GM5.1

Research to understand the drivers of river form and processes has focussed on alluvial sand and gravel-bed channels. However, boulder-bed rivers are also an abundant channel type, particularly in previously glaciated and mountainous regions. Understanding boulder distribution in rivers is important because of their effects on  channel hydraulics and sediment transport processes. Boulder-bed channels in low-relief, previously glaciated landscapes may be considered semi-alluvial since the boulders likely were not deposited by fluvial processes (unlike in e.g., step-pool mountain channels). However, the relative importance of glacial legacy sediment and fluvial processes as drivers of boulder-bed river morphology is poorly understood. This is especially true in northern Sweden where channel clearance for timber floating has resulted in the removal of boulders from most rivers. Restoration of these rivers involves the replacement of boulders but is challenged by a lack of geomorphological understanding.

This study aimed to quantify the morphological characteristics of northern Swedish boulder-bed streams and determine the association between fluvial and glacial legacy controls on these channels. We undertook a large-scale field campaign surveying 20 rivers (drainage area: 15 - 112 km²) that have not been cleared for timber floating. At each reach, we measured channel morphology using a total station over approximately 100 m river length, surveying the channel planform, thalweg and 5 cross sections. In addition, we measured the location, diameter and protrusion of every boulder (> D₈₄) within each reach. We also conducted a survey of the size and density of boulders on the floodplain to compare to in-channel boulder distributions. We coupled this field campaign with analysis of digital elevation models, surficial geology, glacial landform maps, and hydrological data to investigate potential landscape controls on reach-scale geomorphology. Associations between drainage area, channel slope, width and D₈₄ as well as longitudinal clustering of boulders into fluvial bed-forms would indicate fluvial rather than legacy glacial drivers.

Preliminary results show high variability in the morphology of reference sites, from low-gradient reaches with high floodplain connectivity to steep and narrow channels (Slope ranged 1.1 - 8.8%). D₈₄ ranged from 0.4 m to 2.1 m with some sites having as many as 500 large boulders (> 1 m diameter) in a 100 m reach. D₈₄ was not associated with channel slope and boulders were not clustered longitudinally in most reaches. This suggests that boulder spacing is the result of glacial legacy controls. These results are important for understanding geomorphic processes in boulder-bed channels and how channel form relates to reach- and landscape-scale controls. The relative importance of fluvial versus glacial legacy controls on boulder-bed channel morphology is also important to help restoration practitioners more accurately identify reference states of boulder-bed channels in previously glaciated landscapes.

How to cite: Mason, R. and Polvi, L.: Drivers of channel morphology in semi-alluvial boulder-bed streams & implications for river restoration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4568, https://doi.org/10.5194/egusphere-egu21-4568, 2021.

Over the years, there has been tremendous growth in the literature as regards river channel classifications, however, very few studies have been able to engage the use of remote sensing products in channel classification at the reach-scale level especially by combining reflections from satellite sensors with channel morphological variables. This study aims to identify discriminating spatio-morphological variables using machine learning algorithms and classify site-specific channel types at the reach scale. Each reach was broadly classified based on valley settings (confined, partly confined and unconfined) and channel types (alluvial or bedrock). However, variations and site observations were recorded for site-specific classification purposes. For each reach, Global Positioning System devices were used to geo-locate their endpoints. Standard field instruments were used for cross-sectional measurements and established hydraulic equations for the derived variables. A total of 249 points across 83 reaches were sampled during the fieldwork. Landsat 8 and Sentinel-1 bands were retrieved for days the fieldwork was carried out/for days close to those dates using Google Earth Engine (GEE) platform. Hierarchical cluster analysis, HCA, using Ward’s linkages was used to provide a classification for the channel types. For the identification of important variables in predicting channel unit types, the random-forest - recursive feature elimination (RF-RFE) algorithm was used using the rfe() function. To identify the best machine learning algorithm, random-forest (rf), support vector machines (svm), multivariate adaptive regression spline (mars) extreme gradient boosting (xgb) and adaptive boosting (adaboost) were used on the training and test data to identify the best performing algorithm. The rfe() feature selection identified five (5) variables that can significantly help in channel unit type identification. The top five variables are dimensionless stream power, slope, width, wetted perimeter and Band 4. Using ROC curve, sensitivity, and specificity, the mars model has the highest ROC curve. Hence, it appears to be the best performing out of the five. However, if the argument is to be based on positive prediction, then any of the models except adaboost will be preferred given their high sensitivity. The HCA using illustrated the clustering structure of the studied reaches by producing five distinct channel classification types distinguished based on width-depth ratio values (high and low). The five distinct channel types are listed as  M1e, M5e, B1, E5b, and E. These codings are based partly on Rosgen’s classification while, the capital letters (M, B and E) represent mixed channels, bedrock with moderate width-depth ratio and alluvial channels with low width-depth ratio respectively. Numbers 1 and 5 represent bedrocks and sandy beds based on slope variation respectively. The identified channel unit types are a result of the underlying lithology, process-form dynamics and confinement. As streams are expected to respond differently to shocks and recover from damages, it becomes essential to understand these differences in classification which will go a long way in establishing watershed and streamside management guidelines.

How to cite: Olusola, A. and Faniran, A.: Interpreting reach-scale classifications and the role of spatial-morphological variables in river channel mapping using machine learning algorithms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-719, https://doi.org/10.5194/egusphere-egu21-719, 2021.

Alternate bars are large bedforms, characterized by an ordered sequence of scour and deposition zones, which often appear in rivers. It is well proved by many experimental, theoretical and numerical works that the formation of migrating alternate bars results form an intrinsic instability mechanism occurring when the width-to-depth ratio of the channel is larger than a critical threshold. Although a large amount of literature is available to describe equilibrium bar properties under steady flow conditions, much less information exists about the evolution of bars when flow discharge is variable in time. In a recent work we investigated how the long-term, average properties of bars respond to changes of the hydrological regime. This average state represents the net result of a multitude of flood events, each of them producing a different morphological alteration. However, a systematic description of how changes of the bar properties depend on the characteristics of the individual floods is still missing, as existing studies are limited to a small number of flood events, not sufficient to make a statistical description of the riverbed response. In this work, we aim at studying the time evolution of the bar amplitude in a relatively straight, channelized reach of a gravel bed river. Specifically, we considered a 10 km-long reach of the Alpine Rhine River, for which a detailed record of flow stages is available for the period from 1984 to 2010. This is accomplished by modelling the bed evolution through the theoretically-based model of Colombini et al. (1987), here applied by considering a time-varying basic flow and numerically integrating the bar amplitude. Compared with classical approaches based on numerically solving the two-dimensional shallow-water equations, our procedure allows for calculating the bar response over long periods of time with a very low computational cost. This enables for modelling different scenarios of hydrological alterations, due to dam constructions or climate changes, and to statistically analyse the expected impact on bar evolution. Assuming that bars cannot evolve when the flow is too low to fully submerge the bar crests, we identify 200 morphologically-active flood events, covering about 1.1% of the total duration of the flow series. Model results reveal that moderate flow events tend to increase the bar amplitude, while larger floods reduce the bar height. However, the value of the peak discharge alone is not sufficient to explain the morphological changes, as an important (and opposite) role is also played by the duration of the events. Specifically, longer floods tend to promote an increase of the bar height during the receding phase, which implies that a strong reduction of the bar amplitude requires intense, but relatively short flood events.

How to cite: Carlin, M., Redolfi, M., and Tubino, M.: The effect of flood events on the altimetric response of river alternate bars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7957, https://doi.org/10.5194/egusphere-egu21-7957, 2021.

River bifurcations play a crucial role in the morphodynamics of multi-thread channel systems such as braided or anastomosed rivers, deltas and alluvial fans, as they guide the downstream distribution of water and sediment fluxes. Several experimental and theoretical studies have highlighted the unstable character of bifurcations, which tend to produce a differential erosion/deposition in the downstream channels, even in the case of symmetric planform configuration and boundary conditions. This leads to equilibrium states where the flow distribution can be highly unbalanced, depending on the channel width-to-depth ratio. The analyses performed so far have mainly focused on equilibrium configurations, while little information exists about the time evolution of the instability process. In particular, there is no systematic analysis of how the bifurcation timescale depends on the controlling parameters, such as the channel aspect ratio, and the length of the downstream channels. Evaluation of this “intrinsic” time scale is fundamental to study the response of river bifurcations to time variations of any “external” factor that influences the bifurcation dynamics, such as the water discharge, the downstream conditions, or the presence of migrating bedforms. In this work we consider a simple bifurcation, consisting of a straight channel with mobile bed but fixed banks, which splits in two bifurcates that diverge with the same angle. We employ a 1-D shallow-water model for the upstream channel and downstream bifurcates, which are connected by means of the nodal point relation proposed by Bolla Pittaluga et al. (2013). We then numerically analyse the development of the bifurcations instability, starting from an initially-small perturbation of the bed elevation. Finally, we extensively investigate the effect of the key controlling parameters, including the model calibration coefficients, also allowing them to vary in time. The evolution of water discharge asymmetry shows a first exponential growth stage, followed by an asymptotic behaviour that leads to the equilibrium configuration. Model results reveal the key role of the width-to-depth ratio in determining the speed of the bifurcation evolution. Specifically, the evolution is very slow when the system is close to marginal stability conditions, while it becomes increasingly fast when increasing the width-to-depth ratio. Moreover, the timescale of the bifurcation increases with the length of the downstream channel, unless their length-to-depth ratio is sufficiently high.

How to cite: Barile, G., Redolfi, M., and Tubino, M.: Analysing the intrinsic time scale of river bifurcations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8178, https://doi.org/10.5194/egusphere-egu21-8178, 2021.

The morphodynamics of multi-thread fluvial environments like braided and anastomosing rivers is fundamentally driven by the continuous concatenation of channel bifurcations and confluences, which govern the distribution of flow and sediment among the different branches that are reconnecting further downstream. Almost all studies performed to date consider the two processes separately, although they frequently appear as closely interconnected. In this work, we tackle the problem of analyzing the coupled morphodynamics of such bifurcation-confluence systems by studying the equilibrium and stability conditions of a channel loop, where flow splits into two secondary anabranches that rejoin after a prescribed distance. Through the formulation of a novel theoretical model for erodible bed confluences based on the momentum balance on two distinct control volumes, we show that the dominating anabranch (i.e. that carrying more water and sediment) is subject to an increase of the water surface elevation that is proportional to the square of the Froude number. This increase in water surface elevation tends to reduce the slope of the dominating branch, which produces a negative feedback that tends to stabilize the bifurcation-confluence system. A linear analysis of the coupled model reveals that the stabilizing effect of the confluence depends on the ratio between the length of the connecting channels and the average water depth, independently of the channel slope and Froude number. Furthermore, the effect of the confluence is potentially able to stabilize the channel loop in conditions where the classic stabilizing mechanism at the bifurcation (i.e. the topographical effect related to the gravitational pull on the sediment transport) is very weak, as expected when most of the sediment is transported in suspension. The identification of a characteristic length scale that produces a coupling between the confluences and bifurcations opens intriguing possibilities for interpreting the self-adjustment of the planform scale of natural multi-thread rivers.

How to cite: Ragno, N., Redolfi, M., and Tubino, M.: Morphodynamic stability and characteristic length scales of bifurcations and confluences loops, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8017, https://doi.org/10.5194/egusphere-egu21-8017, 2021.

Interactions between riparian vegetation and river morphology are complex as they are often co-dependent, highly dynamic, and vary across both space and time. Vegetation diversity can be partially attributed to factors such as flood regimes and morphology, whilst simultaneously influencing the flow of water and sediment, ultimately impacting morphology and floodplain connectivity. As such, the importance of vegetation within the river corridor is well recognised and has been the subject of a considerable volume of research. However, within ecogeomorphology, most studies to date have been scale invariant, focusing either on characterisation of fine scale hydraulic roughness (e.g. using Terrestrial Laser Scanning; TLS) or on >reach scale patterns of riparian vegetation (using airborne or satellite imagery). Similarly, less attention has been paid to the temporal dynamics of vegetation beyond some appreciation of seasonality in controlling flow dynamics. This leaves a number of unresolved questions relating to the nested spatial and temporal (i.e. 4-dimensional; 4D) interactions of riparian vegetation and river flow.

In this study we seek to establish the temporal and spatial scales of riparian vegetation interaction within a river corridor using a traits based framework. Traits based research characterises plants with similar functional traits into guilds (groups) as opposed to by species or types, and as such provides a more useful basis to group vegetation according to the potential geomorphic impact that they exhibit. Traits based research for ecogeomorphic processes is relatively new in fluvial geomorphology, but has shown promise in its applicability, albeit existing applications are yet to investigate the temporal changes in vegetation. The need for extensive ground survey currently limits the application of traits based methods at reach scale and greater, highlighting the requirement for an approach that is able to classify a range of vegetation sizes and types into appropriate guilds.

Using a novel ULS and multispectral imaging systems, we have collected repeat high resolution (~1000 points per m³) surveys over a 1 km reach of the River Teme, UK, which has a wide variety of seasonally dynamic riparian vegetation. For each survey we use the point cloud data and multispectral imagery to classify vegetation into guilds. We use these in conjunction with the morphological data from the survey to create spatially varying surfaces of ecogeomorphic interactions, allowing us to establish links between guild coverage and morphological evolution across the reach throughout the year. The results show that vegetation-morphological co-evolution exists across scales and that high resolution survey methods are highly beneficial for resolving such interactions. The methods are designed to be transferable to other eco-geomorphic domains in any morpho-climatic regions, highlighting the flexibility and potential of a high resolution 4D traits based approach.

How to cite: Tomsett, C. and Leyland, J.: Exploring the 4D scales of vegetation-morphological interactions along a river corridor using repeat UAV Laser Scanning (ULS), multispectral imagery, and a functional traits framework., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7478, https://doi.org/10.5194/egusphere-egu21-7478, 2021.

A natural riverine corridor has several curls based on its physical and geomorphological characteristics. In most of the scenarios, the bridge construction on a meandering channel aligns along the convergent section. The enhanced secondary flow at convergent sections and the effect of meandering curvature bring the complexity in river turbulent characteristics. This effect may become predominant inside the main channel with variability in size and shape of the bridge pier. The present work discusses the turbulent structures in the main channel due to the variability in pier diameter (1inch and 2 inch ϕ) and a number of bridge piers on floodplain with inclusive of vegetation. Three-dimensional flow vertical and transverse velocity measurements were carried with acoustic Doppler velocimeter (ADV) 100Hz, at apex cross-section in a low sinuous channel. The results of the analysis showed that the combined effect of pier and vegetation on floodplain significantly altered the shear layer mechanisms in the channel with varying flow patterns. The comparison of the difference in secondary velocities between the pier with 1 inch and 2 inch ϕ  is 57% more in the case of lesser diameter pier. Further, the effect of size and number of piers on transverse velocity, Reynold’s shear stress is more susceptible to the mainstream. The convergence induced contraction of the meandering channel along with the bridge pier on its floodplain is observed to affect the turbulent structures formed in the main channel.

How to cite: Modalavalasa, S., Chembolu, V., Nandi, K. K., Kulkarni, V., and Dutta, S.: Effect of bridge pier induced turbulence on vegetated meander river morphology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1465, https://doi.org/10.5194/egusphere-egu21-1465, 2021.

The study of turbulence in a compound channel would address the nature of sedmient transport and bank erosion activity. The study would also give insights of embankment and levee breaches at the time of high flood. Experimental investigations were conducted on two compound channels of 31⁰ and 45⁰ bank angle in the laboratory flume to study the turbulece scale. Velocity data were recorded with Nortek Velocimeter at seven different locations (3 locations at the upstream, 3 locations at the downstream and 1 location at the middle) of the compound channel. Turbulence scale like Taylor microscale (λ_T) estimates the length scale of the inertial sub- range. The Taylor scale is calculated as:

The Taylor microscale analysis showed dominance in the main channel for 45⁰ bank angle as compared to 31⁰ bank angle. In the location of slope midpoint and floodplain region of the compound channel, Taylor microscale was more dominant for 31⁰ bank angle. Another important observation in both the compound channels (31⁰ and 45⁰ bank angle) is the dominance of Taylor microscale at the upstream section of the channel as compared to the downstream part of the channel. The results from the study would help us to get a better understanding of the role of turbulence in the morphological changes in a compound channel with different bank angles.

How to cite: Barman, J., Taye, J., and Kumar, B.: Turbulence Scale in Compound Channel, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10900, https://doi.org/10.5194/egusphere-egu21-10900, 2021.

In gravel-bed rivers the relation between the magnitude and frequency of sediment input, the threshold for motion and channel stability is still not fully understood.

Here we present results from a 280-hour long flume experiment, in which poorly sorted sediment was fed episodically in an 18-m long, 2.2%-steep channel. The experiment included 7 consecutive runs lasting 40 hours each characterized by a constant water discharge but different sediment supply regimes (i.e., with no feed, constant feed and sediment pulses). Several measurements of sediment transport, flow depth and bed structures were taken along the flume, to assess how changes in sediment supply influence particle mobility and channel stability.

Our results show that the surface grain‐size distribution coarsened quickly, developing an armored layer that persisted throughout the entire experiment with only short-lived changes after sediment pulses. Grain clusters and other bed structures developed continuously during the experiments, changing dynamically in response to sediment pulses.

We estimated the thresholds of motion with three different methods, all of which yielded consistent results. Overall, the threshold for motion increased during the experiment, fluctuating in response to changes in sediment input. Our results provide further evidence to the idea that the threshold for motion in gravel-bed rivers is not a constant, but changes as a state parameter. These changes in our experiments are controlled by (a) the sediment supply regime, (b) the degree of bed structuring, and (c) the history of bed evolution. These outcomes suggest that sediment supply regime is a primary control on bed surface evolution and the channel stabilizing function played by surface structures.

How to cite: Saletti, M. and Hassan, M.: Feedbacks between sediment input, bed state and threshold for motion in gravel-bed rivers: an experimental study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12453, https://doi.org/10.5194/egusphere-egu21-12453, 2021.

River recovery is the process that describes the capacity of the river to adjust to the limiting boundary conditions. In the regulated rivers, altered flow-sediment regime controls the trajectory of adjustments along the geomorphic and vegetative attributes. The present study is focused on recovery potential assessment of Mahanadi River, which shows a gradual emergence of in-channel vegetation in the post-dam period. The study area encompasses (i) 10 km reach of Mahanadi River (M₁) having bedrock exposed, anabranching channel pattern and (ii) 102 km reach of Ong river (O₁) with alluvial, compound channel form. In this study, Google Earth Engine cloud computing platform is used to process the Landsat images (1980-2010) and vegetation, water, and floodplain geomorphic classes are derived by Normalized Difference Vegetation Index (NDVI) and modified Normalized Difference Water Index (mNDWI). Finally, the intensity disorder index (IDI) is computed to represent the ‘system state’ in the post-monsoon periods and the influence of vegetation growth on the channel recovery. The results show that M₁ is relatively stable, with cumulative vegetation area increased from 2% in 1980 to 8% in 2010. However, O₁ demonstrates an accelerated increase in vegetation area i.e., 10% in 1980 to 30% in 2010. The system state (IDI) varies between 0.2 and 0.6 and follows a decreasing trend along M₁ and O₁. The findings establish that both regulated reaches may approach channel recovery in the near future, and prevailing boundary conditions indirectly influence the rate and direction of IDI.        

How to cite: Pradhan, C., Padhee, S., Dutta, S., and Bharti, R.: An entropy-based investigation on the river recovery potential in a regulated river basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9362, https://doi.org/10.5194/egusphere-egu21-9362, 2021.

Intermittent floodplain channels are low‐relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low‐level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low‐relief and low‐gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom‐hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission “error”, augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km2 with a detrended regional relief of 1.9 m.

How to cite: Xu, H., van der Steeg, S., and Torres, R.: Automatic Extraction of Intermittent Channel Systems of a Low‐Relief, Low‐Gradient Floodplain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14124, https://doi.org/10.5194/egusphere-egu21-14124, 2021.

In the last years, Albania underwent a rapid development, which resulted in an uncontrolled building boom and general land degradation. For these reasons, an ever-greater portion of the Albanian population is exposed to natural risks, whose major threats are represented by floods and earthquakes. Spatial planning and hydraulic risk management are a worldwide necessity which is best achieved when natural and artificial elements located closely to watercourses are known in detail. A geodatabase is a practical tool to store and manage such information. Land use and land cover changes have negative consequences on watershed management in Buna River Basin. They increase impervious ground surfaces, decrease infiltration rate and increase runoff rate, hence causing flood during the dry seasons. This study was undertaken to achieve the natural and artificial elements connected to hydraulic risk and fluvial dynamics in Buna River. Through a GIS overlay and GPS measurements where mapped elements include buildings, hydraulic works, weirs, drainage outlets, riverbanks, structural damages, fluvial bars and eroding banks. Consequently, a GIS geo database was built to visualize the spatial distribution of the mapped elements and to store a series of technical data, including the present preservation condition for man-made objects. GPS data was integrated in GIS to examine the extent of land use and cover change in the sub catchment of Buna River. Both quantitative and qualitative data were used for this study.  The geo database provides an overview of the territories connected with the fluvial dynamics, highlighting that in the studied territory; the more is urbanized, the more it is exposed to hydraulic risk.

Key word; spatial planning, natural hazards, relief drill, Buna river, Shkoder.

How to cite: Krymbi, E. and Rustja, D.: Spatial planning and flood risk: The case of the Buna River, Shkoder, Albania, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7216, https://doi.org/10.5194/egusphere-egu21-7216, 2021.

The presence of wetlands as a result of local fluvial and hydrological conditions constitutes a frequently observed feature of such rivers. Therefore, they are important elements of the basin, because besides functioning as buffer zones for CO₂ and sediments they also house important ecosystems, playing an important role in the control of water circulation. Brazilian wetlands have different typologies and sizes, varying from huge swamplands such as the Pantanal do Mato Grosso, to flooded savannas called “veredas” or oxbow lakes. Their distribution in inland areas depends on the variety of flood pulses mainly linked to seasonality with the presence of distinct dry and wet seasons (Junk et al., 1989). This strong seasonality affects the São Francisco River (SFR), the 4^th largest river in Brazil, which has frequent marginal lakes and swamps as it passes through five Brazilian states. This research aims to analyze the effect of the variation of the SFR level from 1925 to 2018, on the flow of the Pandeiros River which is one of many tributaries on the left side of SFR and on its wetland (“Pantanal Mineiro”). This wetland is hydrogeomorphologically linked to the SFR and receives water inputs during SFR flood periods. Measurements of the SFR water level performed once daily in the morning were obtained from gauging station n^o 44200000 belonging to the Companhia de Pesquisa de Recursos Minerais (CPRM) [altitude 445 m; 15°56'57.84"S; 44°52'4.68"W. The hydrological year starts at the end of the dry season on October 1st. Time series analyses (level duration curve, Seasonal Trend Decomposition (STL) of the daily level data, monthly level, mean, maximum, minimum level for each day of the year) were conducted to describe the hydrological regime and to assess temporal changes of the SFR levels and how these affect the magnitude, frequency and duration of flooding of the Pandeiros’s River wetland. Field observations (March 14, 2018) show that when SFR, which is Pandeiro’s base level, reaches a level of 5.0 m this leads to flooding conditions of the Pandeiros River wetland. Over the full period of record (1925-2018) the average level of the SFR was 3.86 m, with a minimum annual average of 2.43 m during the dry season (winter) and maximum of 5.98 m during the wet season (summer), with an average annual range of 3.55 m between both seasons. The SFR was above the 5.0 m threshold flooding level for 20% of the time 1925-2018, which corresponds to an average of 77.8 days of flooding per year in the wetland. The longest period of inundation was 178 days in 1926, when the SFR reached its maximum recorded level, and the shortest was 1 day in 2015, when it reached its minimum. The number of days per year of inundation have decreased over the full record, but that this is mainly due to a significant decrease since 1985. Prior to this, cyclic differences between wetter (1925 and 1985) and drier periods (1925 to 1945, 1945 to 1965) are observed.

How to cite: Alves de Oliveira, D., Helena Ribeiro Rocha Augustin, C., Hoey, T., and Persano, C.: Wetland dynamics at the transition between humid and semiarid environments of inland Brazil: São Francisco river morphodynamics and implications for the Pandeiros wetland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9075, https://doi.org/10.5194/egusphere-egu21-9075, 2021.

The Tocantins River is the easternmost fluvial system of the Amazon region, with a watershed draining both the Amazon rainforest and the Cerrado dry forest. This condition makes the Tocantins a natural laboratory to investigate the effects of past climate variability along with the rainforest-savanna ecosystems because its watershed drains both the Amazon rainforest and the Cerrado dry forest, and it is influenced by the Equatorial and central-northeast Brazil hydroclimates. Despite these unique physiographic and climatologic conditions, the Quaternary history of the Tocantins River is poorly known due to a lack of geochronological data about its sedimentary record. Here, we use optically stimulated luminescence (OSL) dating applied to quartz sand grains combined with geomorphological and sedimentological techniques to reconstruct the morphosedimentary evolution of the middle reach of the Tocantins River during the Late Quaternary. Three main geomorphological units were mapped: (i) fluvial plain, (ii) fluvial terraces, and (iii) paleo-alluvial fans. The OSL ages of 33 sediment samples retrieved from these geomorphological units range from 661 ± 42 years to 160 ± 16.3 ka, allowing to reconstruct of the depositional-erosion periods during the Mid-Late Pleistocene and Holocene. Our data indicate three stages of fluvial aggradation and two stages of incision. The older aggradational stage is represented by sediments from Upper Terrace (T1) and the paleo-alluvial fan deposited between 160 and 32 ka. Subsequently, a major incision event occurred at ~31 ka, which resulted in the abandonment of T1. The second phase of aggradation is recorded in the Lower Terrace (T2) and it also promotes reactivation of the paleo-alluvial fans from 31 to 6 ka. A new incision occurred from about 6 to 5 ka, allowing the abandonment of the T2 and reducing the local base level to its current position. The modern floodplain was built from 5 ka to the present, with sediment deposition due to lateral migration of the Tocantins River channel. The phases of aggradation and incision were correlated with regional paleoclimatic data, suggesting that precipitation changes related to the South American Summer Monsoon (SASM) as the main driver of the evolution of the Tocantins river in the last 160 ka. Disturbances in the supply of sediments and the flow of these rivers promote phases of depositional (drier periods) and incision (wetter periods). These depositional and incision phases of the Tocantins River appear to be synchronous with changes recorded by rivers from central and western Amazon, suggesting that the SASM is the main control of the Amazon fluvial systems. The continuous change in the Tocantins River dynamics has molded a high heterogeneity of habitats in the associated floodplains and terraces, which is a fundamental factor to support the diversity of fauna and flora in this transitional environment between Amazon and Cerrado biomes. 

How to cite: Jesus, J., Pupim, F., and Sawakuchi, A.: Quaternary climate variability as the main driver of the fluvial evolution of the Middle Tocantins River, eastern Amazonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6652, https://doi.org/10.5194/egusphere-egu21-6652, 2021.

The last 30 ka is a period marked by well-documented abrupt environmental changes on Earth. Despite the growing efforts to investigate the effects of past environmental changes in the fluvial dynamics, there is a lack of studies in intraplate tropical regions. Here, we applied geomorphological, sedimentological, and optically stimulated luminescence dating (OSL) technics to investigate the effects of environmental factors on the evolution of the Upper and Middle Tietê River during the Late Quaternary. Tietê River is one of the most important rivers of the southeast of Brazil, flowing from steepest to low-relief intraplate terrains, and under tropical climate. In order to understand the responses of the Tietê River system to environmental changes during the Late Quaternary, two main questions were tentatively answered: (i) what are the most important allogeneic factors for the evolution of this system?; (ii) how did climatic fluctuations affect river dynamics over time? We recognized a sequence of seven terraces, from 2 to 105 m above the channel, in the Middle Tietê valley. These terraces are formed by thin deposits (< 10 m), composed of sandy and conglomeratic sediments. The high and intermediate terrace levels of the Middle Tietê River are strath, while the low terraces of the middle reach are cut-and-fill. The formation of seven terrace levels in the Middle Tietê River was controlled by the combination of low erosion resistance of the lithological substrate and high stream power and coarse bedload that increase the erosion efficiency of the channels. OSL dating of sedimentary deposits in different terrace levels indicate 5 periods of aggradation in the Middle Tietê valley since the Last Glacial Maximum: 18.5 ± 2.0 ka; 9.8 ± 1.0 to 8.6 ± 0.8 ka; 7.1 ± 0.7 to 5.8 ± 0.5 ka; 4.2 ± 0.4 to 3.1 ± 0.3 ka; and 0.6 ± 0.06 ka. The results indicate that the activity of the South American Monsoon System induced the occurrence of climatic fluctuations and changes in vegetation cover in the river valleys of southeastern Brazil over the past 20 ka. The aggradation periods are correlated with more arid environmental conditions and sparser vegetation, while the incision events in the valley developed under transitions to humid environmental conditions and stimulated by vegetation recovery.

Key-words: Tietê River, fluvial evolution, fluvial terraces, Quaternary geochronology.

How to cite: Breda, C. and Nascimento Pupim, F.: Change of channel pattern and construction of fluvial terraces driven by SAMS since the LGM in southeastern South America: records from Tietê River, Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9928, https://doi.org/10.5194/egusphere-egu21-9928, 2021.

The Paraná River has being extensively studied in terms of its hydrological and sedimentological aspects, but the geological history of its fluvial plain is still poorly understood due to the lack of geochronological data. Most of the published studies are focused on the low and middle reaches, and the region upstream of the Porto Primavera dam is an area almost unknown in terms of geomorphic evolution. Here, we aim to investigate the succession of geological events responsible for the evolution and current configuration of the Upper Paraná River fluvial system, in the stretch between the Jupiá and Porto Primavera Hydroelectric Plants (São Paulo and Mato Grosso states). Thus, we used an integrated approach including remote sensing data, geomorphology, sedimentology, bathymetric profiles, and chronological methods. Four geomorphological compartments were identified and three of these compartments were dated by Optically Stimulated Luminescence (OSL): Unit 1, raised terrace with circular and semicircular ponds (~150 ka); Unit 2, intermediate terrace with small ponds and waterlogged areas (~60 ka); Unit 3, low terrace with preserved paleochannels (~39–35 ka); and Unit 4, current river plain (>9 ka). The geomorphological units were correlated with previous studies downstream of the studied area and their sedimentary characteristics and depositional ages suggest that their genesis is linked to changes in climatic and hydrological conditions during the Late Quaternary. Units 1, 3, and 4 are considered extensions of the geomorphological units Taquaruçu, Fazenda Boa Vista, and Rio Paraná, respectively. Unit 2 is a compartment with unique morphological characteristics, therefore not correlated with units presented in previous works. Further, two main knickpoints were identified, suggesting an important control in the sedimentation and development of the terrace levels. Thus, this work brings new data on the evolutionary history of the Paraná River, which allows us to understand that the development of the terrace levels and the floodplain of the upper reaches are strongly controlled by the climatic changes that occurred during the Late Quaternary.

How to cite: Oliveira, S. C., Pupim, F. N., Stevaux, J. C., and Assine, M. L.: Late Quaternary evolution of the Upper Paraná River, southeast Brazil: a new geomorphological and chronological database, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11713, https://doi.org/10.5194/egusphere-egu21-11713, 2021.

Climate, tectonic and sea level factors contribute to the geomorphic evolution of large rivers. Rivers draining settings where the tectonic activity and sea level influence are minors allow clearer signals from climate variation to be identified. Thus, rivers that run exclusively in tectonically quiescent areas and away from coastal regions are the most suitable targets to understand the climate drivers. In northeastern Brazil, the São Francisco River is one of the largest cratonic rivers in South America, with an extension of 2,900 km, and its thousand years evolution is registered by Quaternary deposits preserved as terraces. With the upper course in semi-humid settings, the São Franciscos flows northward, but with a watershed mostly under semi-arid conditions. Hence, the São Francisco River’s deposits are an excellent fluvial sedimentary record to shed light on how large tropical rivers responded to climatic changes of the Quaternary. We studied a 200 km stretch of the middle course of the São Francisco in the State of Bahia by using remote sensing methods and field surveys for geomorphological and sedimentological analyses combined with optically stimulated luminescence dating (OSL). We recognized at least four phases of fluvial aggradation (>90 ka; 65 to 39 ka; 18 to 9.5 ka and 380 years to recent) and three phases of incision (I1 - 85 to 65 ka; I2 - 39 to 18 ka and I3 - 9.5 to 1.0 ka). Two aggradation events and the incision event I2 are also observed in the upper course of the São Francisco River. The river incision events agree with precession insolation cycles (~25 ka) at latitude 10° S, which influence the rainfall in the area. The incision events occurred probably due to increased fluvial discharge produced by intensification of the South Atlantic Convergence Zone (SACZ), which has great influence on precipitation over the upper São Francisco river. Thus, we conclude that the aggradation-incision cycles of the São Francisco River during the last 100 ka are likely products of millennial precipitation variation, possibly related to precession cycles. The events of high sedimentation rate in the São Francisco river mouth are partially correlated with incision phases in its middle course. This suggests that sedimentation in plains of large plateau rivers can be decoupled from the coastal area.

Keywords: Late Quaternary, fluvial response, OSL dating, aggradation-incision cycles, precession cycles

How to cite: Mescolotti, P., do Nascimento Pupim, F., Bernardes Ladeira, F. S., Oliveira Sawakuchi, A., Santa Catharina, A., and Assine, M. L.: How Quaternary climate changes build and erode sedimentary deposits in an intraplate large fluvial system: the São Francisco River, Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16498, https://doi.org/10.5194/egusphere-egu21-16498, 2021.

Fluvial sediments are valuable archives of late Quaternary landscape evolution, paleoenvironmental changes and human-environmental interactions. However, given their complex and non-linear character their correct interpretation requires a good understanding of the fluvial architecture. The fluvial architecture describes the spatial arrangement and genetic interconnectedness of different types of fluvial sediments in a floodplain such as channel and overbank deposits. To properly map the different fluvial forms, their variations in composition and geometry must be understood in three dimensions. However, whereas investigations of the fluvial architecture are relatively easy in cohesive floodplain types with incised channel beds and large natural exposures, these are challenging in floodplains with buried stratigraphies where artificial exposures or corings are required.

We studied three cross sections through the floodplain of the middle and upper course of the Weiße Elster River in Central Germany by means of geophysical Electrical Resistivity Measurements (ERT) and closely spaced drillings. These 2D investigations were complemented by spatial geophysical 3D measurements of Electromagnetic Induction (EMI) in the surrounding areas of the cross sections. The latter technique allows fast mapping of larger areas, and was only rarely applied to fluvial systems so far. Our novel and cost-effective combination of core drillings with multidimensional geophysical measurements allowed to systematically reconstruct the fluvial architecture of larger areas of the Weiße Elster floodplain with high resolution, and thereby demonstrates its high value for fluvial geomorphology. Furthermore, in combination with ongoing numerical datings of the fluvial sediments these investigations form the base for precise conclusions about possible climatic and human drivers of the Holocene fluvial dynamics of the Weiße Elster River.

How to cite: von Suchodoletz, H., Zielhofer, C., Ulrich, M., Khosravichenar, A., Miera, J., Fütterer, P., Veit, U., Ettel, P., Werther, L., Ballasus, H., and Werban, U.: A novel combination of core drillings with 2D and 3D geophysical measurements helps to decipher the fluvial architecture of buried floodplain sediments of the Weiße Elster River (Central Germany), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7592, https://doi.org/10.5194/egusphere-egu21-7592, 2021.

This study focuses on the Pliocene-Quaternary sedimentary evolution of the fluvial systems in the Slovenj Gradec, Nazarje, Velenje, Celje, Drava-Ptuj and Krško Basins in the south-eastern Alpine foreland, Slovenia. The main aim was to determine the composition, morphostratigraphy, provenance, sedimentary environment and age of the deposits using geomorphological, sedimentological, geochemical, mineralogical and biostratigraphical methods. Pliocene-Quaternary sediments were deposited in fluvial (braided and wandering river systems) and alluvial/colluvial fan environments. The sediments are preserved in the terrace staircase sequences, formation of which is strongly controlled by tectonic activity. Based on geomorphological analyses, low-, middle- and high-level terrace groups were constrained and tentatively attributed to Late Pleistocene, Middle Pleistocene, and Plio-Early Pleistocene, respectively. The provenance analyses focused on the Plio-Early Pleistocene sediments and included lithological and microfacies analyses of the clasts. Based on the provenance analyses and published data, the long-term development of the drainage network was interpreted. Major changes occurred during the transition from Miocene-Pliocene and at the latest at Plio-Early Pleistocene the drainage network reached conformity with the present one. Overall, the spatial distribution of the Pliocene-Quaternary landforms revealed tectonic activity in intramontane basins during their development, from which the landscape evolution was deduced. 

How to cite: Mencin Gale, E., Jamšek Rupnik, P., Bavec, M., Trajanova, M., Gale, L., Anselmetti, F. S., and Šmuc, A.: Pliocene-Quaternary river-terrace sequences in intramontane basins in the south-eastern Alpine foreland (Slovenia): characterization of morphostratigraphy and provenance, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4979, https://doi.org/10.5194/egusphere-egu21-4979, 2021.

The Seine river (France) drains today a catchment area of 80,000 km² covering almost the northern part of France. Despite its importance, few studies focused on the Seine catchment and its landscape evolution, unlike the Somme basin, which remains a European reference for the Quaternary, because of to the numerous archaeological sites it contains. The middle and lower Seine valley in Normandy shows nevertheless a particular meandering fluvial dynamic and a succession of fluvial terraces over 120 m height, dated back to Early Quaternary. Previous works focused on the stratigraphy of alluvial sequences and led to the accurate characterization of lower fluvial and estuarine levels from Marine Isotope Stage (MIS) 1 to MIS 11. The alluvial terraces comprise also various Acheulean industries, showing human settlements in the valley for at least 400,000 years. Such archaeological remnants were retrieved in Saint-Pierre-Lès-Elbeuf, Tourville-la-Rivière, Vernon and La Celle.

Nowadays, the Seine connects to the drowned lower Seine course which continues in the Channel. This submerged part was subaerial during the last glacial cycle. Presently, the lower Seine course is still under the influence of marine tidal effects up to la Bouille (around 30 km from the coast). Additionally, estuarine deposits filled the valley up to Les Andelys (around 80 km from the coast) during the Holocene transgression and cover the penultimate and last glacial alluvial terraces. Nevertheless, the dynamic of the Seine river is broadly identified with few chronological constraints, but without any morphometric analysis combined with stratigraphical study.

This work provides a review of the stratigraphy of the quaternary alluvial deposits in the lower part of the Seine Valley, together with new morphometrical analysis of the paleo-meanders located at higher altitudes. The analysis of the paleo-morphologies compared with high-resolution digital elevation model (DEM), provides new means for constraining the fluvial incision and deposition over long distances and periods, and helps to discuss the river evolution related with quaternary uplift, catchment evolution and glacio-eustatic dynamics.

How to cite: Genuite, K., Nehme, C., Ballesteros, D., Todisco, D., and Mouralis, D.: Morphological evolution of the middle and lower Seine valley during the Quaternary period., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12502, https://doi.org/10.5194/egusphere-egu21-12502, 2021.