1a river in Southeast Asia that rises in southern China and flows 730 miles (1,175 km) southeast through northern Vietnam to the Gulf of Tonkin. Chinese name Yuan Jiang; Vietnamese name Song Hong. 2a river in the southern US, a tributary of the Mississippi River, that rises in northern Texas and flows 1,222 miles (1,966 km) southeast, forming part of the border between Texas and Oklahoma. It enters the Mississippi River in Louisiana. Also called Red River of the South. 3a river in the northern US and Canada that rises in North Dakota and flows 545 miles (877 km) north, forming for most of its length the border between North Dakota and Minnesota, before entering Canada and emptying into Lake Winnipeg. Also called Red River of the North.