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Two-thirds of Earth’s longest rivers no longer free-flowing

  • Just one-third of the planet’s 242 longest rivers still flow uninterrupted along their entire length, most of them located in remote regions of the Arctic, the Amazon Basin, and the Congo Basin, according to a study to be published in Nature tomorrow.
  • The international team of researchers behind the study, led by Günther Grill of Canada’s McGill University, determined that, of the 91 rivers longer than 1,000 kilometers (about 600 miles) that once emptied out into an ocean, only 21 are still unobstructed from their source to the sea.
  • Dams and associated reservoirs are the biggest causes of river obstruction, the researchers say. There are nearly 60,000 large dams in the world already, and as many as 3,700 more large hydroelectric dams are currently in the planning stages or under construction.
  • Healthy rivers provide a number of benefits to mankind, from recreation to food security. Ensuring the connectivity of the world’s remaining free-flowing rivers is also critical if we’re to preserve biodiversity in freshwater systems.

A first-of-its-kind analysis of Earth’s longest rivers found that they have been severely degraded by human activities and suggests ways for countries to maintain and restore rivers around the world.

Just one-third of the planet’s 242 longest rivers still flow uninterrupted along their entire length, most of them located in remote regions of the Arctic, the Amazon Basin, and the Congo Basin, according to a study to be published in Nature tomorrow. “In densely populated areas only few very long rivers remain free-flowing, such as the Irrawaddy and Salween,” the study states.

The international team of researchers behind the study, led by Günther Grill of Canada’s McGill University, determined that, of the 91 rivers longer than 1,000 kilometers (about 600 miles) that once emptied out into an ocean, only 21 are still unobstructed from their source to the sea.

Dams and associated reservoirs are the biggest causes of river obstruction, the researchers say. There are nearly 60,000 large dams in the world already, and as many as 3,700 more large hydroelectric dams are currently in the planning stages or under construction.

Hydropower Dam Construction, Souanké, Republic of Congo. The town of Sembé will receive power from this newly constructed 19 MW Liouesso Hydropower Dam. Since a centralized power supply is unavailable in this part of Congo, towns are usually reliant on diesel generators for their power. Photo: © Jaap van der Waarde / WWF-Netherlands.

“For millennia, rivers have provided food, contributed water for domestic use and agriculture, sustained transportation corridors and, more recently, enabled power generation and industrial production,” Grill and his co-authors write in the study. In order to access the goods and services furnished by rivers, humans have built a massive amount of infrastructure, including an estimated 2.8 million dams with reservoir areas greater than 103 square meters and more than 500,000 kilometers of rivers and canals used as navigation and transport corridors. “As a result, rivers are exposed to sustained pressure from fragmentation and loss of river connectivity, constraining their capacity to flow unimpeded, affecting many fundamental processes and functions characteristic of healthy rivers and leading to the rapid decline of biodiversity and essential ecosystem services,” the authors note.

“The world’s rivers form an intricate network with vital links to land, groundwater, and the atmosphere,’’ Grill said in a statement. “Free-flowing rivers are important for humans and the environment alike, yet economic development around the world is making them increasingly rare. Using satellite imagery and other data, our study examines the extent of these rivers in more detail than ever before.”

“The saying is true that you can’t manage what you don’t understand or don’t know,” study co-author Michele Thieme, the lead freshwater scientist at WWF, told Mongabay. “We felt like there was a real need to develop a dataset that could be tracked over time, a global inventory of rivers that reflects one of the most important components of freshwater health.”

A Eurasian lynx (Lynx lynx) in a river in Slovakia. Photo: © Tomas Hulik.

Before assessing the connectivity status of 12 million kilometers (about 7.5 million miles) of the world’s rivers, the researchers first had to come up with a definition for what even constitutes a “free-flowing river.”

“There wasn’t a scientifically agreed-upon definition of what a free-flowing river is, so the very first thing that we did was debate and come up with a definition that we all could support and agree to,” Thieme said. The team ultimately decided that a free-flowing river must remain connected in four important ways: longitudinally, so that fish and other species can move upstream while water, nutrients, and sediments can move downstream; laterally, so the river can move out onto its floodplain, delivering important nutrients to fish in other habitats and bringing nutrients back into the river itself; vertically, so the river can flow into and interact with groundwater and aquifers; and seasonally, so that the important ecological functions rivers provide over time are not impaired — such as, for example, the flood pulses that signal fish to spawn.

The team then identified indicators at the global scale that reflect each of those components of river connectivity. The presence of dams, for instance, affects longitudinal, lateral, and seasonal components of connectivity, while roads and urban areas built in flood plains are often responsible for lateral connectivity being lost. The researchers identified global datasets that could be used as proxies for each component of river connectivity, and combined them all into an index that they used to evaluate the connective status of each of the world’s largest rivers from source to outlet.

Map Credit: Grill et al. (2019). doi:10.1038/s41586-019-1111-9

Disrupting river connectivity threatens critical ecosystem services

Healthy rivers provide a number of benefits to mankind, from recreation to food security. They support freshwater fisheries that are relied on by hundreds of millions of people, deliver sediments that keep coastal deltas above rising seas, and help mitigate the impacts of extreme floods and droughts. Disrupting the connectivity of rivers can threaten and even eliminate these important ecosystem services. For instance, Thieme points out that tens of millions of people in Southeast Asia get their protein from fisheries in the lower Mekong River: ”With the dams that are planned on the main stem of the river, we anticipate that a lot of that protein source for local communities is going to go away, as well as their livelihoods.”

Ensuring the connectivity of the world’s remaining free-flowing rivers is also critical if we’re to preserve biodiversity in freshwater systems. WWF’s Living Planet Report 2018 looked at 16,704 populations of wildlife around the world and found that populations of freshwater species have experienced, on average, an 83 percent decline since 1970, the most precipitous declines of any group of vertebrates.

“Freshwater species populations are declining twice as fast as marine and terrestrial populations, and we know one of the main drivers of loss of freshwater species is due to loss of river connectivity,” Thieme said. “Healthy, connected rivers are critical for the life that lives within rivers, which is highly endangered in many parts of the world.”

Aerial view of mangrove and shrimp farms in Isla Escalante, Ecuador. Photo: © Antonio Busiello / WWF-US.

The researchers note in the study that climate change poses yet another threat to the health of rivers, with rising temperatures having already impacted flow patterns, water quality, and biodiversity. Hydropower is a solution countries are increasingly turning to as they seek to grapple with the global climate crisis by decarbonizing their economies. But with 3,700 new hydroelectric dams already in the works and the pace of hydropower development accelerating around the world, it’s even more urgent that we develop energy systems with minimal impact on the planet, Thieme said.

“With the dropping costs of solar and wind energy sources currently, there’s new opportunity opening up to turn to other renewables to meet a lot of our energy needs,” she said. “Hydropower still has an important role to play, but looking at using other renewables as a source of meeting our energy needs while keeping certain rivers connected and free-flowing will allow for a better compliment of all of the benefits that rivers can provide.”

Thieme added that the data and information compiled for the study can provide a foundation for identifying the most critical rivers to protect or restore. “There’s a real window of opportunity for countries to develop in a better way. We believe that you can have development and free-flowing rivers, but you need to be really smart about it. So what does that mean? It basically means siting new infrastructure in ways that you limit the impact to rivers.”

There are a number of examples in the US and Europe of infrastructure that has caused long-term ecological damage while imposing steep social and economic costs, as well, Thieme said. A dam removal movement has already started in the US, where about 1,500 dams have been removed, and Europe is beginning to undertake a similar process. Data on the location and extent of the world’s remaining free-flowing rivers can help: “This type of information overlaid with other information on where historic fish migration pathways were, for example, or where important areas of cultural significance were, can help inform where you would get the most return in terms of restoration of river connectivity.”

Dec. 06, 2015 – Pamok, Laos. Life along the banks of the Mekong River. Photo: © Nicolas Axelrod / Ruom for WWF.

CITATION

• Grill et al. (2019). Mapping the world’s free-flowing rivers. Nature. doi:10.1038/s41586-019-1111-9

• WWF. (2018). Living Planet Report – 2018: Aiming Higher. Grooten, M. and Almond, R.E.A. (Eds). WWF, Gland, Switzerland.