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

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Information about the Amazon Rainforest, the Amazon River, and the Amazon Basin

The Amazon basin is made up of several different geological areas.  There are two upland areas know as shields which are as high as 1000 meters, and these shields are separated by the main river to the east.  The shields which are ancient uplands are the Brazilian shield and the Guiana Shield.  The rest of the region is a sedimentary basin, that is lower than 1000 meters in elevation, and accounts for 35% of the basins drainage.  In some areas the sediment has built up to depths of over 5000 meters.  Additionally the Andes mountains start form the western border of the rainforest, and account for about 15% of the Amazon basin.  The Brazilian and Guiana Shields were actually higher than the Andes mountains at one time, and therefore they supplied most of the sediment to the eastern and central Amazon Basin.  The Andes mountains began to rise about 15 million years ago, when the Nazca and South American plates clashed. 

The heavy rainfall on the eastern slopes of the Andes for the last 10 million years have caused a large amount of erosion, and huge quantities of sediment have flowed eastward.  Seasonal flooding of the Andean Alluvial Extension zone have caused the area to become deeply deposited with sediment.  The headwaters may not follow the same path every year as the river can meander and create new paths cutting through the sedimented soil.
Sedimented soil is constantly being moved across the floodplains and the river through this annual cycle. Over 30% of the Amazon river flows through floodplains, therefore a large amount of the Andean sediment is deposited there.

The Andes mountains are still rising, and the earth is still unstable at the headwaters of the Amazon.  Active volcanoes such as Cotopaxi, Tungurahua, Reventador, Antisana, and Sangay, are all in the Amazon headwater region.  The volcanic materials from the volcanoes can darken the Amazon Rivers tributary rivers such as the Napo River.  Over the last 2 million years, the fluctuations in the earths temperatures have caused the sea levels to rise and since as the polar ice caps melted.  Some scientists believe that the Amazon lowlands were completely submerged when the sea levels were higher than they are today.  The floodplains have been formed over the past 7000 years, an most of them have been filled with sediment from the Andes mountains.  Tributary rivers such as the Negro and the Tapajos are relatively deeper and wider than other tributaries.  They are known as mouth lakes, and it is believed they were formed when the sea level was lower and the currents could flow faster, and dug deep into the land.  These tributaries have not had sufficient time to fill with sediment, and therefore are deeper and wider. 

The basin contains the Amazon rainforest which is more than four times larger than the second largest tropical forest the Congo Basin.  Almost one-sixth of the worlds broadleaf forests is found in the Amazon.  South America has other rainforests besides the Amazon rainforest such as the Orinoco basin, the Atlantic coast of Brazil, and along the Pacific coast of Ecuador and Columbia.  However over 85 percent of the South American rainforests are in the Amazon Basin.