The general perception of small run-of-river hydropower plants as renewable energy sources with little or no environmental impacts has led to a global proliferation of this hydropower technology. However, such hydropower schemes may alter the natural flow regime and impair the fluvial ecosystem at different trophic levels. This paper presents a global-scale analysis of the major ecological impacts of three main small run-of-river hydropower types: dam-toe, diversion weir, and pondage schemes. This review's main objective is to provide an extensive overview of how changing the natural flow regime due to hydropower operation may affect various aspects of the fluvial ecosystem. Ultimately, it will inform decision-makers in water resources and ecosystem conservation for better planning and management. This review analyses data on ecological impacts from 33 countries in five regions, considering the last forty years' most relevant publications, a total of 146 peer-reviewed publications. The analysis was focused on impacts in biota, water quality, hydrologic alteration, and geomorphology. The results show, notably, the diversion weir and the pondage hydropower schemes are less eco-friendly; the opposite was concluded for the dam-toe hydropower scheme. Although there was conflicting information from different countries and sources, the most common impacts are: water depletion downstream of the diversion, water quality deterioration, loss of longitudinal connectivity, habitat degradation, and simplification of the biota community composition. A set of potential non-structural and structural mitigation measures was recommended to mitigate several ecological impacts such as connectivity loss, fish injuries, and aquatic habitat degradation. Among mitigation measures, environmental flows are fundamental for fluvial ecosystem conservation. The main research gaps and some of the pressing future research needs were highlighted, as well. Finally, interdisciplinary research progress involving different stakeholders is crucial to harmonize conflicting interests and enable the sustainable development of small run-of-river hydropower plants.

Ecological impacts of run-of-river hydropower plants—Current status and future prospects on the brink of energy transition

Greater scientific knowledge, changing societal values, and legislative mandates have emphasized the importance of implementing large-scale flow experiments (FEs) downstream of dams. We provide the first global assessment of FEs to evaluate their success in advancing science and informing management decisions. Systematic review of 113 FEs across 20 countries revealed that clear articulation of experimental objectives, while not universally practiced, was crucial for achieving management outcomes and changing dam-operating policies. Furthermore, changes to dam operations were three times less likely when FEs were conducted primarily for scientific purposes. Despite the recognized importance of riverine flow regimes, four-fifths of FEs involved only discrete flow events. Over three-quarters of FEs documented both abiotic and biotic outcomes, but only one-third examined multiple taxonomic responses, thus limiting how FE results can inform holistic dam management. Future FEs will present new opportunities to advance scientifically credible water policies.

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Are large‐scale flow experiments informing the science and management of freshwater ecosystems?

The conclusions of numerous stream restoration assessments all around the world are extremely clear and convergent: there has been insufficient appropriate monitoring to improve general knowledge and expertise. In the specialized field of instream flow alterations, we consider that there are several opportunities comparable to full-size experiments. Hundreds of water management decisions related to instream flow releases have been made by government agencies, native peoples, and non-governmental organizations around the world. These decisions are based on different methods and assumptions and many flow regimes have been adopted by formal or informal rules and regulations. Although, there have been significant advances in analytical capabilities, there has been very little validation monitoring of actual outcomes or research related to the response of aquatic dependent species to new flow regimes. In order to be able to detect these kinds of responses and to better guide decision, a general design template is proposed. The main steps of this template are described and discussed, in terms of objectives, hypotheses, variables, time scale, data management, and information, in the spirit of adaptive management. The adoption of such a framework is not always easy, due to differing interests of actors for the results, regarding the duration of monitoring, nature of funding and differential timetables between facilities managers and technicians. Nevertheless, implementation of such a framework could help researchers and practitioners to coordinate and federate their efforts to improve the general knowledge of the links between the habitat dynamics and biological aquatic responses.

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Detecting biological responses to flow management: Missed opportunities; future directions

Summary
1. Water managers must make difficult decisions about the allocation of streamflows between out-of-channel human uses and environmental flows for aquatic resources. However, the effects environmental flows on stream ecosystems are infrequently evaluated.

2. We used a 13-year experiment in the regulated Bridge River, British Columbia, Canada, to determine whether an environmental flow release designed to increase salmonid productivity was successful. A hierarchical Bayesian model was used to compare juvenile Pacific salmon (Oncorhynchus spp.) abundance before and after the flow release.

3. We found that the total number of salmonids did increase after the release, but most of the gains could be attributed to the rewatering of a previously dry channel located immediately below the dam. In reaches that had flowing water during the baseline period, the response of individual salmon species to the increase in flow was variable, and there was little change in total abundance after the flow release. Our results were inconsistent with both habitat modelling, which predicted a decrease in habitat quality with increasing flow, and holistic instream flow approaches, which imply greater benefits with larger flows.

4. We question whether biotic responses to flow changes can be predicted reliably with currently available methods and suggest that adaptive management or the use of decision tools that account for the uncertainty in the biotic response is required for instream flow decisions when the competing demands for water are great.

Test of an environmental flow release in a British Columbia river: does more water mean more fish?

In mountainous areas, high-head-storage hydropower plants produce peak load energy. The resulting unsteady water release to rivers, called hydropeaking, alters the natural flow regime. Mitigating the adverse impacts of hydropeaking on aquatic ecosystems has become a crucial step in recent water policies. We developed a novel economic-ecological diagnostic and intervention method to assess hydropeaking mitigation measures for fish habitat improvement. This method was applied to an Alpine river downstream of a complex storage hydropower scheme. The approach comprises (1) a hydropower operation model of flow regime generation and cost estimates for different mitigation measures, (2) a two-dimensional hydrodynamic model to simulate the flow conditions in representative river reaches and (3) a dynamic fish habitat simulation tool to assess the sub-daily changes in habitat conditions of three brown trout (Salmo trutta fario) life stages (adult, spawning and young-of-the-year). Simulations showed that operational measures such as limiting maximum turbine discharge, increasing residual flow and limiting drawdown range incur high costs in relation to their ecological effectiveness. Compensation basins and powerhouse outflow deviation achieved the best cost–benefit ratio. Hydropeaking impact was strongly dependent on river morphology. Monotonous river reaches exhibited low habitat suitability for peak discharge, whereas a braided morphology provided high in-stream structure and thus suitable habitat for unsteady flow conditions. The interdisciplinary approach to economic and habitat rating informs decision makers regarding the effectiveness of measures implemented to mitigate the environmental impacts associated with fluctuating hydropower operations. Copyright © 2013 John Wiley & Sons, Ltd.

Mitigation measures for fish habitat improvement in Alpine rivers affected by hydropower operations

Despite the many habitat simulations that have been undertaken around the world, not enough biological monitoring has been performed following flow manipulations. It is difficult, however, to refine flow management decisions without a better understanding of the links between amounts, durations and seasonality of flow deliveries and population dynamics. Trout populations were monitored before and after flow alterations in five trout streams, involving 17 study sites over a 4- to 12-year period, depending on the sites. A comparison of the trout populations observed to theoretical habitat/population models pointed up several helpful lessons. Various factors slow increases in population size, including the availability and quality of spawning grounds, the general connectivity of the bypass section (BPS) and severe spate events. In addition to these site-dependent factors, hydrological dynamics may explain why it is so difficult to clearly identify relationships between habitat availability and real fish stocks. Moreover, opportunities to observe population changes are improved when the pre-enhancement instream flow value is very low, and when there is a considerable difference between pre- and post-enhancement values. A population dynamics model that incorporates different habitat limitations and demographic background can be a very precious tool to improve understanding of the different situations and to build scenarios of population recovery.

Long‐term brown trout populations responses to flow manipulation

Changes in a brown trout (Salmo trutta L) population result from interaction among various mechanisms which are dependent on environmental conditions and biotic processes In reaches influenced by the presence of dams, the instream flow in the bypassed section is not the only parameter which affects the population Flood episodes, the general connectivity of the bypassed section, and the characteristics of the substrate which define the availability and quality of spawning grounds may also have a crucial impact The design and fine-tuning of tools which take environmental parameters into account can improve our understanding of the dynamics of such influenced populations In this perspective, a deterministic model (MODYPOP) has been developed in an attempt to integrate all these factors and to test the effect of different long-term scenarios of influenced flow regimes on the structure of trout populations MODYPOP was applied to three populations and three reaches (on the Roizonne, Neste d'Aure and Lignon du Forez rivers in France) For each stream, experiments were carried out on a bypassed section downstream of a hydropower station, before and after an increase in the minimum instream flow due to relicensing These experiments allowed integrating into MODYPOP local phenomena (impact of flood episodes, impact of flushing, impact of downstream migration of juveniles and adults) affecting the populations during the study period and then calibrating them To estimate the change in the population due to the increase in minimum instream flow, different long-term simulations were run, selecting discharge patterns at random These scenarios help to evaluate the time required for the population to return to a range close to habitat saturation after an improvement in the hydraulic habitat or following a flood event These applications have enabled determining the relative importance of changes in population density due to different types of events Copyright © 2008 John Wiley & Sons, Ltd

Long‐term simulations of the dynamics of trout populations on river reaches bypassed by hydroelectric installations—analysis of the impact of different hydrological scenarios

Successful management and protection of wild animal populations relies on good understanding of their life cycles. Because population dynamics depends on intricate interactions of biological and ecological processes at various scales, new approaches are needed that account for the variability of demographic processes and associated parameters in a hierarchy of spatial scales. A hierarchical Bayesian model for the resident brown trout (Salmo trutta) life cycle was built to assess the relative influence of local and general determinants of mortality. The model was fitted to an extensive data set collected in 40 river reaches, combining abundance and environmental data (hydraulics, water temperature). Density-dependent mortality of juveniles increased at low water temperatures and decreased with shelter availability. High water temperature increased density-dependent mortality in adults. The model could help to predict monthly juvenile and adult mortality under scenarios of global warming and changes in shel...

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Understanding inter-reach variation in brown trout (Salmo trutta) mortality rates using a hierarchical Bayesian state-space model

The key role of hydrological variability in structuring brown trout populations is well established. However, the influence of additional drivers is more difficult to identify. The implementation of long-term monitoring and the development of reliable tools can help to reveal fine local drivers structuring fish populations in contrasted flow regimes. This study used data series for nine reaches monitored for nine to nineteen years in four French salmonid streams. Study reaches were within five bypassed sections influenced by instream flow. A deterministic trout population dynamics model was applied on each reach, with calibration and validation procedures. Results revealed that biological drivers structured all reaches similarly. In addition, seven other drivers were identified. Among these additional drivers, hydrology mainly explained temporal fluctuations in trout density, regardless of reach. Three drivers independent of hydrology were also revealed: poor water quality, limited spawning area and the effect of power plant operations (overtopping, flushing or plant shutdown). All drivers influenced the whole bypassed section and were never limited to the scale of the reach (sampling area). Further analyses of each driver are now needed, to regionalise and quantify their respective impact precisely. Thus, assessment of trout population status would be simplified , enabling implementation of efficient management rules.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/eff.12295

Main potential drivers of trout population dynamics in bypassed stream sections

Deux types d’approches techniques complementaires sont utilisees pour guider l’etablissement des debits ecologiques, a l’echelle des troncons de cours d’eau (ex. : debits reserves) comme a l’echelle de bassins versants (ex. : debits objectifs d’etiage). Les approches « hydrologiques » visent a quantifier les alterations de multiples caracteristiques du regime hydrologique et reposent sur l’identification (delicate) de relations empiriques entre alterations hydrologiques et biologiques. Les approches « habitat hydraulique », ciblees sur les debits bas a moyens, couplent des modeles hydrauliques et des modeles biologiques pour traduire certaines modifications hydrologiques en modification de qualite de l’habitat hydraulique pour les organismes. Elles ont parfois apporte des predictions convaincantes des effets biologiques des modifications de debits d’etiage. Ces deux approches techniques ne fournissent pas directement de valeurs de debits ecologiques. Nous formalisons ici une demarche technique de definition des debits ecologiques, basee sur la comparaison de scenarios de gestion et une meilleure combinaison des deux approches. La demarche comprend quatre etapes : (1) la description du contexte hydrologique naturalise et actuel, des usages actuels et des scenarios de gestion envisages (2) la description du contexte ecologique au sens large, (3) l’identification des metriques pertinentes (hydrologiques et/ou habitats et/ou autres) pour decrire les effets des scenarios (modifications des usages, effets sur le milieu) et (4) la comparaison des scenarios. Cette demarche ne se passe pas d’expertise et doit s’accompagner d’une definition des objectifs partagee par les acteurs ainsi que de retours d’experience, aspects non detailles ici.

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Laurence Tissot

Papers

Long‐term brown trout populations responses to flow manipulation

Long‐term simulations of the dynamics of trout populations on river reaches bypassed by hydroelectric installations—analysis of the impact of different hydrological scenarios

Understanding inter-reach variation in brown trout (Salmo trutta) mortality rates using a hierarchical Bayesian state-space model

Main potential drivers of trout population dynamics in bypassed stream sections

Débits écologiques : la place des modèles d’habitat hydraulique dans une démarche intégrée