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 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 is navigable in short sections. 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, intends to pursue similar joint projects and to explore the establishment of fisheries and the construction of a trans-Indochinese roadway system. 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.

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