<P> 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 124 mi) wide . The fresh water, being lighter, flows on top of the seawater, diluting the salinity and altering the colour of the ocean surface over an area up to 2,500,000 km (970,000 sq mi) 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 . </P> <P> 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 . </P> <P> 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 earth that links two major river systems, a so - called bifurcation . </P> <P> Not all of the Amazon's tributaries flood at the same time of the year . Many branches begin flooding in November and may continue to rise until June . The rise of the Rio Negro starts in February or March and begins to recede in June . The Madeira River rises and falls two months earlier than most of the rest of the Amazon . </P>

What two tributaries of the amazon river flow through colombia