The sources of the Mekong, including its principal headstream, the Za Qu River, rise at an elevation of more than 16,000 feet (4,900 metres) on the north slope of the Tanggula Mountains in Qinghai province. They flow southeast through the Qamdo (Chamdo) region of Tibet, where the Za Qu joins other headstreams to form the main stream, called the Lancang in Chinese. It descends south across the highlands of Yunnan, through which it carves a deep valley, to a point south of Jinghong, where it briefly marks the border between Myanmar and China. The river then bends southwest; over a reach of more than 125 miles (200 km) it forms the Myanmarese-Laotian border. Although two great roads cross it—the caravan route from the southeast to Lhasa and the road from Kunming to Myanmar—much of the river valley in the highlands of Tibet and Yunnan is remote and sparsely populated.
The Mekong River
Below Myanmar, the river basin may be divided into six major sections on the basis of landforms, vegetation, and soils: the northern highlands, Khorat Plateau, eastern highlands, southern lowlands, southern highlands, and delta. Most of the vegetation in the lower basin is of the tropical broad-leaved variety, although the occurrence of individual species varies with latitude and topography.
The northern highlands have highly folded ranges that reach elevations of about 9,000 feet (2,700 metres) above sea level, many with steep slopes. As far south as Vientiane, these dissected uplands (i.e., cut by erosion into hills and valleys) are covered with dense deciduous forest that has deteriorated as a result of frequent cutting and burning for shifting cultivation. The Mekong’s important tributaries in this region include the Tha, the Ou, and the Ngum rivers, all draining northern Laos.
To the south of the east-west course of the river below Vientiane lies the Khorat Plateau, which embraces almost all of the Thai portion of the basin as well as the lower parts of the Mekong’s Laotian tributaries. This is an area of gently rolling hills set amid relatively flat alluvial plains. Soils and deciduous vegetation on the hills are thin, and much of the original forest has been replaced by grassland as a result of grazing and repeated burning. The Songkhram River drains the northern part of the plateau and enters the Mekong above Tha Uthen in Thailand. The Mun River—one of the Mekong’s most important tributaries—drains the majority of the plateau and joins the Mekong at Ban Dan, Thailand.
The eastern highlands form part of the Annamese Cordillera, from which streams drain west into the Mekong. Throughout most of the distance between Ky Son (Muong Sen) in northern Vietnam and Ban Hèt in southern Laos, the watershed forms the border between Vietnam to the east and Laos to the west. There is greater relief in the northern than the southern parts of the watershed, but the highlands in general are characterized by rapid streams that flow through narrow valleys before entering the lowlands bordering the Mekong. The Mekong’s most important tributaries in this region are the Kading, the Bangfai, the Banghiang, and the Kong—which, with its affluent the San, drains a large area of southern Laos, central Vietnam, and eastern Cambodia. Forest degradation, which has resulted from lumbering, shifting cultivation, and grazing, is widespread in this region.
The southern lowlands border both sides of the Mekong below Pakxé (Pakse) in Laos. The Mekong enters Cambodia with a sudden plunge at Khone Falls. Between the falls and Krâchéh there are rapids interspersed with alluvial plains. Below Kâmpóng Cham the river’s gradient becomes gentle, and it flows through wide stretches of alluvium in its floodplain. Near Phnom Penh a junction occurs between the Mekong and the Sab River, which connects it to the Tonle Sap, sometimes called the Great Lake. The direction of flow of the Sab River varies according to the season. During the peak flood season, when the level of the Mekong is high, waters flow through the Sab River to the lake, which then expands from a little more than 1,000 square miles (2,600 square km) to a maximum of about 4,000 square miles (10,400 square km). In the dry season when the floods subside, the Sab reverses its flow to drain southeastward into the Mekong. The Tonle Sap is a highly productive fishing ground.
The Dâmrei (Elephant) and Krâvanh (Cardamom) mountains in southwestern Cambodia constitute the southern highlands. Several streams flow from these uplands into the Tonle Sap.
The river divides into two branches below Phnom Penh: the Mekong proper and the Bassac (Basak). Below this point the delta spreads out to the sea. It has a total area of about 25,000 square miles (65,000 square km) and can be divided into three major sections. The upper section, above Chau Doc (Chau Phu), has strong natural levees (embankments built on either side of the river by accumulated deposits of silt) behind which are low, wide depressions. The middle section has some areas that are well drained, others that are poorly drained and swampy. Along the lower section, formed by the river mouths and by the area to the southwest, sediment carried down from the upper river is in the process of being deposited, and the flooding is less extreme than in the upper sections of the delta. The area north of the Ca Mau Peninsula is forested and swampy.