Amazon rainforest
A definitive guide to the Amazon Rainforest, the Amazon River, and the Amazon Basin

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Information about the Amazon River

There is still debate about exactly where the headwaters of the Amazon River begins.  Most scientists feel that the Ucayali in Peru is where the Amazon begins.  Scientists feel that the Amazon River is between 6000 and 7800 kilometers long. Part of the problem is the fact that expeditions that measured the length of the Amazon headwaters appended their measurements to figures that already included the distance to the Atlantic Ocean. If it can be proven that the Amazon River is indeed 7800 kilometers long, it would in fact be longer than the Nile River.  However it is generally accepted as the second longest river in the world.  If the measurement of the Amazon River is taken from Balique the length is between 4000 and 4200 miles.  The Nile River measures 4100 miles.  Because the tributary rivers of the Amazon meander and constantly change the exact length is in flux as well.

The Amazon River is the only river in the world that has three different tributary rivers each longer than 1850 miles.  The tributary river Madeira, measures 2078 miles, but is the only the second longest tributary river in the world trailing the Missouri River.  Contrary to popular belief rainfall is not dispersed evenly throughout the Amazon basin.  Generally there is a six month period of heavier rain, but the months of heavier rain depend upon the geographic location.  In the southern Amazon March and April are the rainiest, while the land near the equator receives its heaviest rain in June and July.  There is also generally a dry season which may last for several months without heavy rain. 

Over 90 percent of the rainfall comes from the water vapor from the Atlantic Ocean.  The Andes mountains blocks the moisture flow from the Pacific Ocean.   It is believed that up to 60 percent of the rainfall in the Amazon rainforest is recycled by transpiration through the leaves of trees.  The remaining 40 percent of the moisture that falls flows back to the Atlantic via the Amazon River and its tributaries.  The wettest area of the rainforest is in the Andean foothills where over 475 inches of rain falls each year.  Generally between 60 and 120 inches of rain falls in the other areas of the Amazon Basin.  There is locally heavier rain as well in areas near the coast as the moist coastal air can cause afternoon storms anytime.  As a general rule the further south you go in the Amazon basin the less rainfall you will encounter.

The Amazon River has the largest discharge of water in the world accounting for over 15% of the total river water that reaches the ocean.  Scientists believe that the Amazon River discharges over 57 million gallons of water every second.  To put this in perspective if one were to collect all the water that the Amazon discharged in 2 hours it would be enough water to supply all of New York City’s water use for 1 year.  The tributary rivers Madeira and Negro contribute about 29 percent of the water to the Amazon River.  Due to the strong flow of the Amazon saltwater from the Atlantic Ocean is prevented from entering the river through its main channel.  Once the water reaches the Ocean it is forced northward by the Southern Equatorial Current.  Therefore along the coast just north of where the Amazon River meets the Atlantic, the water is brackish where the top layer may be freshwater, and the lower water being saltwater.