Monday, March 12, 2018

Why the Amazon River Has No Delta – Part II

Continued from the previous post regarding the absence of a delta at the mouth of the Amazon River, one of the only major rivers in the world that does not have such a delta.
A Satellite View of the Amazon River Mouth, showing the island of Marajó (size of Denmark or Switzerland), the Mouth of the Xingú River and the dense rain forests

The region around the mouth of the Amazon River reaches west to the mouth of the Xingú River on the south bank where the Amazon starts to enlarge its width. A small portion occurs in French Guiana, just to the north of the Amazon mouth. This entire area contains bits of both seasonally inundated floodplain forest and permanently inundated freshwater swamp forest (igapó).
    This western half of the Marajó Island, located on the south banks of the Amazon mouth, is constructed of recent sediments and hosts the estuarine (tidal) várzea forest. The eastern half of Marajó island, on older Tertiary sediments, is covered by flooded savanna (campo) and humid terra firme forest, the forest being fairly homogeneous and distinct from the inundated forest in the west.
    This estuary, that part of the tidal mouth where the tide meets the stream, is over 202 miles wide. The main river (which is between approximately one and six miles wide) is navigable for large ocean steamers to Manaus, Brazil, more than 900 miles upriver from the mouth. Smaller ocean vessels of 3,000 tons can reach as far as Iquitos, Peru, 2,250 miles from the sea, and smaller riverboats can reach 486 miles higher as far as Achual Point, 2786 miles from the sea.
Pongo de Manseriche on the eastern slopes of the Andes Mountains in Peru

Beyond that, small boats frequently ascend to the three-mile long, white-water Pongo de Manseriche (a pongo is a river gorge water channel through mountain canyon walls connecting two valley basins, usually involving torrents, whirlpools and eddies, along with rocks, almost impossible to cross without motorized vessels), just above Achual Point, almost 3000 miles from the Atlantic.
    The Amazon drains an area of some 2,722,000 square miles, or nearly 40 percent of South America. It gathers its waters from 5 degrees north latitude to 20 degrees south latitude. Its most remote sources are found on the inter-Andean plateau, just a short distance from the Pacific Ocean; after coursing through the interior of Peru and across Brazil, it enters the Atlantic Ocean at the equator. The Amazon has changed its drainage several times, from westward in the pre-historic period to its present eastward direction following the uplift of the Andes Mountains.
    The width of the mouth of the river is usually measured from Cabo do Norte to Punto Patijoca, a distance of some 207 miles; but this includes the 40-mile wide ocean outlet of the Para River, which should be deducted, as this stream is only the lower reach of the Tocantins. It also includes the ocean frontage of Marajó, an island about the size of Denmark lying in the mouth of the Amazon.
    Following the coast, a little to the north of Cabo do Norte, and for 100 miles along its Guiana margin up the Amazon, is a belt of half-submerged islands and shallow sandbanks.
    The floodwaters of the Amazon River are laden with suspended mineral sediments that settle out onto the landscape when the water flow slows. This area is called "region of the islands" because of the intricate labyrinth of sedimentary islands and channels resulting from the constant tidal and fluvial action. This várzea region comprises the western half of Marajó Island, many smaller channel islands, and the surrounding mainland on both the north and south banks of the Amazon.
    It should also be noted that the river current carries tons of suspended sediment all the way from the Andes and gives the river a characteristic muddy whitewater appearance. It is calculated that 106 million cubic feet of suspended sediment are swept into the ocean each day. The result from the silt deposited at the mouth of the Amazon is Majaro island, but a true delta has not occurred.
A tidal bore rushing upriver on the Amazon, measuring about ten feet in height and overflowing the banks of the river

In part, this is due to the tidal phenomenon called the “tidal bore” or pororoca (meaning “great roar” in Tupi, a native Brazilian language), which pushes a tidal wave up the Amazon about 500 miles from the Atlantic—a large wave caused by the funnel of a flood tide as it enters a long, narrow shallow inlet).
    This occurs up the Amazon where the depths are not over 4 fathoms (23 feet). The tidal bore starts with a roar, constantly increasing, and advances pushing up the river against the river’s flow or current downward, at the rate of 10 to 15 miles per hour, with a breaking wall of water 5 to 12 feet high.
Amazon Mouth showing the various paths of the river, which creates numerous islands along its length mouth

This results in a positive surge upriver against a lesser, negative surge downriver, even though the mouth of the Amazon is not narrow, the river still has this uniquely strong tidal bore due to the many low-lying islands and sand bars.
    The Amazon also has a very large tide sometimes reaching 20 feet, and this tidal bore is the reason the Amazon does not have a river delta. Thus, the ocean rapidly carries away the vast volume of silt carried by the Amazon, making it impossible for a delta to grow.
    In addition, we might point out that Geologists all agree that the Andes rose as the result of a collision of the South American plate with the Nazca plate. The rise of the Andes and the linkage of the Brazilian and Guyana bedrock shields, blocked the river and caused the Amazon to become a vast inland sea. Gradually this inland sea became a massive swampy, freshwater lake and the marine inhabitants adapted to life in freshwater. For example, over 20 species of stingray, most closely related to those found in the Pacific Ocean, can be found today in the freshwaters of the Amazon. Some of these waters worked through the sandstone to the west and the Amazon began to flow eastward, and the Amazon rainforest was born. Also around this time, the Central American isthmus rose, allowing a mass migration of mammal species between the north and south to take place. With the central basin exposed, savannas appeared along with divided patches of rainforest into “islands” that later merged as the rain forest expanded.
    The problem lies not in these facts, but in the time frame of when they occurred. This, of course, depends on whether a person believes in evolution and a 4.55-billion year old Earth, or one that involves the creation and organization of the Earth by the lord and uses the chronology of the Patriarchal dates of the Bible and Pearl of Great Price, which then brings these events to within the time of man.

5 comments:

  1. Well nuts! I guess we can't use the argument that no delta means young river in the case of the Amazon. Of course, then again, who would know why there is no delta.

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  2. I did some research, and found that the Ganges river in India has the largest delta of any river in the world. And the Ganges ALSO has very significant tidal bore waves.

    So saying the tidal bore on the Amazon causes it to have have no delta cannot be the whole story.

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  3. I still like the idea of Big River = No Delta= young.

    Good point erichard

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  4. There is no question that the Amazon River is a young river. Finding scientists to agree with that is going to be fruitless, I think. But we know the entire story, they ignore half of the knowledge.

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  5. True Del, these old-earth and catastrophe deniers will always find a way to explain away a detail like no delta.

    Good work!

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