Lowland River Research Articles

Prioritisation of Barriers According to Their Impact on Migratory Fish in the Lowland River Basin District

Artificial barriers are one of the most damaging anthropogenic factors, but are also socio-economic constructions, so the decision between removing a barrier and installing a fishway must be justified. The aim was to develop a system to quantify the amount of suitable habitat for migratory fish above barriers, to assess barriers’ passability, to prioritise them in current and historical terms and to take into account the impact of the reservoir in the selection of barrier management alternatives. For this purpose, the proportion of suitable area and the potential number of spawners were calculated separately for the flooded and free-flowing river sections above the barrier. The effect of the flooded area and fishway efficiency on the potential number of individuals was assessed and the historical importance of the areas above each barrier was evaluated. The results show that the proportion of suitable area in the total area above the barriers varies between 5 and 31%. Short river stretches of high habitat quality have a higher reproductive potential than much longer stretches of lower habitat quality. Dams with fishways can still be among the most negatively impacting barriers if they are located in the migration route of fish into a large part of the basin.

Response of functional diversity of fish communities to habitat alterations in small lowland rivers

AbstractFunctional diversity has proven to be more responsive than traditional taxonomic diversity to alterations for a wide set of communities, especially in aquatic ecosystems. Habitat alterations are among the most relevant causes of biodiversity loss in freshwater ecosystems, nevertheless their effect on functional diversity has been scarcely investigated so far. In this study, functional diversity of fish communities in the River Po basin, north Italy, were related to habitat quality at different spatial scales. Land use and riparian vegetation were identified as the most influent factors on freshwater fish functional diversity. On the other hand, habitat descriptors at smaller scale did not influence functional diversity of fish. The evidenced responses were driven by the contribution of exotic species, which maximized ecological niche occupancy, either by exploiting increased habitat heterogeneity or by interaction with native species. Moreover, traits filtering, disadvantaging large predators such as salmonids, was also observed in the studied communities, but likely this effect was related to other factors than habitat alterations. Overall, this study confirms that land use changes can be considered as valid proxies for anthropogenic alterations. Moreover, possible effects at ecosystem level, also related to the spread of exotic species, could be hypothesized, giving that traits mostly influencing the observed patterns of diversity in communities were related to trophic niche, a factor that has been associated with crucial processes like nutrient cycling or energy transfer.

Detailed analysis of prey taxonomic composition indicates feeding habitat partitioning amongst co-occurring invasive gobies and native European perch

One of the negative impacts of non-native invasive species on trophic interactions in an invaded ecosystem occurs via increased interspecific competition for food resources between the invader and local species of the same food niche. In freshwaters, there are usually several fish species that feed on similar food resources. Ponto-Caspian gobies are amongst the most successful and widespread invaders colonising European waterways. They have a wide food niche and an opportunistic feeding strategy, with a focus on benthic invertebrates and piscivory occurring occasionally mainly in the case of large individuals. Competition with native percids for food resources is predicted on the basis of high dietary overlap. However, studies published so far provide no unequivocal answer. In order to resolve this question, we conducted a comparative taxonomic analysis of gut content, with an emphasis on chironomids and amphipods, of the invasive monkey goby (Neogobius fluviatilis), racer goby (Babka gymnotrachelus) and the native Eurasian perch (Perca fluviatilis) occurring sympatrically in a large lowland European river, the Bug River in Poland. We found that each species forages in slightly different habitats, as indicated by the different composition of prey species in the gut content. This suggests feeding niche partitioning between the studied species facilitating their co-existence and reduction or avoidance of competition for food resources. Resource partitioning regarding prey types and foraging habitats is a mechanism for permitting the co-existence of closely-related alien gobies with similar food preferences in the invaded waters and co-occurrence with local species. This mechanism can contribute to their invasion success, as observed in European waters during the recent decades. We also demonstrate that precise prey identification to the lowest possible taxon is crucial to reveal the dietary overlap between co-occurring fish species and to predict the impact of alien invaders on native species through interspecific competition, as well as to recommend such an approach in studies upon fish foraging strategies.

Effects of mining activities on fish communities and food web dynamics in a lowland river.

Fish communities of streams and rivers might be substantially subsidized by terrestrial insects that fall into the water. Although such animal-mediated fluxes are increasingly recognized, little is known about how anthropogenic perturbations may influence the strength of such exchanges. Intense land use, such as lignite mining, may impact a river ecosystem due to the flocculation of iron (III) oxides, thus altering food web dynamics. We compared sections of the Spree River in North-East Germany that were greatly influenced by iron oxides with sections located downstream of a dam where passive remediation technologies are applied. Compared to locations downstream of the dam, the abundance of benthic macroinvertebrates at locations of high iron concentrations upstream of the dam was significantly reduced. Similarly, catch per unit effort of all fish was significantly higher in locations downstream of the dam compared to locations upstream of the dam, and the condition of juvenile and adult piscivorous pike Esox lucius was significantly lower in sections of high iron concentrations. Using an estimate of short-term (i.e., metabarcoding of the gut content) as well as longer-term (i.e., hydrogen stable isotopes) resource use, we could demonstrate that the three most abundant fish species, perch Perca fluviatilis, roach Rutilus rutilus, and bleak Alburnus alburnus, received higher contributions of terrestrial insects to their diet at locations of high iron concentration. In summary, lotic food webs upstream and downstream of the dam greatly differed in the overall structure with respect to the energy available for the highest tropic levels and the contribution of terrestrial insects to the diet of omnivorous fish. Therefore, human-induced environmental perturbations, such as river damming and mining activities, represent strong pressures that can alter the flow of energy between aquatic and terrestrial systems, indicating a broad impact on the landscape level.

Monitoring surface-water connectivity in a coastal lowland river using field-based data loggers

The concept of hydrologic connectivity has commonly been used as an indicator of river habitat health and considered in environmental flow policies and river management schemes. Field-based data loggers, used to infer flow intermittency, have been used to study the connectivity in ephemeral channels in a variety of physiographic environments; however, no previous studies have utilized and analyzed them in portions of perennial channels and floodplains that experience inundation intermittently. To address this gap, we developed an array of Stream Temperature, Intermittency, and Conductivity Loggers (STICL) and deployed them in a coastal lowland river to analyze temporal changes to hydrologic connectivity induced by surface water in the longitudinal, lateral, and vertical dimensions of the channel and the floodplain. We examine the efficacy of STICLs to monitor surface-water connectivity within the floodplain that experience intermittent inundation. We also examine influence of hydrologic regime and geomorphology on the variations in connectivity. Our results show that the electrical conductivity (EC) records from our STICLs can capture flood inundation dynamics and how these dynamics affect surface-water connectivity. Our estimates of flood inundation duration and the dynamics of surface water connectivity can be improved by better accounting for how EC can vary with temperature and by handling the uncertainty in our STICLs readings in a more explicit manner. The STICLs provide a promising tool to study river hydromorphological processes in detail. Thus, we recommend the development of formal approaches and frameworks for deploying and analyzing these types of data loggers within lowland landscapes.

Sensitivity of a meandering lowland river to intensive landscape management: Lateral migration rates before and after watershed-scale agricultural development

Agricultural development has transformed the vegetation cover of many landscapes around the world, thereby altering water and sediment fluxes to river systems. Past work in the upper midwestern United States, particularly in areas of moderate relief, has shown that increases in water and sediment fluxes associated with agricultural development have dramatically altered river dynamics. Less is known about how agriculture has affected river dynamics, particularly rates of lateral migration, in relatively low relief landscapes of the Midwest shaped by glaciation during the Wisconsin Episode. This research examines rates of lateral migration of a channel bend along a lowland meandering river in Illinois, USA before and after agricultural development. The rate of lateral migration prior to agricultural development is estimated through dating of carbonaceous material within lateral-accretion deposits underlying distinct meander scrolls. The rate of lateral migration after agricultural development is determined from analysis of changes in river-channel position determined from survey records, aerial imagery, and digital elevation data. Average rates of migration before and after agricultural development are similar, suggesting that agricultural development has not substantially affected rates of lateral migration of the river. Some accelerated movement occurred locally following agricultural development, but this movement cannot be definitively tied to landscape transformation. Possible factors responsible for the lack of sensitivity of the river system to agricultural development include high resistance of the cohesive, tree-lined riverbanks to erosion and the low bankfull stream power per unit area of the modern river. From a management perspective, the study highlights the importance of bank vegetation in maintaining channel stability in low-relief agricultural landscapes.

Water Quality Monitoring in the Volga Headwaters

The Volga River is the largest river in Europe in terms of basin area (1.36 mio. km2), length (3531 km), and water content (annual flow 254 km3). We conduct long-term water quality studies in the Volga headwaters: in the Upper Volga Lakes, the free-flowing section, as well as in the Ivankovskoye and Uglich Reservoirs. At the source of the river, the water is very soft, slightly mineralized, characterized by high color indicators (up to 400 degrees Pt-Co scale) and permanganate oxidation (60.3–72 mgO/L). In the Upper Volga Lakes, water mineralization increases to 110 mg/L, and the color of the water decreases significantly. In the Ivankovo Reservoir, higher concentrations of manganese and BOD5 are noted than in the Volga above Tver. Based on total phosphorus concentrations, the Upper Volga Reservoir (Lake Volgo) and the free-flowing section of the Upper Volga (from Selizharovo to Tver) belong to the “mesotrophic” class, and the Ivankovskoye Reservoir belongs to the “eutrophic” class. We characterize the Volga headwaters based on physico-chemical parameters “balanced” and discuss the relevance of “natural pollution”. Especially the free-flowing section comprises an important reference section for lowland rivers; thus, long-term monitoring of abiotic and biotic aspects is an important issue.

Timescale of the Morphodynamic Feedback Between Planform Geometry and Lateral Migration of Meandering Rivers

AbstractAcross varied environments, meandering channels evolve through a common morphodynamic feedback: the sinuous channel shape causes spatial variations in boundary shear stress, which cause lateral migration rates to vary along a meander bend and change the shape of the channel. This feedback is embedded in all conceptual models of meandering channel migration, and in numerical models, it occurs over an explicit timescale (i.e., the model time step). However, the sensitivity of modeled channel trajectory to the time step is unknown. In numerical experiments using a curvature‐driven model of channel migration, we find that channel trajectories are consistent over time if the channel migrates ≤10% of the channel width over the feedback timescale. In contrast, channel trajectories diverge if the time step causes migration to exceed this threshold, due to the instability in the co‐evolution of channel curvature and migration rate. The divergence of channel trajectories accumulates with the total run time. Application to hindcasting of channel migration for 10 natural rivers from the continental US and the Amazon River basin shows that the sensitivity of modeled channel trajectories to the time step is greatest at low (near‐unity) channel sinuosity. A time step exceeding the criterion causes over‐prediction of the width of the channel belt developed over millennial timescales. These findings establish a geometric constraint for predicting channel migration in landscape evolution models for lowland alluvial rivers, upland channels coupled to hillslopes and submarine channels shaped by turbidity currents, over timescales from years to millennia.

Juvenile Pacific salmonid habitat use in two Puget Sound lowland rivers

AbstractObjectiveLarge rivers are complex, productive environments that support numerous species. However, humans have extensively modified these ecosystems, contributing to the decline of Pacific salmonid Oncorhynchus spp. populations. Salmon recovery efforts rely upon an understanding of salmonid habitat needs at different life stages, but data on juvenile salmonid habitat use within large rivers are rare due to the challenges of sampling in large rivers. To help fill this information need, we used a cataraft‐mounted electrofisher to evaluate juvenile salmonid use of natural (bar, backwater, side channel, and unarmored bank) and human‐modified (riprap‐armored bank and biorevetment bank [armored banks with added wood]) channel edge habitats in the Snoqualmie and Green rivers within the Puget Sound region of Washington State.MethodsWe electrofished over 1000 25‐m transects across seven spring rearing seasons (2016–2022) and measured transect water depths and widths of low‐velocity habitat (≤0.45 m/s).ResultBars, backwaters, and side channels were shallower and had wider low‐velocity habitat than armored banks. River flow also influenced water depth and low‐velocity width, but the relationship varied depending on the habitat type. Subyearling Chinook Salmon O. tshawytscha and subyearling Coho Salmon O. kisutch occurred more frequently and were more abundant along natural edge habitats than along armored banks, while their use of biorevetment banks was intermediate between their use of natural edge habitats and armored banks. In contrast, yearling Coho Salmon and trout occurred more frequently and were more abundant along armored, biorevetment, and unarmored banks than along bars and backwaters, but they also used side channels extensively. Habitat use shifted throughout the spring, but specific shifts varied by species or life history stage.ConclusionThese results emphasize the importance of conserving natural edge habitats, removing bank armoring, and reconnecting or restoring side channels. Additionally, our results highlight that a diversity of habitat types and conditions is necessary to support multiple species and life history stages.

Microplastic Contamination of Fine-Grained Sediments and Its Environmental Driving Factors along a Lowland River: Three-Year Monitoring of the Tisza River and Central Europe

The hydro-geomorphological background in microplastic (MP) deposition and mobilization is often neglected, though the sampling environment is the key point in a monitoring scheme. The aim of the study was to analyze the environmental driving factors of MP transport over three years (2020–2022) along a 750 km-long section of the Tisza River, Central Europe. The mean MP content of the fresh clayey sediments was 1291 ± 618 items/kg in 2020, and then it decreased (2021: 730 ± 568 items/kg; 2022: 766 ± 437 items/kg). The upstream and downstream sections were the most polluted due to improper local sewage treatment. In 2020, 63% of the sites were hotspot (≥2000 items/kg), but their number decreased to one-third in 2021 and 2022. MP pollution is influenced by highly variable environmental factors. (1) The geomorphological setting of a site is important, as most of the hotspots are on side bars. (2) The tributaries convey MP pollution to the Tisza River. (3) The bankfull or higher flood waves effectively rearrange the MP pollution. (4) The dams and their operation influence the downstream trend of MP pollution in the reservoir. (5) Downstream of a dam, the clear-water erosion increases the proportion of the pristine sediments; thus, the MP concentration decreases.

Spatial Distribution Patterns of Zooplankton and Macroinvertebrates in a Small River under Strong Anthropogenic Pressure

The main objective of this study was to examine the spatial distribution patterns of aquatic invertebrates in an environment characterized by significant anthropogenic stress. During the entire research period, at all the sites, we noticed 72 taxa of zooplankton and 30 taxa of macroinvertebrates. Variation partitioning analysis reveals that the assemblages of planktonic organisms are much more determined by the time of sampling rather than by the site, in contrary to macroinvertebrates where the sampling time was only slightly more responsible for the composition of the benthic communities than the site. Spatial distribution of aquatic organisms in a small lowland river under strong anthropopressure shows significant deviations from the expectations of the River Continuum Concept (RCC). The benthic macroinvertebrate and littoral zooplankton communities exhibited a strong association with local site conditions, while, in contrast, pelagic zooplankton exhibited a strong dependency on drift and its production in the upper reaches of the river, leading to relatively consistent compositions downstream despite the highly altered river environment. To improve the biodiversity values and ecological state of a river, restoration treatments of bed and shore zones are required.

A comprehensive review of the biodiversity of freshwater fish species in Valleys worldwide and in the Kingdom of Saudi Arabia.

An overview of freshwater fish variety worldwide and the variables influencing trends in variation between and within river basins are given in this review. Continental freshwater ecosystems are highly diverse and species-rich, housing nearly 18,000 species of fish (=50% of all fish species) in <0.5% of the total land area and providing a negligible (<0.01%) share of the planet's water supply. Large lowland tropical river basins such as the Amazon, Congo, and Mekong basins are home to the greatest freshwater fish diversity. Freshwater species of fish depth variation at the global magnitude is correlated with the total amount and variation of aquatic habitats and the environment's equilibrium overtime during the evolution of scales. The river continuum concept states that there is a predictable shift in fish species depth, diversity of species, and functional characteristics along gradients of environment from headwater to estuary. The ongoing trade of minerals and organic matter related to nearby floodplains is a strong factor in the number and variety of riverine fishes in most parts of the world (the flood pulse concept). Without coordinated conservation efforts, freshwater fishes will suffer significant losses in abundance and diversity due to the numerous threats they currently face worldwide. However, further development, adaptation, training, and guidance are needed. New technologies based on water conservation, suitable species, and local traditions are needed. Waste materials and local feed additives can also be used. Farmers should be provided with the necessary training and information.

Changes in Phytoplankton of the Irpin River (Ukraine) as a Result of Military Activities in Its Basin

Phytoplankton structure was studied in a comparative aspect (before and after the beginning of military activities) in the section of the Irpin River belonging to a large lowland rivers flowing in silicate rocks. On the whole, 132 algal species of seven divisions were found in the water column of the Irpin River during the period of investigations. Bacillariophyta and Chlorophyta were highly diverse in their species composition (37.1 and 31.1&amp;#37; of the total number of species). The distribution of plankton algae at the studied stations even within the same massif of surface waters was non-uniform in terms of species composition (the Sorensen coefficient of community similarity on the average was 37&amp;#37;), species richness (18-49 species), cell numbers (1930 thousand cells/L - 24,450 thousand cells/L), and biomass (2.118 mg/L - 20.374 mg/L). Almost different algal species dominated at various stations (14 species among 20 dominated only at one station). Compared to the previous period of investigations (2018), the number of plankton algal species in the studied section of the Irpin River somewhat increased (from 121 to 132). Phytoplankton taxonomic structure also somewhat changed. The contribution of Bacillariophyta increased from 22.3 to 37.1&amp;#37; and that of Cyanoprokaryota - from 9.1 to 12.1&amp;#37;. In this case, the contribution of Chlorophyta decreased from 47.1 to 37.1&amp;#37;. The quantitative indices of phytoplankton development increased. The maximum numbers increased from 19,480 thousand cells/L to 24,450 thousand cells/L, whereas the maximum biomass - from 5.143 mg/L to 20.374 mg/L. As a consequence of the impact of military activities on phytoplankton of the Irpin River, it is possible to consider an increase in the number of its species, the maximum numbers and biomass, as well as the contribution of water bloom forming blue-green algae in the estuarine section of the Irpin River due to the disturbance of hydromorphology and the income of an additional amount of nutrients from flooded territories, which can lead to the formation of such a negative phenomenon as water bloom in this section of the river.

Lateral meander migration of a medium-sized lowland river: Case study on the Rába river, Hungary

Engineering works greatly influence the lateral channel migration (LCM) of meandering rivers. We aimed to characterise the spatiotemporal characteristics of LCM during the last 174 years of the almost freely meandering Upper Rába (Hungary) and to identify units with distinctive LCM histories. The studied Rába's reach has been regulated just at some points. Due to the free meandering, its length varied between 119 and 133 km. The most intensive length increase (+291 m/y) took place between 2005 and 2008, and by the end of the process , the sinuosity had reached its historical maximum (SI 2018 = 1.93). The mean LCM was 3.3 m/y (1844-2018). The periods with intensive channel migration (max: 24-27 m/y) were followed by periods with low migration rates. Based on local channel morphology and LCM rates, 14 units were identified. The highest LCM rate was measured in the freely migrating units (R5: 5.8 m/y; R3: 4.4 m/y and R6 4.0 m/y).

Is it worth the extra mile? Comparing environmental DNA and RNA metabarcoding for vertebrate and invertebrate biodiversity surveys in a lowland stream.

Environmental DNA (eDNA) metabarcoding has emerged as a promising approach to assess biodiversity and derive ecological status classes from water samples. However, a limitation of eDNA surveys is that detected DNA molecules may originate from other places or even dead organisms, distorting local biodiversity assessments. Environmental RNA (eRNA) metabarcoding has recently been proposed as a complementary tool for more localized assessments of the biological community. In this study, we evaluated the effectiveness of eDNA and eRNA metabarcoding for inferring the richness and species distribution patterns of vertebrates and invertebrates in a Central European lowland river. We collected water samples and analyzed them using a 12S marker for vertebrates and a COI marker for invertebrates. We detected 31 fish, 16 mammal, 10 bird and one lamprey species in the vertebrate dataset. While results were largely consistent, we detected a higher number of species when analysing eRNA (mean = 30.89) than eDNA (mean = 26.16). Also, eRNA detections had a stronger local signature than eDNA detections when compared against species distribution patterns from traditional fish monitoring data. For invertebrates, we detected 109 arthropod, 22 annelid, 12 rotiferan, eight molluscan and four cnidarian species. In contrast to the pattern of vertebrate richness, we detected a higher richness using eDNA (mean = 41.37) compared to eRNA (mean = 22.42). Our findings primarily show that eDNA and eRNA-based detections are comparable for vertebrate and invertebrate taxa. Biological replication was important for both template molecules studied. Signal detections for vertebrates were more localized for eRNA compared to eDNA. Overall, the advantages of the extra steps needed for eRNA analyses depend on the study question but both methods provide important data for biodiversity monitoring and research.

Hydrology as a Determinant of Riparian Habitat Structure in Lowland River Floodplains

The objective of the study was to determine the relationship between the structure of phytocenoses in riparian wetland ecosystems and the hydrologic regime in a lowland river floodplain. The hydrobotanical study was conducted over three years—2017, 2018, and 2019—which differed in hydrological conditions (wet, average, and dry) in a middle section of the Supraśl floodplain (NE Poland) as a case study. The results showed that the structure and pattern of phytocenoses in the floodplain are primarily controlled by the hydrological regime of the river and the geomorphological features of the area. The reach and duration of the flood contributed to a specific pattern of riparian vegetation. Based on the plant community structure and riparian habitat indicators such as soil moisture, fertility, reaction pH, soil granulometry, and organic matter content, four habitat types were identified and supported by linear discriminant analysis (LDA): wet, semi-wet, semi-dry, and dry zones. The indicator species analysis (ISA) revealed species characteristic of the zones with the dominance of reed rush, reed canary grass, anthropogenic or partially natural herbaceous communities along watercourses or riparian meadows, respectively. Natural inundation of the river water is an important driver of site-specific vegetation elements and habitat types and determines habitat availability, biodiversity, and ecosystem functions of wetlands. This knowledge can serve as the basis for conservation efforts, sustainable management practices, and decision-making processes aimed at maintaining the biodiversity and ecological integrity of riparian ecosystems in similar regions.

Dominant flood types in mountains catchments: Identification and change analysis for the landscape planning

The classification of floods may be a supporting tool for decision-makers in regard to water management, including flood protection. The main objective of this work is the classification of flood generation mechanisms in 28 catchments of the upper Vistula basin. A significant innovation in this study lies in the utilization of decision trees for flood classification. The methodology has so far been applied in the Alpine region. The analysis reveals that peak daily precipitation in the catchments mainly occurs in summer, particularly from June to August. Maximal daily snowmelt typically happens at the end of winter (March to April) and occasionally in November. Winter peaks are observed in March to April and, in some areas, in November to December, while summer peaks occur in May and, in specific catchments, in October. Higher peak flows for annual floods are noted in March to April and June to August. Most annual floods in the Upper Vistula basin are classified as Rain-on-Snow Floods (RoSFs) or Lowland River Floods (LRFs). LRFs contribute from 19% to almost 72%, while RoSFs range from 18% to 75%. In Season 1 (summer), most seasonal floods are identified as LRFs (51%–100%), with very few as RoSFs (0%–46.9%). In Season 2 (winter), the opposite pattern is observed, with most RoSFs (48.4%–97.9%) and fewer LRFs (0%–20.6%). While there are changes in flood patterns, they are not statistically significant. Conducted studies and obtained results can be useful for the preparation of flood prevention documentation and for flood management in general.

Effect of Teleconnection Patterns on the Formation of Potential Ecological Flow Variables in Lowland Rivers

Climate is probably the most important factor affecting river discharge and flow dynamics. Low flows in rivers during the warm period cause stress to aquatic ecosystems and pose a challenge to sustainable water management. Previous research has shown that the average minimum discharge of the 30 driest continuous days, known as Q30, is a suitable measure for ecological flow estimation in Lithuania. This study aims to examine whether large-scale atmospheric processes, so-called teleconnections, can have an impact on Q30 during the warm period. Hydrological data for 1961–2020 from 25 water gauging stations were used to search for hydrological response signals with five selected climate indices (NAO, SCA, POL, EA/WR, and EA). Pearson correlation and Wilcoxon–Mann–Whitney test approaches were applied. The results suggested that the EA/WR and NAO had the strongest influence on Q30 in the studied region during the warm period. The positive phases of the indices tended to cause a greater decrease in Q30 values due to the prevailing easterly edge of the anticyclonic circulation over the studied region determined by the EA/WR and NAO indices, while the negative phases of the mentioned indices caused an increase and greater dispersion of Q30.

Coexistence of two dune scales in a lowland river

Abstract. A secondary scale of bedforms, superimposed on larger, primary dunes, has been observed in fluvial systems worldwide. This notwithstanding, very little is known about the morphological behavior and characteristics of this secondary scale. This study aims to better characterize and understand how two dune scales coexist in fluvial systems and how both scales adapt over time and space, considering their interdependence. The study is based on analysis of a large biweekly multibeam echo sounding dataset from the river Waal, a lowland sand-bedded river. Results reveal that the secondary dune scale is ubiquitous across space and time and not limited to specific flow or transport conditions. Whereas primary dunes lengthen during low flows, secondary dune height, lee slope angle, and length correlate with discharge. Secondary dune size and migration strongly depend on the primary dune lee slope angle and height. Secondary dunes can migrate over the lee slope of low-angled primary dunes, and their height is inversely correlated to the upstream primary dune height and lee slope angle. In the Waal river, a lateral variation in bed grain size, attributed to shipping, largely affects dune morphology. Primary dunes are lower and less often present in the southern lane, where grain sizes are smaller. Here, secondary bedforms are more developed. At peak discharge, secondary bedforms even become the dominant scale, whereas primary dunes entirely disappear but are re-established during lower flows.

A large-scale passage evaluation for multiple fish species: Lessons from 82 fishways in lowland rivers and brooks

River networks worldwide are highly fragmented and this is particularly prevalent in the human-dominated lowland landscape of the Netherlands. Fishways function as measures to facilitate passage alongside barriers for fish communities and have been evaluated mostly in single case studies. We compared fish monitoring data of 82 fishways with data of fish observed in adjacent water stretches, conducting the first nationwide study on passage for the full spectrum of fish species present. In total, 35 out of 38 (92%) native species recorded in fishways' surroundings used fishways, while per fishway the median was 59% related to a variety of factors (fish behaviour, fishway type, monitoring design). The species using fishways most frequently were perch Perca fluviatilis (71/78 of fishways present), roach Rutilus rutilus (70/79) and gudgeon Gobio gobio (68/77). Logistic regression models showed the effect of monitoring duration and timing in detecting specific fish species ascending fishways. This large-scale analysis highlights the need to consider all native fish species during the design and monitoring of fishways. The obtained information from this study can be used by water managers for improving monitoring schemes and river connectivity which is an essential component for achieving the European Water Framework Directive goals.