The Physical Features Of The Amazon

The Physical Features Of The Amazon

The Upper And Middle Courses

A tiny stream trickles through spongy grass high in the Andes mountains. Even though it is close to the Ecuator, at a height of 5,250 metres, the air is freezing cold. Beginning just 200 kilometres from the Pasific Ocean, this is the source of the Amazon. The tumbling stream joins the the Apurimac river, cutting a deep valley as it flows north - Apurimac means "Great Speaker", the name the Amazon  Indians use to describe the roar of the river.

Like all rivers while they are young, here in the mountains it flows fast, crashing over boulders and tearing at the rocks it passes. Much of the loose material, called sediment, that the river picks up, will be carried thousands of kilometres towards the sea. A scientist once estimated that it would need 9,000 trains pulling 30 ten -tonne trucks every day, to carry the same amount of sediment as the waters of the Amazon carry with it. During its first 1,000 kilometres the river plunges furiously over rapids and through steep-sided gorges.     

Once it leaves the Andes, the Amazon is already twice the width of the Rhine, the largest river in Europe. It still has nearly, 5,500 kilometres to go before reaching the ocean but has only 300 metres to fall. The Amazon slows down to a crawl and starts to flow east across the huge plain towards the Atlantic. In places it widens to several kilometres, appearing more like a giant inland sea than a river. Sometimes you cannot even see the far side.

The small stream is the source of the great Amazon river. High in the Andes mountains, snow covers the peaks and little grows on the slopes below except rough grass. 

The Amazon's Tributaries 

Like all rivers, the Amazon gathers water from other streams as it flows to the sea. These streams are called a river's tributaries. The Amazon has more than a thousand major tributaries: ten of these are more than 1,000 kilometres long.

    Although the Amazon rises in the Andes Mountains, not all its tributaries start there. Some rivers, like the Trombetas, rise in the Guyana Highlands to the north, and others like the Tapajos, begin in the Brazilian Highlands far to the south. The Amazon has the largest river catchment area in the world; the whole area covers more than half the land of South America. 

Here at Santarém, the clearwater Tapajos river meets the whitewater Amazon. For several kilometres the rivers run beside each other without mixing. 

There are three different types of river in the Amazon basin. These are called whitewater, clearwater and blackwater rivers. Whitewater rivers, like the Madeira, are actually a dirty yellow colour. They are like this because the water is full of sediment brought down from the soft rocks of the Andes Mountains.

    Clearwater rivers, like the Xingu, flow from the Brazilian Highlands. The are a blue-green colour and carry little sediment in the water because the Higlands are made of hard rock which the river cannot easily erode.

    There are also blackwater rivers which are the colour of dark tea, such as the Rio Negro ("Rio" means river and "Negro" means black in Portuguese). They too have little sediment. Sediment carries nutrients in it and so whitewater rivers have many more fish and other creatures than the blackwater and clearwater rivers. 

Climate And Water Level

The size of a river is partly due to the distance it covers, because the further it goes the more other important is the amount of rain that falls in the area. The Amazon basin is one the wettest regions in the world, with an average of more than 2,500 mm of rain per year. Rainfall is highest near the Andes - here it can be over 6,000 mm per year. Although there is plenty of rain all year round, there are times when the rainfall is heavier then others. In these wet season the torrential rain swells the rivers.  

The river levels can change by up to 15 metres, twice the height of a two-storey house. In many places the rivers overflow and there are floods. The floods can last from between four to seven months of the year. 

The area which regularly floods is called the floodplain, known in the Amazon region as the varzea. When the river floods, it drops the sediment it has been carrying with it from the Andes. The rich nutrients in the sediment make the varzea one of the most fertile places in the Amazon basin.

The Lower Course

As it nears the sea, the Amazon moves even slowly and drops most of its huge load of sediment. It twists and turns through this mud and silt, forming thousand of islands which shift as sediment is added or washed away. There are channels between the islands, and it is difficult to tell which is the main river any more.

The mouth of the Amazon is vast; its width stretches 330 kilometres across, about the same as the distance from London to Paris. The European explorers who first voyaged to the Amazon's mouth called it "Fresh Water Sea" , because the river's flow is so powerful that it pushes fresh water out into the Atlantic Ocean for some 250 kilometres. In the middle of the river's mouth is Marajo, a huge island about the size of Switzerland.

The Amazon In Prehistoric Times

The Amazon has not always flowed into the Atlantic. 200 million years ago, South America and Africa were joined as part of one huge continent, called Gondwanaland. At that time the Andes did not exist and the Amazon flowed west into the Pasific Ocean.

About 180 million years ago, the Earth's land masses began to break up. The continents of Africa and South America split apart, and the Andes Mountains began to rise. The river could no longer flow into the Pasific Ocean and a huge lake formed, covering most of what is now the Amazon basin. Eventually the water found a way out, spilling over between the Brazilian and Guyana Highlands at Obidos. The Amazon River, as we know it, was formed.