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a river in Peru, a right tributary of the Marañón River (Amazon River basin).
The Huallaga is about 1,200 km long and drains an area of about 95,000 sq km. It rises in the Cordillera Occidental and, for most of its length, flows in a broad valley between the Cordillera Central and Cordillera Oriental. Farther down, it crosses the Cordillera Oriental and flows onto the Amazon lowlands. High water lasts from September through March. The average water discharge is about 3,500 cu m per sec. The river is navigable from the mouth to the city of Yurimaguas, a distance of 250 km. The city of Huánuco is located on the upper Huallaga.