Channel Morphodynamics Research Articles

Downstream Controls on Coastal Plain River Avulsions: A Global Study

AbstractThe avulsion frequency of coastal‐plain rivers is primarily governed by the rate at which channels become superelevated over neighboring plains, which is itself controlled by multiple factors. Notably, the importance of wave and tidal processes, the rates of relative sea‐level (RSL) change, and the bathymetry of the receiving basin are thought to affect channel morphodynamics and channel‐mouth progradation, thereby controlling streambed aggradation and influencing the avulsion frequency and drainage density of coastal plains and deltas. This work tests the significance of these downstream factors on the avulsion histories of 57 Holocene lowland river systems. A quantitative analysis is performed of relationships between variables that quantify downstream controls and estimations of avulsion frequency, based on the number of avulsion events, active or abandoned channel paths, and delta lobes; measures of spatiotemporal avulsion “density” are also derived by normalizing these metrics by the size of study areas and the number of distinct drainage systems. Relationships between avulsion‐frequency metrics and descriptors of process regime indicate that wave and tidal processes may stabilize coastal channel systems, but also that their influence may be modest. No consistent relationship is seen between avulsion‐frequency proxies and the offshore bathymetric gradient, which in the studied examples does not scale with the rate of shoreline progradation. No evident trend exists between measures of avulsion frequency and estimated rates of either eustatic or RSL fluctuations. Overall, the considered variables do not leave a clear statistical signature in Holocene avulsion histories, suggesting that upstream or intrabasinal factors may represent more important controls.

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.

ANTHROPOGENIC INFLUENCE UPON THE SUKIL RIVER BED FUNCTIONING

A river bed is a main indicator of the changes, including manmade ones that take place in their catchment basins. These changes are reflected in the river beds functioning, especially within the catchment basins located in the Ukrainian Carpathians. These basins have been noticeably affected by human impact during last century. The Sukil River (left tributary of the Svicha River) is one of them. By the relief morphology features and geologic-geomorphologic structure the Sukil River basin is divided into three parts: mountain, premountain and plain. The river bed within these three parts is characterized by noticeable difference in morphological and morphodynamical parameters. And the reaction of the riverbed on the manmade changes also is different in all three parts. First of all, it is a deforestation, agriculture, unauthorized gravel intake within the river bed and floodplain, river bed straightening, river bank consolidation and flood protection dikes constructing, melioration and ponds constructing etc. Research results ascertained noticeable difference between manmade influences in different parts of the catchment basin, especially the effects on the river beds and their functioning. In the mountain part the moderate impact dominates and is mainly presented by timber falling which has caused significant sediments runoff increasing during the floods of 2008 and 2010. In the premountain part the strongest impact is done by the unauthorized alluvium mining in the river bed. It caused increasing in the river bed morphodynamics and reinforcing of the linear and local side erosion. The erosion processes threat the roads and buildings within the river valley. The plain part of the river basin is most transformed. The river bed strengthening is carried out since the middle of ХХ-th century which was accompanied by the dikes and drainage network constructing. As the result many boggy areas were drained, the morphology of the Sukil river bed was changed, the meander process was stopped. But the tendencies to renaturalization of the river valley are observed during the last years. Conducted research are important for the rivers renaturalization and providing the strategy of sustainable development in Ukraine in the matter of natural resources protection and management. Key words: channel morphodynamics; Sukil River; anthropogenic influence; mountain, premountain and plain parts of the basin.

Statistical approach of contemporary hydrogeomorphological channel changes of the middle Allier River, France: morphostructural controls, human impacts and flow regime

The Allier River, France, known for its very dynamic river sections in its lower course, was nevertheless subjected to human impacts. The present study aims to better explain the recent hydrogeomorphological channel changes of the middle Allier River considering morphostructural constraints, human impacts and flow regime since the second half of the 20th century. Channel changes are analyzed from six historical aerial photographs (1954, 1974, 1992, 2000, 2010 and 2016) and four longitudinal water surface profiles (1934/35, 1999, 2005 and 2020), based on descriptive statistical analyses and a Principal Component Analysis followed by a Hierarchical Cluster Analysis. The influence of morphostructural controls and human impacts are tested using a Correspondence Factor Analysis, whereas temporal trends of the flow regime are analyzed using Mann Kendall and Pettitt tests. Overall, results show that during the last sixty to eighty years the channel bed incised by 0.86 m, narrowed by 25% and that lateral mobility decreased by 68%. Four morphological evolutionary trajectories were identified for the alluvial plain sections, laterally more mobile than the naturally confined section. These evolutionary trajectories were caracterized by a more or less important presence of human impacts on the channel, such as gravel mining activities and, more locally, in-channel engineering works. Moreover, hydrological changes recorded at the end of the 20th century, characterized by a decrease of the frequency, the duration and the intensity of floods, has led to a general increase of annual abandoned surfaces and, conversely, to a decrease of annual eroded surfaces. In conclusion, future river management schemes will have to consider past and present human impacts on channel morphodynamics as well as current climate change and its consequences on the flow regime.

Global sensitivity analyses of key riparian nitrogen models

Riparian zones can effectively reduce excess nitrogen loading to streams. Modeling nitrogen retention in riparian zones is useful, especially at larger scales. We evaluated select riparian nitrogen models for their robustness in representing hydrology, vegetation, soils, nutrients, and channel morphodynamics. We used global, time-varying sensitivity analyses of REMM (Riparian Ecosystem Management Model) and SWAT+ (Soil and Water Assessment Tool+) to identify the most influential parameters for calculating water table depth and nitrogen processes. Both REMM and SWAT+ were sensitive to topographic and soil parameters such as slope and soil layer thickness, although spatial and temporal scale and hydroclimatic conditions affected parameter sensitivity. Neither model was sensitive to stream channel depth, which is known to affect riparian hydrology and nitrogen cycling. It is necessary to incorporate stream morphodynamics like channel change into both riparian-scale and watershed-scale nitrogen models to provide useful management tools for addressing excessive nitrogen loading in stream networks.

A New Method for In Situ Measurement of the Erosion Threshold of River Channels

AbstractThe vast majority of alluvial deposits have some degree of cohesion, typically due to the presence of clays and/or organic matter. Determining the threshold fluid shear stress, τc, necessary to entrain these sediments is essential for predicting erosion rates and morphodynamics of rivers, tidal channels, and coasts. Cohesive sediments present a greater challenge than noncohesive sand and gravel beds due to the sensitivity of τc to such factors as compaction, aggregation, and particle surface chemistry. All of those factors may be altered if bed and bank sediments are extracted for later analysis in the laboratory. Environments with mixed cohesive and noncohesive materials are common, such as sand‐bedded rivers with muddy banks; it is therefore desirable to have a method for in situ measurement of τc across a very wide range. We present a novel instrument that is capable of reproducibly determining τc for submerged cohesive and noncohesive sediments in situ. The instrument has several advantages over alternative methods, including ease of implementation in the field, and a fluid shear that is more representative of natural flows. The device incorporates common and low‐cost components integrated with an Arduino microcontroller, which may receive commands from a mobile phone. We demonstrate our instrument's capabilities in gravel, sand, and clay‐sand mixtures prepared in the laboratory, and present a proof of concept field deployment in a wadeable stream. Additionally, we provide the necessary schematics, parts lists, code, and calibration procedures for the interested reader to build their own version of the instrument.

Correlating floodplain geochemical profiles with archival historical mining records to establish depositional chronologies of river sediment

Chronological markers in fluvial sediments can provide useful information on geomorphic response to historic catchment disturbance, and help explain historic channel morphodynamics. We concentrate on the River Nent catchment in Northern England, which was heavily impacted by lead and zinc mining in the 18th and 19th centuries. We date a 2 m deep profile of fine (<2mm) floodplain sediments using ore extraction records, covering the period 1845 to 1913, based on a significant correlation between zinc concentrations and zinc-ore extraction (r = 0.78, n = 27; p < 0.01). These dates were constrained with a basal OSL date of 2.7 ± 0.3 ka (Agemin to Agemean 1.4–3.0 ka) and existing lichenometric dates for surface gravels. Earliest lead contaminated sediments at the base of the profile most likely resulted from Roman mining operations within the catchment. Local mine-worker population statistics were used to simulate pre-1845 lead ore extraction; where above-average peaks in lead concentration, recorded at the 125–130 cm, 105–110 cm and 95–100 cm horizons, were linked to respective peaks in lead ore extraction in 1825, 1850 and 1866. Estimates of historic sediment delivery volumes at a 0.062 km2 sink site at Blagill, approximately 6.5 km downstream of the headwaters to the Nent, were estimated through coupling the chronological markers with accumulated sediment thickness and reach area. Sediment delivery was seen to be strongly enhanced by phases of metal mining, with peaks between 1800 and 1825 reflecting the final stages of lead mining, and later between 1892 and 1895, reflecting the peak extraction of zinc. Although lag times between ore extraction peaks and sediment delivery to the Blagill site introduce uncertainty to the proposed chronology, we conclude that where good local mining statistics are available, our approach provides a useful alternative or may be used in conjunction with other commonly used methodologies.

A Field Study on the Lithological Influence on the Interaction Between Weathering and Abrasion Processes in Bedrock Rivers

AbstractLithology is an important control on the efficiency of bedrock incision and thus the pace of landscape evolution. Rock strength is commonly considered the limiting lithologic factor that resists erosion. Yet, rock strength is a dynamic property that oscillates during advection of rock to the channel surface as damaging processes weaken rock and erosion exposes fresh rock. We approach the problem by investigating damage on bedrock surfaces that vary by the frequency they are eroded in channels of different lithologies. Our data set includes measurements of channel slope and width to characterize channel morphology, and Schmidt hammer rebound, P wave velocity, slake durability, and porosity to characterize the mechanical properties of channel surfaces. The average damage accumulation rate of lithologies ranges over 41% of the mean. We find a range of damage patterns among the different lithologies. Local surface damage increases with erosion frequency in channels comprised of coarse‐grained bedrock but decreases with erosion frequency in channels comprised of fine‐grained bedrock. We interpret these patterns to develop from lithological influences on weathering, abrasion, and the threshold of damage to erosion. The cross‐channel damage patterns between channel floors and margins are well correlated with stream power demonstrating links between microstructural rock properties, reach‐scale morphology, and landscape‐scale processes. We conclude that the morphodynamics of bedrock channels are sensitive to the lithological influences on the direction and magnitude of feedback in the coevolution of bedform morphology and the mechanical properties of the surface.

Channel morphodynamics and sediment budget of the Lower Ganga River using a hydrogeomorphological approach

AbstractThe Ganga River is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga River is vulnerable to siltation because of: (a) the naturally low slope in the alluvial stretch; (b) the confluence of several highly sediment‐charged rivers such as the Ghaghra, Gandak, and Kosi; and (c) the reduction in non‐monsoon flows because of upstream abstractions of both surface and groundwater. Additionally, the Farakka barrage has impacted the morphology of the Ganga River significantly, both upstream and downstream of the barrage. Large‐scale siltation in several reaches has reduced the channel capacity, leading to catastrophic floods in this region even at low discharges. This work has utilized historical remote sensing data and UAV surveys to reconstruct channel morphodynamics and compute sediment volumes accumulated in the channel belt along the Lower Ganga River between Buxar and Farakka. The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar–Gandhighat (GW1, 160 km); (b) Gandhighat–Hathidah (GW2, 106 km); (c) Hathidah–Azmabad (GW3, 182 km); and (d) Azmabad–Farakka (GW4, 132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga during the last four to five decades. Sediment budgeting using planform maps provides estimates of ‘extractable’ volumes of sediment in GW1, GW2, GW3, and GW4 as 656 ± 48, 706 ± 52, 876 ± 71, and 200 ± 85 Mm3, respectively. These estimates are considerably lower than those computed from the hydrological approach using observed suspended sediment load data, which assumes uniform sedimentation between two stations. Further, our approach provides reach‐scale hotspots of aggradation and estimates of extractable sediment volumes, and this can be very useful for river managers to develop a strategic sediment management plan for the given stretch of the Ganga River.

Bed and Bank Stress Partitioning in Bedrock Rivers

AbstractApproximation of bed and wall (bank) stresses in confined, narrow bedrock rivers is key to accurately assessing hydraulic roughness, sediment transport, bedrock erosion and the morphodynamics of bedrock channels. Here, we partition bed and wall stresses using the ray‐isovel model (RIM) and field observations. We used the RIM to calculate the distribution of shear stress across an idealized trapezoidal channel and found that the ratio of wall to bed stress () grows slightly with increasing bank angles, but exponentially declines with increasing width‐to‐depth ratio. We applied the RIM to 26 of the canyons along the Fraser River and found that RIM predicts 0.60 0.98. We used field observations of bed and total stress to calculate wall stress in each canyon. The total stress was calculated from 1D momentum balance (depth‐slope product). The distribution of bed stress was calculated from near‐bed velocity profiles and showed that bed stress spikes as water enters constriction‐pool‐widening (CPW) sequences. For the majority of studied canyons, the observed wall stress is larger than the total stress and the observed bed stress. The maximum observed bed stress through a CPW sequence is ∼7.5 times the mean bed stress and ∼4.9 times the total stress. Compared with the observed stresses, the model systematically over‐predicts the observed bed stress and under‐predicts the observed wall stress by ∼55%. Our results reveal that the complex flow structure in bedrock canyons influences the distribution of bed and wall stresses and that bedrock walls contribute more hydraulic roughness than predicted with RIM.

Field study and computer modeling of the Ob River channel morphodynamics to ensure the Barnaul water intake operability in the medium-term future

Field study and computer modeling of the Ob River channel morphodynamics to ensure the Barnaul water intake operability in the medium-term future

Fluvial Organic Carbon Composition Regulated by Seasonal Variability in Lowland River Migration and Water Discharge

AbstractIdentifying drivers of seasonal variations in fluvial particulate organic carbon (POC) composition can aid sediment provenance and biogeochemical cycling studies. We evaluate seasonal changes in POC composition in the Río Bermejo, Argentina, a lowland river running ∼1,270 km without tributaries. Weekly POC concentration and isotopic composition from 2016 to 2018 show that during the wet season, increased lateral channel migration generates an influx of 13C‐enriched and 14C‐enriched floodplain‐sourced material, overprinting the 13C‐depleted and 14C‐depleted headwater signature that is observed during the dry season. These findings demonstrate how channel morphodynamics can drive variability of POC composition in lowland rivers, and may modulate the composition of POC preserved in sedimentary archives.

Large-scale assessment of spatial variability in stream power distribution: An indicator of channel erosion and deposition potential

Channel morphodynamics, governed by erosion and deposition processes, constitute a relevant aspect of integrated water management planning, and development of sustainable fluvial projects. According to the Water Framework Directive, there is a need for hydromorphological recognition and channel geomorphic adjustment at the national level. Such a large scale excludes field measurements as the primary research method. Instead, methods that require less resources and are based on relatively simple process representations are required. The stream power (SP) concept, combining the effect of channel gradient and discharge, has been widely used for quantitative measurements in geomorphic works at different spatial scales, but not at the national level so far. The primary objective of this study was to estimate total and specific stream power, and detect erosion and deposition patterns along the river network in Poland. Given the dominant role of stream slope in SP spatial variations, the specific objective was to assess the vertical accuracy of high-resolution DEM representing water surface elevations along the river course, and to propose a method for extraction of elevation points along river reaches that increase the accuracy of DEM-derived stream slopes. We demonstrated that DEM generally overestimates the elevation by more than 0.5 m, significantly affecting the DEM-derived stream slope estimation. By extracting minimum water surface elevation from DEM within a 15 m radius from the target cell (river segment endpoint), a substantial improvement was achieved. The difference between the actual and DEM-derived water surface elevation dropped to 0.16 m. The results indicated that the distribution of dominating processes in Polish rivers is strongly governed by the character and origin of the terrain relief and lithological diversity. For regions with flat relief, most channels are generally in the state of equilibrium, whereas in upland and mountainous regions, erosion processes are dominant. This study demonstrates the potential use of the SP concept for mapping channel sensitivity to erosion and deposition dominance at a national scale, which can be relevant for integrated river catchment management.

Geomorphological dynamics of tidal channels and flats in mangrove swamps

The tidal channels and mudflats of estuarine wetlands have vital ecological functions for the habitat of aquatic organisms and the exchange of nutrients and sediment. Understanding the spatiotemporal dynamics of their morphology helps to formulate an appropriate management plan for mangrove swamps. The water levels, wetland topography, suspended sediment concentration, sediment particle size, and mangrove stand characteristics were investigated in the Tanshui River estuary to verify the geomorphological model established in this study. The decoupled model includes a hydrodynamic module, a sediment module, and a mangrove module to integrate the effects of tidal hydroperiods and river currents. The simulation results revealed that the model could effectively predict the tidal channel morphodynamics, but the siltation of tidal mudflats was overestimated. Sensitivity analysis results showed that the soil properties and erosion rate had the highest impacts. The flow velocities and bed shear stress of the tidal channels were several times those of the mudflats. The hydrodynamics of ebb tide were more extensive than those of flood tide. The asymmetry between ebb and flood tides contributes to maintaining a self-sustaining tidal channel. The deposition rate during the flood period was 20–80 times that of the normal period. The results indicated that river floods entrained large amounts of sediment to accelerate wetland siltation. The reduced tidal range scenarios and the corresponding suspended sediment input suggested an alternative for preventing channel deposition in the wetland. This study offers a quantitative tool for delivering effective maintenance of tidal channels and tidal flats that may enhance the integrated and adaptive management of mangrove swamps.

Bedrock erosion in subglacial channels.

The Labyrinth in the McMurdo Dry Valleys of Antarctica is characterized by large bedrock channels emerging from beneath the margin of Wright Upper Glacier. To study the morphodynamics of large subglacial channels cut into bedrock, we develop herein a numerical model based on the classical theory of subglacial channels and recent results on bedrock abrasion by saltating bed load. Model results show that bedrock abrasion in subglacial channels with pressurized flow reaches a maximum at an intermediate distance up-ice from the glacier snout for a wide range of sediment grain sizes and sediment loads. Close to the snout, the velocity is too low and the sediment particles cannot be mobilized. Far from the snout, the flow accelerates and sediment is transported in suspension, thus limiting particle impacts at the channel bottom and reducing abrasion. This non-monotonic relationship between subglacial flow and bedrock abrasion produces concave up bottom profiles in subglacial channels and potential cross-section constrictions after channel confluences. Both landforms are present in the bedrock channels of the Labyrinth. We therefore conclude that these geomorphic features are a possible signature of bedrock abrasion, rather than glacial scour, and reflect the complex interplay between transport rate, sediment load, and transport capacity in subglacial channels.

How sediment composition and flow rate influence downstream channel morphodynamics upon dam removal

AbstractThe process of dam removal establishes the channel morphology that is later adjusted by high‐flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport.Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. In total, 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17–18%, while at the high flow it was 31–41%.

High-Resolution Monitoring of Tidal Systems Using UAV: A Case Study on Poplar Island, MD (USA)

Tidal processes regulating sediment accretion rates and vegetated platform erosion in tidal systems strongly affect salt marsh evolution. A balance between erosion and deposition in a restored salt marsh is crucial for analyzing restoration strategies to be adopted within a natural context. Marsh morphology is also coupled with tidal mudflats and channel networks and this makes micro-tidal systems crucial for a detailed assessment of restoration interventions. Here, we present a methodological approach for monitoring channel morphodynamics and vegetation variations over a time frame of six years in a low tidal energy salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (Maryland, USA). The project is a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is used to restore a tidal marsh habitat in mid-Chesapeake Bay. Aerial surveys with an Unmanned Aerial Vehicle (UAV) have been performed for the high-resolution mapping of a small tidal system. Flight missions were planned to obtain a Ground Sample Distance (GSD) of 2 cm. Structure-from-Motion (SfM) and Multi-View-Stereo (MVS) algorithms have been used to reconstruct the 3D geometry of the site. The mapping of channel morphology and an elevation assessment on the mudflat were performed using orthomosaics, Digital Terrain Models (DTMs) and GNSS survey. The results highlight that the workflow adopted in this pilot work is suitable to assess the geomorphological evolution over time in a micro-tidal system. However, issues were encountered for salt marsh due to the presence of dense vegetation. The UAV-based photogrammetry approach with GNSS RTK ground surveys can hence be replicated in similar sites all over the world to evaluate restoration interventions and to develop new strategies for a better management of existing shorelines.

Morphodynamics of Boulder‐Bed Semi‐Alluvial Streams in Northern Fennoscandia: A Flume Experiment to Determine Sediment Self‐Organization

AbstractIn northern Fennoscandia, semi‐alluvial boulder‐bed channels with coarse glacial legacy sediment are abundant, and due to widespread anthropogenic manipulation during timber‐floating, unimpacted reference reaches are rare. The landscape context of these semi‐alluvial rapids—with numerous mainstem lakes that buffer high flows and sediment connectivity in addition to a regional low sediment yield—contribute to low amounts of fine sediment and incompetent flows to transport boulders. To determine the morphodynamics of semi‐alluvial rapids and potential self‐organization of sediment with multiple high flows, a flume experiment was designed and carried out to mimic conditions in semi‐alluvial rapids in northern Fennoscandia. Two slope setups (2% and 5%) were used to model a range of flows (Q1 (summer high flow), Q2, Q10, and Q50) in a 8 × 1.1 m flume with a sediment distribution analogous to field conditions; bed topography was measured using structure‐from‐motion photogrammetry after each flow to obtain DEMs. No classic steep coarse‐bed channel bedforms (e.g., step‐pools) developed. However, similarly to boulder‐bed channels with low relative submergence, at Q10 and Q50 flows, sediment deposited upstream of boulders and scoured downstream. Because the Q50 flow was not able to rework the channel by disrupting grain‐interlocking from preceding lower flows, transporting boulders, or forming channel‐spanning boulders, the channel‐forming discharge is larger than the Q50. These results have implications for restoration of gravel spawning beds in northern Fennoscandia and highlight the importance of large grains in understanding channel morphodynamics.

Channel morphologic processes of a highly sinuous bend approaching neck cutoff by bank erosion in the middle Yangtze River

The distal reach of the Lower Jingjiang River (LJR) in the middle of the Yangtze River consists of five adjacent bends, among which the Qigongling Bend is a Ω-shaped meander with a mean sinuosity of 2.2 and the narrowest neck 525 m in width. This bend is slowly approaching neck cutoff owing to progressive bank erosion. An abnormal phenomenon has occurred in this bend since the Three Gorges Reservoir (TGR) began to operate in 2003 which is erosion in the inner bank zone and deposition in the outer bank zone. This problem has not been fully understood because of the interplay of changes in water-sediment, bank erosion, and artificial bank revetment. In this study, aerial and remote sensing images, hydrological data, channel topography, and an existing bank erosion model are used to reveal channel morphodynamics of this bend and the trend of the potential neck cutoff induced by bank erosion. The study results show that the clear water released from the TGR has provided by forcefully eroded the point bar of inner bank but failed to scour the outer bank due to the protection of bank revetment since the 1990s. Thus far, the outer bank zone near the bend apex has increasingly widened in conjunction with the formation of 2 emerging sand bars. Consequently, the thalweg of the main channel has laterally shifted toward the inner bank by roughly 800 m. More severely, the rate of bank retreat on the upstream side of the bend neck was about 4.5 m/yr in 2010–2019, but the downstream side of this neck was experienced slight deposition. Bank erosion could be accelerated by progressively increasing erosion and eventually trigger the occurrence of neck cutoff in the next few decades, thereby significantly altering the quasi-equilibrium regime of channel morphodynamics in the LJR.

Channel morphodynamics of the Selenga River

The article presents the experimental data of the channel morphodynamics of the Selenga River and the Chikoy River mouth over a 100-year period within the 23.5 km long stretch from the Termyn-Tatur channel to Gustovskii Island. We analyzed changes in the planned pattern of the channel and systemized channel processes for three time sections – 1914, 1987 and 2019 on the stretch from the Termyn-Tatur channel to the Gustovskii Island. The main changes over more than a century have manifested themselves in the form of changes in the elements, types and shapes of bends, dynamics of the width and number of arms, as well as in degradation processes and appearance of new arms, redistribution of the main flow from one arm to another. The most stable areas turned out to be areas of relatively rectilinear channel of undivided and divided types located near the valley side, which was a limiting factor in the development of horizontal deformation. We determined the time of the most significant channel deformations-from 1914 to 1987. This is explained by the number of high-water periods in this period, the total duration of which was 43 years and possibly by the intensification of anthropogenic impact and the associated increase in suspended sediment discharge. The longest period of decrease in the manifestations of channel deformations occurred from 1987 to 2019, and it coincides with a prolonged dry period that began in 1995.