Modelling highly variable environmental factors to assess potential microbial respiration in complex floodplain landscapes.

The hydrological exchange conditions strongly determine the biogeochemical dynamics in river systems. More specifically, the connectivity of surface waters between main channels and floodplains is directly controlling the delivery of organic matter and nutrients into the floodplains, where biogeochemical processes recycle them with high rates of activity. Hence, an in-depth understanding of the connectivity patterns between main channel and floodplains is important for the modelling of potential gas emissions in floodplain landscapes. A modelling framework that combines steady-state hydrodynamic simulations with long-term discharge hydrographs was developed to calculate water depths as well as statistical probabilities and event durations for every node of a computation mesh being connected to the main river. The modelling framework was applied to two study sites in the floodplains of the Austrian Danube River, East of Vienna. Validation of modelled flood events showed good agreement with gauge readings. Together with measured sediment properties, results of the validated connectivity model were used as basis for a predictive model yielding patterns of potential microbial respiration based on the best fit between characteristics of a number of sampling sites and the corresponding modelled parameters. Hot spots of potential microbial respiration were found in areas of lower connectivity if connected during higher discharges and areas of high water depths.

The coupled socio-ecohydrological evolution of river systems: Towards an integrative perspective of river systems in the 21st century.

River systems have undergone a massive transformation since the Anthropocene. The natural properties of river systems have been drastically altered and reshaped, limiting the use of management frameworks, their scientific knowledge base and their ability to provide adequate solutions for current problems and those of the future, such as climate change, biodiversity crisis and increased demands for water resources. To address these challenges, a socioecologically driven research agenda for river systems that complements current approaches is needed and proposed. The implementation of the concepts of social metabolism and the colonisation of natural systems into existing concepts can provide a new basis to analyse the coevolutionary coupling of social systems with ecological and hydrological (i.e., 'socio-ecohydrological') systems within rivers. To operationalize this research agenda, we highlight four initial core topics defined as research clusters (RCs) to address specific system properties in an integrative manner. The colonisation of natural systems by social systems is seen as a significant driver of the transformation processes in river systems. These transformation processes are influenced by connectivity (RC 1), which primarily addresses biophysical aspects and governance (RC 2), which focuses on the changes in social systems. The metabolism (RC 3) and vulnerability (RC 4) of the social and natural systems are significant aspects of the coupling of social systems and ecohydrological systems with investments, energy, resources, services and associated risks and impacts. This socio-ecohydrological research agenda complements other recent approaches, such as 'socio-ecological', 'socio-hydrological' or 'socio-geomorphological' systems, by focusing on the coupling of social systems with natural systems in rivers and thus, by viewing the socioeconomic features of river systems as being just as important as their natural characteristics. The proposed research agenda builds on interdisciplinarity and transdisciplinarity and requires the implementation of such programmes into the education of a new generation of river system scientists, managers and engineers who are aware of the transformation processes and the coupling between systems.

Rheoreaction impacts dispersal of fish larvae in restored rivers.

Connectivity of nurseries and spawning habitats for young of the year life stage is essential for successful recruitment of fish populations and therefore provides a key indicator for river restoration measures. Models for dispersal offer the potential to draw conclusions regarding restoration scenarios and to fill knowledge gaps about possible implications for fish populations. A newly developed rheoreaction-based correlated random walk model (RCRW), in combination with a three-dimensional numerical model and a non-steady-state particle tracing model, was applied for nase carp larvae (Chondrostoma nasus) before and after a restoration project on the river Danube, Austria. Spatio-temporal patterns of dispersal of virtual larvae, attached with rheoreactive behaviour, were analysed within both scenarios. In comparison to the heavily modified river reach, the restored reach enabled a greater amount of upstream movement from the release site and showed a generally higher variability of spatio-temporal distribution patterns. In contrast, estimated total settlement of rheoreactive larvae was substantially higher for the situation prior to the restoration measure. By comparing model results with a previously field experiment it was found that model simulations including rheoreaction as a single behaviour for navigation could not explain the whole pattern of larval dispersal. Therefore it is highly recommended for future studies to develop larval dispersal models by considering other factors (i.e., behaviour, bio-energetics and environmental factors) of existing and future individual-based models, which could serve as a tool to analyse the effect of restoration measures for recruitment of riverine fish populations.

Modelling sediment fluxes in the Danube River Basin with SWAT.

Sediment management is of prior concern in the Danube Basin for provision of economic and environmental services. This study aimed at assessing current (1995-2009) sediment fluxes of the Danube Basin with SWAT model and identifying sediment budget knowledge gaps. After hydrologic calibration, hillslope gross erosion and sediment yields were broadly calibrated using ancillary data (measurements in plots and small catchments, and national and European erosion maps). Mean annual sediment concentrations (SSC) from 269 gauging stations (2968 station-year entries; median 19mg/L, interquartile range IQR 10-36mg/L) were used for calibrating in-stream sediments. SSC residuals (simulations-observations) median was 2mg/L (IQR -14; +22mg/L). In the validation dataset (172 gauging stations; 1457 data-entries, median 17mg/L, IQR 10-28), median residual was 9mg/L (IQR -9; +39mg/L). Percent bias in an independent dataset of annual sediment yields (SSY; 689 data-entries in 95 stations; median 52t/km2/y, IQR 20-151t/km2/y) was -21.5%. Overall, basin-wide model performance was considered satisfactory. Sediment fluxes appeared overestimated in some regions (Sava and Velika Morava), and underestimated in others (Siret-Prut and Romanian Danube), but unbiased elsewhere. According to the model, most sediments were generated by hillslope erosion. Streambank degradation contributed about 5% of sediments, and appeared important in high stream power Alpine reaches. Sediment trapping in reservoirs and floodplain deposition was probably underestimated and counterbalanced by high stream deposition. Factor analysis showed that model underestimations were correlated to Alpine and karst areas, whereas underestimations occurred in high seismicity areas of the Lower Danube. Contemporary sediment fluxes were about one third of values reported for the 1980s for several tributaries of the Middle and Lower Danube. Knowledge gaps affecting the sediment budget were identified in the contributions of some erosion processes (glacier erosion, gully erosion and mass movements), and in-stream sediment dynamics.

Challenges of river basin management: Current status of, and prospects for, the River Danube from a river engineering perspective.

In the Danube River Basin multiple pressures affect the river system as a consequence of river engineering works, altering both the river hydrodynamics and morphodynamics. The main objective of this paper is to identify the effects of hydropower development, flood protection and engineering works for navigation on the Danube and to examine specific impacts of these developments on sediment transport and river morphology. Whereas impoundments are characterised by deposition and an excess of sediment with remobilisation of fine sediments during severe floods, the remaining five free flowing sections of the Danube are experiencing river bed erosion of the order of several centimetres per year. Besides the effect of interruption of the sediment continuum, river bed degradation is caused by an increase in the sediment transport capacity following an increase in slope, a reduction of river bed width due to canalisation, prohibition of bank erosion by riprap or regressive erosion following base level lowering by flood protection measures and sediment dredging. As a consequence, the groundwater table is lowered, side-arms are disconnected, instream structures are lost and habitat quality deteriorates affecting the ecological status of valuable floodplains. The lack of sediments, together with cutting off meanders, leads also to erosion of the bed of main arms in the Danube Delta and coastal erosion. This paper details the causes and effects of river engineering measures and hydromorphological changes for the Danube. It highlights the importance of adopting a basin-wide holistic approach to river management and demonstrates that past management in the basin has been characterised by a lack of integration. To-date insufficient attention has been paid to the wide-ranging impacts of river engineering works throughout the basin: from the basin headwaters to the Danube Delta, on the Black Sea coast. This highlights the importance of new initiatives that seek to advance knowledge exchange and knowledge transfer within the basin to reach the goal of integrated basin management.

Current status and restoration options for floodplains along the Danube River.

Floodplains are key ecosystems of riverine landscapes and provide a multitude of ecosystem services. In most of the large river systems worldwide, a tremendous reduction of floodplain area has occurred in the last 100 years and this loss continues due to pressures such as land use change, river regulation, and dam construction. In the Danube River Basin, the extent of floodplains has been reduced by 68% compared to their pre-regulation area, with the highest losses occurring in the Upper Danube and the lowest in the Danube Delta. In this paper, we illustrate the restoration potential of floodplains along the Danube and its major tributaries. Via two case studies in the Upper and Lower Danube, we demonstrate the effects of restoration measures on the river ecosystem, addressing different drivers, pressures, and opportunities in these regions. The potential area for floodplain restoration based on land use and hydromorphological characteristics amounts to 8102 km(2) for the whole Danube River, of which estimated 75% have a high restoration potential. A comparison of floodplain status and options for restoration in the Upper and Lower Danube shows clear differences in drivers and pressures, but certain common options apply in both sections if the local context of stakeholders and societal needs are considered. New approaches to flood protection using natural water retention measures offer increased opportunities for floodplain restoration, but conflicting societal needs and legal frameworks may restrict implementation. Emerging issues such as climate change and invasive non-native species will need careful consideration in future restoration planning to minimize unintended effects and to increase the resilience of floodplains to these and other pressures.

Educating for action: Aligning skills with policies for sustainable development in the Danube river basin.

Sustainable river basin management depends on knowledge, skills and education. The DANCERS project set out to identify feasible options for achieving education for sustainable water management across the Danube river basin, and its integration with broader education and economic development. The study traced the historic, regulatory and educational landscape of water management in the basin, contrasting it with the complex political decision-making, data-heavy decision support, learning-centred collaboration, and information-based participation that are all inherent components of Integrated Water Resource Management (IWRM). While there is a wide range of educational opportunities and mobility schemes available to individuals, there is no coherent network related to training in water management and sustainable development in the study region. Progress in addressing the multi-layered environmental challenges within the basin requires further aligning of economic, environmental and educational policies, advancing the EU Bologna Process across the region, and the development of dedicated training programmes that combine technical and relational skills. The DANCERS project identified key short and medium term needs for education and research to support progressive adoption of sustainable development, and the necessary dialogue across the public and private sectors to align policies. These include the development of new education networks for masters and PhD programmes, including joint programmes; improved access to technical training and life-long learning programmes for skills development; developing formalized and certified competency structures and associated accreditation of institutions where such skilled individuals work; and developing a co-ordinated research infrastructure and pan-basin programme for research for water management and sustainable development.

Morphodynamic effects on the habitat of juvenile cyprinids (Chondrostoma nasus) in a restored Austrian lowland river.

At the Sulm River, an Austrian lowland river, an ecologically orientated flood protection project was carried out from 1998-2000. Habitat modeling over a subsequent 3-year monitoring program (2001-2003) helped assess the effects of river bed embankment and of initiating a new meander by constructing a side channel and allowing self-developing side erosion. Hydrodynamic and physical habitat models were combined with fish-ecological methods. The results show a strong influence of riverbed dynamics on the habitat quality and quantity for the juvenile age classes (0+, 1+, 2+) of nase (Chondrostoma nasus), a key fish species of the Sulm River. The morphological conditions modified by floods changed significantly and decreased the amount of weighted usable areas. The primary factor was river bed aggradation, especially along the inner bend of the meander. This was a consequence of the reduced sediment transport capacity due to channel widening in the modeling area. The higher flow velocities and shallower depths, combined with the steeper bank angle, reduced the Weighted Useable Areas (WUAs) of habitats for juvenile nase. The modeling results were evaluated by combining results of mesohabitat-fishing surveys and habitat quality assessments. Both, the modeling and the fishing results demonstrated a reduced suitability of the habitats after the morphological modifications, but the situation was still improved compared to the pre-restoration conditions at the Sulm River.