Fragmentation of river systems

Indicator Specification
Indicator codes: SEBI 014
expired Created 26 Feb 2007 Published 21 May 2010 Last modified 21 Oct 2016, 11:31 AM
Topics: ,
This content has been archived on 21 Oct 2016, reason: Content is outdated
The indicator shows in spatial and quantitative terms, fragmentation due to the presence of artificial structures that a) may affect the passage of migratory fish and so restrict their range and/or abundance and b) changes substantially the natural habitat distribution within rivers and modify their ecological capacity. It thus describes the difference between the potential range and actual range of migratory fish in river systems due to artificial obstacles on the one hand and the change in habitats on the other hand.

 The data sets for the indicator are still under construction.


Assessment versions

Published (reviewed and quality assured)
  • No published assessments


Justification for indicator selection


  • This is a direct measure of the impact of fragmentation on biodiversity which is both policy-relevant and resonant with the general public.
  • The second part of the indicator aims at capturing the silent effects of river damming to avoid ill-targeted policy responses (imposing fish ladders everywhere for example) disregarding the habitat destruction resulting from impoundment.

Scientific references

  • No rationale references available

Indicator definition

The indicator shows in spatial and quantitative terms, fragmentation due to the presence of artificial structures that a) may affect the passage of migratory fish and so restrict their range and/or abundance and b) changes substantially the natural habitat distribution within rivers and modify their ecological capacity. It thus describes the difference between the potential range and actual range of migratory fish in river systems due to artificial obstacles on the one hand and the change in habitats on the other hand.


No units have been specified

Policy context and targets

Context description

To be healthy, fish communities require free access to river systems and healthy rivers that offer the different ranges of habitats required to fulfill their life cycles. River fragmentation is understood as more threatening to fish (aquatic) communities than pollution.

All fish species migrate in the water system. Most are short-distance migrants with requirements in the range of 10 to some 100 km Some are amphibiotic and their life cycle requires journeying between sea and specific rivers. For example, anadromous migratory fish (adults living in the sea and migrating up rivers to spawn in freshwater), such as Atlantic salmon (Salmo salar) and sea trout (Salmo trutta), rely for their movement and life cycle upon a favourable conservation status of their waterbodies, including unimpeded access to freshwater spawning sites and adequate river conditions during their fresh-water life. The European eel (Anguilla anguilla) is catadromous (migrating to the sea to spawn and growing in rivers), and relies similarly on movement between the sea where it breeds and within the rivers in which it grows. Obstacles of any kind (dams, cascades, diversions, quality, etc.) affect not only the movement of fish but other groups as well (invertebrates, mammals, plants, etc.).

Several types of habitat modification alter fish populations. A key change is related to the presence of numerous small dams, changing flowing rivers into stretches of river with still water conditions. Even though fish may actually pass the dam wall, they may find adverse living conditions upstream, which may make passing facilities ineffective at the population level.

All causes together alter fish communities by disrupting their structures (size components of the community, functional groups, species diversity and relative abundance) and in extreme cases result in the extinction of a population or even of the species.

Due to data constraints, the first produced indicators will be biased: non-large dams in the height range of 2.5 to 14 meters are impervious to fish journeying upstream; hence they have to be considered as a minimum pressure (underestimated) and not actual pressure. In parallel, comparisons with catchments where all dams are registered will be carried out.

Relation of the indicator to the focal area

Unfragmented rivers support a full range of ecosystem services and the majority of species and habitats within the river. Fragmentation decreases the size of undisturbed rivers and puts the integrity of the ecosystem at risk. This in turn might affect the potential of the river to deliver services.


No targets have been specified

Related policy documents

No related policy documents have been specified


Methodology for indicator calculation

The CBD Subsidiary Body on Scientific, Technical and Technological Advice (SBSTTA) has reported on river fragmentation globally (UNEP/CBD/SBSTTA/10/INF/20, 17 December 2004), based on a fragmentation analysis by the World Resources Institute.

The French authorities, with special mention to the Loire-Bretagne Water Agency has developed a specific approach and carried out surveys on the Loire and Britany systems where LIFE supports Atlantic salmon and European eel habitat and population restorations.

The current development carried out at the EEA consists of implementing in a geographical database system the calculation of migratory routes and the impact of the three variables that affect migration of any kind and apply in both directions:

  • Permeability (multiplicative function expressed as % of passing per obstacle), depending on fish biology and obstacle features. This expresses whether or not a population can reach a target area;
  • Delay (additive function expressing time needed to pass one obstacle), expressing if the targeted area is reached in due time;
  • Fatigue (subtractive function expressing the decrease in physiological status resulting from passing one obstacle), expressing if the fish reaches its target in acceptable condition.

The variables apply individually or jointly on a river system for each species and allow assessing chains of small obstacles. The simplest application consists in assessing to what extent it is possible to access the spawning areas for anadromous fish along time, only based on permeability of large dams.

In a first step, the indicator focuses on the difference between the potential range and actual range of migratory fish in river systems due to artificial barriers and on the change of river structure resulting from works. Salmonids are the most emblematic fishes whose reproductive migration is jeopardised by physical obstacles. Their requirements and routes are the best documented.

By locating and counting the obstacles (large dams first, abstractions and small dams in a second step), the indicator defines the difference between the potential extension and actual range of migratory fish in river systems due to man-made obstacles. The indicator of passing considers both directions of migration because most obstacles do not have a symmetric impact on movement -- they may not pose a problem for upstream movement but a problem for downstream movement, or vice versa.

Methodology for gap filling

No methodology for gap filling has been specified. Probably this info has been added together with indicator calculation.

Methodology references

No methodology references available.

Data specifications

EEA data references

  • No datasets have been specified here.

Data sources in latest figures


Methodology uncertainty

No uncertainty has been specified

Data sets uncertainty

No uncertainty has been specified

Rationale uncertainty


  • From a policy point of view, the absence of explicit targets makes the status assessment possibly controversial (what is the acceptable or sustainable degree of fragmentation vs. the advantages of water impoundment, hydropower production, flood protection, etc?). However, it is the absence of addressing the issue that is the cause of existing gaps in policy that might be documented by the development of the indicators.
  • Data availability: the lack of comprehensive data sets makes it necessary to model most descriptors. To tackle this issue, the model application has been featured with scenario capabilities that facilitate discussion with experts, confrontation of results and improvements.
  • The possible uncertainty (possibly inaccurate indicator) that results from considering either only large rivers or all rivers systems is unknown: for example Atlantic salmon, the most 'tolerant' to obstacles, can be eradicated by dams on small rivers where its spawning areas are situated even though the main river stems are free of obstacles. Hence, a large bias might be experienced if, at the European scale, an arbitrary threshold on data collection is used, for any reason.
  • The large dams represent a subset of obstacles because most dams in the range of 10 - 15 m high are not recorded as large dams and none below 10 m height should be. In number, large dams represent about 10 % only of the total number of dams potentially making obstacles. Hence, at the country scale, these are likely to miss many smaller structures of importance in rivers and streams, such as the accumulation of flow across catchments from diversion reservoirs. They should be investigated, e.g. through the Water Framework Directive risk assessments as a starting point.


Fish is only one dimension of river fragmentation impacts, along with changes in sediment flow and alteration of hydrological cycles. The environmental impacts must be addressed as well because meeting the targets of renewable energy triggers the development of small hydropower plants. These other dimensions however are less directly relevant to biodiversity.

Migratory fish populations may be influenced by a range of pressures and management measures, at sea and within rivers. As a wider biodiversity indicator, the status of fish populations in river systems should be assessed (e.g. as under the Water Framework Directive/Natura 2000 riverine habitats).

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.

Work description

SUGGESTIONS FOR IMPROVEMENT 1. Improve the collection of data related to obstacles (dams, abstractions, etc.), and the impact of other classes of obstacles that may affect the passage and range of long and short distance migratory fish classified according to impact (e.g. based on size of structure / river and their effect on river flow), type (e.g. hydro dam) and effect on short distance migratory fish (e.g. free excursion cumulated length). 2. informing on the quality of the indicator with a sensitivity analysis extending the calculation by including: i) smaller structures ('non‑large' dams) and ii) other obstacles. This would assess bias, and thereby provide input towards a more detailed European analysis. 3. Improve data quality, depending on data availability and country collaboration. A more comprehensive analysis of fragmentation impact on biomes should also take account of the following: a) Other non-migratory organisms whose natural transit through river systems is impeded by engineered structures (with special regards to natural riparian forest regeneration through propagule drifting). b) The influence of other obstacles that may have significant impact on local and long distance migrations. c) Take into account lateral fragmentation in which rivers are isolated from their floodplains by engineered bank structures such as encroachments, bunds, levees and dykes along their length. Finally, it should be examined what would be possible through the monitoring of ecological condition under the Water Framework Directive and the development of water accounts, for example: Hydrological regime in relation with abstractions and diversions operation; Continuity (ability of sediment, plant seed ("propagules") and migratory species to pass freely up/down rivers and laterally with the floodplain); Morphology (i.e. physical habitat — compositions of substrate, width/depth variation, structure of bed, banks and riparian zone, platform).

Resource needs

No resource needs have been specified


Not started


2099/01/01 00:00:00 GMT+1

General metadata

Responsibility and ownership

EEA Contact Info

Katarzyna Biala


No owners.


Indicator code
SEBI 014
Version id: 1
Primary theme: Biodiversity Biodiversity


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DPSIR: Pressure
Typology: N/A
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