Why is the sea "divided in two"?

(Dan Tri Newspaper) - The truth behind this seemingly supernatural phenomenon can be fully explained by science.

The image shows the seawater in Sam Son "divided" into two distinct colors (Photo: Screenshot from a video).

Recently, a video clip showing the sea water in Sam Son "dividing" into two distinct colors, one side clear blue, the other murky, has gone viral on social media.

Filmed around 4 PM on July 13th by a drone, this strange sight has sparked curiosity and led many to speculate about some kind of supernatural phenomenon in the middle of the ocean.

However, from a scientific perspective, this is not a mysterious phenomenon but rather the inevitable result of physical, geological, and biological processes that occur daily in nature.

The sea "divides": When two different currents of water meet.

Sam Son Beach is located near the lower reaches of the Ma River, one of the major rivers in northern Vietnam. After each heavy rain, water from upstream carries a large amount of silt, organic waste, and sediment into the sea.

When this murky freshwater meets the clear, blue saltwater offshore, the interaction between the two fluid masses with different physical properties creates a distinct boundary on the sea surface.

This is the cause of the "two-colored sea" phenomenon that netizens witnessed in Sam Son. This phenomenon is not uncommon in nature and has been recorded in many places around the world .

The confluence of waterways in Brazil (Photo: Getty).

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In Brazil, the confluence of the Rio Negro (black water) and Solimões (brown water) rivers is so distinct that it has been given its own name: "Meeting of Waters".

This area is known as a place where two streams run parallel for kilometers without mixing, due to differences in temperature, flow speed, salinity, and sediment load.

Similarly, the Alaskan coast has also witnessed the phenomenon of murky, sediment-laden glacial water meeting clear blue seawater, creating a striking color boundary that has been captured by NASA satellites.

Laws of fluid dynamics

The color difference between the two streams of water is a result of variations in density, salinity, temperature, and optical properties.

There, freshwater from the river tends to be lighter, less saline, and often contains many suspended sediment particles, making it turbid and giving it a gray, brown, or yellow color depending on the concentration of matter.

Meanwhile, seawater, with its higher salinity and density, is cleaner, so it strongly scatters blue light, making the sea appear a deep blue. This difference maintains a boundary clearly visible to the naked eye between the two bodies of water.

In the case of Sam Son, after heavy rains, water from the Ma River carried a large amount of sediment to the sea, creating a murky area of ​​water. The interaction between the river water and the seawater did not mix immediately, resulting in a two-colored streak of water: one side light blue, the other a darker, silt-laden color.

Satellite imagery reveals a unique coloration of seawater off the coast of Argentina (Image: NASA).

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However, this separation is only temporary. According to Professor Ken Bruland from the University of California, USA, the two distinct streams will eventually mix together through diffusion, ocean waves, and eddy currents.

The dissolution time depends on factors such as density gradient, current speed, wind speed, and coastal geomorphology.

Beyond its visual implications, this phenomenon of color separation also has significant biological meaning.

Specifically, in areas where sediment from rivers carries large amounts of iron or nutrients, it can stimulate the growth of plankton, forming the first link in the food chain of the marine ecosystem, as well as influencing the feeding and migration behavior of many fish species.

Source: https://dantri.com.vn/khoa-hoc/vi-sao-nuoc-bien-chia-doi-20250715090656309.htm