Sediment flux is crucial to river health. Sediment supply and transport processes build and maintain habitats for freshwater organisms, mitigate sea level rise by replenishing deltas and coastlines, and influence carbon and nutrient fluxes across watersheds.
Now, using observational data collected from NASA’s Landsat satellites, scientists have increased understanding of the extent to which human activity has altered how much sediment flows from rivers into the ocean. Many rivers have decreased how much sediment they carry by more than 50%, while others have increased it by more than 100%.
According to the research, sediment flux has decreased in the Global North, a decrease mostly associated with dams. Rivers in the Global South, in contrast, have seen an increase in sediment flux corresponding to erosion associated with agriculture and river mining.
The results were published last month in Science.
Humans as a Geomorphic Agent
This study illustrated the principle of “humans as a geomorphic agent,” said Abdul Hameed Kleo, an assistant professor of geomorphology at Mansoura University in Egypt who was not involved in the research.
“Humans have a vast, visible impact on the landscape. They affect the physical environment, negatively or positively,” Kleo added.
In fact, humans are the most powerful geomorphic agent on Earth, at least in the context of large rivers, said Evan Dethier, a postdoctoral fellow in the Department of Earth Sciences at Dartmouth College and the study’s lead author. “But the magnitude of these changes is even more than we expected in many cases, both in how much sediment flux…has been reduced from northern rivers and [in] how much [it has] increased in southern rivers.”
Monthly Record of Suspended Sediment
Dethier and his colleagues used a suite of algorithms developed over the past decade to take measures of water reflectance captured by Landsat images and convert them to estimates of suspended sediment concentration. Many of the 414 rivers studied do not have active monitoring programs, and some of them have never had formal monitoring of suspended sediment. Regardless, the researchers were able to construct a close to monthly record of suspended sediment concentration starting in the mid-1980s.
“Obtaining such data on the ground is extremely difficult, if not impossible,” said James Leonard Best, a professor of Earth sciences at the University of Illinois Urbana-Champaign who was not involved in the research. “And thus this study marks an important milestone in our understanding of change within the world’s rivers and human impacts on this change.”
The presence of dams and dam construction was noticeable in sediment flux. The sediment flux in the Mississippi River, for instance, did not decrease significantly; there were few new dams constructed during the time period studied (1984–2020). In contrast, sediment flux in the Yangtze River decreased by 65%–80%. Numerous dams were constructed on the Yangtze during this time, including the massive Three Gorges Dam completed in 2003, which the authors specifically tied to sediment flux decline.
Other land use changes altered sediment flux in local rivers. The Ikopa River in Madagascar, for instance, doubled its sediment flux as deforestation, overgrazing, and brush fires contributed to erosion. The authors estimated that the sediment flux of the Essequibo River in Guyana increased even more, between 200% and 300%, during the time a gold mine was established on its banks.
Overall, Best said, the “world’s rivers are responding more to [land use changes] and damming than [to] climate change/precipitation changes at present.”
—Mohammed El-Said (@MOHAMMED2SAID), Science Writer