The health of river ecosystems hinges on sediment transport: how rivers pick up, move, and deposit sediment shapes downstream habitats and estuaries. These sediments also refresh agricultural soils on floodplains with nutrients and help buffer coastlines by delivering sand to deltas.
Yet these essential functions face serious threats. Over the past four decades, human activity has drastically altered the way rivers carry sediment to the ocean. A study from researchers at Dartmouth used satellite imagery and runoff data to examine global sediment transport for 1984 through 2020, covering 414 of the world’s largest rivers.
The findings were stark. Lead author Evan Dethier notes that humans have reshaped the world’s greatest rivers at a scale not seen in the recent geological record.
The amount of sediment moved by rivers depends on natural watershed processes such as rainfall, landslides, and vegetation. The researchers found that human activities have outpaced these natural forces, even surpassing the effects of climate change.
Sediment transport dropped by 49% after dam construction.
The study shows many dams were built across northern regions of the globe, including North America, Europe, and Asia, during the 20th century. These dam projects reduced the global movement of sediment from rivers to oceans by nearly half when compared with conditions before large-scale dam construction.
In southern regions, such as South America, Africa, and Oceania, sediment transport rose in about 36% of rivers, driven by major land-use changes, especially deforestation.
In the north, dams emerged as the principal driver of river change in recent centuries. Co-author Francis Magilligan, a Dartmouth geographer who studies dams and their removal, says one motivation behind the research is the global spread of large-scale dam projects.
Rivers create floodplains, levees, estuaries, and deltas through sediment deposition. When a dam is built, this sediment source, along with essential nutrients, is often cut off, altering downstream ecosystems.
The northern region results raise questions about future changes in the southern hemisphere. The study notes more than 300 dams are planned for major rivers in South America and Oceania, including projects on the Amazon that would alter sediment loads and river dynamics.
Rivers act as accurate indicators of surface processes, functioning like thermometers for land-use change. The researchers emphasize that although there is a well-known crisis of land loss in the United States, sediment export records behind dams often mask this signal, whereas river systems elsewhere reveal the broader trend.
Reference: Science journal, 2020 note on global sediment transport changes. Citation: Science, 2020.
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