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Abstract: Aim To study the composition and population dynamics of Chironomidae (Diptera) from a high-altitude tropical stream in Ecuador. Methods Samples were taken at Saltana River between April 2009 and November 2010. Physico-chemical parameters, such as flow, dissolved oxygen, conductivity, pH, and temperature, were measured in each sampling campaign. Larvae and pupae sampling was conducted with a Hess sampler in different substrates. During each sampling campaign, adults were sampled using amphibious emergence traps for 24 hours. All materials collected were separated and classified in the laboratory. The biomass was calculated using the individual weight of the larvae of different instars multiplied by the density of each instar at each sample. Results A total of 18 larval taxa were found in 14 different genera in the benthic samples. The subfamily Orthocladiinae showed the highest richness (12 genera), followed by Podonominae, Diamesinae, and Chironominae, with two genera each. The most frequent taxa were Cricotopus sp.3, C. (Oliveiriella) rieradevallae, and Genus 1 sp.a. Total densities fluctuate very much from one month to another following the changes of the most abundant species, and only one species showed a relationship with the substrates. Biomass values are lower from June to August of both years, which matches the high flow season. Conclusions The Saltana River’s cold temperature (6-7 °C) and the flow variability determine the low richness, density and biomass throughout the year. It is unclear if individuals showed very short or long-life cycles due to constant temperature throughout the year. However, there is a clear asynchrony in life cycles due to the presence of emerging adults throughout the year.

Dam removal is increasingly considered as a river restoration tool for impoundments that harm the environment or have exceeded their lifespan. However, few studies report the ecological consequences of large dam removal.We performed a multiple before‐after/control‐impact (mBACI) study to investigate the consequences of the decommissioning of a large dam (42 m high) on instream habitat and invertebrate communities in a temperate, forested catchment of northern Spain.Before decommissioning, lack of fine sediments and high concentrations of manganese and iron occurred below the dam but decreased downstream. Invertebrate taxa richness and diversity were reduced, and pollution‐sensitive taxa were missing just below the dam.The drawdown of the reservoir, the first step towards its decommissioning, mobilized stored sediments causing frequent turbidity peaks downstream, which nevertheless, caused no detrimental effects on macroinvertebrate communities. One year after drawdown, the communities downstream from the dam, as well as those in the newly formed stream in the area formerly impounded by the reservoir, became very similar to those in control reaches, showing a successful restoration project.Synthesis and applications. Dam decommissioning helps restore instream habitats and facilitates the recovery of invertebrate communities in a very short time frame if there are nearby sources of potential colonizers. Slow drawdown reduces the transport of the sediments accumulated in the reservoir and their potential downstream impacts, even more if prior to drawdown the reservoir is kept full for years to promote the deposition of sediments in marginal areas that will later be readily colonized by trees. [ABSTRACT FROM AUTHOR]

Mountain streams act as conveyors of sediments within the river continuum, where the physical transport of sediments between river reaches through the catchment or between individual parts (e.g., between hillslopes and channels) of the catchment is assumed. This study focused on sediment connectivity analysis in the Slavíč River catchment in the Moravskoslezské Beskydy Mts in the eastern part of the Czech Republic. The connectivity index and connectivity index target modelling were combined with an analysis of anthropogenic interventions. Additionally, field mapping, grain size of bed sediments and stream power analysis were used to obtain information about connectivity in the catchment. Based on the analysis and obtained results, terrain topography is the current main driving factor affecting the connectivity of sediment movement in the Slavíč River catchment. However, the modelling provided valuable information about high sediment connectivity despite different recent land use conditions (highly forested area of the catchment) than those in historical times from the 16th to 19th centuries when the Slavíč River catchment was highly deforested and sediment connectivity was probably higher. The analysis of anthropogenic interventions, field mapping, grain size of bed sediments and stream power analysis revealed more deceleration of sediment movement through the catchment, decreased sediment connectivity with bed erosion, and gradual river channel process transformation in some reaches. Field mapping has identified various natural formations and human-induced changes impacting the longitudinal and lateral connectivity in the Slavíč River. For instance, embankments along 48% of the river's length, both on the right and left banks, significantly hinder lateral sediment supply to the channel. Stream power index analysis indicates increased energy levels in the flowing water in the river's upper reaches (up to 404.8 W m−2). This high energy is also observed in certain downstream sections (up to 337.6 W m−2), where it is influenced by human activities. These conditions lead to intensified erosion processes, playing a crucial role in sediment connectivity. Similar observations were described in recent studies that pointed out the long-term human interventions on many streams draining European mountains, where a decrease in sediment connectivity in these streams is linked with sediment deficits and the transformation of processes forming channels. [ABSTRACT FROM AUTHOR]

The Shirakami Mountain range, including the largest primeval beech forest in East-Asia, is undergoing ecological change. Dissolved organic matter (DOM) plays an important role in nutrient and material cycling in forest ecosystems. Because the quality of DOM varies based on its origin and diagenetic and runoff processes, changes in the environment surrounding DOM can be rapidly detected by monitoring its quality. Herein, concentrations and fluorescence composition of DOM at 14 sites in 13 streams in the Shirakami Mountain range were monitored monthly for over 2 years, excluding winter (December–March), to gain insight into the catchment hydrological and soil characteristics affecting DOM concentrations and composition in stream water. Based on the pattern of temporal changes in fluorescent component composition, monitoring sites were categorized into four groups (streams with small catchments, large catchments, catchments facing the Sea of Japan, and open waters in the catchment) with similar catchment characteristics affecting DOM dynamics. Multiple linear regression analysis showed that DOM concentrations in each group could be attributed to rainfall on the survey date, short-term (1–2 days) rainfall, midterm (~1 month) accumulated rainfall, midterm (7–11 days) accumulated temperature, and catchment characteristics as explanatory variables. The degree of influence of these variables differed among the four groups. The results of this study show that grouping streams according to catchment hydrological characteristics can help identify the impact of climate and environmental change on DOM dynamics in stream water. [ABSTRACT FROM AUTHOR]

Skulls of alpine bullhead sampled from the Javorinka stream in the Tatra Mountains, West Carpathians, were analyzed to determine concentrations of S, Cl, K, Ca, P, Rb, Zn, Mn, Mb, Fe, Ti, Sn, Co, Ni, Cu, As, Se, Pb, Sb, Ba, Hg, Cr, Ag, and Cd. The stage of development is the most influential factor determining element concentrations in the sampled bullhead, as fry were more polluted than adult fish. The different diets consumed by fry and adult bullhead plays a key role in the accumulation of chemical elements in their bodies. Young bullheads live in small natural embankments containing higher levels of a mixture of sedimentary minerals and microorganisms than in running water. Thus, newly hatched bullheads may serve as excellent indicators of water quality in mountain creeks or streams, as they can indicate the higher pollution of water or prey in their habitats (small bays with sandy bottoms) when compared to the preferred habitat of adult individuals. [ABSTRACT FROM AUTHOR]

Aging dams and the rising efforts to restore stream ecosystems are increasing the number of dam decommissioning programs. Although dam decommissioning aims at improving in‐stream habitat, biodiversity, and ecosystem functioning in the long term, it might also cause ecological impacts in the short term due to the mobilization of the sediment accumulated in the reservoir. Benthic biofilm in particular can be impaired by episodes of high turbidity and scouring. We conducted a multiple before‐after/control‐impact experiment to assess the effects of the drawdown of a large dam (42 m tall), a first step to its decommissioning, on biofilm structure (biomass and chlorophyll‐a) and functioning (metabolism, nutrient uptake, and organic matter breakdown). Our results show that the reservoir drawdown reduced the autotrophic biofilm biomass (chlorophyll‐a) downstream from the dam, which in turn lowered metabolism. However, nitrogen and phosphorus uptake by the biofilm was not affected. Organic matter breakdown was slower below the dam than in nearby undammed reaches before and during drawdown. All drawdown effects quickly disappeared and reaches downstream from the dam approached values found in nearby undammed reaches. Thus, our results indicate that the effects of reservoir drawdown on stream biofilms exist but may be small and disappear rapidly. [ABSTRACT FROM AUTHOR]

Bedload is rarely measured in Indian rivers. It is recommended that 5% of suspended load can be taken as bedload in the absence of measured bedload. The present study validates this by direct physical measurement of bedload using the Helley–Smith sampler in an ephemeral mountain stream of Gujarat, India. It was observed that, on an average, the bedload formed 3.97% of the suspended load. The measured bedload flux was 1.02 tonnes/ day. To overcome the need and dependability on actual physical bedload measurement, a bedload rating curve against specific discharge was developed to predict the bedload rate in the study reach. Few prominent existing bedload equations selected from the literature were tested against the measured bedload, which over-predicted the bedload transport rate with a discrepancy ratio greater than 2 and RMSE 2.4–48. A calibration coefficient ξ = 0.00167 was introduced in the widely used Recking (2013) equation for the study reach resulting in an improvement of the coefficient of variation as 1.92 and RMSE as 1.35. [ABSTRACT FROM AUTHOR]

A macrotidal estuary with mountain streams (MEMS) is characterized by rapidly rising and falling flood peaks and large tidal ranges and is a typical estuary type with strong flow dynamics that is found worldwide. Understanding the morphodynamic evolution of MEMSs has great significance for river management. The roles of the mountain stream river flood process and macrotides on the morphology evolution still needs further quantitative study. Taking the Oujiang Estuary as an example, the evolution mechanisms of bifurcated reaches of a MEMS are studied by analyzing the hydrologic and topographic data and using a two-dimensional numerical model for flow-sediment transport. The results show that (1) under the combined actions of runoff and macrotides, sediment transport is active and morphological evolution is intensive, the main branch and subbranch historically switch frequently, and erosion generally occurs in wet years, while deposition occurs in dry years. (2) The river flood process of mountain streams causes rapid erosion, which rises and falls rapidly with a kurtosis index (K) of approximately 2.9. There is a logarithmic relation between the sediment transport load and K. The volume of sediment transport will be 40% larger than the river flood process with K = 1.2 as for the Yangtze River. (3) Macrotides with large tidal ranges could enhance the discharge asymmetry (ψ). When the ψ is greater than 1, the side branches are dominated by flood tides. Although the current study is site specific, the results are expected to provide a valuable reference for sustainable management in similar estuaries. [ABSTRACT FROM AUTHOR]

Understanding the properties of wildfire-disrupted catchments is crucial for managing river floods and landslide risks. Using a hydrophone, we investigated the changes in sediment supply conditions in small mountainous catchments (30.8 ha) in southern Korea for 6 years (March 2014–December 2019). Bedload transport rates mostly increased in burned catchments 1–2 years post-wildfire (early post-fire sediment regime) but decreased 3 years post-wildfire owing to the coverage of burned slopes (late post-fire sediment regime). Landslides triggered 5 years post-wildfire increased the bedload transport rates by approximately one order of magnitude (post-landslide sediment regime). Although sediment activity decreased in wildfire-disturbed catchments after recovery of the ground-cover layer 1–3 years post-fire, thy increased during events such as landslides. Furthermore, even in undisturbed mountainous catchments (139.7 ha), bedload transport rates increased after intense rainstorm events (≥100.0 mm), with this change lasting for approximately 2 years. Our observations showed that the forest restoration after a wildfire should be planned and implemented from a min- to long-term perspective. In addition, during rainstorms, the relationship between flow and bedload transport showed large variability even in undisturbed catchments. Furthermore, surrogate monitoring using a hydrophone was useful in understanding the changes in bedload transport characteristics according to various supply conditions of the catchment. [ABSTRACT FROM AUTHOR]

The responses of habitat and fish communities to extreme hydrological and habitat disturbance in typhoon-prone mountain streams are not studied much. Such landscape–climate settings may provide a unique opportunity for fish to evolve without special habitat adaptation and involve different assembly rules compared to temperate regions. This study aimed to compare fish communities and habitat factors before and after the typhoon season and to test the influences of various aspects of habitat on fish assemblages between seasons. Fish and habitats were surveyed at 30 wadable stream sites in March and December 2010 in tropical southern Taiwan. Habitat variables differed between the pre-typhoon and post-typhoon seasons. Higher species richness and the total number of fish caught in the pre-typhoon season indicated fast recovery. Benthic fish were more vulnerable to typhoon disturbances than sub-benthic fish. Contrary to the original hypothesis, fish communities were more strongly related to physical habitats than water quality and riparian conditions. In addition, consistently high fish variance explained by habitat measures in the dry seasons indicated that environmental filtering predominated. Dominant fish species were related to main habitat gradients, characterized by low species richness but high beta-diversity. Our findings provide implications for fish conservation in typhoon-prone mountain streams. [ABSTRACT FROM AUTHOR]

Abstract The spatial–temporal patterns of fish assemblages in lotic systems can provide useful information in developing effective conservation measures. This study aimed to explore the spatial and seasonal changes in fish assemblages and their association with environmental factors in mountain streams of the Ren River, southwest China. Field investigations were conducted at 18 sites during the rainy and dry seasons in 2017. A total of 1,330 individuals, belonging to three orders, eight families, 19 genera, and 21 species, were collected. Analysis of similarities (ANOSIM) showed that the structure of fish assemblages varied significantly at the spatial scale, but not at the seasonal scale. In low‐order sites, fish assemblages were mainly dominated by cold‐water and rheophilic species (e.g., Rhynchocypris oxycephalus, Scaphesthes macrolepis, Metahomaloptera omeiensis, and Gnathopogon herzensteini), while those in high‐order sites were predominated by warm‐water and eurytopic or stagnophilic species (e.g., Squalidus argentatus, Hemiculter leucisculus, and Zacco platypus). Canonical correspondence analysis (CCA) showed that the fish assemblages were structured by a combination of large‐scale landscape factors (e.g., altitude and C‐link) and small‐scale habitat features (e.g., channel width, water temperature, and depth). Among these factors, landscape had the greatest influence on fish assemblages, while local habitat variables were less important or were only significant in certain seasons.

We use 11 years of annual surveys in streams of the Southern Rockies of Colorado, USA, to examine the persistence and geomorphic effects of logjams. Each year's survey includes ∼300 logjams along more than 21 km of four mountain streams in primarily old‐growth subalpine forest. Streams alternate longitudinally between laterally confined reaches with a single channel and wider reaches with multithread‐channel planform. We distinguish logjam persistence and site persistence. Logjam persistence is the median timespan over which an individual jam is present. Site persistence describes the tendency for jams to disappear and then re‐form at the same site. We hypothesize that (a) site persistence is greatest in multithread reaches, (b) logjam persistence is greatest in multithread reaches, and (c) average backwater storage at each jam is greater in multithread reaches. We find that spatial and temporal metrics of site persistence differ significantly between single and multithread reaches. Individual logjam persistence does not differ significantly. Backwater storage is significantly greater in multithread reaches. Varying combinations of riparian forest age and average logjams per channel length explain variation in jam and site persistence and backwater storage via multivariate linear regression analyses. Over the 11 years of survey, a total of 429 distinct logjams were observed. Only 2.1% of the population was present for all 11 annual surveys. Median jam persistence is 1–2.5 years; median site persistence is 6–10 years. Despite the transience of most channel‐spanning logjams in the population, these jams create persistent effects in channel planform and backwater storage. Key Points: Logjam persistence (the median timespan over which a jam is present during the 11‐year study) is 1–2.5 yearsSite persistence (the median timespan over which at least one jam is present in retentive sites) is 6–10 yearsDespite the transience of most logjams, the jams create persistent effects in channel planform and backwater storage [ABSTRACT FROM AUTHOR]

The spatial–temporal patterns of fish assemblages in lotic systems can provide useful information in developing effective conservation measures. This study aimed to explore the spatial and seasonal changes in fish assemblages and their association with environmental factors in mountain streams of the Ren River, southwest China. Field investigations were conducted at 18 sites during the rainy and dry seasons in 2017. A total of 1,330 individuals, belonging to three orders, eight families, 19 genera, and 21 species, were collected. Analysis of similarities (ANOSIM) showed that the structure of fish assemblages varied significantly at the spatial scale, but not at the seasonal scale. In low‐order sites, fish assemblages were mainly dominated by cold‐water and rheophilic species (e.g., Rhynchocypris oxycephalus, Scaphesthes macrolepis, Metahomaloptera omeiensis, and Gnathopogon herzensteini), while those in high‐order sites were predominated by warm‐water and eurytopic or stagnophilic species (e.g., Squalidus argentatus, Hemiculter leucisculus, and Zacco platypus). Canonical correspondence analysis (CCA) showed that the fish assemblages were structured by a combination of large‐scale landscape factors (e.g., altitude and C‐link) and small‐scale habitat features (e.g., channel width, water temperature, and depth). Among these factors, landscape had the greatest influence on fish assemblages, while local habitat variables were less important or were only significant in certain seasons. [ABSTRACT FROM AUTHOR]

We use four stream segments along a wood‐rich, pool–riffle mountain stream in the Southern Rockies of Colorado, USA to examine how spatial variations in wood load and variations in discharge during and after the snowmelt peak flow influence the magnitude of surface and subsurface transient storage. Segments range in complexity from a single channel with no large wood to an anabranching channel with closely spaced, channel‐spanning logjams. Discharges at which transient storage was assessed range from base flow to snowmelt peak flow. To explore these relations, we used 10 geomorphic variables representing channel morphology and bed substrate, four wood‐related variables representing wood load and associated backwater storage, and two measures of skewness from instream and bulk electrical conductivity breakthrough curves during tracer tests. Instream curves reflect surface and subsurface transient storage, whereas bulk curves primarily represent subsurface transient storage. Higher values of skewness indicate greater retention, and we used the values here as a metric of increased transient storage. Although limited sample size restricts the power of our results, our findings suggest that stream segments with greater instream large wood loads have more and larger pools, greater storage of fine sediment and particulate organic matter, and higher values of skew from instream conductivity. The results also suggest that the presence of instream wood, rather than changes in channel morphology associated with wood, is the most important driver of transient storage. This implies that river management designed to foster transient storage should focus on retaining instream large wood. We did not find significant correlations between geomorphic or wood‐related variables and the skew estimated from bulk conductivity, which may reflect the relatively thin alluvium present in the field area and the prevalence of surface transient storage in this system. [ABSTRACT FROM AUTHOR]

Bedload transport in natural channels typically shows large fluctuations even for constant hydraulic forcing. It has been shown that the threshold shear stress for initiation of motion depends, for example, on the bed morphology and on channel slope and that it may be influenced by flood history effects. This study is based on a unique data set of continuous field observations of bedload transport acquired during 27 years with impact plate measurements and involving more than 500 sediment‐transporting flood events. A disequilibrium ratio is defined which represents the observed bedload mass per event divided by the bedload mass calculated via a bedload transport equation integrated over the time of a flood event. The disequilibrium ratio varies in a cyclic behavior around equilibrium conditions defined by its median value. The mean duration of a cycle depends on upstream sediment supply or availability on the bed, and longer cycles are associated with a larger active layer on the bed. A memory effect is evident regarding the temporal evolution of both disequilibrium ratio and bed state as reflected by threshold transport conditions, determined from the observations as the critical discharge at the start and at the end of a bedload‐transporting event. The disequilibrium ratio and the threshold transport discharge are inversely correlated with each other, likely providing a feedback mechanism governing the fluctuations around an equilibrium state. Accounting for the memory effect, that is, for the bed state after the previous event, improves the prediction of bedload transport for the following event. Key Points: The disequilibrium ratio represents the observed bedload mass per event divided by the integrated bedload transport capacityDisequilibrium transport ratio and threshold transport discharge are inversely related to each other and vary in cyclesIncreased sediment supply or availability results in longer cycles, a larger active layer depth, and smaller threshold transport discharge [ABSTRACT FROM AUTHOR]

Damming of streams and rivers alters downstream ecosystem processes, and understanding its effects is essential in managing forested mountain streams. This study examined the effects of dams with a reservoir on organic matter decomposition and its seasonality in two neighbouring mountain streams over five seasons. The cotton‐strip assay and measurement of tensile strength loss were used to evaluate decomposition rates. In addition, the environmental factors, such as water level, water temperature, inorganic nitrogen concentrations and contribution of macro invertebrates were measured. The dams with a reservoir lowered decomposition rates downstream compared to upstream and unregulated sites, in all seasons. The decomposition rates also varied by season at all sites, with higher rates in summer and lowest in winter, and the seasonal variation was larger than the effects by the reservoirs in both streams. Seasonal variation in decomposition rates coincided with water temperature variation, suggesting strong influence of water temperature. However, the temperature differences did not explain the effects of reservoirs because water temperature was always higher at downstream sites. The downstream sites had lower nitrate concentrations and contribution of macroinvertebrates, and these factors may have greater effects than water temperature. Thus, damming of streams may lower the decomposition rates at downstream sites due to altered nutrient and biological effects. However, seasonal variation in water temperature may have more pronounced effects, resulting in greater seasonal variation than the difference among sites. [ABSTRACT FROM AUTHOR]

Previous work on the eastern side of Rocky Mountain National Park (RMNP), Colorado indicated correlations among waterfall location, waterfall morphology, and the characteristics of bedrock joints. Characteristics of waterfalls on the western side of the national park do not correlate as strongly with joint geometry. Longitudinal river profiles on the western side are less concave and waterfalls account for a greater proportion of the total elevation loss. We interpret these differences to result from more widely spaced joints, lithological differences, and complex glacial history. These results demonstrate that waterfall shape and typology may change due to both local and regional controls operating in a mountain region. Both regions had alpine valley glaciers, but continued landscape evolution via fluvial erosion has developed waterfalls with diverse locations and morphologies that reflect the influences of glacial deposits, bedrock erosional resistance, and joint geometry. [ABSTRACT FROM AUTHOR]

The Bangudae Petroglyphs is Korea's National Treasure No. 285, carved on a vertical rock face on the riverside of a mountain stream (Daegokcheon) near Ulsan. Since the construction of the Sayeon Dam in the downstream area in 1965, the petroglyphs was repeatedly flooded and submerged, raising concerns of erosion and water damage of the prehistoric rock-art motifs. Recent excavation of the fluvial sediments in front of the petroglyphs provided an opportunity to investigate the changes in sedimentation pattern before and after the construction of the dam and the consequences of human activity on natural environments. Sedimentological observations of trench sections and radiocarbon, OSL, and garbage dating of sediments reveal that the sediments comprises gravelly to sandy fluvial sediments in the lower part that accumulated since the early to middle Holocene or early to middle Neolithic Era until 1965. Above this, 25 units of alternating horizontally laminated sand and massive, mottled mud with abundant rootlets occur. This sand/mud sequence is interpreted to have accumulated after the construction of the Sayeon Dam in 1965, which caused repetitive flooding and submergence of the study area every year or two. The alternations of horizontally laminated sand and massive mud are interpreted to have resulted from rainfall-induced floods and subsequent suspension settling of fines in stagnant water after the area was inundated and temporarily transformed into a lacustrine margin setting. Afterwards, the depositional site was exposed and vegetated until the next flood event. Comparison of the deposit features with the precipitation data in Ulsan area, the water level data of the Sayeon Dam, and age data suggests that this flood-related sedimentation occurred almost every year until 2005 when the water level of the Sayeon Dam was lowered permanently because of the construction of another dam in the upstream section of the stream. This study thus provides a good example of a dramatic environmental change of a mountain stream caused by artificial disturbance of a river system due to construction of dams and an example of high-resolution sedimentary records of meteorologically induced floods in an artificial lake margin. [ABSTRACT FROM AUTHOR]

The objective of this paper is to draw attention to a previously unrecognised habitat for Unio crassus Philipsson, 1788 (Bivalvia: Unionidae), namely beaver dam-and-pond complexes. The construction of beaver ponds will allow the colonisation of high energy streams by providing flow refuges during periods of high near-bed shear stress. The absence of freshwater mussels (Unionidae) in mountain streams may result from steep slopes, shear stress during floods, lack of food suspended in the water column (seston), and unfavourable conditions for the settlement and growth of juvenile mussels. Transportation of parasitic larvae on host fish is the main dispersal mechanism for the mussels. A possible constraint on the colonisation of beaver complexes by freshwater mussels is that beaver dams may hinder the upstream movement of their minnow hosts. In the surveyed beaver ponds, the first specimens of U. crassus were found in 2008. Single mussels were also found in subsequent years. It is uncertain whether the occurrence of U. crassus depends on the periodic support of the population from the main river, but the presence of beaver ponds is certain to have given it the opportunity of colonising new areas. [ABSTRACT FROM AUTHOR]

Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the Δ14C signatures of POC suspended in SMR, we examined the temporal variability of Δ14C and δ13C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the Δ14C of riverine POC. A preliminary analysis of the existing data for a spectrum of rivers supports this hypothesis.

Wolman [1954] pebble sampling is the most commonly used method to estimate surface bed grain sizes in gravel-bed streams. A few studies documented different results between individual operators or repeated measurements within the same channel-reach obtained by this method. We tested potential differences in pebble sample distributions and related grain-size percentiles (D10, D50 and D90) between two fluvial geomorphologists and two almost inexperienced students in three channel-reaches and one gravel bar. None of sampled locations provided statistically consistent particle-size distributions and related percentiles when comparing measurements of all operators. The samples of experienced fluvial geomorphologists were most consistent for the channel-reaches with assumed widest range of particle sizes; a post-hoc test documented significant differences for the gravel bar and the lower plane bed reach. Medians of particle-size distributions for the gravel bar were equal for three of four operators; the fourth operator probably included also coarser particle population between the channel bed and bar. It implies that 100 sampled particles are most likely sufficient only for D50 estimations and homogenous sediment populations (i.e. well-sorted gravel bars). In any other case, much larger number of particles should be sampled in gravel/cobble bed streams to obtain narrower confidence limits of related grainsize percentiles.

Mountain headwater streams are still somewhat on the boundary of interest regarding possible human impact on their morphology or geomorphic processes, which may be caused by our perception of mountains as islands of relatively preserved natural conditions. This paper summarizes the past and present human pressure on the headwater streams that drain the highest mountain ranges of the Outer Western Carpathians in Czechia. Anthropogenic pressure began in this region in the 16th century during a colonization of the mountains and continued by timber harvesting, timber floating, and construction of torrent control works until present. Each of these interventions produced a morphological response of the channels in relation to altered sediment or water fluxes at the whole catchment scale or within longitudinal stream profiles. Because it is highly unlikely to reach pre-settlement conditions of the channels, the management effort should be concentrated to achieve realistic restoration targets under the present socioeconomic circumstances by taking into consideration the morphodynamical specifics of mountain headwater streams.

Elevational gradients are powerful 'natural laboratory' for testing the responses of microbes to geophysical influences. Microbial communities are normally composed of a few abundant and many rare taxa. Abundant and rare taxa play different ecological roles in kinds of environments, but how their diversity and composition patterns response to elevation gradients is still poorly elucidated. In this study, we investigated the elevational patterns of abundant and rare bacterial diversity and composition in a mountain stream from 712 to 3435 m at Gangrigabu Mountain on the Tibetan Plateau, China. Our results revealed abundant and rare bacteria had similar decreasing elevation trend of alpha diversity, and both of them showed a significant elevational distance-decay relationship. However, the turnover rate of the elevational distance-decay of rare bacteria was higher than that of abundant bacteria. The species-abundance distribution patterns of rare taxonomic composition were associated with the elevational gradient, while most of abundant bacterial clades did not display any relationships with elevation. Our results suggested that rare bacteria were more sensitive to changes in elevation gradient. [ABSTRACT FROM AUTHOR]

Abstract Check dams can modify local channel and sedimentological characteristics through sediment deposition in upstream sedimentary wedges and scour processes downstream of individual check dams. However, research focusing on the channel reaches between subsequent check dams (referred to here as intermediate reaches) is limited. We evaluated channel complexity and its selected dimensions (longitudinal and cross-section heterogeneity, sediment characteristics and the presence of instream wood) in 30-m long intermediate reaches (n = 10) between subsequent check dams in comparison with channel reaches that were not treated with check dams (n = 10) in both a stepped-bed stream in a steep confined valley and an originally pool-riffle stream in an unconfined foothill valley. Check dams altered the channel complexity of intermediate reaches when compared with reaches of undisturbed streams. However, in contrast to foothills streams, check dams did not heavily affect longitudinal or cross-sectional heterogeneity of the intermediate reaches in the steep streams. Despite an increase in sediment homogeneity in steep reaches treated with check dams, the presence of coarse bed sediments helped to preserve their stepped-bed morphology. In contrast, the longitudinal profile of the treated foothill stream completely lost its vertical oscillations because of the transformation of pool-riffles to a uniform plane bed morphology. Similarly, cross-sectional heterogeneity in the foothill stream was degraded in comparison with those of untreated reaches. We did not observe differences in instream wood abundance between treated and untreated streams. Graphical abstract Unlabelled Image Highlights • The channel complexity of intermediate reaches between check dams was investigated. • Different adjustments were observed between steep mountain and foothill streams. • Decreased bed sediment heterogeneity was measured in the managed mountain stream. • Cross-sectional and longitudinal heterogeneity was degraded in the foothill stream. • Instream wood abundance was equal in the managed and unimpacted streams. [ABSTRACT FROM AUTHOR]

Natural riffles are formed as a result of fluvial processes in the stream channel, water flow, and sediment movement. In recent years, artificial riffles have also been constructed in order to improve the water flow and sediment transport as well as to initiate the processes that lead to the restoration of natural riffle-pool sequences. In river engineering, the utilization of artificial riffles (rapid hydraulic structures) is still in its experimental stage. Artificial riffles are studied and described in the literature on a case study basis. However, the studies on riffle sequences and their effects on stream channel evolution need to be improved. A system of artificial riffles was created in the mouth section of Krzczonówka Stream in 2013. The new riffles made the stream channel more morphologically diverse and also reduced its longitudinal slope. The check dam on the stream, located upstream of the newly formed riffles, was renovated and lowered in 2014. At this time, a flood occurred and released a large mass of sediment downstream of the check dam. Then, the sediment settled in the sections between the artificial riffles, thus covering some of them. The aim of the study was to examine the effect of the system of riffles on the changes in sediment transport and water flow in the studied stream channel. Calculations indicate that the system of riffles established downstream of the check dam has limited the movement of sediment. The sediment transport is varied throughout the studied stream course and is mostly dependent on the sediment supply. [ABSTRACT FROM AUTHOR]

Extreme storms in forested environments commonly increase inputs of coarse particulate organic matter (CPOM) and large wood (LW) to streams. Protruding boulders and bedforms, mid‐channel bars, and standing trees can trap CPOM and LW. These organic accumulations can become large enough to span the bankfull channel width, or the accumulations can be predominantly along the channel margins. We refer to both types of accumulations as transient organic jams (TOJs). TOJs can create diverse geomorphic and ecological effects in channel and floodplain environments. We use data collected from mountain streams of the Luquillo Mountains of north‐eastern Puerto Rico following September 2017 Hurricanes Irma and Maria. We examine the location, characteristics, and geomorphic effects of TOJs in channel segments representing diverse drainage areas and channel gradients. We ask three questions: (a) Does the downstream spacing of TOJs correlate with variables such as drainage area or channel gradient? (b) What variables best predict the volume of organic matter within individual TOJs or within a channel segment? And (c) is there a threshold within a river network that separates channel segments with channel‐spanning versus marginal TOJs? Datasets include multiple TOJs along each of 12 stream segments and presence/absence of channel‐spanning TOJs along an additional six streams. Data analysis with multiple linear regressions indicates that downstream spacing, average volume, and total volume per channel length of TOJs correlate significantly with bankfull channel width. Using the akaike information criterion with correction (AICc) model selection method, Strahler stream order has the most influence on the probability of TOJs being marginal or spanning. [ABSTRACT FROM AUTHOR]

Boulder block ramps are river engineering structures used to stabilise river beds. Block ramps provide a semi-natural and aesthetically pleasing solution to certain river engineering problems in mountain streams. When constructing block ramps, one can use the dissipative behaviour of large macro-roughness elements randomly placed on the river bed to enhance fish migration in an upstream direction thus, in this sense, meeting the requirements of the EU Water Framework Directive. Block ramps are often designed and constructed to replace damaged drop hydraulic structures in the channels of mountain streams. This paper investigates the resilience of a particular block ramp placed in the Krzczonówka stream (Polish Carpathians) in terms of the engineering design function and its durability against damaging. A hydrodynamic analysis of a block ramp is presented before and after a flood event that changed the configuration of the blocks. The semi-natural unstructured hydraulic structure was built on the Krzczonowka stream to protect gas pipes which are located beneath it. As a result of several floods, the boulder block chute described in this paper was damaged, and some boulders were dislodged and transported downstream. Our post-flood investigations of bathymetry and velocity revealed that even damaged boulder blocks, removed from the chute and displaced downstream of the structure, still provide significant energy dissipation of the flowing water. The novel of our paper is for the first time showing very detailed analysis of unstructured block ramp hydrodynamics parameters done in the field. Also the novel finding of our investigations shows that before and after the flood event the unstructured block ramp structure, is still fish friendly in terms of hydrodynamics. [ABSTRACT FROM AUTHOR]

Check dams have been widely constructed in Thai streams and our study examined effects of dam-caused flow regime changes on aquatic insect communities. The flow regime of Huay Kaew Stream in Doi Suthep-Pui National Park, Chiang Mai, Thailand was separated into two periods: high flow and low flow periods, which characterize the natural flow dynamics in tropical streams. The sampling sites in three altitudes (1500, 900, and 500 m asl.) of the stream showed a similar trend in the effects of check dams during high flow. High discharge reduced the density of aquatic insects but this reduction was locally greater near check dams. Higher changes in water velocity at dammed sites caused greater decreases in aquatic insect community abundance because of alternating accumulation and dislodgement of fine sediment and leaf pack habitat below dams. An increase in cobble habitat because of dam erosion below a non-permanent check dam helped mitigate the effects of high flow and flow disturbance on aquatic insect density. A high density of check dams likely affects streambed structure and insect composition within the stream system. [ABSTRACT FROM AUTHOR]

This study explores how spatial dynamics and frequency of bed mobility events in a headwater stream affect the spatial and temporal variability in stream benthic algal abundance and ultimately the resilience of benthic algae to stream scouring events of different magnitudes. We characterized spatial variability in sediment transport for nine separate flow events (0.1–1.7 of bankfull flow), coupling high resolution (< 0.1 m 2 ) two-dimensional shear stress values with detailed measurements of the channel substrate. The stream bed was categorized into regions of high and low disturbance based on potential mobility of different grain sizes. High resolution (< 0.25 m 2 ), in situ measurements of benthic Chlorophyll- a concentrations (Chl- a ) were taken on 18 sampling dates before and after high flow events in regions of the streambed with contrasting disturbance to understand how benthic algal communities respond to sediment transport disturbance through space and time. According to the modeling results, the percentage of the channel likely to be disturbed varied greatly across the different flows and considered grain sizes between 7.7 and 70.4% for the lowest and highest flow events respectively. Mean shear stress in the channel bed across all sampling dates explained 49% of the variance in Chl- a . Over the 18 sampling dates — encompassing post-disturbance impacts and subsequent recovery — Chl- a differed between disturbance level categories defined based on the relative movement of the median grain size on 14 occasions. However, low disturbance locations were not always associated with higher Chl- a . The algal Chl- a biomass at any given time was a function of the stage of algal recovery following a high flow event and the magnitude of the disturbance itself — impacting algal loss during the event. Resistance of the algal communities to bed disturbance and resilience to recovery following a flow event varied spatially. Areas with low shear stress were less susceptible to scour during moderate disturbance events but were slower to recover when scour occurred. In contrast, high shear stress areas responded rapidly to flood events with rapid declines, but also recovered more quickly and appeared to have high potential for maximum accrual within our study reach. Ultimately, timing along with the inverse relationship between resiliency and disturbance frequency highlights the complexity of these processes and the importance of studying the interactions between geomorphic and ecological processes with high resolution across spatial and temporal scales. [ABSTRACT FROM AUTHOR]

Abstract: Previous measurements of bed load transport in gravel bed streams revealed a large temporal and spatial variability of bed load transport rates. Using an impact plate geophone system, continuous bed load transport measurements were made during 6 years in two mountain streams in Austria. The two streams have a snow‐melt and glacier‐melt dominated hydrologic regime resulting in frequent transport activity during the summer half year. Periods of days to weeks were identified which are associated with approximately constant Shields values that indicate quasi‐stable bed conditions. Between these stable periods, the position of the bed load transport function varied while its steepness remained approximately constant. For integration time scales of several hours to 1 day, the fluctuations in bed load transport decreased and the correlation between bed load transport and water discharge increased. For integration times of about 70–100 days, bed load transport is determined by discharge or shear stress to within a factor of about 2, relative to the 6 year mean level. Bed load texture increased with increasing mean flow strength and mean transport intensity. Weak and predominantly clockwise daily hysteresis of bed load transport was found for the first half of the summer period. [ABSTRACT FROM AUTHOR]

The purpose of the paper is to identify differences in hydraulic conditions in the areas closest to already-developed gravel bars situated in a small, mountain stream in the area of the Polish part of the Carpathian Mountains. Basic hydraulic parameters of flowing water, including velocity, shear stress, Froude number, Reynolds number and flow resistance coefficient were examined within the region of two different gravel bars in a mountain stream. At the same time, sediment samples were drawn from the riverbed in the area in which the hydraulic measurements were taken. After analysing the data, several conclusions were presented concerning sedimentation of gravel and hydraulic parameters within the cross section of a mountainous stream. The study was undertaken on Skawica-Jałowiecki Stream in the Polish part of the Carpathian Mountains called the Beskidy Mountains.

Mountain stream gravel is very often legally and illegally mined and gravel is removed from river beds sometimes on the very large scale which is disastrous for fluvial situation of rivers, for river ecology and river engineering works done for flood protection such as river revetments, bridges and all hydraulic structures. This situation makes a big problem for all river managers. Thus gravel mining of the mountain streams in the Polish Carpathians is the subject of many scientific studies when we observe river problems, but also it has a place in Slovakia. This paper deals with such problems additionally showing examples of such bad practices.