SUMMARY

The major geological and topographic features of the Mekong River and basin can be easily seen on the topographic maps and satellite images of Southeast Asia. These features include the large scale physical landscapes comprised of mountains, highlands, plateaus, deep narrow river valleys separated by steep and very abrupt ridges, broad low lying almost featureless flood plains and the Mekong Delta. This diversity of physical landscapes owes its origins to the events that have taken place during a long and complex geological history. The topographic diversity of the Mekong basin is reflected in the diversity of ecological systems and the physical and ecological diversity is in turn reflected in the diversity of human use of the Mekong River basin.

The present day physical features of Southeast Asia in general and of the Mekong basin in particular, owe their shape to the uplifting and folding of the earth's crust that was initiated about 50 million years ago by the collision of the Indian and Eurasian tectonic plates. From a geological perspective, mainland Southeast Asia represents a jigsaw puzzle made up of ancient blocks of bedrock, relatively young mountain ranges and more recent alluvial plains. Much of the topography resulted from the Indian subcontinent pushing under the crust of the Eurasian subcontinent. This process that created the Himalayan mountains and the Tibetan Plateau. Toward the south and Southeast, the enormous tectonic forces have displaced and shifted old blocks, uplifted and folded younger geological strata of sedimentary rock, and created depressions and lowlands that now act as traps for collecting the sedimentation deposited during flood time by the present day river network.

All of the tectonic activity resulted in a topographic pattern, in which the mountain ranges resemble ribs of a fan separated by the deep trenches carved by the Irrawaddy, Salween, Mekong, Yangzi and Red Rivers. Interspersed within the fan-shaped pattern are plateaus at differing elevations. For the Mekong basin, these include the Korat Plateau of Thailand and Bolovens Plateau of Lao PDR. Topographically and in terms of elevation, the Mekong River basin can be divided into the upper catchment, the middle reaches and the lowlands and delta.

The source of the Mekong is located on the Tibetan Plateau, or "Roof of the World", at approximately 5,000 meters elevation. The catchment area of the headwaters is generally very narrow; the relief and altitude differences exceed several thousand meters. Further downstream, the "Three Rivers" area of the Salween, Mekong and Yangzi contains extraordinary landforms in very rugged terrain with high, north-south trending mountain ranges, steep slopes, and the deeply cut, closely spaced valley floors. The upper catchment area has considerable potential for hydropower development; some are already realized or under construction in Yunnan, China.

In the middle reaches of the Mekong River basin, highland and plateau areas are most prominent, often rising above 2,000 meters in elevation. Within the middle reaches, the Truong Son Mountains (Annamese Cordillera) forms the topographic 'spine' of Viet Nam. The Kontum Massif, as part of the Cordillera, contains the oldest rocks of Southeast Asia. Some of the hard, resistant rocks form rapids and water falls in the middle and upper reaches of the river, which has an impact on migration of some fish species and create hazards to navigation. Among the plateau areas, the Bolovens Plateau is the highest at about 1,500 meters and is composed of hard volcanic rock. The Korat Plateau is the lowest at about 100 meters and largest. It is composed of sediments and eroded bedrock. Low mountain ranges form a distinct rim around the Korat Plateau.

The lower reaches of the Mekong include the Great Cambodian Plain and the Delta region. These are young geological 'traps' containing the sediments of the Mekong and those of its tributaries. The very flat and low-lying terrain of the Plain has resulted in frequent course changes of the Mekong over time. Annual flooding during the monsoon season occupies the floodplain, including extensive back-swamp areas. Flooding also causes the famous reversal of the flow of the Tonle Sap River back into the Great Lake , filling it like a bladder and increasing its surface area enormously while remaining shallow.

The present delta of the Mekong is located south of the "Four Arms" area at Phnom Penh. On its way to the sea, the various branches of the mighty river are contained by natural embankments, or levees. These are only partially flooded during the rainy season, in contrast to the adjacent lowlands where numerous rice paddies cover the rich alluvial soils. Coastal currents in the South China Sea shape the coast line and constantly redistribute the rich sediment load of the Mekong, particularly around the southern tip of Viet Nam.

What physical-geological impacts are going to shape the Mekong region in the future? Lowland areas are likely going to be affected in a serious way by salt water intrusion and sea level rise as a result of climate change. Over long periods of geologic time, volcanic activity has been rare throughout the highlands of the Mekong Basin, but the region will continue to experience earthquakes resulting from the shear stress of tectonic movements, particularly in the middle reaches of the Mekong. On the time scales of geologic history, the enormous erosion powers of the rivers has captured neighbouring watersheds, which has led to either enlarging or decimating the catchment area of the Mekong.

Today although certainly not on the grand scale of geological history, this capture and diversion of rivers can be accomplished by the technology of dam building and water diversion schemes. Consequently, understanding the geology and topographic features of the Mekong basin provides a foundation for understanding the Mekong River as a physical and ecological system and for placing the issues of river development and management into a more realistic framework.