International Journal of Applied Science and Technology Vol. 2 No. 6; June 2012 92 Physico-chemical Characteristics in the Filling Phase of Bakun Hydroelectric Reservoir, Sarawak, Malaysia Nyanti Lee Department of Aquatic Science Faculty of Resource Science and Technology University of Malaysia Sarawak Malaysia Ling Teck Yee Department of Chemistry University of Malaysia Sarawak Malaysia Jongkar Grinang Institute of Biodiversity and Environmental Conservation, University of Malaysia Sarawak Malaysia Abstract Since the impoundment of the Bakun Hydroelectric Dam commenced, there have not been any studies on the water quality of the reservoir. As aquaculture is a potential industry to be developed, a study was conducted to determine the water quality of the reservoir fifteen months after the impoundment started at 12 stations. Results show that temperature in the reservoir decreased 5 o C from surface to 20 m depth. At the station nearest to the dam site, thermocline occurred at 5 m depth whereas further away, thermocline occurred at 4 m depth. In the reservoir, DO at the subsurface (5.49 - 8.51 mg/L) dropped drastically to anoxic level at 2 - 4 m. As the distance from the dam site increased, turbidity at the subsurface and at 20 m depth increased indicating the high suspended solids originating from upstream. The present study shows that the newly filled Bakun reservoir is not suitable for aquaculture due to the anoxic condition at such a shallow depth, low pH and high turbidity. Keywords: hydroelectric dam, turbidity, dissolved oxygen, thermocline, inland aquaculture 1. Introduction World-wide hydroelectric dams have been constructed to provide an alternative source of energy as petroleum reserve is not renewable and is depleting. The Sarawak Corridor of Renewable Energy (SCORE) which is the center piece of the Sarawak state’s plans f or economic growth and development (Sovacool and Bulan 2012) is a key development project of Malaysia situated on the Borneo Island. To meet the energy needs and security, dams are and will be constructed to generate hydroelectricity. Among them is the Bakun Hydroelectric Dam situated on the Balui River, a tributary of the Rajang River near the town of Belaga where construction started in 2002. The Bakun Hydroelectric dam is the largest hydropower project in Malaysia with an installed capacity of 2,400 MW of electricity (http//www.sarawak-hidro.com). It is the second highest concrete faced rockfill dam in the world with height of 207 metres (http//www.bakundam.com). The impoundment commended on 13 th October 2010 and it reached its full supply level of 228 m above sea level on 9 th March 2012 with an area of 695 square kilometers. Research related to Bakun Dam has been conducted on the aspect of energy security and hydropower development (Sovacool and Bulan 2012) and the analysis of the concept of sustainability assessment (Andre 2012). However, no studies have been conducted on the water quality of the newly filled reservoir. Similar to the earlier developed Batang Ai Reservoir, tourism and aquaculture are the two main potential industries that could provide employment opportunities for the resettled population. According to Chapman (1996), reservoirs formed by a dam across the course of a river are intermediate type of water bodies between rivers and natural lakes as there is a control in the contained volume of water at the outlet.