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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 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]

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]

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]

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]

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]

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]

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]