Abstract
The Upper Rhine hydrosystem exhibits a high biodiversity. Since mid-19th century, several engineering projects developed for flood control, agriculture, navigation and hydropower, have drastically impacted aquatic and riparian habitats. In this context, the specific alluvial species of the Rohrschollen natural reserve is declining. Its main watercourse, the Bauerngrundwasser, an anastomosing channel of the Rhine, has now a constant and low discharge (under 0.1m3/s), inducing a high rate of fine sediment deposition. During periods of high flow, the site is flooded but water has no energy due to a dam's backwater effect. The LIFE+ project "Restauration de la dynamique des habitats alluviaux rhénans sur l'île du Rohrschollen", managed by the City of Strasbourg, plans to restore the hydromorphological and ecological functionality of the hydrosystem: bedload dynamics, channel mobility, surface water-groundwater exchanges, renewal of pioneer ecosystems...Thus, the Bauerngrundwasser will be reconnected to the Rhine by building a new upstream connection channel. Water input will attain 80m3/s, leading to dynamic floods and making of this project one of the most ambitious for the French side. To assess the efficiency of this restoration project, a hydromorphological monitoring (included in an inter-disciplinary project) is carried out and is based on the comparison of pre- and post-restoration dynamics. It includes in particular a topographic (LiDAR, theodolite...), a cartographic (aerial photos), a sediment transport (sampling, PIT tags, erosion chains...), a hydrological (water level sensors, gauging) and a hydrogeological (piezometers) studies. This paper will give the first results, as well as the implications of the temporal trajectory of the system (historical study based on old maps, aerial photos and sedimentological investigations), which have notably been got in order to give some prospective guidelines concerning the efficiency and sustainability of the restoration.