The way rivers function is significantly affected by how much sediment they transport and deposit.

River sediment — mostly sand, silt and clay — plays a critical ecological role, as it provides habitat for organisms downstream and in estuaries. It is also important for human life, resupplying nutrients to agricultural soils in floodplains and buffering the rise in sea levels from climate change by delivering sand to deltas and coastlines.

But these functions are under threat: In the past 40 years, humans have caused unprecedented, consequential changes to river sediment transport, according to a new study by scientists at Dartmouth, supported by the U.S. National Science Foundation and published in Science.

Using satellite images from the joint NASA-U.S. Geological Survey Landsat program and streamflow data, the researchers examined changes in how much sediment was carried to the oceans by 414 of the world’s largest rivers from 1984 to 2020.

“Humans have been able to alter the world’s biggest rivers at rates that are unprecedented in the recent geologic record,” said lead author Evan Dethier. “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 findings show that widespread 20th century dam building in Earth’s northern areas — North America, Europe and Asia — reduced the global delivery of water-borne sediment from rivers to the oceans by 49% relative to pre-dam conditions. However, in Earth’s southern areas — South America, Africa and Oceania — sediment transport has increased in 36% of rivers due to major land-use changes, most of which are associated with deforestation.

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.

The news story can be read here and here.

The peer reviewed research paper in Science can be read here.