Mekong River (ទន្លេរមេគង្គ)
Megaung Myit, แม่น้ำโขง (Maenam Khong), Mékôngk, Tonle Thom, Cửu Long, Mê Kông, 湄公 (Méigōng)
River
View of the Mekong before sunset.
Countries	Burma, Cambodia, Laos, People's Republic of China, Thailand, Vietnam
States	Qinghai, Tibet, Yunnan, Shan, Luang Namtha, Bokeo, Oudomxay, Luang Phrabang, Sayabouly, Vientiane, Vientiane, Bolikhamsai, Khammouane, Savannakhet, Salavan, Champasak
Source	Lasagongma Spring
- location	Mt. Guozongmucha, Zadoi, Yushu Tibetan Autonomous Prefecture, Qinghai, China
- elevation	5,224 m (17,139 ft)
- coordinates	33°42.5′N 94°41.7′E / 33.7083°N 94.695°E / 33.7083; 94.695
Mouth	Mekong Delta
- elevation	0 m (0 ft)
Length	4,350 km (2,703 mi)
Basin	795,000 km2 (307,000 sq mi)
Discharge	for South China Sea
- average	16,000 m3/s (570,000 cu ft/s)
- max	39,000 m3/s (1,400,000 cu ft/s)

This article contains Chinese text. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Chinese characters.

This article contains Lao text. Without proper rendering support, you may see question marks, boxes, or other symbols instead of Lao script.

The Mekong is a river in Southeast Asia. It is the world's 12th-longest river^[1] and the 7th-longest in Asia. Its estimated length is 4,350 km (2,703 mi),^[1] and it drains an area of 795,000 km² (307,000 sq mi), discharging 475 km³ (114 cu mi) of water annually^[2]

From the Tibetan Plateau this river runs through China's Yunnan province, Burma, Laos, Thailand, Cambodia and Vietnam. In 1995, Laos, Thailand, Cambodia and Vietnam established the Mekong River Commission to assist in the management and coordinated use of the Mekong's resources. In 1996 China and Burma became "dialogue partners" of the MRC and the six countries now work together within a cooperative framework.

The extreme seasonal variations in flow and the presence of rapids and waterfalls in this river have made navigation difficult. The river is a major trading route linking China’s southwestern province of Yunnan to Laos, Burma and Thailand to the south, an important trade route between western China and Southeast Asia.

Names[link]

In English the river is called "the Mekong River", derived from "Mae Nam Khong", a term of both Thai and Lao origin. In the Lao-Thai toponymy, all great rivers are considered "mother rivers" signalled by the prefix "mae", meaning "mother", and "nam" for water. In the Mekong's case, Mae Nam Khong means Khong, The Mother of Water^[3]. "Khong" is derived from the Sanskrit "ganga", meaning the Ganges. Many Northern Thai and Laos locals refer to it as the "River Khong". Such is the case with the Mae Nam Ping in Chiang Mai which is known as the "Ping River". The Tonle Sap in Cambodia is a similar example – where Tonle translates as "Great lake or river", making the Tonle Sap River an unnecessary repetition of what is in fact the "Sap River".

Since the river flows through a number of countries, it has many different names in local languages:

• Burmese: မဲခေါင်မြစ်, IPA: [mɛ́ɡàʊɴ mjɪ̰ʔ]
• Chinese: Riverhead: 加果空桑贡玛曲, 扎那曲 and 扎曲 Zā Qū, upper reaches: 澜沧江, 瀾滄江 Láncāng Jiāng ("Turbulent River", "Láncāng" is the same as 'Lan Xang' in Chinese), middle and lower reaches: 湄公河 Méigōng hé.
• Khmer: មេគង្គ Mékôngk [meekoŋ], ទន្លេមេគង្គ Tônlé Mékôngk [tʊənlee meekoŋ], ទន្លេធំ‎ Tônlé Thum [tʊənlee tʰom] ("Great River").
• Lao: ແມ່ນ້ຳຂອງ, [mɛː nâːm kʰɔ̌ːŋ], ນ້ຳຂອງ [nâːm kʰɔ̌ːŋ].
• Thai: แม่น้ำโขง, [mɛ̂ː náːm kʰǒːŋ].
• Tibetan: རྫ་ཆུ་, Wylie: rDza chu, ZYPY: Za qu
• Vietnamese: Sông Mê Kông (pronounced [soŋm mekoŋm]), Sông Lớn ("Great River", [soŋm lə̌ːn]), Sông Cửu Long ("Nine Dragons River", [soŋm kɨ̃w lɔŋ]).

Course[link]

The Mekong rises in the "Three Rivers Area" on the Tibetan Plateau in the Sanjiangyuan National Nature Reserve as the Lancang, together with the Yangtze and Yellow (Huang He) Rivers.^[2] It flows southwest through Yunnan Province, and through the Three Parallel Rivers Area in the Hengduan Mountains, along with the Salween River (Nujiang in Chinese) and the Yangtze.

After leaving China, it flows southwest and forms the border of Burma and Laos for about 100 kilometres (62 mi) then turns southeast to briefly form the border of Laos with Thailand. The Mekong then flows east and south into Laos for some 400 kilometres (250 mi) and defines the Laos-Thailand border again for some 850 kilometres (530 mi) as it flows east, turning south through central Southeast Asia, passing in front of the capital of Laos, Vientiane. Thereafter, the river 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 the Tonle Sap. When the Mekong is low, the Tonle Sap is a tributary; water flows from the lake and river into the Mekong. When the Mekong floods, the flow reverses; the floodwaters of the Mekong flow up the Tonle Sap.

Immediately after the Sap River joins the Mekong by Phnom Penh, the Bassac River branches off the right (south) bank. The Bassac River is the first and main distributary of the Mekong; thus, this is the beginning of the Mekong Delta. The two rivers, the Bassac to the south and the Mekong to the north, enter Vietnam very soon after this. In Vietnam, the Bassac is called the Hậu River (Sông Hậu or Hậu Giang); the main, northern, branch of the Mekong is called the Sông Tiền or Tiền Giang. In Vietnam, distributaries of the northern branch include the sông (river) Mỹ Tho, the sông Ba Lai, the sông Hàm Luông, and the sông Cổ Chiên.

Drainage basin[link]

The Mekong from Phou si

The confluence of the Mekong and the Nam Ou river in Laos.

The Mekong Basin can be divided into two parts: the 'Upper Mekong Basin' in Tibet and China, and the 'Lower Mekong Basin' from Yunnan downstream from China to the South China Sea^[4]. From the point where it rises to its mouth, the most precipitous drop in the Mekong occurs in Upper Mekong Basin, a stretch of some 2,200 km (1,400 mi). Here, it drops 4,500 metres (14,800 ft) before it enters the Lower Basin where the borders of Thailand, Laos, China and Burma come together in the Golden Triangle. Downstream from the Golden Triangle, the river flows for a further 2,600 km (1,600 mi) through Laos, Thailand and Cambodia before entering the South China Sea via a complex delta system in Viet Nam^[4]

Upper Mekong Basin[link]

The Upper Basin makes up 24% of the total area and contributes 15 to 20% of the water that flows into the Mekong River. The catchment here is steep and narrow. Soil erosion has been a major problem and approximately 50% of the sediment in the river comes from the Upper Basin^[4].

In Yunnan province in China, 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 1,000 km² (390 sq mi), yet the greatest amount of Loss of forest cover in the entire river system per 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^[4].

Lower Mekong Basin[link]

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 Lao PDR. 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^[4]. 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 of Northern Laos and is reported to account for as much as 27% of the total land under rice cultivation^[4]. As elsewhere in the basin, forest cover has been steadily reduced during the last three decades 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 ^[5].

Loss of forest cover in the Thai areas of the Lower Basin has been the highest in all the Lower Mekong countries over the past 60 years. On the Korat Plateau, which includes the Mun and Chi tributary systems, forest cover was reduced from 42% in 1961 to 13% in 1993^[4]. Although this part of Northeast Thailand has an annual rainfall of more than 1,000 mm, 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^[4]. 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^[4]. Drought is by far the major hydrological hazard in this region^[4].

As the Mekong enters Cambodia, over 95% of the flows have already joined the river^[4]. From here on 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” of water into and out of the Great Lake via the Tonle Sap River. Phnom Penh also marks the beginning of the delta system of the Mekong River. Here the mainstream begins to break up into an increasing number of branches^[4].

In Cambodia, the wet rice is the main crop and is grown on the flood plains of the Tonle Sap, Mekong and Bassac (the Mekong delta distributary known as the Hậu in Viet Nam) rivers^[4]. 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^[4]. Here, the river landscape is flat. Small changes in water level determine the direction of water movement, including the large-scale reversal of flow into and out of the Tonle Sap basin from the Mekong River^[4].

Mekong Delta, Vietnam

The Mekong Delta in Viet Nam is farmed intensively and has little natural vegetation left. Forest cover is less than 10%. In the Central Highlands of Viet Nam, forest cover was reduced from over 95% in the 1950s to around 50% in the mid 1990s^[4]. 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^[4].

Water flow along its course[link]

Table 1: Basic data on country share of Mekong Basin territory and water flows^[4]

By taking into account hydrological regimes, physiography land use, and existing, planned and potential resource developments, the Mekong is divided into six distinct reaches^[4]:

The Mekong in Laos

Reach 1: Lancang Jiang or Upper Mekong River in China. In this part of the river, the major source of water flowing into the river comes from melting snow on the Tibetan Plateau. This volume of water is sometimes called the “Yunnan Component” and plays an important role in the low-flow hydrology of the lower mainstream. Even as far downstream as Kratie, the Yunnan Component makes up almost 30% of the average dry season flow. A major concern is that the on-going and planned expansion of dams and reservoirs on the Mekong mainstream in Yunnan could have a significant effect on the low-flow regime of the Lower Mekong Basin system^[4].

Reach 2: Chiang Saen to Vientiane and Nongkhai. This reach is almost entirely mountainous and covered with natural forest, although there has been widespread slash and burn agriculture. Although this reach could hardly be described as “unspoiled”, the hydrological response is perhaps the most natural and undisturbed in all the Lower Basin. Many hydrological aspects of the Lower Basin start to change rapidly at the downstream boundary of this reach^[4].

Reach 3: Vientiane and Nongkhai to Pakse. The boundary between Reach 2 and 3 is where the Mekong hydrology starts to change. Reach 2 is dominated in both wet and dry seasons by the Yunnan Component. Reach 3 is increasingly influenced by contributions from the large left bank tributaries in Laos, namely the Nam Ngum, Nam Theun, Nam Hinboun, Se Bang Fai, Se Bang Hieng and Se Done rivers. The Mun - Chi river system from the right bank in Thailand also enters the mainstream within this reach^[4].

Reach 4: Pakse to Kratie. The main hydrological contributions to the mainstream in this reach come from the Se Kong, Se San and Sre Pok catchments. Together, these rivers make up the largest hydrological subcomponent of the Lower Basin. Over 25% of the mean annual flow volume to the mainstream at Kratie comes from these three river basins. They are the key element in the hydrology of this part of the system, especially to the Tonle Sap flow reversal^[4].

Reach 5: Kratie to Phnom Penh. This reach includes the hydraulic complexities of the Cambodian floodplain, the Tonle Sap and the Great Lake. By this stage, over 95% of the total flow has entered the Mekong system. The focus turns from hydrology and water discharge to the assessment of water level, over- bank storage and flooding and the hydrodynamics that determine the timing, duration and volume of the seasonal flow reversal into and out of the Great Lake^[4].

Floating market of Cần Thơ, Mekong Delta.

Reach 6: Phnom Penh to the South China Sea. Here the mainstream divides into a complex and increasingly controlled and artificial system of branches and canals. Key features of flow behaviour are tidal influences and salt water intrusion. Every year, 35 to 50% of this reach is flooded during the rainy season. The impact of road embankments and similar infrastructure developments on the movement of this flood water is an increasingly important consequence of development^[4].

Table 2 summarises the mean annual flows along the mainstream. The mean annual flow entering the lower Mekong from China is equivalent to a relatively modest 450 mm depth of runoff. Downstream of Vientiane this increases to over 600 mm as the principal left bank tributaries enter the mainstream, mainly the Nam Ngum and Nam Theun. The flow level falls again, even with the right bank entry of the Mun-Chi system from Thailand. Although the Mun - Chi basin drains 20% of the lower system, average annual runoff is only 250 mm. Runoff in the mainstream increases again with the entry from the left bank of the Se Kong from southern Laos and Se San and Sre Pok from Vietnam and Cambodia.

Table 2: Lower Mekong Mainstream annual flow (1960 to 2004) at selected sites^[4].

Flows at Chiang Saen entering the Lower Basin from Yunnan make up about 15% of the wet season flow at Kratie. This rises to 40% during the dry season, even this far downstream. During the wet season, the proportion of average flow coming from Yunnan rapidly decreases downstream of Chiang Saen, from 70% to less than 20% at Kratie. The dry season contribution from Yunnan is much more significant. The major portion of the balance comes from Laos, which points to a major distinction in the low-flow hydrology of the river. One fraction comes from melting snow in China and Tibet and the rest from over-season catchment storage in the Lower Basin. This has implications for the occurrence of drought conditions. For example, if runoff from melting snow in any given year is very low, then flows upstream of Vientiane-Nongkhai would be lower^[4].

In a large river system like the Mekong, seasonal flows can be quite variable from year to year. Although the pattern of the annual hydrograph is fairly predictable, its magnitude is not. The average monthly flows along the mainstream are listed in Table 3, providing an indication of their range and variability from year to year. At Pakse, for example, flood season flows during August would exceed 20,000 cubic metres per second 9 years out of 10, but exceed 34,000 m³/s only 1 year in ten^[4].

Table 3: Mekong Mainstream monthly discharge 1960 to 2004 (m³/s)^[4].

There is little evidence from the last 45 years of data of any systematic changes in the hydrological regime of the Mekong^[4].

Mekong River modifications[link]

The Mekong is already heavily dammed, with many more dams planned and under construction.

Natural history[link]

Extirpated from most of its pan-Asian range, Cantor's giant softshell turtle can still be found along a certain stretch of the Mekong in Cambodia

The Mekong basin is one of the richest areas of biodiversity in the world. Only the Amazon boasts a higher level of biodiversity.^[2]. Biota estimates for the Greater Mekong Subregion (GMS) include 20,000 plant species, 430 mammals, 1,200 birds, 800 reptiles and amphibians^[6] and an estimated 850 fish species^[7]. In 2009, 145 new species were described from the Mekong Region, comprising 29 fish species previously unknown to science, two new bird species, ten reptiles, five mammals, 96 plants and six new amphibians^[8]. The Mekong Region contains 16 WWF Global 200 ecoregions, the greatest concentration of ecoregions in mainland Asia^[2].

No other river is home to so many species of very large fish. The biggest include the giant river carp (Probarbus jullieni), which can grow up to 1.5 metres (4 ft 11 in) and weigh 70 kilograms (150 lb), the Mekong Freshwater Stingray (Himantura chaophraya), which can have a wingspan of up to 4.3 metres (14 ft), the giant pangasius (Pangasius sanitwongsei), Siamese giant carp (Catlocarpio siamensis) and the endemic Mekong giant catfish (Pangasianodon gigas), all three of which can grow up to about 3 metres (9 ft 10 in) in length and weigh 300 kilograms (660 lb). All of these are in serious decline, because of dams, flood control and overfishing.

One species of freshwater dolphin, the Irrawaddy Dolphin (Orcaella brevirostris), was once common in the whole of the Lower Mekong but is now very rare^[9].

Among other wetland mammals that have been living in and around the river are the smooth-coated otter (Lutra perspicillata) and fishing cat (Felis viverrina).

The endangered Siamese crocodile (Crocodylus siamensis) occurs in small isolated pockets within the northern Cambodian and Laotian portions of the Mekong River. The Saltwater crocodile (Crocodylus porosus) once ranged from the Mekong Delta up the river into Tonle Sap and beyond but is now extinct in the river, along with being extinct in all of Vietnam and possibly even Cambodia.

The fisheries[link]

Aquatic biodiversity in the Mekong river system is the second highest in the world after the Amazon.^[10][11] The Mekong boasts the most concentrated biodiversity per hectare of any river.^[12]

The commercially valuable fish species in the Mekong are generally divided between ‘black fish’, which inhabit low oxygen, slow moving, shallow waters, and ‘white fish’, which inhabit well oxygenated, fast moving, deeper waters^[13]. People living within the Mekong river system generate many other sources of food and income from what are often termed ‘other aquatic animals’ (OAAs) such as freshwater crabs, shrimp, snakes, turtles, and frogs.

OAAs account for about 20 percent of the total Mekong catch^[2]. When fisheries are discussed, catches are typically divided between the wild capture fishery (i.e. fish and other aquatic animals caught in their natural habitat), and aquaculture (fish reared under controlled conditions). Wild capture fisheries play the most important role in supporting livelihoods. Wild capture fisheries are largely open access fisheries, which poor rural people can access for food and income.

Broadly, there are three types of fish habitats in the Mekong: i) the river, comprising all the main tributaries, rivers in the major flood zone and the Tonle Sap, which altogether yield about 30 percent of wild catch landings; ii) rainfed wetlands outside the river-floodplain zone, comprising mainly rice paddy in formerly forested areas and usually inundated to about 50 cm and yielding about 66 percent of wild catch landings; and iii) large water bodies outside the flood zone, including canals and reservoirs yielding about 4 percent of wild catch landings^[2].

The Mekong Basin has one of the world’s largest and most productive inland fisheries^{[14][15][10][16]}. An estimated 2 million tonnes of fish are landed a year, in addition to almost 500,000 tonnes of other aquatic animals^[17]. Aquaculture yields about 2 million tonnes of fish a year^[13].

Hence, the Lower Mekong Basin yields about 4.5 million tonnes of fish and aquatic products annually. The total economic value of the fishery is between USD 3.9 to USD 7 billion a year^[2]. Wild capture fisheries alone have been valued at USD 2 billion a year^[15]. This value increases considerably when the multiplier effect is included, but estimates vary widely.

An estimated 2.56 million tonnes of inland fish and other aquatic animals are consumed in the lower Mekong every year^[13]. Aquatic resources make up between 47 percent and 80 percent of animal protein in rural diets for people who live in the Lower Mekong Basin^[15][18][19]. Fish are the cheapest source of animal protein in the region and any decline in the fishery is likely to significantly impact nutrition, especially among the poor ^{[20][21][18][10]}. The size of this impact has not been established^[20]

It is estimated that 40 million rural people, more than two-thirds of the rural population in the Lower Mekong Basin, are engaged in the wild capture fishery^[2]. Fisheries contribute significantly to a diversified livelihood strategy for many people, particularly the poor, who are highly dependent on the river and its resources for their livelihoods^[14][15][19].