Rivers are unpredictable, and nowhere is this truer than in the Amazon River Basin, which is subject to radical seasonal changes throughout the year.

Amazon rivers provide a range of habitats, including swamps, marshes and streams, each hosting different types of wildlife.¹ These waterways are subject to major annual flood cycles, affecting the ecology and the landscape of the region.

The Amazon River

Coming a close second after the Nile as the world’s longest river, the Amazon River sets the record in terms of the sheer volume of water that it carries – a mind-boggling average discharge of 219,000 m^₃/sec of water.²

It is estimated that approximately one-sixth of all fresh water that drains into the world's oceans goes through the 320-km-wide delt  of the Amazon, where it meets the Atlantic Ocean.

As the seasons change, so does the river. During the dry season, the width of the Amazon River can be 4 km to 5 km in places – and in the wet season, this can increase to 50 km! At the height of the wet season, the current can reach a speed of 7 km/hr.

Major roles of the Amazon River
As the drainage system of the Amazon Basin, the Amazon River and its approximately 1,100 tributaries play major roles in the ecology of the basin.

Before roads and airstrips started appearing in the basin, these waterways were the major access routes to the interior areas of Brazil and the northern half of South America.

For example, the only way you can get to Iquitos, Peru, which is right on the Amazon River, is to board a plane or a boat. There are no roads to get there.³

Origins and course of the river
The Amazon River has its source high in the Peruvian Andes, at an elevation of 5,598 m. There, at a mere 192 km from the Pacific Ocean where it once flowed into, the Amazon River begins as a small tributary called the Carhuasanta.

As it heads east, it flows into and becomes the Hornillos, which merges into the Apurimac, a major tributary that eventually joins the Ene, the Tambo and then the Ucayali.⁴

After an initial drop in elevation, the Amazon River steadies its descent towards the Atlantic Ocean at a rate of 1.5 cm for every kilometre over a distance of over 6,400 km. In some places, the river reaches a width of 10 km, as far as 1,600 km upriver, and large ships can dock all the way up to Iquitos, Peru.⁶

Sighting the river before the land
The brown waters of the Amazon River can be seen as far as 100 km out to sea from the mainland, well before the continent is in sight.

In the early days of colonization, this phenomenon would help ships sailing from Europe to South America ensure they were on good course before sighting the land.

------------------------------------------------
¹ Kricher, 1997
² Voeroesmarty CJ, Moore V, Grace AL, Gildea MP, Melillo JM. 1989. Continental Scale Models of Water Balance and Fluvial Transport: An Application to South America. Global Biogeochemical Cycles (GBCYEP). Vol. 3, No. 3, p 241-265
³ Kricher, 1997
⁴ Kricher,1997
⁵ Kricher, 1997
⁶ Dyk, 1995 in Kricher, 1997