An estimated two-thirds of the Earth's longest rivers have been corrupted by dams, reservoirs or other manmade structures, seriously harming some of the most important ecosystems on the planet, researchers said Wednesday. Using the latest satellite statistics and digital modelling software, the international team looked at nearly 12 million kilometers of rivers worldwide, offering the first global evaluation of the human disruption on the world's streams and rivers.
The study found that out of the 91 rivers longer than 1,000 kilometers in length, just 21 preserved an unbroken connection from high lands to seashores. Just a little over a third-roughly 37 percent-of the 242 longest rivers had preserved their free flow, something that experts said is having an overwhelmingly critical influence 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 of the study. "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 rivers were limited to isolated parts of the Arctic, the Amazon and the Congo basins. This week the UN's panel on biodiversity released a summary of its shocking evaluation on the state of Nature. The report, which will be made public in the coming weeks, found that 50 percent of rivers "manifest severe impacts of degradation" from human occupation.
Wednesday's study laid bare in intricate detail just how severely manmade activity is impacting our waterways. It estimated there was now a total of 60,000 large dams at least 15 meters 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-borne 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. Less than a quarter of free-flowing rivers now connect to oceans, robbing inlet environments from vital nutrients and sediments. The team warned that dams had already led to a significant fall in river fish, which provide nearly all the animal protein eaten by close to 160 million people.
An unrelated assessment last year from conservation group WWF said freshwater species had experienced the sharpest drop of all vertebrates over the past 100 years, falling an average of 83 percent since the 70s. The study also acknowledged roughly 3,700 hydropower projects either planned or are currently under construction, including some on rivers that offer fundamental life support for the human populaces who depend on them.
While hydroelectric power is considerably cleaner than oil, gas or coal, the team stressed that mega power projects involving dams and reservoirs could have surprising harmful consequences.
"Hydropower certainly has more complex environmental impacts than the often-cited positive effects of avoiding fossil fuels," Bernhard Lehner, a professor at McGill, told AFP. "While hydropower inevitably has a role to play...countries should focus on sustainable options like solar and wind which can have less detrimental impacts on rivers and the communities, cities and biodiversity that rely on them."
The health of Earth's rivers will also be affected as climate change hastens, disturbing flow patterns and water quality, as well as causing more disruptive species to move in, the authors said.
