Almost two in three of Earth's longest rivers have been severed by dams, reservoirs or other manmade constructions, severely damaging some of the most important ecosystems on the planet, researchers said Wednesday. Using the latest satellite data and computer modelling software, the international team looked at the connectivity of 12 million kilometres of rivers worldwide, providing the first global assessment of human impact on the planet's waterways.
They found that out of the 91 rivers longer than 1,000 kilometres (600 miles) in length, just 21 retained a direct connection between source and sea. Just a little over a third (37 percent) of the 242 longest rivers had retained their free flow, something experts said was having a profound effect on Earth's biodiversity. "The world's rivers form an intricate network with vital links to land, groundwater and the atmosphere," said Gunther Grill from McGill University's Department of Geography and lead author. "Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare."
Most of the remaining free-flowing rivers were confined to remote parts of the Arctic, the Amazon and the Congo basins, the study found.
This week the UN's panel on biodiversity released a summary of its devastating assessment on the state of Nature. The underlying report, which will be made public in the coming weeks, found that 50 percent of rivers "manifest severe impacts of degradation" from human activity.
Wednesday's study, published in the journal Nature, laid bare in intricate detail just how drastically manmade activity is impacting our waterways. It estimated there was now a total of 60,000 large dams at least 15 metres tall severing rivers, out of a total of 2.8 million worldwide.
The blocking or damming of rivers disrupts the flow of nutrients vital to replace those lost through agriculture, and diminishes the amount of river-bourne species that can complete their life-cycles. -
It also lessens the sediment flows river deltas provide coastal regions with, which currently help to protect millions of people against sea level rises.