Abstract: Se San river is one of main branches of Me Kong river, it has rather rich water resource, and the topographical condition of the catchment and river are very good for hydropower development. The aquatic life in Se San River is rich, both with respect to species diversity and biomass. There are about 120 fish species are recorded, but the number of species is most likely higher, in the range from 200-300 species, fish is a very important food source for the local people. Up to now, in the Vietnam part of the Se San catchment there are 6 hydropower projects along Se San river from upstream to Vietnam - Cambodia boundary, and Se San 4A re-regulation reservoir. In the part catchment in Cambodia, in development plan there will be 3 hydropower projects, they are Se San 5/lower Se San 1, lower Se San 3, and lower Se San 2. The hydropower projects on Se San river have impacts on socio-economical development of the region (Vietnam and Cambodia) such as create new energy sources and product condition for remote and difficult areas of the catchmnet, improve the transportation system and living conditions for the rural areas. These hydropower projects also have impacts on natural environment such as flow regime changes at downstream, sediment in downstream river reaches, and some impacts on the ecosystems along river.

Key words: Se San River, hydropower, environmental issue, mitigation measure, development