first large Lao dam on the main stem of the Mekong, and the first of five planned schemes. It generates 1,285 megawatts. According to Radio Free Asia, around 95% of this electricity is destined for Thailand. AFP describes the dam as owned by Thai interests and puts its cost at $4.47 billion.

The paradox is striking. Thailand finances and consumes electricity produced on the Lao bank, while its own riverside communities fear the consequences. On the eve of its commissioning, the network of residents from the eight Thai provinces bordering the Mekong demonstrated in Loei province.

October 2019: the river laid bare

That same month, the river fell to historically low levels. In Loei, where it is usually about one kilometre wide, it shrank to a few dozen metres. AFP attributed the drop to drought and to upstream dams in China and Laos.

The causal link nonetheless remains disputed. In January 2020, a Thai official quoted by Radio Free Asia considered it ‘too early’ to blame the dams for the drought or the sediment deficit. Two readings therefore coexist. One emphasises the climate. The other stresses the regulation of flow by upstream dams. They are not mutually exclusive, but their respective weight is not settled by the sources consulted.

The voice of the riverside communities

On the ground, the verdict is more direct. Interviewed by Mongabay, the headman of Ban Klang village, in Chiang Khan, states that the number of fish species has fallen from more than a hundred to around twenty since Xayaburi came into operation. This is testimony, not a scientific measurement. It does, however, reflect a perception widely shared in fishing communities.

The same investigation mentions a new 684-megawatt Lao project located about two kilometres from the Thai border. It may be the Sanakham dam, but this name has yet to be confirmed. The closer the dams come to the border stretch, the more concrete the question of their cross-border effects becomes.

Two banks, one river

The river that divides is also the river that is crossed. According to available sources, the first Thai–Lao Friendship Bridge, between Nong Khai and the Vientiane area, opened in 1994. Other bridges followed; their list and dates have yet to be verified. Long before these bridges, dugout canoes already linked the two banks, and the fish market between Chiang Khong and Houay Xai bears witness to this age-old movement.

The border of 1893 has lastingly transformed the territory. It created two distinct states, two administrations and two official national languages. But the river on which it rests continues to connect what it separates. Migratory fish cross it, sediment travels downstream past both banks, and the Naga dwells on either shore. Electricity generated on the Lao bank powers Thai cities, and floods and low water alike affect both countries at once.

It is in this sense that the border is alive. The political line is stable, but the thing on which it rests changes with the seasons, the dams and the climate. Every decision taken upstream is felt downstream, whichever the bank. Radio Free Asia reported in 2020 that Thailand intended to raise its concerns with the Mekong River Commission. The choice of this shared forum reveals an obvious truth: a shared river cannot be managed from one side alone. More than a century after the Franco-Siamese treaty, the issue is no longer who owns each bank. It is how to preserve a river on which those living along both its shores depend together.

Sources and further reading

1. non relevé, The French army and Siam, 1893–1914, Journal of the Siam Society (ThaiJO), non relevé.
2. non indiqué dans l'extrait (attribution à Andrew Walker à vérifier), Borders in motion on the Upper Mekong: Siam and France in the 1890s, New Mandala (ANU), 2006.
3. Parlement britannique, France and Siam (Hansard, House of Commons, 14 November 1893), UK Parliament – Historic Hansard, 1893.
4. collectif, 1893 Franco-Siamese crisis ; Rattanakosin Kingdom (1782–1932), Wikipedia, consulté 2025.
5. Mekong River Commission, Fisheries Programme, MRC (archive IW:Learn), années 2000.
6. Catch and Culture vol. 21 n°3, Lower Mekong fisheries estimated to be worth around $17 billion a year, Mekong River Commission, 2015.
7. S. Sverdrup-Jensen, Fisheries in the Lower Mekong Basin: Status and Perspectives, Mekong River Commission, 2002.
8. non relevé, Understanding the Threats to Fish Migration: Applying the Global Swimways Concept to the Lower Mekong, Reviews in Fisheries Science & Aquaculture (Taylor & Francis), 2024.
9. Open Development Mekong, Fishing, fisheries and aquaculture, Open Development Mekong, c. 2017.
10. Z. Hogan et al., The Imperiled Giants of the Mekong, American Scientist, 2004.
11. N. Sukumasavin, Status of the Mekong giant catfish, Pangasianodon gigas, SEAFDEC, 2006.
12. National Geographic, Giant Catfish Critically Endangered, Group Says, National Geographic, 2003.
13. collectif, Naga fireball, Wikipedia, consulté 2025.
14. Thailand NOW, Naga Fireball Festival 2025, Thailand NOW (rattachement gouvernemental à confirmer), 2025.
15. Time, Hot Spot (Mekong Full Moon Party), Time, 2002.
16. Erik Cohen, The "Postmodernization" of a Mythical Event: Naga Fireballs on the Mekong River, Tourism Culture & Communication, 2007.
17. Boing Boing, Naga fireballs of the Mekong River, Boing Boing, non relevé.
18. AFP, Mekong levels at lowest on record as drought and dams strangle river, Phys.org / Philstar, 2019.
19. Mongabay, Struggling Thai fisherfolk gird for losses as another Laos Mekong dam looms, Eco-Business, c. 2022.
20. Radio Free Asia / BenarNews, Laos's Controversial Xayaburi Dam Begins Operations ; Mekong River Dams Disrupting Lives ; Thailand to Air Concerns with River Commission, Radio Free Asia, 2019-2020.
21. VoiceMap, The Mekong River (Laos - Thai Border), VoiceMap (audioguide, source faible), non relevé.