Changes in land use have caused massive sedimentation of rivers, threatening African waterways, a new study says.
Sediment delivery to rivers and seas in the global hydrologic south – Africa, South America and Oceania – has increased 36 percent since 1980, with suspended sediment concentration growing by 41 percent in the period, according to a survey published in the journal Science last week.
“Humans dramatically change the amount of sediment that makes it to the oceans and seas through land-use changes,” the researchers from the Department of Earth Sciences at Dartmouth in Hanover, New Hampshire, said.
River function is significantly affected by how much sediment they transport and where it gets deposited. The sediment – mostly sand, silt and clay – plays a critical ecological role, providing habitat for organisms downstream and in estuaries. It is also important for human life, resupplying nutrients to floodplain agricultural soils and buffering sea level rise by delivering sand to deltas and coastlines.
However, the study found that “humans have caused unprecedented, consequential changes to river sediment transport in the past 40 years” and now many river functions are under threat.
“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 team used satellite imagery from the mid-1980s onward and ground-truthing to estimate the sediment flux from 414 rivers worldwide.
Dams in North America, Europe and Asia were found to trap sediment, contributing to global sediment decline of 49 percent.
“Our results tell a tale of two hemispheres. The global 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,” says.
The amount of sediment rivers carry is dictated by natural processes like rain or landslides and vegetation cover.
“Direct human activities are overwhelming these natural processes, and even outweighing the effects of climate change,” he added.
River sedimentation in Africa has been driven mainly by deforestation and alluvial mining. Tanzania was cited for its heavy deforestation for agriculture.
The researchers say dams are useful in controlling suspended sediment concentration downstream.