We project climate-driven changes in flow and sediment partitioning across the Rhine delta using a hybrid one-dimensional model informed by two-dimensional sediment-partitioning data. Simulations spanning 150 years and 540 km show a continued shift of discharge toward the Waal branch, while the effects of historical interventions gradually diminish. Climate impacts on flow division emerge around 2050 and intensify thereafter: by 2150, the IJssel is projected to convey approximately up to 17% less discharge under low-flow conditions, whereas the Waal may receive up to 6% more. Although hydrograph changes have limited influence on flow partitioning, they markedly increase channel-bed erosion by coarsening the sediment flux delivered to the bifurcation region and enhanced sensitivity to shear-stress gradients across the bifurcation. Consequently, climate forcing, particularly sea-level rise, overtakes past interventions as the dominant driver of future flow partitioning and bed level adjustment. These results have direct implications for long-term water management, navigation, and ecological resilience in the Rhine delta.

The Pannerdense Kop is a key bifurcation in the engineered Dutch Rhine system where the Bovenrijn divides into the Waal and the Pannerden Canal. The discharge partitioning at the Pannerdense Kop is important for the Room for the River 2.0 programme, as it influences navigation, flood safety, and freshwater availability. Observations show that since the 1990s an increasing share of discharge is routed towards the Waal (Fig. 1), accompanied by a stronger erosional trend in the Waal than in the Pannerden Canal (Becker, 2021; Sloff, 2019; Chowdhury et al., 2023). A mechanism which may have caused this change is related to the peak flows in the 1990s, when the incoming sediment flux may have exceeded the transport capacity in the Pannerden Canal (Chowdhury et al., 2023; Blom et al., 2024). This stresses the importance of the morphological behaviour during peak flows. During peak flows, morphological adjustments around the bifurcation occur on multiple spatial scales, from dune dynamics affecting roughness (Julien et al., 2002; Frings & Kleinhans, 2008) to patterns related to floodplains, groynes, and bends (Ahrendt et al., 2022; Parker et al., 2011), yet existing knowledge is fragmented across individual processes and time periods. This research therefore provides a comprehensive multiscale analysis of morphological behaviour at the Pannerdense Kop by combining multiple field datasets with output from 1D and 2D morphodynamic models and systematically comparing their responses.

Engineered river systems, such as the Lower Rhine, are currently facing pressures from historical channelisation and projected climate change, which, together, exacerbate issues such as channel bed incision and skewed discharge partitioning at bifurcations (Ylla Arbós, 2021; Blom, 2024). In response to these challenges, Rijkswaterstaat is currently exploring various large-scale intervention strategies under the new Room for the River 2.0 programme. To determine how robust these interventions remain under future climate scenarios characterised by increased hydrograph variability and sea level rise, river management requires simulating large-scale measures, such as Longitudinal Training Walls (LTWs) with or without side channels, and floodplain lowering, over a centennial timescale. While one-dimensional (1D) models have been successfully employed for long-term simulations of the Lower-Rhine (Ylla Arbós, 2023; Chowdhury, 2025), they rely on nodal-point relations, of which the formulation contains challenges to adequately capture critical 2D/3D flow structures, lateral morphological changes, and complex bifurcation feedback mechanisms that influence the discharge partitioning in the system. As an alternative, we consider two-dimensional (2D) depth-averaged morphodynamic modelling. However, applying high-resolution 2D models over large spatial domains (~400 km) for century-long periods presents a computational bottleneck.

This paper presents the outcomes of a learning needs assessment for upscaling and mainstreaming nature-based solutions in river systems. This study was undertaken as part of the EU Interreg NWE ResiRiver project (2023–2028) and provided insight in the Technology Readiness Levels and Societal Readiness Levels for nature-based solutions across The Nether lands, Germany, France, Belgium and Ireland. Furthermore, target audiences and learning needs were identified. The outcomes will be used to develop a learning platform and a tailor made training programme for the eleven organizations participating in ResiRiver and their direct partners in the process of implementing nature-based solutions in river systems.

The Rhine River system has been shaped by human interventions for centuries, making it one of the most engineered river networks in Europe. Issues such as ongoing channel bed incision and changes in hydrograph due to climate change (Arbos et al., 2023) are anticipated to significantly affect future river functions, including flood safety, freshwater supply, and inland shipping. In recent decades, large-scale projects such as “Room for the River,” the installation of longitudinal training walls, and sediment nourishments have helped preserve the Rhine’s functionality. At the Pannerdense Kop bifurcation, where the Dutch upper Rhine divides flow and sediment between the Waal and the Pannerden Canal, a gradual change in flow division between the branches is observed (Chowdhury et al., 2023). This change appears to be linked to a series of peak flow events in the 1990s, which resulted in sediment deposition in one bifurcate, setting off a slow shift in flow partitioning ever since. More recently, Blom et al. (2024) proposed that the extreme flows in the 1990s might have pushed the system toward a tipping point and an emerging alternative equilibrium state. Ensuring the anticipated flow partitioning is crucial not only for maintaining water supply and navigability during low-flow periods but also for controlling flood risk during peak events.

Creating Resilient River Systems by Mainstreaming and Upscaling Nature-based Solutions or ‘ResiRiver’ for short, is an EU Interreg project focusing on the creation of Resilient River Systems through the mainstreaming and upscaling of Nature-based Solutions (NbS). Today, most North-West European (NWE) river systems face significant challenges, largely resulting from past anthropogenic riverine alterations (regulations and interventions) which have greatly exacerbated the negative effects of climate change like flooding and drought. These changes also put additional pressures on existing land-use-claims, and other societal/economic interests like shipping, agriculture and water quality and cause continued biodiversity-loss. In recent years, NbS have emerged as a novel and sustainable approach in river management, cap able of mitigating these threats while also strengthening climate resilience. They fit well within the EU’s ‘Blue Economy’ ambition and provide undeniable benefits for both human well-being and biodiversity. However, the widespread integration (mainstreaming) and implementation (upscal ing) of NbS in riverine systems has yet to become common practice. The ResiRiver project there fore aims to accelerate the upscaling and mainstreaming of NbS through a curated partnership of full-scale North West Europe NbS projects by gathering, transferring, and synthesizing know ledge and experiences, whilst ensuring that this is done at all relevant partner/national/EU levels.

Climate change is expected to increase the frequency and magnitude of river floods ¹. Floods not only cause damage by inundation and loss of life ^2,3 but also jeopardize infrastructure because of bank failure and riverbed erosion processes that are poorly understood. Common flood safety programmes include dyke reinforcement and river widening ^{4, 5, 6, 7, 8–9}. The 2021 flood in the Meuse Basin caused 43 fatalities and billions of dollars of damage to infrastructure ¹⁰. Here, on the basis of analysis of the Meuse flood, we show how uneven widening of the river and heterogeneity of sediment deposits under the river can cause massive erosion. A recent flood safety programme widened the river ¹¹, but created bottlenecks where widening was either prevented by infrastructure or not yet implemented. Riverbed erosion was exacerbated by tectonic uplift that had produced a thin top gravel layer above fine-grained sediment. Greatly enhanced flow velocities produced underwater dunes with troughs that broke through the gravel armour in the bottlenecks, exposing easily erodible sands, resulting in extreme scour holes, one more than 15 m deep. Our investigation highlights the challenges of re-engineering rivers in the face of climate change, increased flood risks and competition for river widening space, and calls for a better understanding of the subsurface.

In engineered river systems such as the Dutch Rhine, bifurcation dynamics play a crucial role in providing flood safety, freshwater supply, and inland navigation. While regulation measures in the past caused bed erosion (Ylla Arb´os et al., 2021) and peak discharges may have caused changes in flow partitioning (Chowdhury et al., 2023) at the bifurcation points, climate change is expected to further alter hydrological patterns, impacting sediment transport and increasing bed erosion (Ylla Arb´os et al., 2023) as well as affecting flow partitioning again. The objective of this study is to investigate the effects of climate change on the Dutch Rhine bifurcation system over the next 150 years, focusing on hydrograph variations, sea level rise, and the influence of past engineering interventions. Understanding these dynamics is critical for future river management and adaptation strategies.

This paper explores the mainstreaming and upscaling of Nature-Based Solutions (NbS) in Northwest Europe, focusing on experiences from the INTERREG ResiRiver project. It highlights key concepts such as NbS, mainstreaming, and upscaling, using case studies from nine pilot projects across Belgium, France, Germany, Ireland, and the Netherlands. The study applies the IUCN Global Standard for NbS to establish a baseline, emphasizing the importance of iterative assessments and co-benefit valuation. Challenges in governance, economic feasibility, and adaptive management are discussed. The paper concludes by advocating for robust communication strategies and international collaboration to accelerate NbS adoption.

Nature-based Solutions (NbS) are actions that harness nature to help address major societal challenges. The assessment frameworks for NbS proposed in the literature differ in scope and intended use. In 2020, the International Union for Conservation of Nature (IUCN) introduced their Global Standard for NbS as a framework that can be used by anyone working on different types of NbS. Since research on the applicability of the IUCN Standard remains limited, the aim of this paper is to analyse whether the IUCN Standard may be used as an overarching assessment framework for NbS in river flood management applications and to identify the main differences in content with other NbS-frameworks. This was achieved through a comparison with 29 assessment frameworks for NbS, that are applicable to physical interventions for riverine flood risk reduction. The comparisons showed that the IUCN Standard has the largest breadth in scope of application and may therefore be used as an overarching framework. In addition, we identified a distinction between frameworks for the assessment of project processes (process-oriented) and project results (results-oriented), where the IUCN Standard can be characterized as process-oriented. This implies that the IUCN Standard may be used to assess the processes (e.g. stakeholder engagement and adaptive management) of planned, ongoing or completed NbS projects for a wide variety of environmental contexts and societal challenges. This will help persuade policy makers to consider NbS as one of the solutions in flood management issues, next to or in combination with e.g. engineering solutions or changing land use. We also identified that, while the IUCN Standard is straightforward to use and incorporates stakeholder input, the environmental context specificity as well as guidance depth on resources for assessment can be improved.

Channel adjustment in engineered rivers is often associated with channel bed incision (e.g., Chowdhury et al., 2023, Czapiga et al., 2022a, 2022b, Ylla Arbós et al., 2021). Channel bed incision reduces the stability of in-river structures, exposes river-crossing cables and pipelines, and the spatial variability of channel bed incision due to less erodible reaches creates shipping bottlenecks. Various measures have been implemented to cope with these issues. They range from sediment nourishments to erosion control structures (e.g., Habersack and Piégay, 2007). Our objective is to assess the potential of floodplain lowering and sediment nourishments in mitigating large-scale channel bed incision in engineered rivers affected by climate change, considering a spatial scale of hundreds of kilometres. Our domain of interest is the Rhine River between Bonn, Germany, to Gorinchem, Netherlands. This reach has been extensively channelized during the 18th-20th centuries for improved navigation and flood protection (e.g., Ylla Arbós et al., 2021).

Local river interventions, such as channel narrowing or side channels, are often necessary to maintain safety, ecology, or navigation. Such interventions have different effects on the river's bed morphology during periods of high- and low-discharge events. Mapping the bed-level variations for different discharge levels and understanding these effects can provide new opportunities for the design of interventions in multifunctional rivers. At any moment, the local bed level in a river is composed of bed-level changes that occur at various spatial and temporal scales. These changes consist of bed aggradation/degradation trends on a large scale, on an intermediate scale bed-level variations as a result of discharge fluctuations, and on small-scale moving river bed forms like dunes. Using the river Waal in the Netherlands as a case study, we analyze the intermediate-term bed-level changes resulting from discharge fluctuations (dynamic component) and propose adaptations to the design of floodplain interventions such that possible negative impact on the local bed-level changes is minimized. Time series of bed levels along two 10 km stretches of the case study are considered for a period of 16 years (2005–2020). Using a wavelet transform, we isolate bed-level variations resulting from discharge events. These bed-level variations are presented based on the magnitude of the discharge event and are compiled in an interactive atlas of river morphodynamics, allowing us to mitigate the impact of interventions. This will help river managers in the design of interventions and lead to improved management, operation, and maintenance of multifunctional rivers.

Climate change is responsible for global shifts in precipitation patterns and an overall in-crease in global temperatures. The transi-tions are anticipated to modify the river hydro-graph and sea level. The changes to the hy-drograph are also likely to influence sediment flux. These alterations imply shifts in both up-stream and downstream boundaries for river bifurcations. However, the resulting bifurca-tion response remains uncertain and warrants further investigation. Our objective is to un-derstand the extent of large-scale and long-term response of river bifurcations to climate change. We take the Upper Dutch Rhine bifur-cation region as our case study and develop a 1D hydro-morphodynamic model representing the system to achieve this goal.

River managers today are faced with the challenge of adapting to climate change while also having to sustainably secure all important functions in a healthy river system for society. Nature-based Solutions (NbS) have proven themselves effective across a multitude of contexts; providing integrative approaches for river restoration, conservation and sustainable management, ensuring both climate change adaptation and contribute to climate change mitigation and biodiversity recovery for generations to come. NbS are multi-faceted and more importantly, they are effective when it comes to addressing complex societal challenges (e.g. reducing flood risk, increasing natural values and biodiversity, ecosystem services and human well-being), as they provide a novel, integrative and coherent approach. Despite the significant and rapidly growing base of scientific evidence regarding the effectiveness of NbS in riverine systems management, the actual uptake and application of NbS on a larger (EU) scale is still in its early phase. From where we stand today, a major barrier to the wider uptake and application of NbS in riverine systems remains (a) our limited experience in scaling solutions beyond their local contexts (so called ‘Upscaling’), and (b) make Nbs as a standard work practice within water management organisations throughout North-West Europe (so called ‘Mainstreaming’). Also, our lack of standardised methods for quantitative assessment and monitoring of ecosystem services and benefits related to NbS hinders replication and application at a wider scale.

Erosion-control measures in rivers aim to provide sufficient navigation width, reduce local erosion, or to protect neighboring communities from flooding. These measures are typically devised to solve a local problem. However, local channel modifications trigger a large-scale channel response in the form of migrating bed level and sediment sorting waves. Our objective is to investigate the large-scale channel response to such measures. We consider the lower Rhine River from Bonn (Germany) to Gorinchem (the Netherlands), where numerous erosion-control measures have been implemented since the 1980s. We analyze measured bed level data (1999–2020) around four erosion-control measures, comprising scour filling, bendway weirs, and two fixed beds. To get further insight on the physics behind the observed behavior, we set up an idealized one-dimensional numerical model. Finally, we study how the geometry and spacing of the measures affect channel response. We show that erosion-control measures reduce the sediment flux due to (a) lack of erosion over the measure and (b) sediment trapping upstream of the measure, resulting in downstream-migrating incision waves that travel tens of kilometers at decadal timescales. When the measures are in close proximity, their downstream effects may be amplified. We conclude that, despite fulfilling erosion-control goals at the local scale, erosion-control measures may worsen large-scale channel-bed incision.

Floods can cause punctuated changes to river channel morphology over short time scales. This work investigates whether spatial variation in river floodplain width drives enhanced morphodynamic change during floods. We examine the relationship between longitudinal variation in floodplain width and bed elevation change within and between flood events using high-resolution, biweekly bathymetry measurements from the Waal River, the Netherlands, over the last 20 years across a 10km study reach. We find that bed erosion during floods tends to occur just downstream of floodplain constrictions while deposition during floods tends to co-occur with spatial floodplain widening. Low flows show inverse bed elevation changes at the same locations resulting in a cyclic, along-channel variation in lowvs. high-flow bed elevation variation. This study suggests that spatial changes in planform channel geometry can help predict relative intra- and inter-flood morphodynamic changes.

The International Union for Conservation of Nature (IUCN) published their Global Standard for Nature-based Solutions (NbS) in an effort to further a common understanding and successful application of NbS. Our objective is to analyse the applicability of and considerations and advancements in using the IUCN Standard, as very few studies have examined and reflected on its actual application. As method, we applied the IUCN Standard to three case studies of river restoration projects with a focus on flood risk mitigation: (1) Eddleston Water Project, (2) “Room for the River” Deventer Project, and (3) Missouri River Levee Setback Project. Rather than evaluating the case studies itself, we evaluated the outcome to find the strong and weak points of the IUCN Standard. The gathered data (publicly accessible documents, conducted interviews with experts and stakeholders) was analysed and showed the role of the number of documents and interviews available. This determined the outcome of the IUCN assessment. The consultation of project experts has appeared to be an essential step in the data collection, while stakeholder interviews and field visits were less important, but did increase the degree of substantiation and the ease of data collection, respectively. Although restricted by a limited evaluation of flood risk mitigation studies, using the IUCN Standard for an ex-post assessment can provide credibility to project processes (e.g. stakeholder engagement and adaptive management), reveal project strengths and weaknesses, and provide opportunities for the comparison of projects. Hence, the IUCN Standard aptly evaluates process-based aspects of Nature-based Solutions for riverine flood risk mitigation.

Recent policy initiatives in Europe emphasize a movement towards nature-based solutions in flood management; however, a quantitative relationship between specific flood management measures and indicators of ecological health and biodiversity is difficult to establish (Penning et al., 2023). In the Netherlands, several studies have been conducted on floodplain vegetation monitoring; however, these studies are primarily focused on monitoring changes to hydraulic roughness for flood risk assessment (Harezlak et al., 2020; Penning & van de Vries, 2020). These works provide an opportunity to expand upon existing research to explicitly connect river management practices with indicators of floodplain biodiversity change in the Netherlands. In this study, we utilize publicly available geospatial data to identify changes in land use, vegetation classification and spectral indicators of vegetation health at restoration sites associated with the Room for the River (RftR) program in the Netherlands. Completed in 2018, RftR involved over 30 river management projects constructed to reduce flood risk by lowering peak water levels (Mosselman, 2022). Our objective is to quantify the impact of ecologically focused RftR projects on habitat heterogeneity and river connectivity in the surrounding floodplains.

Tipping occurs when a critical point is reached, beyond which a perturbation leads to persistent system change. Here, we present observational indications demonstrating presently ongoing noise-tipping of a real-world system. Noise in a river system is associated with the changing flow rate. In particular, we consider the upper Rhine River delta, where flow and sediment fluxes are partitioned over the two downstream branches (bifurcates) of an important river bifurcation. Field observations show that a sequence of peak flows in the 1990s resulted in sudden sediment deposition in one bifurcate, triggering a persistent and ongoing change in the flow partitioning. This has caused the system to move toward an alternative equilibrium state or attractor. An idealized model confirms that a river bifurcation system under such conditions is prone to tipping, and provides insight on the onset of tipping.

Typically the time scale of river response to change of the controls (i.e., flow duration curve, sediment flux, and sea level) is of the order of decades to centuries. Understanding temporal change and, in particular, abrupt change in channel response is increasingly important in engineered river systems, as abrupt change may negatively affect flood risk, navigation, and freshwater supply. The analysis of abrupt change in engineered systems with a bifurcation (i.e., a single channel splitting into two branches or bifurcates) is complicated by the fact that insight and measured data on the partitioning of water and sediment over the bifurcates are typically lacking. Our objective is to provide insight on whether observed abrupt change of the Pannerden bifurcation in the upper Rhine delta (Netherlands) may be associated with tipping.