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river, c.1,000 mi (1,600 km) long, rising in Lake Lauricaucha in the Cordillera Occidental, W central Peru. It flows generally NW, then E across the Andes to join the UcayaliUcayali
, river, c.1,000 mi (1,610 km) long, formed by the confluence of the Apurímac and Urubamba rivers, E Peru, and flowing generally north through a mountain and jungle wilderness to the Marañón River, SW of Iquitos.
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 River in NE Peru where it forms the AmazonAmazon,
Port. Amazonas , world's second longest river, c.3,900 mi (6,280 km) long, formed by the junction in N Peru's Andes Mts. of two major headstreams, the Ucayali and the shorter Marañón.
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 River; some consider the Marañón to be the authentic headwater of the Amazon, but an expedition in 2000 determined that the headwaters of the ApurímacApurímac
, river, c.430 mi (690 km) long, rising in the Andes, S Peru. It flows generally northwest in a narrow valley. After joining the Mantaro River, the Apurímac becomes the Ene; after joining the Perené River, it becomes the Tambo.
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 River (a tributary of the Ucayali) are the most distant from the Amazon's mouth. The Marañón is navigable to the Pongo de Manseriche, the gorge in NW Peru through which it flows before reaching the Amazon basin. The Huallaga River is its chief tributary. Pedro de Ursúa, the Spanish explorer, descended the Marañón in 1560.
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