Red River, Chinese Yuan Chiang, Vietnamese Song Hong, chief river of N Vietnam, 730 mi (1,175 km) long, rising in Yunnan prov., S China, and flowing southeast, in deep, narrow gorges, through N Vietnam to form a great delta before entering the Gulf of Tonkin. The river carries a large quantity of silt, rich in iron oxide, that gives it a red color. Northwest of Hanoi the river flows onto the coastal plain and receives the Clear and Black rivers, its chief tributaries. The Red River delta, c.75 mi (120 km) long and 75 mi wide, is the economic center of N Vietnam, whose chief port, Haiphong, is on the delta's north branch. Rice is the principal crop of the river valley; wheat, beans, rapeseed, corn, and subtropical crops are also grown. The Red River has an irregular flow and is subject to flooding, especially during the June–October high water period; dikes and canals protect the delta from floodwaters. A railroad and highway follow the Red River valley, an important transportation route linking China and Vietnam.