Mekong māˈkŏng, mēˈ– [key], 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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