(CN) – Only a third of the world’s longest rivers remains free-flowing from source to sea, and the interruptions in flow by dams and reservoirs adversely affect critical ecosystems, according to a study published Wednesday in the journal Nature.
A team of 34 international researchers from McGill University, World Wildlife Fund and other institutions created the first-ever global assessment of over 7.4 million miles of rivers to identify the location and extent of those that are completely free-flowing.
They mapped rivers in four categories separated by lengths, with the longest rivers more than 600 miles. Only 21 of the 91 rivers in this category flow uninterrupted to the sea, mostly in remote areas of the Arctic, the Amazon Basin and the Congo Basin, according to the study.
Rivers have played an essential role in human well-being for millennia, providing food, water, transportation corridors and power generation, and enabling industrial production, the study authors said.
“Rivers are the lifeblood of our planet,” said Michele Thieme, lead freshwater scientist at World Wildlife Fund and global leader of the organization’s free-flowing rivers initiative. “They provide diverse benefits that are often overlooked and undervalued. This first-ever map of the world’s remaining free-flowing rivers will help decision makers prioritize and protect the full value rivers give to people and nature.”
Structures and alterations that affect the free flow of rivers include dams, reservoirs, and diversions from man-made rivers and canals. Globally, 60,000 of the 2.8 million dams are classified as large, and 3,700 hydropower dams are under construction or planned, according to the authors.
These interruptions to the flow of the river have far-reaching effects on ecosystem services. Freshwater species experienced the most pronounced decline among invertebrates of around 83% in the past 50 years, according to a recent global analysis of wildlife, and protecting free-flowing rivers can also protect these species.
Free-flowing rivers move sediment to deltas to keep them above sea levels, are valuable during times of flooding and drought, and protect biodiversity of the water’s inhabitants.
“The world’s rivers form an intricate network with vital links to land, groundwater, and the atmosphere,” said lead author Günther Grill of McGill’s Department of Geography. “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.”
Rising temperatures from climate change already impact water flow, quality and biodiversity, according to study authors. The situation is worsened by the increasing number of hydropower plants around the world. Researchers stress the urgency of developing other energy systems that have less of an impact on the environment.
“Renewable energy is like a recipe – you have to find the right mix of ingredients to have both a sustainable energy grid and a thriving natural world,” said Thieme. “While hydropower inevitably has a role to play in the renewable energy landscape, well-planned wind and solar energy can be more viable options for rivers and the communities, cities, and biodiversity that rely on them.”
Global climate and land use change can also “increase the pressure on rivers and their connectivity” and change water quality, the authors said. Free-flowing rivers may protect freshwater species from these adverse effects by allowing them open pathways to find suitable habitats elsewhere “in response to rising temperatures or other changing conditions.”
According to the study authors, there is a global movement to protect and restore rivers that includes the creation of Agenda 2030 for Sustainable Development by the international community. This initiative “calls on all countries to track, at a national scale, the spatial extent and condition of water-related ecosystems.”
This current study promotes that agenda by providing methods and data to define the baseline and to track connectivity of rivers in the future, the researchers said. They emphasize the importance of action necessary to protect free-flowing rivers.
Even regional efforts to restore river flow can make a positive difference. In an accompanying document to the study, authors created narratives of a few rivers around the world, and the effects of dam development as well as dam removal.
One example is the removal of two hydroelectric dams from the Elwha River in the Pacific Northwest. The document explains how the dams’ initial construction in the early 1900s blocked passage for migratory salmon, with a subsequent huge decline in their numbers.
After 20 years of planning, one dam was removed in 2014, and removal of the second is in process. According to the document, sockeye salmon and bull trout have already returned and there have been positive changes in the coastal ecosystem.