River Basin Modifications

Aswan High Dam, April 12, 2015  NASA Earth Observatory.  Astronaut photograph ISS043-E-101953 was acquired on April 12, 2015

Environmental Significance

Source:  Grill, et al., Mapping the world’s free-flowing rivers, Nature 569(7755):215-     221, doi: 10.1038/s41586-019-1111-9

  • Destruction/separation of floodplains from rivers alters ecosystem services such as natural flood storage, nutrient retention and flood-recession agriculture
  • Built river infrastructure has been linked to declines in terrestrial and freshwater species
  • Sediment capture by dams may cause alteration of the geomorphic dynamics of rivers and the shrinking of river deltas worldwide
  • Inland fisheries provide the equivalent of all dietary animal protein for 158 million people worldwide
  • Only 37% of rivers longer than 1,000 kilometers remain free-flowing in their entire length
  • Only 23% of rivers longer than 1,000 kilometers flow uninterrupted to the ocean
  • Very long free-flowing rivers are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins
  • In densely populated areas, only a few very long rivers remain free-flowing, such as the Irawaddy and Saleen

Dams and Reservoirs

  • the leading contributors to the loss of river connectivity
  • There are approximately 2.8 million dams (with reservoir areas >1000 cubic meters) regulating and creating over 500,000 kilometers of rivers and canals for navigation and transport and building irrigation and water-diversion schemes
  • More than 3,700 hydropower dams (>1MW) are currently planned or under construction worldwide.
2019 Hydropower Status Report
More than 21.8 Gw of hydroelectric capacity was put into operation in 2018
2019 Hydropower Status Report
Electricity generation from hydropower projects achieved a record 4,200 terawatt hours (TWh) in 2018, the highest ever contribution from a renewable energy source, as worldwide installed hydropower capacity climbed to 1,292 GW, according to the 2019 Hydropower Status Report
China added the most capacity with the installation of 8,540 megawatts, followed by Brazil (3,866 MW), Pakistan (2,487 MW), Turkey (1,085 MW), Angola (668 MW), Tajikistan (605 MW), Ecuador (556 MW), India (535 MW), Norway (419 MW) and Canada (401 MW).
Brazil has now overtaken the United States as the second largest producer of hydroelectricity by installed capacity, after 3,055 MW was put into operation last year at the 11,000 MW Belo Monte complex in the country’s northeast.

Small Hydropower Plants (SHP)

Source:  Couto, T. B., & Olden, J. D. (2018). Global proliferation of small hydropower plants – science and policy. Frontiers in Ecology and the Environment, 16(2), 91–100.doi:10.1002/fee.1746 

Definition:  Considerable variability in definition across countries; refers broadly to facilities that produce less electricity and operate in smaller rivers as compared to large hydropower plants.  Vary greatly in level of flow control, storage capacity, diversion structures.  70% of countries classify as installations with less than 10 MW capacity


  • There are close to 11 SHPs for every large hydropower plants (LHPs) = 11% of global hydropower electricity generation
  • 82,891 SHPs are operating or under construction in 150 countries
  • 181,976 new plants may be installed if all potential capacity were developed
  • 10,569 new projects appear in national plans
  • China has the world’s largest number of SHPs—47,073; 57% of the world’s SHPs
  • Europe has 26,877 SHPs
  • Future plans for SHPs are concentrated in Asia, the Americas, Southern and Eastern Europe and East Africa

            Environmental Impacts

  • Similar to LHPs; but SHPs generally occur in smaller rivers, which is significant given the ecological importance of headwater streams
  • Cumulative ecological impacts of SHPs (multiple installations in the same river basin) appears to be an underappreciated issue

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