Meandering Research Articles

:Ausgrabungen in der frühbronzezeitlichen Siedlung im Heraion von Samos 1966

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On the Morphodynamics of a Wide Class of Large‐Scale Meandering Rivers: Insights Gained by Coupling LES With Sediment‐Dynamics

AbstractIn meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications.

Sedimentary Filling Evolution under Paleoclimate Transition—A Case Study from the Middle Jurassic Zhiluo Formation, Ordos Basin

Under a specific tectonic background, the change in paleoclimate can show different facies associations and depositional architecture. The Jurassic China continent was an important region for transforming the Paleotethys tectonic domain to the circum-Pacific tectonic domain, and its paleoclimate information was entirely preserved in the continental sedimentary successions. The Middle Jurassic Zhiluo Formation, in the Ordos Basin, was at just the critical period of paleostructure and paleoclimate transition, preserving considerable sedimentological evidence; however, little sedimentological research has been conducted under the transition. This study reconstructed the sedimentary filling characteristics of the Zhiluo Formation under paleoclimate transition based on field outcrop survey, thin section observation, geochemical indices, stratigraphic correlation, and depositional environment analysis. The results showed that with the paleoclimate change from warm and humid to hot and arid, the sedimentary facies of the Zhiluo Formation were characterised by the change from the braided river/braided river delta in the J2z1-1 sedimentary period to the meandering river/meandering river delta in the J2z1-2 sedimentary period, and finally the change to the meandering river, meandering river delta, and lacustrine in the J2z2 sedimentary period. The combined action of the southern super monsoon effect, the increasing global CO2 concentrations, the moving southward of the East Asian block, and the terrain elevation difference, changed the basin’s climate from warm and humid to hot and semi-arid/arid. This study provides a crucial basis for reconstructing the interplay between paleoclimate and paleotectonics, and guiding sedimentology and paleoenvironment research on East Asia during the Middle Jurassic period.

Experimental Study on Neck Cutoff in Meandering River under Variable Discharges

Neck cutoff is a sudden phenomenon that occurs during the evolution of meandering rivers, but its process and mechanism remain unclear. In this investigation, the neck-narrowing process before cutoff, the formation of a new river, and the appropriate conditions for cutoff under constant and variable discharges were studied using experimental methods in a laboratory flume. The results indicated that bank erosion upstream and downstream of the neck was the main factor that triggered cutoff, regardless of whether the discharge changed. The sinuosity of the model river was approximately 7, the ratio of the neck width to the average river width reached approximately 0.43, and the experimental material was nonuniform natural sand. Under the conditions that the unit width discharge ranged from 0.0039 to 0.0069 m2/s and the longitudinal gradient was 1‰, neck cutoff could occur after sufficient bank erosion. When neck cutoff was imminent, the sinuosity of the channel increased and the channel rotated downstream. After neck cutoff, the formation of a new river experienced three stages, namely, the short-term longitudinal scouring stage, the long-term horizontal widening stage, and the long-term transverse widening and normal slow development stage. The discharge resulting in neck scouring was the effective discharge contributing to the cutoff, and this discharge directly affected the cutoff time and position. We attempted to explain the factors leading to neck cutoff from an energy consumption perspective. Neck cutoff was a sudden phenomenon that occurred when the energy consumption of a meandering river increased to its maximum value upon entry to the recession stage from the relative equilibrium stage. To a certain extent, this study is significant in understanding river evolution and neck cutoff.

Vulnerability of Nyabarongo River Water Abstraction to Climate Change in Rwanda

Purpose: Climate change in Rwanda is a great challenge resulting from increased temperature and causing abnormal variability in weather variables and threatening the socio-economic welfare of human beings. Water resources have been subjected to degradation and pollution due to intensive and frequent rainfall that have negatively impact on water abstractions in their proximity. This study aimed at analyzing the vulnerability of the Nyabarongo river water abstraction to climate change.&#x0D; Methodology: GIS and remote sensing were used to map and analyze possible changes in geophysical status of the river. Secondary data collection on temperature and rainfall for a forty years period was used to analyze the trend in climate change in Rwanda.&#x0D; Findings: The results indicated that climate change in Rwanda has resulted in extreme weather events that resulted in inundations and floods of Nyabarongo River and its wetland that exacerbated the changes in geophysical status such as river flow pathway and meandering. This has resulted in washing away of some water abstraction facilities around the river and the distancing from such facilities leading to interference with abstraction. It was also found out that dry season’s markedly diminished water available for abstraction in the river and impacting negatively on the groundwater recharge.&#x0D; Unique Contribution to Theory, Practice and Policy: The findings can stimulate decision makers in water resources management to initiate and roll out similar studies countrywide and serve as baseline to adopt measures and strategies for the sustainable management of the resources. The findings also call for Water users to take into consideration of climate impacts during the studies, designs and implementation of water use project for the sustainable exploitation of water resources.

Large wood retention in a large meandering river: Insights from a 5-year monitoring in the Odra River (Czechia)

Large wood (LW) is an integral component of watercourses. However, we still need to gain more knowledge of the geomorphic processes related to LW in large lower-energy, meandering rivers due to the limited number of field studies dealing with LW transport and retention in these widespread fluvial systems. This study aims to detect the predictors of the interannual variability of LW during a period of five years in a meandering river wider than the height of riparian trees. We employed four LW inventories between 2016 and 2021 in a 3.65 km long active meandering reach of the Odra (Oder) River, Czechia. We found a negligible interannual variation in LW volumes (8.3–9.2 m3/ha), but much higher in frequencies (4.4–7.0 LW/100 m). However, the variations of LW volumes in individual meander bends did not allow us to develop any robust model predicting LW volumes. We found a higher number of significant models with two independent variables that included variables from the at-site meander and from the first upstream meander in comparison to the second upstream meander. This suggests a higher importance of local “retention” and “recruitment” variables at the expense of variables of “recruitment” related to the more distant meanders. Variables derived from riparian stands were recognised as important predictors entering into 7 of 13 models with two independent variables. This points to the complex recruitment-retention function of riparian stands for LW. We found evidence that the presence of stable LW pieces can drive local channel morphodynamics, and the observed spatiotemporal variation in jams was not only a product of the suggested frequent transport of LW and instability of jams but also of the possible burial of jams in sediments. Our results documented the complex links between the morphodynamics of meandering rivers and the biogeomorphic impact of living or dead vegetation.

The effect of a local mesh refinement on hydraulic modelling of river meanders

AbstractLarge‐scale river models are generally discretized by relatively large mesh cells resulting in bathymetry discretization errors and numerical effects. These hydraulic models are generally calibrated by altering the bed roughness to compensate for these errors and effects. Consequently, the calibrated roughness values are mesh‐dependent while generally local mesh refinements are executed after model calibration to study the effects of river interventions. This study shows both the errors caused by bathymetry discretization and numerical effects for locally refined meshes. First, schematised river meanders with a flat bed in the transverse flow direction are analysed to isolate the induced numerical effects by the mesh. Afterwards, a case study is considered to verify if similar mesh influences are found in natural river meanders. Curvilinear, triangular and hybrid (combination of curvilinear and triangular cells) meshes are used with different resolutions. The analysis shows that in the schematised river meanders lower depth‐averaged flow velocities and larger water depths are simulated with coarser meshes. In the case study, substantial differences in hydrodynamics between the meshes are obtained suggesting that the bathymetry discretization is more influential than the numerical effects. Finally, it was found that triangular meshes, and rivers with narrow meander bends, are most sensitive to mesh resolution. Especially in these cases, it is desirable to refine the mesh at the desired locations before model calibration.

Leaving a plastic legacy: Current and future scenarios for mismanaged plastic waste in rivers

Mismanaged plastic waste (MPW) entering the riverine environment is concerning, given that most plastic pollution never reaches the oceans, and it has a severe negative impact on terrestrial ecosystems. However, significant knowledge gaps on the storage and remobilization of MPW within different rivers over varying timescales remain. Here we analyze the exposure of river systems to MPW to better understand the sedimentary processes that control the legacy of plastic waste. Using a conservative approach, we estimate 0.8 million tonnes of MPW enter rivers annually in 2015, affecting an estimated 84 % of rivers by surface area, globally. By 2060, the amount of MPW input to rivers is expected to increase nearly 3-fold, however improved plastic waste strategies through better governance can decrease plastic pollution by up to 72 %. Currently, most plastic input occurs along anthropogenically modified rivers (49 %) yet these represent only 23 % of rivers by surface area. Another 17 % of MPW occur in free-flowing actively migrating meandering rivers that likely retain most plastic waste within sedimentary deposits, increasing retention times and likelihood of biochemical weathering. Active braided rivers receive less MPW (14 %), but higher water discharge will also increase fragmentation to form microplastics. Only 20 % of plastic pollution is found in non-migrating and free-flowing rivers; these have the highest probability of plastics remaining within the water column and being transferred downstream. This study demonstrates the spatial variability in MPW affecting different global river systems with different retention, fragmentation, and biochemical weathering rates of plastics. Targeted mitigation strategies and environmental risk assessments are needed at both international and national levels that consider river system dynamics.

Characterizing the response of the coastal Rio Grande to upstream damming

Dams are major obstructions to the flow of water and the transport of sediment in rivers. Much work has shown the construction of dams, and storage of sediment in reservoirs, can lead to significant changes in the morphodynamic behavior of alluvial rivers; however, in most cases, the effect of damming has a finite downstream limit as sediment is supplied back to the river through channel bed/bank erosion and/or tributary inputs. Here we present a study of a natural experiment that isolates one source of sediment downstream of a dam – the lower Rio Grande, which has no major tributary inputs of sediment over a ~450 km reach, from Falcon Dam to the coast. Analysis of the morphology and topography of the Rio Grande shows that prior to damming in the 1950s the coastal Rio Grande was a kinematically active channel with large, subaerial point bars. Subsequent to damming, lateral migration of the river shut down and exposed point bars shrunk in size, becoming static and vegetated. Bathymetric survey data show meter-scale incision of the channel bed for hundreds of kilometers downstream of the dam. Entrenchment of the river is also evidenced by the development of steep banks on the inner portions of channel bends. Damming also impacted the interaction between the river and the floodplain, resulting in the formation of persistent gullies incised into the channel banks, suggesting transport of water and sediment is net positive into, not out of, the channel. Based on all of these observations we conclude that the full ~450 km reach of the Rio Grande, from Falcon Dam to the coast, became net degradational following dam closure. We conclude that the presence of such a long degrading reach is caused by the exposure of bedrock in the Rio Grande channel for nearly 100 km downstream of the dam, which, combined with the lack of tributaries, limits downstream sediment supply to the river. Our results provide new insights into the response to damming for rivers with no tributary sediment input and thin alluvial cover, and more broadly to the kinematic and channel geometry responses to a major drop in water discharge and sediment supply for coastal meandering rivers.

Benthic metabolism and nutrient uptake vary with geomorphology and season in a lowland river

Meandering rivers are characterized by geomorphic units like cut banks, point bars, and thalwegs. These units arise from interactions between hydrological and geomorphological forces. However, the individual contributions of geomorphic units to whole-river metabolism or nutrient processing are unclear because these quantifications are often done at larger spatial scales. We used closed recirculating chambers to measure benthic gross primary production (GPP), respiration (R), N uptake or release, and P uptake or release at bimonthly intervals over 1 y at different geomorphic units in the Mulde River, Germany. We compared GPP, R, and nutrient processing among a cut bank, a point bar, and the thalweg at a natural meander. We also compared the cut bank of this natural meander with a cut bank fixed by riprap at a human-altered meander. In the natural meander, GPP, R, and nutrient processing rates were higher at the point bar than the cut bank or thalweg. These differences are likely related to larger sediment grain sizes that provide a more stable substrate for microbial communities. A strong interaction between geomorphic units and time for GPP and NH4+ fluxes suggested that differences in nutrient processing rates among geomorphic units were restricted to specific times during the year. Specifically, we found that the nutrient processing rates differed among geomorphic units during the summer, but not winter. Furthermore, in June and August 2017, R was 2 to 3× lower at the cut bank stabilized by riprap than at the natural cut bank. Our results demonstrate that rivers are composed of functionally distinct geomorphic units susceptible to human-induced hydromorphological degradation. However, strong interactions between space and time and large within-geomorphic unit variability propose that local drivers influence ecosystem function, suggesting that we need additional research to resolve these drivers at the scales of geomorphic units.

Research on sand body architecture at the intersection of a bidirectional sedimentary system in the Jiyuan area of Ordos Basin

The exploration and development of the dual-provenance lower assemblage of the Yanchang Formation in the Jiyuan area has progressed rapidly. At the intersection of this bidirectional provenance system, a complex and variable spatial combination of sand bodies formed, resulting in significant structural heterogeneity in the development and distribution of reservoirs. Based on previous studies, this paper combines core data and logging data with a large number of analytical tests and production performance data to carry out research on the Chang 82–Chang 9 reservoir group in the lower assemblage of the Yanchang Formation in the Shijiawan-Buziwan area. Based on the analysis of sedimentary conditions, the sand body development pattern at the intersection of the bidirectional sedimentary system in the study area was analysed by stepwise dissection of the sand body architecture. After the types and characteristics of the 4th- to 5th-level architectural elements were determined, the spatial distribution of the combinations of these elements was assessed and combined with logging discriminant analysis and geometric shape prediction methods to identify a ‘prism’ architectural distribution pattern. The architectural elements are connected with the distribution of diagenetic facies, the spatial distribution patterns of different types of diagenetic facies under the constraints of the architecture are summarized by region, and the locations of potential favourable reservoir development are discussed. The results show that the degree of superposition and combination of the eight skeletal architectural elements in the target layers gradually deteriorate from the bottom to the top. In addition, the development scale and degree of architectural elements in the braided river delta system in the west are better than those in the meandering river delta system in the east. In the different sedimentary areas, the spatial combination styles of the architectural elements are quite different, and the combination of these elements gradually changes from a combination of braided channels (FA1) and abandoned channels (FA2) to a combination of underwater distributary channels (FA4). Matching of the distribution of diagenetic facies with the distribution of architectural elements reveals that the diagenetic facies dominated by intergranular pores and dissolution pores (associated with good reservoir physical properties) are usually found at the bottom or in the lower to middle parts of the skeletal architectural elements.

A surge in obsidian exploitation more than 1.2 million years ago at Simbiro III (Melka Kunture, Upper Awash, Ethiopia).

Pleistocene archaeology records the changing behaviour and capacities of early hominins. These behavioural changes, for example, to stone tools, are commonly linked to environmental constraints. It has been argued that, in earlier times, multiple activities of everyday life were all uniformly conducted at the same spot. The separation of focused activities across different localities, which indicates a degree of planning, according to this mindset characterizes later hominins since only 500,000 years ago. Simbiro III level C, in the upper Awash valley of Ethiopia, allows us to test this assumption in its assemblage of stone tools made only with obsidian, dated to more than 1.2 million years (Myr) old. Here we first reconstruct the palaeoenvironment, showing that the landscape was seasonally flooded. Following the deposition of an accumulation of obsidian cobbles by a meandering river, hominins began to exploit these in new ways, producing large tools with sharp cutting edges. We show through statistical analysis that this was a focused activity, that very standardized handaxes were produced and that this was a stone-tool workshop. We argue that at Simbiro III, hominins were doing much more than simply reacting to environmental changes; they were taking advantage of new opportunities, and developing new techniques and new skills according to them.

New coefficient for water quality modelling in meandering rivers: Fatigue Factor

The accuracy of water quality predictions is essential, especially in countries affected by climate change and ecological water diversity. Water quality modelling in rivers is a valuable tool for enabling decision-making in surface water management because water quality prediction using sampling methods is expensive and time-consuming. The collection of technical knowledge of river characteristics and information about the sources of pollution plays a vital role in this context. This research focused on the effects of river geometry and meandering on the one-dimensional pollutant transport process. Flow velocity magnitude and direction in meandering rivers are frequently variable, leading to uncertain dispersion coefficients and massive changes in pollution concentration even over short distances of these rivers. So, the geometry of meandering rivers has a significant effect on their ecological indicators. A new coefficient called Fatigue Factor was introduced and defined in this study to consider this effect. Colidale Beck (CB) and Tyne rivers were selected for water quality modelling and implementation of the Fatigue Factor. The simulation-optimization method was employed to calculate zinc concentrations along the CB river using measured data for performance assessment of the model. The genetic algorithm performed well in predicting measured zinc concentration with high accuracy. Results of the model demonstrated that the mean effect of the Fatigue Factor in reducing the peak concentration of zinc increases by 3.8% compared to ignoring the Fatigue Factor along the CB length. With the Fatigue Factor consideration, the Mean Percentage Error between model outputs and measured data is 4%, while without it is 18%. Also, the Fatigue Factor had a greater impact on river pollution transport than the dispersion coefficient. With a 50% increase in the Fatigue Factor, the zinc concentration decreased by 6.1% more than the same increase in the dispersion coefficient. Moreover, results indicated that a 100% increment in the Fatigue Factor increases the assimilation capacity up to 3.5 times in CB.

A multi-spatiotemporal scale strategy to evaluate factors controlling pebble mobility and its interactions with bedforms in a lowland gravel-bed river

This work analysed the factors controling the spatiotemporal variability in bedload mobility in a low-energy gravel-bed meandering river, with a particular focus on interactions between bedload transport distance and bedforms (bar-riffle-pool units). We first addressed the influence of pre-existing morphology, especially bars, on event transport distance, and examined to what extent the maintenance of bedforms requires hydrological events that can move bedload over distances at least equivalent to the length of two successive identical geomorphic units. Specifically, we tracked the displacement of passive integrated transponder (PIT)-tag particles 10 years after their injection, and applied bedload formulae to reconstruct the event transport distance for a 32-year study period. The results showed that event bedload distances for bed surface median size sediments (D50) could not exceed an active channel width, roughly equivalent to 0.3 times the median distance between riffles and between pools. This has two main implications; first, trapping by bars is rare, or perhaps even absent, at the scale of competent events for most particles located at a distance greater than an active channel width from a bar; second, because no event is able to transport bedload over distances at least equal to the length of one riffle-pool unit, maintenance of bedforms must arise from successive minor changes. Furthermore, our data reveal important variability in bedload velocity at the decadal scale according to the year of injection, type of geomorphic unit of injection, and method of injection. This highlights the long-term effects in low-energy rivers of location, timing, and method of injection on velocity estimate. Lastly, we inferred the long-term bedload velocity from PIT-tag data and applied it to bedload formulae. The resulting velocity of 0.95–1.3 km century-1 is considered an overestimate because the slowdown of tracers seeded at the bed surface due to their progressive mixing in the active layer and their trapping in bars is still ongoing.

Free and underfit-scavenger river dynamics dominate the large Amazonian Pacaya-Samiria wetland structure

The Peruvian Amazon is known for harboring the greatest biodiversity on the planet, with a world record for biodiversity per unit area. Previous studies suggested that the high ecological value depends on correlations between ecosystem functionality and seasonal inundation control vegetation patches. However, the knowledge on how river morphodynamics and its complex erosion-depositional processes influence the aquatic mosaic and fishing activity in the region is still incipient. This study examines the hydrogeomorphology of the Peruvian tropical wetland of Pacaya Samiria, located in Western Amazonia, and its role in the distribution of aquatic habitats. By using remote sensing techniques, the hydrogeomorphological connectivity that bounds the Pacaya Samiria National Reserve is characterized by ancient to modern river processes. Additionally, river signatures developed by the Ucayali, Marañon, Huallaga, Pacaya, and Samiria Rivers overlap with fish extraction and dominant vegetation to describe how geomorphology is associated with the spatial distribution of fishing zones. Results indicated that paleochannels regulate wetland drainage within the Ucamara Depression, supporting stational water stagnation, vegetation cover, and formation of carbon rich detritus, relevant aspects to understand fish traits. Moreover, the Ucayali River dominates river dynamics in the Pacaya Samiria wetland, thus playing a pivotal role in shaping the complexity of streams and lakes. Furthermore, underfit-scavenger meandering rivers are observed in areas where paleochannels from large rivers are found. A geomorphological characterization of drainage patterns in freshwater environments, such as Amazonian wetlands, is crucial to develop sound management strategies. This methodological approach is expected to support decision-making in conservation actions in Amazonian environments based on understanding wetland connectivity and hydrogeomorphological behavior and their influence on commercial fisheries.

Architecture, genesis, and the sedimentary evolution model of a single sand body in tight sandstone reservoirs: A case from the Permian Shan-1–He 8 members in the northwest Ordos Basin, China

A single sand body is defined as a geological unit that is continuous vertically and horizontally but separated from the upper and lower sand bodies by mudstone or impermeable intercalation. The architecture of a single sand body is significant in the determination of hydrocarbon accumulation mechanisms in gas reservoirs, especially for exploitation of multiple tight sandstone gas (TSG) reservoirs. One such example is the gas reservoirs in the Tianhuan Depression, China, where the architecture and genesis of sand bodies are poorly understood. Based on the geologic background and sedimentary characteristics, the evolution of the distributary channel in the Tianhuan Depression has been examined using data from geological outcrops, cores, and well logs. The results showed that sand body architecture depends on the evolution of channel systems, and the scale and size of the channel are controlled by the sedimentary environment. Three kinds of sedimentary microfacies (distributary channel, channel mouth bar, and interdistributary bay) are mainly developed in the study area, and four types of single sand body stacking patterns (isolated, vertically superimposed, laterally tangentially superimposed, and horizontally bridged sand bodies) have formed in such a depositional environment. The target strata (Shan-1 and He-8 members) provide an ideal object for studying the evolution of the river and the architecture of the sand bodies. During the early stage of deposition, the sediment supply was insufficient, with restricted meandering river deltas dominating and sand bodies mostly existing as isolated types. Until the middle period of deposition, the sediment supply suddenly increased, the sedimentation rate accelerated with the decrease in the lake water base level, and the channel evolved into a large-scale braided river delta, generally forming superimposed sand bodies. By the late period of deposition, the provenance supply was reduced again; although braided river delta deposits were still dominant, the channel scale was restricted, and the sand bodies were predominantly isolated and horizontally bridged types. This work establishes a sedimentary evolution model for tight sandstone gas reservoirs, that is, a complete cycle of river evolution from small scale to large scale to terminal weakening, and discusses the genetic mechanism of single sand body architecture in such a depositional model.

How Is Time Distributed in a River Meander Belt?

AbstractRiver meandering controls the age of floodplains through its characteristic paces of growth and eventual cutoff of channel bends, forming oxbows. Hence, floodplain‐age distributions should reflect a river's characteristic size and migration rate. This hypothesis has been previously tested in numerical simulations, yet without systematic comparisons with natural systems. Here we analyze oxbow spacing and timescales of bend evolution and abandonment in natural and numerically simulated meander belts. In both cases, a saturated state is achieved whereby oxbows are spaced ∼1 meander radius apart. At saturation, the distribution of floodplain ages and probability of sediment‐storage time can be constrained from characteristic timescales of bend evolution and abandonment. Owing to the similar relationships between floodplain width and characteristic timescales in natural and simulated rivers, we postulate that this approach should apply to unconfined meandering rivers elsewhere—a hypothesis to be tested with independent geo‐ or dendrochronological data.

The Large Rivers of the Past in West Siberia: Unknown Hydrological Regimen

The hydrological regime of large meandering rivers of the West Siberian Plain in the Late Pleniglacial/Late Glacial was reconstructed from the hydraulic geometry of palaeochannels. The main tools for the reconstruction were the power law relationship between channel bankfull width and mean maximum discharge, taken in the downstream direction, and relationships between peak flood discharge and the contributing basin area. Reconstructed values of daily maximum surface runoff depth during the snow thaw period in the Late Pleniglacial/Late Glacial were 60–75 mm/day in the north of the plain with tundra and sparse forest and 20–40 mm/day in the south with periglacial steppe. The mean daily maximum surface runoff depth for the entirety of West Siberia was about 46 mm, which is more than five times greater than the modern value. Annual river runoff was calculated with the ratio between mean annual and mean maximum runoff depths, estimated for the modern region’s analogues of ancient periglacial landscapes and climates. Total annual flow of the Ob into the ocean was about 1000 km3. This is three times the current flow from the same basin, so the river was a significant source of fresh water to the Arctic Ocean during the last deglaciation.

Channel abandonment alters trophic characteristics of highland rivers

The Qinghai-Tibet Plateau (QTP) serves as a collection of unique ecosystems featured as oligotrophic and hypometabolic conditions, and is particularly vulnerable to threats posed by anthropogenic and natural disturbances. Active fluvial processes on the QTP, especially the lateral migration of rivers and channel abandonment accompanied by variations in the hydrological connectivity, and changes in river geomorphology, intensively modify this highland river-floodplain system. However, little is known about how these processes alter the trophic characteristics of highland river system on earth. In this study, we conducted field investigations on a typical meandering river, the Quanji River (QR), located in the northeastern QTP by sampling macroinvertebrates, surveying trophic sources, and measuring a range of environmental conditions. Based on the collected data, we identified four biotopes for the QR through hierarchical clustering, established the representative food web for each biotope through the allometric diet breadth model, and estimated the biomass storage and flux within food web for each biotope through the biomass balance model. Our results show that the identified biotopes differed markedly in hydrological connectivity. Biotopes 1 and 2, which were in the main river channel represented the condition of high connectivity, Biotope 3 in the open channel represented the median connectivity, while Biotope 4 in the semi-open/closed channel represented the low connectivity. In contrast to the unimodal pattern commonly observed in lowland rivers, indices for the highland QR, including the taxa richness, trophic group richness, biomass flux, biomass transfer efficiency, and trophic stability demonstrated a single-valley response pattern to the hydrological connectivity. We argue that the intermediate disturbance hypothesis still works on highland river ecosystems, whereas the optimal disturbance occurs in the low connectivity rather than median connectivity. We attribute this skewness to the low resistance and resilience of highland macroinvertebrate community in the face of hydrodynamic disturbances brought by flood events, and the biological disturbances from the predation by endemic migratory fish (Gymnocypris przewalskii) in QR. This study reveals that lateral migration and channel abandonment play important roles in highland river systems in the QTP in the context of biological and energetic perspectives and suggests that management strategies for highland rivers should be made different from those of lowland rivers.

Experimental assessment of velocity and bed shear stress in the main channel of a meandering compound channel with one-sided blocks in floodplain

Meandering rivers have an important role in transporting runoff from rainfall, especially during floods. With many cities and villages along rivers and the construction of water-management structures in floodplains, the flow pattern of the meandering rivers changes in time of floods. Here the flow behaviors in meandering compound channels with smooth floodplains and one-sided building floodplains are investigated using laboratory experiments. The experiments have been performed at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. The results show that the block density in a floodplain has a significant effect on flow behavior in the main channel. So that, with the increase of the relative depth, the ratios of main channel discharge compared to the total discharge (Qmc/Q) decreases in the bends with minimal extension of blocks floodplain, the crossover areas and the bends with maximal extension of blocks floodplain. For a given Dr, the significant increase in the Manning's n values as the density of the blocks increases (approximately 30%). With increasing the blocks density from zero to 23.7%, the maximum Darcy–Weisbach friction factor in sections CS1 and CS7 increases by 110% and 2%, respectively, which shows that increasing the density of blocks does not have a great effect on f in the section CS7. Under one-sided blocked floodplain conditions, the maximum longitudinal velocity in the main channel in the section with maximal extension of the blocks in the floodplain (CS7) is greater than the section with minimal extension of blocks in the floodplain (CS1). At the relative depth of 0.49, the maximum value of depth-averaged velocity in sections CS1-4 decreased by an average of 52% compared to the smooth floodplain. The maximum shear stress on the bed at the apex section for all channels occurs near the convex side and the bed shear stress at all sections decreases with decreasing relative depth and in the section with minimal extension of blocks floodplain (CS1) and cross-over sections. The maximum shear stress of the bed decreases with increasing the block density and these changes are more near the convex side than the concave side.