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one of the chief tributaries of the Red River (Song Hong) in southeastern Asia. Nearly 500 miles (800 km) long, the river rises in central Yunnan province in southwestern China and flows southeastward into northwestern Vietnam on a course parallel to the Red River. Near the city of Hoa Binh, 37 miles (60 km) southwest of Hanoi, the Black River is dammed to form a reservoir that is the largest in Vietnam. The Hoa Binh Dam went into operation in 1988, though work on its hydroelectric generating station was not completed until 1994. The dam's 1,920-megawatt power plant generates a significant portion of Vietnam's total electricity requirement. After passing the dam, the Black River turns sharply north and unites with the Red River about 35 miles (55 km) northwest of Hanoi.