Springing forth over 4000km from the sea, high up on the Tibetan Plateau, the Mekong River so dominates Lao topography that, to a large extent, the entire country parallels its course. Although half of the Mekong's length runs through China, more of the river courses through Laos than through any other Southeast Asian country. At its widest, near Si Phan Don in die south, the river can expand to 14km across during the rainy season; spreading around thousands of islands and islets on its inevitable course south.

The Mekong's middle reach is navigable year-round, from Heuan Hin (north of the Khemmarat Rapids in Savannakhet Province) to Kok Phong in Luang Prabang. However these rapids, and the brutal falls at Khon Phapeng in Si Phan Don, have prevented the Mekong from becoming the sort of regional highway other great rivers have.

The fertile Mekong River flood plain, running from Sainyabuli to Cham-pasak, forms the flattest and most tropical part of Laos. Virtually all of the domestic rice consumed in Laos is grown here, and if our experience seeing rice packaged up as 'Produce of Thailand' is any indication, then a fair bit is exported via Thailand, too. Most other large-scale farming takes place here as well. The Mekong and, just as importantly, its tributaries are also an important source offish, a vital part of the diet for most people living in Laos. The Mekong valley is at its largest around Vientiane and Savannakhet, which, not surprisingly, are two of the major population centres.

Major tributaries of the great river include the Nam Ou (Ou River) and the Nam Tha (Tha River), both of which flow through deep, narrow limestone valleys from the north, and the Nam Ngum (Ngum River), which flows into the Mekong across a broad plain in Vientiane Province. The Nam Ngum is the site of one of Laos's oldest hydroelectric plants, which provides power for Vientiane area towns and Thailand. The Se Kong (Kong River) flows through much of southern Laos before eventually reaching the Mekong in Cambodia, and the rivers Nam Kading (Kading River) and Nam Theun (Theun River) are equally important in central Laos.

All the rivers and tributaries west of the Annamite Chain drain into the Me­kong, while waterways east of the Annamites (in Hua Phan and Xieng Khuang Provinces only) flow into the Gulf of Tonkin off the coast of Vietnam.