The Amazon Basin         Geology        History

The River Amazon is by far the largest river in the world. About 20 % of all the freshwater flowing into the oceans of the world enter via the River Amazon, which has an average flow of 200,000 cubic meters per second at its mouth. This is more than ten times the flow of the Mississippi. Even given the heavy rainfall that can occur within the Amazon basin (average rainfall varies between 1500 and 3000 mm per year over the majority of the catchment) such a flow still requires a huge catchment. The Amazon drains an area of more than 7 million square kilometers and includes within its catchment Brazil, Venezuela, Colombia, Ecuador, Peru and Bolivia. The Amazon can be considered as a collecting channel for a series of large rivers.
The largest of these tributaries is the Rio Negro. Other major tributaries have their origin in the high Andes and these give the river its essential character because they contribute much of the sediment and nutrients present in the lower Amazon river. The depth of the River Amazon down stream of its confluence with the Rio Negro averages 25-30 m.
River Solimoes (Amazon) in the vicinity of Tefe, Brazil.
For much of its course through the Amazon basin it flows through low lying land and thus is able to meander across a wide floodplain forming a great mosaic of lakes, ponds and channels: the template from which the aquatic diversity of the region is formed.


The Amazon is essentially a giant river valley bordered to the north and south by the Guiana and Brazilian shields respectively, which comprise hard Precambrian rock. The total area of the Amazon Basin is 7.5 x 106 km (795 km2) of which about 80% is rain forest. During the Paleozoic the Basin was a huge marine inlet into which many tributaries flowed.

Map of the Amazon geological shields (71 kB).
Until the Andes began to form at the end of the Miocene this inlet opened into the Pacific ocean and thus some elements of the Amazonian fauna are related to marine fish from the Pacific rather than Atlantic ocean. During the Quaternary water levels within the Amazon basin changed with the sea level. When the sea level was high, huge lakes formed in the valley into which large amounts of sediment formed.

During periods of low sea water level the rivers cut through the sediments forming river valleys. The modern sediments of the Amazon valley were formed during this period. These sedimentary deposits are up to 300 m thick and are called the Barreiras formation. Because these sediments have been heavily leached they hold low levels of nutrients and easily soluble cations such as calcium, and thus water flowing from the sedimentary deposits in the basin always has a low conductivity.


The first European to visit the Amazon was Vicente Yanez Pinzon, who sailed up the river in 1500. The river Amazon was first ascended from its source in the Andes to its mouth by the party led by Don Francisco de Orellana in 1541-42. This journey was forced on the Spaniards who had traveled ahead of the main body to seek provisions. However, they traveled down-stream to a point where they did not have the resources to make their way back up the Andes to Peru. After reaching the mouth they made the journey back to Spain via Trinidad. Orellana later returned to the Amazon, but the expedition was a disaster and he drowned when his ship capsized in the mouth of the Amazon. The first scientist to travel the entire length of the river was Ch. M. de La Condamine.

Intensive biological study of the region commenced in the 19th century. First, the German explorer Alexander von Humboldt, accompanied by the French botanist Aimé Bonpland, mapped the connection between the Amazon and Orinoco systems through the Casiquiare River. The British natural historians Henry Walter Bates and Alfred Wallace followed in these studies mid century. Bates spent the years from 1848 to 1859 along the Amazon, collecting thousands of species of animals. Much of his time was spent in Tefe, then called Ega. This is the region where many of the photographs included on this CD were taken. He wrote a book, The Naturalist on the River Amazons, which is one of the best accounts of the ecology ever written. Wallace was less successful than Bates and lost all his samples in a shipwreck. However, he moved on to Asia where he had the idea of evolution which he later co-presented with Charles Darwin.

At the beginning of the 20th century Theodore Roosevelt took a great interest in the region and led an expedition to explore what became the Roosevelt river. Between 1910 and 1924 expeditions sponsored by Harvard University undertook much work. Today Brazilian scientists undertake research supported by visitors from many countries. The major research institutions are Museo Paraense Emílio Goeldi in Belém and INPA in Manaus.

While there has been considerable habitat degradation over this century the Amazon still remains the greatest green wilderness in the world and attracts many biologists and ecologists to its riches.