The Mekong is one of the world's major rivers. It is the 12th-longest in the world, and the 10th-largest by volume (discharging 475 km³ of water annually). It drains an area of 795,000 km²: From Tibet it runs through China's Yunnan province, Myanmar, Thailand, Laos, Cambodia and Vietnam. All except China and Myanmar belong to the Mekong River Commission. The extreme seasonal variations in flow and the presence of rapids and waterfalls have made Navigation extremely difficult.
The river's source, and therefore its exact length, is uncertain, due to the existence of several tributaries in an inaccessible environment. Chinese researchers believe that the source is located in the Jifu Mountains in Zaduo County, Yushu Tibet Autonomous Prefecture of northwest China's Qinghai Province, which is some 5,200 meters above sea level. An earlier expedition, led by Michel Peissel, placed the source at the head of the Rupsa-La pass (further west, at an altitude of 4975 meters). Figures for the river's total length therefore vary between 4350 and 4909 km.
Approximately half the river's length is in China, where it is called the Dza Chu in Tibetan in its upper course in Tibet, and more generally the Lancang in Chinese (澜沧江; Pinyin Láncāng Jiāng; Wade-Giles Lan-ts'ang Chiang), meaning the "turbulent river". Much of this stretch consists of deep gorges, and the river leaves China at an altitude of only 500 meters.
The river next forms the border between Myanmar and Laos for 200 km, at the end of which it meets the Tributary Ruak River at the Golden Triangle. This point also marks the division between the Upper and Lower Mekong.
The river then divides Laos and Thailand, before a stretch passing through Laos alone. It is known as Mènam Khong in both Lao and Thai (แม่น้ำโขง). The Lao stretch is characterised by gorges, rapids and depths of as little as half a meter in the dry season. It widens south of Luang Prabang, where it has been known to flood to 4 km in width and reach 100 meters in depth, although its course remains extremely inconsistent. The endangered Giant Mekong Catfish was traditionally caught in this region once yearly, following auspicious rites officiated by the quondam royal family.
The river again marks the Lao-Thai border in the stretch which passes Vientiane, followed by a short stretch through Laos alone. This includes the Si Phan Don (four thousand islands) region above the Khone Falls near the Cambodian border, where endangered dolphins can be viewed. The falls are all but impassable to river traffic.
In Cambodia, the river is called the Mékôngk or Tonle Thom (great river). The Sambor rapids above Kratie are the last to impede navigation. Just above Phnom Penh is the confluence with the Tonle Sap, the main Cambodian tributary. Below Phnom Penh, it divides into the Bassac and the Mekong proper, which both flow into the Mekong Delta in Vietnam, near Ho Chi Minh City. In Vietnamese it is the Sông Lớn (great river), Cửu Long (nine dragons river) or Mê Kông.
About 90 million people rely on the river.
The difficulty of navigating the river has meant that it has divided, rather than united, the people who live near it. The earliest known civilisation was the 1st century Indianised Malay culture of Funan, in the Mekong Delta. Excavations at Oc Eo, near modern Rach Gia, have found coins from as far away as the Roman Empire. This was succeeded by the Khmer Chenla state by around the 5th century. The Khmer empire of Angkor was the last great Indianized state in the region. From around the time of the fall of the Khmer empire, the Mekong was the frontline between the emergent states of Siam and Vietnam, with Laos and Cambodia torn between their influence.
The first European to discover the Mekong was the Portuguese Antonio de Faria in 1540; a European map of 1563 depicts the river, although even by then little was known of the river upstream of the delta. European interest was sporadic: the Spaniards and Portuguese mounted some missionary and trade expeditions, while the Dutch Gerrit van Wuysthoff led an expedition up the river as far as Vientiane in 1641-42.
The French took a serious interest in the region in the mid-19th century, capturing Saigon in 1861, and establishing a protectorate over Cambodia in 1863.
The first systematic exploration began with the French Mekong Expedition led by Ernest Doudard de Lagrée and Francis Garnier, which ascended the river from its mouth to Yunnan between 1866 to 1868. Their chief finding was that the Mekong had too many falls and rapids to ever be useful for Navigation. The river's source was located by Pyotr Kuzmich Kozlov in 1900.
From 1893, the French extended their control of the river into Laos, establishing French Indochina by the first decade of the 20th century. This lasted until the First and Second Indochina Wars ended French and American involvement in the region.
After the Vietnam War, the tensions between the US-backed Thai government and the new Communist governments in the other countries prevented co-operation on utilisation of the river.
The story of the film Apocalypse Now is based around a fictional journey up the Mekong River into Cambodia. The film uses the journey up the river as a metaphor for a journey into the darkness of humanity.
A number of dams have already been built on the river's tributaries, notably the Pak Mun dam in Thailand. This has been criticised on grounds of cost as well as damage to the environment and to the livelihoods of affected villagers.
China is engaged in an extensive program of dam-building on the river itself: it has already completed one at Manwan, a second is under construction at Dachaoshan, and another twelve are under consideration. It is widely feared that these will prevent sediment from flowing, which would seriously harm agriculture and fishing downstream. The evening out of seasonal fluctuations in water volume could affect the Tonle Sap, which depends on this differential.
Dams are also planned by the Cambodian and Lao governments. Some groups oppose them.
The Chinese government has also carried out work clearing rocks and sandbars from its stretch of the river, in order to aid navigation, and it has encouraged Laos to do the same. Environmentalists fear that this will mean an increased flow of water, which in turn would cause increased Erosion, as well as damaging fish stocks.
According to researchers the river houses more species of giant fish than any world river, most notably the Giant Mekong Catfish.
The low tide level of the river in Cambodia is lower than the high tide level out at sea, and the flow of the Mekong inverts with the tides throughout its stretch in Vietnam and up to Phnom Penh. The very flat Mekong Delta area in Vietnam is thus prone to flooding, especially in the provinces of An Giang and Dong Thap (Đồng Tháp), near the Cambodian border.