The Red River (Vietnamese: Sông Hồng; Chinese: 红河; pinyin: Hóng Hé), also known as the Sông Cái - Mother River (Vietnamese), or Yuan River (Chinese), is a river that flows from southwest China through northern Vietnam to the Gulf of Tonkin. According to C.Michael Hogan, the associated Red River Fault was instrumental in forming the entire South China Sea at least as early as 37 million years before present.
The Red River begins in China's Yunnan province in the mountains south of Dali. It flows generally southeastward, passing through Dai ethnic minority areas before leaving China through Yunnan's Honghe Autonomous Prefecture. It enters Vietnam at Lào Cai Province. It forms a portion of the international border between China and Vietnam. Once reaching the lowlands near Viet Tri, the river and its distributaries spread out to form the Red River Delta. The Red River flows past the Vietnamese capital Hanoi before emptying into the Gulf of Tonkin. Tonkin is the former name of the northern provinces of Vietnam and thus the eponymous body of water receiving the main river of "Tonkin".
The reddish-brown heavily silt-laden water gives the river its name. The Red River is notorious for its violent floods with its seasonally wide volume fluctuations. The delta is a major agricultural area of Vietnam with vast area devoted to rice. The land is protected by an elaborate network of dikes and levees.