Why the Amazon River Has No Delta – Part I

There has been some discussion lately about the Amazon River (Rio Amazonas) having no delta (a wetland formed as rivers empty their water into the ocean), and thus, if one has not had time to form, might prove its young age. In this way some have thought it might show the late rise of the Amazon Basin concurring with when the Andes rose. While the fact that the Amazon has no delta might, in part be due to a young age, we need to better understand the Amazon River before we jump to any such concrete conclusions. 
     In understanding the Amazon, we need to keep in mind certain well known facts, such as this river being the mightiest in the world by volume, with six times greater total river flow than the next six largest rivers combined, and eleven times greater than the Mississippi, and the most extensive drainage basin in the world.

Darker green area shows the Amazon Drainage Basin; where waters over a 2.7 million-square-mile area drain, amounting to 40% of South America, into the Amazon River and flow into the Atlantic Ocean at the river mouth where it empties into the sea; dark blue line is the actual Amazon River (light blue lines are some of the 1100 tributaries, 17 of which are longer than 1000 miles)

Because of its vast dimensions it is sometimes called The River Sea. In fact, it stretches across eight countries (Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, Guyana, and Suriname as well as French Guiana—a department of France), with the great majority located in Brazil, Peru and Ecuador. The Basin, roughly the size of the 48-contiguous United States, is the largest in the world by volume (a mind-boggling average discharge of 219,000 m³/per second of water, and for the most part barely above sea level, with much of it swamp area where 6-month or longer wet weather floods much of the area.
    The reason the Basin floods these six months of the year is because it is barely above sea level and relatively flat, consequently, when heavy seasonal rainfall, concentrated in the Eastern Andes and the Northwest area of the Basin, begin to flow into the lowlands, there is more water than the waterways can contain, forcing the excess water outside the riverbanks and into the low-lying floodplains. As the water spills over from the rivers, new bodies of water are created, such as ponds and oxbow lakes.
    The uneven distribution of the seasonal rainfall causing the inundation of different parts of the river system at different times, the overall floods last longer than would be the case if temporal distribution of precipitation were the same throughout the basin. This causes flooded areas to extend 10 to 15 miles from the river banks, and water to rise 25 to 50 feet in height above normal. Thus these radical seasonal changes throughout the year forces the water level all over the Basin to fluctuate annually as much as 50 feet along the middle and lower Amazon.
    Initially, the Amazon had no general name; instead, indigenous peoples had names for the sections of the river they occupied, such as Paranaguazu, Guyerma, Solimões, and others. In the year 1500 A.D., Vicente Yañez Pinzon, in command of a Spanish expedition, became the first European to explore the river, investigating its mouth when he discovered that the ocean off the shore was freshwater.
    Pinzon called the river the Rio Santa Maria de la Mar Dulce, which soon became abbreviated to Mar Dulce, and for some years, after 1502, it was known as the Rio Grande. Originally, the early Spanish explorers were bewildered by the difficulties they met in navigating not only the entrance to the Amazon, but the entire island-bordered, river-cut and indented coast, and called the river El Río Marañón, since maraña meant “a tangle,” or “a snarl.”
    However, the actual name arose from a battle that Francisco de Orellana had with a tribe of Tapuyas where the women of the tribe fought alongside the men, as was their custom. This caused Orellana to name the river the “Amazonas” in recollection of the ancient Greek accounts of the Amazons of Asia described by Herodotus and those of Libya claimed by Diodorus.
    This mighty river, runs about 4,000 miles from source to mouth, and most sources arguably regard the Amazon as the second longest river in length, a close second to Africa’s Nile River. The important part of this is that the area covered by the Amazon and its tributaries more than triples between the dry season and wet season over the course of a year. In an average dry season, 68,350 square miles of land are water-covered, while in the wet season the flooded area of the Amazon basin rises to 217,480 square miles.
    At its widest point the Amazon River can be 6.8 miles across during the dry season, but during the rainy season when the Amazon River floods the surrounding plains it can be up to 24.8 miles wide. In fact, the average depth of the river in the height of the rainy season is 120 feet and the average width can be nearly 25 miles.
    The river starts to rise in November, and increases in volume until June, then falls until the end of October. The rise of the Rio Negro (Black River) branch, which links two major river systems, is not synchronous; the rainy season does not commence in its valley until February or March. By June it is full, and then it begins to fall with the Amazon, while the Madeira rises and falls two months earlier than the Amazon.
    Another critically important point is that the quantity of freshwater released by the Amazon into the Atlantic Ocean is enormous: up to 11.8 million square inches per second in the rainy season, making the Amazon responsible for one-fifth of the total volume of freshwater entering the oceans worldwide. In fact, early sailors could drink freshwater out of the ocean before sighting the South American continent.

Freshwater from the Amazon can reach as much as 200 miles into the Atlantic from the river mouth

In fact, offshore of the mouth of the Amazon, potable water can be drawn from the ocean while still out of sight of the coastline, and the salinity of the ocean is notably lower a hundred miles out to sea. One of the reasons for this low salinity, it is believed, is that a recent discovery of a second river flowing the length of the Amazon, some 13,000 feet below the surface, was evidently discovered by Dr. Valiya Hamza and his team of researchers from Brazil’s National Observatory in 2011 . This subterranean river is claimed to run west to east a little less than one-fourth the flow of the surface Amazon, and enters the Atlantic beneath the surface river in some of the muddiest waters in the world.
    At this Amazon River mouth, or the inundated land in and around it, is called the Marajó Várzea. The huge river, having completed its 4,000-mile journey here, empties into the Atlantic Ocean. This area contains numerous islands, the largest being Ilha Marajó at 29,826 square miles. Other islands include Ilha dos Porcos, do Pará, Mututí, and Uituquara. This Amazon estuary is a dynamic lowland consisting of old sediments surrounded by slightly older deposits. Flooding occurs across the landscape twice daily when the ocean tide pushes a large volume of river discharge onto the landscape to a height of 6½ to 10 feet. There are both low-lying tidal floodplain areas and slightly higher ground that is not normally flooded. The entire area is characterized by an abundance of poorly-drained heavy mottled clay soils. This type of várzea is different from the seasonal várzea of the middle and upper Amazon floodplains, which are inundated annually by rivers.
    As mentioned earlier, despite all this, the Amazon River Mouth is not a delta in the true sense of the word. The question still remains, why does it not have a delta, given the enormous time of its development according to Geologists?
(See the next post, “Why the Amazon River Has No Delta – Part II,” to understand why there is no delta at the mouth of the Amazon and whether this means the Amazon River is a recently developed water course)