Plastic waste as a persistent contaminant of our environment is a matter of increasing concern due to the largely unknown long-term effects on biota. Although freshwater systems are known to be the transport paths of plastic debris to the ocean, most research has been focused on marine environments. In recent years, freshwater studies have advanced rapidly, but they rarely address the spatial distribution of plastic debris in the water column. A methodology for measuring microplastic transport at various depths that is applicable to medium and large rivers is needed. We present a new methodology offering the possibility of measuring microplastic transport at different depths of verticals that are distributed within a profile. The net-based device is robust and can be applied at high flow velocities and discharges. Nets with different sizes (41 µm, 250 µm, and 500 µm) are exposed in three different depths of the water column. The methodology was tested in the Austrian Danube River, showing a high heterogeneity of microplastic concentrations within one cross section. Due to turbulent mixing, the different densities of the polymers, aggregation, and the growth of biofilms, plastic transport cannot be limited to the surface layer of a river, and must be examined within the whole water column as for suspended sediments. These results imply that multipoint measurements are required for obtaining the spatial distribution of plastic concentration and are therefore a prerequisite for calculating the passing transport. The analysis of filtration efficiency and side-by-side measurements with different mesh sizes showed that 500 µm nets led to optimal results.

The Vjosa River in Albania represents one of Europe's last free‐flowing wild rivers, comprising a mosaic of various habitats and forming a highly dynamic natural ecosystem for flora and fauna, that has largely disappeared from other European rivers outside of Russia. However, due to economic interests, this sensitive and heterogeneous river system is under pressure from potential future use in hydropower, threatening the good hydromorphological and ecological status of the river with reservoir flushing, hydropeaking, colmation, loss of habitats and loss of connectivity. Beside the ecological impacts, this study aims to point out the boundaries for technical and economic issues related to hydropower. Hence, for the first time, sediment transport in the Vjosa River was measured. The results indicate possible consequences of a disturbed sediment regime and loss of capacity caused by the sedimentation of reservoirs. Moreover, subsequent consequences, such as river bed incision downstream of dams as well as progressive coastal erosion, are discussed. By combining data from direct bedload and suspended sediment measurements with long‐term hydrological data, an attempt is made to describe the sediment characteristics of the Vjosa catchment and evaluate potential hydropower plants and resulting reservoir sedimentation using limited data. Based on the presented study, it is aimed to define sediment monitoring standards for requested environmental impact assessments and to highlight possible negative impacts of hydropower projects across the entire Balkan region.

During rest periods, bedload tracers can be buried, while transport can move them to locations with different bed shear stresses or a different riverbed composition. This affects the mobility of the tracers compared to that of the bedload at the location where the tracers were seeded and has so far limited the explanatory power of field tracer studies on the virtual bedload velocity. This paper proposes a method to assess both the unsteady virtual tracer velocity and the unsteady virtual bedload velocity from field tracer studies. First, the virtual bedload velocity was conceptualized as the velocity of a relay run and contrasted with the velocity of the decelerating runs of bedload tracers. Then, a regression method for deriving the unsteady virtual velocity of bedload tracers was extended to account for tracer slowdown by including a corresponding function of the distance traveled. Finally, data from 65 bedload tracers in the Upper Drava River with very‐high‐frequency (VHF) transmitters were used for method testing. By linking the measured tracer mobility to the hydraulics and bed surface grain size near the seeding location, it was possible to determine the unsteady bedload velocity function as the unsteady tracer velocity function at a travel distance of zero. The tracer travels exhibited increasing slowdown effects with increasing tracer grain size, probably due to the dominant role of advection effects at the study site. The derivation of the bedload velocity ensures comparability to laboratory results and between tracer studies.

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.

Reservoir sedimentation, a global challenge causing an annual loss of 0.8–1% of reservoir storage capacity, disrupts fluvial sediment continuity and impacts ecology and societal needs. This study focuses on the Pulangi IV reservoir in the Philippines, a shallow and wide reservoir facing significant sedimentation issues. The research aims to investigate drawdown flushing and dredging of a flushing channel for future sustainable drawdown sluicing. A test flushing event was conducted and monitoring data, including discharge, suspended sediment concentration, bathymetry, and grain size distribution, were collected. Laboratory analyses, such as critical shear stress tests, were performed for model calibration. A 3D reservoir model and a 1D sediment transport model were applied incorporating cohesive sediment behavior. Scenarios were simulated to assess drawdown flushing, dredging and downstream impacts. Results highlight the importance of drawdown level, with cohesive sediment properties playing a critical role. Sedimentation downstream of the dam, resulting from dumped or flushed sediments, was low. However, downstream ecological and morphodynamic monitoring was found to be essential for all modeled strategies. This study demonstrates potential for establishing a flushing channel enabling future sustainable drawdown sluicing during floods by conducting repeated drawdown flushing in combination with dredging in the upper reservoir.

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.