Southeast Asian Deltas

Deltas and estuaries are actively evolving suites of landforms formed where rivers meet the sea. Deltas are characteristically subaerial (and subaqueous) sediment wedges that protrude from the shoreline, whereas estuaries are typically tidally influenced lower parts of rivers in which the shoreline recedes inland. However, the individual distributaries of deltas, which may themselves be cuspate, exhibit estuarine characteristics, and it is convenient to use the term ‘deltaic–estuarine’ to describe river mouth tidal and alluvial plains. There are extensive low-lying coastal and deltaic–estuarine plains throughout Southeast Asia. These represent productive and relatively easily settled land, which has led to clearance of the natural vegetation of many of these plains for agriculture, silviculture, or settlement. Deltaic–estuarine plains are geologically young, responding to Late Quaternary sea-level and climatic fluctuations, and actively undergoing change in the modern landscape. Most have adopted their present form only in the past few thousand years, and are still active centres of deposition. Worldwide expansion of deltas occurred in the early to mid-Holocene as a result of deceleration of postglacial sea-level rise and the coincidence of sea level with extensive low-gradient shorelines (Stanley and Warne 1994). The formation of deltaic–estuarine plains in semi-arid areas may have been a catalyst for the appearance of civilizations based upon cultivation (Stanley and Warne 1993). Deltas in Southeast Asia, however, presented major challenges to pre-technical societies, as a result of their propensity to flood, poor access across the many bifurcating channels, and malaria, and were slower to be colonized (Büdel 1966). However, they have subsequently become important areas supporting large populations, particularly as a result of successful management of inundation for the cultivation of rice (van de Goor 1966). Overbank flooding is a prominent feature of most deltas and assures nutrient re-enrichment of fertile, but immature, soils supporting intensive farming. On the other hand, such flooding can also represent a major hazard, damaging property and in some cases resulting in loss of life. It is often controlled, or control over the extent of flooding is sought through engineering works.