3.2.1 Barriers

3.2.1  Barriers

Overview

Humans have fragmented European water bodies with artificial barriers such as dams and weirs for centuries, as a means of ensuring water supplies, generating energy, facilitating navigation, and controlling flooding. Such human-made barriers reduce the ecological connectivity of a water body, impeding the flows of water, nutrients and sediment, create obstructions for species movement (particularly migratory species), often alter the quantity, quality and timing of river flows, both upstream and downstream, and can impact surrounding riparian zones and flood plains (Freshwater Information System, 2019).

There are different types of barriers, including dams, sluices, weirs, culverts, fords and ramp-bed sills and the extent differs to which these are recorded in the different national river assessment systems across Europe.

submit comment

In the second RBMPs, barriers are a significant pressure for almost 30 000 surface water bodies (20 % of total) in WFD countries, with the highest numbers being reported in Sweden, Germany, Austria, France, Denmark, and Spain. Furthermore, barriers are the reason or one of the reasons for designating approximately 10 000 water bodies as heavily modified, which amounts to of heavily modified water bodies in Europe.

submit comment

A large number of barriers are reported in the RBMPs to be used for hydropower (dams for hydropower production), flood protection and irrigation (water storage reservoirs). However, for a large share of water bodies affected by barriers (ca. 40 %), the uses which the barriers serve are unclear, either being unknown or not explicitly reported or obsolete. Indeed, many barriers on European rivers originated in the 10th to 19th centuries to operate mills and a high proportion of these are by now redundant. It is estimated that alone in France, Spain, Poland and the UK, there are up to 30 000 mainly small dams which are now obsolete (Gough et al., 2018). In addition, there are many weirs without a practical use.

submit comment

The most comprehensive overview of river fragmentation in Europe is provided by the recently published Pan-European Atlas of In-Stream Barriers.[1] The Atlas contains information on 630 000 barriers including not only large dams, but also hundreds of thousands of smaller weirs, ramps, fords and culverts. However, researchers have recently found that more than one third of barriers on European rivers are unrecorded, bringing the total to well over 1 million. This scale of river fragmentation is alarming and makes Europe the most fragmented river landscape in the world, with hardly any unfragmented, free-flowing rivers left (WWF, 2020). [2]

[1] Produced by the EU Horizon 2020 project Adaptive Management of Barriers in European Rivers (AMBER): https://amber.international/european-barrier-atlas/

[2] WWF, 2020, More than 1 million barriers destroying Europe’s rivers, new research shows, accessed 27th July 2020, https://www.wwf.eu/wwf_news/media_centre/?uNewsID=364559

submit comment

Figure 7             Man-made river barriers in Europe included in the AMBER Atlas

 

Notes: Insert notes here

Source: https://amber.international/european-barrier-atlas/

submit comment

Impacts

Man-made barriers such as dams, weirs and other impounding structures typically have the following negative effects on the environment of rivers:

  •  Habitat loss: Natural dynamics and river habitats are lost upstream of dams as they are ‘drowned’ or suffer depleted flows downstream due to the alteration of water flow conditions. As a result, aquatic flora and fauna are dramatically altered (Gough et al., 2018).
  • Flow regulation is one of the main adverse ecological consequences of dams and reservoirs to rivers. This is evident in downstream river ecosystems and is a result of dam operations reducing natural flows, eliminating peak flows, changing seasonal flow patterns, regulating low flows or other regulatory practices. Flow regulation may have significant negative effects on fish fauna and benthic invertebrate communities.
  • Fragmentation: Rivers are transformed into a series of ponded sections; dams block migration routes for fish in both up and downstream directions and habitats are isolated through fragmentation. This transforms natural fish fauna and leads to local extinction of fish species (Gough et al., 2018).
  • Sediment: Dams block transport of sediments in rivers, leading to accumulation and poor water quality in the reservoir, deprivation of sand and gravels downstream of dams, higher risk of erosion downstream of dams and in river deltas, and to a decrease in habitat quality upstream and downstream of the dam (Gough et al., 2018).
  • Water quality: Storage of organic material and nutrients in reservoirs and also in backwater from smaller dams often leads to a decrease in water quality, changes in temperature and the capacity to dissolve oxygen, and sometimes to seasonal stratification (Gough et al., 2018). Ponded sections have a longer water residence time, thus enhancing eutrophication effects such as phytoplankton blooms.

submit comment

The impacts of barriers vary according to the height and location of barriers. A major impact on a river could be caused by a single, very damaging structure or by the accumulated effects throughout the length of the river of a series of small structures, which may have only a small impact individually (EEA, 2018). The location of barriers in a catchment determines, to a large extent, their impacts on sediment fluxes, fluvial habitats such as floodplains and deltas, and on the abundance and diversity of freshwater biota. For example, barriers in lowlands can prevent or delay fish migrations, while headwater barriers can alter downstream flows and sediment transport (Jones et al., 2019).

The height of barriers also plays a major role in determining the impacts on freshwater biota and the surrounding ecosystem. High-head structures (large structures), typically higher than 8 m or 15 m, often create large impoundments, which can cause shifts in the composition of biota communities within the reservoir as well as downstream. Low-head structures (small structures) can also impact key ecological processes just as strongly. Because of their very large number, small structures are likely to cause greater cumulative impacts and a more significant loss of river connectivity than high-head structures (Jones et al., 2019).

submit comment

Measures and management challenges

Already in the first RBMPs, several European countries planned measures to improve the ecological conditions of rivers impacted by barriers. The planning of measures in the second RBMPs indicated substantial further effort to improve longitudinal continuity in river basin districts. The most common measures planned in this respect include the building of fish ladders and bypass channels, the removal of artificial structures such as barriers, the setting of ecological flows and measures for sediment management .

The implementation of such measures is closely linked to the environmental objective of the WFD to restore continuity for migrating species in regulated rivers. A number of other EU policies are also supporting the restoration of river continuity and the rehabilitation of surface waters that are impacted by barriers, including the Birds and Habitats Directives (2009/147/EC and 92/43/EEC), the new EU Biodiversity Strategy for 2030 (EC, 2020) and the Eel Regulation (Council Regulation (EC) No 1100/2007). The new EU Biodiversity Strategy 2030 has actually included a specific commitment to restore at least 25 000 km of free-flowing rivers by 2030 through the removal of primarily obsolete barriers and the restoration of floodplains and wetlands (EC, 2020).

submit comment

Overall, due to the very high number of barriers present on rivers in Europe, there is a need for prioritisation of measures to restore continuity. Some national and regional strategies for restoring continuity are in place to ensure a phased approach in dealing with the issue of barriers. Examples include the Benelux treaty on free fish migration (adopted in 1996), continuity restoration initiatives in the international river basins of the Rhine[3] and the Danube (Shepherd, 2012) as well as national programmes and priority networks for river continuity restoration in specific countries such as France, Austria, Germany and Finland (Kampa et al, 2017; Ollikainen & Vilhunen, 2019).

[3] http://www.salmoncomeback.org/

submit comment

The implementation of measures is affected by significant gaps in knowledge concerning barriers, their abundance, distribution in the European river networks (especially of small barriers) and their ecological effects. Recent AMBER study (see above) might have summarised many of the needed basic information. Also, knowledge still needs to be solidified on the effects of some of the key measures. For instance, barrier removal is increasingly viewed as a necessary management measure to reinstate natural connectivity. However, we so far have little knowledge to make predictions about the geomorphological and biological trajectory of a river system once a barrier has been removed (Birnie-Gauvin et al., 2017). Also, knowledge is lacking on measures to mitigate impacts on downstream migration of fish at hydropower turbines especially in large rivers.

submit comment

An additional implementation challenge arises from the large number of barriers with an unknown or obsolete use. Funding measures to make barriers with obsolete use passable is a challenge, because of the lack of a specific water use sector assigned to these modifications in the rivers.

submit comment

In parallel to planning measures for dealing with the impacts of existing barriers, new barriers and dams are built elsewhere in Europe driven by policies for energy production, transport, flood protection and securing water supply (e.g. new hydropower plants in the Balkans, see WWF (2019))[4]. In this respect, a much closer coordination of river basin management planning under the WFD and the planning of new river infrastructure to serve sectoral development is essential to safeguard river continuity.

[4] WWF, 2019. Hydropower pressure on European. The story in numbers.

submit comment