The Marañón, Mantaro and Apurímac Rivers originate in the high mountain area of the Peruvian Andes at altitudes over 5,000 metres (16,000 ft). All three sources are considered to be the source of the Amazon, but from different points of view.

The Amazon River is primarily fed by massive quantities of rainfall across the Amazon Basin and the melting snow and glaciers of the Andes Mountains. The river’s "gold standard" source is generally considered to be the Apurímac River in Peru, specifically the headwaters at Mount Mismi, though recent studies also point to the Mantaro River as a more distant source. As the river flows eastward across South America, it is joined by over 1,100 tributaries, including giants like the Rio Negro, the Madeira, and the Marañón. The Amazon rainforest itself plays a critical role through a process called "transpiration," where trees release moisture into the atmosphere, creating "aerial rivers" that fall back as rain, effectively allowing the forest to "feed itself." During the rainy season, the river’s volume increases so dramatically that it accounts for approximately 20% of the total river water entering the world's oceans. This vast network of snowmelt, thousands of smaller streams, and intense tropical precipitation makes the Amazon the largest river in the world by discharge volume.