The compensation reservoir Les Marécottes, which holds the turbined water of the Châtelard hydroelectric plant for further power generation at the Vernayaz hydroelectric plant, is owned by the Swiss Federal Railways. The downstream multiple arch dam is the characteristic element of the compensation reservoir, which was designed in 1926 by the engineer A. Sarrasin. The multiple arch dam consists in 43 thin, inclined reinforced concrete arches supported by buttresses. The total length of the structure reaches 200 m. Although no unusual behavior of the dam has been detected since commissioning, a general rehabilitation of the plant aims to increase the service life of the fairly sensitive structure. In close collaboration with the expert Prof. E. Brühwiler (EPFL), the feasibility study was completed and optimized in order to cautiously preserve the historic monument. The first intended step is the removal by high-pressure water jetting of the pore sealing layer currently covering the surface of the buttresses. This water vapor impermeable layer caused various spalling phenomena in concrete and corrosion on the original steel reinforcement. After local treatment of the damaged zones and reprofiling with a color-matched repair mortar, the entire concrete surface is saturated with a deep impregnation. The purpose of the depth impregnation is to ensure the corrosion protection of the existing reinforcement. The upstream surface of the buttress dam is sealed by an application of synthetic PUR-based layers. The damaged zones of the ground slab of the basin and on the upstream retaining walls are restructured locally. The rehabilitation is planned for 2019 and 2020 and includes the motorization of the vertical gates at the intake of the compensation reservoir as well as the reconstruction of the bottom outlet.