Flüsse, Auen und ihre Grundwasserkörper gehören zu den artenreichsten, ökologisch bedeutsamsten und gesellschaftlich unverzichtbarsten Ökosystemen Europas. Zugleich gehören sie zu den am stärksten bedrohten Ökosystemen. Um den zunehmenden multiplen Belastungen durch Klimawandel, Landnutzung, Schadstoffeinträge und Habitatverlust wirksam zu begegnen und nachhaltige Nutzungsperspektiven sicherzustellen, sind integrierte, langfristige Beobachtungsansätze dringend erforderlich. Dieser Artikel stellt DANUBIUS-ERIC vor, die seit 2025 als „European Research Infrastructure Consortium“ formalisierte europäische Forschungsinfrastruktur für Fluss-Meer-Systeme, und beschreibt die österreichische Forschungs-Supersite „Upper Danube Austria“ als einen ihrer zentralen Standorte. Die Supersite umfasst zwei komplementäre Forschungsgebiete: die Donauauen östlich von Wien (Nationalpark Donau-Auen) und das Ybbs-Einzugsgebiet. Die Supersite hat auf mehreren Ebenen Messinfrastruktur bestehend aus stationären Langzeitmessstationen in beiden Forschungsgebieten, mobilen Feldmessgeräten und Laborinfrastruktur aufgebaut, die eng mit dem gesamten europäischen Netzwerk verbunden sind. Erste Anwendungsbeispiele illustrieren das thematische Spektrum der Supersite: von der integrativen Grundwasserökologie und der Kohlenstoff- und DOM-Dynamik in Auengewässern bis zur einzugsgebietsweiten Modellierung von PFAS-Transportprozessen. Die Infrastruktur ist explizit auf die Unterstützung naturbasierter Lösungen, Renaturierungsmaßnahmen, nachhaltige Nutzungsansätze sowie die Umsetzung der EU-Wasserrahmenrichtlinie sowie des EU-Renaturierungsgesetzes ausgerichtet und ergänzt die bestehende behördliche Gewässerüberwachung um ökologische, biogeochemische und hydromorphologische Dimensionen, für die bislang keine systematische Langzeiterfassung bestand.

Building on the methodological development carried out within the “Alplast” project, a newly developed isokinetic pump was successfully applied in combination with established net-based sampling methodology at several riverine measurement sites. This integrated approach allows for a comprehensive assessment of microplastic transport across a wide range of particle sizes, including the finest fractions that cannot be captured by net-based methods.The combined application of net sampling and isokinetic pump sampling has proven to be robust and operational under varying field conditions form small streams to large rivers. While net sampling continues to effectively target coarser microplastic particles and enables the filtration of large water volumes, the isokinetic pump has delivered reliable results for the smallest size fractions. First experiences with laboratory analysis and data evaluation indicate that these fine particles represent a significant proportion of the total microplastic mass, hence they contribute substantially to overall microplastic transport, also due to their high mobility and widespread spatial distribution within the flow. The isokinetic sampling principle ensured that flow conditions at the intake were representative of the ambient river velocity, thereby minimizing sampling bias and enabling direct weighting of transport across the cross-section. This proved especially advantageous for capturing the variability in microplastic concentrations while keeping the number of required samples manageable.Overall, the results confirm that the combination of net-based sampling and pump sampling with the isokinetic pump significantly enhances the representativeness of microplastic transport assessments in rivers. The methodology provides a sound basis for future studies aiming to quantify the role of fine microplastic fractions and contribute to standardized monitoring approaches.

Introduction River regulation schemes generally led to changes in riverine assemblage composition and their trophic status, which often is reflected in a general loss or decrease of characteristic species and a concomitant increase or dominance of generalist species.Objectives This study examines changes of the hydrodynamic conditions, plankton biomass as well as changes in feeding guilds and the assemblages of macrozoobenthos and of fishes before and after the re-connection of a sidearm of the main stem in a free-flowing section of a large river (Danube in Austria).Methods In order to analyze the effects in the sidearm, a before-after control-impact design was applied; a river section in the main stem served as a reference site.Results The higher and increased connectivity had sizable effects on hydraulic parameters, with the expected shift of prevailing lentic to year-round lotic conditions in the sidearm. The composition of suspended solids changed significantly. Likewise, chlorophyll-a concentrations and zooplankton abundance decreased markedly. The new conditions in turn modulated the pathways and the structure of the food web.Conclusions The permanent flow-through in the sidearm decreased the importance of detritus and forced the one of periphyton and biofilm as an energy and nutrient source for insect larvae which also affected higher trophic levels, that is, fishes.

Plastic waste as a permanent pollutant in the environment is of increasing concern due to its largely unknown long-term effects on biota. The occurrence in rivers, has, compared to research in the oceans, only become the focus of scientific investigations in the last few years. The Austrian Alps in particular are largely unexplored in this respect. Therefore, the Alplast project addresses microplastic transport from the glaciers at the summit over steep mountain torrents to the lowland rivers and aims in conducting a first inventory of the alpine area. Specifically, analyses of microplastic occurrences are being carried out from the Sonnblick glacier via the Rauriser Ache, the Salzach, the Inn and the Danube and are intended to expand the understanding of processes with regard to the behaviour of microplastics in the water cycle from the glacier to the valley. The influence of snowmelt as well as the temporal development, which can be determined from ice cores, are of great interest. In addition, questions regarding the origin and distribution of plastic in flowing waters as well as the possible biological degradation by microorganisms will be clarified.Since the sampling areas cover entire catchments at different altitudes, different methodologies and devices are used. For the studies on the glaciers, the snow cover as well as ice cores are sampled and analysed. In the rivers a multi-point method is used due to the spatial distribution of plastic particles in the river cross-section. But the net samples at different depths are combined with isokinetic pump sampling in order to detect the widest possible size range. Isokinetically taken pump samples have the great advantage that a weighting process takes place directly during sampling. This means that samples can be taken in different areas (high and low flow velocities) of the cross-section (together with the nets) and then a composite sample can be analysed for the profile. Particle counts, classification and the measurement of concentrations and loads are then used to determine quantities and the most common types of plastics in the alpine environment. The measuring stations were selected in such a way that more and more potential microplastic sources are added in the course of the catchment in order to achieve the best possible process understanding regarding the origin and fate of the plastic waste.

In der Wachau, einem der zwei freifließenden Abschnitte der Donau in Österreich, wurde erstmals eine umfassende Tracerstudie zur Untersuchung des Geschiebetransports durchgeführt. Die Studie zielte darauf ab Transportdistanzen, Geschwindigkeiten und Verweildauern von Sedimentpartikeln unter variierenden Abflussbedingungen zu analysieren. Hierfür wurden 45 mit VHF-Sendern ausgestattete Tracersteine eingebracht und über einen Zeitraum von 17 Monaten regelmäßig geortet. Die Ergebnisse zeigen, dass der Geschiebetransport maßgeblich von der Korngröße, der Startposition und hydrologischen Bedingungen beeinflusst wird, aber auch durch die lokale Flussmorphologie gesteuert wird. Auf Grund des größenselektiven Transportverhaltens wurden die kleineren Steine im Mittel über größere Distanzen transportiert. Die Methodik bewährte sich durch eine hohe Wiederfindungsrate (> 99

Quantifying bedload transport and its processes is a key challenge in river research and has been a topic of study for decades. In Austria, bedload transport is continuously monitored at seven sites, ranging from high‐altitude Alpine regions to the Danube River, for over one decade. The investigated catchment areas range from 56 km 2 to 104,177 km 2 . Direct measurement methods, including various bedload samplers, as well as indirect measurement methods, in this case, plate geophones, were used. Thus, the bed load transport of all flood events that occurred during the project period from 2010 to 2021 was recorded. Bedload transport and bedload yield could be determined on a station‐specific basis using calibrated plate geophones and if possible, at the monitoring site with the bedload rating curve approach. A total of 2,848 individual measurements were conducted, using various bedload basket samplers, and 49 measurements with slot samplers. The mean annual bedload load at the observed sites ranges from 4,500 t to 360,000 t at the monitored catchment area. However, the annual loads are subject to significant fluctuations, depending on hydrology and catchment size. Long‐term monitoring has shown that reliable conclusions can be drawn about bedload transport and its dependence on discharge and anthropogenic influences. The correlation between discharge and bedload transport tends to be weaker in smaller catchments.

Die Kiesmächtigkeit spielt eine entscheidende Rolle für die Stabilität der Flusssohle und den Geschiebehaushalt der Donau östlich von Wien. Durch anthropogene Eingriffe wie Begradigungen, Querbauwerke und Regulierungsmaßnahmen für die Schifffahrt kam es in diesem Bereich zu einem signifikanten Sedimentdefizit, das in weiterer Folge zu Sohlerosion führt. Besonders problematisch sind dabei Bereiche mit geringer Kiesüberdeckung, in denen feinere Sedimente tertiärer Ablagerungen anstehen und wo die Voraussetzungen für einen potenziellen Sohldurchschlag gegeben sind. In der vorliegenden Arbeit wurde ein systematisches Monitoring der Kiesmächtigkeit durchgeführt, um eine fundierte Datengrundlage für das Geschiebemanagement zu schaffen. Mithilfe von GIS-gestützten Interpolationsverfahren wurde aus 1097 Bohrkerndaten ein hochauflösendes digitales Geländemodell der Kiesunterkante erstellt und mit den Sohlgrundaufnahmen der letzten 18 Jahre verschnitten. So konnten kritische Bereiche mit geringer Kiesüberdeckung identifiziert und deren Entwicklung über die letzten Jahre analysiert werden. Die Ergebnisse zeigen, dass einige Kolkbereiche eine deutliche Verbesserung der Kiesüberdeckung aufweisen, während andere weiterhin kritisch bleiben. Zusätzlich wurden vier neue potenzielle Risikobereiche identifiziert. Die Untersuchungen bestätigen, dass gezielte Sedimentzugaben und Geschiebemanagementmaßnahmen positive Effekte haben, jedoch weiterhin ein präzises Monitoring erforderlich ist, um das Gefahrenpotenzial zu reduzieren. Das neu entwickelte GIS-basierte Monitoring-Tool bietet eine wertvolle Grundlage für die langfristige Überwachung der Flussmorphologie und ermöglicht eine effektive Planung zukünftiger Maßnahmen. Die gewonnenen Erkenntnisse tragen dazu bei, nachhaltige Strategien zur Stabilisierung der Flusssohle zu entwickeln und damit sowohl die ökologische Funktion als auch die Infrastrukturfunktion der Donau zu sichern.

Temporal and spatial variability and a wide range of measured transport rates at comparable flow rates are well-known characteristics of the bedload transport process but represent substantial difficulties in practice and for load calculation. The possibility of measuring the bedload transport process has increased in recent years through the use of technically advanced methods. Since 2011, an integrative bedload monitoring system has been operating on the Urslau River in Austria and has been continuously observing the transport process. This long-term integrative dataset provides the basis for comparing calculation results from commonly used bedload transport formulae with measured data. On the basis of 27 high sediment transport efficiency events, as well as 75 events with reduced sediment availability and 34 events with low sediment availability, this study compares the measured and calculated bedload volumes. In addition, a wide range of measured bedload transport rates are presented for comparable discharges, and values in this range are represented by the formulae shown. The performance of the transport equations is improved by considering measured data from integrative bedload monitoring and by classifying bedload events. The results presented here indicate that, depending on the question considered and by selecting the appropriate formula, it is possible to achieve improved calculation results for practice that are comparable to the measured values.

Since the introduction of the Sustainable Development Goals, and, in particular, with the goal of reducing marine pollution (SDG 14.1), riverine microplastics are attracting public and scientific attention. But standardized monitoring methods and comparable data are still missing. Therefore, the opportunity was taken to test three of the most common monitoring methods (multiple depths net-method, pressurized fractionated filtration and sedimentation-box) at seven sites in five countries along the Danube and the Tisza Rivers. Different boundary conditions (hydrological and morphological conditions, economic situation, equipment available, etc.) were considered for the evaluation, as well as different sampling methods and sample pre-treatments together with different methodologies for microplastic identification. The sampling methods were evaluated for their suitability to be used as a standard monitoring tool in the future. Only net sampling and pressurized fractionated filtration allow for the determination of microplastic concentration as well as load, and can therefore be recommended. The multi-depth net device, as a labor-intensive method, is recommended if the focus of the monitoring is on larger particles and it is important to calculate particle and mass concentrations. Pressurized fractionated filtration is a practical tool recommended for routine monitoring, having the advantage of less effort being required for sample preparation and simply considering small particle sizes below 500 µm. From a scientific perspective it is recommended to combine both the pump sampling and the net-based device.

Plastic pollution in aquatic environments is a growing concern, with rivers recognized as major pathways. However, rivers themselves are also subject to pollution. Hence, understanding riverine plastic transport dynamics is essential for mitigating environmental impacts. Although plastic-related research focus has shifted from marine environments towards rivers, challenges remain in standardizing methods for monitoring and integrating spatio-temporal variabilities of riverine plastic occurrence into flux determination. This study addresses these challenges by adopting established methods from sediment research. Utilizing data from a net-based cross-sectional multi-point approach, it examines spatio-temporal and discharge-dependent variations. It comprehensively analyzes the complex dynamics of plastic transport in the Danube River, contrasting an impounded section near Aschach, Austria, with a free-flowing reach near Hainburg, Austria. The paper emphasizes the significance of applying these methodologies for accurate flux determination and underscores the risks of neglecting them. By incorporating average microplastic particle weights, we aim to overcome limitations in prior methodologies that solely emphasize qualitative aspects or rely on item numbers. Spatial distribution analysis revealed a pronounced stratification at low flow and a more variable distribution in the free-flowing section, attributed to higher turbulence. As discharge increased, vertical mixing occurred, along with distinct lateral patterns displaying increased concentrations toward the riverbanks. Encountering plastic particles throughout the river profile underscores their properties of both suspended and floating matter, emphasizing the importance of hydro-morphology and multi-point cross-sectional measurement approaches. Microplastic loads were calculated to be <6.9 t a−1 in Aschach and <17.1 t a−1 in Hainburg, compared to total plastic loads of <14.3 t a−1 in Aschach and <41.6 t a−1 in Hainburg. Consequently, plastic loads were doubled to tripled within the Austrian section of the Danube River. The study contributes valuable insights into the complex nature of plastic transport in river systems, emphasizing comprehensive spatial, temporal and discharge-dependent assessments for characterizing and managing plastic pollution. It suggests that rivers can function as sources, pathways and sinks of plastic pollution, contingent upon hydro-morphological conditions. This underscores the need for longitudinal, basin-wide assessments to accurately understand plastic transport dynamics.

Riverine ecosystems are profoundly influenced by hydrological dynamics and natural flow regimes, which dictate the temporal variability of water levels and the amplitude of fluctuations. Human activities, particularly navigation and hydropower generation, have significantly altered these natural patterns, leading to detrimental impacts on the physical, chemical, and biological integrity of river ecosystems. The littoral zone, in particular, is highly susceptible to anthropogenic disturbances, experiencing disruptions in biological activity and biogeochemical processes. This study evaluates the effects of ship-induced wave trains on the structural and functional properties of periphytic algal communities in a regulated river environment. Using data from the Austrian Danube River, periphytic algae's immediate and long-term responses to wave events generated by different types of ships were investigated. Immediate reactions of periphytic algae to wave trains were characterized by reductions in the effective quantum yield of PS II, indicating stress-induced down-regulation of photosystem II photochemistry. Abrasion and remobilization of periphytic algae due to wave action led to increased resuspension of chlorophyll-a into the water column. Furthermore, ship-induced wave trains influenced the pigment composition of periphyton, with photoprotective mechanisms being activated in response to fluctuating light conditions. Long-term effects of wave impact on periphytic algae biomass varied depending on water depth and exposure to aerial stress. While wave action mitigated desiccation stress in shallow areas, it resulted in biomass reduction and alterations in community composition in deeper zones. Notably, the occurrence of diatoms decreased in wave-impacted areas, potentially shifting the community towards Chlorophyceae dominance. Overall, this study underscores the complexity of ship-induced wave impacts on riverine ecosystems and highlights the importance of considering both immediate and long-term responses of periphytic algal communities. Understanding these dynamics is crucial for informing sustainable management strategies aimed at mitigating the adverse effects of navigation activities on riverine biodiversity and ecosystem functioning.

River sediment budgets are prepared globally, under different conditions and requirements. This paper proposes a general methodology for sediment budgeting – the River Sediment Budget Approach (RSBA), to demonstrate the challenges and opportunities involved in establishing a sediment budget. The methodological approach focuses on the bed load, suspended sediment load as well as changes in the river bed and morphology. Four approaches are provided for sediment budgeting, which can be used individually or in combination and are selected based on the time scale and data availability. If measurement data is available or can still be undertaken, a (i) field-based approach is selected. Specifically, the morphological changes or data required for formulas can be gathered using a (ii) GIS-based approach. When calculations are carried out without measured data, a (iii) formula-based approach is pursued. Due to the numerous budgets already conducted and sediment data already measured, an approach dependent on (iv) literature data and estimations can also be considered. Due to the complexity of sediment dynamics in rivers, strict rules are not beneficial in sediment budgeting and, to avoid misinterpretation, it is essential to validate the results.

Transport, accumulation, and degradation of microplastics (MiPs) in the aquatic environment represent a significant concern to the researchers and policy-makers, due to the detrimental impact on biota and human health through food ingestion. Although consistent investigations and research data are available worldwide, comparing the results is still challenging due to the need for more regulations regarding the sampling methods, analysis, and results reporting. The European regulatory efforts include studies on the MiPs transport in the western basin of the Danube River developed with active nets-based multipoint sampling methods from suspended sediments and proposed for standardization. In this context, the present study aimed to address for the first time the transport of MiPs in the Romanian sector of the Danube, starting after entering the country (Moldova Veche) and before the formation of the Danube Delta (Isaccea). The multipoint nets sampling procedure facilitated the collection of suspended sediments in the water columns as deep as 0.0–0.6 and 3.0–3.6 m depths and near riverbed sediments (autumn 2022 sampling) during an extensive spatio-temporal study from spring 2022 until spring 2023. The estimate of the maximum annual transport of 46–51 and 93–100 t·y−1 for MiPs and total (micro–meso–macroplastics) MPs at Moldova Veche was based on 135 collected and processed samples using 2021 water flow data. Polyethylene (58–69

Zusammenfassung Die Donau und die sie umgebende Landschaft liegen heute im Bereich verschiedener Interessen wie Hochwasserschutz, Wasserkraft, Schifffahrt und Ökologie. Aufgrund dieser Tatsache ist das ehemals ungezähmte und wilde Flusssystem seit Ende des 19. Jahrhunderts mit einer Vielzahl anthropogener Veränderungen konfrontiert, die vielfältige hydromorphologische Auswirkungen zur Folge haben. Zusätzlich führen – trotz bereits umgesetzter Renaturierungsprojekte und teilweise adaptiertem Sedimentmanagement – Einengung durch Regulierungen sowie Beeinflussungen des Sedimenthaushalts wie Rückhalt im Einzugsgebiet und Unterbrechung des Sedimentkontinuums, zu sedimentbezogenen Problemen entlang der Donau. Daraus ergibt sich auch die Notwendigkeit, den Sedimenttransport an der Donau länderübergreifend zu untersuchen. Im Rahmen der EFRE-finanzierten EU-Projekte „Sedimentforschung und -management an der Donau II“ (SEDDON II) und „Forschung und Management an der Donau in der Slowakei und in Österreich“ (DREAM SK-AT) wurde daher eine länderübergreifende Analyse der aktuellen sedimentbezogenen Probleme in drei Projektabschnitten in Österreich, der Slowakei und Ungarn durchgeführt, um die Grundlage für ein gemeinsames Sedimentmanagement zu schaffen. Neben der Analyse der sedimentrelevanten Probleme bildet ein kontinuierliches und einheitliches Monitoringsystem für Geschiebe und Schwebstoff die Basis für die Umsetzung nachhaltiger Maßnahmen im Bereich Flussbau. Durch die Kooperation im Fließgewässermonitoring konnten neben der Implementierung neuer Messstationen an der ungarischen und slowakischen Donau umfangreiche Messreihen zur optimierten Berechnung des Sedimenttransports durchgeführt werden. Die dadurch erhaltenen Daten stellen nicht nur wichtige Erkenntnisse für ein besseres Prozessverständnis dar, darüber hinaus dienen sie auch als Grundlage für numerische Modellierung und physikalische Modellversuche zur Entwicklung und Optimierung von flussbaulichen Maßnahmen, die die vielfältigen Probleme der verschiedenen Interessensgruppen adressieren und negative Auswirkungen in Flusssysteme kompensieren sollen.

&lt;p&gt;Microplastics are already part of our environment - even in the most inaccessible and remote places we find plastic particles that also remain there due to their longevity. The finer the scale at which we analyse samples, the more particles we find. It also became clear in the recent years, that rivers transport microplastics and that they also represent a principal pathway for plastic transport from land to the sea.&lt;/p&gt; &lt;p&gt;Initial measurements with benthic nets on the Austrian Danube have shown that the plastic particles are not only found on the surface, but are distributed unevenly with maximum concentrations partly near the bottom, partly on the surface and partly concentrated on one side of the river. Given the spatial and temporal distribution, a multipoint method seems inevitable and was chosen for further measurements, in which three measuring points each are distributed over the depths of several measuring verticals along a cross-section. Furthermore, it was found that the microplastic concentration also strongly depends on the discharge conditions and that by far the largest quantities of microplastics are mobilized and then transported during flood events.&lt;/p&gt; &lt;p&gt;After the first comprehensive measurements on the Austrian Danube between 2014 and 2015, further samplings were carried out in Hungary, Serbia and at 3 locations in Romania over the recent years. Despite the limitations of the data set, regarding the longitudinal, cross-sectional and hydrological representation of the micro plastic transport in the Danube River, an attempt is made to describe its characteristics on a basin wide scale. The measurements, reaching from impounded sections in the Upper Danube all the way down to the Danube delta, have shown, that the concentrations fluctuate strongly longitudinally according also to the discharge level. Therefore, at least at certain times, the Danube &amp;#8211; and river systems with their inundation areas in general &amp;#8211; are not only pathways, but can also be regarded as a microplastic sink. However, to increases process understanding and derive reliable statements, more data are needed.&lt;/p&gt;

Zusammenfassung Numerische Modelle stellen ein wichtiges Werkzeug zur Analyse und Bewertung von Gewässerprozessen sowie von flussbaulichen Maßnahmen dar. In den beiden Projekten DREAM SK-AT (Interreg V‑A Slowakei – Österreich) und SEDDON II (Interreg V‑A Österreich – Ungarn) wurden Nutzungsansprüche bezüglich Wasserkraft, Hochwasser, Schifffahrt, Ökologie und Trinkwasser analysiert und anschließend mittels numerischer Modelle unter Zuhilfenahme von Feldmessungen und physikalischen Experimenten in Österreich, Slowakei und Ungarn untersucht. In dieser Studie wurden an der Donau in Österreich und in der österreichisch-slowakischen Grenzstrecke beispielhaft morphologische Eigenschaften, Geschiebemanagementmaßnahmen (Buhnen, Geschiebezugabe) sowie eine verbesserte Rauigkeitsermittlung aus Luftbildaufnahmen untersucht. Unterschiede zwischen der frei fließenden Strecke östlich von Wien und der vom Rückstau des Kraftwerks Gabčíkovo beeinflussten Strecke wurden numerisch bezüglich Geschiebetransport und Morphodynamik gefunden. Buhnen wurden als wichtige Stellschraube für die Erreichung eines morphologischen Gleichgewichts anhand einer nachträglichen Bewertung von früheren Pilotmaßnahmen und anhand weiterer Szenarioanalysen identifiziert. Die erste Modellierung einer Geschiebezugabe im numerischen Sedimenttransportmodell sowie die Rauigkeitsermittlung mittels Luftbildern für die Präzisierung von hydrodynamischen Modellen in Seitenarmen wurden erfolgreich getestet. Die Ergebnisse und die angewendeten numerischen Werkzeuge aus dieser Studie stellen eine wichtige Grundlage für die Planung und Bewertung künftiger flussbaulicher Maßnahmen, vor allem in Hinblick auf die Anpassung an den Klimawandel sowie die Verbesserung der Biodiversität dar.

The monitoring of bedload transport processes in rivers is a still challenging task in research and the parameter effective transport width (ETW) allows, for the first time, a process description of the local variability of the bedload transport. In this paper, the distribution of bedload transport over a river cross-section was studied by statistically analyzing long-term monitoring data. Using an innovative integrated measurement system, combining geophones distributed over the river width with bedload traps and basket sampling, allows for continuous, high-resolution, long-term monitoring data of bedload transport characteristics to be generated. Geophone data can be used to record and describe bedload transport processes such as intensity, temporal variability, and local variability. One parameter observed was the ETW, which describes the width where bedload transport is taking place for different discharge classes in gravel bed rivers. With the help of a data series recorded over more than a decade, it is now possible, for the first time, to statistically describe the continuously and directly measured ETW. Hence, it became evident that the effective bedload transport width depends on the discharge rate and local transport capacity, and that bedload transport over the entire river width only occurs at high flows that prevail a few days a year.

Plastic waste finds its way to the ocean often through rivers: it is estimated that between 1.15 and 2.41 million tons of plastic waste enters the ocean every year from rivers. Out of the 1500+ rivers that are estimated for being responsible for 80% of the riverine plastic emissions, around 70 of these rivers are located in Vietnam which highlights the importance of further investigating the plastic waste situation in its rivers. The aim for this study was to develop a method for machine learning based measuring several types of plastic litter (in particular floating, trapped and submerged) in riverine systems. By considering the combined information of various litter categories this methodology is able to draw a holistic picture of plastic transport in riverine systems. Two different methodological components were set up: (i) an AI (artificial intelligence) based litter detection algorithm which analyses imagery gathered by bridge-installed action cameras for floating and trapped plastic waste items in terms of abundances and waste types and (ii) a net-based sampling method which measures floating as well as submerged plastics at the bridge locations. The applied AI-based litter detection algorithm was originally developed for plastics detection in an aquatic environment in Cambodia for drone imagery. Within this framework, this approach was further developed and applied to detect floating and trapped plastic litter in polluted rivers captured with action cameras in Vietnam. The complementing net-based sampling for submerged plastics was applied in parallel to calibrate the continuous camera-based sampling with direct measurements. Within this study it was shown that the combination of the two presented approaches provides a suitable methodology for the measurement of plastic transport along a river. Calibration of the continuous camera-based method showed that about 50% of the litter was transported at the surface and was thus directly detectable by AI. The methodology is relevant to the remote sensing community focusing on plastics detection and to researchers addressing plastic waste. The continuous assessment of plastic quantities transported by rivers will be key for policy makers to identify main polluters and to understand the impacts of any taken measures to reduce plastics pollution. Increasing the understanding of plastic types through these measurements is key for policy makers to develop the right measures which can target the items responsible for the majority of plastics pollution in rivers, as typically only few items are responsible for the majority of plastics leakage. The achievements of this study aim to fill these knowledge gaps by enhancing the litter detection method. As a next step, this method could be scaled up to be tested for a longer time period and at additional sites. The results of such longer-term measurements of surface and submerged plastics may allow for extrapolation of floating plastics to total transported plastics.