Two-thirds of Earth’s 242 longest rivers are no longer free-flowing due to human activities, a study mapping over 12 million kms of watercourses reveals.
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Why are free-flowing rivers important?
Besides dams and reservoirs, activities like water extraction and sediment trapping disrupt rivers’ natural flow. Free-flowing rivers feed hundreds of millions of people, deliver sediments crucial to agriculture and mitigate the impact of floods and droughts. River fragmentation and alteration threaten vital ecosystems for people and wildlife.
This groundbreaking study by hydrologists from McGill University, published in Nature, is the first comprehensive global assessment of the connectivity of Earth’s largest rivers. Scientific wisdom postulates that free-flowing rivers must remain connected across four dimensions: 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 — for example, the flood pulses that signal fish to spawn.
Using satellite imagery and hydrological modelling, researchers mapped over 12 million kilometers of watercourses worldwide. The team identified indicators at the global scale that measured any of the four ways of river connectivity. Measurements showed how the presence of dams affects longitudinal, lateral, and seasonal components of connectivity. Roads and urban areas built in flood plains also disrupted lateral connectivity.
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Figure 1 River connectivity of global rivers threatened by the five dominant pressures: DOF (degree of regulation), SED (sediment trapping), USE (consumptive water use) and URB (urban areas) (Source: Grill et al. 2019)
Data analysis revealed that river fragmentation, flow regulation, sedimentation, water consumption, and urbanisation were the five dominant constraints rivers face worldwide. Only rivers in remote regions like the Arctic and the Amazon rainforest were found to have remained untouched and could flow unimpeded along its entire course. In densely populated areas only a few very long rivers remain free-flowing, such as the Irrawaddy in Myanmar and the Salween in China.
There are over 2.8 million large and small dams constructed around the world; these are the leading cause of river fragmentation. Dams, intentionally designed to impede river flow, not only alter terrestrial and freshwater biodiversity by preventing species migration, but also accelerate the shrinking of downstream river deltas and expose low-lying regions to increased flood risk by preventing the exchange of sediments.
This study adds to growing evidence highlighting how human activities are fundamentally changing the natural landscape and the water cycle. With more than 3,700 dams in the works and the pace of hydropower development accelerating around the world, the ecological consequences of dams should push us to develop an energy system that minimises negative impacts on our ecosystem. The results of this study, freely available in an interactive map portal, should serve as a wake-up call to policymakers, engineers, and planners and encourage them to redesign the infrastructure development projects.
Life in and around a river evolves and is conditioned by free-flowing water. A dam disrupts life.
The best way forward is to adopt nature-based solutions (NbS), which are defined as ‘actions to protect, sustainably manage, and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits’. Countries should embrace NbS like floodplain restoration, ecological engineering, and integrated water resources management to ensure river flow connectivity and protect the ecosystem.
Removing dams is another way to restore the rivers. A dam removal movement has already started in the US, where about 1,500 dams have been removed. Support for river restoration through dam demolition is also growing in Europe and Japan. A project called Dam Removal Europe focuses on clearing rivers of the 30,000 old or obsolete dams that still exist across Europe. These projects should be embarked upon more widely to ensure the protection and longevity of Earth’s last free-flowing rivers.
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