Geography and the Environment in Southeast Asia: Proceedings of the Geology Jubilee Symposium, The University of Hong Kong, 21-25 June 1976

15. The Environmental Impact of some Dams In Southeast Asia

XV THE ENVIRONMENTAL IMPACT OF SOME DAMS IN SOUTHEAST ASIA Prachoom Chomchai Rapid technological development and man's attempt to control nature can cause changes in the natural environment. The advent of manmade lakc.s as a result of construction of dams to harness water resources 1S known to cause such beneficial environmental changes as the development of hydro-electric power, of a more reliable water supply, and of fishing, transport and recreation. On the other hand, the consequential changes in the environment may be harmful in that they may a1ter natural beauty, remove extensive areas from agriculture or lumber productlon, wipe out wildlife habitats, lead to public health problems and result in a net economic and social loss to the country as a whole. With growing scarcity of natural resources a11 over the wor1d, it is imperative that they are used and preserved in a rational manner. Thus, once the nature and magnitude of thc resultant environmental problems are known, remedial measures may be introduced and preventive or corrective features may be built into future water-resources proJects. Unfortunately, few tropical reservoirs have been thoroughly studied from the point of view of their environmental impact. Existíng studies,apart from those in Southeast Asia, are confmed to Africa and South America, namely, the Vo\ta Lake in Ghana (Obeng, 1973 , Freeman, 1974), Lake Kariba in Rhodesia (Balon and Cocl嗨， 1974) and the Brokopondo Reservoir in Surinam (Leentva缸， 1973). These studies have identified problems that may be characteristic of man-made lakes within the tropics. Prominent among these are increased populations of disease vectors, poor water qua1ity caused by decaying vegetation, invasion of troublesome macrophytes and difficulties in developing a stable fishery. The initial water qua1ity and productivity of a man-made lake are closely tied to the productivity of the terrestria1 environment and to whether or not vegetation was cleared prior to flooding. The nutrient-poor lateritic 218 PRACHOOM CHOMCHA1 soils of the tropics would not yield nutrient inputs to a lake via tributaries or direct runoff. Lakes lying in a watershed with nuwent-poor soils would thus be expected to be less productive than lakes lying in nutrient-rich soils. These clear relationships are initia1ly observed in the dlSruption of the riverine ecosystem by 加poundment and in the subsequent decay and release of nutrients from the drowned vegetation. Possibly two of the more extensively studied reservoirs of Southeast Asia are those of the Nam Ngum dam, Laos, and the Nam Pong dam, Thailand. NAMNGUMDAM Phase 1 of the Nam Ngum dam was completed in 1972, though impoun也nent had begun in 1971. The resulting reservoir flooded an area of 324 squ缸e kilometres, with the water level varying from 209.2 metres in September 1973 to 201.5 metres in April 1974: the average depth of the reservoir is about 60 metres. Phase 11 of the proje仗， which is in progress, will increase the maximum water level to 212 metres and flood about seven per cent more of the area,or a total of approximately 347 squ訂e kilometres,the ìmpact ofwhich is expected to be felt from 1976 onwards. A post-impoundment assessment of the ecological effects of 甘le Nam Ngum dam and reservoir, carried out for the Mekong Committee (Comrnittee for Co-ordination of Investigations of the Lower Mekong Basin), is summarized below (Srni血sonian Institution, 1975). Impact on vegetation Since the trees of the original forest were not removed prior to 個1poun 也ne肘， the reservoir has been strewn with trunks and naked branches, the leaves having fallen off during the first year of impoundment. In addition to the submerged rice fields and villages, the submerged vegetation of the reservoir consists of the same type of forest as that found on the s10pes - either forest on sandstones or degraded forest with bamboo. The nature and rate of their decay vary. At depths greater than six metres, they remain intact as long as anaerobic conditions persist, while trunks and branches near the surface and emerging from the water will decay rapidly,especially those parts subject to fluctua世ons in the water level. ENVIRONMENTAL IMPACT OF OAMS ‘..... ~---­ ﹒呵呵呵 -,-""..... 一，叫間制圖 FψlreJ5.1: Thelo啊r Mekong b/JIm 1I. 芸E去工 220 PRACHOOM CHOMCHAI Among the submerged species are aquatic plants, namely, Azolla, Eichhom旬， Hydrilla. Pistia and Salvin旬， which have caused severe problems in other tropical man-made lakes. While it is true that aquatic vegetation in the Nam Ngum...