Category: River Basin Modifications

Large and small hydro

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

Freshwater Planetary Boundary

Freshwater consumption and the global hydrological cycle

“The freshwater cycle is strongly affected by climate change and its boundary is closely linked to the climate boundary, yet human pressure is now the dominant driving force determining the functioning and distribution of global freshwater systems. The consequences of human modification of water bodies include both global-scale river flow changes and shifts in vapour flows arising from land use change. These shifts in the hydrological system can be abrupt and irreversible. Water is becoming increasingly scarce – by 2050 about half a billion people are likely to be subject to water-stress, increasing the pressure to intervene in water systems.  A water boundary related to consumptive freshwater use and environmental flow requirements has been proposed to maintain the overall resilience of the Earth system and to avoid the risk of ‘cascading’ local and regional thresholds.”
Stockholm Resilience Planetary Boundaries


  • Two control variables:
    • Global–Maximum amount of consumptive blue water use
      • 4000 cubic km/year (4000-6000 km3/yr)
    • Regional (River Basin)–Blue water withdrawal as % of mean monthly river flow
  • Current Global value (river basin not determined): 2600 km3/year

Hydrological Cycle in the Anthropocene

Source:  (Global Hydrological Cycle in the Anthropocene) This commonly reproduced image from the USGS of the averaged depiction of the hydrological cycle does not represent important seasonal and interannual variation in many pools and fluxes.
A hydrologic cycle in the Anthropocene should include:

  • Include anthropogenic influences
  • Include global teleconnections
  • Multiple catchments
  • Endorheic basins
  • Estimates of green, blue, and grey water use

Estimates of Global Pools and Fluxes of Water

Estimates of pools and fluxes are based on a synthesis of approximately 80 recent and global-scale studies; volumes represent the central point of the most recent or comprehensive individual estimates. Adapted from Abbott, et al., Nature Geoscience 12, 533–540. 2019
  • Based on the figures in the previous table, human appropriation (~24,000 km3yr-1) redistributes the equivalent of half of global river discharge or double global groundwater recharge
  • Irrigated agriculture is the largest water consumer, accounting for ~85-90% of water consumption, followed by industrial, and domestic water use
  • Global water consumption rose 40% between 1980 (~1,200km3) and 2016 (~1,700km3).

Global and Sectoral Water Consumption 2016

Adapted from: Qin, et al., Flexibility and intensity of global water use,Nature Sustainability 2, 515-523 (2019)

Top Six Water Stressed River Basins by Continent 2012-2016 5-year Index

In each continent, the basin is selected as the one having the largest water stress index among the top 10% basins that have substantial electricity generation, crops production, human population, livestock and dam capacity in each continent.

Source:  Source:  Qin, et al., Flexibility and intensity of global water use, Nature Sustainability 2, 515-523 (2109)

Global Demands on Freshwater

Source:  Wu, N., Wang, C., Ausseil, A. G., Alhafedh, Y., Broadhurst, L., Lin, H. J., Axmacher,J., Okubo, S., Turney, C., Onuma, A., Chaturvedi, R. K., Kohli, P., Kumarapuram Apadodharan, S., Abhilash, P. C., Settele, J., Claudet, J., Yumoto, T., Zhang, Y. Chapter 4: Direct and indirect drivers of change in biodiversity and nature’s contributions to people. In IPBES (2018): The IPBES regional assessment report on biodiversity and ecosystem services for Asia and the Paci c. Karki, M., Senaratna Sellamuttu, S., Okayasu, S., Suzuki, W. (eds.). Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Bonn, Germany, pp. 265-370.

Over 2 billion people live in countries experiencing high water stress. Recent estimates show that 31 countries experience water stress between 25% (which is defined as the minimum threshold of water stress) and 70%. Another 22 countries are above 70% and are therefore under serious water stress.

It has been estimated that about 4 billion people, representing nearly two-thirds of the world population, experience severe water scarcity during at least one month during the year.

Global overview of countries experiencing different levels of water stress
(the ratio of total freshwater withdrawn annually by all major sectors, including environmental water requirements, to the total amount of renewable freshwater resources, expressed as a percentage). Source: WWAP (UNESCO World Water Assessment Programme). 2019. The United NationsWorld Water Development Report 2019: Leaving No One Behind. Paris, UNESCO

Trends in Terrestrial Water Storage (TWS) April 2002-March 2016

Adapted from Rodell, et al. 2018; based on Figure 1 and Table 1
Terrestrial Water Storage = Groundwater + soil moisture + surface waters + snow + ice