The scale of historical river modification in Europe is staggering. Human intervention began far earlier than most people realise: the late Bronze Age, around 1300 to 800 BCE, marked the start of significant changes in river sediment transport and discharge patterns due to extensive agricultural activity and deforestation, which accelerated during the Roman period. By the Middle Ages, these interventions had caused so much sediment accumulation in river valleys that the first human-made channels had to be constructed on valley floors simply to re-establish water flow. The Netherlands pioneered innovative river management techniques in the late Middle Ages, including dredging for land reclamation, building retaining dams and groynes to direct rivers toward the North Sea, and installing floodgates to regulate water levels. But it was the Industrial Revolution of the nineteenth and twentieth centuries that fundamentally transformed Europe’s river systems, as demand for energy and efficient transportation led to the widespread construction of dams, weirs and locks for hydroelectric power, flood control and commercial shipping.

The cumulative impact of these centuries of engineering is visible today in the numbers. Over 6,000 dams and approximately 1.2 million instream barriers disrupt river connectivity across the continent. Land reclamation, drainage, excessive irrigation and river embankment have contributed to the loss of nearly half of Europe’s wetlands and floodplains, which serve as critical nurseries for a wide range of species. As a result of these anthropogenic stressors, the hydrology and ecology of many European rivers have become more homogeneous, leading to decreased biodiversity. The scientists argue that these levels of degradation are especially alarming in light of climate change, as healthy, diverse river ecosystems are essential for natural resilience to climate impacts such as increased flooding and droughts.

Against this backdrop, European environmental policy has begun to respond. The Water Framework Directive, adopted in 2000, already includes objectives related to river connectivity, although these primarily focus on the longitudinal dimension along the river’s course. More recently, the EU Biodiversity Strategy for 2030 set a target to restore at least 25,000 kilometres of free-flowing rivers within the EU by 2030. This target was subsequently incorporated into the legally binding EU Nature Restoration Regulation, adopted in 2024, substantially strengthening the policy framework for river restoration across Europe. Under the regulation, EU Member States are required to inventory artificial barriers affecting the connectivity of surface waters and to identify those that should be removed to achieve the Union-wide target, with barrier removal implemented through National Restoration Plans focused primarily on obsolete structures.

But what exactly counts as a free-flowing river? The review proposes using the holistic definition developed by Grill and colleagues in 2019 together with the European Commission, which defines free-flowing rivers as fluvial systems in which ecosystem functions and services are not affected by any human-induced change in connectivity. This allows for the unrestricted movement and exchange of water, energy, material and biodiversity within the river system and across surrounding landscapes. Connectivity must be considered along four dimensions: longitudinal, along the river’s course; lateral, across the floodplain; vertical, between the river and groundwater; and temporal, across different time periods and seasons. The researchers integrated these four dimensions into a Connectivity Status Index, quantifying connectivity from 0 to 100 percent, and suggest that only rivers scoring at least 95 percent along their entire length should be classified as free-flowing. The European Commission’s ECOSTAT working group has since developed field criteria for determining whether a river stretch is already free-flowing or whether this status can be achieved through specific restoration measures, explicitly including the assessment of sediment connectivity, migration barriers and hydromorphological functioning.

The review stresses that simply restoring longitudinal connectivity is insufficient for effective river restoration. The lateral and vertical dimensions are of vital importance for ecological functioning, particularly where river valleys intersect with riparian lands and aquifers heavily influenced by urbanisation and agriculture. Without the physical processes of flooding and erosion, the formation of type-specific structures like oxbow lakes, floodplain forests and alluvial islands is severely limited, undermining the diverse habitats these features sustain. The scientists also highlight the importance of the temporal component, as seasonal and interannual flow variations maintain ecological balance and support aquatic life at various stages. Furthermore, the dendritic, branching network structure of rivers — which the classic river-floodplain ecosystem concept of Ward and Stanford largely ignores — is essential for biodiversity and ecosystem function, with the fractal structure of river networks influencing hydrological processes and ecosystem dynamics in ways that local connectivity impacts flow patterns, sediment transport and resource exchange throughout entire catchments.

Given this complexity, the researchers argue that creating a one-size-fits-all criterion for river restoration is impractical and unlikely to help conserve freshwater biodiversity. Instead, restoration methods and criteria should be tailored to each river’s unique situation, prioritising longer and more ecologically valuable river sections while incorporating river order, size and network properties such as centrality into restoration objectives. Building on this network perspective, the review promotes meta-ecosystem thinking, which focuses on interactions between aquatic systems, riparian zones and terrestrial areas, and helps explain how rivers connect different ecosystems across multiple spatial and temporal scales. Strategies such as creating migration corridors, reconnecting rivers with floodplains and establishing support areas for both land and water ecosystems strengthen these connections and improve resilience to extreme weather events. This approach is particularly relevant for transboundary river basins spanning EU and non-EU countries, such as the Vjosa–Aoos Basin in the Balkans, and the International Commission for the Protection of the Danube River offers one example of coordinated collaborative management across an entire river basin.

The review is emphatic that protecting the last natural free-flowing rivers must take precedence over restoring connectivity in degraded systems. While removing obsolete barriers may appear to be a simple way to meet policy targets, it is insufficient to reverse the decline in freshwater biodiversity. Dam construction plans in untouched Balkan rivers pose a direct threat to Europe’s remaining pristine rivers, and the scientists point to the Vjosa Wild River National Park as a proven successful conservation model. In contrast, restoration projects have produced mixed results: the Allier River in France recovered successfully, while the costly Ems River restoration in Germany failed to address larger-scale connectivity problems essential for the migration and dispersal of aquatic species, leading to limited biodiversity recovery even over multi-decadal time frames. The high financial investment did not yield proportional ecological benefits, suggesting resources could have been better allocated to preserving free-flowing rivers. Local habitat restoration often fails to provide significant biodiversity benefits due to ongoing larger-scale connectivity issues, underscoring the need for approaches that consider both local and regional ecological dynamics.

Understanding the ecological impacts of barriers is central to prioritising restoration effort. Barriers affect natural flow regimes, sediment transport, water quality, channel morphology, stream temperature and nutrient cycling, as well as biodiversity and population connectivity. The number of physical barriers in river systems is frequently underestimated due to a lack of a universally accepted barrier definition and the dynamic interaction between seasonal flow variations and river network structures, which can create temporal natural barriers during low-flow periods that expose riverbeds or high-flow periods that create impassable velocities for biota. Not all barriers have the same impact: large storage dams disrupt sediment transport and organism movement, while even small barriers only 20 centimetres high can obstruct the upstream migration of weaker swimmers, and low-head barriers impede the downstream dispersal of sediment, fish larvae, benthic invertebrates and macrophytes. Multiple barriers within a river reach have cumulative effects that complicate the measurement of riverscape permeability. Because the ecological success of barrier removal varies across species and fluvial systems, the review warns that limited restoration efforts — such as lowering instead of removing barriers, or implementing technical solutions aimed at a single organism group like fish ladders — should be avoided.

Setting achievable restoration targets is equally critical for effective resource allocation and stakeholder support. In densely populated regions where most rivers are strongly influenced by long-term anthropogenic change, identifying comparable reference conditions is often not possible, and restoration success may need to be defined in terms of rehabilitation or functional improvement rather than a full return to an initial natural state. Reference conditions should be defined using ecological criteria such as the presence and abundance of typical species, habitat area coverage and free-flowing river properties like channel migration, hydromorphological dynamics and flow patterns. Across Europe, only a limited number of river systems still approximate such conditions and can serve as contemporary reference systems. The Tagliamento River remains one of the few Alpine rivers with a largely intact, laterally connected and morphodynamically active braided channel network, providing an important benchmark for defining reference conditions in braided river systems. Targets should also reflect future-oriented objectives related to ecosystem resilience and adaptation to climate change, including the capacity to buffer extreme floods and droughts, maintain connectivity under altered flow regimes and support climate-resilient species assemblages. It is crucial to reliably establish thresholds beyond which a water body’s resilience is exceeded, potentially leading to a transformation from one alternative state to another — for example, if upstream damming significantly reduces a river’s flow to the point where fish species cannot access their spawning grounds, the local fish population may collapse, causing a permanent shift in the river’s ecological dynamics.

Practical restoration programmes across Europe demonstrate both the promise and the pitfalls of this work. Following major flooding events in the Netherlands in 1993 and 1995, the Room for the River programme was established to reduce flood risks and improve the ecological quality of Dutch rivers and floodplains, leading to more than 1,400 river restoration projects across Europe. From 2017 to 2020, a comprehensive monitoring programme examined habitat conditions and young-of-the-year fish populations in 46 restored floodplains in the lower Rhine, showing that restoring flowing conditions and permanent lateral connectivity with the main channel greatly benefits fish biodiversity. There was no single restoration approach that worked everywhere, as each restoration type created distinct nursery habitats supporting different ecological fish groups. In Germany, the Blue Belt programme, adopted in 2017 and running until 2050 with interim targets for 2035, is creating a network of restoration projects along federal waterways, focusing on sections no longer required for commercial navigation — about 2,800 kilometres in length — and allocating roughly 50 million euros annually for river and embankment restoration alongside 12 to 15 million euros for floodplains. Integrating environmental flow regimes into river management can further help address drought pressures, as management-prescribed flow releases from dams designed to closely mimic natural river flow patterns support aquatic ecosystem integrity and biodiversity. Well-designed implementations have demonstrated clear ecological benefits, including improved community composition of fish, macroinvertebrates and macrophytes in restored floodplain systems of the upper Danube, although implementation is frequently hampered by limited resources, knowledge gaps, institutional barriers, conflicts of interest and a lack of political will.

Ultimately, the review concludes that the lack of awareness about the ecological significance of rivers remains a broader challenge, as many freshwater processes are invisible to the naked eye, leading to widespread ignorance among stakeholders and the public about the roles free-flowing rivers play in sustaining aquatic and terrestrial ecosystems. Addressing this requires proactive stakeholder engagement, enhanced dissemination of information and strong partnerships with the agricultural sector, whose high relevance for wetland management makes collaboration especially critical. Achieving long-term results requires a multi-actor approach and co-creation of restoration plans involving local communities, research institutions, environmental organisations, government agencies and the private sector. Improving the science-policy interface is also central: the BioAgora project, funded by the EU Horizon Europe programme, aims to create an online platform serving as the scientific foundation of the EU Knowledge Centre for Biodiversity, with one demonstration case centred on the 25,000-kilometre free-flowing river target. Rivers cross political boundaries, so supra-regional and international communication must be promoted to utilise their ecological functions effectively, with NGOs playing a crucial facilitating role in initiating dialogue, building trust and bringing diverse stakeholders to the same table. Yet recent political developments in parts of Europe indicate increasing pressure on NGOs, which may undermine their capacity to contribute to participatory river restoration. The scientists call for immediate action to protect remaining natural rivers and accelerate implementation of the Biodiversity Strategy and Nature Restoration Regulation goals, for the benefit of both the environment and humanity.

Subject of Research: Challenges and opportunities in restoring European free-flowing rivers

Article Title: Challenges and opportunities in restoring European free-flowing rivers

Article References: Stoffers, T., Schultze, A.-K., Ehlert, T., Kaiser, L., Scholz, M., & Nagelkerke, L. A. J. (2026). Challenges and opportunities in restoring European free-flowing rivers. Nature Conservation, 62, 355-381. https://doi.org/10.3897/natureconservation.62.173762

Image Credits: AI Generated

DOI: 10.3897/natureconservation.62.173762

Keywords: Challenges, opportunities, restoring, European, free-flowing, rivers, scientific research