Salween River much of its length, the Salween's course is nearly parallel to that of the much larger Mekong in the east. Although the commonly accepted name is Salween, the river is known by a handful of other names regionally: "Nu" in China, "Thanlwin" in southern Burma, and "Salawin" on the border of Thailand and Burma. Several major sub-continental drainage basins border on the Salween. The Mekong basin forms nearly the entire eastern boundary of the watershed, while the Irrawaddy River flows southwards on the west side of the basin. The smallest boundary is formed by the Yangtze to the extreme north.
Sai River (Thailand) flowing through the Tachileik/Mae Sai urban area, where there is a sign marking the northern most point of Thailand just east of the border-crossing bridge —at the meeting point of Thailand Route 1 and National Highway 4 (Burma)— over the river. Finally it flows northeastwards before joining the right side of the Ruak River, a tributary of the Mekong, less than southeast of Tachileik Airport. Sai River (Thailand) Sai River (; ; ), formerly known as the River of Lawa (; ), is a river that forms the natural border between Thailand and Myanmar at Tachileik and Mae Sai Districts.