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How the world’s rivers are changing

The rivers of the world are changing, here's how

The Three Gorges Dam has drastically reduced the amount of sediment transported by the Yangtze River in China after its completion in 2003. The top image shows the dam during construction in 1999, when sediment turned the free-flowing river brown. The bottom image shows the completed dam in 2010. Dark blue water flows through the dam with no sediment, trapped upstream in the reservoir, one of an estimated 50,000 in the watershed. Credit: NASA Landsat/United States Geological Survey; Image: Evan Dethier

The way rivers function is significantly affected by the amount of sediment they carry and where it is deposited. River sediment – mostly sand, silt and clay – plays a crucial ecological role, as it provides habitats for organisms downstream and in estuaries. It is also important for human life, supplying nutrients to farmlands in floodplains and buffering sea level rise due to climate change by supplying sand to deltas and coastlines. However, these functions are under threat: Over the past 40 years, humans have caused unprecedented, consistent changes in river sediment transport, according to a new Dartmouth study published in Science

Using satellite images from NASA Landsat and digital archives of hydrological data, Dartmouth researchers examined changes in the amount of sediment carried to the oceans by 414 of the world’s largest rivers between 1984 and 2020.

“Our results tell a story across two hemispheres. In the north, river sediment transport has declined sharply over the past 40 years, while the south has seen a major increase over the same period,” said lead author Evan Dethier, a postdoctoral researcher at Dartmouth. † “Humans have been able to alter the world’s largest rivers at speeds unprecedented in the recent geologic record.

“The amount of sediment that rivers carry is generally determined by natural processes in watersheds, such as how much rain there is or whether there are landslides or vegetation. We find that direct human activities overwhelm these natural processes and even outweigh the effects of climate change .”

The findings show that massive 20th-century dam construction in the global hydrological north — North America, Europe/Eurasia and Asia — has reduced the global delivery of sediment in river suspension to the oceans by 49% from pre-dam conditions. . This global decrease has occurred despite a large increase in sediment supply from the global hydrological south – South America, Africa and Oceania. There, sediment transport on 36% of the region’s rivers has increased as a result of major land use changes.

The changes in sediment transport in the south are mainly caused by intensive land use changes, most of which are related to deforestation. Notable examples are logging in Malaysia; alluvial gold mining in South America and sub-Saharan Africa; sand extraction in Bangladesh and India; and palm oil plantations in much of Oceania. †In previous researchDethier discovered that artisanal gold mining in Peru is associated with increases in suspended sediment levels).

In the north, dam construction has been the dominant factor for river change over the past centuries.

“One of the motivations for this research was the global expansion of building large dams,” said study co-author Francis Magilligan, a professor of geography and the Frank J. Reagan ’09 Chair of Policy Studies at Dartmouth, who studies dams and dams. studies. removal. “In the US alone, more than 90,000 dams are listed in the National Inventory of Dams.” Magilligan says, “One way to think about this is that as a nation, we’ve built an average of one dam a day since the signing of the Declaration of Independence.”

Rivers are responsible for creating floodplains, sandbanks, estuaries and deltas because of the sediment they carry. However, once a dam is installed, that supply of sediment, including the nutrients, is often cut off.

The rivers of the world are changing, here's how

The Maroni River flows through tropical rainforest along the border of Suriname and Guyana. The basin was relatively unchanged until the 1990s. Over the past 25 years, extensive deforestation, mainly through mining, has increased erosion in the basin. The previously dark brown or black water river now carries extra sediment all year round, even during the dry season. These images from 1993 (left) and 2021 (right) show some of the transformation of land use by mining activities and the resulting flow of muddy water into the river. Images: NASA Landsat/United States Geological Survey. Figure composed by Evan Dethier. Credit: NASA Landsat/United States Geological Survey; Image: Evan Dethier

However, in the US and other countries in the Northern Hemisphere, many dams are more than half a century old and fewer dams are being built in the 21st century. The recent decreases in sediment transport are therefore relatively minimal. The construction of dams in Eurasia and Asia over the past 30 years, especially in China, has led to a continuous reduction in global sediment transport.

“For low-lying countries (countries that live at, near, or below sea level) in delta regions, past sediment supply from rivers has been able to offset the effects of sea level rise due to climate change,” Magilligan says. “But now you have the dual drivers of declining sediment from dam construction and rising sea levels.” He says, “This is especially worrisome for densely populated places like Vietnam, where sediment supply has been significantly reduced due to dam activity along the Mekong River.”

The results in the north are striking and could foreshadow future changes for the south, as the study reports that more than 300 dams are planned for major rivers in South America and Oceania. The Amazon River carries more sediment than any other river in the world and is a prime target for these dams.

“Rivers are pretty sensitive indicators of what we’re doing to Earth’s surface — they’re kind of a thermometer of land use change,” said study co-author Carl Renshaw, the Evans Family Distinguished Professor of Earth Sciences at Dartmouth. “But for rivers in the Northern Hemisphere, dams now block that signal for sediment coming to the ocean.”

Renshaw says, “It’s common knowledge that there is a bottom loss crisis in the US, but we just don’t see it in the sediment export record because it’s all bogged down behind these dams as we signal for rivers in the global south.”

Dethier says, “In many cases around the world, we have built our environment around rivers and the way they work, for use in agriculture, industry, recreation and tourism, and transportation, but when human activity suddenly changes the way rivers function.” , it can become difficult to adapt to such effects in real time.”

How dams retain sediment and how land use increases downstream are principles the researchers hope can be used to help guide future planning decisions and policies for land use and environmental management in riparian and coastal areas.


Paper examines how swamp sediment forms and influences river deltas


More information:
Evan N. Dethier et al, Rapid changes in the global river-suspended sediment flow by humans, Science (2022). DOI: 10.1126/science.abn7980

Provided by Dartmouth College


Quote: How the world’s rivers are changing (2022, June 29) retrieved June 29, 2022 from https://phys.org/news/2022-06-world-rivers.html

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