The Nam Ngum is a major tributary to the Mekong and is over 1,400 kilometers long, draining an area of 17,000 square kilometers with an average annual outflow of 647 cubic meters per second. While this tributary constitutes about two percent of the total area of the Mekong, it provides up to five percent of the total contributing flow in the Mekong. The Nam Ngum has one existing hydropower dam with 4.7 billion cubic meters of storage and nine more reservoirs either currently under construction or planned for an additional storage of 5.7 billion cubic meters.