Operational methods for prioritizing the removal of river barriers: Synthesis and guidance

Optimizing river restoration: A multi-scenario approach to barrier removal prioritization

River barriers can disrupt connectivity, sediment transport, water flow, and biodiversity, leading to widespread degradation of river ecosystems. Hence, the EU Nature Restoration Law (NRL) urges river connectivity restoration by removing barriers. However, its implementation requires robust, data-driven prioritization strategies. In this study, we apply a multi-scenario spatial optimization framework using multi-criteria decision analysis to prioritize barrier removal across seven conservation scenarios-focusing on biodiversity enhancement, erosion reduction, and human impact mitigation-within the Morava sub-basin of the Danube Basin. Our findings reveal substantial variability in prioritization across scenarios, driven by differing costs. At the same time, certain areas consistently emerge as high priority, indicating their role in improving connectivity and riparian restoration. Removing barriers in these areas could restore up to 1011 km of free-flowing river length and 604 ha of riparian zones. These results provide actionable insights for identifying areas that maximize ecological benefits while minimizing socio-economic trade-offs, supporting effective restoration planning. Despite the utility of this approach, challenges remain, particularly the lack of high-resolution data for small and undocumented barriers and the exclusion of smaller streams. Expanding this analysis to basin-wide scales and incorporating detailed datasets-such as sediment dynamics and climate change scenarios-will refine future outcomes. Additionally, transboundary coordination will be essential to achieve the NRL's target of 25,000 km of free-flowing rivers. This study highlights the trade-offs between ecological restoration and socio-economic constraints, demonstrating how scenario-based spatial optimization provides a structured framework for balancing these priorities.

Spatiotemporal changes in river network connectivity in the Nile River Basin due to hydropower dams

Dams offer indispensable services, including hydropower generation, water for irrigation, and flood mitigation. These barriers disrupt natural river flows, affecting river ecosystems structural and functional connectivity. The number of hydropower dams in the Nile River Basin has increased during the last decades. However, little is known regarding the spatiotemporal variation in the river network fragmentation and the individual dam contributions. We used the Reach Connectivity Index (RCI) and network centrality measures to assess temporal and spatial changes in longitudinal connectivity. We selected the Equatorial Nile and Blue Nile basins, the major hotspots for present and future hydropower developments. We collated 101 existing, under construction, and proposed dams in the Equatorial Nile Basin (ENB) from 1954 to 2035 and 19 dams in the Blue Nile Basin (BNB) from 1925 to 2035. The dams in the ENB have significantly increased over time, with a sharp increase recorded between 2010 and 2015. The mean RCI for the BNB, considering impassable dam scenarios, ranged from 62.5% (SD = 12.5%) in 1925 to 21.35% (11.6%) in 2035. Similarly, in the ENB, the mean RCI for impassable dam scenarios ranged from 50.1% (SD = 2%) in 1954 to 18.1 (12.1%) in 2035. River segments in the middle section of both basins were the most affected. The decline in the mean RCI was significantly higher in the ENB compared to BNB. The reduced connectivity is mainly in the ENB, threatening the basin's biodiversity. Existing dams, including the Grand Ethiopian Renaissance Dam on the BNB and Kakono on the ENB, should have effective fish migratory corridors that allow the passage of fish either upstream or downstream. We also recommend establishing a detailed basin-wide database for barriers and assessing their passability to understand the full extent of the river network fragmentation. We also recommend regular monitoring of barrier impacts by integrating safe, cost-effective methods such as remote sensing and environmental DNA (eDNA) to assess both flora (macrophytes, phytoplankton) and fauna (macroinvertebrates, fish, zooplankton).

Riverine Barrier Removals Could Proliferate Biological Invasions

Multiple stressors, such as pollution, climate change, invasive species and fragmentation, threaten global ecosystems, requiring holistic management actions. Freshwater ecosystems are disproportionately biodiverse and particularly impacted by fragmentation and biological invasions. Artificial barriers, such as dams and weirs, are long-standing features of global landscapes, with a divergence of views on their benefits and disbenefits. Recognition of the negative impacts of barriers on the river continuum and native biota, particularly for migratory aquatic species, has led to a rapid rate of barrier removals in recent decades, especially in North America and Europe. However, since the rise in riverine barrier construction centuries ago, global biological invasion rates have concurrently surged. Artificial barriers can paradoxically slow the spread of invasive species through freshwaters, and barrier removal efforts thus risk proliferating invasive species that disperse rapidly through connected habitats. Despite well-intended plans for river restoration through barrier removals, the subsequent spread and colonisation of invasive species have been largely overlooked. This presents a 'connectivity conundrum': the removal of barriers intuitively addresses the issues of native species migrations and dispersals, but could perversely exacerbate the spread of invasive species. Basin-scale data collection around the short- and long-term impacts on invasive species will help to underpin future restoration projects and maximise the potential beneficial outcomes of barrier removals for native species.

An annotated satellite imagery dataset for automated river barrier object detection

Millions of river barriers have been constructed worldwide for flood control, hydropower generation, and agricultural irrigation. The lack of comprehensive records on river barriers' locations and types, particularly small barriers including weirs, limits our ability to assess their societal and environmental impacts. Integrating satellite imagery with object detection algorithms holds promise for the automatic identification of river barriers on a global scale. However, achieving this objective requires high-quality image datasets for algorithm training and testing. Hence, this study presents a large-scale dataset named the River Barrier Object Detection (RBOD). It comprises 4,872 high-resolution satellite images and 11,741 meticulously annotated oriented bounding boxes (OBBs), encompassing five classes of river barriers. The effectiveness of the RBOD dataset was validated using five typical object detection algorithms, which provide performance benchmarks for future applications. To the best of our knowledge, RBOD is the first publicly available dataset for river barrier object detection, providing a valuable resource for the understanding and management of river barriers.

Bending the curve of global freshwater biodiversity loss: what are the prospects?

Freshwater biodiversity conservation has received substantial attention in the scientific literature and is finally being recognized in policy frameworks such as the Global Biodiversity Framework and its associated targets for 2030. This is important progress. Nonetheless, freshwater species continue to be confronted with high levels of imperilment and widespread ecosystem degradation. An Emergency Recovery Plan (ERP) proposed in 2020 comprises six measures intended to "bend the curve" of freshwater biodiversity loss, if they are widely adopted and adequately supported. We review evidence suggesting that the combined intensity of persistent and emerging threats to freshwater biodiversity has become so serious that current and projected efforts to preserve, protect and restore inland-water ecosystems may be insufficient to avert substantial biodiversity losses in the coming decades. In particular, climate change, with its complex and harmful impacts, will frustrate attempts to prevent biodiversity losses from freshwater ecosystems already affected by multiple threats. Interactions among these threats will limit recovery of populations and exacerbate declines resulting in local or even global extinctions, especially among low-viability populations in degraded or fragmented ecosystems. In addition to impediments represented by climate change, we identify several other areas where the absolute scarcity of fresh water, inadequate scientific information or predictive capacity, and a widespread failure to mitigate anthropogenic stressors, are liable to set limits on the recovery of freshwater biodiversity. Implementation of the ERP rapidly and at scale through many widely dispersed local actions focused on regions of high freshwater biodiversity and intense threat, together with an intensification of ex-situ conservation efforts, will be necessary to preserve native freshwater biodiversity during an increasingly uncertain climatic future in which poorly understood, emergent and interacting threats have become more influential. But implementation of the ERP must be accompanied by measures that will improve water, energy and food security for humans - without further compromising the condition of freshwater ecosystems. Unfortunately, the inadequate political implementation of policies to arrest widely recognized environmental challenges such as climate change do not inspire confidence about the possible success of the ERP. In many parts of the world, the Anthropocene future seems certain to include extended periods with an absolute scarcity of uncontaminated surface runoff that will inevitably be appropriated by humans. Unless there is a step-change in societal awareness of - and commitment to - the conservation of freshwater biodiversity, together with necessary actions to arrest climate change, implementation of established methods for protecting freshwater biodiversity may not bend the curve enough to prevent continued ecosystem degradation and species loss.

The Global Dam Watch database of river barrier and reservoir information for large-scale applications

There are millions of river barriers worldwide, ranging from wooden locks to concrete dams, many of which form associated impoundments to store water in small ponds or large reservoirs. Besides their benefits, there is growing recognition of important environmental and social trade-offs related to these artificial structures. However, global datasets describing their characteristics and geographical distribution are often biased towards particular regions or specific applications, such as hydropower dams affecting fish migration, and are thus not globally consistent. Here, we present a new river barrier and reservoir database developed by the Global Dam Watch (GDW) consortium that integrates, harmonizes, and augments existing global datasets to support large-scale analyses. Data curation involved extensive quality control processes to create a single, globally consistent data repository of instream barriers and reservoirs that are co-registered to a digital river network. Version 1.0 of the GDW database contains 41,145 barrier locations and 35,295 associated reservoir polygons representing a cumulative storage capacity of 7,420 km3 and an artificial terrestrial surface water area of 304,600 km2.

Enhancing migratory potential in fish passes: The role of pier shape in minimizing debris accumulation

Biodiversity preservation is a primary challenge of the 21st century, focusing on restoring unobstructed river flows and mitigating the effects of barriers, supported by European biodiversity strategies up to 2030. Maintaining ecological continuity, such as unblocking fishways clogged by floating debris disrupting natural fish migration paths, remains a challenge despite conventional protective methods. This study, taking a vertical slot fish pass in Wrocław on the Odra River as a case study and based on research on bridge piers, suggests modifying pier shapes from rectangular to rounded in order to reduce debris accumulation. Field studies, utilizing an OTT MF Pro flow meter, were conducted to validate the numerical model. The measured flow rate in the field was 3.15 [m³·s-1], while the numerical modeling yielded a flow of 3.19 [m³·s-1]. Focusing on optimizing the shape of cross-wall piers to enhance fish migration conditions, the study examined six different pier configurations, analyzing flow speed in the main slot, crucial for migration. Using 2D hydraulic modeling with Iber, it assessed the migratory potential of different pier designs by analyzing hydraulic conditions and comparing them with the swimming capabilities of fish species native to the Odra River. Results indicate that rounding the pier edges positively affects flow speeds in the main slot, enhancing fish migration possibilities, contributing to fish pass functionality improvement and supporting broader biodiversity and ecosystem health goals.

Purification of dredged water by magnetic coagulation: Response surface optimization and dissolved organic matter removal characteristics

In the present study, magnetic coagulation was used to treat dredged water and the response surface method was used to optimize process parameters. The dissolved organic matter (DOM) removal characteristics were characterized by three-dimensional fluorescence spectrometry and ultra-high resolution mass spectrometry. During the magnetic coagulation process, the suspended solids (SS) removal rate increased initially and then decreased under conditions of increasing magnetic powder dosage and stirring rate. After magnetic coagulation and precipitation for 20 min, the contents of SS, ammonia nitrogen, chemical oxygen demand, and total phosphorus in the treated dredged water met the requirements of the discharge standard (GB8978-1996, China). Three-dimensional fluorescence results showed that magnetic coagulation selectively removed fulvic acids and humic acid substances. After magnetic coagulation with precipitation for 10 min and 20 min, the total relative content of lignins, tannins, proteins, lipids, aminosugars, unsaturated hydrocarbons, condensed aromatic structures, and carbohydrates decreased by 26.3% and 39.4%, respectively. After magnetic coagulation, the distribution range of small molecule DOM shifted to the low H/C and high O/C regions. This study provides a novel perspective for studies on the removal of DOM in dredged water by magnetic coagulation. PRACTITIONER POINTS: SS and DOM removal were significantly enhanced by the use of magnetic coagulation. SS removal efficiency was affected by stirring rate and magnetic powder dosage. Magnetic coagulation selectively removed fulvic acids and humic acid substances. DOM molecule shifted to low H/C and high O/C regions after magnetic coagulation.

Over 200,000 kilometers of free-flowing river habitat in Europe is altered due to impoundments

European rivers are disconnected by more than one million man-made barriers that physically limit aquatic species migration and contribute to modification of freshwater habitats. Here, a Conceptual Habitat Alteration Model for Ponding is developed to aid in evaluating the effects of impoundments on fish habitats. Fish communities present in rivers with low human impact and their broad environmental settings enable classification of European rivers into 15 macrohabitat types. These classifications, together with the estimated fish sensitivity to alteration of their habitat are used for assessing the impacts of six main barrier types (dams, weirs, sluices, culverts, fords, and ramps). Our results indicate that over 200,000 km or 10% of previously free-flowing river habitat has been altered due to impoundments. Although they appear less frequently, dams, weirs and sluices cause much more habitat alteration than the other types. Their impact is regionally diverse, which is a function of barrier height, type and density, as well as biogeographical location. This work allows us to foresee what potential environmental gain or loss can be expected with planned barrier management actions in rivers, and to prioritize management actions.

Boosting large-scale river connectivity restoration by planning for the presence of unrecorded barriers

Conservation decisions are invariably made with incomplete data on species' distributions, habitats, and threats, but frameworks for allocating conservation investments rarely account for missing data. We examined how explicit consideration of missing data can boost return on investment in ecosystem restoration, focusing on the challenge of restoring aquatic ecosystem connectivity by removing dams and road crossings from rivers. A novel way of integrating the presence of unmapped barriers into a barrier optimization model was developed and applied to the U.S. state of Maine to maximize expected habitat gain for migratory fish. Failing to account for unmapped barriers during prioritization led to nearly 50% lower habitat gain than was anticipated using a conventional barrier optimization approach. Explicitly acknowledging that data are incomplete during project selection, however, boosted expected habitat gains by 20-273% on average, depending on the true number of unmapped barriers. Importantly, these gains occurred without additional data. Simply acknowledging that some barriers were unmapped, regardless of their precise number and location, improved conservation outcomes. Given incomplete data on ecosystems worldwide, our results demonstrate the value of accounting for data shortcomings during project selection.

Impacts of existing and planned hydropower dams on river fragmentation in the Balkan Region

The Balkan region has some of the best conserved rivers in Europe, but is also the location of ~3000 planned hydropower dams that are expected to help decarbonise energy production. A conflict between policies that promote renewable hydropower and those that prioritise river conservation has ensued, which can only be resolved with the help of reliable information. Using ground-truthed barrier data, we analysed the extent of current longitudinal river fragmentation in the Balkan region and simulated nine dam construction scenarios that varied depending on the number, location and size of the planned dams. Balkan rivers are currently fragmented by 83,017 barriers and have an average barrier density of 0.33 barriers/km after correcting for barrier underreporting; this is 2.2 times lower than the mean barrier density found across Europe and serves to highlight the relatively unfragmented nature of these rivers. However, our analysis shows that all simulated dam construction scenarios would result in a significant loss of connectivity compared to existing conditions. The largest loss of connectivity (-47 %), measured as reduction in barrier-free length, would occur if all planned dams were built, 20 % of which would impact on protected areas. The smallest loss of connectivity (-8 %) would result if only large dams (>10 MW) were built. In contrast, building only small dams (<10 MW) would cause a 45 % loss of connectivity while only contributing 32 % to future hydropower capacity. Hence, the construction of many small hydropower plants will cause a disproportionately large increase in fragmentation that will not be accompanied by a corresponding increase in hydropower. At present, hydropower development in the Balkan rivers does not require Strategic Environmental Assessment, and does not consider cumulative impacts. We encourage planners and policy makers to explicitly consider trade-offs between gains in hydropower and losses in river connectivity at the river basin scale.