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Understanding the concept of ecological continuity

The ecological continuity of watercourses is currently defined as "the free movement of living organisms and their access to areas for reproduction, growth, feeding or shelter, as well as the natural transport of sediments from upstream to downstream areas and from one bank to the other". Hence, when one talks about ecological continuity in rivers, one is talking about biological, sedimentary and hydrological continuity.

This continuity exists in two dimensions:

• A longitudinal dimension, upstream-downstream, illustrated in particular by the upstream and downstream migrations of numerous migratory fish species.
• A lateral dimension, from one bank to the other, from the major bed to the minor bed, illustrated, for example, by the interactions between aquatic and terrestrial environments and the resulting succession of animal and plant species.

The impact of obstacles on watercourses

The presence of obstacles in watercourses (dams, locks, weirs, mills) or on their banks (dykes, bank protection) leads to the fragmentation of watercourses and a breakage in ecological continuity. In other words, it leads to an ecological discontinuity. This discontinuity can be more or less significant. A mill occupying only part of a watercourse will cause only a minor discontinuity in the watercourse, even though it already exists. On the other hand, the presence of major obstacles such as dams will prevent the migration, reproduction and settlement of many species, and create artificial lakes of stagnant water. These changes will lead to a chain of other changes in the river ecosystem and its catchment area:

• Biodiversity loss
• Disruption of the food web
• Morphological and sedimentary changes to the banks and the watercourse
• Overflows during flood periods (lateral obstacles)
• Degradation of water quality
• Reduced exchanges between groundwater and rivers
• Instability of ecosystems (loss of resilience)
• etc.

Solutions for restoring ecological continuity

Ecological continuity of watercourses was defined by the Water Framework Directive in 2000 as an element contributing to the good ecological status of watercourses (Annex V, Directive 2000/60/EC). Since then, many texts have taken up this notion of ecological continuity. These include the 2006 law on water and aquatic environments (a French law translating the objectives of the WFD), and the national management plan for eels. However, it was only after the introduction of a green and blue framework by the Grenelle laws of 2009 and 2010 that restoration of the ecological continuity of rivers became a priority. In France, this led to the introduction of a national plan to restore ecological continuity in 2009.

In France, since 2009, the Office Français de la Biodiversité (formerly ONEMA, now AFB and OFB) and the Water Agencies have been drawing up an inventory of obstacles in watercourses. This inventory is accompanied by an assessment of the use and impact of these obstacles. This detailed diagnosis makes it possible to propose various restoration solutions "to cancel or at least reduce the negative impacts associated with the presence of obstacles in watercourses" (see ONEMA brochure). These solutions range from simply modifying the obstacle to removing it altogether.

Lowering

This solution allows keeping part of the obstacle in place by proposing either a reduction in its height, or the opening of a localised gap to allow for water and aquatic species circulation. This solution is commonly considered for structures of great heritage or landscape interest, such as mills. It reconciles ecological continuity with heritage conservation.

Opening the floodgates

Some obstacles that have become obsolete (old mills or disused hydraulic structures) retain a strong heritage interest. Their complete or partial removal (lowering) is not an option. To achieve this, it may be proposed to open the gates permanently or seasonally. This solution allows retaining the structure of interest while restoring the ecological continuity of the watercourse.

Installation of fish passage devices

Where the main objective is to restore fish continuity, the installation of devices to enable fish to pass over the obstacle may be considered. There are many different ways of enabling fish to swim up or down watercourses: technical or natural fish passes, lifts, behavioural barriers with light, sound or electric signals. Although useful, these facilities are specific to certain species and require regular and costly maintenance. Their installation also depends on numerous local and logistical factors, sometimes limiting their effectiveness. A great deal of research and development work is underway to optimise these methods.

Interruption of maintenance operations

Some obstacles are removed naturally, due to their age and the natural erosion of the environment (e.g. floods washing away abandoned structures). In this particular case, no action is taken, but measures are taken to support their natural removal.

Complete removal

This is the most effective way of restoring the ecological continuity of watercourses. Complete removal is recommended in particular for obstacles that are abandoned, ageing, unused and of no major heritage or landscape interest. This solution is usually accompanied by restoration of the watercourse to support the system restoration process. For example, many hydroelectric structures built in the early 1910s-1920s are now ageing, with safety and profitability problems. Against this backdrop, there are an increasing number of decommissioning projects around the world. However, the complete removal of barriers is also one of the most controversial solutions. It is therefore necessary to initiate a thorough management and good communication for the restoration project.

To find out more about the work of the OFB and the Water Agencies, see our "Further information" page.