The Moei River, also known as the Thaungyin River is a tributary of the Salween River. Unlike most rivers in Thailand, the Moei River flows north in a northwest direction. It originates in Phop Phra District, Tak Province, flowing then from south to north across Mae Sot, Mae Ramat, and Tha Song Yang Districts, finally entering the Salween River within the limiā¦The Moei River, also known as the Thaungyin River is a tributary of the Salween River. Unlike most rivers in Thailand, the Moei River flows north in a northwest direction. It originates in Phop Phra District, Tak Province, flowing then from south to north across Mae Sot, Mae Ramat, and Tha Song Yang Districts, finally entering the Salween River within the limits of Sop Moei District of Mae Hong Son Province. The river is 327 kilometres long.