How do humans change the world's rivers?

The way rivers function is significantly affected by the amount of silt they transport and the sediment it contains, according to a new Dartmouth University study published in the journal Science.

Three Gorges Dam in China changes the flow after 11 years (1999 - 2010)

The researchers used satellite images from NASA Landsat and digital archives of hydrological data to examine changes in the amount of sediment delivered to the oceans of 414 of the world's largest rivers. world from 1984 to 2020.

Lead author Evan Dethier, a postdoctoral fellow at Dartmouth, said the amount of sediment in rivers is often determined by natural processes in the watershed, such as rainfall, landslides or vegetation. object.

Hoover Dam on the Colorado River, located on the border between the states of Nevada and Arizona, is America's most famous dam at 221m high.

The researchers found that direct human activities are crowding out these natural processes, even outweighing the effects of climate change.

The findings show that major dam construction during the 20th century in the global hydrological north - which includes North America, Europe, Eurasia and Asia - has resulted in a 49 per cent reduction in calcareous sediments. mezzanine from rivers to the ocean compared to pre-dam conditions.

This global decline occurred despite a significant increase in the supply of sediments from the global hydrological south - including South America, Africa and Oceania. Here, sediment transport has increased in 36% of the rivers in the region due to large land-use changes.

Oroville Dam in California is the highest in the US with 235m

These changes are all due to human activities, such as gold mining in alluvium in South America and sub-Saharan Africa; sand mining in Bangladesh and India; and palm oil plantations across much of Oceania.

In the north, dam construction has been a major driver of river changes over the past few centuries. In the US alone, more than 90,000 dams were built.

However, in the United States and other countries in the Northern Hemisphere, many dams are more than half a century old, and fewer dams have been built in the twenty-first century. Therefore, the recent decline in sediment transport is relatively small.

Yacyretá Dam, on the Parana River between the Argentine province of Corrientes and the city of Ayolas in Paraguay

But dam construction in Eurasia and Asia over the past 30 years, especially in China, has led to an ongoing reduction in river sediment transport across the globe.

Study co-author Francis Magilligan, a professor of geography, stressed: "This is especially worrisome for densely populated places like Vietnam, where the sediment supply has been significantly reduced due to operation of dams along the Mekong River".

Carl Renshaw, professor of Earth sciences at Dartmouth, said: "River activity provides pretty sensitive indicators of what we're doing to the Earth's surface - they're like a temperature measure change in land and water use".