Fluvial Transport Research Articles

Extreme dispersal or human-transport? The enigmatic case of an extralimital freshwater occurrence of a Southern elephant seal from Indiana.

Elephant seals (Mirounga spp.) are the largest living pinnipeds, and the spatial scales of their ecology, with dives over 1 km in depth and foraging trips over 10,000 km long, are unrivalled by their near relatives. Here we report the discovery of an incomplete Holocene age Southern elephant seal (M. leonina) rostrum from Indiana, USA. The surviving material are two casts of the original specimen, which was collected in a construction excavation close to the Wabash River near Lafayette, Indiana. The original specimen was mostly destroyed for radiometric dating analyses in the 1970s, which resulted in an age of 1,260 ± 90 years before the present. The existence of sediments in the original specimen suggests some type of post depositional fluvial transportation. The prevalent evidence suggests that this male Southern elephant seal crossed the equator and the Gulf of Mexico, and then entered the Mississippi River system, stranding far upriver in Indiana or adjacent areas, similar to other reported examples of lost marine mammals in freshwater systems. Based on potential cut marks, we cannot exclude human-mediated transportation or scavenging by Indigenous peoples as a contributing factor of this occurrence. The material reported here represents by far the northernmost occurrence of a Southern elephant seal in the Northern Hemisphere ever recorded. The unusual occurrence of a top marine predator >1,000 km from the closest marine effluent as a potential extreme case of dispersal emphasizes how marine invasions of freshwater systems have happened frequently through historical (and likely geological) time.

HyLands 1.0: a hybrid landscape evolution model to simulate the impact of landslides and landslide-derived sediment on landscape evolution

Abstract. Landslides are the main source of sediment in most mountain ranges. Rivers then act as conveyor belts, evacuating landslide-derived sediment. Sediment dynamics are known to influence landscape evolution through interactions among landslide sediment delivery, fluvial transport and river incision into bedrock. Sediment delivery and its interaction with river incision therefore control the pace of landscape evolution and mediate relationships among tectonics, climate and erosion. Numerical landscape evolution models (LEMs) are well suited to study the interactions among these surface processes. They enable evaluation of a range of hypotheses at varying temporal and spatial scales. While many models have been used to study the dynamic interplay between tectonics, erosion and climate, the role of interactions between landslide-derived sediment and river incision has received much less attention. Here, we present HyLands, a hybrid landscape evolution model integrated within the TopoToolbox Landscape Evolution Model (TTLEM) framework. The hybrid nature of the model lies in its capacity to simulate both erosion and deposition at any place in the landscape due to fluvial bedrock incision, sediment transport, and rapid, stochastic mass wasting through landsliding. Fluvial sediment transport and bedrock incision are calculated using the recently developed Stream Power with Alluvium Conservation and Entrainment (SPACE) model. Therefore, rivers can dynamically transition from detachment-limited to transport-limited and from bedrock to bedrock–alluvial to fully alluviated states. Erosion and sediment production by landsliding are calculated using a Mohr–Coulomb stability analysis, while landslide-derived sediment is routed and deposited using a multiple-flow-direction, nonlinear deposition method. We describe and evaluate the HyLands 1.0 model using analytical solutions and observations. We first illustrate the functionality of HyLands to capture river dynamics ranging from detachment-limited to transport-limited conditions. Second, we apply the model to a portion of the Namche Barwa massif in eastern Tibet and compare simulated and observed landslide magnitude–frequency and area–volume scaling relationships. Finally, we illustrate the relevance of explicitly simulating landsliding and sediment dynamics over longer timescales for landscape evolution in general and river dynamics in particular. With HyLands we provide a new tool to understand both the long- and short-term coupling between stochastic hillslope processes, river incision and source-to-sink sediment dynamics.

Fluvial transport of coarse particulate organic matter in a coastal mountain stream of a rainy‐temperate evergreen broadleaf forest in southern Chile

AbstractCoarse particulate organic matter (CPOM, i.e. particles such as leaves, wood fragments, twigs, branches, flowers, seeds and fruits) in aquatic systems influences the flow, provides an important food source, and at the catchment scale, may significantly contribute to total carbon export. CPOM exports have rarely been quantified in subtropical, broadleaf forest streams. We captured CPOM in Bunte traps in the rainfall‐dominated Vuelta de Zorra stream in southern Chile to (a) propose a novel classification to characterize the different CPOM components, (b) analyze the frequency of each matter class (i.e. leaves, wood fragments, and ‘others’) and its seasonal variability, (c) quantify the CPOM transported, (d) derive a model to quantify CPOM transport rates, and (e) compare the transported CPOM data with those from a unique long‐term (> seven years) large wood monitoring dataset. Results showed that leaves were significantly more abundant than other types of CPOM in all seasons. The dry CPOM transport rate ranged over three orders of magnitude, and there was a significant relationship with mean discharge. Mean dry CPOM yield for the study period 2015–2017 was 4.6 kg/ha/yr when normalizing to the total forested catchment area. The 2009–2018 decadal average yield was 6.8 kg/ha/yr when normalizing to the total forested catchment area. These values are similar to measurements from deciduous and coniferous forests in streams in the United States and the Brazilian Mato Grosso and ~1/10 of the yields obtained in a Swiss torrent. Over a three‐year period, the CPOM exports in Vuelta de Zorra ranged between 13 and 36% of the exported large wood (particles with diameters ≥ 100 mm and length ≥ 1 m) exports. Our data collected from an underrepresented area improve the understanding of global carbon budgets and cycling. © 2020 John Wiley & Sons, Ltd.

Provenance and transport process for interdune sands in the Kumtagh Sand Sea, Northwest China

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.

Miocene C4 Grassland Expansion as Recorded by the Indus Fan

AbstractIn the late Miocene, grasslands spread across the forested floodplains of the Himalayan foreland, but the causes of the ecological transition are still debated. Recent seafloor drilling by the International Ocean Discovery Program (IODP) provides an opportunity to study the transition across a larger region as archived in the Indus submarine fan. We present a multiproxy study of past vegetation change based on analyses of the carbon isotopic composition (δ13C) of bulk organic carbon, plant wax n‐alkanes and n‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX86 index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C4 grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C4 expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C4 expansion.

Surface textures of quartz and ilmenite grains from dune and beach sands of the Gulf of Mexico Coast, Mexico: Implications for fluvial, aeolian and marine transport

Surface textures of quartz and ilmenite grains provide an insight in assessing current mechanical, chemical and mechanical-chemical processes in dune and beach sands of the Gulf of Mexico Coast (GMC) Mexico. The GMC area produces hybrid sands enriched in monocrystalline quartz and ilmenite-free grains. Quartz grains are observed in all dunes and beach localities of the central GMC close to the Trans-Mexican Volcanic Belt characterized by large conchoidal fractures (>100 μm), angular to subangular outlines, arcuate steps, high relief, and sharp edges associated with fluvial transport. Aeolian quartz features like meandering ridges, upturned plates, bulbous edges and adhering particles on quartz are only present in some GMC sites, whereas marine surface textures like straight and curved grooves are sparse. Chemical surface textures on quartz and ilmenite are a consequence of silica saturated low-energy subaqueous conditions and subaerial exposure, diagenesis and intermediate weathering processes in the dunes and beach sands. Ilmenite grains are common in all studied localities of the GMC, characterized by large conchoidal fractures (>100 μm), angular to subangular outlines, flat cleavage surfaces, arcuate steps and high relief. In some ilmenite grains, upturned plates, meandering ridges, bulbous edges and abrasion fatigue characterize aeolian surface features. Straight and curved grooves only characterize marine surface textures. To assess the prevailing transport mechanisms along the GMC, quartz and ilmenite grains from dunes and beach sands were selected based on surface texture observations. Our findings suggest that quartz and ilmenite grains are primarily transported and deposited along the coast by riverine input, a second-cycle of aeolian transport onshore, and recycling by marine longshore currents and waves.

River plume monitoring in a deep valley reservoir using HJ-1 A/B images

River discharge carries suspended matter into lakes, reservoirs, estuaries, and coasts, resulting in obvious plumes in river mouths. River plumes are more likely to form in valley reservoirs due to the steep slopes in the basin, which may accelerate fluvial transport from upstream to the river mouth, causing a considerable particles load and increasing the appearance times and scale of river plumes during rainfall events. Information on the intensity and evolution of river plumes in valley reservoirs has important implications for understanding nearshore geomorphological processes and dynamics in the water quality surrounding river mouths. Taking the Xin’anjing Reservoir, a typical deep valley reservoir, as a case study, a plume index based on the relative reflectance height in the red band of HJ-1 A/B images was first proposed. Subsequently, we used seventeen HJ-1 A/B images during 10 heavy rainfall events from 2009 to 2014 to characterize the spatial–temporal variability in river plumes and illustrate the relation of the river plumes and total suspended matter (TSM) concentration to the amount of rainfall. The results indicated that 10 heavy rainfall events caused different plume areas varying from 41.26 to 148.12 km2, with an average area of 76.45 ± 35.03 km2. Significant positive correlations were observed between plume areas, TSM concentrations and rainfall amount (P < 0.005). The plumes would simultaneously emerge when the rapid surface flow reached the river mouth but increase to the maximum size after 2–3 days of these heavy rainfalls. These results confirmed that the HJ-1 A/B images combined with our proposed plume index could detect and monitor the formation and diffusion of river plumes in valley reservoirs during heavy rainfall events.

Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin

AbstractSoil erosion modulates the atmospheric CO2 level by affecting the redistribution of young biospheric organic carbon (OCbio) and ancient petrogenic organic carbon (OCpetro) in different order streams. However, the fate of soil organic carbon (SOC) in low‐order stream systems is still uncertain due to the complex influences of terrain, land uses, and anthropogenic disturbances. Here, we used the geochemical properties fingerprinting method and a radiocarbon‐based two‐member mixing model to clarify the source, budgets, and composition of SOC in a low‐order stream catchment in the Yellow River basin. The results showed that the primary source of SOC in the catchment was agricultural activity area, which accounted for 68.5%. After vegetation restoration, the SOC delivery rate and loss rate decreased from 0.38 ± 0.06 to 0.26 ± 0.04 and 0.19 ± 0.06 to 0.14 ± 0.04 t·ha−1·yr−1, respectively. The SOC mobilized by soil erosion in the source area was 1,085.8 ± 170.5 t from 1969 to 2015, and the SOC lost during the transport process was 552.9 ± 170.1 t. The lost carbon was dominated by OCbio, whereas the loss of OCpetro was minimal. Soil erosion can mobilize the OCpetro sequestered in terrestrial ecosystems and deliver it to a river system if it is not intercepted. The oxidation of OCpetro during long‐distance fluvial transport may represent a significant carbon source for atmospheric CO2 on a geological timescale. This study contributes to our understanding of the carbon sink/source issue and quantifies the SOC budgets in stream systems.

Straight from the source's mouth: Controls on field‐constrained sediment export across the entire active Corinth Rift, central Greece

AbstractThe volume and grain‐size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grain‐size exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grain‐sizes of channel bars and full‐weighted grain‐size distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grain‐size increases westward along the southern coast of the Gulf of Corinth, with the coarse‐fraction grain‐sizes (84th percentile of weighted grain‐size distribution) ranging from approximately 19 to 91 mm. We find that the median and coarse‐fraction of the sieved grain‐size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grain‐size export is primarily controlled by the input grain‐size within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarse‐fraction of the grain‐size distribution are predominantly transported in bedload; however, typical sand‐grade particles are transported as suspended load at bankfull conditions, suggesting disparate source‐to‐sink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grain‐size‐specific bedload discharged into the Gulf of Corinth using the grain‐size measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475–16 cm). Our results suggest that the grain‐size of sediment export at the rift scale is particularly sensitive to catchment lithology and fluvial mophodynamics, which complicates our ability to make direct inferences of tectonic and palaeoenvironmental forcing from local stratigraphic characteristics.

Long‐term large wood load fluctuations in two low‐order streams in Southern Chile

AbstractThe eco‐hydrogeomorphic significance of large wood (LW) and its potential for increasing downstream hazards during extreme floods have been widely recognized. We used LW data collected for a 10‐year period from the two low‐order streams of Pichún (Pi) and Vuelta de Zorra (VZ) in Southern Chile to (a) determine if the abundance and dimensions of individual LW pieces change with time, (b) quantify wood load fluctuations during the 10‐year period, and (c) assess the role of LW recruitment from the riparian forests to explain wood load fluctuations during the study period. Nine years after the first survey, the number of LW pieces in Pi and VZ diminished by 60 and 40%, respectively. Despite the reduction in these numbers, in Pi, the LW dimensions did not change significantly during the study. In VZ, the dimensions exhibited statistically significant differences, despite being within the same class. In both catchments, the LW load fluctuated during a 10‐year period, but the drivers of change differed. Although tree toppling was the recruitment mechanism responsible for LW in both stream cases, the high wood load measured in Pi at the beginning of the study suggested massive tree recruitments before the first survey, followed by wood exports which were higher than inputs in the subsequent 10‐year period. In VZ, LW load decreased during the first 9 years (mean annual rate of ~9.2 m3 year−1) and then increased by ~12.1 m3 year−1 in year 10. At VZ, the inputs consisted of single trees that were recruited from the riparian area and by upstream flotation, while exports occurred by downstream fluvial transport. Wood inputs and exports occurred asynchronously and led to LW load fluctuations at decadal and annual intervals. Land management and tree species thus exert a major influence on wood inventory and budget in streams. © 2020 John Wiley &amp; Sons, Ltd.

Detrital zircons and sediment dispersal in the eastern Midcontinent of North America

Abstract Results of detrital-zircon analyses (U-Pb ages and initial Hf values, εHft) of Mississippian–Pennsylvanian sandstones in the Michigan, Illinois, and Forest City basins are remarkably similar to data for coeval sandstones in the Appalachian basin, indicating dispersal of sediment from the Appalachian orogen through the Appalachian basin to the eastern Midcontinent during the late Paleozoic. The similarities of results include matches of the two most prominent age groups (1300–950 Ma and 490–350 Ma), as well as matches of the less abundant age groups. Comparisons of the data are from observations of probability density plots and multidimensional scaling of U-Pb age data and of εHft values. Despite the dominance of an Appalachian signature in all samples, some samples contain grains with ages that suggest intermittent additional sources. Four samples (three ranging in depositional age from Morrowan to Atokan–Desmoinesian in the Illinois basin, and one of Desmoinesian age in the Forest City basin), in addition to typical Appalachian age distributions, have prominent age modes between 768 and 525 Ma, corresponding in age to Pan-African/Brasiliano rocks in Gondwanan accreted terranes in the Appalachian orogen, suggesting intermittent dispersal from the Moretown terrane of the northern Appalachians. Sandstones in the Appalachian basin and those in the Midcontinent basins have very few grains with ages that correspond to the Alleghanian orogeny in the Appalachian orogen. Nevertheless, three sandstones each in the Illinois basin and Forest City basin with depositional ages of 312–308 Ma have a few zircon grains in the age range of 321 ± 5 to 307 ± 4 Ma. The nearly identical crystallization and depositional ages suggest reworking at the depositional sites of air-fall volcanic ash from the Alleghanian orogen, rather than fluvial transport from the orogen. The basal Pennsylvanian sandstones lap onto a regional unconformity around the northern rims of the Illinois and Forest City basins, suggesting sources for recycled grains. Along the northern edge of the Illinois basin, Ordovician sandstones beneath the unconformity may have contributed minor concentrations of Superior-age zircons in the basal Pennsylvanian sandstones. Basal Pennsylvanian sandstones in the Forest City basin lap onto Mississippian strata, suggesting possible recycling of zircons from eroded Mississippian sandstones.

Sr-Nd-Pb isotope systematics of Australasian tektites: Implications for the nature and composition of target materials and possible volatile loss of Pb

The Australasian tektite (AAT) strewn field is the largest strewn field on the Earth with about ∼10–30% coverage, both land and ocean, but a clearly identified source impact crater is absent despite the young age of AAT of ca. 790 ka. A genetic link between the Australasian tektites and their unequivocal parental materials is therefore largely impossible to establish. Nevertheless, the nature of the parental materials and the extent of volatilization can be constrained using the splash form tektites, carrying the chemical signatures of high-temperature processes, and the layered (so-called Muong Nong-type) tektites, which are less chemically homogenized and exceptionally abundant in the AAT field. New high-precision Sr, Nd and Pb isotopic measurements were obtained for a chemically and petrographically well-characterized suite of AAT, which included the Muong Nong-type (MN-AAT) with precisely known field locations in Laos and splash forms (SF-AAT) from different parts of the strewn field. In addition, optically dark and light zones of the MN-AAT were also separately analyzed. Homogeneous εNd values from −11.8 to −11.2, combined with a narrow range of two-stage Nd model ages from 1.67 to 1.72 Ga for the entire AAT suite, point to a well-mixed source, in terms of REE, of the crustal segment from which the sedimentary material for tektites was ultimately derived. The Sr isotopic data largely overlap for SF-AAT and MN-AAT (87Sr/86Sr = 0.71636–0.72021) and indicate Paleozoic to Mesozoic sedimentary parentage. However, late Neogene to early Quaternary re-deposition and formation of a thick silt-sized sedimentary section with vertical stratification is required to comply with 10Be data. Lead isotope systematics documents at least three different components which can perhaps be represented by different mineral phases, such as feldspar, zircon, organic matter adsorbed on young sediments etc., sorted during fluvial transport and final deposition. In addition, the SF-AAT have systematically lower Pb contents than the MN-AAT, and generally show isotopically heavier Pb isotopic ratios. This is theoretically consistent with a preferential volatilization of lighter Pb isotopes during evaporation and considerably larger Pb loss from SF-AAT when compared to MN-AAT. Nevertheless, further experimental work would be necessary to unambiguously distinguish kinetic fractionation from source mixing.

Ricostruzione 3D della distribuzione delle tessiture nel sottosuolo della Pianura Lombarda centro-occidentale

Po Plain is the result of complex endogenous processes which induced the growth of a large and deep subsiding foredeep basin for the south-vergent thrusting from the Alpine Chain first, and then for the Apennine structure, verging in the opposite direction. The Po Plain foredeep was nourished by large amount of debris coming from exogenous processes of erosion of both mountain chains. During Pleistocene, the foredeep has assumed a W-E direction along the Appenine margin, corresponding to the Po River course, deeper eastward. At the same time, at the northern margin of the plain, isostatic processes have been responsible of relevant raising. Sediments in the area have been deposited firstly in a marine basin, then in a wide delta-lagoon region, and more recently, a continental sedimentation prevailed. The area was dominated by large alluvial fans coming from the main Alpine valleys while in the southern part of the plain, where finer sediments are widespread, intense subsidence phenomena due to compaction of the sedimentary deposits occurred. During Quaternary, fluvial transport and sedimentation was strongly influenced by climate changes and, in the cold stages, large amount of debris were produced on the mountain chains and remarkable sea-level changes caused marine ingression in the middle of the Po Plain alternating to as much relevant regressions. Through the use of GOCAD software, using SGRID frame, voxels 500x500 m were obtained interpolating between Topo and A surfaces subdivided in 90 intervals, between A and B surfaces in 84 intervals and between B and C surfaces in 74 intervals respectively. The surfaces A, B and C, (Regione Lombardia & ENI-Divisione AGIP, 2002) were dated back respectively to 450, 870 and 1240 Ky. In each voxel, gravel, sand and clay texture distribution have been interpolated using percentage of texture for each metre assumed from stratigraphic data of the wells (11,993) stored in TANGRAM database. Interpolation has been carried out using kriging methods into GOCAD environment. To each voxel, the value of texture percentage has been assigned and the distribution in space and time has been modelled. 3D analysis related to clastic deposits distribution in the whole studied area testify that: 1) gravels deposited in the piedmont area over sand and clay materials from MIS 22 (870 Ky), and prograding gradually southward, are the effects of the beginning of glacial phases; 2) sand levels interbedded with clay ones, along the south-eastern Po river axis, have a geometric sequences, along axis Z, according to temporal sequences, effects of climate changes during Pleistocene.

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

AbstractPredicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: Is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus‐challenging developments in sediment transport experiments, two‐phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system‐spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle‐bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well‐known Shields diagram, where measurements of fluid entrainment thresholds could actually correspond to entrainment‐independent cessation thresholds.

Dynamics of fluvial suspended sediment transport and yield in volcanic highland’s agricultural basin: case of the Noun River, Western Cameroon

The dynamics of suspended sediment transport in the Noun River has been quantified using 93 runoff samples. Daily flow and monthly normalized difference vegetation index (NDVI), leaf area index (LAI), rainfall, and dam management data allowed the quantification of the total suspended solid (TSS), on the one hand, and the interpretation of the dynamics of solid transport within the drainage basin, on the other hand. The TSS concentrations range from 3 to 1375 mg L−1 for a volume weighted mean concentration of 90.33 to 110.49 mg L−1 and a specific sediment yield (SSY) of 41 to 57 t km−2 year−1. The basin’s hydrological characteristics between March to June and December to February have been reversed, revealing a strong impact of a dam. The maximum TSS concentration obtained during the second hydrological year at the beginning of the rainy season (April) is paradoxically three times higher compared to that recorded in July during the first measurement period. This occasional high TSS concentration for the single month of April released sedimentary fluxes superior to those recorded over the period of July to November combined, in comparison to those obtained during the first year. This situation causes sediment depletion not only on the next regular maximum TSS concentrations but also on the quantities of sediments exported later. The monthly TSS concentration/discharge does not present a clockwise hysteresis curve observed on tropical streams such as the Mbam and Sanaga in Ebebda, with annual variations in its evolution.

Geochemical characteristics of Gondwana shales from the Barapukuria basin, Bangladesh: implications for source-area weathering and provenance

The major, trace, and rare earth element (REE) compositions of shales from the drill hole GDH-40, Barapukuria basin, northwestern Bangladesh have been examined to evaluate their compositional variations, source rock weathering, and provenance. The shale samples are characterized by very low to moderate abundance of SiO2 (38.84–62.91 wt.%) and high Al2O3 (19.08–30.36 wt.%). The Gondwana shales are mainly enriched in Al2O3, TiO2, Th, Ce, Zr, Nb, Pb, Sc, Y, Ni, Cr, and V relative to average upper continental crust (UCC) and marked depleted in CaO, Na2O, K2O, MgO, and Ba. Depletion of these labile elements relative to UCC indicates destruction of feldspar during fluvial transportation. The Index of Compositional Variability (ICV) values range from 0.08 to 0.21, indicating high compositional maturity. Chondrite-normalized REE patterns for the shales are very similar to the average UCC, with significant LREE enrichment (LaN/YbN 10.03–13.58), flat HREE, and negative Eu anomalies (Eu/Eu* 0.36–0.73) representing a felsic source-rock provenance. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), K2O/Rb, and Rb/Sr ratios indicate that the Gondwana shales were largely derived from deeply weathered source rocks. Discriminant function diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE parameters (∑LREE/HREE and GdN/YbN) in the Gondwana shales further support felsic detritus input, with compositions close to the average rhyolites, UCC, and I- and S-type granites.

Genesis of giant, bouldery bars in a Miocene gravel-bed river: Insights from outcrop sedimentology, UAS-SfM photogrammetry, and GPR

ABSTRACTBedsets of bouldery intraformational conglomerate partly fill a narrow, 40-m-deep paleovalley in the Arikaree Group (lower Miocene) in Nebraska, USA. Entire bar successions are exposed in 3D, offering an opportunity to improve upon sedimentological models for coarse-grained fluvial deposits. Using unmanned aircraft, structure-from-motion photogrammetry, and ground-penetrating radar, we offer the first comprehensive assessment of the lithofacies and stratigraphic architecture of this regionally unique fluvial conglomerate. We interpret three lithofacies associations: (1) beds of chaotic breccias lining the exhumed valley wall (colluvium), (2) &amp;lt; 5-m-thick units dominated by horizontal to gently inclined (2–3°) beds of granule to pebble conglomerates, and (3) 10–24-m-thick units of steeply inclined (∼ 30°) beds of bouldery conglomerates, which dominate the channel fill. The deposit comprises at least four stories, but its thickest part is a deep scour filled by a single story of steeply inclined bouldery deposits. The direction of accretion of the giant bars was toward the paleovalley wall, where contemporaneous valley-side colluvium and bar deposits are juxtaposed. We propose that increased discharge in a high-order trunk stream rapidly eroded the friable sandstone of the underlying Monroe Creek–Harrison formations, amassing resistant, large concretions that were subject to valley-slope mass movements and fluvial transport (subaqueous traction, grainflow, and debris flow). Giant accretion surfaces formed as fluvial bars migrated over the edges of deep scours. Thus, the funneling of increased discharges through a confined paleovalley amplified the magnitudes of valley and bed erosion, water depth, stream competence, and bar thickness, preserving an exceptionally thick, coarse-grained deposit.

Martian fan deposits: Insights on depositional processes and origin from mass balance survey

Fan deposits located at the mouths of Martian valleys have been interpreted as indicators of wet conditions during Mars history. However, the processes, time and amount of water needed to carve the valleys and form the fans are still debated. Here we present a detailed morphometric and mass balance analysis of valleys and fan-shaped deposits using high resolution topography, which provides new insights into past depositional environments.Based on morphometric and volumetric measurements, we found that several Martian fans previously interpreted as deltas can be categorized into two main types. Type I fans are more numerous, relatively well preserved, are associated with smaller and less mature drainage networks and were deposited on steeper gradients. The balance between eroded and deposited materials indicates that this set of fans might have been formed mainly in subaerial settings by depositional processes other than typical deltaic sedimentation (e.g. alluvial, glacial, mass-wasting in general), since a significant loss of sediment during deposition did not occur (we obtained a volume ratio of eroded vs. deposited materials of approximately 1). Therefore, we propose that the identified Type I fans were not deposited in prevailing subaqueous deltaic settings, but mainly in subaerial conditions with perhaps sporadic presence of ephemeral bodies of standing water within the basins.In contrast, Type II fans are less abundant, highly eroded, were deposited on flatter areas and formed downstream of more mature drainage networks with longer, deeper and wider valleys. The mass balance for this set of fans clearly shows that considerable amounts of sediment were not retained in the fans, implying large offshore transport of sediment during their formation (eroded vs. deposited volume ratios between 3 and 10 are probable). This evidence supports a fluvio-deltaic origin for this class of fans, indicating the existence of paleolakes over substantial periods of time.Based on the collected morphometric measurements and mass balance modeling we conclude that only a small percentage of the fans (Type II fans correspond to ∼1/3 of the sampled areas) are consistent with the occurrence of favorable and durable conditions for life, i.e. long-lived integrated fluvial, deltaic and lacustrine environments. Whereas, the majority of fans might have been formed in subaerial settings with significant contributions of alternative processes besides fluvial transport and deposition thus not necessarily requiring the occurrence of extended epochs of clement climatic conditions.

Tributary‐junction fans as buffers in the sediment cascade: a multi‐decadal study

AbstractAlluvial fans at tributary junctions modulate sediment flux through river networks, by buffering the mainstem channel from disturbance in the tributaries. Buffering occurs through the storage (and release) of sediment in fans. Here, we use an extensive historic dataset to characterise the ways in which fan buffering can change as sediment supply varies.In New Zealand's East Coast region, sediment supply and fluvial transport are prolific by global standards. We reconstruct how tributary‐junction fans in this region have responded to sediment generated by deforestation and extreme storms. The dynamics of five fans along the Tapuaeroa River are examined for the period 1939–2015. In response to major sediment loading, fans aggraded by up to 12 m and prograded by up to 170 m. Net sediment accumulation ranged from near zero to 1.5×106 m3.Fan size, gradient, sediment storage and buffering were influenced by both upstream and downstream controls. Key upstream (tributary) influences were sediment supply and stream power; downstream (mainstem) influences included distal confinement and, importantly, the nature of fan interaction with the mainstem, which aggraded by up to 6 m. The fans' ability to buffer the Tapuaeroa River from change in the tributaries was largely governed by this downstream interaction: as the mainstem aggraded, it increasingly curtailed fan progradation, thus limiting buffering.Previous studies of tributary‐junction fans have related fan morphometry to basin characteristics. However, we find that fan slope and area can vary considerably at decadal, annual or even monthly timescales. Consequently, we suggest that such studies could benefit by examining regional histories of disturbance. © 2019 John Wiley &amp; Sons, Ltd.

Transporte fluvial en Colombia: operación, infraestructura, ambiente, normativa y potencial de desarrollo

El transporte fluvial constituye un eje de desarrollo y crecimiento económico en países con vías navegables aprovechables, como los europeos en donde la adecuada explotación de un transporte fluvial eficiente ha redundado en su mayor competitividad en el comercio exterior. Estos desarrollos requieren que cada nación posea un claro entendimiento de sus potencialidades en el sector fluvial, en razón a que el desconocimiento de estos aspectos es el principal obstáculo de cara a la planeación, gestión y explotación óptimas de este sistema de transporte. El objetivo de este artículo es proveer una perspectiva del transporte fluvial en Colombia y una prospectiva sobre su potencial de desarrollo. La metodología se orienta al estudio de su origen, progreso, su forma de operación actual, las características de su infraestructura, la situación ambiental y el marco normativo aplicable en el contexto colombiano. Sobre lo anterior se proporciona una prospectiva de desarrollo del transporte fluvial orientado al comercio, la infraestructura y nuevos planes y políticas públicas. Se concluye que este modo de transporte tiene gran viabilidad en Colombia, para lo cual se requiere la inversión en infraestructuras prioritarias y servicios logísticos, así como el desarrollo de nuevas políticas públicas.