Ecological restoration of tufa ridges on a karst stream

Flood prevention and agricultural policy have led to the physical modification of watercourses through recalibration and rectification. These techniques were frequently used in rural and peri-urban areas, particularly between 1950 and 1980, to reduce the frequency of flooding. In most cases, these works were carried out without any environmental diagnosis, and without taking into account the natural dynamics of the river and the potential impact on the aquatic ecosystem. The result today is a profound deterioration in all the functions and ecosystem services provided by the environment. This work has resulted in a general homogenization of the riverbed, flows and, consequently, aquatic habitats. They have also contributed to the significant incision of riverbeds and the accentuation of summer low-water periods, which are unfavorable to aquatic life.

A left-bank tributary of the Dordogne, the Céou stream is no exception to this trend. This 50 km-long river, which flows through a karstic watershed, is subject to major disturbances caused by past works. To improve the condition and functioning of this watercourse, the Programme pluri-annuel de gestion du Céou (PPG., 2007) has enabled the restoration of what are known as "barres tufeuses", small sills formed naturally from limestone tuff. The aim of these structures is to reverse the sinking of the riverbed induced by active self-incision since the extensive dredging work carried out in the past, and to diversify the functionality of the environment by creating fish habitats in the most disturbed sectors, serving as refuge areas in the summer.

The need to actively intervene in terms of ecological restoration in this given context stems from several factors: summer drying-up of the watercourse, which seems to worsen in the driest years, significant mortality of aquatic fauna (need for annual rescue fishing), a difference between the abundance of fish populations actually observed and those potentially expected in view of the capacities of the environment and historical data. In order to obtain characterization data for each development, monitoring was carried out in 2014 and 2015. The aim was to assess the capacity of the restored tufa bars to respond to the problems identified: limiting incision and restoring certain ecological functions. A protocol has been set up to assess the effects of the developments on the watercourse, using a number of indicators that will be described below.

In this article, we will briefly present the processes involved in the formation of tufa bars, based on selected bibliographical data. We will then outline the various key stages in their restoration, from project design to implementation. We will present the difficulties encountered, whether technical, economic...