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Large wood (LW) and log jams have important implications for sediment dynamics within river systems. Although many case studies explore this topic, far fewer synthesize these results at the reach and basin scales. In this paper, we present a conceptual model that relates valley setting and landscape history of forested, mountain streams to spatiotemporal patterns of sediment storage and transport via the formation and decay processes of logjams. This conceptual model explores the function of logjams as sediment transport moderators, significantly influencing sediment retention, channel morphology, and lateral and longitudinal connectivity. Detailed wood inventories, channel topographic surveys, and channel morphology from four case studies are used to illustrate the range of channel responses to logjam formation, evolution, and decay in different valley contexts. Valley setting is shown to influence logjam formation and dynamics by determining the mechanisms and timing of sediment and wood recruitment. In steep and narrow (and thus coupled, confined) reaches, colluvial processes are the dominant mechanisms of wood and sediment delivery, and logjams grow vertically as sediment is stored in a wedge behind the jam (longitudinal disconnectivity). In contrast, in unconfined reaches (decoupled hillslopes), wood and sediment recruitment occurs primarily through fluvial processes, and logjams grow laterally as the channel shifts across the valley bottom. These logjams have long lifespans, often over a century, and gradually release sediment as the logjam decomposes. Sediment deposits stabilized by riparian vegetation behind these structures may persist long after logjam decay. This conceptual model is a testable framework and practical tool that offers insight into wood-sediment interaction mechanisms within steep, forested basins. [ABSTRACT FROM AUTHOR]

The Holocene sedimentary history of the Turbio River valley in semi-arid north-central Chile and its relationship to climate variability is presented in this study. Fluvial landforms and sedimentary deposits serve as essential records for studying the long-term dynamics of fluvial systems in response to external factors such as tectonic and climatic influences. The study area, located in the Andean Mountain range, is highly sensitive to hydrodynamic changes and sediment transport processes, making it suitable for studying fluvial dynamics and their relationship to climate fluctuations over millennia. In particular, the transitional semi-arid conditions of this region provide an opportunity to link Holocene fluvial sediments to climatic shifts. This study presents the analysis of well-preserved sedimentary deposits that outcrop along the valley walls, including grain-size distribution, stratigraphy, geochemical composition and radiocarbon dating to establish the chronological context. Facies association analysis, along with geochemical data, allowed the interpretation of depositional sub-environments and their evolution through time. We interpret the Holocene sedimentary evolution for the Turbio River valley as six time periods that reflect shifts in sediment source, energy regime, flow types, and sedimentary processes linked to fluctuations between wet and dry climatic conditions. The sedimentary sequence is interpreted as reflecting an initial high-energy period with continuous surface runoff followed by two periods of formation of extensive lakes, likely separated by periods of runoff intensification. The later part of the sequence reflects a more stable fluvial landscape that led to the excavation of the sedimentary fill. This sedimentary record provides valuable insights into past climate variations and their impact on the hydrodynamic conditions of the fluvial system in the region. Comparisons with other regional records support a broader understanding of climate change in the semi-arid Andes of Chile. • Rio Turbio sedimentary record is studied to reconstruct Holocene river dynamics. • Fluctuations in fluvial processes are related to alternating wet and dry periods. • A new model explains how the river evolved in response to Holocene climate variations. • Fluvial record provide valuable insights into past climates. [ABSTRACT FROM AUTHOR]

The structure and function of riparian ecosystems generally result from feedbacks between plant dynamics and fluvial processes and landforms, i.e., river morphodynamics. Taxonomic approaches do not allow direct identification and quantification of the mechanisms involved in the interaction between plant communities and the geomorphological environment. Although riparian ecosystems show enormous taxonomic variations, comparable patterns of plant response traits across different taxa may evolve in response to the exposure to similar hydrogeomorphological processes in relation to flow dynamics, sediment transport, and nutrient and water availability. Biogeomorphological functional classifications for plant responses to, and effects on, river morphodynamics at the levels of individual plants, populations, and communities have been proposed in the literature. They serve herein as a basis for identifying and quantifying, within a presented standardized functional trait framework, key plant response and effect traits that can help to explain feedbacks between plants and morphodynamics. In particular, functional guilds and functional diversity metrics can be applied at the population and community levels for exploring feedbacks between vegetation and river morphodynamics worldwide. In this framework, the variability in plant effects across different environments and spatiotemporal scales, related to the biological characteristics of the plants (i.e., morphological reconfiguration of plants into the flow, seasonal phenology, phenotypic plasticity, and vegetation succession), can be explored. New opportunities including remote sensing and numerical modelling approaches, coupled with the inclusion of plant traits through our original framework, will improve our understanding of feedbacks between vegetation and morphodynamics and contribute to an improved prediction of biogeomorphological river trajectories. • Taxonomical approaches do not permit the analysis of plant-geomorphology feedbacks. • Functional traits links hydrological, geomorphological and biological drivers and processes which shape river systems. • Riparian plant traits converge despite taxonomic variation. • The functional trait approach is an opportunity for quantifying riparian vegetation-river interactions worldwide. • The functional trait approach can be supported by remote sensing and numerical modelling of river trajectories of change. [ABSTRACT FROM AUTHOR]

The grain size of sediment delivered to a river by hillslope processes is an important factor which could impact fluvial erosion, sediment transport, and associated geomorphic changes. Grain size distribution (GSD) is increasingly recognised as an important factor for the impact of landslides on sediment pulses and long-term erosion rates. Therefore a better comprehension of grain size control on landslide-generated sediment transport and dynamics will help with understanding post-seismic fluvial processes and landscape evolution. In this study, we modelled the recovery of the Hongxi River basin affected by the Wenchuan earthquake under different landslide GSD scenarios. The CAESAR-Lisflood (CL) model was first modified to enable the spatial input of sediment GSD, which allows the model to distinguish the grain size composition of landslide sediment and the background sediment sourced from other areas. Using the modified CL model, we simulated three different landslide GSD scenarios (Original, Coarser, Finer) by altering the original landslide sediment GSD observed from a post-earthquake basin. In particular, we analysed the fate of landslide-generated sediment using a new sediment tracing function embedded in CL. This enabled us to evaluate the role of landslide GSD variation on the spatio-temporal heterogeneity of sediment transport. Our results show that the GSD of landslide material exerts a clear control on both sediment yield and channel deposition, the sediment transport of landslides disconnected to the fluvial system is more sensitive to grain size variation, while the GSD of landslides connected to the fluvial system has less impact on sediment dynamics. The duration of landslide disturbances on basin sediment dynamics can largely be dependent on the period of time (e.g., 20 years in this study basin) taken for grain coarsening (the grain size is increasing upward), which is dominated by the evacuation of fine sediments. The modelling results and findings highlight the importance of grain size distribution of landslide material and provide information to support basin sediment management and environmental risk mitigation. [ABSTRACT FROM AUTHOR]

• New geomorphological and geochemical records from the southern Mu Us Desert (MUD), northern China. • The sources of southern MUD sediments have evident temporal variation. • Early–Mid-Holocene southern MUD sediments are mainly derived from a local source. • The maximum CIA occurs at 8.4–6.4 ka BP and is related to enhanced monsoonal rainfall. • Late-Holocene mixed sources in the southern MUD are associated with an enhanced winter monsoon. • Summer and winter monsoons alternate in controlling the sediment source and geomorphic evolution in the southern MUD. Identifying the sources and weathering degree of sediments in the northern deserts of China is crucial to understand the evolution of the Asian monsoon system and corresponding induced changes in surface processes. Great progress has been made in the reconstruction of paleoclimate and paleoenvironment from the Early Holocene onwards based on chronostratigraphy and proxies of aeolian sedimentary sequences in the Mu Us Desert (MUD). However, previous studies have given rise to some controversial over the timing of the Holocene optimum period, and there is a scarcity of studies on the sediment provenance evolution of the desert during the Holocene and its internal relationship with the East Asian summer (EASM) and winter (EAWM) monsoons. Here, we provide new elemental geochemical data from a fluvial–aeolian sedimentary sequence in the southern MUD; we then combine these data with other available elemental data and hydroclimatology information to investigate the characteristics of sediment source evolution and their possible connection with the EASM and EAWM on the southern margin of the MUD-desert/loess transition zone since the latter part of the Early-Holocene. The elemental composition and geochemical characteristics of the studied sedimentary sequence differed markedly in the periods before and after 6.4 ka BP, suggesting a change in the source of the sediments across this boundary. From the late Early-Holocene (10.6 cal. ka BP), sediments were dominated by the chemical weathering products of local source material, with a gradual increase in chemical weathering over time, indicating a gradual strengthening of the EASM; the EASM reached its peak Holocene strength at 8.4–6.4 ka BP, accompanied by a moderate level of chemical weathering. After 6.4 ka BP, with the weakening of monsoon precipitation, the EAWM began to strengthen, and long-distance transport of source material from the west and northwest increased; however, chemical weathering of local source material driven by the EASM remained dominant until 4.0 ka BP. After 4.0 ka BP, the EAWM became dominant, and sediments showed strong sedimentary cycles, mainly transported from long-distance sources by aeolian activity, and the MUD became an important sedimentary sink for the northern-central deserts. After 0.7 ka BP, the sediments in the MUD became a mixture of different-age sediments due to river scouring and the transport of pre-existing sediments along the shoreline. Collectively, the elemental geochemical and other proxy data from Holocene sediments on the southern margin of the MUD record an inverse relationship between the EASM and EAWM; these two monsoonal circulations have alternated in their control of the sedimentary source characteristics and geomorphological environmental evolution processes across the southern margin of the MUD during the Holocene. [ABSTRACT FROM AUTHOR]

• A strong signal of reduced winter conditions i.e. duration, snow accumulation and frost days. • Hydroclimatic regime shift alters the sediment flux seasonality in near future. • Thaw-seasons have important role controlling the annual sediment flux. Climate change alters high latitude hydrological cycle by diminishing the snow accumulation and spring flood magnitude, and by altering temperature and rainfall finally leading to hydroclimatic regime shift from snow-dominated to rain-dominated. These changes in temperature, precipitation patterns, and discharge strongly influence geomorphological processes in fluvial environments, leading to alterations in sediment transport, erosion, accumulation, and landscape changes. We conducted comprehensive analysis of hydroclimatic trends and shifts on boreal Oulanka River system, spanning over the past five decades. A strong signal of warming temperatures (+0.61 Celsius/decade), reduced winter conditions and warmer summers (+0.41 Celsius/decade) was detected. The spring flood magnitude diminished 7 %, but high discharge peaks (Q > annual p90m3/s) during other seasons increased 22 % together with 28 % increase of annual minimum discharge. Simultaneously, precipitation intensity increased during summer. The meander migration rate (mean 0.89 or 2.55 m/year) and bank erosion volume were interconnected to ground frost, high snow sum, high discharge, and high-water level during spring flood, but no significant long-term trends were observed. Our findings underscore that climate is the first-order control on fluvial geomorphology and emphasizes the complex interplay between various hydrological and climatic factors that shape the dynamics of river systems. Based on the results, we expect to see changes in the spatial–temporal distribution of high latitude rivers sediment flux in future. In addition, more attention should be addressed to the thaw seasons controlling the sediment transport, as majority of the observed shifts took place in these months. [ABSTRACT FROM AUTHOR]

Up to several meters thick fine-grained Holocene overbank deposits are ubiquitously found in most Western and Central European lowland floodplains. However, despite their large importance for the geomorphological and geoecological floodplain properties, the interplay of different possible causes for their formation are not well understood yet. Most authors suggest human-induced deforestation as the main precondition for sediment mobilization and transport from the slopes to the floodplain, whereas others suggest a stronger influence of climatic factors. This current research gap is caused by often missing well-resolved fluvial chronostratigraphies and spatio-temporal information about former human activity within the studied catchments. To fill this gap we exemplarily studied Holocene overbank sedimentation and possible human or natural drivers in the meso-scale Weiße Elster catchment in Central Germany. To do so, we applied numerical dating as well as sedimentological and micromorphological analyses to Holocene fluvial sediments along three floodplain transects. Furthermore, we built up an unprecedented systematic spatio-temporal database of former human activity within the catchment from the Neolithic until the Early Modern Ages. Together with published paleoclimatic data, this database allowed an unprecedented, systematic comparison of Holocene overbank sedimentation phases with possible external controls. Our data show that some overbank sedimentation phases were directly linked with human activities in the affected site sub-catchments, whereas others were not. Instead, all phases seemed to be linked with natural factors. This difference with most former studies could possibly be explained by previously often limited numerical dating of the fluvial sediments and by largely missing spatio-temporally well-resolved regional settlement records, hindering a precise temporal link of fluvial sedimentation with former human settlement. Furthermore, this difference could possibly also be explained by a relatively high natural sensitivity of the landscape dynamics in the Central German lowlands, showing a subcontinental climate, towards climatic external controls. [Display omitted] [ABSTRACT FROM AUTHOR]

Sediment transport (bedload and suspension) plays a relevant role in the morphological response of river channels to large floods. Sediment load controls erosion and aggradation patterns during high flows, drives the migration of macroforms and contributes to the definition of thresholds for bank erosion and channel instability. Given this influence on channel morphology, it is clear that sediment transport influences both the channel geometry and water stage reached during floods and should therefore be considered in flood hazard analysis. So far, however, legislation on flood hazard and flood management plans still continues to typically disregard sediment transport. This is due mainly to the paucity of available data on sediment transport and the lack of standardized approaches for its integration into flood hazard analysis. The present work, which aims to provide some guidance on how to advance in this issue, has undertaken a bibliometric study and thorough review of the published scientific literature that had previously addressed the implications of sediment transport on flood hazards assessment. This review showed that: i) most studies on the influence of sediment transport on flood risk have focused on mountain streams, as they are highly sensitive systems to changes in sediment supply; ii) research has also focused on explaining historical changes in channel-conveyance capacity and flood frequency based on long-term trends in sediment supply; and iii) recent developments in hydrodynamic and morphodynamic numerical models provide opportunities to explicitly incorporate sediment transport in flood hazard analysis. However, despite the recent advances, we have identified important challenges and discussed needs to better consider sediment transport in flood hazards at different spatial and temporal scales. [ABSTRACT FROM AUTHOR]

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

Fluvial sediment fingerprinting employs various methods, directly link sink and source. However, the identification and quantification of the transformation that sediments experience during fluvial transport remain unclear. The luminescence properties of quartz and K-feldspar change during the burial/exposure to sunlight cycles undergone by fluvial sediments, potentially allow scientists to trace fluvial sediment transport. Here, we report on changes in luminescence properties in active river bars from source to sink along the Yarlung Tsangpo-Brahmaputra River system. Optically stimulated luminescence (OSL) sensitivities and residual doses of quartz and K-feldspar within the fluvial sediments are used for the fingerprint method. Source rock is identified as the primary determinant of regional variations in quartz OSL sensitivities within fluvial sediments along the Yarlung Tsangpo River, while transportation distance governs local changes. Samples from narrower river channels with higher normalized channel steepnesses (Ksn) along the Yarlung Tsangpo River exhibit higher residual doses. Furthermore, the Brahmaputra River exhibits higher residual dose values compared to the Old Brahmaputra River. The sediments' residual doses are therefore employed to fingerprint fluvial erosion ability. Our luminescence fingerprint results indicate that coarse-grained fractions experience a "dilution" effect due to the input of new sediments after traveling approximately 50-200 km along the Yarlung Tsangpo River and about 50-100 km along the Brahmaputra-Old Brahmaputra River course. The comparison of luminescence (quartz and feldspar) and detrital zircon fingerprinting within the fluvial sediment holds the potential to trace weathering and erosion cycles during transportation in large river contexts. • Luminescence fingerprints fluvial sediment transport along the Yarlung Tsangpo River. • Source rock determine regional variations in quartz sensitivities. • Transportation distance governs local changes in quartz sensitivities. • The residual doses are employed to fingerprint fluvial erosion capabilities. • Coarse-grained fractions would likely be "diluted" by the input of local sediments. [ABSTRACT FROM AUTHOR]

Topographic relief and bedrock fractures caused by fault activity can accelerate surface erosion, suggesting that erosion rates increase with decreasing distance to the fault core. To test this hypothesis, we used 10Be cosmogenic nuclides to quantify catchment-wide erosion rates at various distances from a fault in the footwall of the Daqing Shan fault system, a 200-km-long active normal fault in Inner Mongolia, northern China. In addition, in order to examine the influence of topography and bedrock fractures on the surface erosion processes in the fault-weakened zone, their relationships with hillslope erosion, fluvial sediment grain size and transport efficiency, erosion rates, and erosion coefficients were studied. The resulting catchment-wide erosion rates ranged from 0.01 ± 0.00–0.13 ± 0.01 mm yr−1. These erosion rates and coefficients were unaffected by distance from the fault. The analysis of topography, bedrock fractures, and hillslope erosion yielded distinct hillslope erosion processes at different distances from the fault. Rapid mass wasting events, such as landslides, occurred at <15 km from the fault in steep, rocky hillslopes with abundant bedrock fractures. In contrast, at >15 km from the fault, hillslopes gradually become more gentle and soil-mantled. These spatial-slope configuration differences result in a decrease in sediment grain size delivered to river channels with increasing distance from the fault. Once sediments enter river channels, they impact fluvial transport capacity by influencing the initial motion threshold of sediment. Correlation analyses revealed that sediment transport efficiency in channels directly controls the catchment-averaged erosion rate and coefficient. This mechanism explains why the erosion rate remains insensitive to variations in topography and bedrock fractures in fault-weakened zones. • Study of patterns and controls on erosion rate across Daqing Shan fault zone. • Cosmogenic nuclide 10Be used to derive catchment-averaged erosion rate. • Erosion rates do not vary with distance from the fault. • Erosion rate remains insensitive to variations in topography and bedrock fractures. • Sediment transport efficiency in channels controls erosion rate. [ABSTRACT FROM AUTHOR]

The gross morphology of a free face and talus rampart occurs in a range of landscape settings. Semi-arid mesas, river gorges and coastal cliffs all share this geometry, but differ in how the components respond to their respective environments, to each other and to the capacity to export material from the talus rampart. Differences are reflected in the rates and processes of cliff retreat, and in the boundary conditions for rampart growth and removal. For mesas and buttes, export from the talus is controlled by the rate at which coarse material weathers to a size that can be readily washed away, leading to a stable proportional height for the rampart, which then consists of a boulder controlled slope at the angle of repose. For river gorges, a high incremental increase in fluvial sediment transport allows incision to accompany cliff retreat, while low rates lead to valley widening and rampart formation. Coastal cliffs may either lack ramparts, with supply limited development controlled by wave attack on cliffs; or be limited by the transport capacity of transverse offshore sediment processes, allowing development of a persistent rampart. In all contexts, the transport term should not be ignored in understanding both the rates of retreat and the morphology of cliffed landforms. [Display omitted] • Cliffs with a talus rampart occur in diverse settings. • Mesas, river gorges and steep coasts. • Boundary conditions depend on the process setting and geometry. • Evolution may be driven by supply- or transport-limited removal. • The rate of material transport from rampart base is always important. [ABSTRACT FROM AUTHOR]

[Display omitted] • The grain size and major element composition in sedimentary sequences of the Yellow River Basin are presented. • The provenance, transport dynamics and sedimentary characteristics of sediments of the Yellow River is disclosed. • The processes and geomorphological effects of aeolian-fluvial interactions in the Yellow River Basin are identified. • The intensity of aeolian-fluvial interactions is related to sediment erosion and transport. Understanding environmental changes requires the characterization of potential interactions between aeolian and fluvial systems. This study aimed to identify the sedimentary characteristics, provenance, and transport of sediments of the Yellow River Basin, as well as the processes and geomorphological effects of aeolian-fluvial interactions using end-member modelling analysis (EMMA), multidimensional scaling (MDS), and principal component analysis (PCA). Samples (n = 107) collected from sedimentary sequences in the source area, upper, middle, and lower reaches were analyzed for grain size and major element contents. The results showed that sediments in the source area of the Yellow River mainly originated from the Tibetan Plateau, whereas the input of materials from the Chinese Loess Plateau (CLP) gradually increased from the upper to lower reaches. The sediment transport was jointly controlled by simultaneous or alternating aeolian and fluvial processes. Asynchronism in aeolian activity and rainfall and the non-uniformity of runoff and sediment sources resulted in a concentration of aeolian-fluvial interactions in the river reaches traversing through the deserts and CLP. Fluvial sediments in the upper reaches consisted of wash loads deposited in slow-moving water, whereas rapid replenishment of sediment from the CLP in the middle reaches increased sediment transport capacity, thereby enhancing the uniformity of sedimentary characteristics from the upper to lower reaches. Moreover, numerous gullies in the CLP resulted in increased intensities of aeolian-fluvial interactions, thereby accelerating severe sedimentation in the lower reaches. This study can act as a reference for paleoclimatic reconstruction and for soil and water conservation in large-scale watersheds across a range of climatic zones. [ABSTRACT FROM AUTHOR]

Subaqueous slopes of many actively prograding deltas are heavily modified by widespread submarine sediment failures, forming highly complex geomorphic features that undergo continuous evolution even over short time scales. However, the effects of wave-seabed interactions (especially liquefaction) on the evolution of seabed sediment failures are rarely studied. Here, we use side-scan sonar, sub-bottom profilers, multibeam bathymetry, and in situ and laboratory geotechnical tests to map the bottom morphology and sediment structure of the Yellow River subaqueous delta. Our results indicate that: 1) The morphology along the Yellow River subaqueous delta is highly complex and can be categorized as a moderately disturbed area and heavily disturbed area; 2) Heavily disturbed area shows notable lines of evidences of liquefication-related sediment failure and four separate morphological zones (Zones 1–4). Zone 1 consists of differently sized sediment highs, which considered to be scarp or monadnock features. Zone 2 undulates with widely distributed tabular sediment blocks, gullies, and small-scale seafloor irregularities. Zone 3 is characterized by irregular, elongate sediment ridges. Finally, Zone 4 is characterized by relatively flat terrain with some localized highs; 3) Sub-bottom profiles used to visualize failure features in Zone 1 show acoustically continuous high-amplitude parallel or subparallel reflection, and the geomorphic features in this area are mainly caused by erosion. In comparison, Zones 2, 3 and 4 lack internal structures and display chaotic/disturbed signatures in corresponding sub-bottom data, which might be an indication of liquefaction; and 4) A combination of preconditioning and triggering factors lead to sediment failures and the complex morphological features in this area; however, silt liquefaction under storm conditions is the most important controlling factor. Based on this evidence, we propose formation mechanism for the liquefaction-related sediment failure features in this area. Sediment liquefaction forms an initial depression and gully when fine particles are resuspended along the arc-shaped liquefaction boundary as sediment vents and re-solidification of the liquefied sediment leads to a stiffer bed. Gullies are then widened and deepened by the predominant currents, and the disturbed depressions develop into elongated highs because of the denser structure and higher strength following liquefaction. While disturbed by engineering activities, the sediment highs become fragmented due to intense localized erosion, having the potential for downslope transport and the formation of sediment ridges. The geometries and distributions of the sediment failure features on the Yellow River subaqueous delta provide insights into process mechanisms and causative factors, especially when compared with other, similar phenomena reported elsewhere. • The geomorphology of the Yellow River subaqueous delta was characterised. • Distinct morphological zones are identified and associated processes explored. • Silt liquefaction under storm conditions dominates. • An evolutionary model for liquefaction-related sediment failure is proposed. [ABSTRACT FROM AUTHOR]

Quantifying the timescale of sedimentary processes in river basins, such as erosion, transport and deposition, and their relationships with geomorphic parameters is important in the context of ongoing climate change. In this study, we combine uranium activity ratios (234U/238U) for fine-grained (<63 μm) fluvial sediments from the Var River basin (Southern French Alps) and inferred sediment residence times, to catchment-scale spatial analysis of geomorphic parameters with the aim to investigate the factors that influence the timescale of hillslope storage and alluvial transport. Our results show that the U isotope composition of sediments and inferred sediment residence time are mostly controlled by geomorphic parameters such as catchment elevation, slope and curvature, rather than catchment lithology. We show that sediment residence times are shorter in steep, high elevation terranes, but markedly increase wherever catchment-averaged slopes <30°, reflecting the transition from bedrock-dominated to soil-covered landscapes. Independent estimates of sediment residence times using spatial data for soil thickness and 10Be-derived denudation rates show good agreement with those calculated from U isotopes. This observation validates the suitability of U isotopes to quantify sediment residence times in river catchments and for investigating how geomorphic processes control the duration of hillslope storage and fluvial transport. Furthermore, these results show that the application of U isotopes to sedimentary deposits provides robust information on the relationship between climate variability and catchment erosion, allowing us to assess the role of intrinsic vs extrinsic controls on sediment routing at the millennial timescale. • Uranium isotopes were measured in sediments of the Var River basin (France). • Catchment elevation and slope are main controls on U isotopes, not lithology. • Elevation and slope also control sediment residence times derived from U isotopes. • Independent estimates of sediment residence times show agreement with U isotopes. [ABSTRACT FROM AUTHOR]

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. • We investigate the response of the coastal Rio Grande to 1950s dam construction. • Image, topography and bathymetric data show the river has become net degradational. • Degradation affects the river all the way to the coast, a length of ~450 km. • This reach has no tributary sediment input and the upstream channel exposes bedrock. • These attributes limit sediment supply resulting in large length of degradation. [ABSTRACT FROM AUTHOR]

Megafans are ever-evolving landforms and sedimentary bodies produced by rivers that regularly change their courses through avulsions, in which the processes that cause the avulsions and their rates are tightly linked to the formation and evolution of the fans. The commonly cited condition required for the formation of megafans is a highly variable climate setting (monsoonal) which results in variable discharge and sediment transport capacity. However, some megafans occur in tropical settings in rivers with lower discharge variability but high sediment loads due to input from volcanic or tectonically active sources. We examine the Santa Clara Megafan (Costa Rica) to establish a model of explosive volcanism-driven megafan evolution, an alternative model to the climate-driven one. Studying chronological, archaeological, historical and remote sensing data we infer that major avulsions and shifts in the main rivers of this megafan are temporally and spatially correlated to strong eruptions (VEI ≥ 3). Furthermore, a period of ca. 1700 years with few changes in the fluvial system of the eastern sector of the megafan coincides with a low volcanic activity in its source area (i.e. Turrialba Volcano). The megafan avulsion rate was estimated as ∼1 major avulsion per century in the last five centuries, when four strong eruptions (∼0.8 eruptions/century) took place. These shifts in the rivers in response to the volcanic activity produced the activation or abandonment of megafan sedimentary lobes at a secular pace. The record of these changes will support future archaeological studies and lahar hazard assessments. Furthermore, the Santa Clara Megafan provides a model to explain the formation of megafans on Earth or on other planets through non-climatic processes. • A model for the evolution and development of a volcanogenic megafan is established. • Santa Clara Megafan exemplifies the formation of megafans through non-climatic processes. • Explosive eruptions can drive megafan formation instead of a highly variable climate regime. [ABSTRACT FROM AUTHOR]

• Quantitative reconstruction of soil erosion, sediment transport and delta deposition. • Soil erosion intensity was weak before the late Holocene, and increased afterwards. • The sediment deposition and soil erosion intensity were synchronous. • Soil erosion was dominated by climate as natural process before the middle Holocene. • Soil erosion was accelerated by human activities in the late Holocene. The Yellow River Basin is ideal for studying fluvial landscape evolution under climate change and human activities. Quantitative reconstruction of soil erosion, sediment transport, and delta deposition in the basin enhances understanding of the dynamic mechanisms of fluvial surface processes. This study reconstructs the spatial and temporal distribution of precipitation and vegetation cover based on geological profile records, paleoclimatic datasets, and historical documentation in the Yellow River basin during the Holocene. A landscape evolution model (Landlab) is introduced to simulate soil erosion and sediment deposition processes. Modeling results show that soil erosion intensity in the middle reaches of the Yellow River was weak during the early and middle Holocene. Since the late Holocene, soil erosion has increased due to vegetation degradation, as the climate became drier, but with more fluctuations. Since 2.0 ka (1 ka = 1,000 calibrated a B.P.), and especially since 1.0 ka, soil erosion in the middle reaches of the Yellow River increased rapidly due to strong human activity, with the intensity-six times higher than the average level before the late Holocene. Average sediment deposition thickness in the lower reach of the Yellow River (North China Plain) was about 2.4 m, and sediment volume was about 3.6 × 1011 m3. The deposition rate and soil erosion intensity showed synchronous changes during the Holocene. Since 2.0 ka, the sediment load and deposition rate have increased rapidly, to 6–10 times higher than in the early and middle Holocene. On this basis, the effects of climate change and human activities on the evolution of the fluvial landscape are discussed. [ABSTRACT FROM AUTHOR]

A 100–150 km-wide clustering of six large Late Cretaceous to Paleogene basin margin slope incisions along the northern Gulf of Mexico, up to 1000 m deep and 100 km long suggest a structural rather than eustatic control on their development. Counterintuitively to the existent models for canyon formation, the incisions align with the basinward trend of the San Marcos Uplift instead of forming in front of large adjacent sedimentary depocenters of the Rio Grande and Houston embayments where large rivers were discharging. Three plausible mechanisms are proposed for canyon formation, both triggered by tectonics. A shelf edge bulge scenario proposes, somewhat similar to previous continental flexure theory, a slow vertical rise of the sub-regional uplift onto the shelf and shelf-edge area that increased the gradient, triggered the initial incision and supported long term headward erosion. The high uplift rate scenario suggests a fast-vertical rise and extension of the uplift into the inner shelf area, whereby the high uplift rate diverted the rivers away from the area and protruded the shorelines as a headland. A third model envisions a deltaic headland caused by a tectonically locked (by uplift) river valley. The structural or deltaic headland diverted the longshore currents and sediment transport basinward, eventually causing a cascading over the shelf-edge and initiating incision and formation of the canyons. Erosion by sediment-laden flows played a critical role in initiating canyon formation, but with tectonics playing a preconditioning role. Other uplift regions around the Gulf of Mexico, Sabine, LaSalle and Tamaulipas (in Mexico) arches also align with large Wilcox Formation sedimentary incisions offshore, strongly suggesting that tectonism was a main control on canyon formation. • The arches in northern Gulf of Mexico relate to the Paleocene-Eocene canyons • The tectonic arches (uplifted areas) precondition the formation of a canyon • The recurrent large slope incisions formed by tectonics and shelf processes • Canyons are not aligned with fluvial valley axes but with tectonic arches [ABSTRACT FROM AUTHOR]

Central Chile has been affected by a sustained deficit in precipitation since 2010, the so-called mega drought, that has led to decreased water discharge in the fluvial basins of this region, affecting sediment fluxes. We investigate changes in sediment fluxes in the Itata River Basin (36-37°S, 11,326 km2) in central Chile over a period of 30 years (1986–2017) to evaluate the role of the mega drought in this region. We use a dataset of water discharge and suspended sediment concentration measured on a daily basis from a network of gauging stations that cover the basin from the mountain area to the river mouth. We estimated temporal variations in sediment fluxes within the basin. When comparing the pre-drought (1986–2009) and drought (2010–2017) periods, we found that annual sediment fluxes during the mega drought decreased between 28% and 67% from pre-drought values with variable intensity among the main tributaries. In addition, the season of maximum sediment transport in the basin has shifted from May–June to August during the mega drought along with a shift in the seasonality of maximum water discharge. We interpret that the precipitation deficit during the mega drought delays the recharge of the soil moisture and the establishment of favorable conditions to mobilize sediment in hillslopes. Because sediment transport in the basin occurs under conditions of lower water discharge compared to the pre-drought period, we also interpret the drought inhibits sediment transport in this system. Considering future climate projections for central Chile, it is likely that sedimentary dynamics in this basin, as well as similar fluvial basins in the region, will continue changing as the dry hydroclimatic conditions continue in the coming decades. • The mega drought has led to reduced sediment fluxes in the Itata River Basin. • Seasonality of peaks in sediment flux shifted from May–June to August. • Sediment is transported under conditions of relatively low water discharge. [ABSTRACT FROM AUTHOR]

Soil erosion, sediment transport, and delta deposition are of great importance to the evolution of fluvial landforms. In this study, we carry out a quantitative and comprehensive analysis to identify dynamic processes and mechanisms of soil erosion, fluvial plain deposition, and river delta formation in response to Holocene environmental changes over the past 10,000 years. In this investigation, the following results are achieved. First, spatial variations of the monsoon border (400 mm isohyet) resulted in dynamic changes in soil erosion on the Loess Plateau, sediment transportation downstream, and fluvial deposition on the plains and delta during the Holocene. Climate change and human activities played essential roles in variations of the plateau-plain-delta system during different periods. In the early Holocene, the number and scale of Neolithic cultural sites in the Yellow River basin increased. In the past 3000 years, human activities have strongly influenced and dominated environmental evolution in this region. Second, soil erosion was the critical contributor to the construction of the North China Plain. The Yellow River Delta provided the preliminary living environment where historical civilization in the Yellow River basin formed and developed, giving birth to the Yellow River civilization. Sediment discharge in the Loess Plateau was the primary source for river channel development and shaping of the river delta. • To identify processes and mechanisms of the Holocene soil erosion, fluvial plain deposition, and river delta formation in the middle and lower reaches of the Yellow River. • Spatial variations of the monsoon border resulted in dynamic changes in soil erosion on the Loess Plateau, sediment transportation downstream, and fluvial deposition on the plains and delta during the Holocene. • Climate change and human activities played essential roles in variations of the plateau-plain-delta system during different periods. [ABSTRACT FROM AUTHOR]

This study examines the role of topography as a dynamic boundary condition that limits the efficiency of fluvial erosion in the post-glacial Teton Range landscape. The volume of sediment currently stored in two major catchments was estimated using high-resolution LiDAR and geometric reconstructions of depth to bedrock. Seismic reflection data in Moran Bay reveals post-glacial sediment preserved behind a submerged moraine, which isolates the bay from the larger Jackson Lake depocenter. The volume of post-glacial sediment stored in the canyons and bay totals 173.82 ± 19.5336.0 (×10−3 km3), which translates to a catchment-wide sediment production rate of 0.17 ± 0.02 mm/yr. The rock-equivalent sediment volume in Moran Bay is 4.4 ± 0.9 (×10−3 km3), only ∼2.6% of the total post-glacial volume. While the estimated sediment production rate in the canyons is similar to the uplift rate, the denudation rate derived from Moran Bay sediment is 0.004 ± 0.001 mm/yr, implying highly inefficient post-glacial sediment transport. The fluvial system has been disequilibrated by glacial erosion such that interglacial valley profiles lack the steepness needed to transport sediment, delaying sediment evacuation until the next glacial advance, or until uplift sufficiently steepens the fluvial system so that it regains efficiency. Furthermore, colluvial production rates in the deglaciated valleys are close to long-term denudation and uplift rates, suggesting that once topography has been equilibrated to glacial erosion processes, subsequent glaciers do not need to produce much bedrock erosion, but mainly sweep out accumulated sediment to maintain equilibrium. • Sediment budget quantified for two catchments in the Teton Range, Wyoming. • Flattened post-glacial topography limits efficient fluvial erosion. • Sediment transport delayed until next glacial advance or uplift steepens system. • Colluvial production rates are close to long-term denudation and uplift rates. • Subsequent glaciers may not need to produce much bedrock erosion. [ABSTRACT FROM AUTHOR]

• A coupled model is presented of flow and sediment transport in sewer flushing channels. • Modeling results agree with observed data in a combined sewer channel in Paris. • Sediment transport may deviate from capacity, necessitating non-capacity modeling. • Flushing sediment deposits may affect flow considerably, inviting coupled modeling. Flushing is cost-effective for mitigating sediments and constraining environmental problems in sewer systems. Previous mathematical models are almost exclusively built upon simplified governing equations evoking the assumptions of slow bed evolution and sediment transport capacity, of which the applicability remains open to question. Here a 1D coupled non-capacity model is presented for non-uniform sediment transport in sewer flushing channels, as adapted from recently established shallow water hydro-sediment-morphodynamic models for fluvial processes. The present model is tested for an experimental flushing case in Paris's Des Coteaux catchment system. The computational results agree with observed data more closely than those of a previous decoupled capacity model. While the differences between decoupled and capacity models and the present coupled non-capacity model are minor for flushing processes of short-durations, they are more pronounced for sustained long-duration flushing processes. Physically, the adaptation of suspended sediments to capacity regime cannot be fulfilled quickly, though bedload sediments can adapt to capacity regime instantly. Also, the bed deformation rate is comparable to its counterpart of the flow depth. Therefore, coupled non-capacity modelling is suggested for general applications to sewer flushing channels, of which the computing cost is essentially equivalent to simplified models built upon decoupling and capacity assumptions. [Display omitted] [ABSTRACT FROM AUTHOR]

River confluences are sites that operate with complex interactions between material and energy, provided by the combination of different flow patterns that present continuous changes in flow structure and sediment transport. In addition, river confluences have intrinsic ecological value, where biophysical processes and ecosystem services may be concentrated. This study aimed to analyze the morphology, flow structure, and hydrosedimentary interaction along tributary-trunk river confluences in a bedrock-alluvial plateau basin. The study included a field investigation into the mixed bedrock-alluvial basin of morphology, bed sediments, and hydrologic condition (including the flow structure) of 29 confluences to identify and analyze the similarities and differences between these confluences at the basin scale. For example, we considered the Ivaí River Basin, a medium-sized dam-free river that exhibits a mixed bedrock-alluvial bed and heterogeneity in both geology and longitudinal profile. Our findings show that the flow structure and morphology are conditioned by geological aspects, and the composition of the flow is extremely affected by the Ivaí River. In general, our results presented distinct differences from field-based data described in the literature for alluvial river confluences. • Study of flow structure, morphology, and bed sediments in river confluences based on field measurements. • Influence of physiographic properties on hydro-sedimentary aspects in river confluences. • The characteristics of river confluences in mixed bedrock-alluvial channels. [ABSTRACT FROM AUTHOR]

Low-accommodation foreland basins have been of recent interest because their stratigraphic architectures provide insight into the interaction of tectonic subsidence and eustasy. Aptian-aged strata in Alberta, Canada (McMurray Formation and Wabiskaw Member) were deposited under the influence of a continental-scale river that longitudinally transported and debouched sediment into the southward-transgressing Boreal Sea in the distal, low-accommodation Western Canada Foreland Basin (WCFB). In this study, we present a high-resolution stratigraphic framework for the McMurray-Wabiskaw interval, which was deposited during a long-term transgression. Using wireline logs from >20,000 wells and more than 500 cores, regional mapping focused on resolving stratigraphic relationships and the distribution of depositional environments across approximately 60,000 km2 in the Athabasca Oil Sands Region of northeastern Alberta. The studied stratigraphic interval is part of a third-order sequence: the McMurray Formation developed during a period of relative sea-level fall, lowstand, and early transgression, while the overlying Wabiskaw Member was deposited during late transgression. Within the McMurray Formation, superimposed fourth-order sea-level fluctuations in this low accommodation setting created a complex amalgam of deltaic strata vertically and laterally juxtaposed with valley-fill deposits. Relative sea-level rises resulted in rapid transgressions in which the shorelines migrated >400 km landward. Thin (5–15 m thick), widespread deltaic parasequence sets were deposited during subsequent sea-level highstands. During relative sea-level falls, fluvial meandering channel belts developed in downcutting valleys, with associated shorelines dramatically prograding basinward. In the Wabiskaw Member, continued sea-level rise coupled with potentially decreasing sediment supply resulted in the development of tide-influenced/dominated channels and bars in estuaries. The final phase of the third-order transgression was marked by a regional transgressive ravinement event and several southwestward backstepping wave-dominated shorefaces/strandplains. This low-accommodation foreland basin succession demonstrates a transition from deposition in continental fluvial and marginal-marine settings subject to high-frequency eustatic changes, to open-marine conditions over a long-term transgression. • The McMurray-Wabiskaw interval demonstrates a high-resolution stratigraphic framework in a distal, low-accommodation foreland basin • The stratigraphy illustrates a transition from deposition in continental fluvial and marginal-marine settings to open-marine conditions over a 3rd-order transgression. • 4th-order sea-level fluctuations led to the deposition of repeated packages of meandering channel belt strata and deltaic deposits in the basin • The juxtaposition of fluvial meandering channel belts and deltas was more influenced by eustatic changes given its low accommodation and topographic relief • Later stage of the overall transgression was marked by tide-influenced channels and bars in estuaries and backstepping wave-dominated shorefaces/strandplains [ABSTRACT FROM AUTHOR]

• Geomorphic evolution of the deltaic channel and active river mouth is evaluated. • Morphologic variability of the active delta has a distinct spatial variance. • Offshore fine sediment dispersal processes are simulated and quantified. • The new river regime can improve channel stability and intensify deltaic recession. Intensified human interventions in river basins and deltas lead to more complexities of environmental changes during the Anthropocene. Changes in river regime especially a dramatic reduction in sediment delivery increase challenges of the morphological and ecological sustainability of river deltas. In evaluating deltaic risks and sustainable solutions, researches are often limited to single geomorphic units of the deltaic system, and investigations of sediment source to sink transfer at river deltas under recent river regimes are often missing. The Yellow River Delta (YRD) presents as a typical megadelta under stressors induced by changing environments. This study utilizes a period of 20-yr high-resolution topography data of the deltaic channel and its subaqueous delta to investigate sediment transport and source to sink process by integrated methods of field measurements and numerical simulations. The results indicate that the deltaic channel has transitioned from net accretion to erosion after the implementation of the Water-Sediment Regulation Scheme (WSRS) in 2002. The active river mouth experienced a slow accretion phase since the river channel diverted to Qing 8 channel, with a reduced vertical deposition rate of 0.15 m/yr, whilst its adjacent Gudong littoral zone had a −0.11 m/yr erosion rate. Under the new fluvial regime, the river-borne suspended sediment tends to transport southwards to the Laizhou Bay, followed by the river-derived sediment transport eastward and northward to the offshore delta. It is clear that with the continued human activities in the region, the YRD is at the potential state of deltaic transition both in the deltaic channel and its subaqueous delta. This transition is believed to be beneficial to the deltaic channel stability, but it could significantly impact on the geomorphic and ecologic sustainability of the entire deltaic system. [ABSTRACT FROM AUTHOR]

River deltas and enclosed lagoons represent a zone where fluvial and littoral processes interact through the redistribution, erosion, and deposition of sediment, which have a large impact on coastal management and engineering. The focus of the study is to understand the correct balance between strategies that maintain the navigational efficiency of tidal inlets and those that respect the ecological and economical functions of coastal lagoons. We applied an integrated modeling system that links multiple hydrodynamic and morphodynamic models to understand how coastal processes and associated sediment transport influences the functioning of the southern inlet of the Barbamarco Lagoon in the Po River Delta, Italy. Moreover, our study provides engineering solutions to improve the inlet functioning efficiency and a proposal for a monitoring plan. Our results highlight the importance of the seasonal effects of wave climate on the littoral sediment transport. Model outcomes show that the dredging volume is approximately 15,000 cubic meter/year for the southern inlet, which may vary with wave climate. However, creating a wider seaward tidal channel will reduce dredging activities over a longer interval than would sediment removal. A deeper and wider channel will deflect the along shore current seaward and cause sediment to bypass the inlet. Therefore, the sediment will reach the erosional side of the inlet and enhance the redistribution of the sediment, which may reduce over wash during storms and high-water levels. Our results illustrate the ephemeral equilibrium of tidal inlets and coastal lagoons in deltaic systems impacted by large riverine sediment delivery. Shore management scenarios and decisions rely on hydro-morphodynamic numerical models that predict best practices for coastal sustainability. [ABSTRACT FROM AUTHOR]

This applied study investigated the recovery trajectory of a channelized, lowland stream following removal of channel dredging and livestock grazing. Fine-scale relationships between macrophyte establishment and key physical descriptors (flow velocity, water depth and substrate) were analysed to explain channel adjustments and associated pioneer landform development. Through a process of biogeomorphic succession, macrophytes regulated the cohesiveness, erosion and transport of sediments, thereby affecting morphological responses and improving physical habitat diversity. The observations and analysis in this study capture the role of flow-sediment-vegetation interactions in self-organisation of a lowland stream, and demonstrate how better understanding of this phenomenon may inform river management and restoration science. Coupled relationships between macrophyte succession, flow and sediment patterns promoted hydromorphological recovery and re-naturalisation of the stream form. This dynamic was reflected in greater depth and substrate diversity characterised by increasing substrate coarseness and higher mean velocities. By regulating variations in local flow velocities, macrophytes created both low velocity-fine bed substrate and high velocity-coarser substrate habitat patches, and a range of shallow and deeper depth habitat patches. The process encouraged landform development on the channel margins and extension of the riverbank into the channel, thereby enhancing planform sinuosity as a two-stage channel formed. Such natural progression towards more heterogeneous habitat may sustain greater species diversity than artificially simplified rivers and deserves greater attention in restoration science. • Coupled relationships between plants and fluvial processes promoted physical recovery. • Macrophyte colonisation and succession regulated variations in local flow velocities. • This process enhanced depth, flow and substrate diversity. • Interactions at the fine scale promoted landform development at the landscape scale. [ABSTRACT FROM AUTHOR]

With one of the largest watersheds in Europe, draining complex geological terrains within the Alps, Bohemian Massif, Carpathians, Dinarides, and the Balkan Mountains, the Danube River valley has long been linked to the formation of thick loess deposits, particularly within the Middle and Lower Danube basins. However, uncertainty over the provenance of loess-palaeosol sequences along the Danube impacts our understanding of sediment generation mechanisms and hinders interpretation of paleoenvironmental proxies preserved in loess. To date, most of the studies investigating loess provenance in Europe have not attempted a standardised characterisation and synthesis of loess deposits with potential source rocks. Further, despite clear links identified between loess and rivers in Asia and Europe, detrital zircons have not yet been used systematically to investigate the relationship between loess and the fluvial sediments of the Danube and its tributaries. Finally, in European loess research, provenance fingerprinting has often been conducted using indirect approaches or bulk sample geochemical analyses, which have been shown to have a limited application in well mixed sedimentary bodies such as loess. This provenance study of loess along the Danube River integrates existing zircon U-Pb ages and Hf datasets for loess, rivers, and bedrock, with new loess zircon U-Pb and Hf results from loess sequences in Croatia, Serbia and Bulgaria. The results show that all surrounding mountain belts (i.e. the Alps, Bohemian Massif, Carpathians, Dinarides Alps, and Balkan Mountains) contribute primary sediment to loess deposits in the Danube valley via its modern tributary network. Critically sedimentary sources remain relatively homogenous along the river, with no apparent major change in source with tributary confluence, further highlighting the role of fluvial transport in homogenising sediment prior to final aeolian deposition. Whilst some small site variations can be observed, they are likely explained by contributions restricted to very local rock outcrops. Moreover, geomorphological results support floodplain sediments as the proximal sediment source and suggest that short-distance aeolian transport dominates sediment delivery to loess sequences, challenging distant sources hypotheses such as major Saharan sources. The identification of sediment sourced from lower elevation regions such as the Bohemian Massif, Dinarides, and Balkans, which did not support ice-caps, suggests that the role of glacial action in silt-size sediment production has been previously exaggerated. Therefore, the glacial and desert loess division inadequately separates and describes sediment generation processes. This research supports and furthers previous work, which suggests "mountain sourced and transported by-rivers" as a more appropriate term for the particles forming loess, at least in the Danube basin. [ABSTRACT FROM AUTHOR]

As evidenced by recent earthquakes in Chi-Chi, Wenchuan and Gorkha, seismic triggered landslides can have a major impact on river basins and landscape evolution. The location of landslides with respect to river channels plays an important role in post-earthquake sediment dynamics. As well as direct impacts such as damming rivers, the connectivity of landslides to the fluvial network can alter how quickly sediment is moved and stored within river basins. Such "landslide-landscape relationships" are dominated by factors including geology, hydrological and geomorphic characteristics of the landscape, the location of landslides, and the geometric properties (e.g., size and runout length) of landslides. These factors will therefore control the magnitude and retention of associated sediment transportation. But key characteristics of this relationship and how it controls sediment dynamics are poorly understood. In this research, we probed the impact of landslide location on sediment dynamics and the response of sediment yields through numerical modelling. We modified the landscape evolution model, CAESAR-Lisflood (CL) to enable sediment from landslides to be tracked throughout the basin, and to include spatial changes in landslide failure angle. The model was then used to investigate impact of the different landslide location scenarios on basin morphology and sediment yields. Using a study area in the Hongxi river basin, four different regions (upstream-high, upstream-low, downstream-high, and downstream-low) were determined in terms of the distance to main stream for an upstream and downstream basin - and in each region landslides were generated for twenty times at random locations. The model results indicate that the location of landslides could alter the fluvial sediment yields and spatial pattern of the sediment dynamics, with low location landslides being more likely to link or connect to the drainage system (higher connectivity) thus leading to greater sediment yields at basin outlet. Whereas material generated by high location landslides tended to accumulate on hillslopes, which could lead to a lag sediment evacuation from the basin. Simulations also show a big latency of sediment yield between upstream and downstream scenarios, which is largely due to the differences of the flow length from landslides source to the basin outlet. These findings are of great importance to understand the characteristics especially the location of landslides and measures for landslide risk prevention and mitigation. • Using a novel tracing technique embedded in CAESAR-Lisflood model. • Landslide location impact on sediment transport and dynamics was investigated. • Relationship between the connectivity of landslide and sediment yields was analyzed. [ABSTRACT FROM AUTHOR]

As an important component in East Asia sediment source-to-sink systems, small mountainous rivers in Taiwan deliver disproportionately large amounts of sediments to oceans. Although the modern fluvial sediment transport processes, discharge fluxes and sediment compositions have been well investigated, the drainage evolution of these mountainous rivers remains understudied and sediment fluxes are expected to vary greatly in glacial-interglacial cycles due to the tremendous climatic fluctuations. To define how the drainage of Taiwan mountainous rivers has evolved since the last glaciation, we target a sediment core (98 m in length) from the Zhuoshui River delta, western Taiwan and conduct sediment petrography, heavy mineral analysis, detrital zircon U Pb geochronology and clay mineralogy to investigate provenance variations and river basin evolution since 60 ka. Sediments of the last glaciation show comparatively high illite crystallinity index values, low metasedimentary lithic fragment and stable heavy mineral contents and similar detrital zircon U Pb ages with downstream signatures, indicating prominent sediment contributions from the Coastal Plain and Western Foothills regions (elevation <1 km) during the glaciation. However, characteristics of the deglacial and Holocene sediments indicate high contributions from the higher Hsueshan Range and Central Range regions (elevation >1 km). We suggest that headward extension and drainage capture since the deglaciation, which was most likely due to the increasing monsoon rainfall, account for the provenance variations in the discharged sediments. This implies a climate-driven drainage reorganization of the small mountainous rivers in Taiwan since the last glaciation. Our findings highlight the previously-overlooked, variable provenance information from Taiwan in glacial-interglacial cycles, and the dynamic source signatures are important to East Asia sediment source-to-sink studies. [Display omitted] • The provenance of Zhuoshui River sediments has changed since the last glaciation. • The provenance variation was caused by climate-driven drainage reorganization. • The river system of Zhuoshui River was not developed during the last glaciation. • The Zhuoshui River headward extended and captured other basins in the Holocene. [ABSTRACT FROM AUTHOR]

• An efficient 2D shallow water hydro-sediment-morphodynamic model is presented. • Morphodynamic processes under the impact of cascade movable weirs are studied. • Disparate morphological changes may result under different operation schemes. • An alternating operation scheme is proposed to constrain morphological changes. Cascade movable weirs have been increasingly widely used for river landscaping, water storage and ecological wellbeing. The weirs disturb the prior equilibrium in line with fluvial hydrology, sediment transport and geomorphology, and may lead to profound changes in the river, of which, however, the understanding has so far remained poor. Here, the first of its kind, a computational modelling study is presented on the morphodynamic processes under the impact of cascade hydraulic lifting dams, a type of movable weirs, as to the middle Fen River, China. A two-dimensional shallow water hydro-sediment-morphodynamic model is applied to resolve the fluvial processes, which is physically coupled and computationally efficient based on Finite Volume Method, unstructured mesh, local time stepping and parallel computing. Computational results show that the cascade dams alter the flow and sediment transport locally and on the reach scale in a distinct manner under different operation schemes, which in turn incur disparate morphological changes. Collapse of all the dams all year round would lead to net degradation in the study reach, with the floodplains aggrading and the main channel degrading. Operation of all the dams all year round may induce an overall aggradation both on the floodplains and in the main channel. If all the dams collapse in flood seasons and operate in dry seasons (CF), degradation occurs in the main channel whereas the floodplains experience aggradation, with a net degradation. When only the dams in the main channel operate all year round (OM), the floodplains degrade while the main channel aggrades, resulting in a net aggradation in the study reach. Either aggradation or degradation may prevail locally on floodplains, in main channel and individual subreaches. The present findings lead us to propose a complex operation scheme of annually alternating CF and OM to constrain significant morphological changes. [ABSTRACT FROM AUTHOR]

Studies of post-disturbance recovery in alluvial channels have considered form- and/or process-based paradigms to describe pathways to dynamic equilibrium. Yet, an integrated understanding of the covariation between form and process as well as the mechanisms that drive the recovery of each remains incomplete. We used a small (4 m tall) upland dam removal in New England as a natural experiment to investigate the timing and arc of a channel's return to dynamic equilibrium conditions, using both form- and process-based perspectives. We observed rapid post-removal adjustments in the former reservoir and downstream channels, then relative stability after a 2-year flow that occurred 6 months post-removal. This pattern, previously described as the "two-phase" model of channel recovery, was the result of temporal variations in the ratio of Shields values at the 'bankfull' flow (θ Q2) to critical Shields (θ cr). Immediately post-dam removal, elevated sediment flux was driven by the high ratio of θ Q2 to θ cr that resulted from decreased θ cr values and elevated θ Q2 values. As fine-grained sediments were winnowed, θ cr quickly recovered back to typical values within weeks to months. Subsequent adjustments to channel geometry by higher flows (i.e., the 2-year flow) redistributed bed shear stress and facilitated the relaxation of θ Q2 so that θ Q2 : θ cr ≈ 1, the threshold channel criterion. Despite the rapid return to process-based equilibrium, nearly 3 years post-removal the former reservoir reach remains morphologically immature with respect to reach-scale characteristics of natural channels. Our results indicate that process-based recovery is rapid, facilitated by relatively frequent, modest flows, and dependent on the adjustment of the characteristic bed grain size (d 50) to reflect prevailing hydraulics. In contrast, the slower attainment of form-based equilibrium depends on the timing of less frequent flows equal to and/or larger than the bankfull discharge as well as extrinsic channel controls such as local valley topography and the availability of recruitable large wood. [ABSTRACT FROM AUTHOR]

Ancient shoreline–shelf depositional systems are influenced by an unusually wide array of geological, biological and hydrodynamic processes, with sediment transport and deposition primarily determined by the interaction of river, wave (including storm) and tidal processes, and changes in relative sea level. Understanding the impact of these processes on shoreline–shelf morphodynamics and stratigraphic preservation remains challenging. Numerical modelling integrated with traditional facies analysis provides an increasingly viable approach, with the potential to quantify, and thereby improve understanding of, the impact of these complex coastal sedimentary processes. An integrated approach is presented here that focuses on palaeotidal modelling to investigate the controls on ancient tides and their influence on sedimentary deposition and preservation – one of the three cornerstones of the ternary process classification scheme of shoreline-shelf systems. Numerical tidal modelling methodology is reviewed and illustrated in three palaeotidal model case studies of different scales and focus. The results are synthesised in the context of shoreline–shelf processes, including a critique and modification of the process-based classification scheme. The emphasis on tidal processes reflects their global importance throughout Earth's history. Ancient palaeotidal models are able to highlight and quantify the following four controls on tidal processes: (1) the physiography (shape and depth) of oceans (1000s km scale) determines the degree of tidal resonance; (2) the physiography of ocean connections to partly enclosed water bodies (100–1000s km scale) determines the regional-scale flux of tidal energy (inflow versus outflow); (3) the physiography of continental shelves influences shelf tidal resonance potential; and (4) tides in relatively local-scale embayments (typically 1–10s km scale) are influenced by the balance of tidal amplification due to funnelling, shoaling and resonance effects versus frictional damping. In deep time, palaeogeographic and palaeobathymetric uncertainty can be accounted for in palaeotidal models by performing sensitivity analyses to different scenarios, across this range of spatial scales. These tidal process controls are incorporated into an updated predictive decision tree for determining shoreline–shelf process regime in terms of the relative interaction of wave, fluvial and tidal processes. The predictive decision tree considers the effects of basin physiography, shelf width and shoreline morphology on wave, fluvial and tidal processes separately. Uncertainty and ambiguity in applying the widely used three-tier process classification scheme are reduced by using the decision tree in conjunction with a proposed two-tier classification of process regime that is limited to primary and secondary processes. This two-tier classification scheme is illustrated in the three case studies, showing how integration of numerical modelling with facies analysis of the preserved stratigraphic record improves confidence in prediction of tide-influenced shoreline-shelf process regimes. Wider application of this approach will further improve process-based classifications and predictions of modern and ancient shoreline–shelf systems. [ABSTRACT FROM AUTHOR]

During the Quaternary period, the eastern Sahara's hydroclimate oscillated between wet and dry intervals. These oscillations caused drastic changes in precipitation rates, often associated with ancient human migrations. In particular, significant migration of riparian populations from the Nile Valley to the west and northwestward of the Sahara occurred during the African Humid Period (AHP), an episode of increased monsoons, which characterized North Africa in response to increasing insolation. Several fossil rivers, now preserved as ridges throughout southern Egypt due to their floodplains' deflation, contain archeological artifacts and thus represent a potentially important record of fluvial activity during this episode of past human dynamics and environmental change. Here we present 14C and Optically Stimulated Luminescence (OSL) ages of sediments preserved in these palaeorivers, which cluster within the AHP and are thus consistent with increased fluvial activity during this distinct humid period. Palaeohydraulic reconstructions based on grain size, channel geometry, and drainage area suggest typical precipitation intensities of 55–80 mm/h during sediment transport events. Given previous annual rainfall estimates, these hydrologic conditions may have lasted, or occurred, during the AHP up to 3–4 times more frequently than before and after this period. Such intense fluvial activity is consistent with monsoon intensification and may have rendered the area inhospitable for human settlements, congruent with population migration out of the Nile Valley during the AHP. These findings highlight links between past human ecodynamics and environmental signals, providing a concrete narrative of human population response to warming with potential echo in the current situation. • A chronological and palaeohydrological approach has been applied to six fossil rivers. • Our chronological constraints cluster within the AHP (13–5.2 ka BP). • Average rainfall intensity of 55–80 mm/h during channel sediment transport events occurred up to 3–4 times more frequently during the AHP. • Human migration from the Nile Valley may have been driven by increased flooding for ca. 3000 years. [ABSTRACT FROM AUTHOR]

A landscape's sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization. • We synthesize work to date exploring how boulders affect the evolution of nonglaciated, eroding hillslopes and channels. • Boulder dynamics are one facet of landscape self-organization, influencingtransient adjustment and steady-state form. • The effects of boulders on landscape shape can be quantified by combining field data with simplified theoretical models. • The key research need is for more field data on boulder dynamics spanning gradients in boundary conditions. [ABSTRACT FROM AUTHOR]