The rivers and their remediation: issues, potential and limits

According to EauFrance.fr, the total length of French rivers in metropolitan France is 428,906 km, or 1.13 km of watercourses per square kilometer of surface area. This density immediately indicates the importance of these ecosystems for our territory. They represent essential resources for human societies, whether in terms of supplying water for consumption, agriculture or industry, or as nutritive resources, border demarcations or communication and transportation routes. All the services provided by watercourses have been strongly affected by the development of human societies, particularly since the industrial acceleration and demographic explosion of just over a century ago. Their ecological status is deteriorating under the multiplication of physical, chemical and biological pressures. By 2021, the European Environment Agency estimates that only 37% of surface water bodies in Europe are in good or very good ecological condition. In quantitative terms, water stress is on the increase. Every year, 20% of the European territory and 30% of the population are affected, and these figures are likely to increase in the future due to climate change. The situation is therefore alarming, despite the many efforts made over the last twenty years and the overall improvement in the quality of surface waters (rivers and lakes). According to a WWF (World Wildlife Fund) report, by 2024, 67% of France's surface water bodies could fail to meet the 2027 target for good ecological status (the initial target was 100% in 2015). However, the budgets involved are substantial. Indeed, annual expenditure on water policy in France is in the region of €25 billion a year, or €500 billion over 20 years, with sanitation and potabilization being the two main items of expenditure, far ahead of environmental restoration measures per se.

The situation varies greatly depending on the type of watercourse: while the quality of rivers and large watercourses has clearly improved (as a result of advances in sanitation and effluent collection), that of small watercourses has deteriorated as a result of the intensification of agricultural practices, the artificialization of watersheds, the disappearance of hedgerows.... Yet these represent the vast majority of watercourses in France, the victims of decades of overexploitation of their resources, whether the watercourse itself or its margins and associated wetlands. This historical legacy is a cause for concern, and has led to the urgent need for action to remediate watercourses. The scientific and technical knowledge is there, as are the awareness and the means, but probably still at an insufficient level.

The aim of this article is to provide a basic understanding of how running water ecosystems function, particularly under the impact of anthropogenic...