Search

• 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]

The Paleozoic arkoses of the Inkisi Group in both the Republic and the Democratic Republic of the Congo has been investigated in order to examine whether they can represent an alluvial fan originating from the Gondwana supermountains in Northern and East Africa. This study focuses on outcrops' description, lithofacies, bounding surfaces, architectural and sequential organization of sedimentary bodies and paleocurrents. The aim is to determine the nature of the Inkisi sandstones, the provenance of its sediments and the depositional setting, in order to test whether the East African orogen was also the sediments source for the formation of sedimentary fan in the western part of Central Africa. The study shows that the Inkisi Group consists of various sandstone lithofacies, amalgamated by conglomerates. The sediments are immature composed of quartz and feldspars associate with lithic fragments and micas. The sediments originated from the erosion of the West-Congolian belt and the Congo Craton, and were transported by braided fluvial system under mainly upper flow regime and secondarily lower flow regime. Sediments were deposited in the Inkisi basin, which was extensional and affected by burial. The Inkisi sediments setting up in a proximal to medial distributary zone of an alluvial megafan. The flows were directed southward, as opposed to paleofans originating from the Gondwana supermountain in Northern and East Africa. Thus, the Inkisi sandstone constitutes a new alluvial megafan distinguished from the Gondwana superfan derived from the Gondwana supermountain. It originated from the Pan-African belt of Mayombe and the Congo Craton. We name this megafan " the Cambro-Ordovician alluvial megafan of Central Africa ". • The Inkisi Group are arkoses and sub-arkoses associated with conglomerates. Conglomerates are gravel bars and gravity flows. • The Inkisi sediments originated from the peneplanation of the Pan-African West-Congolian/Mayombe and the Congo craton. • The Inkisi sediments were transported by a braided fluvial system and deposited in an extensional basin affected by burial. • The Inkisi Group is an alluvial megafan deposits. • The Inkisi Groupe constitutes a Lower Paleozoic alluvial megafan in Central Africa. [ABSTRACT FROM AUTHOR]

The properties of sediment grain shape provide valuable information about the transport mechanisms in different sedimentary and geomorphological environments. With the emergence of new, high-resolution analytical techniques, it has become possible to quickly examine the grain shape properties of a large number of individual mineral grains. In this study, we used automated image analysis (Malvern Morphologi G3SE-ID) to investigate mineral particles of four sediment types from different depositional environments (sand sheet (1), floodplain (2) and fluvial channel deposits (3), Pleistocene infilling material of sand wedges (4), n = 20) in the Carpathian Basin (Central Europe). Our primary objective was to identify quantitative key variables that can help objectively distinguish certain geomorphological environments located in the Carpathian Basin. In our analysis and data processing (which included techniques such as hierarchical cluster analysis, Wilks' λ, Kruskal–Wallis, multivariate analysis of variance and principal component analysis) we focused on four variables related to grain shape: circularity (form), convexity (surface texture), solidity (roundness) and elongation (form). The form of sedimentary grains depends largely on the physical properties of their source area, while the roundness depends on the energy of the transport medium and the distance of transportation. Surface texture or convexity can change in a relatively short time in a fluvial environment. The study revealed that distinguishing geomorphological environments can be achieved by analyzing the circularity, convexity and solidity parameters of the sediment grains. Based on the established grouping, the analyses carried out with hundred repetitions showed that high sensitivity circularity, convexity and solidity variables were the most effective attributes regarding Kruskal–Wallis test statistics that provided significant (p < 0.001) results between the analyzed sedimentary environments, while the elongation was not able to provide significant results between the grouped samples. Statistical analyses of the MANOVA test with hundredfold repetitions showed significant differences between the derived groups. Wilks' λ test statistics and PCA showed that convexity and high sensitivity circularity discriminate the groups. Separate analyses of aeolian and fluvial sediments have been carried out. The Kruskal–Wallis analysis of variance showed the significant differences considering all four variables, and differences were also significant in the case of the MANOVA test. Wilks' λ test statistics and PCA showed that convexity, high sensitivity circularity and solidity discriminate the groups. According to our results, the circularity parameter can provide information about the transport distance, while the solidity parameter can indicate the transport energy. The convexity parameter can serve as an indicator of both transport distance, as well as post-depositional processes. Some infilling materials underwent multiple transport processes, including high energy aqueous, wind transport mechanisms, and the post-depositional alteration process (frost weathering), while others originated from sand-sheet covered areas (active during Pleistocene glacials). The solidity parameter proved effective in separating sediments with similarly high convexity values (smooth surface), which were, in our case, from recent aeolian and fluvial environments. This result was due to the investigated fluvial sediments that inherited their form and low level of roundness from their source area. Our research supports that aeolian transport is more effective in rounding the grains than the aqueous environment. Using automated static image analysis producing statistically stable results with hundreds of analyzed mineral grains provides useful indicators for paleogeographical reconstruction studies by investigating paleo and recent sediments. • The high HS circularity values can indicate a longer transport distance. • The solidity factors can be used to identify relative transport energy. • The convexity variable may indicate the post-depositional process. • The low level of roundness can be inherited from the source environment. • Automated static image analysis produces statistically robust results on mineral grains. [ABSTRACT FROM AUTHOR]

We have measured the He isotope composition in detrital native gold, copper, silver, Pt 3 Fe and OsIr alloy grains from placer deposits in order to assess the extent to which cosmogenic He can be quantified and used to determine the exposure history of grains in sedimentary systems. 3He/4He ratios (0.1–273 R a) exceed values typical of radiogenic He. 4He is dominantly the result of the decay of U and Th, or 190Pt in the case of Pt-rich alloys. 3He concentrations in grains from alluvial deposits range from 0.05 to 2 × 108 at g−1 and appears to be dominantly cosmogenic in origin. The absence of significant nucleogenic or mantle-derived 3He in metals is tentatively confirmed by the analysis of grains from underground mine workings. The implantation of cosmogenic He is not the main source of 3He in alluvial metals but can be relatively important for small grains. New GEANT4-based calculations predict the production rate of 3He (including in precursor 3H) from Au and Pt to be approximately 25 at g−1 yr. Using this we determine that cosmogenic 3He concentrations in the detrital grains record minimum exposure times of between 0.5 and 7 million years that generally increase with distance from the putative source. Paired fragments from three large grains show that there is considerable heterogeneity in cosmogenic He concentrations that may reflect the complex origin of detrital metal grains. This work suggests that cosmogenic He in detrital metals and minerals may find use for providing the chronology of sedimentary processes. Concentration of 3He plotted versus deposit type (i.e. the relative distance the grain had traveled from source). Using this we determine that cosmogenic 3He concentrations in the detrital grains record minimum exposure times of between 0.5 and 7 million years, that generally increase with distance from the putative source. Unlabelled Image • Helium isotopes are measured in alluvial Au, Pt 3 Fe, Cu and OsIr. • 3He/4He ratios (0.1–273 R a) exceed values typical of radiogenic He • Production rates of 3He in these metals are determined using GEANT4 • Minimum transportation-storage ages of alluvial metals vary from 0.5 to 7 Myr. [ABSTRACT FROM AUTHOR]

Abstract Megaclast-dominated fluvial systems have been largely ignored by researchers. Yet megaclasts exert an important control on channel geomorphology, flow behaviour and sediment transport, and such systems are probably far more common than this neglect would suggest. Despite the explicit identification of megaclasts as the products of cataclysmic flow, observation of modern fluvial systems confirms that megaclasts can be entrained by flows that lie within the normal flow regime. The pattern of fining along the megaclast-dominated Sabeto River of western Viti Levu, Fiji reveals a strong negative exponential relationship between particle size and distance downstream, and a rate of downstream fining comparable with that of other gravel-bed rivers. The downstream fining pattern is straightforward and step-like discontinuities in the fining gradient are absent. This is suggestive of a sedimentary system only minimally disrupted by the supply of sediment from tributaries or the reworking of material from sources such as hillslopes and alluvial terraces. There is evidence, however, of anomalously coarse sub-populations at two sites. These coarser components must pre-date the other deposits at the sites. We speculate that they represent the products of earlier and higher-magnitude events. The bed sediments may thus be composite features, made up of individual components each transported by a specific event of particular magnitude. Our work confirms the relationship between the rate of downstream fining and particle size. This may arise because the distance over which individual particles are transported is likely to be size dependent. It is also possible that abrasion may be more effective in the case of larger particles than smaller ones, with the result that the coarser fraction of the river's bed sediment fines downstream more rapidly than the finer fraction. There is a long-standing debate about the relative roles of abrasion and sorting in downstream fining. Our data reveal little evidence for a downstream increase in the roundness of the river's gravels, implying that abrasion is likely to play a minor role in the fining process in this system. Highlights • Megaclast-dominated rivers occur widely, but have been largely ignored by researchers. • Megaclasts may be entrained by normal flows and not solely by catastrophic events. • Megaclast downstream-fining rates are comparable with those of other gravel-bed rivers. • There is a close relationship between bed-sediment size and rate of downstream fining. • Bed-sediment assemblages consist of discrete components, each transported by individual events. [ABSTRACT FROM AUTHOR]

Abstract Fluvial suspended sediment concentration and discharge is most commonly estimated by applying rating curves developed over a given base period of sediment monitoring to longer, continuous records of water discharge. This practice invokes the tacit assumption of stationary suspended sediment concentration–water discharge relationships (i.e. suspended sediment behavior). Here the ramifications of this assumption are explored through an investigation into the temporal dependence of suspended sediment concentration–discharge relationships for all hydrologic stations on small rivers draining the west coast of the continental US that have been monitored for suspended sediment over a period of 10 years or more. Persistent suspended sediment behavior was found to be the norm, with 19 of 24 stations expressing apparent monotonic trends over their base periods, and 21 stations with finer scale (subannual to interdecadal) persistent patterns. The identified persistent periods of suspended sediment behavior corresponded to distinct rating curves that differed significantly from other periods in terms of offset, slope, or both. Assuming stationary suspended sediment behavior resulted in 1% to 63% biasing of suspended sediment load. Applying rating curves developed for single persistent periods to broader discharge records resulted in up to 10× potential error in estimated sediment load. The risk of falsely characterizing the long-term (interannual-scale) suspended sediment behavior of a river through a single persistent period of elevated or depressed behavior may be much higher than expected. The average duration of identified persistent periods was 8.6 years, which is longer than the base period of suspended sediment monitoring for most river stations in the US. Peak discharge (high discharge events 20 to 1000 times mean discharge) was usually followed by periods of increased sediment concentration across US Pacific Coast. Seven rivers in Oregon and northern California displayed the same pattern of decadal- to interdecadal-scale elevated suspended sediment concentrations following the Great Christmas Floods of 1964. Highlights • Persistent patterns of suspended sediment dynamics appear to be the norm. • 21 of 24 investigated rivers displayed persistent suspended sediment patterns. • Persistent suspended sediment patterns were 4 months to 36 years in duration. • Persistent patterns resulted in up to a 10× difference in sediment load. • Peak discharge events were usually followed by increases in sediment concentration. [ABSTRACT FROM AUTHOR]

Abstract The particle size, shape, surface texture and composition of aeolian and fluvial sedimentary samples were analyzed and compared using optical microscope-based automatized image analysis, in order to suggest indicators that could discriminate these two modes of transport on Mars. The principal goal is to assess if such indicators – established on Earth – could be used on Mars thereby aiding interpretations on the origin of sediments analysed by the ExoMars 2020 rover during its mission. Wind and water transported sand from, respectively, the Sahara Desert and the Maros river bar in Hungary were investigated and treated as mainly aeolian and fluvial end-members. In addition, two Mars-analogue aeolian and fluvial basaltic sediments from Iceland were analysed similarly. Our analyses demonstrated that useful parameters for the distinction of aeolian and fluvial transport included the degree of sorting, the grain diameter/perimeter ratio, the circularity (reflecting the sphericity of the grains) and convexity (showing roughness of the grains) of particles. Mature aeolian sands are always well-sorted with high diameter/perimeter ratios since they are dominated by rounded grains. Differences were observed in both surface textures and the range of mineral compositions, wherein the aeolian samples reflect a more mature state. Separation of the two Icelandic samples with different origins was less straightforward due to both shorter transport distances and the fact that the fluvial sample was made of scoria, although the intermixing of grains with a probable aeolian origin could be also determined. The characteristics of grain size and shape parameters are potential indicators allowing the separation of the two transport modes, and their determination is shown to be feasible based on high resolution Close-UP Imager (CLUPI) observations. Nonetheless, knowledge of the geological context – for instance sedimentary facies, layering, sorting of the sediments and grain shape – is a requirement for proper interpretations. These sedimentary features could be party determined by MaMISS, CLUPI and PanCam observations. Differences between aeolian and fluvial grains are expected to be more pronounced on Mars than on Earth due to the episodic and likely short-term subaqueous transport, as opposed to the long-term activity of wind. However, future interpretations of the ExoMars 2020 rover images of martian sedimentary grains will be hampered by the limited knowledge of fluvial/aeolian transport on Mars. Based on our results, future research should focus on determining the dominant transport mode of any sediment and the degree of grain mixing, which strongly based on the transport mode. Highlights • Mars-analogue particulate sediments, including basaltic in origin were analyzed. • Morphometric differences in the shapes of aeolian and fluvial grains were identified. • The ExoMars CLUPI camera will be able to distinguish the agents of deposition for grains of 100–200 μm size. • Aeolian grains are expected to be more mature, and differ from fluvial grains on Mars. • Local sedimentary features give important context information to assist the identification of grain origin. [ABSTRACT FROM AUTHOR]

Two episodes of intense flooding and sediment movement occurred in the Westmorland Stream alluvial system near Caveside, Australia in January 2011 and June 2016. The events were investigated in order to better understand the drivers and functioning of this composite alluvial system on a larger scale, so as to provide awareness of the potential hazard from future flood and debris flow events. A novel combination of methods was employed, including field surveys, catchment morphometry, GIS mapping from LiDAR and aerial imagery, and hydraulic modelling using RiverFlow-2D software. Both events were initiated by extreme rainfall events (< 1% Annual Exceedance Probability for durations exceeding 6 h) and resulted in flooding and sediment deposition across the alluvial fan. The impacts of the 2011 and 2016 events on the farmland appeared similar; however, there were differences in sediment source and transport processes that have implications for understanding recurrence probabilities. A debris flow was a key driver in the 2011 event, by eroding the stream channel in the forested watershed and delivering a large volume of sediment downstream to the alluvial fan. In contrast, modelled flooding velocities suggest the impacts of the 2016 event were the result of an extended period of extreme stream flooding and consequent erosion of alluvium directly above the current fan apex. The morphometry of the catchment is better aligned with values from fluvially dominated fans found elsewhere, which suggests that flooding represents a more frequent future risk than debris flows. These findings have wider implications for the estimation of debris flow and flood hazard on alluvial fans in Tasmania and elsewhere, as well as further demonstrating the capacity of combined hydraulic modelling and geomorphologic investigation as a predictive tool to inform hazard management practices in environments affected by flooding and sediment movement. [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]

Small mountainous rivers (SMRs) play a crucial role in the global sediment and carbon cycle by rapid transfer of huge particulate and dissolved loads into the sea. Gaoping river-sea system is a good representative of SMRs in Oceania, having an extreme high denudation rate (15,000 t km − 2 yr − 1 ) and a high burial efficiency of terrestrial particulate organic carbon (POC) in the Gaoping submarine canyon (GPSC). However, it is contentious whether silicate weathering or POC burial plays more important role in long-term carbon cycling in such a high active orogen. Clay minerals of fluvial sediments not only act as an important carrier of POC, but also encode valuable information of weathering regimes on land and dispersal routes in the seas. To uncover these informations, surface and core sediments are systematically collected from the Gaoping river-sea system for clay-mineral analysis. Comparative studies are carried out on the basis of new results and vast published data to better understand weathering processes and products in Oceanian islands and Southeast Asian continent. We find that clay-mineral compositions in the Gaoping River (GPR) and other Taiwan rivers are a legacy from provenance rocks in the alpine reach, characterized by predominant illite and chlorite and limited compositional change throughout the river courses. These sharply contradict the downstream increase in compositional complexity (rich in kaolinite and smectite) and chemical weathering intensity in Southeast Asian large rivers because of strong hydrolysis and active sediment recycling at the lowland reach with extensive floodplains. High consistency of clay-mineral compositions is also observed in the interior and exterior of GPSC. Careful examination shows that a slight offshore decrease in cumulative illite and chlorite content is accompanied by a slight increase in exotic clay-mineral inputs by ocean circulations. The combination of this finding with previously reported data on the offshore change in δ 13 C org and Δ 14 C org values suggests that petrogenic POC predominates in the river-dominated Gaoping continental margin with high preservation efficiency due to its tight bandage with clay minerals. Moreover, a tardy response of chemical weathering fluxes to high elevated physical erosion rates is remarkably examined in the Oceanian and global river gauging data. We therefore postulate that rapid transfer and high burial efficiency of terrestrial POC in the tectonically active high-standing islands and their adjacent continental margins play much more important role in the long-term carbon cycle than silicate weathering. [ABSTRACT FROM AUTHOR]

Recent studies have shown the super saturation of fluvial networks with respect to carbon dioxide, and the concept that the high carbon dioxide is at least partially the result of turnover of sediment organic carbon that ranges in age from years to millennia. Currently, there is a need for more highly resolved studies at stream and river scales that enable estimates of terrestrial carbon turnover within fluvial networks. Our objective was to develop a new isotope-based metric to estimate the quality of sediment organic carbon delivered to temperate streams and to use the new metric to estimate carbon quality across landscape disturbance gradients. Carbon quality is defined to be consistent with in-stream turnover and our metric is used to measure the labile or recalcitrant nature of the terrestrial-derived carbon within streams. Our hypothesis was that intensively-disturbed landscapes would tend to produce low quality carbon because deep, recalcitrant soil carbon would be eroded and transported to the fluvial system while moderately disturbed or undisturbed landscapes would tend to produce higher quality carbon from well-developed surface soils and litter. The hypothesis was tested by applying the new carbon quality metric to 15 temperate streams with a wide range of landscape disturbance levels. We find that our hypothesis premised on an indirect relationship between the extent of landscape disturbance and the quality of sediment carbon in streams holds true for moderate and high disturbances but not for un-disturbed forests. We explain the results based on the connectivity, or dis-connectivity, between terrestrial carbon sources and pathways for sediment transport. While pathways are typically un-limited for disturbed landscapes, the un-disturbed forests have dis-connectivity between labile carbon of the forest floor and the stream corridor. Only in the case when trees fell into the stream corridor due to severe ice storms did the quality of sediment carbon increase in the streams. We argue that as scientists continue to estimate the in-stream turnover of terrestrially-derived carbon in fluvial carbon budgets, the assumption of pathway connectivity between carbon sources to the stream should be justified. [ABSTRACT FROM AUTHOR]

Aeolian deposits are frequently used for palaeoenvironmental change studies. Their formation depends on an array of requirements: the supply of material suitable for aeolian transport and favorable conditions of sediment availability and wind strength. In order to infer palaeoenvironmental information from aeolian sand deposits these factors need to be carefully evaluated. We present a study from northern Chinese Hexi Corridor, based on 11 optically stimulated luminescence (OSL) dated sediment sections. These represent interchanging aeolian and alluvial deposits under gravel surfaces and aeolian sand in dune fields interrupted by interdunal flood deposits. Investigations in two subareas reveal contrasting geomorphologic and sedimentary histories: (1) sediment deposition during the Pleistocene–Holocene transition (~ 12 ka) followed by deflation during the Holocene and (2) frequent sediment recycling revealed by a wide spectrum of ages throughout the Holocene. The late glacial sediment pulse recorded in the western Hexi Corridor is attributed to high sediment supply, generated by efficient (peri-)glacial sediment production during glacial times in the adjacent Qilian Shan (< 5700 m asl) and a moisture increase inducing the reworking of those (glacio-)fluvial deposits during the Pleistocene–Holocene transition. The absence of a powerful reworking agent preserved these late glacial deposits in the western Hexi Corridor in contrast to moister eastern parts where Holocene sediment reworking prevailed. Geomorphological and hydrological preconditions of the subareas are discussed and reveal the controlling influence of fluvial processes on sand supply for the aeolian system. While a perennial drainage is missing in the drier western part, the Hei River drainage is fed by higher monsoonal precipitation in the central Hexi Corridor. It maintains a sediment recycling system and has ensured a sufficient sediment supply throughout the Holocene. The study promotes closer consideration of the fluvial influence on aeolian archives in palaeoenvironmental studies from central Asian and other drylands. [ABSTRACT FROM AUTHOR]

Luminescence dating of sediment requires the sand grains to have been exposed to sunlight prior to their most recent burial. Under fluvial transport, the amount of sunlight exposure may not always be sufficient to reset the luminescence signal, a phenomenon known as ‘partial bleaching'. The extent of bleaching is dependent on a combination of geomorphic, sedimentological and fluvial processes. If bleaching can be quantified, and the relationship with these processes understood, it could potentially be used as a new environmental proxy for changes in the dynamics of river systems. Here, we use a recently developed statistical model to evaluate the extent of bleaching, by inferring the proportion of well-bleached grains in the small-aliquot population. We sampled low-flow and flood deposits at a single site on the River Sabie, South Africa. We show that the low-flow sediment is almost perfectly bleached (>80% of grains well bleached), while sediment at flood elevations is partially bleached (20–70 % of grains well bleached). The degree of bleaching may show a relationship with flood magnitude as defined by elevation above normal river level, and we speculate on the causes of variability in bleaching between flood samples. [ABSTRACT FROM AUTHOR]

Alluvial fans are depositional systems that develop because of a disparity between the upstream and downstream sediment transport capacity of a system, usually at the base of mountain fronts as rivers emerge from the constrained mountain area onto the plain. They are dynamic landforms that are prone to abrupt changes on a geomorphological (decades to centuries) time scale, while also being long-term deposition features that preserve sedimentary strata and are sensitive indictors of environmental change. The complexity of interactions between catchment characteristics, climate, tectonics, internal system feedbacks, and environmental processes on field alluvial fans means that it is difficult to isolate individual variables in a field setting; therefore, the controlled conditions afforded by experimental models has provided a novel technique to overcome some of these complexities. The use of experimental models of alluvial fans has a long history and these have been implemented over a range of different research areas utilising various experimental designs. Using this technique, important advances have been made in determining the primary factors influencing fan slope, understanding of avulsion dynamics, identifying autogenic processes driving change on fan systems independent of any change in external conditions, and the mechanics of flow and flood risk on alluvial fans, to name a few. However, experiments cannot be carried out in isolation. Thus, combining the findings from experimental alluvial fans with field research and numerical modelling is important and, likewise, using these techniques to inform experimental design. If this can be achieved, there is potential for future experimental developments to explore key alluvial fan issues such as stratigraphic preservation potential and simulating extra terrestrial fan systems. [ABSTRACT FROM AUTHOR]

The Brazos River is second only to the Mississippi River in terms of sediment load discharged to the Gulf of Mexico (GOM). This study focused on the interaction between a salt wedge and suspended sediment in the lower Brazos River, and the implications for sediment export to the coastal ocean. Salinity and turbidity (optical backscatter sensor) data with depth, swath bathymetry, and CHIRP sub-bottom profiling data were collected from 2007 to 2012 over various river discharge stages. Sampling locations included the lower-most 15 km river reach and the coastal ocean proximal to the mouth. Results indicated that during low to moderate river discharges a salt wedge was present in the lower river, but absent during elevated fluvial discharge. Quantitative analysis showed the salt wedge within the river modulated sediment export to the GOM by trapping sediment along the lower most 9 km reach. Within this reach the trapped sediment formed a decimeter-thick mud layer observed in the CHIRP and bathymetric data. At elevated fluvial discharges the salt wedge was no longer observed in the river, and the lower river reach transitioned from depositional to erosional. Based on the observations and analysis from this study, a fluvial discharge threshold where the salt wedge is not present in the river and sediment export to the coastal ocean was maximized was estimated. This threshold has been met or exceeded only ~ 10% of time since the 1960s. These conditions imply that sediment transport to the coastal ocean is highly variable, and occurs during irregular, pulsed events. This study highlights how estuarine conditions at the river mouth can impact terrestrial material flux to the coastal ocean. [ABSTRACT FROM AUTHOR]

Understanding the pace of erosion through space and time may provide insight into how tectonic and climatic processes conspire to drive landscape evolution. However, determining whether erosion rates in a given locality have varied with past climate change remains challenging, and the mechanisms invoked to explain variations in past erosion rates are not well understood. In this study, we evaluate past rates of erosion during the late Quaternary along the northern Qilian Shan, the range bounding the northeastern margin of the Tibetan Plateau. We measured the concentrations of cosmogenic 10Be from fluvial deposits atop 5 terraces along the Hongshuiba River and average 10Be concentrations at the time of terrace formation. We also determined terrace ages by combining 5 10Be depth profiles and 15 OSL samples of overlying loess. Our results confirm previous dating of this terrace sequence using cosmogenic exposure ages of boulders and depth profiles in terrace gravels and indicate that terrace treads span glacial-interglacial cycles over the past ~200 ka. We exploit this exceptionally dense sampling of 10Be to evaluate differences in cosmogenic 10Be concentrations at the time of terrace formation along the Hongshuiba River. Analysis of 48 individual determinations of "inherited" 10Be concentrations reveal that fluvial sediment in terraces abandoned during the last glacial and penultimate glacial periods are 3–4 times greater than concentrations in sediment atop terraces formed during interglacial times. Paleoerosion rates within the Hongshuiba watershed appear to have been ~200 m/Myr during glacial episodes but increased to ~500–700 m/Myr during interglacial times, similar to recent erosion rates inferred from 10Be in modern sediment. These results point to likely changes in the mean residence time of sediment with the watershed that fluctuate in association with 100-ka glacial-interglacial cycles. We suggest that reductions in erosion rates associated with glacial conditions may have been driven by changes in catchment hydrology during cold and dry periods. • Late Quaternary erosion study from the northern Qilian Shan, Tibetan Plateau margin • 10Be-derived catchment-wide erosion rates from dated Hongshuiba River terraces • Erosion rates vary from ~200 m/Myr (glacial) to ~500–700 m/Myr (interglacial/modern) • Erosion rate fluctuations occur across 100-ka glacial-interglacial cycles • Hydrological changes drive variations in catchment sediment production and transport [ABSTRACT FROM AUTHOR]

• An analytical model for the suspended sediment concentration in the ice-covered channel flows is proposed. • A modified sediment fall velocity considering the boundary effect is presented. • The proposed model performs well in predicting the suspended sediment concentration. Ice cover formed on an alluvial channel can significantly alter the flow characteristics, such as the vertical distributions of streamwise velocity and shear stress, and hence the water and sediment transport process. The vertical profile of the suspended sediment concentration in the ice-covered alluvial channels with steady uniform flows is investigated in this study. To calculate the suspended sediment concentration, we are based on the Schmidt O'Brien equation and deduce an analytical model that employs an existing eddy viscosity model and a modified formula of the sediment fall velocity considering the common effects of the upper and lower boundaries. The proposed analytical model is then validated by using available experimental data reported in the literature. The predicted accuracy of the proposed model is evaluated through error statistics by comparing to previous modeled results. The relative concentration profiles of the suspended sediment are subsequently simulated by applying the validated analytical model with different characteristic parameters. Results show that the relative concentration decreases with the increase of both the ice cover roughness and the sediment fall velocity. The uniformity of the relative concentration distribution is closely related to the value of the proportionality parameter σ , revealing the physical mechanism that the more prominent the turbulent diffusion effect is, the more uniform the relative concentration profile is. [ABSTRACT FROM AUTHOR]

Geomorphic evolution of the Wahiba Dune Field, Oman, during the Quaternary has occurred within a set of boundary conditions that include climatic forcing of fluvial, aeolian and eustatic cycles within an active tectonic basin. Because of basin down-warping and sediment transport into the basin, evolution of the geomorphic surface has been accompanied by the generation of a distinctive stratigraphic record. The coupled geomorphic and stratigraphic record of the northeastern portion of the dune field illustrates wadi-aeolian interactions, in which a channel avulsion, likely initiated during a flood, scoured through the interdune corridor between linear dunes. Interdune outcrops (7 m thick) consist of a lower interval interpreted as deposited by ephemeral fluvial flow, but an upper interval consists of six fining-upward units, each of which is interpreted to represent a flood event that culminated in ponding followed by desiccation. Luminescence dating indicates that the channel remained open for 2–3 ka during the Holocene, but ground-penetrating radar imaging shows that dunes encroached into the channel between floods and suggests that the transition from ephemeral flow to ponding resulted from dune damming. Maximum channel width and length are unknown, but width was greater than the current interdune area, and a speculative extended channel course is identified. Subsequently, interdune strata and linear dunes were buried by crescentic dunes sourced by an influx of sand with wadi affinity. The resultant complex stratigraphic architecture illustrates the role of existing surface topography in providing local geomorphic accommodation space for short-lived, concentrated patterns of sedimentation. [ABSTRACT FROM AUTHOR]

In large watersheds, sediment storage imposes geologic timescales on the downstream movement of suspended sediment and associated contaminants. Modeling these processes requires temporal and spatial averaging, a task often accomplished using concepts derived from reservoir theory, where residence times and age and storage time distributions of stored sediment are treated as constants, even though all of these quantities should be time-variable under conditions of unsteady sediment transport. Here, concepts from reservoir theory are replaced by deposition and erosion laws to develop a parsimonious quantitative framework for watershed-scale routing of sediment and contaminants that explicitly accounts for time-dependent alluvial storage. Solution of the new equations, combined with appropriate initial and boundary conditions, predicts the age and storage time distributions of stored sediment, the sediment residence time, the mass of stored sediment, and the variations in all of these quantities through time. Model calibration is illustrated by reinterpreting previously published data from the Little Missouri River in North Dakota to create a 3-dimensional chronostratigraphic model of stored sediment, supplemented by observations of contemporary patterns of erosion. Model parameters from the Little Missouri River are then used to demonstrate that approximately 750 yr will be required to remove arsenic-contaminated alluvium supplied by a century of mine tailings discharges to a 112-km reach of the Belle Fourche River in South Dakota. The new approach described here requires very modest computing resources to simulate long timescales across large watersheds, and the fidelity of storage parameters, which ultimately control the timing of sediment delivery, is supported by site-specific stratigraphic observations and sediment dating. Unlabelled Image • New erosion and deposition laws quantify transfers into and out of alluvial storage. • The new framework predicts sediment age and transit time distributions. • Equations are parameterized using 3-d stratigraphic and age data. • Arsenic will persist in floodplains of the Belle Fourche R., S.D., USA, for 750 years. [ABSTRACT FROM AUTHOR]