Longitudinal barriers alter a river’s natural flow. They can block the migration routes of fish and aquatic species both up- and downstream, with habitats becoming isolated through fragmentation.
Disrupted continuity affects the reproduction patterns of migratory fish. The transport of sediments in rivers is also blocked by barriers. This leads to accumulation of sediments upstream and a lack of sand and gravel downstream. As a result of all these factors, ecosystems and the physical processes that maintain them can be severely affected, and the habitat of aquatic flora and fauna can be dramatically altered. This also contributes to biodiversity losses in riparian areas and flood plains along rivers. In addition, the build-up of organic material and nutrients in reservoirs and in backwater from smaller dams often leads to a decrease in water quality, changes in temperature and the capacity to dissolve oxygen, and eutrophication.
The environmental objectives of the Water Framework Directive (2000/60/EC) are a strong driver for restoring river connectivity, and this particular objective has been further strengthend by the EU biodiversity strategy 2030, under the European Green Deal.
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Information on barriers in Europe’s rivers has been collected through two sources. EU MS has reported on barriers as part of their second river basin management plans and the research project AMBER has collected extensive information on location of barriers (AMBER, 2020). It is estimated that there are more than one million longitudinal barriers in Europe’s rivers, many of which are obsolete.
FIGURE 1 shows European river water bodies with significant pressures from barriers. ‘Significant’ means that the pressure contributes to an impact that may result in failing to meet the WFD objectives of not having at least good status.
If you want to learn more
Agriculture is an important sector for the European economy and, particularly in the second half of 20th century, agricultural yields have increased, thanks to changes in crop varieties and breeding techniques, new technologies and machinery, new farming practices and increased use of inputs such as fertilisers, pesticides and irrigation water.
However, growth in agricultural productivity has been accompanied by increased pressures and impacts on water and aquatic ecosystems in the form of pollution from nutrients and pesticides, together with over-abstraction of water for irrigation, and hydromorphological alterations.
By adopting the European Green Deal in order to achieve sustainable development, EU will need to reduce the environmental impact of the agricultural sector, in particular on freshwater ecosystems.
EPA Catchment Unit, 2016, 'Hydromorphology: what is it?'
Houlden, V., 2018, 'Hydromorphology: the forgotten facet of the Water Framework Directive', HR Wallingford
See also the EEA Reports
- Water and agriculture. Towards sustainable solutions. EEA Report No 17/2020
- European waters - Assessment of status and pressures 2018. EEA Report No 17/2020