Misplaced Pages

#259740

60-579: The Mekong or Mekong River is a trans-boundary 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 4,909 km (3,050 mi) and a drainage area of 795,000 km (307,000 sq mi), discharging 475 km (114 cu mi) of water annually. From its headwaters in the Tibetan Plateau ,

120-787: A connection to the South China Sea. Although the hydraulic relationships between the Mekong and the Tonle Sap Great Lake systems during the Holocene are not well understood, it is clear that between 9,000 and 7,500 years ago the confluence of the Tonle Sap and the Mekong was in proximity to the South China Sea. The present river morphology of the Mekong Delta developed over the last 6,000 years. During this period,

180-467: 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 - Nong Khai would be lower. In a large river system like

240-589: A maximum of about 4.5 m (15 ft) above present levels in the early Holocene about 8 ka. At this time the shoreline of the South China Sea almost reached Phnom Penh and cores recovered from near Angkor Borei contained sediments deposited under the influence of tides, and salt marsh and mangrove swamp deposits. Sediments deposited in the Tonle Sap Great Lake about this time (7.9–7.3 ka) also show indications of marine influence, suggesting

300-456: 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

360-645: Is a series of rapids along a 1.6 km (1.0 mi) section of the Mekong River dividing Chiang Rai 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

420-678: Is also the point of confluence between the Ruak River (which follows the Thai–Myanmar border) and the Mekong. 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 a drug producing region. From the Golden Triangle tripoint, the Mekong turns southeast to briefly form the border of Laos with Thailand. Khon Pi Long

480-547: Is an archaic word meaning "river", loaned from Austroasiatic languages , such as Vietnamese sông (from * krong ) and Mon kruŋ "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 . The local names for

540-404: Is by far the major hydrological hazard in this region. As the Mekong enters Cambodia, over 95% of its flows have already joined the river. 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

600-476: 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

660-526: Is heterogeneous and active, and is the major factor controlling the course of rivers and the landscapes they carve out. The elevation of the Tibetan Plateau during the Tertiary period was an important factor in the genesis of the south-west monsoon , which is the dominant climatic control influencing the hydrology of the Mekong Basin. Understanding the nature and timing of the elevation of Tibet (and

SECTION 10

#1732772824260

720-528: Is little evidence from the last 45 years of data of any systematic changes in the hydrological regime of the Mekong. The internal drainage patterns of the Mekong are unusual among those of large rivers. Most large river systems that drain the interiors of continents, such as the Amazon , Congo , and Mississippi , have relatively simple dendritic tributary networks that resemble a branching tree. Typically, such patterns develop in basins with gentle slopes where

780-568: The Central Highlands of Vietnam) therefore helps explain the provenance of sediment reaching the delta and the Tonle Sap Great Lake today. Studies of the provenance of sediments in the Mekong delta reveal a major switch in the source of sediments about eight million years ago (Ma) . From 36 to 8 Ma the bulk (76%) of the sediments deposited in the delta came from erosion of the bedrock in the Three Rivers Area . From 8 Ma to

840-590: 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 enters the mainstream within this reach. 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 sub-component of

900-463: The Richter magnitude scale and is unlikely to cause material damage. The difficulty of navigating the river has meant that it has divided, rather than united, the people who live near it. The earliest known settlements date to 210 BCE, with Ban Chiang being an excellent example of early Iron Age culture. The earliest recorded civilization was the 1st century Indianised -Khmer culture of Funan , in

960-630: 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",

1020-834: 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 Tibetan Autonomous Region and then southeast into Yunnan Province, and then the Three Parallel Rivers Area in the Hengduan Mountains , along with

1080-475: 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 in the Mekong 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

1140-544: 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. Table 1: Country share of Mekong River Basin (MRB) and water flows By taking into account hydrological regimes, physiography land use, and existing, planned and potential resource developments,

1200-777: 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. In Cambodia, 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 Vietnam) Rivers. 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,

1260-729: The Kingdom of Thailand which is the mechanism for cooperation with regard to riverine trade on the upper stretches of the Mekong. The Mekong basin is one of the richest areas of biodiversity in the world. Only the Amazon boasts a higher level of bio-diversity. Biota estimates for the Greater Mekong Subregion (GMS) include 20,000 plant species, 430 mammals, 1,200 birds, 800 reptiles and amphibians, and an estimated 850 freshwater fish species (excluding euryhaline species mainly found in salt or brackish water, as well as introduced species ). The most species rich orders among

SECTION 20

#1732772824260

1320-630: The Mekong delta. Excavations at Oc Eo , near modern An Giang , have found coins from as far away as the Roman Empire . This was succeeded by the Khmer culture Chenla state 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 front line between the emergent states of Siam and Tonkin (North Vietnam), with Laos and Cambodia, then on

1380-400: The Mekong in the 2000s through the 2020s has caused serious problems for the river's ecosystem , including the exacerbation of drought . The Mekong was originally called Mae Nam Khong from a contracted form of Tai 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

1440-466: The Mekong is divided into six distinct reaches : 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,

1500-820: The Mekong to the east, enter Vietnam shortly after this. In Vietnam, the Bassac is called the Hậu River (Sông Hậu or Hậu Giang); the main, eastern, branch of the Mekong is called the Tiền River or Tiền Giang. In Vietnam, distributaries of the eastern (main, Mekong) branch include the Mỹ Tho River , the Ba Lai River , the Hàm Luông River , and the Cổ Chiên River . The Mekong Basin is frequently divided into two parts:

1560-650: 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 m/s (5,300,000 US gal/s) nine years out of ten, but exceed 34,000 m/s (9,000,000 US gal/s) only one year in ten. Table 3: Mekong Mainstream monthly discharge, 1960–2004 (m/s). There

1620-566: The Thai areas of the lower basin has been the highest of all the lower Mekong countries over the past 60 years. 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

1680-658: 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

1740-469: The capital of Laos, Vientiane , then turns south. A second time, the river leaves the border and flows east into Laos soon passing the city of Pakse . Thereafter, it 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 the Tonlé Sap . When the Mekong is low, the Tonle Sap is a tributary : water flows from

1800-600: The coast, torn between their influence. The first European to encounter the Mekong was the Portuguese António 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 Spanish and Portuguese mounted some missionary and trade expeditions, while the Dutch Gerrit van Wuysthoff led an expedition up

1860-496: The delta advanced 200 km (120 mi) over the continental shelf of the South China Sea, covering an area of more than 62,500 km (24,100 sq mi). From 5.3 to 3.5 ka the delta advanced across a broad embayment formed between higher ground near the Cambodian border and uplands north of Ho Chi Minh City . During this phase of its development the delta was sheltered from the wave action of long-shore currents and

Mekong - Misplaced Pages Continue

1920-451: The delta. For much of its length the Mekong flows through bedrock channels, i.e., channels that are confined or constrained by bedrock or old alluvium in the bed and riverbanks. Geomorphologic features normally associated with the alluvial stretches of mature rivers, such as meanders , oxbow lakes , cut-offs, and extensive floodplains are restricted to a short stretch of the mainstream around Vientiane and downstream of Kratie where

1980-484: The freshwater fish in the river basin are cypriniforms (377 species) and catfish (92 species). Trans-boundary river A transboundary river is a river that crosses at least one political border , either a border within a state or an international boundary. Bangladesh has the highest number of these rivers, with at least 58 major rivers that enter the country from the Republic of India , including two of

2040-539: 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. Major tributary systems develop in the lower basin. These systems can be separated into two groups: tributaries that contribute to

2100-568: The height of riverbeds, thereby causing flooding. International conventions governing water sharing have led to complex political disputes. Ruak River The Ruak originates within the hills of the Daen Lao Range , Shan State ( Burma ), and becomes the boundary river between Thailand and Burma at the confluence with the Mae Sai River near the northernmost point of Thailand. It then meanders eastwards until it empties into

2160-436: 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 are particularly concerned as July is in the wet season, when mainstream flows are abundant historically. Locals are blaming low water on

2220-522: 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 (west) bank. The Bassac River 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

2280-437: 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

2340-453: The lower basin where the borders of Thailand, Laos, China, and Myanmar 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 Vietnam . The upper basin makes up 24% of the total area and contributes 15–20% of

2400-466: 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. 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

2460-408: 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 of northern Laos and is reported to account for as much as 27% of

Mekong - Misplaced Pages Continue

2520-401: 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

2580-559: The newly constructed Xayaburi Dam , as it enters its test phase prior to the start of commercial operation in October 2019. 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

2640-727: The present, however, the contribution from the Three Rivers Area fell to 40%, while that from the Central Highlands rose from 11 to 51%. One of the most striking conclusions of provenance studies is the small contribution of sediment from the other parts of the Mekong basin, notably the Khorat Plateau, the uplands of northern Laos and northern Thailand, and the mountain ranges south of the Three Rivers area. The last glacial period came to an abrupt end about 19,000 years ago (19 ka ) when sea levels rose rapidly, reaching

2700-795: 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. Flows at Chiang Saen entering

2760-495: The river as far as Vientiane in 1641–42. The French invaded the region in the mid-19th century, capturing Saigon in 1861, and establishing a protectorate over Cambodia in 1863. The first systematic European 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 and 1868. Their chief finding

2820-448: The river develops alluvial channels that are free of control exerted by the underlying bedrock. The Mekong basin is not normally considered a seismically active area as much of the basin is underlain by the relatively stable continental block. Nonetheless, the parts of the basin in northern Laos , northern Thailand , Myanmar and China do experience frequent earthquakes and tremors. The magnitude of these earthquakes rarely exceeds 6.5 on

2880-562: The river include: The Mekong rises as the Za Qu ( Tibetan : རྫ་ཆུ་ , Wylie : rDza chu , ZYPY : 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 old name of Lao kingdom Lan Xang ; the characters may also be literally understood as "turbulent green river"). It originates in

2940-463: 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. 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

3000-412: 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. Even so, the river is a major trade route between Tibet and Southeast Asia. The construction of hydroelectric dams along

3060-631: The support of the Office of Weapons Removal and Abatement within the US State Department Bureau of Political-Military Affairs , to conduct underwater explosive removal. The many maps of the river basin produced throughout recorded history reflect the region's changing human geography and politics. In 1995, Laos, Thailand, Cambodia, and Vietnam established the Mekong River Commission (MRC) to manage and coordinate

SECTION 50

#1732772824260

3120-571: 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. 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

3180-513: The total land under rice cultivation. 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. Loss of forest cover in

3240-477: The underlying geological structure is fairly homogeneous and stable, exerting little or no control on river morphology . In marked contrast, the tributary networks of the Salween , Yangtze , and particularly the Mekong, are complex with different sub-basins often exhibiting different, and distinct, drainage patterns. These complex drainage systems have developed in a setting where the underlying geological structure

3300-687: The use and care of the Mekong. In 1996 China and Myanmar became "dialogue partners" of the MRC and the six countries now work together in a cooperative framework. In 2000, the governments of China, Laos, Thailand and Myanmar signed a Agreement on Commercial Navigation on Lancang-Mekong River among the Governments of the People's Republic of China, the Lao People's Democratic Republic, the Union of Myanmar and

3360-407: The wars in Indochina in the 1970s, a significant quantity of explosives (sometimes, entire barges loaded with military ordnance ) sank in the Cambodian section of the Mekong (as well as in the country's other waterways). Besides being a danger for fishermen, unexploded ordnance also creates problems for bridge and irrigation systems construction. As of 2013, Cambodian volunteers are being trained, with

3420-471: 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 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 km (400 sq mi), yet

3480-400: The world's largest rivers, the Brahmaputra and the Ganges . The Naf River is the only river that flows via Bangladesh into Myanmar . The hydrologic and political effects of rivers that cross significant boundaries are enormous. Rivers have positive effects in that they carry a significant amount of sediment, which aids in building land in estuarine regions. However, this sediment raises

3540-414: Was constructed largely through fluvial and tidal processes. At this time the delta was advancing at a rate of 17 to 18 m (56 to 59 ft) per year. After 3.5 ka, however, the delta had built out beyond the embayment and became subject to wave action and marine currents. These deflected deposition south-eastwards in the direction of the Cà Mau Peninsula , which is one of the most recent features of

3600-438: Was that the Mekong had too many falls and rapids to ever be useful for navigation . The river's source was found 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 expelled French from its former colony and defeated US-supported governments. During

#259740