A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 2006

Humans have long been fascinated by the dynamism of free-flowing water. Yet we have exercised to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at invaluable cost. Ecological responses to altered flow regimes in a specific stream or river depend on how the components of flow have changed relative to the natural flow regime. This paper addresses the flow regime of the Mekong river and its environmental consequences. The Mekong River is the largest river of the South East Asia, with the total length of 4,800km and the basin area of 795,000km2. People may suffer from the Mekong's anomalistic floods, yet the rich soil it distributes/deposits and the fish it nurtures sustain them. However, China's plan to build colossal dams on the upper reaches of the Mekong in Yunnan Province, which is known as Lancang in China, will change the river's natural flow regime. This work is an attempt to reckon the hydrologic alteration on natural flow regime of lower Mekong river over the post-dam period which marks the completion of Manwan and Dachaoshan dams in upper Mekong using Range of Variability Approach while assuming that the pre-dam period was sound ecologically. The required environmental flow for healthy ecosystem and the applicability of standard setting methods for lower Mekong river are demonstrated. To reflect the completion of Xiaowan dam in 2010, few wellgrounded scenarios are considered in Mike 11 modeling environment to predict the future flow regime. Subsequently, its effect on environmental flow requirement is reported. Further, the impact of altered flow regime on fish production and the role of water resources developments of lower Mekong riparian countries in altering the natural flow regime are reported