As rivers flow, they transport suspended sediment, such as sand, silt or clay, which is crucial for downstream ecosystems. The sediment is deposited in places along the way, forming habitats for organisms in the river, as well as in estuaries and deltas. It also supplies nutrients to floodplains where humans till the soil and grow crops, and provides a buffer against sea level rise when it is deposited along coastlines.
Human activities in the past 40 years have drastically changed the amount of river sediment that makes it to the world’s oceans and seas, thereby affecting all of these functions. A new study, led by scientists from Dartmouth College, finds that humans have caused unprecedented, consequential changes to river sediment transport, and to the way that nutrients are cycled into the oceans.
The scientists used NASA Landsat imagery and digital archives of hydrologic data, to estimate changes in how much sediment is carried to the oceans by 414 of the world’s largest rivers. The satellite images covered river hydrology from 1984 to 2020, and field work on the ground delivered over 100,000 measurements to corroborate the estimates.
“Our results tell a tale of two hemispheres. The north has seen major reductions in river sediment transport over the past 40 years, while the south has seen large increases over the same period,” said study lead author Evan Dethier, a post-doctoral fellow at Dartmouth. “Humans have been able to alter the world’s biggest rivers at rates that are unprecedented in the recent geologic record,” Dethier says.
“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 find that direct human activities are overwhelming these natural processes, and even outweighing the effects of climate change.”
These findings, published in the journal Science, attribute the reduction in river sediment transport in the Northern Hemisphere mostly to 20th century dam building activities. The construction of dams in North America, Europe/Eurasia and Asia has reduced the global delivery of river sediment to the oceans by a massive 49 percent, when compared to pre-dam conditions. This reduction has occurred despite major increases in sediment delivery from Southern Hemisphere rivers.
“One of the motivations for this research has been 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 dam removal. “In the U.S. alone, there are more than 90,000 dams listed in the National Inventory of Dams.”
“One way to think about this is that we, as a nation have been building on average, one dam per day, since the signing of the Declaration of Independence.”
Dam building in the U.S. and Europe has decreased since the start of the 21st century, however, and many of the current dams are more than half-a-century old. Sediment transport by rivers in the U.S. and Europe has declined only minimally in recent decades, as a consequence. However, large dams have been built in Eurasia and Asia in the past 30 years, especially in China, and these new constructions have largely driven ongoing reductions in global sediment transport in the Northern Hemisphere.
Reductions in river sediment flux can have serious consequences for low-lying countries that are at risk from rising sea levels. “For low lying countries (countries that lie at, near or below sea level) in delta regions, sediment supply from rivers has in the past, been able to help offset the effects of sea level rise from climate change,” said Professor Magilligan.
“But now you’ve got the double drivers of declining sediment from dam construction and rising sea levels. This is particularly worrisome for densely populated places like Vietnam, where sediment supply has been reduced significantly by dam activity along the Mekong River.”
By contrast, sediment transport in the global hydrologic south – South America, Africa and Oceania – has increased in 36 percent of the rivers. This situation has been driven by changes in land use, many of which are associated with deforestation and the increase in erosion that results from this practice. Examples of changes in land use around rivers include logging in Malaysia, alluvial gold mining in South America and sub-Saharan Africa, sand mining in Bangladesh and India, and palm oil plantations across much of Oceania.
The researchers found that rivers studied in the global hydrologic south carried suspended sediment at a concentration that was, on average, 41 percent greater now than it was in the 1980s. This north-south divergence has affected global patterns of sediment flux into the oceans, with the dominant sources of river sediment no longer being rivers in Asia, but now being rivers in South America.
Interestingly, this situation could change in the near future as there are more than 300 new dams planed for large rivers in South America and Oceania. The Amazon River carries more sediment than any other river in the world and is a major target for these dams.
“Rivers are pretty sensitive indicators of what we’re doing to the surface of the Earth – they are sort of like a thermometer for land use change,” said study co-author Carl Renshaw, the Evans Family Distinguished Professor of Earth Sciences at Dartmouth. “Yet, for rivers in the Northern Hemisphere, dams are now blocking that signal for sediment coming to the ocean. 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.”
“In many cases throughout the world, we have built our environment around rivers and the way that they operate, for use in agriculture, industry, recreation and tourism, and transportation; but when human activity suddenly disrupts the way rivers function, it may become difficult to adapt in real-time to such impacts,” said Dethier.