Les relations karst-rivière dans les calcaires barrémo-bédouliens du Diois (Drôme – France) Exemple de la Gervanne

The karst in the Gervanne river is a well-developed system found in a limestone formation from the Barremian-Bedoulian that is about 400?m thick. Although its limits are not known to any great precision, they are very similar to those of the geographical area of the Gervanne watershed. Explorations have brought to light a dense system of cavities and one main outlet: Les Fontaigneux spring, a permanent spring. The relationships between the river and the karst are complex. They can be analysed by studying the hydrological data and tracer information. During low waters, seepage from the river often reduces river flow to nothing. Tracer tests were carried out to check the conditions in which water circulates in the karst. The rate between the first arrivals of the tracer and the peak velocity shows that the flow takes place in a channel pattern with very low longitudinal dispersal during low waters. Dispersion was only seen to occur during high waters, when the karst feeds water back into the river in a flow that takes into account the fissuring and joints between the rock. The seepage discharge in the Gervanne therefore depends on the amount of water in the karst. In fact, it does not seem possible to conduct artificial maintenance of the water level in order to avoid losses from the river into the karst. If artificial measures were to be taken to maintain a minimum water level in the river, for instance, by building a dam, watertightness could not be guaranteed because the basin is much too permeable.