Connect with us


The oceans lack their rivers




GFrom the eighth floor of a hotel in Georgetown, Guyana, the wide expanse of the Atlantic Ocean looked dirty brown. Only a thin blue border on the horizon showed the true color of the ocean; the rest were swirled by sediments coming out of the mouth of the Essequibo River.

In a rhythm that pulsates through the ages, the river plume carries sediment and nutrients from the interior of the continent to the ocean, the main exchange of resources from land to sea. More than 6,000 rivers around the world carry fresh water to the oceans, delivering nutrients, including nitrogen and phosphorus, that feed phytoplankton, giving rise to the flourishing of life, which in turn feeds ever larger creatures. They may even influence ocean currents in ways that researchers are only just beginning to understand. But today, in rivers around the world, people are aging from this critical phenomenon.

In many places, the main culprit is the dam: a wall of concrete and stone that bisects a river and diverts its energy and water to human needs. There is 58,000 “big ladies”– 50 feet tall or taller – worldwide, 3,700 more planned, mostly in low-income countries in Asia and South America.

Sea level rise deserves headlines, but sunken land is no less a problem.

Many of the harms caused by dams are well documented. They block the passage of fish and starve the fishermen who fish; radically change the natural regimes of rivers and the life cycles of aquatic creatures; and flooded forests, wetlands, villages and historic sites. (so they less climate friendly1 another reliable2 than is widely believed.) Scientists now describe another impact that has received relatively little attention, but which also appears to be serious: dams block sediment-carrying river surges into the ocean.

The researchers used satellite data to evaluate sediment changes over the past 40 years in 414 rivers around the world and found that dams in the global north—North America, Europe and Asia—blocked 49 percent of sediment delivery to coasts.3 This conclusion, which was published last year in the journal The science, is even more startling when you consider the frenzy of dam building that took place in the 1970s. The baseline survey of the early 1980s probably already showed a significant deviation from the natural state of the rivers.

“People have built structures that have vastly outperformed the effects of climate change in many ways,” says Evan Dethier, an oceanographer at Bowdoin College and lead author of the study.

Sediments play a vital role in river and coastal systems. As rivers reach their floodplains, the water slows down, shedding the silt that shapes its course and nourishes the vegetation that grows among the canals. The sediments also bring nutrients to the floodplain, helping to speed up algal blooms, which in turn feed the phytoplankton and juvenile fish. At the edge of the sea, sediments are constantly reclaiming coastal land otherwise eroded by ocean waves. The rising seas deserve the headlines, but the sunken land no less serious problem for people living in river deltas, now devoid of sediment.4

The stories of rivers and the sediment they carry do not end there. The ocean is turbulent, which can cause its currents to meander back and forth like torrents on land. Meanders become unstable, forming whirlpools where strong currents meet weak currents and where fresh water meets denser, often colder, salt water. When rivers empty into the oceans, they form a plume of fresh water that can extend up to 80 miles from the coast; the resulting collision of fresh and salt water, their difference in temperature, and the sediments they carry create chains of whirlpools, similar to the lines of the Congo whirlpools, that affect larger ocean circulation patterns.

In body image
RIVER IN THE SEA: Fresh water from the Mekong River circulates through the South China Sea, pictured here between August and October 2014. Image from Zeng, X., et al. Dynamic impact of the Mekong plume in the South China Sea. JGR Oceans (2022).

Researchers led by Annalize Bracco, an oceanographer at the Georgia Institute of Technology, have studied these dynamics. in the study plume created by the Mekong River, the 12th longest river in the world.5 It flows nearly 3,000 miles from its headwaters in the Tibetan Plateau through China, Myanmar, Thailand, Laos, Cambodia and Vietnam before reaching the South China Sea.

Over 150 dams have been built in the Mekong Basin, including 13 in the main riverbed, and more than 100 additional dams are planned. To understand how they might affect the South China Sea, Bracco’s team created a computer model of how the Mekong plume affects the sea’s circulation today. They then modeled how the plume would behave if more dams were built, drastically reducing the Mekong’s average annual flow and its seasonal cycle.

“You do get different transportation,” says Bracco. Because fresh water is less dense than salt water, plume water tends to stay at the surface, where it can be more easily moved by wind. If the Mekong plume decreases, she said, the winds will need more energy to move sea water, which will slow down the speed of the currents.

Bracco’s team found that in the South China Sea, summer monsoon winds drive currents northeastward, bringing nutrients, food and warmer temperatures with them. Future dams will cause the eddy current to oscillate, weakening the northward movement of currents and reducing the productivity of the marine ecosystem.

Bracco recalls the time she was on the boat, measuring ecosystem changes along the river’s plume. “You see plankton that emerge and flourish under the influence of river water,” she says. This in turn feeds larger species. “If you drastically change a river so it no longer produces a plume, you prevent the ecosystem from flourishing.”

Dams are less environmentally friendly and reliable than is commonly believed.

Glen Gavarkiewicz, a physical oceanographer at the Woods Hole Oceanographic Institution who studies the Northwest Atlantic, notes that eddy currents carry sediment from river plumes across continental shelves to ocean basins, and fish concentrate on these nutrient fronts. “The decrease in eddies could mean that fewer nutrients are entering ocean basins. And in Southeast Asia, fish is very important for both protein and culture,” says Gavarkevich. Fishing in the South China Sea is already a blast diplomatic disputeshe says, and changes in nutrient circulation could spark new conflicts.

While Bracco’s findings about a likely decline in ocean productivity in the South China Sea are sobering, she cautions that the dynamics she observes are not universal. “Nothing happens the same way everywhere,” she says. For example, the plume of the Mississippi River does not change the direction of currents in the Gulf of Mexico. “The loop current, which is the main ocean current into the Gulf of Mexico, is just so big and so strong.” (Of course, the Mississippi plume has another well-documented impact: nutrient overload from agricultural runoff. creates an extensive dead zone Every year.)

However, it is difficult to accurately estimate the broader impact because today’s climate models – the only tools available to predict the behavior of the Earth system for decades to come – are too low resolution to accurately show the impact of river plumes. The oceanic turbulence tracked by Bracco in the South China Sea, which affects the patterns of eddies and currents, has a scale of one to two kilometers. Most climate models have a resolution of 50 to 100 kilometers. With such a coarse resolution, turbulence from river plumes is not recorded. When existing models try to account for freshwater input from large rivers, they “mix that water in a way that is very different from reality,” says Bracco.

Gavarkevich notes a related issue. In his own research on how fresh water gets into the depths of the ocean, he found that “there are a lot of surprises. The dynamics are very complex.” The interaction of wind and water, the mixing of tides, the presence and intensity of sunlight, eddy movements all affect what happens. But climate models typically use only one variable—the density difference between fresh river plume and salt sea—to represent all of these factors.

Human-induced changes in the quantity, quality and timing of river plumes can have other unpredictable consequences for the ocean. Dam building is not the only way humans are changing river plumes. Instead of holding back sediment, sometimes people release huge amounts of excess sediment.

A study documenting a dramatic reduction in sediment delivery to coasts in the global north found the opposite problem in the global south. Coasts in South America, Africa and Oceania are now receiving 36 percent more sediment than four decades ago, largely due to runoff and erosion caused by clearing forests to make way for palm oil, soybeans, sugar cane and mining.

Over the past decade, the Gulf of Mexico and the Caribbean have experienced a nasty explosion of sargassum seaweed; Bracco believes this will eventually be due to runoff caused by deforestation, which brought excess sediment into the Amazon River plume.

Most ocean impacts caused by anthropogenic changes in river plumes are local rather than global, Bracco said. But with dams already damming two-thirds of the world’s major rivers and thousands more planned, these local impacts could affect ocean life just about anywhere, and we’re only just beginning to reckon with them.

First image: Mekong Delta in Vietnam. Credit: European Space Agency.


1. Fernside, P. M. and Pueyo, S. Greenhouse gas emissions from dams in the tropics. NatureClimate Change 2382-384 (2012).

2. Gies, E. Can wind and solar power secure Africa’s future? Nature 53920-22 (2016).

3. Dethier, E.N., Renshaw, K.E., and Magilligan, F.J. Rapid changes in the global river flow of suspended sediments under human influence. Science 376, 1447-1452 (2022).

4. One Earth Editorial. Keep up with changing deltas. One Earth 6183-184 (2023).

5. Zeng H., Brakko A. and Tagklis F. Dynamic effects of the Mekong plume in the South China Sea. jgr oceans 127e2021JC017572 (2022).

Published in partnership with:

Continue Reading
Click to comment

Leave a Reply

Your email address will not be published. Required fields are marked *


NASA’s Webb Space Telescope discovers water around mysterious main belt comet



NASA’s James Webb Space Telescope has helped astronomers find gas around a comet in the main asteroid belt for the first time.

In a press release, the agency said the presence of water vapor was confirmed using the observatory’s near-infrared spectrograph instrument.

The findings indicate that water ice from the early solar system, which formed about 4.5 billion years ago, may be preserved in the region.

However, unlike other comets, scientists have stated that comet 238P/Read does not contain carbon dioxide.


An artist’s concept of comet 238P/Read shows the main belt comet sublimating, its water ice evaporating as it orbits the Sun. Sublimation is what distinguishes comets from asteroids, creating their characteristic tail and hazy halo. (NASA, ESA)

“Our water-drenched world, teeming with life and unique in the universe, to our knowledge, is something of a mystery — we’re not sure how all that water got here,” Stephanie Milam, Associate Scientist for Planetary Science at the Webb Project. and co-author of the study reporting the discovery, published in the journal Nature, the statement explains.

“Understanding the history of the distribution of water in the solar system will help us understand other planetary systems and see if they might be on their way to creating an Earth-like planet,” she said.

A comet is an object that is found in the main asteroid belt and periodically displays a halo and tail like a comet.

These main belt comets are a fairly new classification, and Comet Reed was one of three comets used to establish this category.

Prior to this classification, comets were known to be outside the orbit of Neptune, where ice could persist further from our Sun.

emission spectrum data

This graphical representation of the spectral data highlights key similarities and differences between the 2022 observations of Comet 238P/Read by NASA’s James Webb Space Telescope Near-Infrared Spectrograph and those of Comet 103P/Hartley 2 by NASA’s Deep Impact mission in 2010. show a distinct peak in the region of the spectrum associated with water. (NASA, ESA, CSA and J. Olmsted (STScI))


The frozen material that evaporates as comets approach the sun is what distinguishes them from asteroids, giving them their characteristic halo and tail.

Scientists have previously speculated that water ice might persist in the warmer asteroid belt – inside the orbit of Jupiter – but NASA said the evidence has so far been elusive.

Comet in the night sky

This image of Comet 238P/Read was taken by NASA’s James Webb Space Telescope Near Infrared Camera on September 8, 2022. The dusty coma and tail are the result of the evaporation of ice as the sun heats the main body of the comet. (NASA, ESA, CSA, M. Kelley/University of Maryland | Image Processing: H. Hsieh/Planetary Science Institute), A. Pagan (STScI)

“Thanks to Webb’s observations of Comet Reed, we can now demonstrate that water ice from the early solar system can persist in the asteroid belt,” said University of Maryland astronomer Michael Kelly, lead author of the study.


The lack of carbon dioxide, which typically makes up about 10% of the volatiles in a comet, has two possible explanations presented by the researchers.

“Being in the asteroid belt for long periods of time can do this – carbon dioxide evaporates more easily than water ice and can seep out over billions of years,” Kelly suggested, also suggesting that Comet Reed could have formed in a particularly warm part of the planet. A solar system where there was no carbon dioxide.

Continue Reading


Paleontologists have discovered a new species of spinosaurus dinosaur



A new genus and species of spinosaurid dinosaur, named Protathlitis cinctorrensis, was discovered by Dr. John Wilson. Andres Santos-Cubedo of Jaume I University and colleagues.

The post Paleontologists discover new spinosaurian dinosaur species first appeared on Sci.News: Breaking Science News.

Continue Reading


United States returned to Mexico the sculpture of the Olmec “Earth Monster”



MEXICO CITY (AP) — Mexico announced Friday that a huge 2,500-year-old Olmec stone sculpture has been returned from the United States.

Nearly six feet (two meters) tall, the “Earth Monster” sculpture appears to represent the gaping maw of a monster large enough to swallow humans, and may represent a symbolic entrance to the underworld.

Experts say the sculpture is important because it provides insight into the cosmological vision of the Olmecs, considered the founders of Mesoamerican culture.

Foreign Secretary Marcelo Ebrard said the sculpture was handed over to the Mexican consulate in Denver, Colorado on Friday.

Experts believe the sculpture was smuggled out in the 1960s from Chalcatzingo, a lesser-known ruin south of Mexico City.

“It was like an open wound when I didn’t have this artifact,” Ebrard said.

Continue Reading