The demonstration sites network: 16 years of monitoring to assess the effectiveness of hydromorphological river restoration in France

France's rivers have been largely altered by human activities. Hydromorphological restoration aims to re-establish their physical and dynamic characteristics, which support the good ecological status required since 2000 by the Water Framework Directive for all European rivers. The increasing number of restoration projects and studies assessing their effectiveness are showing positive overall effects on certain biological indicators (species abundance and diversity), but ecological responses remain variable. Since 2010, the French Biodiversity Office, the Water Agencies, INRAE and a number of technical partners, including river managers, have been setting up a network of demonstration sites for hydromorphological restoration. This network includes around fifty restored sites, covering seven ambitious types of restoration : remeandering, removal of cross-cutting obstacles, modification of bed geometry, ponds bypass, return to the talweg, removal of bed and bank fixations, addition of sediments. Within this network, standardised monitoring, known as minimum scientific monitoring, is being implemented on restored sites. Protocols make it possible to collect data over ten years on three compartments: hydromorphology (channel geometry, habitats, riparian vegetation, granulometry), physico-chemistry (pH, temperature) and hydrobiology (fish, macroinvertebrates). A first appraisal carried out on 26 restored sites highlights the wide variability in ecological and hydromorphological responses, depending on the type of restoration, objectives, time elapsed after the work and the local context. Works aimed at reconstituting the alluvial mattress or removing cross-cutting structures offer faster ecological responses. Works that encourage the reactivation of processes, lateral mobility and the diversity of facies and habitats by remeandering would have more lasting ecological responses. Finally, the importance of rigorously implementing long-term monitoring is highlighted for a proper assessment of restoration effectiveness. The aim is to improve practices and makes river managers able to design appropriate, ambitious and replicable projects of restoration to achieve good ecological status as required by the Water Framework Directive.