Physical features

The Mekong River drains more than 313,000 square miles (810,600 square kilometers) of land, stretching from the Plateau of Tibet to the South China Sea. Among Asian rivers, only the Yangtze and Ganges have larger minimum flows. The Mekong River's narrow width in the first 1,215 miles of its course, and the contrast between the physical conditions that prevail above and below the reach where it flows down off the Yunnan highlands, divide it into two major parts.

The upper Mekong is a long, narrow valley comprising roughly one-fourth of the total area, cutting through the mountains and plateaus of China. The lower Mekong, below the point where it forms the border between Myanmar and Laos, is a stream 1,485 miles in length that claims the drainage from the Khorat Plateau of Thailand, from the westward slopes of the Annamese Cordillera (Chaîne Annamitique) in Laos and Vietnam, and from most of Cambodia, before reaching the sea through the distributary channels of its delta in Vietnam.

In its upper reaches the Mekong is one of the cluster of great streams rising in the plateau between the Salween and Yangtze rivers; the streambed has cut deeply into the rugged landscape through which it flows. Between Myanmar and Laos, the Mekong drains about 8,000 square miles of Myanmar territory, all of which consists of rough and relatively inaccessible terrain. In its more gentle lower stretches, where for a considerable distance it constitutes the boundary between Laos and Thailand, the Mekong forms a subject of both friction and cooperation among the four countries of Cambodia, Laos, Thailand, and Vietnam.

Physiography

The upper sources, known locally as the Ang and Cha streams, rise at elevations of more than 16,000 feet (4,900 metres) in the T'ang-ku-la Mountains on the southern border of Tsinghai province. They flow southeasterly through the Ch'ang-tu (Chamdo) region of Tibet. The main stream, called the Lan-ts'ang River, descends in a southerly direction across the highlands of Yunnan, which are cut by erosion into hills and valleys, to a point south of Ching-hung, where it becomes the border between Myanmar and China. The river then moves in a southwesterly direction; over a reach of more than 125 miles it forms the Myanmar-Laotian border. Although two great roads cross it--the caravan route from the southeast to Lhasa and the road from K'un-ming to Myanmar--much of the river valley in the highlands of Tibet and Yunnan is extremely inaccessible and sparsely populated.

Below Myanmar, the river basin may be divided into six major sections--the northern highlands, Khorat Plateau, eastern highlands, southern lowlands, southern highlands, and delta. All these sections have somewhat similar landforms, vegetation, and soils. 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 above sea level, many with steep slopes. As far south as the latitude of 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 inroads made by shifting cultivation.

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 traversed by relatively flat alluvial plains. Soils and deciduous vegetation on the uplands are thin, and much of the original forest has been replaced by grassland as a result of grazing and repeated burning.

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 Vietnam and Krâchéh (Kratie) in Cambodia, the watershed forms the border between Vietnam to the east and Laos and Cambodia 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 Khorat Plateau or other lowlands. Forest degradation, which has resulted from lumbering, the temporary use of land for cultivation, and grazing, is widespread.

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 Tonle Sap, sometimes called the Great Lake. The direction of flow of the Sab River varies according to the season. In the height of the flood season, when the level of the Mekong is high, waters flow through the Sab River to the lake, which at this time increases its area from a little more than 1,000 square miles to a maximum of about 4,000 square miles. In the dry season when the floods subside, the Sab reverses its flow to drain southeastward into the Mekong. Tonle Sap is a highly productive fishing ground.

The Dâmrei (Elephant) and Krâvanh (Cardamom) mountains in southwestern Cambodia constitute the southern highlands. Several tributaries of the Sab rise in these uplands.

The river divides into two branches below Phnom Penh: the Mekong proper and the Bassac. From this point onward the delta spreads out to the sea. The delta, which has a total area of about 25,000 square miles, has 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 Cà Mau peninsula is forested and swampy.

Climate and hydrology

The Mekong's flow comes chiefly from rainfall in its lower basin and is a function of the variation in seasonal rainfall caused by the monsoon winds. This variation generally forms a regular annual pattern. In April the flow is ordinarily at its lowest. In May or June--as the rain-bearing monsoon moves in from the south--the flow begins to increase, doing so most rapidly in the eastern and northern highlands. The highest water levels are reached as early as August or September in the upper reaches and as late as October in the southern reaches. The northeasterly monsoon wind, beginning ordinarily in November in the southern areas, brings dry weather until May.

Temperatures in the lower Mekong basin are uniformly warm throughout the year. Daily highs at Phnom Penh average 89º F (32º C), and lows average 74º F (23º C). In the upper basin, temperatures are moderated somewhat by altitude and generally are lower and exhibit more seasonal variation than those found farther south.

The mean annual flow of the river at Krâchéh in Cambodia is about 500,000 cubic feet (14,200 cubic metres) per second, which is about twice the flow of the Columbia River in North America. The recorded minimum at Krâchéh is about one-twelfth of the mean, and the annual peak flow about four times the mean. Below Krâchéh the peak flows diminish as the water spreads out into the distributary channels and backswamps.

The annual sediment load is recorded as being highest at Pakxé, where it amounts to some 187 million tons; it is about half that amount at the Myanmar border and about two-thirds that at Phnom Penh. The dominant hydrologic fact affecting agriculture is the long dry period in which rice cultivation is impossible without irrigation.

The people

The inhabitants of the lower basin constitute about one-third of the population of the four riparian countries. Nearly all are engaged in agriculture, and rice is the major crop. Heaviest population concentrations are in the delta and on the Khorat Plateau. The small urban population has been growing rapidly, chiefly through migration to the capital cities. There is no common ethnic tie among the basin populations. Ethnic groups range from Sino-Tibetan, including Karen and Miao (Meo) mountain groups, in the upper Mekong region to Khmer, Cham, Tai, Mon, and Vietnamese lowland groups in the lower Mekong basin. The Vietnamese are heavily concentrated in the delta, and the Khmer and Tai are the most widely distributed in the lower basin.

The economy

Irrigation and flood control

In the lower basin the management of water offers major opportunities to increase the economic productivity of the tributary lands. Farmers practicing shifting cultivation on the uplands and the rice growers on the rain-fed lowlands are able, under normal conditions, to grow only one crop a year, taking advantage of wet-season precipitation. Half of the cultivated land is dependent upon some form of inundation by flood flows. Control of water, however, makes it possible to store water during the dry season and thus permits the harvesting of a second or third crop. In addition, irrigation combined with flood control has improved the cultivable land by reducing the losses and delays caused by floodwaters pouring over the river's banks. Where storage facilities and the degree of downward slope are favourable, hydroelectric power has been developed.

Much of this development work has been undertaken under the auspices of the Committee for Coordination of Investigations of the Lower Mekong Basin (Mekong Committee), organized in 1957 by Cambodia, Laos, Thailand, and South Vietnam (since 1978 consisting of Laos, Thailand, and Vietnam). The commission has sponsored a series of preinvestment and general scientific investigations and has undertaken construction of a number of water projects. These projects include the multipurpose dam near Nam Phong in northeastern Thailand and the hydroelectric dam near Nam Ngum in Laos. The countries of the commission have continued to cooperate despite the political stresses produced by the war in Vietnam and its aftermath and have enlisted the assistance of other countries and international organizations.

Navigation

There is an elaborate system of canals in the Vietnamese part of the delta. Smaller seagoing vessels can sail upstream as far as Phnom Penh, and vessels drawing almost 15 feet can reach Kâmpóng Cham during high water. Continuous water transport is blocked chiefly by the barriers of the Khone Falls and other falls between Sâmbor and Pakxé, and upstream uses of the river are limited to local traffic. Navigational conditions on the Mekong's main stream and on some of its tributaries also have been improved through the activities of the Mekong Committee.

Study and exploration

A rich literature describing the upper and lower Mekong basins has existed for some time, but until the 1950s the river's resources were treated chiefly in local studies of navigational access to urban areas. Following the organization of the Mekong Committee, information on the river was consolidated, and in the late 1960s a bibliography and atlas were published. In the 1950s, surveys of the lower Mekong were carried out by the Bureau of Flood Control of the United Nations Economic Commission for Asia and the Far East (later renamed the Economic and Social Commission for Asia and the Pacific) and the U.S. Bureau of Reclamation, and in the early 1960s a new program for integrated study took shape under the Mekong Committee.

Investigations undertaken have included basic mapping, hydrologic observations, flood forecasting, soil surveys, fisheries studies, health studies, engineering-feasibility studies, power-market surveys, and agricultural research and pilot farms. The engineering studies have provided reconnaissance appraisal of all the tributary basins and a more detailed examination of selected projects. The focus of Mekong development as a whole has shifted since the mid-1970s to planning comprehensive programs for agricultural and community development in areas where water supply was available, with each country working out its individual financial arrangements