The objective of the study carried out by the Etablissement Public Loire is to update the status of the water bodies concerned by the hydroelectric development of Montpezat, by integrating Lake Issarlès, and to deepen knowledge on the development. The specificity of the water transfer between the Loire basin and the Rhône basin makes the Montpezat complex a major and strategic structure in the perimeters of the SAGE Loire upstream and Ardèche. It is at the heart of the concerns of the members of the Local Water Commission (CLE). The data acquired as part of this study will feed into the reflections initiated in parallel on the sharing of water between the Loire basin and the Ardèche, and more generally on water management on the Loire basin upstream, by integrating the prospects of climate change.