A sharp bend west and another south takes the Salween between the great mountain ranges of eastern Burma, the Daen Lao Range, a subrange of the Shan Hills, followed by the Dawna Hills in the east and the Karen Hills in the west, the river then receives the Pai River also from the left and flows through Salawin National Park to join the Moei River from the east as it approaches Thailand, where it is called the Salawin, forming the Burma-Thailand boundary for about , before re-entering Burma, passing through Karen State and Mon State. The river finally breaks out of its gorge about from the mouth, and slows down dramatically as it courses through a series of agricultural valleys. About from the mouth the river finally widens and deepens enough to become navigable for large watercraft. In quick succession it receives Dontham River from the right and Gyaing River from the left, from where the river turns west, flowing under Thanlwin Bridge and widening into a small river delta at Mawlamyaing (formerly Moulmein). This region is the most heavily developed on the river and contains most of the basin's population.