Piyain River  a tributary of the surma river, originates from the Umgat river of Assam. The Umgat originates from the hilly areas of Assam. Flowing southward, the river enters Bangladesh through Sylhet district and branches out into two: the Piyain and the dauki or Jaflong. After entering into Bangladesh, the Piyain follows a 7km-route westward along the international boundary and slighty turns southwest to meet the Surma river at chhatak. At Pratappur border outpost, the Piyain branches out into three courses: the Old Piyain, the Pabitradhala and the Naiyadala. Currently the original flow of the Piyain follows the course of the Naiyadala and meets the shari-goyain river at downreach. The Old Piyain and the Pabitradhala receives life during the rainy season.

The total length of the Piyain is about 145 km. The river contains 22 meanders on its 80km course from Jaflong to Chhatak. As the river is a hilly stream, it often causes flash floods and transports a huge amount of boulders from its source area. [Masud Hasan Chowdhury]