Investments in water resources infrastructure, especially hydropower dam, have been essential for economic development by generating electricity, but when they are improperly planned, designed, or operated, they can cause problems for downstream ecosystems and communities because of their impact on the volume, pattern, and quality of flow. This study aims to firstly assess the flow variations and secondly to characterize water quality variations resulting from dam construction and operation in the Sesan River. The findings from this study showed significant hydrologic alteration both during low flow and during high flow after the construction of Yali Falls Dam in Vietnam. Temporal change in maximum water level showed large differences before the dam construction of 6.8 m but accounting for only 5.2 m after dam operation. The fluctuation has been seen more severe after more several dams starting their operation. A significant reduction in flow duration in particular from 5 to 70% during the period 2001-2005 and more reduction in flow duration from 2009 to 2012 due to more water impoundments from new hydropower dam operation. The change in flow duration and flow characteristics could have more adverse impacts on the community livelihoods and ecosystem along the river system. Water quality dynamics have been also influenced by water release from the dam. The temporal variability of water quality in the Sesan River from June 2012 to May 2013 showed that TSS ranged from 1.5 to 333 mg/l, TP from 0.01 to 0.18 mg/l, NO3-N from 0.05 to 0.36 mg/l, and NH4-N from 0.03 to 0.26 mg/l. Our results can be used to enhance the understanding of the potential hydrologic changes due to the cascades of dam construction and rapid hydropower development in the Sesan River Basin. The study recommends for transboundary water resources management which includes disaster prevention and mitigation, dam operation rule, water quality, and environmental flow monitoring.
