Free-flowing rivers (FFRs) support diverse, complex and dynamic ecosystems globally, providing important societal and economic services. Infrastructure development threatens the ecosystem processes, biodiversity and services that these rivers support. Here we assess the connectivity status of 12 million kilometres of rivers globally and identify those that remain free-flowing in their entire length. Only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length and 23 per cent flow uninterrupted to the ocean. Very long FFRs are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins. In densely populated areas only few very long rivers remain free-flowing, such as the Irrawaddy and Salween. Dams and reservoirs and their up- and downstream propagation of fragmentation and flow regulation are the leading contributors to the loss of river connectivity. By applying a new method to quantify riverine connectivity and map FFRs, we provide a foundation for concerted global and national strategies to maintain or restore them.
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The geometric dataset of the global river network and the associated attribute information for every river reach—that is, the values of all pressure indicators (DOF, DOR, SED, USE, RDD and URB)—as well as the main results of the study—that is, values for the CSI, the dominant pressure factor and the FFR status— are available at https://doi.org/10.6084/m9.figshare.7688801 under a CC-BY-4.0 license. The dataset can be used together with the published source code (see ‘Code availability’) to recalculate the main study results and to run existing and new scenarios. The databases of dams required to calculate the DOF, DOR and SED indicators are not in the data repository owing to licensing issues, but are freely available at http://www.globaldamwatch.org. Original data that supported the study—that is, raw datasets of roads, urban areas, water use, waterfalls, erosion data and floodplain information—and their sources are summarized in Extended Data Table 1. Additional higher-resolution maps of Figs. 1–3 are available at http://www.hydrolab.io/ffr.
The source code of the main tools, scripts and algorithms used in this research is available under the GNU General Public License v3.0 at https://github.com/ggrill/Free-Flowing-Rivers. Other procedures and GIS steps (as described in Methods) were conducted manually and are therefore not part of the code repository.
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Funding for this study was provided in part by World Wildlife Fund (WWF), the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant RGPIN/341992-2013) and McGill University, Montreal, Québec, Canada.
Nature thanks Edward Park, N. LeRoy Poff and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Methodological steps to define and assess the CSI of individual river reaches (steps 1–5) and decision tree used to assess the free-flowing status of entire rivers (step 6 and following).
a–c, The baseline river network consists of individual ‘river reaches’ (1–32 in a), defined as line segments separated by confluences (black dots). River reaches can be aggregated into ‘rivers’ according to a ‘backbone’ ordering system, which classifies river reaches as the mainstem or a tributary of various higher orders (b). Following this system, the river network can be distinguished into distinct rivers (1–16 in c), defined as contiguous stretches of river reaches from source to outlet on the mainstem or from source to confluence with the next-order river. d, CSI values for individual river reaches, as calculated with our model. If a value is at or above the CSI threshold (95%), the river reach is declared to have good connectivity status; if it is below the threshold, it is declared to be impacted. e, If an entire river (as defined in c) has good connectivity status, it is defined to be an FFR (blue). A river can be partly above the CSI threshold, and thus contiguous stretches can have good connectivity status (green).
a, b, The DOF index ranges from 0% (no fragmentation impact) to 100% (completely fragmented) and is shown for the conceptual approach (a) and the river example (b) in the colour coding shown in b. It is calculated for all river reaches connected to the barrier location in both the up- and downstream directions (but tributaries to the mainstem downstream of the barrier are not considered affected). The impact is largest in connected river reaches that are similar in discharge to the barrier site and diminishes as rivers become increasingly dissimilar in size, that is, larger in the downstream or smaller in the upstream direction. c, DOF decay functions, as considered and evaluated by the expert group.
The SED ranges from 0% to 100%, assessing the degree to which sediment connectivity in any river reach is altered by upstream dams. a, River network with individual river reaches and PSL ranges. b, The SED, which accounts for the relative contribution of tributaries to the total sediment budget of the river network, and its changes in response to changes in longitudinal sediment connectivity.
a–f, Individual indicators within their range of occurrence, between 0% and 100%. The colour schemes are nonlinear and vary between indicators. The blue shades represent the magnitude of river discharge for river reaches with pressure values of 0% (that is, darker shades for larger rivers).
a, Averaged CSI standard deviations for CSI ranges. b, Number of benchmark FFRs correctly classified at different CSI thresholds.
Supplementary Table 1: List of free-flowing rivers longer than 500 km by continent.
Supplementary Table 2: List of reference rivers evaluated for benchmarking. Sources: ‘Expert nominated’ (BENCH_SCR = ‘EXP’) and Nilsson et al.27 (BENCH_SCR = ‘NLS’).
Supplementary Table 3: Results of benchmarking and key statistics of 100 scenarios.
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Grill, G., Lehner, B., Thieme, M. et al. Mapping the world’s free-flowing rivers. Nature 569, 215–221 (2019) doi:10.1038/s41586-019-1111-9
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