considerably during the dry season as water is drawn off for power generation. The Nakai Reservoir draws down to a minimum operating size of around 70 km2 by the end of April each year. Nam Theun 2 also features a smaller reservoir and dam, the Re-Regulating Dam and Pond, built to control releases from the Nam Theun 2 Power Station to the Xebangfai River. One of the most productive hydro power projects in Laos has been the Theun-Hinboun scheme in the center of the country, which was built in two phases opening respectively in 1998 and 2012. The initial project featured a relatively small dam and reservoir, transferring water from the Theun River to the neighbouring (and lower) Hinboun River by tunnel, producing 220 MW. Theun-Hinboun provided substantial economic benefits to Laos through energy sales to neighboring Thailand at a time when the country had few exports, and its success encouraged other international investments in the country, including the Theun-Hinboun Expansion Project. This project added another 220 MW turbine at the Theun-Hinboun powerhouse and also built a 60 MW powerhouse on the Gnouang River, a tributary of the Theun River. The water for this expanded power generation is supplied by a new 105 km2 reservoir on the Gnouang River, created by construction of the 65 m high Nam Gnouang Dam. In 2012 construction began on the 1,285 MW Xayaboury hydroelectricity power plant, the first dam to be built in Laos on the mainstream of the Mekong River. The plant is scheduled to begin operations in 2019 and will be the biggest hydro power station in the country. Its dam is to be 32.6 m tall, stretching 820 m across the Mekong. The project is designed to be run-of-the river and as such will not create a large reservoir. However, both the Xayaboury and Don Sahong Dams have attracted international attention due to the trans-boundary effects of the Mekong River on people and the environment downstream in Cambodia and Vietnam. The governments of both countries have challenged Laos to justify the construction of the projects through the Mekong River Commission. Construction of a 290 MW project that will build two dams and two reservoirs on the Ngiep River in Bolikhamsai Province began in October 2014. The Nam Ngiep 1 hydropower plant, scheduled to begin operations in January 2019, will be the second project on the Ngiep River. An additional 21 dam