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The influence of riparian zones on the good status of rivers

For the first time in France and in Europe, a study carried out jointly by Onema and Ecolab in Toulouse confirmed the fundamental role played by riparian zones for rivers. The study showed that any interference in the continuity of the vegetation along rivers affects the correct operation of rivers.

Riparian zones contribute to maintaining the physico-chemical properties of water and regulating its temperature, they also ensure the equilibrium of fish and benthic invertebrate populations. These results were recently published in a study run by Ecolab (a joint research unit between the CNRS and the Paul-Sabatier University in Toulouse) and Onema. The study addressed the importance of the integrity (continuity) of riparian zones and of their ecological restoration for biodiversity and the correct operation of rivers. Similar observations have been made in Canada and the U.S., but this study provided detailed information and new knowledge on the physical role of riparian zones and their links with life in rivers.

Maintaining good ecological status
What ecological role do riparian zones play? What impact do they have on good ecosystem operation? Which indicators should be used when restoring damaged zones? The study, launched in 2009, explored these zones with abundant plant life that lie at the border between land and water. It revealed their influence on rivers in the upper sections of basins and their role in maintaining the good ecological status of such rivers. Eight sites were selected in the heavily wooded Montagne noire region to the east of Toulouse. The rivers have been only slightly impacted by human activities, with the exception of forestry. They are home to a population of brown trout and have diversified vegetation, with occasional clear areas, between 130 and 550 metres long, caused by cutting along the river.

During the study, the scientists noted that the small clearings had an ecological impact on the surrounding area. For example, if the riparian vegetation is not dense enough, parasites develop and infiltrate the community of benthic invertebrates (mayfly larvae in this case) and then, via the food chain, the trout population. Invertebrates are thus good indicators of the status of the environment. Similarly, the decomposition process of organic matter in rivers and the speed of decomposition can also serve as indicators because excessively high or low speeds reveal an ecological malfunction or imbalance. This study paves the way for the development of a fairly simple method to define a set of indicators required to measure the ecological status of ecosystems during the restoration of riparian zones.

Impact on river morphology
The absence of vegetation results in more linear river beds and tends to limit the width of rivers. The creation of a method to monitor results following restoration will, for example, enable local stakeholders to observe the beneficial effects and will demonstrate the value of the work to restore the vegetation along rivers.

The study will soon be available at http://www.documentation.eaufrance.fr/.

Contact: jean-marc.baudoin@onema.fr

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