Although microplastics (MPs; < 5 mm) have been recognized as one of the most challenging environmental pollutants in the ocean, our understanding of the environmental fate of freshwater MPs falls far ...behind, particularly on smaller MPs (<0.1 mm). Here, we seek to reveal the latest MP pollution status in the largest freshwater lake of China, Poyang Lake, by comprehensively assessing the abundance, distribution, size, shape, polymer composition, and micro-morphology of MPs in water and sediment, covering a large geographic area of the Lake and its five main river tributaries. High levels of MPs were detected in water (up to 1064 ± 90 MP/m3) and sediment (up to 1936 ± 121 MP/kg), with the highest concentrations in the Gan River and the lowest in the national Nature Reserves. While a positive correlation was identified between MP abundance in water and sediment, the size distribution of MPs in between water and sediment was distinct. The dominant MP form in sediment and water was fragment and fiber, respectively. Infrared spectroscopy analysis confirmed the dominant polymer types including polypropylene, polyvinyl chloride, polyethylene, polystyrene, and polyvinyl alcohol. Moreover, both μ-FTIR and SEM results suggested significant features of weathering and fragmentation of MPs. This study provides comprehensive data to understand the environmental behavior and pollution magnitude of MPs in China’s largest freshwater lake and highlights the significant contribution of smaller-size fractions (0.03–0.1 mm) to improve future MP studies in freshwater systems.
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•Microplastics (MPs) are abundant in China’s largest freshwater lake.•The abundance of MPs in water and sediment samples is positively correlated.•The shape and size profiles of MPs in water are different from those in sediment.•Fragment, film, and fiber MPs composed of PP, PVC, PE, and PS are dominant.•SEM and μ-FTIR results indicate significant weathering and fragmentation of MPs.
Freshwater mussels are among the world’s most imperilled species, and much effort has been expended to understand their precipitous decline. The current paradigm is that relative river bed stability ...over decades is critical to maintaining mussel beds at a given river reach. Such information, however, is in stark contrast to the fundamental understanding of self‐formed rivers.
Here, we examine the relationship between long‐term occurrence of aggregated freshwater mussel communities (mussel beds) and the dynamic rivers in which they live.
We surveyed mussel assemblages in two streams with historical records to document their long‐term persistence, and we simulated bed mobility with a two‐dimensional flow and sediment transport model to estimate bedload transport during channel‐forming discharges.
We found evidence of long‐term mussel bed persistence, in time and in kind, within these two streams and that significant bed mobility within mussel beds occurs every 1–2 years, as to be expected for self‐formed rivers with labile beds. Flow refugia, or bed immobility, thus cannot completely explain the persistence of these mussel beds.
Our results suggest that mussels have adapted to proliferate in river channels that often experience significant bed mobility, but further studies are needed to understand the biophysical mechanisms enabling the long‐term persistence of mussel communities.
Hyporheic exchange in riverbeds is driven by current‐bed topography interactions. Because riverbeds exhibit topographic roughness across scales, from individual grains to bedforms and bars, they can ...exhibit fractal patterns. This study analyzed the influence of fractal properties of riverbed topography on hyporheic exchange. A set of synthetic fractal riverbeds with different scaling statistics was used as inputs to sequentially coupled numerical simulations of turbulent channel flow and hyporheic flow. In the analysis, the maximum power spectrum (dune size) and the fractal dimension (topographic complexity) were considered as independent variables and we then investigated how interfacial fluxes and hyporheic travel times are functionally related to these variables. As the maximum power spectrum increases (i.e., dune height to flow depth ratio), the average interfacial flux increases logarithmically whereas it increases exponentially with an increase in fractal dimension. Hyporheic exchange is more sensitive to additional roughness (larger fractal dimensions) than to bedform size (larger maximum power). Our results imply that fractal properties of riverbeds are crucial to predicting hyporheic exchange. The predictive relationships we propose could be integrated with reduced complexity, large‐scale models. They can also be used to design artificial topographies that target hyporheic ecosystem services.
Key Points
A series of numerical simulations were conducted to explore how the fractal properties of bedforms are related to hyporheic exchange
The results show that hyporheic exchange flux increases exponentially with respect to the bed topography fractal dimension
Fractal properties of riverbeds are crucial to predicting hyporheic exchange
Grade control structures (GCSs) are one of the most often used hydraulic structures in a river regulation in mountain catchments. The purpose of their use is to reduce the river bed gradient and ...prevent downcutting. The aim of this study is to evaluate the influence of GCS on changes in river bed morphology. The study was conducted in a downstream stretch (5.66 km long) of the Carpathian river – the Mszanka, regulated with 25 GCSs. Research has shown that erosion is here a predominant process despite the use of GCSs. The river bed has been cut into bedrock along approximately 40% of a studied stretch. A total of 56 scour holes were identified, which vary substantially in terms of their depth, length, and mechanism of formation. The erosion depth depends mainly on the length of reaches between GCSs and on the water surface gradient. A surprising research discovery was scour holes found upstream of the crests of the GCSs – 16 such scour holes were found. The formation of these scour holes is explained through the drawdown profile characterized by the increasing of the velocity that favored the bed erosion. The studied river is characterized by deficit of sediment. The insufficient sediment supply and imbalance between the river's load and sediment transport are two of the greatest problems affecting the most Carpathian rivers. The novelty of this research is the analysis of such a long and complex reach of mountain river engineered with GCSs in terms of variances in erosion processes. Also a new knowledge presented here is an information about scour holes located upstream of GCSs and explanation of the mechanism of their formation. Results from this research could serve geomorphologists, engineers and ecologists as well as help river managers in decision-making processes when river regulation is planned.
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•Mountain river regulated with grade control structures (GCSs) was examined.•Erosion is a predominant process in studied river bed despite the use of GCSs.•A surprising phenomenon is the occurrence of scour holes upstream of GCS crests.•The mechanism of the formation of scour holes upstream of GCS crests was analyzed.•The main problem in maintaining river with GCSs is deficit of river sediments.
Rivers are one of the main sources to supply sand and gravel for construction projects. Depending on river morphology and hydraulic characteristics, its sediment transport capacity, and mining ...operation method, the extraction of river bed materials may affect its ecosystem through bank and bed erosion. To advance the mechanisms of river pit infilling, the effects of various parameters (i.e., the distance between pits, the pit plan shape, the pit depth, sediment size, and approaching flow velocity) on pit infilling volume are investigated in this research. The results of this research show that infilling volume of upstream pit is insignificantly affected by the distance between the pits, and it is completely refilled for different distances. However, the infilling volume of downstream pit decreases by increasing the distance between the pits. In addition, by reducing the ratio of pit length to its width (pit shape extension in spanwise direction), the pits can be excavated in a shorter distance from each other; when this ratio decreases by 15%, the infilling volume increases up to 30%. Subsequently, as a cost-effective option, the pit distance can be reduced up to 50% in these conditions. According to the obtained results, although the sediment size has negligible effect on infilling volume in the studied range, the infilling volume increases up to 20% by an increase of 8% in the approaching flow velocity. Increasing the ratio of pit length to its width (pit shape extension in streamwise direction) highlights the effectiveness of smaller depths, so that the infilling volume increases up to 20% by a decrease of 20% in the pit depth. In this regard, it is recommended that the pit depth be restricted to 70% of the channel flow depth to have a complete pit refilling.
The Xiangjiaba and Xiluodu reservoirs, located on the upper Yangtze River, have been operational and the changes in the downstream river regime and waterway conditions have attracted great attention. ...A two-dimensional horizontal (2DH) flow–sediment mathematical model of the Shuifu to Lanjiatuo reach (Shuilan reach) downstream of the Xiangjiaba Dam was developed. The spatio-temporal characteristics of sediment scouring and siltation after completion of the cascade of reservoirs were simulated using the model, and then the river regime changes in several typical reaches as well as their possible impacts on navigation channels were analyzed. The results show that, after operation of the cascade of reservoirs, the downstream channel is in the state of scour. The general change of the river regime is that the flow in curved reaches tends to be straight though the river pattern still remains curved. Sub-branches tend to experience deposition, while the main channel will be scoured deeper, which can help to increase the navigation depth in the shoals in the transition sections. Deposition in some sections could result in new shoals. In addition, the bottom elevation difference between floodplains and channels will increase, which could aggravate local adverse flow patterns; therefore, navigation conditions in torrential shoals will tend to deteriorate.
Summary
Many rivers are oversaturated in methane (CH4) and carbon dioxide (CO2) relative to the atmosphere, but we know little about the biological controls on the balance between these two important ...greenhouse gases and how they might respond to warming.
We characterise the potential response to temperature in the biological production of CO2 and CH4 and the subsequent microbial oxidation of that CH4, that is the sink and source components of the CH4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels.
In the fine sediments, anaerobic production of both CH4 and CO2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH4:CO2 production for a 1 °C increase in temperature.
In the coarse gravels, microbial CH4 oxidation showed no response to temperature at CH4 concentrations characteristic of these gravel beds (30–200 nmol CH4 L−1), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH4 concentrations, more characteristic of the fine sediment patches (2–4 μmol CH4 L−1), CH4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH4 production.
In the fine sediment, the surface layers had a CH4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH4 concentrations meant CH4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels.
The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO2 to CH4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH4 to CO2 are unlikely to be due to the incapacity of methanotrophy to respond to CH4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer.
The development of methodologies for analyzing the evolution and pressures exerted on the river channel network is one of the main concerns of researchers today. The assessment of natural or ...artificial changes of river channels and beds plays an important role in environmental protection, but also in the implementation of integrated water resource management plans. Given the episodic and dynamic nature of river bank and bed erosion, along with the difficulty of reaching certain reaches, a methodological approach that uses aerial imagery, initially from satellite sources and afterwards from unmanned aerial vehicles, is proposed. This approach was utilized in a perennial river in Romania but also in an ephemeral torrent channel in Greece, in order to test the prevalent types of hydrographic network in the Mediterranean and Black Sea region. The methodology used was able to identify the location and the volume of the bed material extracted and the time frame in which it occurred. These encouraging results showcase an accurate but also relevantly low-cost monitoring method for illegal anthropogenic activities that can be easily adopted by the responsible authorities. The adoption of the method will contribute to the more efficient monitoring of river protection, by accurately and timely identifying areas of illegal river bed extraction that will enable authorities to enforce European Union and national legislation.
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