The Marañón River (Spanish: Río Marañón, IPA: [ˈri.o maɾaˈɲon]) 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 degrees 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. Although historically, the term "Marañon River" often was applied to the river all the way to the Atlantic Ocean, nowadays the Marañon River is generally thought to end at the confluence with the Ucayali River, after which most cartographers label the ensuing waterway the Amazon River.
Marañón River as the source of the Amazon
The Marañon River was considered the source of the Amazon River starting with the 1707 map published by Padre Samuel Fritz, who indicated the great river “has its source on the southern shore of a lake that is called Lauricocha, near Huánuco." Fritz’ reasoning was based on the fact that the Marañon River is the largest river branch one encounters when journeying upstream, something clearly evident on his map. For most of the 18th–19th centuries and into the 20th century, the Marañon River was generally considered the source of the Amazon. The Marañon River continues to claim the title of the "mainstem source" or "hydrological source" of the Amazon due to its contribution of the highest annual discharge rates, a distinction similar to the Mississippi's "source" at/near Lake Itasca in Minnesota and the Nile's "source" at/near Lake Tana on the Blue Nile in Ethiopia.