Marañón, 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 Ucayali River in NE Peru where it forms the Amazon River; some consider the Marañón to be the authentic headwater of the Amazon, but an expedition in 2000 determined that the headwaters of the Apurímac 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.