Natural river landscapes can be biodiversity hotspots but are one of the most human altered ecosystems with habitats significantly damaged around the world, and a third of fish populations threatened with extinction. While riparian ecosystems have been negatively altered by anthropogenic activities, effective planning and restoration strategies can reverse negative impacts by improving habitat quality. However, restoring rivers requires appropriate data on current riparian health while also considering priorities for different stakeholders. To address this, a Geographic Information System (GIS) was used to create a new and transferable restoration priority model based on a section of the river Linth in Switzerland as a case study. The restoration priority model is founded on connectivity, river condition, national priority species and species hotspots. Landscape change of the riparian zone was analyzed using aerial imagery and landscape metrics. Almost a quarter of rivers within the study area were considered high or very high restoration priority, with many aquatic species set to benefit from restoration. From 1946 to 2019, the riparian landscape became highly fragmented due to significant growth in impervious surfaces and a concomitant loss of agricultural land. The GIS model provides a tool by which environmental agencies can manage natural features over large scales, while also planning priorities and targeting conservation strategies to the areas of greatest need.

Understanding the relationship between a dam\'s size and its ecological effects is important for prioritization of river restoration efforts based on dam removal. Although much is known about the effects of large storage dams, this information may not be applicable to small dams, which represent the vast majority of dams being considered for removal. To better understand how dam effects vary with size, we conducted a multidisciplinary study of the downstream effect of dams on a range of ecological characteristics including geomorphology, water chemistry, periphyton, riparian vegetation, benthic macroinvertebrates, and fish. We related dam size variables to the downstream-upstream fractional difference in measured ecological characteristics for 16 dams in the mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic residence times (HRTs) ranging from 30 min to 1.5 years. For a range of physical attributes, larger dams had larger effects. For example, the water surface width below dams was greater below large dams. By contrast, there was no effect of dam size on sediment grain size, though the fraction of fine-grained bed material was lower below dams independently of dam size. Larger dams tended to reduce water quality more, with decreased downstream dissolved oxygen and increased temperature. Larger dams decreased inorganic nutrients (N, P, Si), but increased particulate nutrients (N, P) in downstream reaches. Aquatic organisms tended to have greater dissimilarity in species composition below larger dams (for fish and periphyton), lower taxonomic diversity (for macroinvertebrates), and greater pollution tolerance (for periphyton and macroinvertebrates). Plants responded differently below large and small dams, with fewer invasive species below large dams, but more below small dams. Overall, these results demonstrate that larger dams have much greater impact on the ecosystem components we measured, and hence their removal has the greatest potential for restoring river ecosystems.

The Periyar River, a vital component of Kerala\'s ecosystem in India, serves as a lifeline supporting agriculture, hydropower generation, and ecological equilibrium. This study adopts a multifaceted approach to address critical challenges in the Periyar basin, with a primary focus on flood mitigation due to the region\'s susceptibility to devastating floods. Covering a length of 67.85 km, the study intricately segments the Periyar River into distinct reaches for a comprehensive steady flow analysis, considering factors such as seasonal monsoon fluctuations, diverse catchment topography, and human-induced alterations. Utilizing advanced modeling techniques, particularly HEC-RAS software, the study effectively predicts and simulates shifts in hydraulic behavior. The results, including velocity plots and cross-sectional maps, offer accurate insights into critical parameters, enabling the identification of areas with high velocity occurrence. This information proves instrumental in making informed decisions for the construction of river restoration structures, crucial for mitigating the impact of floods. The study\'s findings contribute valuable tools for future forecasting and sustainable management of the Periyar River, addressing the complex interplay of natural and anthropogenic factors.

Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x¯ = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x¯ = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

Flow regulation in gravel-bed rivers impacts the hydrology, sediments and morphology, riparian vegetation, and vertical connectivity with the hyporheic zone. In this context, previous works have suggested that flood events may have riverine morphological and ecological benefits. In a Mediterranean-climate river system, we analyzed the impact of a 18-year return period flood on river morphology, riparian vegetation, fish aquatic habitat quality, and hyporheic exchange in a dam-regulated gravel-bed river, Serpis River (Spain). We collected pre- and post-flood riparian vegetation distributions and bathymetries, which were used to develop two-dimensional surface and three-dimensional subsurface numerical models to map surface and hyporheic hydraulics. Results show that the large flood removed the invasive giant reed from large areas, reshaped the in-channel morphology by forming new bars and pools, and enhanced the complexity of the flow field and the hydro-morphological diversity. The habitat availability for the endemic Eastern Iberian chub (Squalius valentinus) and invasive bleak (Alburnus alburnus) increased. Hyporheic exchange showed limited change under losing conditions, but noticeable under neutral ambient groundwater condition. This study corroborates the beneficial effects that flood events or high flow releases may have on regulated streams and the potential use of high flow pulse as a restoration tool.

Longer durations of warmer weather, altered precipitation, and modified streamflow patterns driven by climate change are expected to impair ecosystem resilience, exposing freshwater ecosystems and their biota to a severe threat worldwide. Understanding the spatio-temporal temperature variations and the processes governing thermal heterogeneity within the riverscape are essential to inform water management and climate adaptation strategies. We combined UAS-based imagery data of aquatic habitats with meteorological, hydraulic, river morphology and water quality data to investigate how key factors influence spatio-temporal stream heterogeneity on a diurnal basis within different thermal regions of a large recently restored Danube floodplain. Diurnal temperature ranges of aquatic habitats were larger than expected and ranged between 14.2 and 28.0 °C (mean = 20.7 °C), with peak median temperatures (26.1 °C) around 16:00 h. The observed temperature differences in timing and amplitude among thermal regions were unexpectedly high and created a mosaic pattern of temperature heterogeneity. For example, cooler groundwater-influenced thermal regions provided several cold water patches (CWP, below 19.0 °C) and potential cold water refuges (CWRs) around 12:00 h, at the time when other habitats were warmer than 21.0 °C, exceeding the ecological threshold (20.0 °C) for key aquatic species. Within the morphological complexity of the restored floodplain, we identified groundwater influence, shading and river morphology as the key processes driving thermal riverscape heterogeneity. Promoting stream thermal refuges will become increasingly relevant under climate change scenarios, and river restoration should consider both measures to physically prevent habitat from excessive warming and measures to improve connectivity that meet the temperature requirements of target species for conservation. This requires restoring mosaics of complex and dynamic temperature riverscapes.

Over the past two decades, dam removal has become an increasingly important aspect of aquatic ecology. As a result of this work, ecological studies have arisen that monitor the changes to riverine ecosystems as a result of removal. Unfortunately, given the uncertain nature of funding and public concerns over dam removal, long term longitudinal studies that cover multiple trophic levels are difficult to find. Fortunately, the University of Michigan Biological Station has been involved in the ecological monitoring of a headwater river (the Maple River) in the northern part of the lower peninsula of Michigan. The physical, chemical, and some biological aspects of this river\'s ecology was measured for eight years prior to dam removal, during dam removal, and for two years post-dam removal. The results presented here show that the ecology of the river recovered within this two-year period, but had a different ecological set point. This new habitat is primarily driven by increases in flow, ammonia, silica, and increases in the populations of two macroinvertebrate feeding guilds. Discharge increased seven-fold in the year that the dam was removed in two sampling sites furthest from the dam but returned to pre-dam removal conditions a year after removal occurred. Turbidity followed this same temporal pattern as turbidity increased during dam removal but decreased to pre-removal levels once the dam was removed. pH decreased at all sites post-removal. In addition, ammonia showed a five-fold increase following dam removal at the two most upstream sites, while phosphate increased at all sites. Last, the number of filterers and shredders increased at all sampling sites, though the significance of increase varied spatially for each guild. The results and observations presented here may provide some guidance for other long term monitoring studies.

River floodplain channels can serve as reproduction, nursery or refuge areas for fish. Although the complementary use of floodplain and main channels is known, few studies attempted to quantify this use and even fewer analysed its controlling factors. The objectives of this study are (1) to describe the spatio-temporal use of floodplain habitats and to identify their roles as complementary habitats for fish and (2) to analyse how abiotic variations and their modifications under restoration impact habitat use by fish. To meet these objectives, we analysed (Principal Components Analysis and Coinertia Analysis) multi-site data collected over 20 years in eight main channels and 23 floodplain channels of eight restored sectors of the French Rhône River. Results show that habitat use by fish is mainly related to spatial effects, with 37 % of within-sector variance in taxonomic assemblages explained by the stations. As expected, rheophilic species were more abundant in lotic stations and limnophilic species in lentic ones. In addition, we identified an euryecious guild, grouping young of the year taxa (roach, gudgeon, chub, bleak) that used all types of habitats and particularly lentic floodplain channels with short life-span. Temporal effects (with ~10 % of the variance explained by years) combine (1) the effect of restoration, that increased the diversity of fish assemblages across the floodplain, with stronger changes in floodplain channels whose connectivity regime was modified, (2) the effect of high flows on fish habitat use, that reinforces the nursery and refuge functions of floodplain channels. Our results demonstrate the importance of restoring the diversity of habitats and connectivity because floodplain habitats have complementary functions for fish. Furthermore, our results also suggest to account for temporal variations in order to better estimate the potential effects of restoration on river and their floodplains.

The EU Water Framework Directive (WFD) has emphasized that altered stream/river morphology and diffuse pollution are the two major pressures faced by European water bodies at catchment scales. Increasing efforts have been directed toward restoration to meet WFD standards for ecological health, but this work has achieved limited success. One challenge is that little is known about how morphological changes (i.e., re-meandering) may affect nitrate retention within whole stream networks. We investigated this issue in the well-monitored Bode catchment (3200 km2) in central Germany. First, we implemented a fully distributed process-based mHM-Nitrate model, exploring its performance over the period from 2015 to 2018. Second, we simulated the effects of restoring more natural stream morphology (i.e., increasing sinuosity) on nitrate retention. The mHM-Nitrate model performed well in replicating daily discharge and nitrate concentrations (median Kling-Gupta values of 0.78 and 0.74, respectively). Within the stream network, mean and standard deviation (SD) of gross nitrate retention efficiency was 5.1 ± 0.61 % and 74.7 ± 23.2 % in the winter and summer, respectively; this measure took into account both denitrification and assimilatory uptake. In the summer, the denitrification rate was about twice as high in a lowland sub-catchment dominated by agricultural lands as in a mountainous sub-catchment dominated by forested areas (median ± SD of 204 ± 22.6 and 102 ± 22.1 mg N m-2 d-1, respectively). Similarly, in the same season, the assimilatory uptake rate was approximately five times higher in streams surrounded by lowland agricultural areas than in streams in higher-elevation, forested areas (median ± SD of 200 ± 27.1 and 39.1 ± 8.7 mg N m-2 d-1, respectively). This suggests that restoration strategies targeted at lowland agricultural areas may have a greater potential for increasing nitrate retention. In our simulation, restoring stream sinuosity was found to increase net nitrate retention efficiency by up to 25.4 ± 5.3 %; greater effects were seen in small streams. Taken together, our results indicate that restoration efforts should consider augmenting stream sinuosity to increase nitrate retention and decrease nitrate concentrations at the catchment scale.

Gualaxo do Norte River (GNR), in southeastern Brazil, was impacted by iron ore tailings from the Fundão Dam rupture (November 2015). The deposition of tailings on the riverbed has changed the hydrogeomorphological characteristics of the GNR, resulting in a decrease in the diversity of physical habitats and ecological biodiversity. As part of the process of restoration and management of this damaged ecosystem, the river restoration project ReNaturalize was implemented to restructure the geomorphological characteristics and the physical habitat and to enhance the reestablishment of biota, mainly for macroinvertebrates and fishes. For this goal, 203 wooden structures, such as tree trunks, branches, and grass were installed in two sections of GNR (T6R and T7R), totaling 1.8 km long. The effectiveness of the project was evaluated by an assessment that followed a before and after and control and impacted (BACI) design. Upstream of each Restored reach there is a Control and a Reference reach. Four campaigns were carried out, two before and two after the restoration process. After 14 months of the woody installation, an increase in hydraulic retention in the restored reaches was observed (T6R-20.2%; T7R-63.5%), when compared with the Control reaches, which favored the accumulation of sediments (T6R-388 metric tons; T7R-396 metric tons). This enhanced the formation of natural tailings barriers and promoted the enrichment of substrate types (T6R-39.2%; T7R-43%). The benthic macroinvertebrate community showed an increase in the total abundance (T6-110%), including the most sensitive groups (T6R-124%; T7R-124%). For fish, the increase was up to 81.38% with hand nets capture, indicating the recruitment of juveniles, and the abundance and the biomass of some species were also higher (up to 100%) than the Control reaches. The results indicated that the Restored reach is already qualitatively and quantitatively better than the Control reach and similar to the Reference reach, indicating the success of the study. Integr Environ Assess Manag 2023;19:648-662. © 2022 SETAC.