Amazon River
Origins

The most distant source of the Amazon was thought to be in the Apurímac river drainage for nearly a century. Such studies continued to be published even as recently as 1996, 2001, 2007, and 2008, where various authors identified the snowcapped 5,597 m (18,363 ft) Nevado Mismi peak, located roughly 160 km (99 mi) west of Lake Titicaca and 700 km (430 mi) southeast of Lima, as the most distant source of the river. From that point, Quebrada Carhuasanta emerges from Nevado Mismi, joins Quebrada Apacheta and soon forms Río Lloqueta which becomes Río Hornillos and eventually joins the Río Apurímac.

A 2014 study by Americans James Contos and Nicolas Tripcevich in Area, a peer-reviewed journal of the Royal Geographical Society, however, identifies the most distant source of the Amazon as actually being in the Río Mantaro drainage. A variety of methods were used to compare the lengths of the Mantaro river vs. the Apurímac river from their most distant source points to their confluence, showing the longer length of the Mantaro. Then distances from Lago Junín to several potential source points in the uppermost Mantaro river were measured, which enabled them to determine that the Cordillera Rumi Cruz was the most distant source of water in the Mantaro basin (and therefore in the entire Amazon basin). The most accurate measurement method was direct GPS measurement obtained by kayak descent of each of the rivers from their source points to their confluence (performed by Contos). Obtaining these measurements was difficult given the class IV–V nature of each of these rivers, especially in their lower "Abyss" sections. Ultimately, they determined that the most distant point in the Mantaro drainage is nearly 80 km farther upstream compared to Mt. Mismi in the Apurímac drainage, and thus the maximal length of the Amazon river is about 80 km longer than previously thought. Contos continued downstream to the ocean and finished the first complete descent of the Amazon river from its newly identified source (finishing November 2012), a journey repeated by two groups after the news spread.

After about 700 km (430 mi), the Apurímac then joins Río Mantaro to form the Ene, which joins the Perene to form the Tambo, which joins the Urubamba River to form the Ucayali. After the confluence of Apurímac and Ucayali, the river leaves Andean terrain and is surrounded by floodplain. From this point to the confluence of the Ucayali and the Marañón, some 1,600 km (990 mi), the forested banks are just above the water and are inundated long before the river attains its maximum flood stage. The low river banks are interrupted by only a few hills, and the river enters the enormous Amazon rainforest.
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