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Only one third of world's longest rivers remain free-flowing
Human activity disrupts river connectivity, which threatens ecosystems, services
Published in Daily News Egypt on 16 - 05 - 2019

Rivers play a prominent role in supporting human communities through the provision of drinking water, and agricultural activities. But recent years have seen a decline in the ability of major rivers around the world to flow freely, because of human interventions, the establishment of dams, and the obstruction of the streams of these rivers.
According to a recent study published in the Nature journal, 91 rivers of the longest rivers in the world, with a length of more than 1,000km and they number 242 rivers only, are still flowing freely, an estimated 37.6%, while 63% of these 91 rivers are obstructed by water flow due to dams and reservoirs created by man.
Fluvial conductivity in the waterway helps to facilitate communication between components of the river system, whether water or organic matter, marine species, energy, and light.
The study, prepared by an international team of 34 researchers led by Canada's McGill University and the World Wide Fund for Nature (WWF), provides the first assessment of fluvial conductivity by monitoring the state of 12m km of river flow around the world.
Free-flowing rivers are very valuable ecosystems and contribute a lot to nature and humans. Dam construction is the major reason why river connectivity has been declining worldwide, with often negative consequences on river health and on the capacity of rivers to provide eco-services, such as food, clean water, and nutrients, according to Günther Grill, professor of geography at McGill University, and first author of the study.
Biodiversity is also rapidly declining in rivers. We need to better protect these rivers and find smart and sustainable solutions for future infrastructure development to minimise the impact on rivers.
The study showed that 12 of the 91 rivers that continue to flow freely and retain the direct link between upstream and downstream on the sea coast eventually end up in the ocean. The remaining 79 free rivers are confined to remote areas of the Arctic, the Amazon basin, and the Congo basin.
"Our methods are very sensitive to any kind of infrastructure development. One road near a river would show up as a signal in our data. But there is a grey zone between a river that is free-flowing and one that is not. Even though these rivers have been touched by human infrastructure development, most of these were touched only lightly. Diminished connectivity may not be a major problem for them," Grill told Daily News Egypt.
He added that the data published in this paper is the culmination of more than a decade of data processing and preparation. The researchers first needed to create digital maps of millions of rivers, as these were not available on a global scale. They also needed millions of lakes, and tens of thousands of reservoirs, as well as information about water use, roads, and urban areas for each of the millions of rivers.
Then the second phase of the work started, which was about four years ago: bringing this information together which involved processing the data in geographic information systems (GIS).
"A major challenge was to verify this information and eliminate errors that inevitably occur in these types of global datasets. This work included a lot of manual labour, done by us, or colleagues and students," according to the researcher.
"For example, the location of each of more than 25,000 dams has been verified manually against satellite imagery by our colleagues in several years of work, and by using crowdsourcing techniques," he explained.
But once the team had the data in one database, another challenge was to process this enormous amount of data using their connectivity methodology. This task required lots of computer memory and processors, so the researchers needed to constantly improve the efficiency of the model code as well as automating the steps as much as possible.
Finally, they used statistical models to calculate an index on connectivity for each river worldwide.


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