A collaborative research agenda for restoring free-flowing rivers

A collaborative research agenda for restoring free-flowing rivers

Rivers are increasingly fragmented and degraded, yet the European Union Nature Restoration Regulation calls for restoring at least twenty-five thousand kilometers of free-flowing rivers by twenty thirty. Translating this ambition into effective implementation remains challenging because restoration priorities differ across ecological, social, economic, and governance contexts. Here, we synthesize expert knowledge from forty-five countries through a structured, multi-step prioritization process to identify research priorities for restoring free-flowing rivers in Europe. We identified twenty-seven priorities and analyzed how expert background and spatial context influenced their ranking. Restoration priorities differed systematically depending on whether experts emphasized ecological integrity, community engagement, economic considerations, or governance capacity, revealing clear patterns in how disciplinary and professional perspectives shape implementation pathways. This demonstrates that restoration strategies cannot be universal but must be adapted to local and regional political, institutional, and ecological conditions. Building on these findings, we propose a structured prioritisation framework that links barrier removal, connectivity restoration, governance mechanisms, and policy instruments to context-specific needs. Together, our results provide an empirically grounded and implementation-oriented roadmap to support European Union Member States in delivering ambitious river restoration targets in a context-sensitive and socially robust manner.

Rivers are the lifelines of the planet, supporting biodiversity and providing essential contributions to people and their well-being. Yet, river ecosystems are suffering from rapid biodiversity loss caused by fragmentation, habitat degradation, invasive alien species, and pollution, all exacerbated by the current context of climate change. For centuries, infrastructure has been built to harness goods and services provided by rivers, at a high ecological cost. Structures, such as dams, weirs, dykes, and channelized river sections, have not only disrupted the natural flow regimes along river courses, but have also altered sediment transport, and negatively impact biodiversity and ecosystem functioning along all connectivity dimensions. Connectivity, defined as the movement and transformation of materials, energy, and organisms, is vital to river health and operates along four key dimensions: longitudinal (from headwaters to river mouth), lateral (between the channel and floodplains), vertical (from surface water to groundwater), and temporal (seasonal or annual). Thus, disruptions to this connectivity impede, for instance, access to critical habitats for resident and migratory species, with cascading effects that ripple far beyond the rivers themselves. Today, over sixty-three percent of the world's rivers longer than one thousand kilometers do not flow freely, and global migratory freshwater fish populations have declined by eighty-one percent over the last fifty years. In Europe, over one million physical barriers continue to impede river connectivity, with ninety percent of the original floodplains degraded, and only forty-four percent of European rivers meeting the supposedly binding targets of the EU Water Framework Directive.

Global initiatives such as the UN Decade on Ecosystem Restoration, twenty twenty-one to twenty thirty, the Ramsar Convention on Wetlands, and the Convention on Biological Diversity seek to guide and coordinate freshwater ecosystem recovery. At the European level, the Nature Restoration Regulation was enforced with the aim of restoring at least twenty-five thousand kilometers of rivers to a free-flowing state by twenty thirty, among other goals. Although the NRR builds on established policies like the WFD and the Habitats Directive, its success depends on the ability of responsible parties to translate its ambitious objectives into practical, effective action on an unprecedented scale. Penca and Tănăsescu warn that restoration initiatives will be unsuccessful unless the root causes of biodiversity loss are addressed. Other authors also consider it important to look beyond ecological aspects: Hering et al. emphasize that, while the NRR takes a traditional conservationist approach, meeting its objectives requires adequate funding, institutional support, and stakeholder engagement. In fact, responsible parties must go beyond resource allocation to address fundamental conflicts regarding values and priorities. Therefore, to successfully implement the NRR, restoration efforts must integrate ecological, social, and political dimensions through adaptive, cross-sectoral planning, aligning with recent calls for "fundamental, system-wide shifts in views, structures, and practices" and supported by growing empirical evidence that socio-ecohydrological and multi-actor restoration initiatives yield more resilient and lasting outcomes.

In light of these persistent policy obstacles to implementation and complex socio-ecohydrological interactions, recent advances in structured methodologies have emerged as promising tools to bridge the gap between (and within) research and the practical implementation of restoration initiatives. Techniques such as horizon scanning, collaborative research prioritization, and structured expert elicitation have been demonstrated to be effective in identifying key research needs. For instance, Harper et al. distilled twenty-five essential research questions aimed at reversing freshwater biodiversity decline. This underscores the imperative for evidence-based management and strong policy frameworks. Similarly, Van Rees et al. emphasized that freshwater biodiversity is disproportionately threatened and under-prioritized relative to marine and terrestrial systems, advocating for policies that address environmental flows, water quality, and integrated water management. While their work emphasizes that research and conservation actions should mutually reinforce one another through sound evidence and multidisciplinary collaboration, our study extends this approach by explicitly aligning research priorities with tangible conservation and restoration objectives for free-flowing rivers. These are defined as four-dimensional fluvial systems, extending from source to sea, in which ecosystem functions and services are affected by minimal human-induced disruptions in connectivity.

Yet, despite growing recognition of the need for integration, genuine collaboration between natural and social sciences remains rare. Persistent barriers such as disciplinary paradigms, insufficient institutional support, and limited collaborative skills continue to constrain integration. Empirical analyses show that multidisciplinary research still represents only a small fraction of publications, even in fields where coupled human-natural processes are central. Engaging social sciences is key to producing outcomes that are more legitimate, salient, and effective. Building on these insights, our study advances an implementation-oriented, cross-disciplinary research agenda that explicitly links scientific priorities to (i) the typology and context of river barriers, (ii) mechanisms for collaboration across natural and social domains, and (iii) actionable policy frameworks such as the NRR and BDS twenty thirty. In doing so, we move beyond conceptual appeals for interdisciplinarity toward a context-sensitive roadmap for restoring free-flowing rivers and translating research into practice, building on recent European initiatives that bridge scientific, policy, and stakeholder domains in river restoration. This roadmap explicitly acknowledges that restoration decisions are shaped by stakeholder perspectives, governance constraints, and political trade-offs across spatial scales.

Building on this need for more effective integration, we identify research priorities that emerge from the practical requirements of implementing river restoration in Europe as mandated by the legally binding NRR, which is a direct outcome of the EU Biodiversity Strategy twenty forty-seven. Our aim is to identify interdisciplinary research priorities for successfully and efficiently restoring free-flowing rivers in Europe, thereby bridging the gap between scientific knowledge and practical implementation and accelerating evidence-based restoration efforts. To achieve this, we employed a four-step approach: one, an online scoping survey to identify key research topics and priorities, two, an expert workshop to refine these topics, three, systematic ranking of research areas, and four, advanced analyses of the ranking results through correspondence analysis to visualize topic clustering and assess how participant characteristics influence topic selection. This integrative methodology, incorporating horizon-scanning, collaborative prioritization, and best practices in expert-elicitation, ensures that the research agenda is scientifically robust and aligned with the needs of policymakers and practitioners working to restore Europe's river ecosystems. We hypothesize that achieving transformative change in the restoration of free-flowing rivers, as well as in the restoration of other ecosystem types, requires an interdisciplinary approach that integrates insights from the natural and social sciences. Our approach therefore serves as a best-practice example by linking research priorities to diverse restoration contexts and barrier-related challenges, aligning them with EU policy frameworks. This provides a clear, operational roadmap for translating research priorities into context-dependent restoration action.

Methods

To establish a research agenda that bridges the gap between scientific insights and practical policy implementation, we conducted a multi-step expert consultation and ranking process using a modified Delphi protocol. The Delphi method is a structured, iterative process used to obtain consensus among experts through multiple rounds of questionnaires. We applied a modified Delphi approach that combined open-ended scoping, in-person refinement, and structured ranking in a single cycle to balance inclusivity and feasibility. The following sections outline our approach.