Khammouan is a province of Laos, located in the center of the country. It has borders to the north with the Province of Bolikhamxai. To the south, it borders the Province of Savannakhet. To the west it has international borders with Thailand, while to the east it has international borders with Vietnam. The capital is Thakhek. It has 16,315 km² of surface and a population of 367,562 inhabitants according to estimate 2010. The huge Nakai-Nam Theun forests in the Biodiversity Conservation Area are an important watershed that feeds many tributaries of the Mekong as well as forms the catchment area for Nam Theun 2, the largest hydroelectricity project in Laos. The Xebangfay, Hinboun and Nam Theun rivers are the main rivers of the province.