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.

The main source of the largest river in the world has been a subject of exploring and speculations for centuries and continues to cause arguments even today. Determining the origin of the Amazon River has evoked broad debates among scholars, explorers, and travelers all over the world. Different definitions of a river's source have been used and continue to be used. Generally, four main criteria can be applied to determine the main source of a river: source flow rate, source length, watershed area of the source, and an altitude of its spring. At present, the Amazon River is not considered to have one unique source but a number of headstream areas. These are headwaters of three different Peruvian rivers that can be found in the high Andes: the Marañón, the Apurímac, and the Mantaro.