Abstract :

Rivers are hotspots for biological diversity and sources of ecosystem services. Because of the close interactions between riverine ecosystems and human activities, rivers are recognized as a socio-ecological system. Centuries of intensive exploitation of the ecosystem services, urbanization, and water management focusing on the use of water and safeguarding humans from floods and diseases, rather than on ecological health, have led to severe degradations and functional losses. In recent years, restoration has been recognized as essential to reestablish the quality of the rivers and an increasing number of restoration projects have been implemented. In Europe, the Water Framework Directive orchestrates restoration efforts, and demands that all water bodies achieve their good ecological status or potential. In urban areas, most rivers are heavily impacted by human activities and social demand for restoration is high. However, little knowledge exists about urban restoration practices. This research aims (1) to identify the different restoration practices and drivers, and examine the particularities of urban river restorations, and (2) to assess the potential conflicts inside the socio-ecological system.

In order to address these objectives, different methods from social and ecological sciences were applied. A first set of analysis concerned 153 river restoration projects located in France and Germany. A comparison between urban and rural restoration practices and a project typology using hierarchical clustering procedure and examination of the restoration drivers using textual analyses were performed. A second set of analysis concerned the case of the Isar restoration in Munich. The analysis of restoration success for target fish (Thymallus thymallus L., Hucho hucho L., and Chondostroma nasus L.) and plant species (Myricaria germanica L.) taking into account potential conflicts with human uses (recreation, hydro-electrical production) were based on field surveys (plant growth, user pressures, habitat structures), semi-experimental studies, and habitat modelling using Computer Aided Simulation Model for Instream Flow Requirements (CASiMiR).

Key findings concerning the first research objective identified five types of river restoration (RR) projects : Fish RR, Blue RR, WFD RR, Flood protection RR, and Human RR (Paper A). Surprisingly project types distinguished themselves from their social rather than ecological quality goals. Urban river restorations intend to restore domesticated ecosystems but differ from rural counterparts based on the diversity of restoration goals, the project motivation, and the restored area (only aquatic in rural and often aquatic and riparian in urban areas). Interestingly, while the WFD is the obvious driver of the European restoration effort, it drives the national restoration trends with different intensities. Furthermore social drivers highly influence the trends, particularly in the case of urban river restorations (Paper B). Key findings on the second research objective suggested that restoration projects can recreate suitable habitats for sensitive plant (Paper C) and fish species (Paper D) but that recreational uses partly conflict with ecological quality goals. However, outcomes of the modeling procedure indicated that the identification of the best restoration scenario may depend of the specie target. Interestingly, the suppression of the water diversion did not succeed in recreating suitable habitats for all fish species targeted by the restoration (Paper E).

The research produced a dataset of projects that was not yet recorded in the national and European Dataset doubling the number of French river restoration projects recorded in RiverWiki and increasing of ten times the number of urban projects of the previously published databases. The study highlights the importance of societal driving forces in urban restoration projects and shows the need for policy adjustments according to a socio-ecological approach (Paper A and B). The research also enabled the reintroduction of an endangered plant species in the Isar (Myricaria germanica L), providing important conservation benefits. The inclusion of social parameters into the modeling procedure is a novelty that substantiates important insights to understand the failures and successes of the projects and to provide guidance on management strategies (Paper C, D and E).

To conclude, the socio-ecological system collapsed after decades of intensive exploitation of the ecosystem service provided by rivers. The current restoration approach had limited results and the return to a prior degradation status is, at least in urban area, a utopia. Better consideration of the socio-ecological system should furnish better outcomes and enable achievement of a hybrid and futuristic status that allows the maximum benefit for both humans and the environment.