Rivers of Southeast Asia

Southeast Asia, in general, is a subcontinent with surplus water, as evidenced by the formerly widespread tropical rainforests. Most of the region receives at least 2000 mm of rainfall annually, and a positive water balance prevails for the majority of months. Four very large rivers (the Irrawaddy, Salween, Mekong, and Sông Hóng or Red) originate close to each other on the eastern Tibetan Plateau north of the region, and flow through large structure-guided valleys towards the southeast like outstretched fingers. Other major rivers of the region (Chao Phraya, Pahang, Brantas, Mahakam, etc.) start and end within Southeast Asia. The upland slopes are drained by a large number of tributaries, and short, wide estuaries wind through coastal plains. Table 4.1 lists selective physical dimensions of the large rivers of Southeast Asia. Except the Mekong, a part of whose discharge consists of seasonal snowmelt from the Tibetan Plateau, the rivers are rain-fed; and the majority tend to show a seasonal pattern of discharge corresponding to either the southwestern or the northeastern monsoon, depending on the location. The wide riverine lowlands of the previous chapter are structural depressions, filled in mostly by the alluvium of the major rivers that occupy them. The Irrawaddy and its main tributary, the Chindwin, flow through the Central Myanmar Lowland. The channel of the Chao Phraya is located within the Central Plain of Thailand. Further to the east, the Mekong has filled the eponymous lowland. The Salween, in contrast, flows almost entirely in 1000 m gorges cut into plateaux and mountains. The Sông Hóng flows in a narrow valley except for the last 250 km from the coast, where it traverses the coastal plain of north Viet Nam. The present coastline of Southeast Asia, however, is a temporary pause in the geological evolution of the drainage system, and as described in Chapter 2, only appeared in the Holocene. The rivers of the ice age Pleistocene used to continue further. What now are individual major streams in many instances used to be parts of the channel network of a much larger system.