Rapid sub-daily flow fluctuations caused by hydroelectric power generation (hydropeaking) pose a significant challenge to aquatic ecosystems in Alpine rivers, particularly for young-of-the-year (YOY) fish. Hydropeaking reduces recruitment rates through reduced spawning success and habitat availability and persistence, as well as increased downstream drift and stranding rates. This study assessed the abundance of YOY brown trout (Salmo trutta) and European grayling (Thymallus thymallus) in multiple Austrian Alpine rivers sampled mainly in August and September. Hydropeaking was analysed alongside other factors, including land use, river morphology, hydrology and tributary influences. The results show that sub-daily flow fluctuations, characterised by event frequency and down-ramping rates, were the main predictor associated with lower YOY fish abundance. To capture their combined pressure, event frequency, down-ramping rate, and amplitude were integrated into a hydropeaking index. Along the gradient of the hydropeaking index, the abundance of YOY brown trout and European grayling declined from median values of 10.6 and 4.8 individuals per 100 m, respectively, to less than 1 individual per 100 m. Consistent with this pattern, the literature-based down-ramping threshold (0.2-0.3 cm/min) became more common with increasing hydropeaking index and coincided with higher event frequency. Suitable tributaries showed only limited mitigating effects on conditions in the main river channel. From a management perspective, the findings highlight the need to reduce down-ramping rates and event frequency, particularly during the critical emergence period of YOY fish, in order to mitigate the effects of hydropeaking. Furthermore, the findings show that YOY fish are a suitable indicator of hydropeaking.

ABSTRACT Photograph of the River Saane showing hydropeaking water-level fluctuations, with a visible low-flow waterline and a wetted gravel bank indicating the higher peak-flow stage. Storage hydropower is central to the energy transition, yet its flexible mode of operation can generate frequent flow fluctuations, known as hydropeaking. This paper synthesises recent empirical and methodological advances to show that the ecological impacts of hydropeaking depend not only on the magnitude of individual flow fluctuations, but also on the frequency of these fluctuations. In Alpine hydropeaking rivers, flow fluctuations occur on average three times per day, with only minor seasonal differences. This frequency is 20–60 times higher than under natural flow conditions on an annual basis and 49–78 times higher during spring, an ecologically sensitive period. This high frequency of flow fluctuations drives cumulative ecological impacts, including repeated fish stranding and invertebrate drift. The underlying mechanisms operate at the patch scale, where recurrent shifts in habitat conditions lead to a pronounced increase in habitat dynamics. In practical terms, habitat persistence and temporal availability decline, as does the spatio-temporal connectivity of habitats. Three recently developed metrics (M1, M2, M3) quantify patch-scale habitat dynamics and provide a practical tool for assessing cumulative ecological impacts in a spatially explicit manner.

The integration of shaft hydropower plant (SHPP) modules into existing weir structures offers a compact and potentially environmentally compatible solution for low-head hydropower generation. However, site-specific geometric constraints and operational variability can substantially affect intake hydraulics and, consequently, ecological performance. This study presents a structured three-dimensional CFD-based hydraulic optimization framework applied to the At-Bashy demonstration site (Kyrgyzstan), representing a SHPP configuration integrated into an existing gated weir. Nine intake configurations were systematically evaluated by varying trash rack length (6–8 m) and submergence above the rack (2.00–2.85 m) while maintaining constant turbine and bypass discharges. Fish-relevant hydraulic indicators were quantified within defined near-rack volumes of interest using time-averaged metrics. In addition to mean velocity components, tail-based indicators of extreme downward-directed velocities and statistical measures of free-surface variability were assessed. Results demonstrate that increasing submergence consistently reduces extreme downward velocities and homogenizes the near-rack flow field. Increasing rack length primarily lowers average downward deflection. Strong upstream flow asymmetry caused by geometric constraints induces localized flow separation and heterogeneous vertical flow velocity cores, highlighting the importance of site-specific hydraulic assessments. A rack length of 7 m combined with operational submergence levels between 2.35 m and 2.85 m provides a balanced compromise between ecological performance and structural-economic feasibility. The study confirms that SHPP intake optimization cannot rely on generalized empirical rules but requires site-adapted numerical evaluation. The presented CFD-based framework provides a transparent and transferable methodology for hydraulic optimization of SHPP intake configurations.

Central Asia faces increasing pressure on transboundary water, energy and food systems, particularly in the Aral Sea Basin, where hydropower, agriculture and downstream water availability are closely linked. This study applies a spatially disaggregated Water–Energy–Food (WEF) Nexus System Dynamics Model under combined climate–socioeconomic scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5) to assess hydropower development and policy pathways at basin and sub-basin scales in the Amu Darya and Syr Darya basins during 2015–2050. At basin scale, mean annual water supply remains stable across scenarios, at approximately 42 and 87 million hm3/year, respectively. Under national hydropower plans, installed capacity nearly doubles by 2050, reaching 10.45 and 10.65 GW, respectively. Food supply is more sensitive to climate–socioeconomic pathways than to hydropower expansion, with 2050 values ranging from 1120 to 1370 tonnes in the Syr Darya and 1405 to 1780 tonnes in the Amu Darya. Spatial simulations show that upstream-only development concentrates energy gains, whereas integrated basin-wide planning delivers more balanced WEF outcomes and the highest Aral Sea inflows, reaching 8.41 million hm3/year and 17,319 hm3/year, respectively. Persistent Amu Darya trade-offs underscore the need for transboundary coordination in reservoir operation, irrigation planning and downstream-flow safeguards. Spatial disaggregation reveals trade-offs concealed by basin-scale averages.

Freshwater biodiversity in Central Asia faces increasing pressure from anthropogenic impacts. A group of fish species of ecological and economical importance in the area are snow trout, cyprinids of the genus Schizothorax. However, fundamental knowledge to aid species conservation is limited, such as information about their seasonal movement patterns and habitat preferences. Consequently, in this study we tracked 27 snow trout (Schizothorax eurycephalus) equipped with radio transmitters for almost two years in the transboundary Shakhimardan River basin in the Alay mountain range, Uzbekistan and Kyrgyzstan. Movement distances and directions were analyzed using descriptive statistics and linear mixed models, and habitat use were assessed using ordinal logistic regression. Our results show that snow trout had a movement range up to 18 km and exhibited a seasonal migration pattern, moving downstream in spring and upstream in autumn. We observed that many fish migrated upstream into the groundwater-dominated Koksu River for wintering, potentially highlighting the role of thermal refugia. They mostly used habitats in medium-to-fast flowing sections with large substrates. This study is the first biotelemetry-based research of fish in Central Asia, revealing details on seasonal migration and habitat use of Schizothorax eurycephalus. Our findings underline the need to preserve aquatic connectivity and structurally diverse habitats in mountain rivers. Incorporating these insights into hydropower development, design and river restoration programs can help maintain ecological integrity while supporting sustainable development.

Hydropeaking, the intermittent operation of hydropower, generates rapid flow fluctuations that disrupt river ecosystems. However, the mechanisms driving stranding and displacement of young fish under varying hydropeaking conditions remain poorly understood. Using a nature-like experimental facility, we conducted ∼1,000 hydropeaking trials involving >120,000 larval and juvenile fish from four species to assess how hydropeaking intensity, environmental conditions, and biotic factors influence fish behavior. Stranding and downstream displacement increased with hydropeaking intensity, particularly at night, in smaller fish, and lower water temperatures. Flow peaks with cold-water releases (thermopeaking) further amplify displacement. Fish demonstrated behavioral adaptation, reducing displacement over successive peaks, suggesting learning effects. The patterns were largely consistent across species. By identifying critical ecological thresholds, our findings inform hydropower strategies that minimize ecological harm while supporting renewable energy production.

Central Asia is experiencing increasing energy demand, which is intended to be met through the development of renewable energy sources, including expanded hydropower generation. Yet no comprehensive, high-resolution regional assessment of remaining sustainable hydropower potential exists to date. This study fills that gap by developing an open-access, reproducible geospatial framework. In a step-wise approach, the theoretical, technical, and exploitable hydropower potential is estimated, ultimately deriving the remaining sustainable potential. River networks were delineated from a digital elevation model, and discharge estimates from a global hydrological model were combined with regional gauging data. The analysis incorporates efficiency losses, site-specific spill rates, exploitability constraints, and currently operating hydropower plants. Finally, geophysical hazards and detailed ecological criteria, including threatened species, protected areas, free-flowing rivers, and rare river types, were integrated. The theoretical potential amounts to 703 TWh yr-1, but successive reductions toward sustainable potential lower this value to 92 TWh yr-1. More than 80% of this remaining sustainable potential is concentrated in Tajikistan (43.7 TWh yr-1) and Kyrgyzstan (28.7 TWh yr-1). Uzbekistan (10.8 TWh yr-1) and Kazakhstan (8.4 TWh yr-1) hold smaller shares, Turkmenistan nearly none (0.01 TWh yr-1). The resulting highresolution map can support both regional-scale strategic planning and site-specific feasibility analyses. By identifying where hydropower development is feasible and environmentally compatible, this study provides a robust foundation for sustainable hydropower expansion within Central Asia's Water-Energy-Food-Ecosystem Nexus.

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.

Hydropower plants operated on demand cause rapid and frequent flow fluctuations (hydropeaking), which negatively impact downstream ecosystems. However, an improved understanding of the ecological effects of hydropeaking and confounding riverine stressors is needed to provide benchmark knowledge to mitigate impacts of hydropeaking. To this aim, multi-scale approaches are needed to better understand ecological processes. This article synthesizes key advances in our understanding of flow-ecology relationships in hydropeaked rivers at the national and the river bar scale, integrating data-driven, field, and experimental approaches, all targeting fish as an ecological indicator. These findings are placed in the context of hydropeaking mitigation by highlighting ways to bridge the gap between science, management, and policy. The article also points to urgent research questions, particularly regarding recurring hydropeaking events.

The mountainous rivers of Central Asia host diverse ichthyofauna threatened by increasing anthropogenic pressures, particularly water pollution, abstraction, and hydropower development. This study provides valuable morphometric and ecological data for Schizothorax eurycephalus (snow trout) and Triplophysa ferganaensis (stone loach) in the Shakhimardan River basin, Uzbekistan. S. eurycephalus exhibited positive allometric growth, while T. ferganaensis showed negative near-isometric growth. The mean Fulton’s Condition Factor was 1.0 for S. eurycephalus and 0.7 for T. ferganaensis, with site-specific variations. Strong correlations among morphometric parameters, particularly length–height relationships, support non-invasive monitoring techniques. Dietary analysis revealed S. eurycephalus was predominantly herbivorous, with around 70% algae consumption. Early sexual maturity was observed in S. eurycephalus males, whereas T. ferganaensis showed no clear maturity signs, but swollen bellies suggested ongoing or recent reproductive activity. These baseline morphometric and ecological data establish a solid foundation for future ecological assessments, conservation strategies, and the design and monitoring of mitigation measures to address anthropogenic impacts in this vulnerable region.

The operational practice of "hydropeaking" allows hydropower plants to cover peaks and deficits in energy demand, but it also impacts river ecosystems. The assessment of hydropeaking impacts plays an important role in safeguarding ecosystem services, but is challenging due to the relative importance of impacts at different sites. To compare impacts in hydropeaking rivers, we elicit expert judgment on the relative impacts of hydropeaking on river ecosystem services. Using the best-worst scaling (BWS) method, we compare the impact on the three categories of river ecosystem services (provisioning, regulating and cultural). Our respondents include 98 hydropower experts. Our analysis accounted for individual heterogeneity to assess how perceptions vary across regions, attitudes and representative river characteristics. We find trade-offs between provisioning and regulating services at the regional and local levels, which represents a key issue in dealing with climate change and ecosystem degradation. The best-affected services were water for power generation, raw materials, water for industrial activities and water for irrigation. The worst-affected services were fisheries and aquaculture, maintenance of population and habitat, and wild animals. Our results have implications for the safeguarding of river ecosystem services and the design of regulatory and incentive schemes for mitigation.

Wasserkraftwerke, die bedarfsabhängig betrieben werden, führen zu schnellen und häufigen Abflussschwankungen (Schwall und Sunk), welche flussabwärts gelegene Ökosysteme negativ beeinträchtigen. Dieser Artikel fasst aktuelle Forschungserkenntnisse über dessen Auswirkungen auf verschiedenen räumlichen Ebenen, von großen Flusseinzugsgebieten bis hin zur Kiesbankebene, zusammen. Feld-, Labor- und Modellierungsansätze werden integriert, um zielgerichtete Sanierungsmaßnahmen für ein nachhaltiges Gewässermanagement aufzuzeigen.

Infolge des hohen Ausbaugrades der Wasserkraft in Österreich haben sich die Lebensraumbedingungen für Fische in den letzten Jahrzehnten drastisch verschlechtert. Im Zusammenwirken mit anderen anthropogenen Belastungsfaktoren (Flussregulierungen, Klimawandel, Schadstoffe, Schifffahrt etc.) führt dies dazu, dass früher weit verbreitete Fischarten, wie der Huchen, an den Rand des Aussterbens gedrängt wurden, Massenfischarten, wie Nase und Barbe, in ihrer Verbreitung und Bestandsgrößen drastisch reduziert wurden und ca. zwei Drittel der heimischen Fischarten auf der Roten Liste stehen. Es besteht daher dringender Bedarf, die wenigen verbliebenen ungestauten Fließstrecken vollständig zu erhalten und neben der Sanierung der anderen Belastungsfaktoren nicht nur ausgewählte, sondern alle ökologisch wirksamen Maßnahmen bei Wasserkraftanlagen umgehend umzusetzen, da ansonst Fischarten in Österreich in naher Zukunft unwiederbringlich verloren gehen würden.

Tributaries play a vital role in fish spawning and recruitment, significantly influencing mainstem river fish populations. However, in the Anthropocene era, tributaries within river networks suffered ecological degradation due to fragmentation and channelization. This has led to reduced distribution ranges and declining populations of various riverine species, underscoring the urgency of conservation and rehabilitation efforts. Our study explores the potential for reintroducing fish from the mainstem Danube River into the Wien River, an urban tributary that has undergone partial rehabilitation. We assessed habitat use and movement patterns of 20 adult barbel (Barbus barbus), a species classified as ‘near threatened’ in Austria, collected from the Danube River during the spawning season. These barbel were tagged with radio telemetry tags and relocated into the upper reaches of the Wien River, surmounting several artificial barriers in the lower sections. Although spawning activity was not observed among the barbel, possibly due to prevailing water temperatures, our data suggest that the Wien River could function as a viable temporary habitat. Barbel were notably inclined to inhabit deeper pool and run habitats. Their daily movements and home ranges were relatively limited, ranging from 0.0 to 1.1 km and 100 m to 2.9 km, respectively. One contributing factor to restricted movement was the presence of beaver and knotweed dams, which created temporary migration obstacles. On average, the barbel remained within the system for more than a month. Most of these fish migrated downstream at the onset of a significant high-flow event. Our assessment suggests that while the Wien River may not serve as a permanent habitat, it could function as a temporary habitat for migratory fish. However, to ensure accurate assessments of the restored Wien River as a spawning habitat, it is imperative to reevaluate the findings under stable spawning conditions and gather comprehensive data on relevant abiotic factors. This study advocates for the restoration of longitudinal connectivity between tributaries and mainstem rivers as a means to counteract biodiversity loss in Anthropocene river ecosystems.

This study presents a comprehensive analysis of the length-weight relationship, condition factors, and age of Schizothorax eurystomus in the Shakhimardan River basin in Central Asia, along with a comparative perspective to other Schizothorax species in the region. The study found that S. eurystomus exhibits positive allometric growth, which is consistent with similar patterns observed in this species from the Syr Darya River basin. The two analyzed condition factors showed mean values within the normal range, indicating good feeding and environmental conditions. However, significant disparities between minimum and maximum values of these factors indicated varied growth conditions which may be influenced by anthropogenic factors. Age estimation using opercular bones showed variations in the total length among fish of the same age, and a clear age distribution pattern across different sites. Younger fish predominantly inhabited the shallower, warmer, and lower sections of the river, which is impacted by agricultural water diversion, while older specimens were found in areas with higher discharge and deeper pools. Overall, this research provides valuable insights into the life history traits of S. eurystomus, underlining the need for sustainable fishery management and conservation strategies in the Shakhimardan River basin. The findings also emphasize the importance of considering habitat quality and anthropogenic pressures regarding understanding both fish population dynamics and growth patterns.

Unnatural changes in river flow patterns resulting from peak-operating hydropower plants adversely impact freshwater ecosystems. In particular, the rapid dewatering of shoreline habitats during artificial flow down-ramping puts early fish life stages at a high risk of becoming stranded if they fail to follow receding water levels in time. While extensive research has been conducted on the effects of hydropeaking on salmonid species, there is limited knowledge on the diverse cyprinid family, particularly on vulnerable early life stages. Hence, this study aims to compare the larval stranding of two cyprinid species, the common barbel (Barbus barbus L.) and common nase (Chondrostoma nasus L.), in response to bank dewatering. We conducted larvae experiments in near-natural mesocosms, simulating single flow down-ramping events with varying down-ramping rates (0.3-1.8 cm & sdot;min(-1)) during the day and at night to quantify stranding rates, also including water temperature and fish development. Our results reveal distinct diurnal patterns for both species, with higher stranding rates during the night than during the day in all experimental scenarios. The data also show higher stranding rates at faster down-ramping, with interaction effects between down-ramping rates and time of day. The stranding rates between the two species are similar across most of the scenarios. Scenarios with colder water temperatures show that nase larvae tend to strand more frequently than with warmer temperatures. In conclusion, the study results contribute to the ongoing discourse on hydropeaking mitigation by providing new perspectives on flow-reduction effects on early cyprinid life stages. Mitigation measures should prioritize the periods during early larval development and factor in prevailing water temperatures. Lowering down-ramping rates, especially during nighttime, will help minimizing negative impacts on aquatic ecosystems, particularly when combining flow rules and habitat restoration measures.

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.