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Habitat destruction is a global driver of biodiversity loss. In rivers, damming and river regulation for hydropower have caused extensive loss of high gradient, riffle, rapid and waterfall habitats. Restoring these habitats, which support unique biodiversity, should be an urgent priority, but inadequate documentation hampers evaluation of different management strategies. Focusing on Sweden, where river regulation affects most catchments, we mapped and quantified characteristics (e.g., slope, length, discharge) of 968 bypassed reaches (BRs), that is, river sections that are dewatered due to diversion of discharge for hydropower production. The extent of habitat loss associated with BRs is substantial, summing to a total length of 1,256 km, 94% of which is predominantly comprised of former riffles, rapids and waterfalls. BRs are typically located in larger rivers at central river network positions, highlighting their potential importance for hydrological and ecological connectivity. These habitat losses are poorly addressed by current management: three quarters of Swedish BRs have no mandated minimum discharge. Of the remainder, 88% have a discharge <2 m³/s, a flow threshold below which, on average, the proportion of rheophilic fish decline rapidly in Swedish BRs. Based on these findings for Sweden, and given the ubiquity of hydropower elsewhere, we suggest that the thousands of kilometers of mostly dry high-gradient habitat linked to diversion hydropower worldwide likely represent high value restoration targets. Increased, ecologically meaningful, flow releases in these reaches has great potential to rehabilitate essential habitat for threatened rheophilic organisms and ecosystem functioning of regulated rivers.

Hydropower is a vital renewable energy source but has substantial ecological impacts on rivers, lakes, and surrounding ecosystems. It alters hydrogeomorphology, disrupts connectivity, and changes water physicochemical properties such as temperature and dissolved gas concentrations. Historically, the environmental impact has been of less concern compared to energy production, and there is an urgent need to adapt hydropower production to reduce impacts on aquatic ecosystems. While various mitigation measures exist, a systematic understanding of their efficiency is lacking. Here, we extensively review both the environmental effects of hydropower and the scientific base for mitigation measures. We then list key abiotic and biological candidates for systematic monitoring before outlining a programmatic monitoring approach to evaluate the efficiency of mitigation measures. This programmatic monitoring approach involves monitoring packages based on specific mitigation measures. A set of abiotic parameters and biological indicators are monitored with standardized methods and monitoring designs over the long-term and at several sites, covering different river types and hydropower configurations. The proposed program serves to inform ongoing and future remedial measures, expand our mechanistic understanding of the ecological effects, facilitate knowledge transfer, and allow for more reductionist monitoring approaches outside of the program.

This work examines the relationships between flow restoration, natural flow regimes and riparian vegetation utilizing a case study in northern Sweden. The riparian zone is one of the most species-rich ecosystems and forms an important link between aquatic and terrestrial systems. The integrity of the riparian zone and its vegetation is harmed by flow alteration in numerous rivers, which calls for enhancing riparian management and ecological mitigation measures. Here, we take an ecohydraulic approach, combining a hydraulic model and data on inundation tolerance of riparian vegetation to evaluate the effects of reintroducing seasonal flow variation in a bypassed reach with minimum discharge. The results show that implementing the seasonal flow variation is projected to benefit riparian vegetation by extending the riparian zone and to lead to the development of distinct vegetation belts similar to riparian vegetation along free-flowing rivers. Additional simulations demonstrated that a further increase in riparian area could be achieved by increasing the magnitude of the minimum flow release. While the method assumes the riparian vegetation to be in equilibrium with the flow regime, continued monitoring is needed to assess how fast the riparian vegetation adjusts to new flow conditions.

Hydropeaking causes unnatural flow and water level fluctuations in the riparian zone. Biological processes in the riparian zone, such as nutrient cycling, are affected by changes in temperature, oxygen, and moisture, which are all to some extent influenced by the water level. Here, we studied the relationship between hydropeaking and decomposition of organic matter in the riparian zone. Specifically, we measured the decomposition rate (k) and decomposition endpoint (the stabilization factor, S), and examined associations between k, S, hydropeaking, and different environmental variables. Decomposition was estimated using the Tea Bag Index at 33 sites in Swedish rivers and streams with different degrees of hydropeaking. Rivers and streams were sampled in the summer of 2020 (N = 26) and autumn of 2023 (N = 16), with nine sites sampled on both occasions. In summer 2020, k was negatively related to high hydropeaking intensity, but there was no effect of hydropeaking on S. In autumn 2023, there was no effect of hydropeaking on k or S. Of the other environmental variables tested (substrate grain size, temperature, and length of growing season), the growing season had a positive relationship with S in autumn 2023. Our findings indicate that reducing hydropeaking during summer, when biological activity is high, may be beneficial for maintaining riparian function.

Hydropower production leads to losses of species and ecosystem types in rivers. A major knowledge gap is represented by bypassed reaches where all or most discharge is diverted to hydropower stations. Bypassed reaches may have some flow, derived from tributaries and dam leakage, along with minimum flow. In Sweden, mandates for minimum discharge are lacking for most bypassed reaches, and when present, generally amount to only a few percent of mean annual discharge. We asked how fish communities in bypassed reaches differ from comparable non-regulated reference reaches, and whether minimum discharge can help sustain fish faunas typical of non-regulated rivers. We assembled a unique data set of 2693 samples collected via electrofishing by wading from 166 bypassed reaches and 247 free-flowing reaches spread across Sweden. To quantify flow-related community changes, we compared the relative dominance of rheophilic (preference for high flow conditions) versus limnophilic (preference for slow-flowing to stagnant conditions) fish species, fish density and fish species richness between bypassed and free-flowing reaches and investigated the effects of minimum discharge on community structure. Bypassed reaches had lower rheophilic dominance and fish density, but slightly higher fish species richness than comparable free-flowing reaches. Minimum discharge had positive effects on rheophilic dominance and fish density but not fish species richness. For relatively low values of minimum discharge, small changes in flow corresponded to large changes in the predicted values of fish density (up to about 0.5 m3/s) and rheophilic dominance (up to 2 m3/s), after which curves flattened. When minimum discharge was expressed as a percentage of mean annual discharge, rheophilic dominance and fish density increased almost linearly with increasing discharge, but the effects on fish density were dependent on river size. Synthesis and applications. We conclude that small increases in minimum discharge can be particularly beneficial to the restoration of rheophilic community structure, especially at low levels of minimum flow, even though the area of lotic habitat has been reduced to a fraction of natural conditions. Implementation of minimum discharge in bypassed reaches can thus be an effective rehabilitation measure, providing ecological benefits helping to meet environmental objectives.

Hydropower is central to renewable electricity systems, but degrades ecosystems, calling for environmental flow schemes to enhance the ecological status of river systems. Environmental flow assessments need to account for climate change, since climate-driven changes in runoff affect both hydropower operation and riverine ecosystems. Here, we quantify expected changes in hydropower production and environmental benefits of introducing environmental flows in a large regulated river system in northern Sweden in a future climate. Compared with the hydrology of 1981–2010, runoff is projected to increase with climatic conditions projected for 2040, leading to a 2.2 % increase in hydropower production with present rules for turbine and reservoir operation. Implementing environmental flows will result in lower hydropower production losses in with the 2040 climate than at present: Introducing restrictions against zero flow events, discharge to technical fishways and bypassed reaches throughout the year (with seasonal flow variation), as well as having more natural water-level variation in all run-of-river impoundments, would reduce annual hydropower production with 3.5 % with present conditions, and by 3.3 % in 2040. At the same time, the net effect of higher runoff and introducing environmental flows means that the annual hydropower production in the 2040 climate would be only 0.8 % lower compared to 1981–2010. In all scenarios, reservoir filling degree in 2040 was projected to increase compared to scenarios for 1981–2010, and flow requirements were met for both environmental flows and hydropower production over an 83-year scenario-based time series. This study demonstrates the feasibility of introducing environmental flow actions in Sweden, and other regions where increases in runoff are projected, with sustained hydropower production, having large benefits for riverine biodiversity and enhancing resilience of riverine ecosystems to climate change. For this to be successful, collaboration among stakeholders in riverine management is needed.

Freshwater ecosystems and their bordering wetlands and riparian zones are vital for human society and biological diversity. Yet, they are among the most degraded ecosystems, where sharp declines in biodiversity are driven by human activities, such as hydropower development, agriculture, forestry, and fisheries. Because freshwater ecosystems are characterized by strongly reciprocal linkages with surrounding landscapes, human activities that encroach on or degrade riparian zones ultimately lead to declines in freshwater–riparian ecosystem functioning. We synthesized results of a symposium on freshwater, riparian, and wetland processes and interactions and analyzed some of the major problems associated with improving freshwater and riparian research and management. Three distinct barriers are the lack of involvement of local people in conservation research and management, absence of adequate measurement of biodiversity in freshwater and riparian ecosystems, and separate legislation and policy on riparian and freshwater management. Based on our findings, we argue that freshwater and riparian research and conservation efforts should be integrated more explicitly. Best practices for overcoming the 3 major barriers to improved conservation include more and sustainable use of traditional and other forms of local ecological knowledge, choosing appropriate metrics for ecological research and monitoring of restoration efforts, and mirroring the close links between riparian and freshwater ecosystems in legislation and policy. Integrating these 3 angles in conservation science and practice will provide substantial benefits in addressing the freshwater biodiversity crisis.

Restaurering av Juktån är ett samverkansprojekt som har drivits på frivillig basis av den ideella föreningen Samverkan Umeälven (www.umealven.se) tillsammans med Umeälvensvattenregleringsföretag, Vattenfall och Umeå Universitet. Juktån är en 60 kilometer lång torrfåra med minimitappning om 12% av det oreglerade flödet (MQ) vid reglerings. I produktionssystemet har den en funktion som spillfåra. Projektets ambition och mål var att Juktån skulle återfå ekologisk funktion med fungerande reproduktion av öring och harr, mer naturlig och bredare strandvegetation och svämskogar, mer naturlig artsammansättning av makrofyter samt en sedimentationsprocess med mindre deponering av silt och finsediment. Juktånprojektet har kartlagt och genomfört biotopåtgärder samt restaurerat huvudfåran sträckan nedströms Tjangarn och sidofåran Lickotgrenen under 2019 och 2020. Totalt har dryga 35 kilometer torrfåra restaurerats, hundratals trösklar och flottledsobjekt har rivits ut. Juktåprojektet ansökte hos Mark och Miljö Domstolen (MMD) om att ändra den relativt statiska minimitappningen till en säsongsanpassad minimitappning, vilket beviljades i augusti 2020 och infördes våren 2021. Metoderna för att genomföra restaureringen bygger på restaureringsekologi och sambandet till naturlig flödesregim. Juktåns restaurering har inneburit en komplex restaurering från utrivning av grunddammar och trösklar, restaurering av den fysiska miljön för att gynna öring, harr, makrofyter, strandvegetation och processer. Vi har utgått från ekologi i oreglerade vattendrag och utvecklat metoder speciellt anpassat för torrfåror med minimitappning. Preliminära resultat visar på en ökad reproduktion av öring, mer naturliga processer kopplat till flöde, sedimentation, vattenhastighet och som har påverkat både makrofyter samt strandvegetation.

To enable prioritization among measures for ecological restoration, knowing the expected benefits and consequences of implementation is imperative but rarely explicitly quantified. We developed a novel method to prioritize among environmental flow measures to rehabilitate ecosystems in the Ume River catchment in northern Sweden, a river system heavily regulated for hydropower production. Our strategy was to identify measures with minimal impact on hydropower production while providing substantial environmental benefits. Based on field surveys of remaining natural values and potential for ecological rehabilitation, we quantified the projected gain in habitat area of implementing environmental flows for target organism groups, for example, lotic fish species and riparian vegetation, along the whole river length. We quantified the consequences for hydropower production by identifying a set of hydropower operational rules reflecting the constraints added by environmental flows. We then used production optimization software to calculate changes in hydropower production and revenues. Implementing restrictions on zero-flow events by mandating minimum discharge at all run-of-river hydropower stations and allocating 1%–12% of mean annual discharge to bypassed reaches in the entire catchment would result in a 2.1% loss of annual electricity production. Adding flow to fishways would increase the loss to 3.1% per year. With implementation of more natural water-level fluctuations in run-of-river impoundments, the loss increases to 3.8%. These actions would increase the habitat for lotic species like the grayling Thymallus more than threefold and increase the area of riparian vegetation by about 66%. Our method forms a basis for ongoing implementation of nationwide environmental rehabilitation schemes.