Amazon River - Drainage Area

Drainage Area

The Amazon Basin, the largest in the world, covers about 30% of South America, an area of approximately 7,050,000 square kilometres (2,720,000 sq mi). It drains from west to east, from Iquitos in Peru to all the way across Brazil to the Atlantic. It gathers its waters from 5 degrees north latitude to 20 degrees south latitude. Its most remote sources are found on the inter-Andean plateau, just a short distance from the Pacific Ocean.

The Amazon River and its tributaries are characterized by extensive forested areas that become flooded every rainy season. Every year the river rises more than 9 metres (30 ft), flooding the surrounding forests, known as várzea ("flooded forests"). The Amazon's flooded forests are the most extensive example of this habitat type in the world. In an average dry season, 110,000 square kilometres (42,000 sq mi) of land are water-covered, while in the wet season, the flooded area of the Amazon Basin rises to 350,000 square kilometres (140,000 sq mi).

The quantity of water released by the Amazon to the Atlantic Ocean is enormous: up to 300,000 cubic metres per second (11,000,000 cu ft/s) in the rainy season, with an average of 209,000 cubic metres per second (7,400,000 cu ft/s) from 1973 to 1990. The Amazon is responsible for about 20% of the Earth's fresh water entering the ocean. The river pushes a vast plume of fresh water into the ocean. The plume is about 400 kilometres (250 mi) long and between 100 and 200 kilometres (62 and 120 mi) wide. The fresh water, being lighter, flows on top of the seawater, diluting the salinity and altering the color of the ocean surface over an area up to 1,000,000 square miles (2,600,000 km2) in extent. For centuries ships have reported fresh water near the Amazon's mouth yet well out of sight of land in what otherwise seemed to be the open ocean.

The Atlantic has sufficient wave and tidal energy to carry most of the Amazon's sediments out to sea, thus the Amazon does not form a true delta. The great deltas of the world are all in relatively protected bodies of water, while the Amazon empties directly into the turbulent Atlantic.

The tidal bore is the reason the Amazon does not have a protruding delta; the ocean rapidly carries away the vast volume of silt carried by the Amazon, making it impossible for a delta to grow past the shorelines.

There is a natural water union between the Amazon and the Orinoco basins, the so-called Casiquiare canal. The Casiquiare is a river distributary of the upper Orinoco, which flows southward into the Rio Negro, which in turn flows into the Amazon. The Casiquiare is the largest river on the planet that links two major river systems, a so-called bifurcation.