The headwaters of the Apurímac River on Nevado Mismi had been considered for nearly a century as the Amazon’s most distant source, until a 2014 study found it to be the Cordillera Rumi Cruz at the headwaters of the Mantaro River in Peru. The Mantaro and Apurímac join, and with other tributaries form the Ucayali River, which in turn meets the Marañón River upstream of Iquitos, Peru, to form what countries other than Brazil consider to be the main stem of the Amazon. Brazilians call this section the Solimões River above its confluence with the Rio Negro to form what Brazilians call the Amazon at the Meeting of Waters (Portuguese: Encontro das Águas) at Manaus, the river's largest city.
At an average discharge of about 209,000 cubic metres per second (7,400,000 cu ft/s; 209,000,000 L/s; 55,000,000 USgal/s)—approximately 6,591 cubic kilometres per annum (1,581 cu mi/a), greater than the next seven largest independent rivers combined—the Amazon represents 20% of the global riverine discharge to the ocean. The Amazon basin is the largest drainage basin in the world, with an area of approximately 7,050,000 square kilometres (2,720,000 sq mi). The portion of the river's drainage basin in Brazil alone is larger than any other river's basin. The Amazon enters Brazil with only one-fifth of the flow it finally discharges into the Atlantic Ocean, yet already has a greater flow at this point than the discharge of any other river.
During what many archaeologists call the formative stage, Amazonian societies were deeply involved in the emergence of South America's highland agrarian systems. The trade with Andean civilisations in the terrains of the headwaters in the Andes, formed an essential contribution to the social and religious development of the higher altitude civilisations of among others the Muisca and Incas. Early human settlements were typically based on low-lying hills or mounds.
Shell mounds were the earliest evidences of habitation; they represent piles of human refuse and are mainly dated between 7500 and 4000 years BP. They are associated with ceramic age cultures; no preceramic shell mounds have been documented so far by archaeologists. Artificial earth platforms for entire villages are the second type of mounds. They are best represented by the Marajoara culture. Figurative mounds are the most recent types of occupation.