Rivers are one of the most visible examples of connectivity in the landscape, linking together people, plants, animals, nutrients, sediments, and more. Maintaining river connectivity is beneficial to both the health of the environment and human well-being.
One of the greatest threats to river connectivity is the almost 3 million dams and reservoirs installed around the world, with thousands more in planning or under construction. Other contributing factors, such as habitat loss and urbanization, combine to put sustained pressure on river networks and threaten to fragment them further.
A new study uses novel and integrated methods to quantify connectivity across more than 12 million kilometers of rivers across the globe. Advances in the accessibility and resolution of global hydrological data mean that free-flowing rivers can now be mapped at a high resolution, and in a way that better informs decision makers who must balance human demands and conservation needs.
This assessment of rivers was based on a multi-step process: explicitly defining terms, identifying pressure factors on river networks, and developing a weighted model to assign river reaches a connectivity status index. In this framework, a river was considered free-flowing only if it had an index value at or above 95% over its entire length.
The results reveal striking information about global river connectivity:
- 63% of the world’s very long rivers (>1,000 km) are no longer free-flowing.
- Free-flowing rivers still connected to the ocean show a loss of connectivity in 77% of very long rivers (>1,000 km) and in 54% of long rivers (500–1,000 km).
- Dams and reservoirs are the biggest contributors to major connectivity loss.
- Large contiguous river networks can be found only in remote regions of the Arctic, the Amazon Basin and (to a lesser degree) the Congo Basin.
These results provide a starting point for deciding where river conservation strategies should be focused. Both the maintenance of free-flowing rivers and the restoration of fragmented ones can provide large-scale benefits to the landscape. With this work as a baseline assessment, infrastructure planning and decision making can more effectively focus on how to address energy and spatial needs while keeping rivers connected.