The main power supply of Laos.

In 1997 I returned to Vientiane from Luang Prabang. These two cities and others Laos' and a few Thailand's cities are supplied by the electric power from one source, Nam Ngum dam.
from http://www.mekong.es.usyd.edu.au/case_studies/nam_ngum/nam_ngum_dam.htm

Nam Ngum Dam was the LPDR’s first hydro dam. It was financed with assistance from ten countries, under the auspices of the United Nations. It was constructed by a Japanese firm and was completed in 1971. The dam has a generating capacity of 150 megawatts.

Nam Ngum Dam generates most of Laos' electricity, including all the power used in the capital, Vientiane. Even more important is the 70 to 80 per cent of electricity that is exported to Thailand, accounting for about a quarter of Laos' foreign exchange earnings. While bringing in significant foreign exchange earnings, Nam Ngum is economically questionable on several counts. Between 1982 and 1992, electricity generated and exported declined, a result uncorrelated to rainfall. Its revenues have not been enough to cover the cost of repairs, most of which has been covered by Japanese aid. Already apparent is the fact that its watershed will require significant ongoing financial and management input.

Nam Ngum Dam is a resource user in a number of respects. The reservoir occupies what used to be fertile land and forest, and communities affected by flooding continue to suffer direct and indirect consequences a quarter of a century later. Nam Ngum Dam thus competes for land, forest and water resources, but also bears the consequences of resource use in other parts of the watershed.