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Mekong (māˈkŏng, mēˈ–), Chinese Lancang, one of the great rivers of SE Asia, c.2,600 mi (4,180 km) long. From its marshy source (definitively identified in 1994) on the Rup-sa Pass in the highlands of Tibet, it rises as the Za Qu (Dza Chu) and flows generally S through Yunnan prov. in deep gorges and over rapids. Leaving Yunnan, the Mekong forms the Myanmar-Laos border, then curves E and S through NW Laos before marking part of the Laos-Thailand border.

From SW Laos the river descends onto the Cambodian plain, where it receives water from Tônlé Sap during the dry season by way of the Tônlé Sap River; during the rainy season, however, the floodwaters of the Mekong reverse the direction of the Tônlé Sap River and flow into Tônlé Sap, a lake that is a natural reservoir. The Mekong River finally flows into the South China Sea through many distributaries in the vast Mekong delta (c.75,000 sq mi/194,250 sq km), which occupies SE Cambodia and S Vietnam. The delta, crisscrossed by many channels and canals, is one of the greatest rice-growing areas of Asia. It is a densely populated region; Vinh Long, Can Tho, and Long Xuyen are the chief towns there. Ho Chi Minh City (Saigon) is located just east of the delta. The Mekong delta was the scene of heavy fighting in the Vietnam War. The river's watershed is the world's largest inland fishery.

The Mekong River is navigable for large vessels c.340 mi (550 km) upstream; Phnom Penh is a major port. North of the Cambodian border, the Mekong was navigable in short sections, but dams and other developments have increased the reach of river traffic in the 21st cent. At Khone Falls, a series of rapids (6 mi/9.7 km long) in S Laos, the Mekong drops 72 ft (22 m). The falls are the site of a hydroelectric power station, part of the Mekong Scheme, a project undertaken by the United Nations in the early 1960s to develop the potentials of the lower Mekong basin. The project sought to improve navigation, provide irrigation facilities, and produce hydroelectricity.

The Mekong River Commission, whose members consist of Cambodia, Thailand, Laos, and Vietnam, have agreed to pursue sustainable hydropower and irrigation projects and to explore the establishment of fisheries and the construction of a trans-Indochinese roadway system. Laos in particular is undertaking an extensive hydroelectric development of the river and its tributaries, with dozens of dams completed, under construction, or planned. China is developing the upper Mekong, constructing a series of dams to provide hydroelectric power and a navigable waterway; the first, at Manwan, was completed in 1993, and ten more had been built by the late 2010s. In 2010 a report to the commission recommended that no dams that span the full breadth of the river channel be built on the lower Mekong for 10 years because of projected losses to fisheries and other environmental damage that could worsen poverty despite the revenues from producing hydroelectricity. Nonetheless, several mainstream dams are under construction or planned in Laos and Cambodia, though Cambodia put its plans on hold in 2020. Two dams in Laos began producing electricty in 2019. The Lancang-Mekong Cooperation, which was established in 2015, includes Cambodia, China, Laos, Myanmar, Thailand, and Vietnam and is officially committed to regional cooperation for sustainable development, but its impact on the river's health and the people who depend on the river is unclear.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a river in China, Laos, Campuchia (Cambodia) and Vietnam that partially forms the border between Laos and Burma and Thailand; the largest river in Indochina. Length, approximately 4,500 km; basin area, 810,000 sq km.

The Mekong originates in the Tangkula Mountains in the Tibetan highlands. The upper course of the river is called the Dzi Chu; the middle course (in China), the Lantsan Chiang. The river flows into the South China Sea, forming a delta. The Mekong’s main tributaries are, on the right, the Mun and Tonle Sap; on the left, the Wu, Theun, and Sen.

In its upper and middle courses, the Mekong flows mainly through deep gorges, and its channel has many rapids. One of the world’s largest waterfalls, Khone Falls (elevation to 21 m), is situated where the river flows into the plain. The rapids end near the city of Kratie in Campuchia. Below the city of Phnom Penh, the Mekong Delta begins (area, approximately 70,000 sq km); the river divides into two large streams linked by numerous channels. Approximately 75 percent of the Mekong’s total flow passes through the main left stream, the Bassac. The delta projects into the sea in the shape of an arc with a coastline approximately 600 km in length; as a result of the river’s abundant regular flow, the delta’s size increases by 80–100 m a year. The delta is marshy and in many places is covered with mangrove thickets.

The Mekong is fed primarily by rain, and in the upper course also by snow and partly by glaciers. High water occurs in summer and fall. In the river’s middle course, maximum water level is usually recorded in August (in the lower reaches, in October); the lowest flow occurs in April. The fluctuation in water level reaches 10–15 m in the mountains, and up to 10 m on the plain and in the delta. The mean annual flow rate in the middle course, at Vientiane, is 4,600 cu m per sec; the maximum annual flow rate is approximately 21,000 cu m per sec. In the lower course, at Kratie, the mean annual flow rate is 14,800 cu m per sec, with a maximum annual flow rate of 33,000 cu m per sec (some years it reaches 67,000 cu m per sec). The Mekong freezes only in the upper course for 1–2 months. It carries an average of 1.5 cu km of drift a year. Tonle Sap, a lake that is connected with the river by the Tonle Sap tributary, exerts a great regulatory influence on the river’s flow. In the humid season (June-November) the lake fills up with water from the Mekong; in the dry season (November-June), the water level of the river falls lower than that of the lake, which results in an intensive overflow of lake water into the Mekong channel, uncovering vast areas of farmland fertilized with productive silt.

The lower Mekong is used for irrigation. Its extensive flooding is conducive to rice growing. The Mekong’s huge hydraulic power resources (estimated at approximately 75 million kilo-watts) are not much used. The rivers and lakes in the Mekong basin are rich in fish, primarily of the carp family; there are many waterfowl, and crocodiles are still common. The river is navigable year round for 700 km, and for 1,600 km (to Vientiane) during high water. Ocean vessels can reach Phnom Penh (350 km). The cities of Luang Prabang, Vientiane (Laos), and Phnom Penh (Campuchia) are on the Mekong.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


a river in SE Asia, rising in SW China in Qinghai province: flows southeast forming the border between Laos and Myanmar, and part of the border between Laos and Thailand, then continues south across Cambodia and Vietnam to the South China Sea by an extensive delta, one of the greatest rice-growing areas in Asia. Length: about 4025 km (2500 miles)
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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