1.2 BASIC TOPOGRAPHIC AND GEOLOGIC FEATURES

• The present-day landscapes of the Mekong River basin are very diverse, ranging from the rugged mountains, deep valleys and plateaus at high elevations, down to the broad low-lying flat river valleys, coastal plains and river deltas of the lower Mekong basin and along the South China Sea.

• The geological and topographic features for both the Mekong River Basin and the Southeast Asian mainland have resulted from a long and complex geological history that included the plate tectonic movements resulting from the "collision" of India and Asia about 50 million years ago and led to the patterns of mountain ranges, plateaus and other geologic and topographic features of today.

• These tectonic movements created the fan-shaped topography of Southeast Asia, where the mountain ranges resemble the ribs of a fan separated by the deep valleys carved by the Irrawaddy, Salween, Mekong, Yangtze and Red Rivers.

• Interspersed into this fan-shaped pattern are plateaus at various altitudes. Some of the most distinct plateaus in the vicinity of the Mekong River basin are the Korat Plateau of Thailand, and the Bolovens Plateau of Lao PDR.

Mainland Southeast Asia represents a geological jigsaw puzzle with the pieces made up of ancient blocks of geological land surfaces or 'terranes', relatively young mountain ranges and recent alluvial plains. Much of the present-day topography is a result of the 'under-thrusting' of the Indian subcontinent under the crust of the Asian continent which created the Himalayas and uplifted the Tibetan plateau.

In the middle reaches of the Mekong basin, highland areas have been affected by earthquakes resulting from the shear stress of tectonic movements. Toward the south and southeast, tectonic forces have displaced and shifted the ancient blocks, uplifted and folded younger geological strata of sedimentary rock, and created depressions and lowlands that now act as traps for sedimentation by the present-day river network. The enormous river erosion and deposition process of today's Mekong River has been operating over a long period of time, leading to stream capture of neighbouring watersheds, which has resulted in either enlargement or reduction of the catchment area of the Mekong.

The Mekong River Basin is part of the Southeast Asian landmass. The continental portion of the Mekong basin's landscape has many distinct topographic features extending from the highest elevations of the Himalayas and the Tibetan Plateau, south and east to the shores of the South China Sea and the Gulf of Thailand. The present day features of the basin's landscape were created to a large extent by the massive tectonic movements of three major portions of the Earth's crust; the Eurasian, Indian-Australian, and Pacific plates. The Mekong basin, as part of this diverse Southeast Asian landmass, has been subjected over long periods of time to geologic activities such as faulting, folding, uplifting, and volcanic activity. In more recent geologic time the landscapes of the Mekong basin were strongly influenced by erosion and deposition and other climatic and river related processes.

Topography of the mainland portion of Southeast Asia This digital elevation model provides a colour-coded representation of topographic features. Lowland areas appear in green; hilly and mountainous terrain appears in yellow; and high-altitude and alpine terrains appear in brown and light gray, respectively. Source: NOAA Globe Project [www.ngdc.noaa.gov]

Geological evolution and tectonics of East and Southeast Asia This tectonic model of Asia shows the directional movement of tectonic plates (yellow arrows); major plate margins and subduction zones (red lines with arrowsheads); and major tectonic fault systems (black lines). The latter two are potential zones of earthquakes. The area of the Mekong basin generally experiences a south-eastward movement as a result of the Indian subcontinent's collision with Asia. credit: Tapponnier

The most important geologic process that is responsible for creating the major geologic features of Southeast Asia was the collision between the tectonic plates of the Indian subcontinent and Eurasia. This process that began about 50 million years ago and continues to the present day.

Although the mainland as a whole is similar in a geological sense, its specific geologic and topographic features and the time periods of mountain-building (orogenic) activity differ. The mainland portion of Southeast Asia is characterized by a series of generally north-south-trending mountain ranges separated by a number of major river valleys and their associated deltas. In many ways these ranges resemble ribs in a fan, that are separated by deep valleys carved by the process of river erosin. Mainland Southeast Asia is characterized by a series of generally north-south-trending mountain ranges separated by several major river valleys and their associated deltas. Three primary ranges fan out southward from the southeastern corner of the Plateau of Tibet, where they are tightly spaced. A major, S-shaped branch of this system extends through the entire western margin of Myanmar.

Another major mountain system extends along a straight north-south axis from eastern Myanmar east of the Salween River through northwestern Thailand toward the Malay Peninsula. It consists of a series of elongated blocks that were pushed upward from tectonic activity, rather than one continuous mountain ridge. The altitudes of the mountains are more than 2,500 metres near the Chinese border in the north and they decrease to below 1,200 metres through the Malayan Peninsula in the south. Much of the catchment area of the Mekong River belongs to this system.

The easternmost chain of mountains on the mainland is the Truong Son Mountains in Lao PDR and Viet Nam. This mountain chain forms several nearly straight ranges that trend northwest to southeast. Their eastern slopes rise abruptly from the South China Sea while more gradual slopes prevail toward the Mekong basin in the west.

The fan-shaped pattern of the three major mountain ranges is interrupted in several areas by ancient blocks that were pushed upward by tectonic forces along fault lines or by large flows of lava. Today, these geologically very old massifs form the two prominent plateaus found in the region adjacent to the Mekong basin. The largest by far, is the Korat Plateau in eastern Thailand and west-central Lao PDR. The plateau itself is only 100 meters above the floodplains of the surrounding rivers, but the hills along its perimeter range from 200 to 700 metres. The main drainage for the Korat plateau region is eastward through the Mun River tributary of the Mekong.

In contrast, the Bolovens Plateau of southern Lao PDR more than 1,500 metre above sea level. It is composed of basaltic rock formed by great lava flows when tectonic movement stressed the Earth's crust more than a million years ago. The Bolovens plateau drains south west through the Se Done River and other smaller tributaries of the Mekong.

From the perspective of the regions geological history, the river related lowland areas comprise the youngest of the large scale physiographic features of mainland Southeast Asia. There are two primary lowlands environments. The first includes the broad river valleys that extend seaward from the northern uplands. The second lowland feature so characteristic of Southeast Asia coastal landscapes is the vast deltas that form at the mouth of the Mekong and other large river systems. The alluvial sediments found in the broad river valleys and deltas allow intense cultivation and support dense human population. The largest coastal lowland is that of the Mekong River. The area encompasses most of Cambodia and southern Viet Nam.

When the geographic sectors of the Mekong River from the delta to the Tibetan Plateau are considered, it becomes possible to gain an insight into the incredible complexity of the river's physical environment. It is this environment that supports the Mekong basin's biological diversity as well as the cultures and economies of the Mekong basin. The more specific details of the physical environments of the Mekong River and its basin can be better understood when divided into three geographical sectors:

• The upper catchment area
• The middle reaches
• The lower reaches and delta

The Mekong River: an introductory short video. Click on the image to view the video clip