The Sesan, Sre Pok, and Sekong basins (3Ss) straddle Cambodia, Vietnam, and Lao PDR, and have been targeted for rapid development of hydropower.  Understanding the  impact of flow changes due the operation of cascading dams is critical to address transboundary issues, including power generation and potential alteration of downstream ecosystem productivity.

The Sesan, Sre Pok and Sekong river basins (3Ss) are located in the south-eastern part of the Mekong Basin and have a total drainage area of 78,650 km². The 3Ss straddle Cambodia (33%), Lao PDR (29%), and Viet Nam (38%). The Sre Pok and Se San rivers flow from the Central Highlands of Viet Nam through Cambodia and join with Sre Kong, which flows from Annamite mountain in Lao PDR about 40 km before merge with the Mekong River mainstream at Stung Treng. Annual rainfall over the basin varies from 1,100-3,800 mm, with the higher rainfall intensities occurring in the Highlands of Lao PDR and Viet Nam. Annual flow at the outlet is about. Annual discharge from the 3Ss represents approximately 17-20% of total annual flows of the Mekong mainstream (91,000 mcm or an average of 2,886 m³/s), making it the largest tributary contribution to the Mekong basin and therefore of significant importance.

The 3S basin area also has important aquatic biodiversity and provides ecosystem services through habitat for migrating fish (rapids, deep pools and sand bars).  National protected areas cover about 20% of the basin. There are also significant Important Bird Areas which have high biodiversity value. 

Over 2 million people live in Viet Nam part while in Lao PDR and Cambodia there are only 260,000 and 230,000 people, respectively. Most people in Lao PDR and Cambodia  still live close to the river system and remain highly depend on natural resources and ecosystem services while the Central Highlands of Viet Nam have a dense population and are undergoing a rapidly accelerating development.

The impact of dam development along the 3Ss and dam operation scenarios were modelled with the HEC-HMS (Hydrological Modelling System), SWAT, and HEC-ResSim (Reservoir Simulation) models to quantify the magnitude of potential changes to water flows and levels. Simulations were conducted on a daily time step.

 Sesan, Sre Pok, and Sekong basins with existing, under construction, and proposed dam locations.

 An overview of results from some of our modelling efforts were presented at the Stimson Center in August, 2011.  The video interview and powerpoint presentation can be seen here:

http://www.stimson.org/video/interview-with-dr-tom-cochrane/

 Associated journal publication:

Arias, M.E., Piman, T., Lauri, H., Cochrane, T.A., Kummu, M. (2014) Dams on Mekong tributaries as significant contributors of hydrological alterations to the Tonle Sap Floodplain in Cambodia. Hydrological and Earth System Sciences Discussion 11, 2177-2209. doi:10.5194/hessd-11-2177-2014.

Piman, T., Cochrane, T.A., Arias, M.E., Green, A., and Dat, N.D. (2013) Assessment of Flow Changes from Hydropower Development and Operations in Sekong, Sesan and Srepok Rivers of the Mekong Basin.  Journal of Water Resources Planning and Management, ASCE,  http://dx.doi.org/10.1061/(ASCE)WR.1943-5452.0000286