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.

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 were to decrease, she said, the winds would need more energy to move seawater, slowing 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 in it. The dynamics are very complex.” The interaction of wind and water, the mixing of tides, the presence and intensity of sunlight, and 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.

Recommendations

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).

• Erica Gies

  Published May 3, 2023

  Erica Gies – Author Water Always Wins: Prosperity in an Age of Drought and Flood freelance journalist who writes for Scientific American, Nature, The New York Timesand other publications. Find her on LinkedIn, TwitterAnd mastodon.

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