Amazon rivers

Amazon rivers, major pathways for wildlife, people and water

Sometimes they rush, sometimes they just creep along, and in some places they may almost die out. At other times of the year, they will suddenly overflow with water.

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 mean annual discharge of 207,000 m^₃/sec of water.²

It is estimated that approximately one-fifth 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 drainpipe of the Amazon Basin, the Amazon River and its approximately 1,100 tributaries, such as the Rio Negro, the Madeira and the Napo, 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 6,696 km.⁵ 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.

The Orinoco River

The Orinoco Basin, extending from the Andes and the plains of the Llanos and the Guiana shield of Colombia and Venezuela to the Atlantic, covers 980,000 km². The river flows 2,140 km from its source in the extreme south of the Guianan massif until reaching the ocean.

With its tributary basins, the Orinoco represents one of the most biologically and hydrologically diverse areas of the world. It is considered to be the third most important river system on the planet, essentially due to the volume of water that flows through it before spilling into the sea – an average of 36,000 m³/sec.

The Orinoco through the seasons
Like the Amazon River, the Orinoco changes greatly from season to season.

Between May and August, rainfall oscillating between 5,000 and 7,000 mm and the relatively flat topographic conditions of the basin generate a flooded area covering almost 10% of the basin.

This creates new habitats and resources for many aquatic species. In Ciudad Bolivar (the capital of the eastern Venezuelan state of Bolivar), the annual variation in water level ranges from 15 m to 18 m and the discharge rate varies from one season to the next.⁷

A tributary of the Orinoco River connects with the Rio Negro, a tributary of the Amazon River, forming a 'natural canal' between the Orinoco and the Amazon River.⁸

A geological history along the Orinoco River
The Orinoco River runs through two distinct geological areas: one is the oldest geological formation on the continent, to the south of the river, and the other is a relatively recent formation of land, created by sediment washed down from the Andes.

Over time, the eroding action of the river has contributed to the isolation of an area of high sandstone mesas, locally known as the tepuis, some with an altitude of 1,500 m. One of these is home to Angel Falls, which at 1,000 m is the highest waterfall on Earth.⁹

The Orinoco and Amazon river systems provide a range of habitats, including swamps, marshes and streams, each hosting different organisms.¹⁰ These waterways are subject to major annual flood cycles, regulating the ecosystems and the landscape of the region.



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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
⁷ Kricher, 1997
⁸ Wikipedia. The Orinoco River. "http://en.wikipedia.org/wiki/Orinoco". Accessed 17/11/05.
⁹ Kricher, 1997
¹⁰ Kricher, 1997

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