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For centuries we have been adapting our waterways to use the fertile land of the major river beds for agriculture, to facilitate navigation and promote trade, to produce energy using watermills and dams, and, more recently, for leisure activities. At the same time, to protect against flooding, dykes were built to the extent that Europe's rivers have lost 90% of their flood plains. And, of course, rivers have too often served as an outlet for our waste!
The concept of 'ecological restoration' seeks to correct these impacts and suggests that an ecosystem can be restored to its 'original' or 'balanced' state. One can understand that restoring a castle consists of taking it back to its original state because there is a reference point, from the date of its creation. But when we are dealing with an ecological system that has evolved and changed over thousands of years, the question is more complex: there is no date of creation and we don't know where to place the marker on the timeline! Another approach is to assimilate ecosystems with organisms and to seek a 'healthy state', a rather fuzzy notion akin to that of restoring a functional status before the 'stress'. In both cases 'natural ecosystems' are construed as non-artificialised/humanized. But for socio-ecosystems which have co-evolved for centuries, this reference is not very meaningful as many species have since either disappeared or adapted to the new conditions. When the goal is to remove undesirable aesthetical, ecological, health or economic states, we talk of rehabilitation or reassignment. But then, the 'reference' does not exist and must be defined: what are the criteria to be used and what natures do we want to achieve?
Human actions are most often described by environmentalists as 'degradations', and human use of a system as 'pressure'. Indeed, according to the dominant paradigm of some conservationists, nature is perfect when it is free of human artefacts. Hence, they seek to 'remove' the infrastructures that have been built on watercourses so as to reveal 'wild' or 'natural' rivers... But this calls into question many other issues, such as the safety of local residents, the aquatic ecosystems that have grown over the centuries, and the future of the built heritage. Thus, some ecologists (not environmentalists) differentiate between levels of adapted systems, many of which are functional and should be studied as such rather than systematically considered as 'degraded'. What some have called 'new ecosystems'[i], as opposed to non-humanised systems, contribute to our living environment and also constitute an ecological heritage. This is not to say, of course, that all modified ecosystems are the same, or equally legitimate; indeed, case-by-case evaluations are required. The priority in terms of restoration then becomes clear: limiting pollution and maintaining high environmental heterogeneity, while accounting for an expected reduction in water flows due to climate change.
The large dams that were constructed to produce hydroelectricity and/or store water to protect against flooding, or to supply irrigation networks, together with massive withdrawals, have significantly affected the functioning of rivers: disrupting upstream-downstream continuity, particularly with regard to sediment transport, and greatly shrinking alluvial plains and 'fluvial annexes'. While in the popular imagination a river consists of flowing water, for ecologists a river system is foremost a heterogeneous set of more or less flowing or stagnant sub-systems. A hydro-system is the river together with its floodplain and fluvial annexes, such as backwaters, residual ponds, connected wetlands, etc.
Reclaiming these now largely urbanised areas in order to reintegrate them into the functioning of the river tends to be impossible for reasons of land use and safety. Infrastructures have a role in the current context and, as a rule, there are no plans to remove them in the near future. Neither is there talk of removing navigational facilities or the dykes that protect cities, such as the reservoirs on the Seine River shielding Paris. Lastly, these debates should not allow us to forget the issue of water quality, and, despite great efforts to clean up our waterways, the battle against pollution must remain the priority.
Therefore, the infrastructure discussed above have all been in place for a long time and, with few exceptions, the rediscovery of 'pristine' watercourses is possible only in our imaginations. Thus, particularly in France, attention has been turned to small weirs and their annexes, whether idle/abandoned or still in use, which are said to hinder 'ecological continuity' (i.e. the movement of fish and sediment). One of the objectives is the return of migratory fish (somewhat hastily considered to epitomize aquatic biodiversity) and the movement of trout, despite their being able to cross most weirs (naturally or using fish passes). However, not all aquatic species live in flowing water. Amphibians, for example, prefer stagnant environments and do not mix well with fish. This is also true of many invertebrates that would disappear with the removal of the weirs and associated wetlands, which are known to be rich in species, including those at risk. The removal of these last refuges is in any case only a meagre compensation for the fluvial annexes of the alluvial plain that we have erased, and it condemns part of the aquatic biodiversity that does not live in running water. Despite no assessment having been made of what stands to be gained or lost in terms of aquatic species and ecological functions, the French government has introduced a centralised policy of weir removal.
Supporters of ecological continuity also seem to fail to appreciate the ongoing processes of climate change, which, forecasts suggest, will lead to both more severe flooding and longer dry spells. From this perspective, is it preferable to have a 'natural' dry river or one in which there are still some small water bodies, which, despite their drawbacks, can serve as a refuge during a drought? My experience with other intermittent river systems has taught me the importance of such refuges for the recolonisation of the stream through drift, when the water rises again.
In conclusion, the entire debate revolves around two opposing ecological visions. One can be considered eco-centric, where 'true nature' is non-humanised to the extent that any development is seen as damaging the integrity of the system. The paradigm of nature without humans is the ideal within conservationist circles and is communicated by the concept of 'wild rivers'. Restoration then means erasing every human imprint. This long-dominant vision has been brought into question by those who believe that humans are actors in their environment and that river systems are co-constructed through natural processes and human actions.
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