evidence: Laos, landlocked country of northeast-central mainland Southeast Asia. It consists of an irregularly round portion in the north that narrows into a peninsula-like region stretching to the southeast. Overall, the country extends about 650 miles (1,050 km) from northwest to southeast. The capital is Vientiane (Lao: Viangchan), located on the Mekong River in the northern portion of the country. [...] The general slope of the land in Laos is downhill from east to west, and all the major rivers—the Tha, Beng, Ou, Ngum, Kading, Bangfai, Banghiang, and Kong—are tributaries of the Mekong (Mènam Khong). The Mekong flows generally southeast and south along and through western Laos and forms its boundary with Myanmar and most of the border with Thailand. The course of the river itself is severely constricted by gorges in northern Laos, but, by the time it reaches Vientiane, its valley broadens and exposes wide areas to flooding when the river breaches its banks, as it did most notably in August 1966. A few rivers in eastern Laos flow eastward through gaps in the Annamese Cordillera to reach the Gulf of Tonkin; the most important of these is the Ma River, which rises in the northeast, just inside the Vietnam border.