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 江 whose Old Chinese pronunciation has been reconstructed as /*kˤroŋ/ 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. 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.[citation needed] The local names for the river include:
- From Thai:
- Thai: แม่น้ำโขง, mɛ̂ɛ-náam-kǒong, [mɛ̂ː náːm kʰǒːŋ], or just แม่โขง, mɛ̂ɛ-kǒong [mɛ̂ː kʰǒːŋ].
- Northern Thai: ᨶᩣᩴ᩶ᩯᨾ᩵ᨡᩬᨦ, [náːm mɛ̂ː kʰɔːŋ], or just ᨶᩣᩴ᩶ᨡᩬᨦ [náːm kʰɔːŋ].
- Northeastern Thai: แม่น้ำของ, [mɛ̄ː na᷆ːm kʰɔ̌ːŋ], or just แม่ของ [mɛ̄ː kʰɔ̌ːŋ].
- Lao: ແມ່ນ້ຳຂອງ, mǣ nam khǭng, [mɛ̄ː nâːm kʰɔ̌ːŋ], or just ນ້ຳຂອງ, nam khǭng [nâːm kʰɔ̌ːŋ].
- Tai Lue: น้ำแม่ของ [nâːm mɛː kʰɔ̌ːŋ], น้ำของ [nâːm kʰɔ̌ːŋ].
- Shan: ၼမ်ႉၶွင်, nâ̰m khǎung [nâm.kʰɔ̌ŋ] or ၼမ်ႉမႄႈၶွင်, nâ̰m māae khǎung [nâm.mɛ.kʰɔ̌ŋ].
- Other:
- In Vietnamese, the river was originally referred to as Sông Khung or Khung Giang (chữ Hán: 穹江, [ʂɔŋ˧ xuŋ˧], [xuŋ˧ zaŋ˧]) in classical Vietnamese texts (Đại Nam thực lục). Modern terms Sông Mê Kông ([ʂɔŋ˧ me˧ koŋ˧]) or Sông Cửu Long, (九龍 Nine Dragons River [ʂɔŋ˧ kɯu˧˩˧ lɔŋ˧]).
- Mandarin Chinese: 湄公河, Méigōng Hé [meɪ̯³⁵ kʊŋ⁵⁵ xɤ³⁵].
- Burmese: မဲခေါင်မြစ်, mai:hkaungmrac, IPA: [mɛ́ɡàʊɰ̃ mjɪ̰ʔ].
- Khmer: ទន្លេធំ Tônlé Thum [tɔnlei tʰum] (lit. "Big River" or "Great River") or មេគង្គ Mékôngk [meːkɔŋ], ទន្លេមេគង្គ Tônlé Mékôngk [tɔnlei meikɔŋ].
- Khmuic: [ŏ̞m̥ kʰrɔːŋ̊], 'ŏ̞m̥' means 'river' or 'water', here it means 'river', 'kʰrɔːŋ̊' means 'canal'. So 'ŏ̞m̥ kʰrɔːŋ̊' means 'canal river'. In the ancient time Khmuic people called it '[ŏ̞m̥ kʰrɔːŋ̊ ɲă̞k̥]' or '[ŏ̞m̥ kʰrɔːŋ̊ ɟru̞ːʔ]' which means 'giant canal river' or 'deep canal river' respectively.
Course
The Mekong rises as the Za Qu (Tibetan: རྫ་ཆུ་, Wylie: rDza chu, ZWPY: Za qu; Chinese: 扎曲; pinyin: Zā Qū) and soon becomes known as the Lancang River (simplified Chinese: 澜沧江; traditional Chinese: 瀾滄江; pinyin: Láncāng Jiāng, 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 10 km (6 mi) 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 100 km (60 mi) 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 920 km (570 mi) until the Khong Chiam district in northeastern Thailand. Khon Pi Long is a series of rapids along a 1.6 km (1.0 mi) 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 400 km (250 mi) before meeting the border with Thailand again. Once more, it defines the Laos–Thailand border for some 850 km (530 mi) 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 2,200 km (1,400 mi).
Here, it drops 4,500 m (14,800 ft) before it enters the lower basin at 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 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 1,000 km2 (400 sq mi), 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 1,000 mm (40 in), 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:
Reach 1: Lancang Jiang or Upper Mekong River in China. 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 Kratié, the Yunnan component makes up almost 30% of the average dry season flow.
A major concern is that the ongoing 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.
Reach 2: Chiang Saen to Vientiane and Nong Khai. This reach is almost entirely mountainous and covered with natural forest although there has been widespread slash and burn agriculture. Although this reach cannot be termed "unspoiled", the hydrological response is perhaps the most natural and undisturbed of all the lower basin.
Many hydrological aspects of the lower basin start to change rapidly at the downstream boundary of this reach. On 19 July 2019 this reach of the river dropped to its lowest level in a century. Officials were particularly concerned as July is in the wet season, when mainstream flows are abundant historically. Locals blamed low water on the newly constructed Xayaburi Dam.
Reach 3: Vientiane and Nong Khai 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. The Mun-Chi river system from the right bank in Thailand enters the mainstream within this reach.
Reach 4: Pakse to Kratié. 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 sub-component of the lower basin. Over 25% of the mean annual flow volume to the mainstream at Kratié comes from these three river basins.
They are the key element in the hydrology of this part of the system, especially to the Tonlé Sap flow reversal.
Reach 5: Kratié to Phnom Penh. This reach includes the hydraulic complexities of the Cambodian floodplain, the Tonlé 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, overbank 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.
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, 35–50% 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.
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 (18 in) depth of runoff. Downstream of Vientiane this increases to over 600 mm (24 in) 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 (10 in). 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.
- Discharge m3/s — Volume km3 — Runoff (mm)
- Chiang Saen — 189,000 — 2,700 — 85 — 450 — 19
- Luang Prabang — 268,000 — 3,900 — 123 — 460 — 27
- Chiang Khan — 292,000 — 4,200 — 133 — 460 — 29
- Vientiane — 299,000 — 4,400 — 139 — 460 — 30
- Nong Khai — 302,000 — 4,500 — 142 — 470 — 31
- Nakhon Phanom — 373,000 — 7,100 — 224 — 600 — 49
- Mukdahan — 391,000 — 7,600 — 240 — 610 — 52
- Pakse — 545,000 — 9,700 — 306 — 560 — 67
- Stung Treng — 635,000 — 13,100 — 413 — 650 — 90
- Kratié — 646,000 — 13,200 — 416 — 640 — 91
- Basin Total — 760,000 — 14,500 — 457 — 600 — 100
Flows at Chiang Saen entering the lower basin from Yunnan make up about 15% of the wet season flow at Kratié. 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 Kratié.
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 would be lower.
In a large river system like the Mekong, seasonal flows can be quite variable. 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 m3/s (5,300,000 US gal/s) nine years out of ten, but exceed 34,000 m3/s (9,000,000 US gal/s) only one year in ten.
Table 3: Mekong Mainstream monthly discharge, 1960–2004 (m3/s).