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Mapping the world’s free-flowing rivers

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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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ARTICLE https://doi.org/10.1038/s41586-019-1111-9
Mapping the world’s free-flowing rivers
G. Grill1*, B. Lehner1*, M. Thieme2, B. Geenen3, D. Tickner4, F. Antonelli5, S. Babu6, P. Borrelli7,8, L. Cheng9, H. Crochetiere10,
H. Ehalt Macedo1, R. Filgueiras11,36, M. Goichot12, J. Higgins13, Z. Hogan14, B. Lip15, M. E. McClain16,17, J. Meng18,19, M. Mulligan20,
C. Nilsson21,22, J. D. Olden23, J. J. Opperman2, P. Petry24,25, C. Reidy Liermann26, L. Sáenz27,28, S. Salinas-Rodríguez29, P. Schelle30,
R. J. P. Schmitt31, J. Snider10, F. Tan1, K. Tockner32,33,37, P. H. Valdujo34, A. van Soesbergen20 & C. Zarfl35
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.
Rivers are essential sources of environmental health, economic wealth
and human well-being. For millennia, rivers have provided food, con-
tributed water for domestic use and agriculture, sustained transpor-
tation corridors and, more recently, enabled power generation and
industrial production
1
. These goods and services generally require built
infrastructure, and society has addressed this demand by constructing
an estimated 2.8 million dams (with reservoir areas >10
3
m
2
)
2
, regu-
lating and creating over 500,000km of rivers and canals for navigation
and transport3,4 and building irrigation and water-diversion schemes.
As a result, rivers are exposed to sustained pressure from fragmentation
and loss of river connectivity, constraining their capacity to flow unim-
peded, affecting many fundamental processes and functions character-
istic of healthy rivers5 and leading to the rapid decline of biodiversity
and essential ecosystem services6.
The capacity of rivers to flow freely is governed by the connectivity
of pathways that enable the movement and exchange of water and of
the organisms, sediments, organic matter, nutrients and energy that
it conveys throughout the riverine environment. River connectivity
extends in four dimensions: longitudinally (up- and downstream in
the river channel), laterally (between the main channel, the floodplain
and riparian areas), vertically (between the groundwater, the river
and the atmosphere) and temporally (seasonality of flows)7,8. River
connectivity is also spatially and temporally dynamic, largely driven
by the natural flow regime
9
, enabling and regulating hydrological, geo-
morphic and ecological processes in river networks and providing the
aquatic medium for matter and species to move along the river and into
adjacent habitats10. Humans have altered natural river connectivity in
multiple ways, either directly, by placing structures into the longitudinal
or lateral flow paths, such as dams and levees, or indirectly, by altering
the hydrological, thermal and sediment regimes of the river11,12.
Although it is inherently complex to quantify the value of services
provided by FFRs or to measure the devaluing effect of impeding infra-
structure, many examples exist that underline the importance of con-
nectivity for the provision of natural riverine ecosystem functions and
processes. For instance, floodplains are among the most productive and
diverse riverine ecosystems globally13, and their disconnection from
the upstream catchment or river channel alters ecosystem services such
as natural flood storage, nutrient retention and flood–recession agri-
culture14. Built river infrastructure has also been linked to declines
in terrestrial and freshwater species
11,1517
, and sediment capture by
dams may cause the alteration of the geomorphic dynamics of rivers
and the shrinking of river deltas worldwide
18
. Although advances in
the socio-economic valuation of river connectivity have emerged—for
example, inland fisheries provide the equivalent of all dietary animal
protein for 158 million people globally, particularly for poor and under-
nourished populations
19
—more comprehensive and detailed studies
are needed20.
Acknowledging the importance of river connectivity, a decade ago
the Brisbane Declaration21 called for the identification and conser-
vation of “a global network of FFRs”, and in 2015 the world’s govern-
ments committed to “protect and restore water-related ecosystems
under the United Nations’ Sustainable Development Goals (target 6.6).
Nevertheless, continued and accelerating declines in river connectiv-
ity, aquatic biodiversity and associated ecosystem services remain a
1Department of Geography, McGill University, Montreal, Québec, Canada. 2WWF-US, Washington, DC, USA. 3WWF-NL, Zeist, The Netherlands. 4WWF-UK, Woking, UK. 5WWF-Mediterranean, Rome,
Italy. 6WWF-India, New Delhi, India. 7Environmental Geosciences, University of Basel, Basel, Switzerland. 8European Commission, Joint Research Centre, Directorate for Sustainable Resources,
Ispra, Italy. 9WWF-China, Beijing, China. 10WWF-Canada, Toronto, Ontario, Canada. 11WWF-Zambia, Lusaka, Zambia. 12WWF Greater Mekong Programme, Ho Chi Minh City, Vietnam. 13The
Nature Conservancy (TNC), Chicago, IL, USA. 14Department of Biology and Global Water Center, University of Nevada, Reno, NV, USA. 15WWF-Malaysia, Sarawak, Malaysia. 16Department of
Water Science and Engineering, IHE Delft, Delft, The Netherlands. 17Department of Water Management, Delft University of Technology, Delft, The Netherlands. 18WWF-Germany, Berlin, Germany.
19HTWG Konstanz University of Applied Sciences, Konstanz, Germany. 20Department of Geography, King’s College London, London, UK. 21Department of Ecology and Environmental Science,
Umeå University, Umeå, Sweden. 22Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, Umeå, Sweden. 23School of Aquatic and Fishery Sciences,
University of Washington, Seattle, WA, USA. 24The Nature Conservancy, Hollis, NH, USA. 25Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA. 26UW Center for Limnology,
University of Wisconsin-Madison, Madison, WI, USA. 27Conservation International (CI), Arlington, VA, USA. 28Environmental Engineering, Michigan Technological University (MTU), Houghton,
MI, USA. 29WWF-Mexico, Mexico City, Mexico. 30WWF International, Gland, Switzerland. 31Natural Capital Project, Department of Biology and the Woods Institute for the Environment, Stanford
University, Stanford, CA, USA. 32Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany. 33Institute of Biology, Freie Universität Berlin, Berlin, Germany. 34WWF-Brazil,
Brasília, Brazil. 35Center for Applied Geoscience, Eberhard Karls University of Tübingen, Tübingen, Germany. 36Present address: Rewilding Europe, Nijmegen, The Netherlands. 37Present address:
Austrian Science Fund, FWF, Vienna, Austria. *e-mail: guenther.grill@mail.mcgill.ca; bernhard.lehner@mcgill.ca
Corrected: Author Correction
9 MAY 2019 | VOL 569 | NATURE | 215
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Streamflow and sediment loads undergo remarkable changes in worldwide rivers in response to climatic changes and human interferences. Understanding their variability and the causes is of vital importance regarding river management. With respect to the Changjiang River (CJR), one of the largest river systems on earth, we provide a comprehensive overview of its hydrological regime changes by analyzing long time series of river discharges and sediment loads data at multiple gauge stations in the basin downstream of Three Gorges Dam (TGD). We find profound river discharge reduction during flood peaks and in the wet-to-dry transition period, and slightly increased discharges in the dry season. Sediment loads have reduced progressively since 1980s owing to sediment yield reduction and dams in the upper basin, with notably accelerated reduction since the start of TGD operation in 2003. Channel degradation occurs in downstream river, leading to considerable river stage drop. Lowered river stages have caused a ‘draining effect’ on lakes by fostering lake outflows following TGD impoundments. The altered river–lake interplay hastens low water occurrence inside the lakes which can worsen the drought given shrinking lake sizes in long-term. Moreover, lake sedimentation has decreased since 2002 with less sediment trapped in and more sediment flushed out of the lakes. These hydrological changes have broad impacts on river flood and drought occurrences, water security, fluvial ecosystem, and delta safety.
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Changes in riverine suspended and riverbed sediments have environmental, ecological and social implications. Here, we provide a holistic review of water and sediment transport and examine the human impacts on the flux, concentration and size of sediment in the Yangtze River in recent decades. We find that most of the fluvial sediment has been trapped in reservoirs, except for the finest portion. Furthermore, soil-conservation since the 1990s has reduced sediment yield. From 1956-1968 (pre-dam period) to 2013–2015 (post-dams and soil-conservation), the sediment discharge from the sub-basins decreased by 91%; in the main river, the sediment flux decreased by 99% at Xiangjiaba (upper reach), 97% at Yichang (transition between upper and middle reaches), 83% at Hankou (middle reach), and 77% at Datong (tidal limit). Because the water discharge was minimally impacted, the suspended sediment concentration decreased to the same extent as the sediment flux. Active erosion of the riverbed and coarsening of surficial sediments were observed in the middle and lower reaches. Fining of suspended sediments was identified along the river, which was counteracted by downstream erosion. Along the 700-km-long Three Gorges Reservoir, which retained 80% of the sediment from upstream, the riverbed gravel or rock was buried by mud because of sedimentation after impoundment. Along with these temporal variations, the striking spatial patterns of riverine suspended and riverbed sediments that were previously exhibited in this large basin were destroyed or reversed. Therefore, we conclude that the human impacts on sediment in the Yangtze River are strong and systematic.