What You Didn’t Know About the Amazon River

Below our clunky plane, within the creaks of the clouds, a twisted pattern of muddy orange arose. The colour had a clay-like dullness but tinted […]

Below our clunky plane, within the creaks of the clouds, a twisted pattern of muddy orange arose. The colour had a clay-like dullness but tinted a beautiful pink in the sunlight. The Amazon River. It twisted and curled into an image so random a toddler could replicate and yet so exact nature could never again.

The Amazon River discharges over 200,000 cubic meters of water per second, crowning it the largest river in the world. The distant origins of the river water came from a glacial stream in the West Peruvian Andes. The river now has a series of other river contributors, yet pinning down the exact ones is still heavily disputed. Nonetheless the mighty river flows from west to east, through Brazil, until it deposits itself into the Atlantic Ocean.

200 million years ago this image, however, would literally have been reversed. At such a time, the river instead flowed from east to west. Due to the past fusion of the continents, known as Gondwana, the origins of the river used to be inhabited by Africa. Evidence for this theory includes sediment found in the current river having an Eastern origin. The river would therefore have flowed westerly into the Pacific Ocean. This flow however was cut short 50 million years ago when the Andes Mountains became uprooted from a collision between the Nazca and South America plate. This upcoming of ground caused the river to be blocked on both sides; to the east was the Amazon Rivers’ African origins and to the west the large Andes preventing further flow. What remained was a large inland sea- around the size of the United States of America. Gradually sediment running off the Andes built up to form a large swamp-like freshwater lake.

The ever twisting tale of the Amazon River

The ever twisting tale of the Amazon River 

Moving forward to 10 million years ago the powerful lake eventually eroded the sandstone to the east allowing the continuous flow into the Atlantic. Completing this epic story was the Ice Age. The Ice Age caused a fall in sea levels leading to this great lake rapidly draining into the Atlantic and so forming the intense array of river systems we see today.

The history of the Amazon River, bizarre enough to first be acknowledged as an old wives tale, provides the explanation for a number of the Amazon’s characteristics.

The Amazon once consisted of a huge lake, due to this, and the rivers annually bursting their banks, the surrounding land is completely flat. The highest amount of land I had to climb during my entire expedition at the Pacaya-Samiria National Reserve was that of a mud bank to get into the forest. This was only due to me arriving in the dry season when the water levels can decrease by 15 metres. The Amazon is so flat that during the night large flashes of lightening can be observed across the horizon even with a clear sky above you. This lightening is from vast distances but due to the flat land it appears as if ready to sweep over you in minutes.

The rise of the Andes not only caused a reversal in the flow of water direction but another equally important feature of the Amazon. The run-off sediment from the mountains into the river basin resulted in the Amazon having a heavily overloaded composition of mud. This mud layer is so thick that it requires a good 100 metres of digging until a single rock can be found- a challenge even for the over ambitious. Rocks were so precious that they were used by the indigenous people of the forest as a currency of exchange with those in the Andes. A few bucket loads of rainforest gifts would gladly have been swapped for a simple small rock.

Trees in the Pacaya-Samiria also have a very small life span due to a combination of the soft unstable mud and high winds which can occur. Thus the forest becomes a constant obstacle course of avoiding and persevering through fallen trees. Though the lifespan is young, trees can still reach incredible sizes due to the high productivity of the rainforest giving fast growth rates.

Not only a consequence of Amazon River’s past but also providing evidence for the theory is the stingray. Over 20 species of stingray live in the Amazon River existing in the family Potamotrygonidae. Findings studied the parasites of these stingrays and found that the closest relative of these lived in the Pacific marine habitat. This indicated that these stingrays are derived from Pacific stingrays. This ancestor stingray would have invaded the Amazon River from the West through the Pacific- only feasible when the River flowed in the opposite direction. The uplifting of the Andes then caused those stingrays inhabited to be trapped and so diverge widely from its previous ancestors, making any sort of past characteristic similarity absent in today’s world.

The vast differences between the Amazonian stingrays and the Pacific stingrays are due to one major environmental difference; that of fresh and that of salt water. The change of habitat from salt to fresh water forced major evolutionary change to occur in the Amazonian stingrays. The reason such a strong change was required was due to the osmotic change in the stingray’s environment. Salt water in the Pacific causes water to move out of the stingray into the surrounding water. The Pacific stingray in response has evolved to retain high levels of urea in their blood; this balances the osmotic difference with the surrounding water thus preventing the flow of fluids occurring through their skin into the water. They also have highly adapted rectal glands to secrete excess salt. The Amazonian stingrays however live in fresh water and so have lost the ability to retain high levels of urea along with the degeneration of their rectal glands.

This incredible path from Pacific to Amazonian River home has been suggested to occur in a number of species including anchovies and needlefish. The dramatic story of the Amazon River therefore appeared to give its inhabitants an ultimatum- evolve or become extinct.

Those that could undergo equally dramatic adaptations now have the opportunity to live in one of the most diverse and nutrient rich places in the world. Not a bad price to pay for a few major organ adjustments.

About Julia Galbenu

Studying Biology at Oxford University.