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2 - Physical landforms of the Mekong BasinPhysical landforms of 2the Mekong BasinThe Mekong River Basin stretches about 2,600 km from the Tibetan Plateau to the South China Seaand comprises some 795,000 km², ranking it the 21 st largest river basin worldwide. 1 It incorporatesareas of six countries (Table 1), including six broad physiographic regions which are briefly describedbelow (Figure 1).Table 1. Territory of the six Mekong River Basin Countries within the catchmentDescriptionCountry or provinceYunnan Myanmar Lao PDR Thailand Cambodia Viet Nam MekongProvince,River BasinPRCArea (km²) 165,000 24,000 202,000 184,000 155,000 65,000 795,000Catchment as 38 4 97 36 86 20% of countryor provinceCatchment as 21 3 25 23 20 8 100% of MRBTwo primary forces have structured the basin. In the upper basin, tectonic forces arising fromthe northeastward movement of the Indian continental plate pushing against the Asian mainlandhave created mountain ranges and deep valleys. Rivers and erosion from high rainfall is theother major factor and has been especially important in the lower basin. Consequently, thereare three major geological components: the older rocks of the Indian subcontinental plate; theyounger geological strata from the Asian continental block which have been uplifted and folded;and the more recent sedimentary deposits that have filled the depressions and lowlands createdby the uplift and folding.Mainland Southeast Asia is characterised by a series of high mountain ridges and deep valleyswhich extend from the northwest to the southeast (Fig 1). The mountains of the Tibetan Plateauform an eastern extension of the Himalayas, and the deeply incised valleys which fan out from it,form the headwaters of a number of major rivers. These include the Irrawaddy, the Salween, theMekong, the Red and the Yangtze. 27