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Indicator Facts

CBD Strategic Goal: B. Reduce the direct pressures on biodiversity and promote sustainable use

Aichi Biodiversity Target: Target 5. By 2020, the rate of loss of all natural habitats, including forests, is at least halved and where feasible brought close to zero, and degradation and fragmentation is significantly reduced.

Headline Indicator: Trends in extent, condition and vulnerability of ecosystems, biomes and habitats

CBD AHTEG Operational Indicator: Trends in the fragmentation of natural habitats

Key Indicator Partners: 

 

Data Available: Global, regional or river basin baseline

Development Status: Ready for global and river basin use

Reason

Background: Freshwater ecosystems including rivers, lakes and wetlands are extremely rich in species, but unfortunately, are also amongst the most altered and threatened ecosystems in the world. The natural flow regime and the longitudinal and lateral connectivity of rivers, both essential characteristics that sustain the biophysical and ecological processes necessary for life in freshwaters, are disrupted when rivers are fragmented by dams and their reservoirs. This fragmentation and the consequent loss of ecosystem processes do not only affect ecosystems and species, but humans as well. For example, the loss of floodplain inundation patterns affects both native ecosystems and human communities dependent on floodplain fisheries and flood recession agriculture.

Policy questions that the indicator addresses: The indicator provides a snapshot of the condition of riverine ecosystems around the world and identifies those rivers that remain free flowing and therefore worth conserving, as well as rivers where environmental flows, restoration and other interventions could improve the ecosystem functioning and address the loss of riverine species and habitats.

Indicator relationship to main Aichi Biodiversity Target: It is directly related to Target 5 on extent of habitats – it is one of the few indicators that looks at freshwater ecosystem condition.

Data Information

Data Available: Global, regional or river basin baseline

Global indicator aggregated from: National and River Basin level data

Indicator can be disaggregated at: Regional and River Basin level

Status

The River fragmentation and flow regulation indicator was developed in 2005 to provide a global picture of the degree of alteration of the world’s large river systems.
The methodology used in the indicator allows it to be calculated for regions and nations provided that there are sufficient data available at river basin scale.
A national level application of the indicator has been done for Sweden, by Department of Ecology and Environmental Science at Umeå University.  For more information about producing regional and national river fragmentation and flow regulation indicators, contact Christer Nilsson from Umeå University (christer.nilsson@emg.umu.se).

Scale

 

Levels at which the indicator is currently used: Global, river basin

Levels at which the indicator could be used: Global, national

 

The purpose of the river fragmentation and flow regulation indicator is to provide a measure of the degree to which rivers’ connectivity and flow regime have been altered by dams and reservoirs. The impacts of dams and reservoirs vary depending on the number and size of the dams, where they are located on the river system, as well as the storage capacity of the associated reservoirs. In its current version, the indicator partners assessed 292 large river systems with drainage basins representing 54 percent of the world’s land area. Fragmentation rankings (i.e., highly affected, moderately affected and unaffected) are assigned to each large river system using the number, location, and storage capacity of dams to estimate the proportion of free flowing miles of river length, as well as the annual runoff that is stored behind dams throughout the system.
Unaffected rivers are those without dams in the main channel of the river and, if tributaries have been dammed, the flow of the river has not changed substantially (less than 2% of the natural flow has been affected). Highly fragmented and regulated rivers, on the other hand, include those with less than one quarter of their main channel left without dams, where the largest tributary has at least one dam, and where the reservoirs retain a considerable portion of a year’s flow.
 

The Indicator

Impact classification based on river channel fragmentation and water flow regulation by dams on 292 of the world’s large river systems.

Unaffected rivers —shown in green— are those without dams in the main channel of the river and, if tributaries have been dammed, the flow of the river has not changed substantially (less than 2% of the natural flow has been affected). Highly fragmented and regulated rivers (shown in red) include those with less than one quarter of their main channel left without dams, where the largest tributary has at least one dam, and where the reservoirs retain a considerable portion of a year’s flow.

Source: Nilsson et al. 2005

Compiled by UNEP-WCMC, 2010.

How to interpret the indicator

River fragmentation disrupts the natural biophysical and ecological processes essential for maintaining life in freshwaters. Dams disconnect rivers from the surrounding floodplains, block migratory fish passage, and retain sediments and nutrients that nourish downstream ecosystems. Reservoirs turn running water habitats into lake-like systems, resulting in the disappearance of fish species adapted to riverine habitats and the proliferation of other, often exotic species adapted to still waters. In highly fragmented rivers the diversity of life is likely to have been reduced and the adaptive capacity of native species impacted. Therefore an increase in river fragmentation indicates a higher likelihood of biodiversity loss as well as a disruption in ecosystem service delivery. Sediment trapping behind dams, for instance, disrupts the delivery of essential nutrients and sediments to coastal ecosystems, affecting the services they provide, such as fisheries and beach replenishment.

Current Storyline

Globally, two-thirds of all large river systems are moderately to highly fragmented by dams and reservoirs. Industrialized regions such as the United States and Europe and heavily populated countries like China and India encompass the most fragmented rivers. Arid regions also tend to have some of the highest levels of river fragmentation, since people in these regions have managed scarce water resources by building dams and reservoirs. Free flowing rivers only remain in the world’s most remote and less-populated regions of Alaska, Canada, and Russia, and in small coastal basins in Africa and Asia.

National Use

The River fragmentation and flow regulation indicator provides a global picture of the degree of alteration of the world’s large river systems. However the methodology used in the indicator allows it to be calculated for regions and nations provided that there are sufficient data available at river basin scale. 

A national level application of the indicator has been completed for Sweden, by the Department of Ecology and Environmental Science at Umeå University. For more information about producing regional and national river fragmentation and flow regulation indicators, contact Christer Nilsson from Umeå University (christer.nilsson@emg.umu.se) and/or Carmen Revenga from The Nature Conservancy (crevenga@tnc.org).

Future development

The recent development of new, high resolution global data sources describing the location of rivers (provided by the HydroSHEDS database; Lehner et al. 2008) and dams (provided by the GRanD database; Lehner et al. 2011) enables the calculation of new indices related to river fragmentation. Current research at McGill University aims at calculating a new global ‘River Connectivity Index’ as a measure of network fragmentation, and a ‘Degree of Regulation Index’ as a measure of flow regulation. Both indices may replace the current river fragmentation index in the near future and are applicable at various scales, ranging from global to individual watersheds and river reaches. For more information contact Bernhard Lehner from McGill University (bernhard.lehner@mcgill.ca).

Indicator Publications
 TitleDescription
High-resolution mapping of the world’s reservoirs and dams for sustainable river-flow management (2011)Journal Article: Lehner et al., Front. Ecol. Environ., 9, 494–502.
Pilot Analysis of Global Ecosystems: Freshwater Systems (2000)Report: Revenga et al. Washington, DC: World Resources Institute
Fragmentation of Riparian Floras in Rivers with Multiple Dams (2000)Journal Article: Jansson et al., SEcology, 81. no. 4, 899-903
“Fragmentation and Flow Regulation of River Systems in the Northern Third of the World.” (1994)Journal Article: Dynesius and Nilsson, Science, 308. no. 5720, 405 – 408
Fragmentation and Flow Regulation of the World's Large River Systems (2005)Journal Article: Nilsson et al., Science, 308. no. 5720, 405 – 408
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