The Mekong

The Mekong River and its associated basin are the mainstay of the region’s rice and fish economy, as well as providing water for domestic, municipal and industrial use. The river is an approximate 4350 kilometers in length, originating high in the Tibetan plateau and passing through six countries: China, Myanmar, Thailand, Laos, Cambodia and Vietnam. It is the seventh longest river in Asia and the 12th longest in the world. The average mean discharge for the Greater Mekong Basin is about 460 km3 of water annually, making it 10th in water volume in the world. Between 60-65 million people live within the Lower Mekong Basin, and around 80 percent are dependent on the river system for their food and livelihoods.

The Mekong basin and associated river network. Distribution coverage by country: China (21 percent), Myanmar (3 percent), Lao People’s Democratic Republic (25 percent), Thailand (23 percent), Cambodia (20 percent) and Vietnam (8 percent). Map created by ODM.

The Greater Mekong River Basin is 795,000 km2 in area, and is commonly divided into two parts: the Upper Basin, primarily in China (where it is called the Lancang), and the Lower Mekong Basin, generally described as south of China from Myanmar all the way to southern Vietnam, where the river empties into the South China Sea. While there are some differences of opinion on how to demarcate the upper and lower basins, the most common division appears to be by national boundaries, with the Upper Basin entirely within China, and the Lower Mekong Basin everything below that. This is consistent with the Mekong River Commission’s descriptions of the Upper and Lower Mekong Basins.

The entire Greater Mekong Basin may be further divided into seven sub-areas based on physical characteristics, including drainage patterns, landform and geology. These seven areas are: the Tibetan Plateau, the Three Rivers Parallel Area, the Lancong Basin, the Northern Highlands, Khorat Plateau, Tonle Sap Basin and Mekong Delta.1

The Upper Basin

The Upper Basin is divided into three areas: the Tibetan Plateau, Three Parallel Rivers Area, and Lancang Basin, all within China.

The Tibetan Plateau or Three Rivers Source: The Mekong, called the Lancang in China, originates high in the glaciers of the Tibetan Plateau, in southern Quinhai province. An area of about 316 km2 drains into the headwaters of the Mekong. Starting from the Jifu mountains, the river flows for about 450 km through Quinhai, and continues through the Tibetan Autonomous Zone.

The Mekong’s headwaters are protected by China’s Sanjiangyuan National and Nature Reserve—an area also known as the “Three Rivers Source” and the “Three Rivers’ Headwaters Natural Reserve Area”, since the Yellow and Yangtze rivers also originate there. The headwaters of the Salween (called the Nu River in China) originate in the Tibetan plateau to the south of the Mekong.

The Three Parallel Rivers Area: The Mekong then flows through the Three Parallel Rivers Protected Area in Yunnan Province; the three parallel rivers being the Mekong/Lancang, the Salween/Nu and Yangtse rivers. The eight composite protected areas form a 1.7 million hectare UNESCO heritage site. This section of the river runs through a deep ravine, with no tributaries joining it.

The Upper Mekong, the Three Parallel Rivers, their basins, and the Sanjiangyuan National Nature Reserve in Yunnan. Map created by ODM.

The Lancang Basin: From here the river flows into the Lancang Basin in lower Yunnan where it transitions from steep gradients to mid and lowland plains, broadening as it goes.

South of the Three Rivers Area, the river continues to drop through highlands and plateaus that are still relatively elevated (2000-3000 meters above sea level), transitioning to mid and lowland reaches and broadening as it goes. Small tributary catchments drain into the river from both sides of the mainstream.

About 50 percent of the sediment necessary for the downstream ecologies enters the river in the Upper Basin. The Upper Basin contributes about 20 percent of the total river flow, while during the dry season it accounts for 45 percent.

Overall, the Upper Basin catchment is long and narrow, characterized by steep inclines. This is advantageous for the development of hydroelectric dams, which capture the power generated by water falling through steep gradients. China has built seven dams on the river and has plans for as many as 21 more. There has been considerable controversy about these dams since they have disrupted the natural flow of water and sediment to the lower Mekong countries.

The Lower Mekong Basin

The Lower Mekong Basin (LMB), stretching from the Chinese border to the South China Sea in southern Vietnam, is a massive 630,000 km2 area, and is critical to maintaining the river’s sensitive ecology, and the ecosystems services that underlie the region’s water and food security.

The interface between the mainstream Mekong, its tributaries, and the native forest habitat, is critical for the regulation of surface water, ground water and sediment. As water flows through the forest, organic matter from the forest floor is also carried to the flood plains, helping to replenish soil fertility. The LMB’s forests are also host to an astonishing range of biodiversity.

The LMB is divided into four areas.

The Northern Highlands: An upland region that includes northeastern Myanmar, northern Thailand, and northern Laos. Tributaries entering the Mekong at this point include the Nam Ta, Nam Ou, Nam Soung, and Nam Khan on the left bank, and the Nam Mae Kok and Nam Mae Ing on the right bank. This region’s steep topography makes it attractive for hydropower dams. The controversial Xayaburi Dam in Laos is in this stretch of the river. Thailand has also conducted feasibility studies for diverting water from the Kok and Ing basins into the Chao Praya basin.

The Khorat Plateau: A low-lying plain, mostly within northeastern Thailand (Isaan) and covering about one third of the country. Major tributaries are the Songkhram and Mun rivers on the right bank, and the Nam Ca Dinh, Se Bang Fai, and Se Bang Hiang Rivers on the left bank. The area previously supported dry deciduous forest, but is now mostly deforested.

This is one of the driest regions in the Lower Mekong. In 2016, the Thai government conducted feasibility studies to increase agriculture production to the arid Isaan region by diverting water through a giant tunnel from the Loei estuary, which drains into the Mekong.2 In February 2016, the Thai government began pumping water from the Huai Luang River—a Mekong tributary in Nong Khai province—with the intent to extract 47 million m3 of water in three months, to provide for irrigation during the drought.3 While that action was temporary, Thailand’s National Water Resources Board also approved a much larger, permanent pumping station that may divert 150 m3 per second from the Mekong River,4 or 10 times the capacity of the pumping carried out in February.5

The Lower Mekong Basin. Map created by ODM.

The Tonle Sap Basin: Beginning in southern Laos just north of Pakse, and includes northern and central Cambodia. This encompasses the 3S Basin. which is made up of major tributaries the Se Kong, Se San, and Sre Pok rivers—which emanate from Vietnam’s central highlands—and the great Tonle Sap Lake, which also drains into the Mekong. About 20 percent of the Mekong’s total sediment flow enters from the 3S Basin. 95 percent of its water flow has joined the mainstream by the time it enters Cambodia.

The Tonle Sap Lake, located in Cambodia’s central floodplain, is the largest lake in Southeast Asia and the most important inland fishery in Cambodia. It is one of the most critical ecosystems in the Lower Mekong Basin due to its role in sustaining vital fisheries, the floodplains where most of Cambodia’s rice is produced, and the Delta below.

The Mekong’s flood pulse is key to this ecosystem. During the dry season, the Tonle Sap Lake drains into the Mekong via the 120 kilometer-long Tonle Sap River. During the wet season, however, the Mekong swells to as much as 30 times its dry season volume, causing the Tonle Sap to reverse the direction of its flow and spill into the floodplain. On average, the Tonle Sap expands from about 2500 km2 to as much as 15,000 km2, and increases in volume from 1.5 km3 (or a depth of about 1 meter) to between 60-70 km3 (a depth of about 10 meters). At the end of the wet season, when the Mekong River flow subsides, the Tonle Sap once again reverses and flows downstream draining the floodplain. This pulse flushes the lake and plain, preventing stagnation and redistributing organic matter and sediment that increases the fertility of the plain, while also creating an exceptionally productive ecosystem for fish. The biodiversity of the lake and floodplain is uniquely adapted to this cycle. In addition, 40 percent of Cambodia’s population depends on the Tonle Sap Lake and floodplains for food security and livelihoods.

Significant changes in the lake’s cycle brought about by alterations in hydrology and water quality—whether upstream or around the lake itself&mdash:could have disastrous affects. Heavy deforestation around the floodplains and chemical pollutants are taking their toll on the habitat. Upstream dams and water extraction, exacerbated by climate change, are also changing the amount and timing of water and sediment entering the sensitive system. Many indicators already point to problems in the ecosystem, and sharp declines have been documented in several large keystone fish species.6

The Mekong Delta: Beginning at Phnom Penh where the Bassac River diverges from the mainstream Mekong and the two rivers fan into a 62,520 km2 alluvial plain covering southern Vietnam from just north of Ho Chi Minh City. The mainstream Mekong crosses the border at Dong Thap province, where it is known as Song Tien or Tien Giang. It flows through Long Xuyen, Sa Dec and Vinh Long and then divides into four distributaries before exiting to the South China Sea. The Bassac, which flows south, enters Vietnam at Chau Doc, where it becomes known as the Sông Hậu or Hậu Giang and passes through the major delta city of Can Tho. It disperses into three distributaries before exiting to the sea in Soc Trang. In Vietnam, the Mekong Delta is known as the Đồng Bằng Sông Cửu Long or ‘Nine Dragon River Delta’; the name is derived from the nine major channels.

The Delta is the most densely populated section of the Mekong.

References

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