Focal Area: Ecosystem integrity & ecosystem goods & services

Headline Indictor: Connectivity/fragmentation of ecosystems

Key Indicator Partner:      

Data Available: Global, regional or river basin baseline

Development Status: Ready for global and river basin use

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. 

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 total, 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.

This is a global level indicator that captures the degree of alteration of large river systems (Virgin Mean Annual Discharge of at least 350m3/s). The methodology and concepts, however, can be applied at national or regional scales if the necessary information on dam location and reservoir storage capacity are available. Some examples of more detailed national level applications have been carried out in Sweden by the Department of Ecology and Environmental Science at Umeå University. 

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.

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.

‘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. Rivers flow most freely in the world’s most remote and less-populated regions of Alaska, Canada, and Russia, and in small coastal basins in Africa and Asia.’

There is no future development of the indicator planned at this time.