The Huallaga River is a tributary of the Marañón River, part of the Amazon Basin. Old names for this river include Guallaga and Rio de los Motilones. The Huallaga is born on the slopes of the Andes in central Peru and joins the Marañón before the latter reaches the Ucayali River to form the Amazon. Its main affluents are the Monzón, Mayo, Biabo, Abiseo and Tocache rivers. Coca is grown in most of those valleys, which are also exposed to periodic floods.

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 northwest across plateaus 3,650 m high, it runs through a deeply eroded Andean valley, 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 Ande in its midcourse, until at the Pongo de Manseriche it flows into the flat Amazon basin. Although historically, the term "Marañón River" often was applied to the river all the way to the Atlantic Ocean, nowadays the Marañón River is generally thought to end at the confluence with the Ucayali River, after which most cartographers label the ensuing waterway the Amazon River.