Red River (Asia), river in southeastern Asia, principal river of northern Vietnam. The river rises in the mountains of Yunnan Province in southern China, where it is called the Yuan Jiang. It then flows southeast across northern Vietnam, where it is called the Sông Hong. The river passes through Hanoi, the nation's capital, and empties into the Gulf of Tonkin. The Red River spans 800 km (500 mi). Its chief tributaries are the Black (Sông Dà) and Clear (Sông Lô) rivers. The delta of the Red River forms one of the most fertile and populous areas in Vietnam.