The Marañón River is the principal or mainstem source of the Amazon River, arising about 160 km to the northeast of Lima, Peru, and flowing through a deeply eroded Andean valley in a northwesterly direction, along the eastern base of the Cordillera of the Andes, as far as 5° 36′ southern latitude; from where it makes a great bend to the northeast, and cuts through the jungle Andes, until at the Pongo de Manseriche it flows into the flat Amazon basin. Wikipedia