This paper synthesises published literature on run-of-river hydropower, highlighting its potential to affect both the physical and ecological conditions of river systems. The paper considers the limited number of direct studies and reviews a wider literature on the two principal impacts of such schemes on river systems: the introduction or maintenance of in-channel barriers and water abstraction/flow regime alteration. We outline how river systems are likely to be impacted by such schemes and identify the key issues arising from their continued development. Potential mitigation approaches are highlighted and the areas of future research required to adequately address current knowledge gaps are identified.

Summary 1.Research has highlighted the importance of buoyancy for hydrochorous propagule dispersal, but recently the river bed has been identified as a significant store of viable propagules. 2. Over four consecutive 4-month periods, deposited propagules (predominantly seeds) and sediment were sampled at 78 river bed, bank face and bank top sites within three river reaches in two catchments. Species of deposited propagules were compared with the propagule bank and standing vegetation to identify 'new' species. 3. Forty-four percent of deposited propagule species were not present in the vegetation and the largest proportions were deposited in winter. New propagules showed a higher proportion of species that establish long-term seed banks and produce light seeds, a higher proportion of Rstrategists, and a lower proportion of C-strategists than species in the vegetation. 4. Ordination of propagule species abundance data revealed differences in samples between reaches; bed and bank locations; and sampling periods. Within depositional samples there were significant positive correlations between average C scores and the proportion of species producing relatively heavy, short-lived seeds and scores on an ordination axis describing a gradient from channel bed to bank top. 5. Correlations and multiple regression models between species richness and abundance of deposited propagules (total and new), and the quantity and calibre of river-deposited sediment, demonstrated a direct link between propagule deposition and hydraulic conditions during inundation. 6. Synthesis . Riparian connectivity of river flows, varying hydraulic conditions and temporary storage of propagules are all complex components of hydrochorous propagule dispersal in the study reaches. Flood flows can transfer sediment particles and non-buoyant, viable propagules from the river into the riparian zone. Propagules can pass in and out of storage within the channel bed and margins through a variety of dispersal processes and can then settle out of the water in suitable depositional environments generating distinct spatial and temporal patterns in total propagule deposition. New propagules show greatest floristic similarity to propagule bank and bed samples. They are preferentially smaller, lighter and form longer term seed banks, making them particularly suitable for long distance transport over prolonged time periods and for widespread dispersal across river margins.

No abstract

Interactions between vegetation, flow and sediment are a key ingredient for the development of vegetated islands in highly dynamic, fluvial alpine ecosystems such as the Tagliamento River, north-east Italy. There has been substantial research on factors influencing the establishment of vegetation and feedback mechanisms between vegetation, hydraulic, and geomorphological processes in such environments. This has yielded the development of conceptual models identifying different trajectories of vegetation and landform development from bare gravel to established floodplain forest. Nevertheless, some of the finer-scale processes underpinning such interactions are not well understood and parameterisation concepts that augment our knowledge from process understanding to quantified data and prediction models are not available until now. This paper identifies mechanisms and parameters of vegetationflow interaction at the individual scale that are reflected at a patch or even at the channel scale. These mechanisms are reviewed from a multi-disciplinary perspective and concepts and analogies are proposed that provide ideas to progress research towards the development of predictive vegetationflow models. Such models must incorporate both hydraulic and ecological components and this is demonstrated for a simplified force-bending model of Salicaceae seedlings. The development of such models demands advances in the individual disciplines of hydraulics, morphology, plant ecology and biomechanics, which offers many possibilities for multidisciplinary research between these disciplines.

Question: How important is hydrochory for dispersing propagules along riverbanks and to what extent do the quantity and species composition of deposited propagules reflect the riparian vegetation, represent “new” species that are not present in the vegetation, and vary with river flow and season? Location: River Frome, Dorset, UK. Methods: Over 13 consecutive 6‐week time periods, during which river water levels were continuously monitored, aerial inputs of propagules to riverbanks were sampled using funnels, hydrochorous propagule transport was sampled using drift nets, and deposition across the riverbanks was sampled using astroturf mats. A survey of the riparian vegetation enabled comparison between samples and the standing vegetation, so that “new” species could be identified. Differences in propagule abundance and diversity between sampling methods, time periods and locations were tested using Mann‐Whitney U‐tests and Kruskall‐Wallis ANOVA. DCA established contrasts in the floristic composition of all deposited propagules and “new” propagules between different sample types, time periods and locations. Results: Aerial seed fall generated few propagules of low species richness. Hydrochory introduced large numbers of propagules and new species, resulting in high propagule deposition on the riverbank. The number and diversity of deposited propagules was governed by seasonal patterns of seed release and the hydrological regime. Propagule deposition was significantly greater on the most frequently inundated parts of the riverbank and autumn floods were particularly important for transporting “new” species to the study site and for remobilizing previously released propagules. Conclusions: The abundance and diversity of propagules deposited along riverbanks is dependent upon high river flows, which facilitates connectivity between the channel and the riparian zone.

Salicaceae are key pioneer riparian tree species that have the ability to reproduce sexually and asexually. Recent research has suggested that Salicaceae act as ecosystem engineers, modifying hydrological and geomorphological processes, resulting in the stabilisation and growth of landforms. Understanding these interactions requires knowledge of the controls on Salicaceae regeneration. This paper describes a study of Salicaceae establishment and growth along a reach of a highly dynamic, island-braided river. The sexual and asexual regeneration of three species were investigated using experimental planting of cuttings and observation of seedlings. Plots were located at a range of elevations, in different habitats associated with the established riparian vegetation and in contrasting sediment types. Survival and growth were monitored over two growing seasons. Asexual regeneration was more successful than sexual regeneration, with cuttings demonstrating faster growth rates and tolerance of broader environmental conditions than seedlings. Cutting survival and growth was highest in sediments with a relatively high organic content and in plots located between patchy Salicaceae stands or in the lee of islands. Seedling mortality was extremely high due to fluvial disturbance, although seedlings in habitats that were protected from fluvial disturbance survived. Seedling growth showed preferences for particular sedimentary conditions, which varied between species. The major control on regeneration was the upstream presence of established Salicaceae, particularly on islands, which provided open sites that were protected from fluvial disturbance and suitable for regeneration. Thus, asexual regeneration facilitated sexual regeneration by rapidly colonising sites that provided habitats protected from fluvial disturbance for seedling establishment. This supports previous work suggesting that Salicaceae can act as ecosystem engineers.

The establishment of vegetation in riparian zones is dependent upon the availability of suitable habitat and the presence of viable propagules. Studies have shown that propagules are transported by rivers (hydrochory) and deposited in the riparian zone. However, there has been no detailed integrated study of fluvial propagule transport and deposition in relation to the hydrological regime and relative to other key dispersal mechanisms, such as wind dispersal (anemochory) and direct fall from local riparian vegetation.This paper presents the findings of a 1 year study of the hydrological controls on propagule input, transport and deposition in the riparian zone along two morphologically contrasting reaches of the River Frome (UK) and the potential importance of these processes for the riparian vegetation.Analysis of samples, taken every 6 weeks, shows that aerial seed dispersal delivered relatively few propagules to the riparian zone and was highly spatially and temporally variable. The river transported and deposited large numbers of propagules of high diversity, including many species which were not present in the local vegetation, although this varied with river water levels and with season. Winter high flows were particularly important for depositing diverse propagules into the riparian zone along with local seed production during the summer. Despite contrasts in the planform and cross profile morphology of the two study reaches, they showed great similarity in patterns of propagule deposition, which were largely driven by the inundation regime, the elevation of the sampling site, and the season of sampling.

Regional Climate Models (RCMs) are an essential tool for analysing regional climate change impacts as they provide simulations with more small-scale details and expected smaller errors than global climate models. There has been much effort to increase the spatial resolution and simulation skill of RCMs, yet the extent to which this improves the projection of hydrological change is unclear. Here, we evaluate the skill of five reanalysis-driven Euro-CORDEX RCMs in simulating precipitation and temperature, and as drivers of a hydrological model to simulate river flow on four UK catchments covering different physical, climatic and hydrological characteristics. We use a comprehensive range of evaluation indices for aspects of the distribution such as means and extremes, as well as for the structure of time series. We test whether high-resolution RCMs provide added value, through analysis of two RCM resolutions, 0.44° (50 km) and 0.11° (12.5 km), which are also bias-corrected employing the parametric quantile-mapping (QM) method, using the normal distribution for temperature, and the Gamma (GQM) and Double Gamma (DGQM) distributions for precipitation. The performance of these is considered for a range of meteorological variables and for the skill in simulating hydrological impacts.In a small catchment with complex topography, the 0.11° RCMs outperform their 0.44° version for precipitation and temperature, but when used in combination with the hydrological model, fail to capture the observed river flow distribution. In the other (larger) catchments, only one high-resolution RCM consistently outperforms its low-resolution version, implying that in general there is no added value from using the high-resolution RCMs in those catchments. GQM decreases most of the simulation biases, except for extreme precipitation and high flows, which are further decreased by DGQM. Bias correction does not improve the representation of daily temporal variability, but it does for monthly variability, in particular when applying DGQM, which reduces most of the simulation biases. Overall, an increase in RCM resolution does not imply a better simulation of hydrology and bias-correction represents an alternative to ease decision-making.An important driver for increasing RCM resolution is the need to improve the analysis of climate change impacts for decision-making (e.g. Macadam et al., 2016;Qian et al., 2015). For hydrology, the standard analysis of climate change impacts generally involves coupling uncorrected or bias-corrected GCM or RCM precipitation and temperature outputs with hydrological models to simulate river flow scenarios (e.g.