The Mekong river: Morphology, evolution and palaeoenvironment

Abstract

The Mekong, the largest river in Southeast Asia, shows striking variations in channel form and behaviour along its course. It appears to link several different types of channel rather than continuing in an orderly progression. Eight different river units have been identified along the lower 2000+ km of the river from the Chinese border to the sea across Southeast Asia. The exercise was carried out using satellite images, field visits, topographical maps, and various publications of the Mekong River Commission including large-scale river maps. The boundaries between such units are sharp and not gradual. They are different from each other depending on their morphological characteristics, their behaviour and whether the channel is bedrock-controlled. The Mekong flows for nearly 4000 km before being a freely-moving river with overbank flooding and course changes across the wide plain of Cambodia. Prior to that the channel is essentially structure-guided. It is a seasonal river with high flows during the southwestern monsoon and periodic floods late in the wet season. This linkage of several identifiable river units and the location of the present course of the river have not been properly explained. The estimated stream power of the Mekong in large floods seems to be very low when compared to what is required to erode a large channel in rock even along geological lineations. The opening of the South China Sea and the lowlands of Southeast Asia are associated with extrusion tectonics resulting from the Indian Plate colliding with the Eurasian Plate and building of the Himalaya Mountains from Eocene onwards The present course of the river is likely to have been determined at least primarily from this event, and possibly also associated with stronger monsoon systems in the Early Holocene as recognised for South Asia. Morphology comparable to that of the Mekong seems to exist in several other major rivers of Southeast Asia and there could be a regional pattern, but investigation on this topic has hardly started. © Geol. Soc. India.