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.

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Engineered rivers are often prone to channel bed incision. This decreases the channel-floodplain connection, hampers navigation where nonerodible reaches increasingly protrude from the bed, and can destabilize structures. Here we inventorize causes and characteristics of channel incision measures. We elaborate on how channel bed incision is a transient channel response toward a new equilibrium channel state. Causes of incision comprise base level fall, channel narrowing (e.g., due to river training), channel shortening (bend cut-offs), an increased channel-forming discharge (e.g. due to climate change), and a decrease (or fining or coarsening) of the sediment flux from the upstream part of the basin. Finally, we discuss two measures that may mitigate channel bed incision: sediment nourishments and longitudinal training walls.

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Over the past century, the main channel of the Waal has experienced erosion of approx-imately 1-2 metres (Ylla Arb´os et al., 2021; Chowdhury et al., 2023). This erosion leads to various problems such as instability of struc-tures or disruption to shipping. To address this ongoing degradation, a potential solution is the implementation of sediment nourishments. Recent pilot studies have been conducted in 2016 and 2019 to investigate the feasibility of using sediment nourishments in the main channel of the Dutch Rhine (Becker, 2023). Another possibility of nourishing is to add sed-iment to the groyne fields. Under the influence of currents and ship waves, sediment is ex-pected to be transported to the main channel, causing a groyne field to act as a sand mo-tor. To explore this concept, Rijkswaterstaat initiated a pilot project with sediment nourish-ments in three groyne field clusters along the Waal during the fall of 2023. The pilot includes an extensive measurement campaign.@en

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.@en

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).@en

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.

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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.@en

River restoration is an established method for the rehabilitation of river ecosystems in order to combat the current declines of freshwater biodiversity (Wohl et al., 2005; WWF, 2022). The urgency of restoration is recognized internationally, as the IUCN has proclaimed 2021-2030 to be the ‘Decade on Ecosystem Restoration’ (Cooke et al., 2022). So far only few restoration projects have been evaluated based on monitoring data (England et al., 2021), and there is a need for monitoring techniques to assess restoration practices. The analysis of environmental DNA (eDNA) has gained popularity in the last decades, as it allows for rapid standardized biomonitoring across the tree of life, requires a reduced dependence on taxonomic expertise for species identification, and it is cheaper than traditional monitoring methods. Depending on the organism, eDNA is shed by its host in forms such as mucous, shed skin cells, and faeces. After release, eDNA is exposed to a wide spectrum of environmental variables that may impact its state, transport capacity, fate, and the subsequent inference made by the practitioner (Barnes and Turner, 2016). Our objective is to study how eDNA quantities are affected by flow and sediment transport in river ecosystems. @en

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.@en

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.

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The dynamics of the bifurcating Karnali river in the western plains of Nepal and India is governed by the geomorphological processes in an alluvial fan. The dynamic branches showcase a notable degree of braiding, dominant channel switching and unequal discharge partitioning. Since recent switching of the dominant channel of Karnali system occurred after an intense monsoon in 2009, the eastern Geruwa branch of the system, which used to be dominant channel passing through the Bardiya National Park, is now receiving a lower share of discharge. This situation exacerbates in the low flow periods when there is very small flow in the Geruwa branch. This decreasing discharge has been associated with depleting diversity of wildlife habitat in Bardiya National Park (Bijlmakers et al., 2023). For sustainable habitat management in the Bardiya National Park, there is a necessity to study the dynamic Karnali river and its two branches, the eastern Geruwa branch and the western Kauriala branch. Activities such as sediment mining, construction of irrigation and hydropower and inter-basin water transfer projects will potentially influence the system dynamics. Our objective is to understand the switching behaviour of the Karnali system to the natural dynamics such as bend sorting (Baar et al., 2020; Parker & Andrews, 1985) of sediments at the location where water from the main Karnali enters the Geruwa branch, and offer understanding of system response to human interventions especially with regards to the distribution of discharge between the Geruwa and Kauriala branches. We combine the technique of field observations and numerical modelling to study the system. @en

River bifurcations divide the water and sediment over two downstream branches or bifurcates. As the changing climate adjusts the boundary conditions (i.e., base level, hydrograph, and sediment flux) for bifurcations, it will affect their flow and sediment partitioning over the bifurcates. Our objective is to provide insight into the response of a bifurcation to sea level rise (SLR). To this end, we compare the response of an idealized bifurcation in an engineered river (i.e., with a fixed planform and width) to SLR to the one of a single channel, using a one-dimensional numerical model system.@en

A monitoring technique capable of assessing the status of an aquatic ecosystem is needed for reversing negative trends in river biodiversity. Recently, an innovative technique for detecting the degree of biodiversity based on environmental DNA traces (mucus, shed skins etc.) has been proposed (Carraro et al., 2020). This eDNA-based biomonitoring relies on the collection and processing of water samples containing genetic material released by organisms. In recent years, the research community has made significant efforts to advance the identification of species from biological samples, for instance by expanding genetic reference databases. However, eDNA technique implementation is hampered by a lack of knowledge about the dynamics of biological traces in rivers. Here, the aim is to investigate the transport of eDNA in water streams, while considering processes such as degradation and spreading.@en

A bifurcation in an engineered river system (i.e., fixed planform and width) has fewer degrees of freedom in its response to interventions and natural changes than a natural bifurcation system. Our objective is to provide insight into how a bifurcation in an engineered river responds to peak flows and human interventions. To this end, we analyze the change in hydraulics, bed level, and bed surface grain size in the region of two bifurcations in the upper Rhine delta in the Netherlands over the last century. We show that, over the last two decades, the water discharge in one bifurcate (the Waal branch) has steadily increased at the expense of the other. This gradual increase in the water discharge of the first branch is associated with its erosion rate being larger than the other branch. The quick succession of two or three peak flow events (1993, 1995, and 1998) caused rapid sediment deposition over the upstream part of the bifurcate that has gradually lost discharge, which seems to have triggered the slow change in flow partitioning.@en

Climate change puts pressure on river systems, as it increasingly alters the river controls. Engineered rivers with a fixed planform respond to climate change and human intervention by adjusting the channel slope and bed surface grain size distribution. This response often consists of channel bed incision, over hundreds of kilometres, and during decades to centuries, resulting in serious disruptions of inland navigation, increased flood risk, and ecological degradation. Here we investigate how the lower Rhine River (Bonn, Germany – Vuren, the Netherlands, including the Pannerden bifurcation) continues to adjust to channelization measures of the 19th century (Ylla Arbós et al., 2021), and responds to different climate scenarios of control change over the 21st century, using a schematized one-dimensional numerical model for mixed size sediment.@en

Rivers flowing from the highlands to the lowlands, characterize the Terai Arc Landscape (TAL) in Nepal and India, but also the neighbouring region in Bhutan. The sudden change in slope results in deposition of nutrient-rich sediment, which forms large alluvial or fluvial fans that enhance biodiversity and economic activities in the region. Our objective is to identify similarities and differences in hydro-morphodynamics between the TAL rivers, with a focus on the Karnali and Koshi systems in Nepal. It is expected that such insight in river dynamics will aid the sustainability and management of the TAL rivers. To this end, we investigate and compare discharge characteristics, historical flow paths, and morphodynamic characteristics such as channel shifting, braiding, and flow partitioning to identify and understand the fan scale dynamics of the two systems. Hereto we used optical remote sensing images and river discharge time series from Chisapani (Karnali) and Chatara (Koshi) gauging stations. In addition, we conducted a field campaign in November 2022.@en

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.@en

Human intervention makes river channels adjust their slope and bed surface grain size as they transition to a new equilibrium state in response to engineering measures. Climate change alters the river controls through hydrograph changes and sea level rise. We assess how channel response to climate change compares to channel response to human intervention over this century (2000–2100), focusing on a 300-km reach of the Rhine River. We set up a schematized numerical model representative of the current (1990–2020), non-graded state of the river, and subject it to scenarios for the hydrograph, sediment flux, and sea level rise. We conclude that the lower Rhine River will continue to adjust to past channelization measures in 2100 through channel bed incision. This response slows down as the river approaches its new equilibrium state. Channel response to climate change is dominated by hydrograph changes, which increasingly enhance incision, rather than sea level rise.

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