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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]

Lateral inputs from hillslopes and tributaries exert a variable impact upon the longitudinal connectivity of sediment transfer in river systems with differing drainage network configurations. Network topology influences channel slope and confinement at confluence zones, thereby affecting patterns of sediment storage and the conveyance of sediments through catchments. Rates of disturbance response, patterns of sediment propagation, and the implications for connectivity and recovery were assessed in two neighbouring catchments with differing network configurations on the East Cape of New Zealand. Both catchments were subject to forest clearing in the late 1940s and a major cyclonic storm in 1988. However, reconstruction of landslide runout pathways, and characterization of connectivity using a Tokunaga framework, demonstrates different patterns and rates of sediment transfer and storage in a dendritic network relative to a more elongate, herringbone drainage network. The dendritic network has a higher rate of sediment transfer between storage sites in successive Strahler orders, whereas longitudinal connectivity along the fourth-order mainstem is disrupted by lateral sediment inputs from multiple low-order tributaries in the more elongate, herringbone network. In both cases the most dynamic (‘hotspot’) reaches are associated with a high degree of network side-branching. [ABSTRACT FROM AUTHOR]

Reservoirs behind dams act as deposition sites for much of the sediment being transported by rivers. As a result, the downstream river flow can be well below the transport capacity for bed-material. This promotes bed erosion and other geomorphic changes over some length of river located immediately downstream from a dam. These adjustments have been characterized for the Trinity River, TX, downstream of Livingston Dam. Field measurements and results from a 1D numerical model define a 50–60 river kilometer segment of river undergoing bed erosion as the transport capacity for bed material is reestablished. Consequences of this erosion include lowering of the channel bed, reduction in the sediment volume of channel bars, coarsening of sediment on bar tops, steepening of channel banks, and reduction in lateral migration rates of river bends. Repeat surveys of the river long profile reveals that 40 yr of dam closure has produced up to seven meters of channel-bottom incision downstream of the dam, transforming an initially linear profile into a convex-up long profile. The model output matches this observed change, providing confidence that calculated estimates for spatial and temporal changes in bed-material sediment flux can be used to explore the long-term signature of dam influence on the geomorphology of a sand-bed channel. Measurements of channel geometry, profile, lateral migration, and grain size of the lower Trinity River with distance downstream define both the trend and expected variability about the trend associated with the disruption to the bed-material load. [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]

Several works have discussed the morphological evolution in the Changjiang Estuary (CJE) in recent years. The erosion of its subaqueous delta in recent decades has been ascribed to a decline in fluvial sediment input. However, the interaction between the reduction of riverine sediment load and human activities in the estuary that could have caused morphological change has not been considered. In this work we provide evidence on the morphological evolution around the delta front zone since 1986 and use a numerical model to explore the correlation between the change in hydrodynamics and the evolution pattern. Bathymetric data analysis suggests a decrease of net accretion rate from 16.7 mm/year (1986–1997) to 9.1 mm/year (1997–2010) in the study area. Spatially, the tidal flats accreted whereas the subaqueous delta switched from deposition between 1986 and 1997 to erosion between 1997 and 2010. We used two indicators, tidal energy dissipation and erosion rate, to quantify the change in hydrodynamics and found that the erosion of the subaqueous delta in recent decades can readily be explained by the alteration of the hydrodynamics. The newly built navigation training works in the North Passage had a significant effect on the estuarine hydrodynamics, resulting in a local morphological adjustment. This erosion generated a new source of sediments to maintain the high suspended sediment concentration and tidal flat progradation. The erosion of the subaqueous delta may continue and gradually slow down until the altered hydrodynamics and morphology reach an equilibrium state in the future. [ABSTRACT FROM AUTHOR]

This study examines the fluvial readjustment of a Javanese river impacted by the major eruption of the Merapi volcano (Indonesia) in October and November 2010. The basin of the Opak River, located on the southern flank of the Merapi, was subject to substantial sediment input related to massive pyroclastic deposits that were remobilized by numerous lahars during the year after the eruption. Two study sites were equipped in order to evaluate the morphodynamic evolution of the riverbed of the Opak River. Topographic surveys, bedload particle marking, and suspended sediment sampling revealed an important sediment mobilization during efficient flash floods. Surprisingly, no bed aggradation related to the progradation of a sediment wave was observed. Two years after the eruptive event, marked bed incision was observed. The Opak River readjustment differs from that of other fluvial systems affected by massive eruptions in two ways. Firstly, local population extracted the sand and blocks injected by the eruption as they represent a valuable economic resource. Secondly, several dams trapped the major part of the sediment load remobilized by lahars. [ABSTRACT FROM AUTHOR]

This study describes channel changes following completion of the Provo River Restoration Project (PRRP), the largest stream restoration project in Utah and one of the largest projects in the United States in which a gravel-bed river was fully reconstructed. We summarize project objectives and the design process, and we analyze monitoring data collected during the first 7 years after project completion. Post-project channel adjustment during the study period included two phases: (i) an initial phase of rapid, but small-scale, adjustment during the first years after stream flow was introduced to the newly constructed channel and (ii) a subsequent period of more gradual topographic adjustment and channel migration. Analysis of aerial imagery and ground-survey data demonstrate that the channel has been more dynamic in the downstream 4 km where a local source contributes a significant annual supply of bed material. Here, the channel migrates and exhibits channel adjustments that are more consistent with project objectives. The upstream 12 km of the PRRP are sediment starved, the channel has been laterally stable, and this condition may not be consistent with large-scale project objectives. [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]

The dominant processes of sediment transport and morphological changes are different between rivers and coastal areas. In many situations rivers, estuaries and coasts need to be modelled together in an integrated way. This paper investigates the capability of a freely available, open source, coastal morphodynamic software (XBeach) to estimate sediment transport and morphological changes in fluvial environments. Four benchmark tests were designed to test code performance and included simple unidirectional flow cases, complex topography, fluvial flood flows (hydrographs) and dam break scenarios (fast transient, supercritical flow fields). The results were compared to laboratory experimental results or simulations results from industry standard software. Analysis suggested that the coastal morphodynamic code is able to simulate sediment transport and morphological changes in a fluvial environment, but there are limitations to what can be modelled and the accuracy to which they are modelled. General morphological trends are replicated reasonably well by the code however specific bed forms and rapid erosive responses are less well modelled. Suggestions are made for applicability of the code, code improvement and future work. [ABSTRACT FROM AUTHOR]

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974. This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts. [ABSTRACT FROM AUTHOR]

We document the detailed morphology and bed sediment size distribution of a stream channel upstream and downstream of a 200-year-old run-of-river dam on the Red Clay Creek, a fifth order stream in the Piedmont of northern Delaware, and combine these data with HEC-RAS modeling and bedload transport computations. We hypothesize that coarse bed material can be carried through run-of-river impoundments before they completely fill with sediment, and we explore mechanisms to facilitate this transport. Only 25% of the accommodation space in our study site is filled with sediment, and maximum water depths are approximately equal to the dam height. All grain-size fractions present upstream of the impoundment are also present throughout the impoundment. A characteristic coarse-grained sloping ramp leads from the floor of the impoundment to the crest of the dam. A 2.3-m-deep plunge pool has been excavated below the dam, followed immediately downstream by a mid-channel bar composed of coarse bed material similar in size distribution to the bed material of the impoundment. The mid-channel bar stores 1472 m 3 of sediment, exceeding the volume excavated from the plunge pool by a factor of 2.8. These field observations are typical of five other sites nearby and suggest that all bed material grain-size fractions supplied from upstream can be transported through the impoundment, up the sloping ramp, and over the top of the dam. Sediment transport computations suggest that all grain sizes are in transport upstream and within the impoundment at all discharges with return periods from 1 to 50 years. Our computations suggest that transport of coarse bed material through the impoundment is facilitated by its smooth, sandy bed. Model results suggest that the impoundment is currently aggrading at 0.26 m/year, but bed elevations may be recovering after recent scour from a series of large floods during water year 2011–2012. We propose that impoundments upstream of these run-of-river dams behave as long pools that adjust their bed elevation and texture to transport the load supplied by the watershed, rather than as impounded reservoirs with little bed material transport capacity. Scour may only occur during episodic high flows, followed by aggradation during periods of low flow. [ABSTRACT FROM AUTHOR]

The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21 ± 3 million m 3 of sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000 ± 3,400,000 tonnes of transported sediment, with 1,100,000 and 7,100,000 t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000 ± 80,000 t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292 m 3 /s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300 mg/L and gauged bedload transport as large as 24,700 t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000 t in year 1 and 5,400,000 t in year 2. Of the total 6,300,000 ± 3,200,000 t of suspended-sediment load, 3,400,000 t consisted of silt and clay and 2,900,000 t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000 ± 360,000 t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of < 2-mm bedload was estimated to be on the order of 1.5 Mt. [ABSTRACT FROM AUTHOR]

Various field-based studies of meander development have been realised, yet fluviomorphological processes and their spatial patterns over bends are not fully understood. In this paper, the processes of three natural meander bends during a flood event are dissected using a unique study approach, which combines traditional field surveying, close-range remote sensing, and computational modelling. The combined study approach improves the spatial and temporal resolutions of the data compared to traditional survey methods, although the bathymetric data collection needs improvement. The analyses indicate that the influence of a flood on the morphological changes of point bars depends on the stage of the bend's development. The point bars may also experience lateral growth during low discharge. However, the point bar is too large a unit to find simplified relationships between the discharge and morphological changes. Instead, one should concentrate on different parts of the bend and the point bar. In general, the longer the inundation period of one part, the more probable that the area will experience net deposition. The magnitude of the sediment transport and the net morphological change are not interconnected: Areas of net erosion and deposition can experience either high or low sediment transport rates during a flood. The results are compared with earlier theories of meander development, which are also assessed. [ABSTRACT FROM AUTHOR]

This paper reports findings from experimental studies of sediment transport and bed morphology at asymmetrical confluences with hyperconcentrated tributary flows in the upper Yellow River. The results indicate that the hyperconcentrated flow confluence can be divided into four hydraulic regions, including the backwater zone above the upstream junction corner, the maximum velocity area, the separation flow zone, and the post-confluence region downstream of the junction corner. The bed morphology also consists of four basic elements, including the sandbar in the backwater zone, the bar in the separation flow zone, the thalweg for flow conveyance and sediment transport, and bars in the reach downstream of the separation zone. The sediment load of the hyperconcentrated flow from the tributary was the most important control on fluvial processes at such confluences. The increase in deposition in the backwater zone as the sediment load increased was almost linear, and the depth of sediment deposition in the backwater zone was approximately normal in distribution. The validity of a conceptual model for discriminating the status of the backwater effect, developed earlier from field data using the relationship between the sediment load and water volume of hyperconcentrated flows, was confirmed by the experiments. Deposition in the reach downstream of the junction, sandbar height in the backwater zone, and the width and length of the separation zone bar all tended to increase as the sediment load in the tributary increased. An obvious upstream-directed density current occurred in the backwater zone when the sediment concentration of the hyperconcentrated flow exceeded a critical value. The travel distance of the density current increased as the sediment load in the tributary increased. A formula was proposed, based on sediment continuity, for estimating the deposition volume in the reach downstream of the junction. Compared with ordinary sediment-laden flow confluences, hyperconcentrated flow confluences have a sandbar in the backwater zone associated with an upstream-directed density current that may sometimes block the main channel. Hyperconcentrated flow confluences have a thalweg, and so are different from debris flow confluences, which have a fan-shaped deposit. [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]

In this paper, we used a bedload transport data set collected at North Fork Caspar Creek, California, to examine temporal variation in sediment transport rate over a 7-year period. Using a hierarchical mixed-effects model, we examined across and within-event variation to determine whether the bedload-shear stress relation trends over time. The relation between bedload transport and shear stress was modeled using log(Qb)α+β*log(τ)+ε, where α and β are constants and ε is an error term. Depending on the length of observation, α and β can vary over several orders of magnitude, making modeling of transport based on flow challenging and highly inaccurate. We found a higher order yearly relation between bedload and shear stress, indicating systematic changes to the system over time. In the absence of significant additions to the system, α decreases roughly linearly over time, while β does not show any trend. From the systematic decline in α, we infer changes to sediment availability in the stream over time. Mixed-effects models have the potential to be a useful predictive tool in fluvial geomorphology, as they are more powerful at detecting trends in sediment transport rates than individual linear regressions. [ABSTRACT FROM AUTHOR]

Abstract: Alpine regions are strongly affected by the global climate change. Alpine glaciers have had a negative net balance since the end of the Little Ice Age (LIA). Proglacial areas with freshly exposed subglacial sediments are expanding due to the retreat of glaciers. These sediments (moraines, tills, glaciofluvial deposits, etc.) are unconsolidated, nearly unvegetated and therefore unstable and highly vulnerable to surface changes triggered by geomorphological processes. Particularly during heavy rainfall events, glacial and glaciofluvial deposits are remobilized and transported within the fluvial system. This study is focused on rapidly changing surfaces in the proglacial fluvial system of the Fagge River, which drains the Gepatschferner, one of the biggest glaciers in Austria, and is located in the Kaunertal/Austria. The field site covers an area from the snout of the glacier (2206ma.s.l.) to the outlet of the Fagge River into the Gepatsch Reservoir at (1750ma.s.l.). The main goal of this study is to measure surface changes and quantify mass balances of important sediment sources (alluvial plains, bars) in the proglacial area, which are directly connected to the fluvial system. For this purpose, multiple terrestrial laser scans are performed with an Optech ILRIS-36D laser scanner. During the field season in 2011 and 2012, several sediment sources were scanned at least twice. Significant surface changes occurred during the investigation period, mainly caused by an extreme flood event after heavy rain on August 26, 2012. Large amounts of sediment (>70,000m3) were remobilized, especially in the upper parts of the proglacial area, and were accumulated further downstream during this event. [Copyright &y& Elsevier]

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]

Effective discharge plays a major part in sediment and nutrient transport, landscape modification, and river restoration. However, a complete understanding of effective discharge is still missing because of heterogeneity in various fluvial processes. The present study investigates the role of effective discharge for suspended sediment transport in a regulated macrochannel river system, the Brahmani River, India. The natural flow-sediment regime and channel pattern of the Brahmani River have been altered by the Rengali dam. The effective discharge is estimated with both analytical and deterministic methods using long-term hydrological data. The results show the control of regulated flow and sediment regime in the post-dam period contributes to the reduction in effective discharge (1849 m3/s to 820 m3/s), where frequent moderate floods are able to transport a maximum portion of sediment load. An effective discharge integrated stream power curve is formulated, which accurately predicts the channel transition (from sinuous to weakly braided) in the post-dam period. Finally, the proposed probability of braiding captures the chute formation correctly, which further highlights the hierarchy of energy dissipation and morphological continuum in the Brahmani River. A conceptual plot showing the link between observed geomorphic units, morphological analysis and process-based stream power estimation. [Display omitted] • Effective discharge converges to a moderate flood in the macrochannel Brahmani • Effective discharge integrated stream power curve predicts the channel transition • River goes through transition to accommodate effective discharge in inset channel • Proposed probability of braiding accurately captures the chute formation [ABSTRACT FROM AUTHOR]

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34 × 107 m3 and a cumulative deposition of 8.79 × 107 m3, resulting in a net deposition of 2.45 × 107 m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35 km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47 m in the upper 98 km, but increased by 0.96 m in the final 35 km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise. • Assessment of channel bed dynamics of a large lowland alluvial river on the Mississippi River Delta • Changes in volume and mass of the channel beds are quantified, and factors affecting them are analyzed. • Channel degradation occurs in the upper reach of the Lower Atchafalaya, mainly affected by flood frequency. • Channel aggradation mainly occurs in the river's final 35 km despite of reduced sediment delivery. [ABSTRACT FROM AUTHOR]

Interdune areas are an important part of dune systems, as they affect dune evolution and sand sea development. However, there has been little work on the provenance of interdune sediments. Our lack of knowledge of these sediments prevents a more complete understanding of sand sea formation and evolution. In the present study, we focused on the provenance of interdune sediments in the Kumtagh Sand Sea based on their granular, mineral and geochemical characteristics, then further identified the spatial scope of the contribution of these sources to the interdune sediments and the associated transport mechanisms. Although sediment supply analysis suggested that the interdune sand may derive from the Altyn Tagh Mountains, Beishan Mountains, and Sanlongsha dune field, the provenance signature based on the K/Rb and K/Ba ratios revealed that the ultimate sources were the Altyn Tagh Mountains (in the south) and the Beishan Mountains (in the north). By plotting the identified provenance results on a map of the sand sea, we found that the spatial scopes of contributions from these sources were separated at around 40°N. However, the existence of a fluvial load (EM1) and of reworked coarse particles (EM3) in the northern sand sea demonstrated that the interdune sands in this region were partly derived from the Altyn Tagh Mountains in the south. Consequently, by combining our results with the transport agents condition, we concluded that the Altyn Tagh Mountains supplied sediments to interdune areas by fluvial transport throughout the sand sea, but that sediment in the northern sand sea was subsequently reworked and mixed with windblown sand originated from the Beishan Mountains. • We studied the provenance of interdune areas, which is often ignored. • New and robust methods were used to identify sediment provenance. • We defined the spatial scopes of the contribution from different sources. • Sediment transport represented a synthesis of fluvial and aeolian processes. [ABSTRACT FROM AUTHOR]

• We present an analysis of chute cutoff along a non-vegetated, ephemeral river. • Twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. • Chute cutoffs occur despite low stream powers (<10 W/m2) and cohesive sediments. • Episodic La Niña-driven flood events drive clusters of chute cutoffs. • Findings contribute to improved understanding of dryland river meander dynamics. The origin and development of meandering river planforms has long been a focus of research in the geosciences. Most attention has focused on perennial meandering rivers with well developed riparian vegetation assemblages, and while increasing interest is turning towards the dynamics and sedimentology of ephemeral meandering rivers with sparse to no vegetation, additional field data are required. As a contribution, this study presents a remote-sensing and field-based analysis of chute cutoff-driven abandonment and sedimentation of meander bends along the fine-grained, non-vegetated, ephemeral Río Colorado on the Bolivia Altiplano. Along the 25 km long sinuous study reach, located between 25 and 50 km upstream of the present-day margins of Salar de Uyuni, quasi-regular flood events (typically at least one per year) drive bend cutoffs and wider channel-floodplain dynamics, despite low specific stream power (<10 W/m2) and cohesive (dominantly silt and clay) bed and bank sediments. Along the reach, twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. Three chute cutoffs (CC1, CC2, CC3) occurred between 1996 and 2016; for one bend (CC3), high-resolution (<0.65 m) satellite imagery and field investigations reveal the details of pre-, mid- and post-chute cutoff processes and sedimentary products. Together, the findings suggest that for any given bend, an increase in bend amplitude (mean ratio of meander bend length to chute channel length ∼4) combines with a typically high diversion angle between the channel-belt axis and the upstream limb of the meander bend (mean ∼98°) to result in declining flow efficiencies. This enhances overbank flooding and promotes deposition along the upstream limb of the bend. Overbank flooding promotes the development of chute channels, which commonly initiate as shallow (typically <1 m) headward eroding channels, while sediment derived from overbank flow and headward erosion tends to be deposited in the downstream limb of the bend. By obstructing flow, such deposition reduces sediment transport capacity within the meander bend as a whole, thereby further inducing deposition within the upstream limb. During subsequent floods, deepening and widening of a dominant chute channel and sedimentation and shallowing of the meander bend continues. Along the reach, episodic La Niña-driven flood events drive phases of more rapid bend migration and clusters of chute cutoffs. Our findings contribute to a more comprehensive understanding of meandering river dynamics across the full range of Earth's conditions, and also may help to improve interpretations of Earth's pre-vegetation rivers and meandering fluvial forms on other planetary bodies. [ABSTRACT FROM AUTHOR]