A new study has found over just the past four decades, human activity has been responsible for causing unprecedented, consequential changes to rivers, and the flow of sediment. Using satellite images from the joint NASA-U.S. Geological Survey Landsat program and streamflow data, the researchers studied changes in how much sediment was transported to the oceans by 414 of the world’s largest rivers from 1984 to 2020.
The scientists warned that these changes could place the normal functioning of rivers under threat, as the way rivers flow is significantly affected by how much sediment they transport and deposit.
River sediment, which mostly comprises of sand, silt and clay, plays a vital role in our ecosystem by providing habitats for creatures downstream and in estuaries.
Sediment is also vital to supporting human life, by resupplying nutrients to agricultural soil in the floodplains, while also stemming the rise in sea level due to climate change, by transporting sediment to deltas and coastlines.
Lead author Evan Dethier said: “Humans have been able to alter the world’s biggest rivers at rates that are unprecedented in the recent geologic record.
“The amount of sediment rivers carry is generally dictated by natural processes in watersheds, like how much rain there is or whether there are landslides or vegetation.
“We found that human activities are overwhelming these natural processes and outweighing the effects of climate change.”
The researchers found that widespread dam building during the 20th century played a major role in changing the rivers, particularly in Northern continents like North America, Europe and Asia. This construction is responsible for reducing the global delivery of sediment from rivers to the ocean by around 49 percent.
Meanwhile in the southern hemisphere, which includes South America, Africa and Oceania, sediment transport had increased by 36 percent of the rivers surveyed, which researchers attribute to major land-use changes, most of which were associated with deforestation
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In the northern hemisphere, dam building had been one of the biggest factors causing rivers to change over the past few centuries.
Co-author Francis Magilligan, a geographer at Dartmouth who studies dams and dam removal said: “One of the motivations for this research has been the global expansion of building large dams.”
By transporting sediment, rivers are responsible for the formation of floodplains, sandbars, estuaries and deltas. However, once a dam is built, this sediment flow is completely halted, as fresh soil and nutrients can no longer be deposited.
The researchers believe that the patterns observed in the northern could be used to predict future changes in the south, particularly given that there are over 300 dams that have been planned around large rivers in South America and Oceania, according to the study.
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The most important of these is the Amazon, which carries more sediment than any other river in the world.
Co-author Carl Renshaw, an earth scientist at Dartmouth said: “Rivers are pretty sensitive indicators of what we’re doing to the surface of the Earth — they are like a thermometer for land-use change.
“It’s well-established that there’s a soil loss crisis in the U.S., but we just don’t see it in the sediment export record because it’s all getting stuck behind these dams, whereas we can see the signal for rivers in the global South.”
How dams retain sediment and how land-use practices are increasing downstream erosion are principles the researchers hope can be used to help inform planning decisions and environmental management policies in the future.