REVIEW ARTICLE https://doi.org/10.1038/s41561-018-0262-x Departments of Geology, Geography and Geographic Information Science, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, USA. e-mail: jimbest@illinois.edu T he world’s great rivers have been foci for the origin and growth of human culture 1–5 , support huge populations 6 (Fig. 1; Supplementary Table 1) and constitute some of the most diverse ecosystems on the surface of the globe 7,8 . These big riv- ers (Fig. 1, see definition; Supplementary Information) are largely transboundary 9,10 , and can help promote both regional collabora- tions and conflicts 11–17 . Standing on the riverbanks at the mouth of a big river gives a sense of spatial connections — in the water, sedi- ment, ecology and cultures that the river unites — as well as tempo- ral corridors into the evolution of the landscape, linked ecosystems and human civilization. The world’s big rivers hold huge societal importance in the benefits they bring through food production, hydropower generation and providing trade routes. For instance, hydropower provides 16% of the world’s total electricity and 70% of renewable electricity 18 . In addition, the annual use of freshwater from surface and ground waters of the world’s artificial reservoirs, for the purposes of hydropower generation, irrigation, industrial and domestic water supply, flood protection, fishing and recreation, is valued at US$265 × 10 9 per year 19 . The economic importance of big rivers is shown by evaluation of economic dependence and risk for transboundary rivers 9 as a function of urban and agricultural (irrigation) activity (Supplementary Table 1). Such analysis shows that 15 (Congo, Nile, La Plata, Niger, Volga, Zambezi, Ganges- Brahmaputra, Orinoco, Tigris, Indus, Danube, Mekong, Ganges, Irrawaddy and Rhine) of the 24 transboundary river basins consid- ered herein have a very high or high economic dependence on these waterways, with the Amazon and Mississippi rivers possessing a moderate dependence 9 . However, population growth, and the rising demand for water, power, food and land, have generated increas- ing stresses on the world’s great waterways 9,17 , and we have reached a time when the integrity of many of the world’s largest rivers is being irrevocably threatened by a combination of anthropogenic stressors 20–23 . At the downstream termination of many big rivers, the world’s great deltas are home to 500 million people 1,24 , but are threatened by relative sea-level rise, due to a combination of ris- ing sea level, land subsidence through groundwater abstraction and upstream water and sediment starvation 1,24 . The large-scale controls on the location, morphology and eco- systems of big rivers centre around their plate tectonic setting and relationship to topographic gradients, geology, controlling climatic factors and the influence of relative sea level 25,26 . Some researchers have proposed that big rivers possess distinctly different character- istics from smaller channels 27 , and perhaps adopt an anabranching channel pattern that is the end-member adjustment for large fluvial systems 28 . Such large channels also display considerable complexity in their planform and floodplain-channel connectivity 29 . The mor- phology of, and sediment flux from, the world’s great rivers have changed radically over periods of thousands to millions of years, with the sea-level minimum at the Last Glacial Maximum having extended channel networks, especially in Southeast Asia 30 . Such long-term changes have also had an indelible imprint on biologi- cal evolution, creating a direct link between species differentiation and large river basin development 31–33 . On a shorter timescale, river channel migration has been shown to be responsive to the imposed water discharge and sediment flux 34 . For example, the 1950 Assam earthquake introduced c. 45 × 10 9 m 3 of sediment into the Jamuna– Padma–Meghna river system 35 . This sediment pulse has been argued to have created a wave of bed-load material, with a celerity between 16 and 32 km yr -1 , which caused channel widening, and associated societal and engineering pressures, over the succeeding 50 years 35 . However, anthropogenic change is enforcing more rapid, and more long-lasting, radical changes on the world’s great freshwater corri- dors. This paper provides a review of aspects of the world’s big riv- ers (see Fig. 1 and Supplementary Table 1 for details of the world’s 32 largest rivers) and details the principal anthropogenic stressors. This synthesis reveals that unless concerted and truly multidisci- plinary and intergovernmental efforts are forwarded rapidly, several of these river basins will suffer immense change within decades and from which there will be no recovery. Damming The last two decades have witnessed a resurgence in the plans for, and construction of, new hydroelectric power schemes and ‘mega- dams’ 36–38 (Fig. 1; Supplementary Table 1; megadams are those with a height >15 m (ref. 37 ); dams depicted in Fig. 1 have a maximum design capacity of 1 MW or greater 38 ), with the worldwide installed GW hydropower capacity having increased 55% from 2000 to 2015 (ref. 39 ). Such growth has been fostered by energy demands to Anthropogenic stresses on the world’s big rivers Jim Best The world’s big rivers and their floodplains were central to development of civilization and are now home to c. 2.7 billion peo- ple. They are economically vital whilst also constituting some of the most diverse habitats on Earth. However, a number of anthropogenic stressors, including large-scale damming, hydrological change, pollution, introduction of non-native species and sediment mining, challenge their integrity and future, as never before. The rapidity and extent of such change is so great that large-scale, and potentially irreparable, transformations may ensue in periods of years to decades, with ecosystem collapse being possible in some big rivers. Prioritizing the fate of the world’s great river corridors on an international political stage is imperative. Future sustainable management, and establishment of environmental flow requirements for the world’s big riv- ers, must be supported through co-ordinated international funding, and trans-continental political agreement to monitor these rivers, finance their continual upkeep and help ameliorate increasing anthropogenic pressures. To have any effect, all of these must be set within an inclusive governance framework across scales, organizations and local populace. NATURE GEOSCIENCE | www.nature.com/naturegeoscience