The Mekong or Mekong River ( , ) is a transboundary river in East Asia and Southeast Asia. It is the world's twelfth-longest river and the third-longest in Asia with an estimated length of and a drainage area of , discharging of water annually. From its headwaters in the Tibetan Plateau, the river runs through Southwest China (where it is officially called the Lancang River), Myanmar, Laos, Thailand, Cambodia, and southern Vietnam. The extreme seasonal variations in flow and the presence of rapids and waterfalls in the Mekong make navigation difficult, though the river remains a major trade route between Tibet and Southeast Asia. The construction of hydroelectric dams along the Mekong in the 2000s through the 2020s has caused serious problems for the river's ecosystem, including the exacerbation of drought.
Names
The Mekong was originally called Mae Nam Khong from a contracted form of Kra-Dai shortened to Mae Khong. In Thai and Lao, Mae Nam ("Mother of Water[s]") is used for large rivers and Khong is the proper name referred to as "River Khong". However, Khong is an archaic word meaning "river", loaned from Austroasiatic languages, such as Vietnamese sông, Katu karuːŋ, May ʑuːŋ³, and Mon kruŋ, all which derived from Proto-Austroasiatic root *krɔːŋ, meaning "river", which led to Chinese whose Old Chinese pronunciation has been reconstructed as Baxter-Sagart. and which long served as the proper name of the Yangtze before becoming a generic word for major rivers. To the early European traders, the Mekong River was also known as Mekon River, May-Kiang River and Cambodia River.Reid, H. (1857). A System of Modern Geography ... with Exercises of examination. To which are added treatises on Astronomy and Physical Geography. United Kingdom: (n.p.).Universal Gazetteer of the World: A Dictionary, Geographical, Historical and Statistical, of the Various Kingdoms, States, Provinces, Cities, Towns, Forts, Harbors, Rivers, Lakes, Seas, Mountains, &c., in the World ... Also the Census of 1850. (1852). United States: Z. & B. F. Pratt.A Gazetteer of the World: Brazil–Derry. (1856). United Kingdom: A. Fullarton.Roberts, G. (1834). The Elements of Modern Geography and General History, on a Plan Entirely New ... New Edition ... Improved, Etc. United Kingdom: (n.p.).Hamilton, W. (1815). The East India Gazetteer: Containing Particular Descriptions of the Empires, Kingdoms, Principalities, Provinces, Cities, Towns, Districts, Fortresses, Harbours, Rivers, Lakes, &c. of Hindostan, and the Adjacent Countries, India Beyond the Ganges, and the Eastern Archipelago; Together with Sketches of the Manners, Customs, Institutions, Agriculture, Commerce, Manufactures, Revenues, Population, Castes, Religion, History, &c. of Their Various Inhabitants. United Kingdom: J. Murray. Historian William Dalrymple suggests that Mekong comes from "Mā Gaṅgā" (Khmer: ម៉ែគង្គា; UNGEGN: Mê Kôngkéa) which means Mother Ganges. Hindus think of the river Ganges as mother. The local names for the river include: From Thai: , , or just . , , or just . , , or just . , , or just . Tai Lue: , . or . Other: In Vietnamese, the river was originally referred to as Sông Khung or Khung Giang (, , ) in classical Vietnamese texts (Đại Nam thực lục). Modern terms Sông Mê Kông () or Sông Cửu Long, ( Nine Dragons River ). . , . Tônlé Thum (lit. "Big River" or "Great River") or Mékôngk , Tônlé Mékôngk . Khmuic: , '' means 'river' or 'water', here it means 'river', '' means 'canal'. So '' means 'canal river'. In the ancient time Khmuic people called it '' or '' which means 'giant canal river' or 'deep canal river' respectively.
Course The Mekong rises as the Za Qu (; ) and soon becomes known as the Lancang River (, from the Lao kingdom Lan Xang; the characters may also be literally understood as "turbulent green river"). It originates in the "three rivers source area" on the Tibetan Plateau in the Sanjiangyuan National Nature Reserve. The reserve protects the headwaters of (from north to south) the Yellow (Huang He), the Yangtze, and the Mekong rivers. It flows through the Tibet Autonomous Region and then southeast into Yunnan Province, and then the Three Parallel Rivers area in the Hengduan Mountains, along with the Yangtze to its east and the Salween River (Nu Jiang in Chinese) to its west.
Then the Mekong meets the China–Myanmar border and flows about along that border until it reaches the tripoint of China, Myanmar and Laos. From there it flows southwest and forms the border of Myanmar and Laos for about until it arrives at the tripoint of Myanmar, Laos, and Thailand. This is also the confluence with the Ruak River (which follows the Thai–Myanmar border). The area of this tripoint is sometimes termed the Golden Triangle, although the term also refers to the much larger area of those three countries that was notorious as an opium-producing region.
From the Golden Triangle tripoint, the Mekong turns southeast form the border of Laos with Thailand for until the Khong Chiam district in northeastern Thailand. Khon Pi Long is a series of rapids along a section dividing Chiang Rai in Thailand and Bokeo province in Laos. The name of the rapids means 'where the ghost lost its way'. It then turns east into the interior of Laos, flowing first east and then south for some before meeting the border with Thailand again. Once more, it defines the Laos–Thailand border for some as it flows first east, passing Vientiane, then turns south. A second time, the river leaves the border and flows east into Laos passing Pakse.
The river turns and runs more or less directly south, crossing into Cambodia. At Phnom Penh the river is joined on the right bank by the river and lake system Tonlé Sap. When the Mekong is low, the Tonlé Sap is a tributary and it flows into the Mekong. When the Mekong floods, the flow reverses, with the floodwaters flowing up the Tonlé Sap.
South of Phnom Penh, the Bassac River branches off the right (west) bank. The Bassac is the first and main distributary of the Mekong. This is the beginning of the Mekong Delta. The two rivers, the Bassac to the west and the Mekong to the east, enter Vietnam shortly after this and the Mekong is called the Tiền River or Tiền Giang. Additional distributaries include Mỹ Tho River, Ba Lai River, Hàm Luông River, and Cổ Chiên River.
Drainage basin
The Mekong basin is frequently divided into two parts: the "upper Mekong basin" comprising those parts of the basin in Tibet, Yunnan and eastern Myanmar, and the "lower Mekong basin" from Yunnan downstream from China to the South China Sea. From the point where it rises to its mouth, the most precipitous drop occurs in the upper Mekong basin, a stretch of some . Here, it drops before it enters the lower basin at the Golden Triangle. Downstream from the Golden Triangle, the river flows for a further through Laos, Thailand, and Cambodia before entering the South China Sea via a complex delta system in Vietnam.
Upper basin The upper basin makes up 24% of the total area and contributes 15–20% of the water that flows into the Mekong River. The catchment here is steep and narrow with soil erosion being a major problem and as a result of this, approximately 50% of the sediment in the river comes from the upper basin.
In Yunnan the river and its tributaries are confined by narrow, deep gorges. The tributary river systems in this part of the basin are small. Only 14 have catchment areas that exceed , yet the greatest amount of loss of forest cover in the entire river system per square kilometer has occurred in this region due to heavy unchecked demand for natural resources. In the south of Yunnan, in Simao and Xishuangbanna prefectures, the river changes as the valley opens out, the floodplain becomes wider, and the river becomes wider and slower.
Lower basin Major tributary systems develop in the lower basin. These systems can be separated into two groups: tributaries that contribute to the major wet season flows, and tributaries that drain low relief regions of lower rainfall. The first group are left bank tributaries that drain the high rainfall areas of Laos. The second group are those on the right bank, mainly the Mun and Chi rivers, that drain a large part of northeast Thailand.
Laos lies almost entirely within the lower Mekong basin. Its climate, landscape and land use are the major factors shaping the hydrology of the river. The mountainous landscape means that only 16% of the country is farmed under lowland terrace or upland shifting cultivation. With upland shifting agriculture (slash and burn), soils recover within 10 to 20 years but the vegetation does not. Shifting cultivation is common in the uplands and is reported to account for as much as 27% of the total land under rice cultivation. As elsewhere in the basin, forest cover has been steadily reduced by shifting agriculture and permanent agriculture. The cumulative impacts of these activities on the river regime have not been measured. However, the hydrological impacts of land cover changes induced by the Vietnam War were quantified in two sub-catchments of the lower Mekong River basin.
Loss of forest cover in the Thai areas of the lower basin has been the highest of all the lower Mekong countries in the 20th century. On the Khorat Plateau, which includes the Mun and Chi tributary systems, forest cover was reduced from 42% in 1961 to 13% in 1993. Although this part of northeast Thailand has an annual rainfall of more than , a high evaporation rate means it is classified as a semi-arid region. Consequently, although the Mun and Chi basins drain 15% of the entire Mekong basin, they only contribute 6% of the average annual flow. Sandy and saline soils are the most common soil types, which makes much of the land unsuitable for wet rice cultivation. In spite of poor fertility, however, agriculture is intensive. Glutinous rice, maize, and cassava are the principal crops. Drought is a major hydrological hazard in this region.
When the Mekong enters Cambodia, over 95% of its flows have joined the river. Downstream the terrain is flat, and water levels rather than flow volumes determine the movement of water across the landscape. The seasonal cycle of changing water levels at Phnom Penh results in the unique "flow reversal" with the Tonlé Sap River. Phnom Penh also marks the beginning of the delta system of the Mekong River, where the main stream breaks up into an increasing number of branches.
In Cambodia, wet rice is the main crop and is grown on the flood plains of the Tonlé Sap, Mekong, and Bassac. More than half of Cambodia remains covered with mixed evergreen and deciduous broadleaf forest, but forest cover has decreased from 73% in 1973 to 63% in 1993. Here, the river landscape is flat.
The Mekong Delta in Vietnam is farmed intensively and has little natural vegetation left. Forest cover is less than 10%. In the Central Highlands of Vietnam, forest cover was reduced from over 95% in the 1950s to around 50% in the mid-1990s. Agricultural expansion and population pressure are the major reasons for land use and landscape change. Both drought and flood are common hazards in the delta, which many people believe is the most sensitive to upstream hydrological change.
Water flow along its course Table 1: Country share of Mekong River Basin (MRB) and water flows China Myanmar Laos Thailand Cambodia Vietnam Total Basin area (km2) 165,000 24,000 202,000 184,000 155,000 65,000 795,000 Catchment as % of MRB 21 3 25 23 20 8 100 Flow as % of MRB 16 2 35 18 18 11 100
By taking into account hydrological regimes, physiography land use, as well as existing, planned, and potential resource developments, the Mekong is divided into six distinct reaches: