Modern Fluvial Systems Research Articles

An assessment of oxbow lakes and their potential in reconstructing past river discharge: Implication to reconstruct past climate in Southern West Bengal

For modern fluvial systems, stage height, velocity, and cross-section help estimate a river’s discharge. However, understanding the amount and fluctuations of past discharges remains a challenge. The dimen-sions of the meander loops/oxbow lakes are directly proportional to the river’s discharge and the sedi-ment load type. Central West Bengal, India, has a complex network of numerous oxbow lakes as a rem-nant of the river Hooghly, a major distributary of the River Ganga. The present work revisits Schumm’s classic work to estimate river discharges from the meander loops and explores a potential proxy for es-timating past discharges. Thus, it is extended to reconstruct past climate using grain size data, facies analysis, and the dimension of the meander loops coupled with the luminescence chronology of oxbow lakes. The study reveals that the region received good rainfall during reported periods of intense mon-soon, and the increased discharge at different time intervals has given rise to numerous oxbow lakes. The discharge values have shown considerable fluctuations, rising to 15 to 20 times the current value and very few anomalously high, even up to 250 to 300 times, left unrecorded from other archives. The study shows the preservation of systematic growth of discharges and the meander loops with climate re-vealing events with anomalously high (and so highly disastrous) discharges at the scale of tens of thou-sands of years, usually missed from other archives. Results from Ichhamati and Hooghly River meander loops in West Bengal, India, indicate a manifold increase of discharges from the current during 1–1.5 ka (known as the Medieval Warm Period); around 2.2 ka and around 3.9 ka with low or gaps of enhanced monsoon, e.g., Little Ice Age and during 2.5–3.5 ka. This is an effort to show the potential of past mean-der loops to be explored well for comprehensive records.

Fluvial character and architecture of an outcrop using sedimentology combined with UAV-based modeling, Cretaceous McMurray Formation, NE Alberta, Canada

ABSTRACT It is widely accepted that most occurrences of inclined heterolithic stratification (IHS) in the rock record form by laterally accreting point bars in freshwater fluvial, tidally influenced fluvial, or tidally dominated estuary channels. Despite the widespread distribution of IHS in the subsurface and outcropping strata of the lower Cretaceous McMurray Formation, the large-scale depositional architecture and lateral facies variability of these deposits remains controversial. The relatively limited lateral extent of many of the outcrops is a challenge, particularly when point-bar deposits on the scale of hundreds of meters to kilometers are interpreted in outcrops spanning anywhere from 100 to 300 meters laterally. This has in turn led researchers to leverage other datasets such as 3-D seismic to analyze the large-scale depositional architecture of the IHS, leading to two main interpretations for the IHS in the McMurray Formation: 1) a fluvially dominated environment owing to geomorphological features comparable to those in large modern fluvial systems, or 2) an estuarine environment owing to the presence of trace fossils characteristic of marine-derived faunal colonization in brackish-water settings and strong evidence of tidal modulation. The purpose of this study is to investigate the sedimentology and depositional architecture of IHS in a unique, kilometer-wide outcrop exposure of McMurray Formation strata and compare it to IHS observed at other McMurray Formation outcrops previously interpreted as estuarine channels. This is achieved by combining traditional field-based methods with Unmanned Aerial Vehicle-based outcrop modeling to create a 3-D outcrop model to visualize and analyze large point-bar geobodies deposited in a channel upwards of 25 meters deep and 750 meters wide exposed in outcrop at Crooked Rapids of the Athabasca River, west of the City of Fort McMurray. Importantly, this methodology uses bed orientation trends, paleocurrent data, and sedimentological observations to identify and map architectural elements, which constitute an eastward-accreting point bar crosscut by a southwestward-accreting counter point bar in the outcrop. The results strongly suggest that the IHS at Crooked Rapids was deposited in a freshwater fluvial environment. When compared to IHS deposited in estuarine depositional environments, fluvial IHS is driven by seasonal river discharge as opposed to the interplay between river discharge and the extent of the tidal prism. Therefore, fluvial IHS is: 1) dominantly sandstone with very minor waning-flow siltstone interbeds resulting from erosion by the succeeding freshet phase, and 2) completely devoid of bioturbation until flat-lying bar top or overbank strata is encountered. Using 3-D outcrop modeling to supplement sedimentological and ichnological observations strengthens the interpretation of complex fluvial geobodies and increases the overall understanding of the large-scale depositional architecture of point bars across the tidal–fluvial transition zone.

The Effect of Sediment Storage in Glaciated Catchments on Multimineral Detrital Geochronology: Deciphering Conflicting Zircon and Apatite U‐Pb Dates

AbstractDetrital geochronology and thermochronology are widely used to study erosion and the exhumation of rocks. In glaciated landscapes, these studies have shown exhumation to be highly heterogeneous; however, these studies have typically only applied apatite low‐temperature thermochronology, a mineral prone to mechanical and chemical breakdown, and a method with lower precision or data dispersion. As such, it is unclear if the detrital apatite signal represents derivation from the entire catchment. In this study, we present both zircon and apatite U‐Pb dates from a modern detrital sample of the Saco River, New Hampshire, USA. The river flows through a catchment that was highly modified during the last glacial maximum. The underlying geology is characterized by two contrasting groups from different eras; Palaeozoic metasediments and granites, and Mesozoic granitoids. Zircon U‐Pb dates are consistent with near‐uniform erosion of the catchment, encompassing both Proterozoic–Palaeozoic and Jurassic–Cretaceous grains consistent with the catchment's geology. However, detrital apatite U‐Pb dates are predominantly Palaeozoic. Detrital U‐Pb modeling shows that zircon grains are indeed derived from erosion of the entire catchment, while apatites likely originate from areas at high elevation. This stark contrast in ages likely indicates apatite's fragility. Dissolution resulting from acidic soils in the river's catchment is a likely cause, though mechanical breakdown during transport is also possible. As such, the detrital apatite signal appears to only record erosion from the modern fluvial system, while zircon represents a combination of uniform erosion during glaciation and the modern fluvial system. These findings imply the detrital apatite signal is prone to significant modification in areas of sediment storage and acidic soils with implications for sediment generation in other formerly glaciated landscapes worldwide.

Controls on the morphology of braided rivers and braid bars: An empirical characterization of numerical models

ABSTRACTBraided rivers exhibit highly variable morphologies, morphodynamic behaviours and resulting depositional records. To evaluate relationships between characteristics of braided‐river channel belts and river depth, water discharge and streambed gradient, 39 numerical modelling experiments were conducted with the software Delft3D to simulate braided‐river evolution under a broad range of boundary conditions. Data from model outputs were integrated with observations from 63 natural braided rivers differing with respect to river depth and streambed gradient. The modelled rivers each underwent similar evolutions, yet each culminated in markedly different final river morphologies, dependent on discharge and riverbed gradient. The rivers underwent evolutionary stages of: (i) formation of transverse unit bars with limited relief from an initially featureless bed; (ii) channel development around bars and in some cases dissecting transverse unit bars; (iii) formation of relatively simpler compound bars; and (iv) amalgamation of these simpler compound bars into more complex compound bars. Quantitative relationships relating to braided‐river channel‐belt morphology and organization are established, and the following results are noted: (i) bar elongation (length‐to‐width ratio) is correlated positively with riverbed gradient; (ii) bar height and area are correlated positively with discharge, and negatively with riverbed gradient; (iii) the river depth is the main predictor of mean braid‐bar area; and (iv) the degree of braiding is primarily associated with river width‐to‐depth ratio and riverbed gradient. Results arising from this research improve our understanding of controls on the morphology and architectures of braided fluvial channel belts; they provide a novel empirical characterization that can be applied for predicting channel depth, bar morphology, streambed gradient, and degree of braiding of modern fluvial systems and of the formative rivers of ancient preserved successions.

Quantitative analysis of crevasse-splay systems from modern fluvial settings

ABSTRACTAlthough crevasse splays are a common constituent of many fluvial and fluvio-deltaic systems, they remain less well understood than the channel deposits in those settings, especially with respect to controls on their occurrence, distribution, and geometry. The current study aims to redress this balance and investigate controls on 1) splay formation and occurrence and 2) splay size and geometry. The study has used Google Earth-based satellite imagery to examine crevasse-splay deposits from eight modern fluvial systems. A total of 1556 crevasse splays were identified using imagery from 1984 to 2020. Most of the splays (c. 70%) occur on the outer sinuous river bank with offtake angles ranging from 10° to 140° (mean 75°) to the channel flow direction. Three different types of splays have been identified: i) single crevasse splays, ii) laterally amalgamated crevasse splays, and iii) crevasse-splay complexes. The areal extent of splay bodies varies widely and ranges from less than 1 km2 up to 221 km2. The single crevasse splays are the primary and smallest form of splay, with an average area of 0.61 km2. Compensational stacking or progradation significantly increase the splay area and form laterally amalgamated splays and splay complexes, respectively. The average areal extent of laterally amalgamated splays is 1.33 km2, and of splay complexes, 39 km2.The climate, discharge, floodplain morphology, vegetation, trunk channel slope, sinuosity, and sediment load primarily control the occurrence, geometry, and dimensions of crevasse splays. Results demonstrate that sparse or no floodplain vegetation favors the formation of elongated tongue-shaped crevasse splays while densely vegetated floodplains produce more lobate splays. The highest splay frequency occurs in systems where the river experiences sudden high magnitude variation in discharge, has a low cross-sectional area, and noncohesive bank materials. Larger splay size is correlated with lower river slope angles and higher sinuosity, discharge, and floodplain relief. Channel size has little influence on the extent of splays. This work suggests that autogenic factors such as trunk-channel slope and sinuosity are more influential in arid–semiarid settings while allogenic factors such as discharge are important in temperate–equatorial settings.

Factors governing travertine deposition in fluvial systems: The Bagni San Filippo (central Italy) case study

Although fossil fluvial travertines have been described, factors controlling their formation are poorly understood because of the paucity of their modern counterparts. To disclose processes affecting their deposition, a modern fluvial travertine system at Bagni San Filippo (Siena, central Italy) was studied here and compared with other cases. The studied travertines occur in a valley with an ephemeral stream, but continuous water influx from hot springs on the hillside makes part of the stream become perennial. Four sub-environments were recognized: slopes, waterfalls, pools, and channels. The first two are mainly composed of laminated crystalline crust-boundstone. In contrast, pools are mainly covered by lime mud and some post-flood travertine-encrusted breccia, while channels commonly display abundant travertine-encrusted breccia. Many erosional features and products were also found and are largely the result of episodic erosion triggered by heavy rainfalls and accompanied floods. The predominant erosional processes might include abrasion and plucking. Based on environment distribution, stream bed morphology, and erosional distinctions, the fluvial system was distinguished into slope-pool-waterfall and channel-pool-waterfall subsystems. Such system differentiation is attributed to the original stream bed difference: wide beds promoted the development of slopes, while narrow beds encouraged travertine erosion and subsequent gravel accumulation during flood events, favoring the formation of channels. The comparison shows that fluvial travertine deposition largely occurs within existing rivers. The relative contribution of spring water to original river water controls the deposit composition (the higher contribution ratios, the more abundant carbonate/travertine facies). Furthermore, erosion might be common and unavoidable because the reported fluvial travertines were all formed in non-arid regions. These findings suggest that fluvial travertine deposition is influenced by topography, hot spring contribution, original river bed geometry, and fluvial erosion, and might aid in the interpretation of ancient fluvial travertine systems. Additionally, downstream fluvial travertine systems might show similar characteristics to fluvial tufas, but their formation is clearly hot spring influenced, indicating the importance of analyzing facies distribution and δ13C-δ18O signatures in fluvial carbonate studies.

Sedimentology of the modern seasonal lower Ganges River with low inter-annual peak discharge variance, Bangladesh

The Ganges River, one of the largest rivers on Earth, is a typical monsoonal and flood-controlled system but has low inter-annual peak discharge variability. The seasonal discharge can reach 70 000 m 3 s −1 during the wet season but maintains a low base flow of 500–3000 m 3 s −1 during the dry season. However, the constancy in peak discharge every year categorizes the lower Ganges River as a river with low inter-annual peak discharge variability. This paper examines the modern lower Ganges River by conducting a detailed process-oriented investigation of the main channel, channel margin and overbank deposits, supplemented by satellite image observation and comparison with other modern fluvial systems. The channel and braid bar deposits show a dominance of small-scale to medium-scale cross-sets, with a variety of accretion processes constructing braid bars. The braid bar and channel deposits are typical of facies models of rivers with low inter-annual peak discharge variance. In contrast, the channel flank deposits are dominated by planar lamination, massive sand and mud couplets, and some ripple cross-lamination, with very little cross-bedding. Characteristic channel margin deposits represent sediments that accumulated by high-speed flows, multiple-surge and rapidly depositing flows, rapid or regular waning flows and hyperconcentrated flows. The overbank deposits predominantly comprise current ripples with long, thin bedforms and soft sediment deformation structures, which record flow transformation on the muddy flat topography and the processes of an unstable river bank. Our study shows that the channel margin and floodplain deposits are entirely different from those of the braid bar and channel. The bedform distribution of the fluvial deposits here (main channel, channel margin and overbank) may be an important tool in the identification of similar seasonal rivers with low inter-annual peak discharge variance and in the interpretation of fluvial processes. Supplementary material : The sediment texture and the depositional age is available at: https://doi.org/10.6084/m9.figshare.c.5144403

Application of channel-belt scaling relationships to early Middle Jurassic source-to-sink system evolution in the southern Junggar Basin

Abstract A wide range of key scaling relationships obtained from modern fluvial systems can be used to estimate the drainage areas in ancient source-to-sink systems. In this study, these relationships are applied to reconstruct source-to-sink systems of the early Middle Jurassic Xishanyao Formation in the southern Junggar Basin where meandering fluvial and deltaic successions are developed. Both the lower channel-belt thickness and the bankfull channel-belt thickness are measured and are used to estimate the scale of drainage systems that were associated with the seven fluvial systems in three sequences (Sequence 1, 2 and 3 in ascending order). The results indicate that the channel-belt thickness had significant variation through time and space. The average values of the bankfull thickness of the seven fluvial systems are translated into drainage area, which range from 63,000–157,000 km2, and the maximum drainage length, which range from 559 to 934 km. The delta volume of the seven fluvial systems range from 2.01 km3 to 10.35 km3. From Sequence 1 to Sequence 3, detrital mineral compositions of sandstones show a gradual change from lithic arkose to feldspathic litharenite. Provenance fields on ternary plots and the characteristics of sandstone components show that the sandstone sources gradually shifted from the southwest-dominated Yilinhabierga Mountains to the southeast-dominated Bogda Mountains. Sediment provenance analysis, together with paleogeographic maps and analysis of sedimentary records, suggests that orogenic building of the Bogda Mountains was responsible for the progressive eastward shifting of the main fluvial channels, and for the modification of drainage areas. The mismatched pattern of channel sizes between the rivers developed in the early Middle Jurassic and in the modern northern Tian Shan front is due to late Cenozoic north-south crustal shortening, which had caused the drainage network reorganization and the evolution of fluvial styles. Our work provides a complete profile of sediment dispersal from source to sink for better understanding of the paleogeographic evolution in the southern Junggar Basin. This source-to-sink approach could also be applicable to other lacustrine basins in China.

Stratigraphic architecture and hierarchy of fluvial overbank splay deposits

Splay deposits represent an important sand-prone component of the otherwise fine-grained stratigraphic record of fluvial overbank systems. This work presents a hierarchical approach to the classification and palaeoenvironmental interpretation of ancient preserved splay deposits supported by the analysis of the stratigraphic architecture of 11 exhumed examples from the Jurassic Morrison Formation and the Cretaceous Mesaverde Group (western USA) and analysis of the morphology of splays from nine modern fluvial systems. A hierarchical arrangement of splay deposits is proposed, categorized into lithofacies, beds, elements and complexes. Recognition criteria for each tier of the hierarchy include identification of bounding surfaces, thinning and fining trends of splay elements and complexes, and palaeocurrent variability. Progradational and compensational stacking trends control the stratal architecture of splay deposits, and these are influenced by the following factors: (1) the rate of local accommodation generation, which influences the erosive power of floodwaters and whether splay elements are laterally offset owing to compensational stacking; (2) the nature of the topographic confinement of the floodplain; (3) the preservation potential linked to migration direction of channel. Splay bodies can contribute volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby enhancing reservoir connectivity. Supplementary material: Thickness and facies types with global positioning system (GPS) coordinates, length and width v. thickness, and modern data and coordinates are available at https://doi.org/10.6084/m9.figshare.c.4461026

Downstream fining in a megaclast-dominated fluvial system: The Sabeto River of western Viti Levu, Fiji

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.

THE FLUVIAL SYSTEM ON THE EAST EUROPEAN PLAIN: SEDIMENT SOURCE AND SINk

The modern fluvial system on the lowland East European Plain is of depositional type. Sediment transport to the seas is only a few percent of the total erosion, and the main part of eroded material is accumulated in the channels. The recent deposition of suspended sediments is caused by accelerated soil erosion on the arable slopes, which led to a high rate of lateral sediment input and deposition at the river headwaters and on the floodplains. The process of accumulation is facilitated by the unfilled “negative” volume of the net of dry valleys formed during the Late Glacial catastrophic erosion event. Such events of catastrophic erosion of the sediments deposited in the lowland fluvial systems occur with a frequency of 100-120 thousand years. In the conditions of both scarce vegetation and extremal surface runoff, the entire fluvial systems become the area of intensive erosion, with the deep incision of gullies and of the river channels. Therefore, despite the modern intensive deposition, delivery ratio for the fluvial systems on this lowland territory is close to one in the long-term perspective.

Relationships between discharge parameters and cross-sectional channel dimensions of rivers in an active margin influenced by tropical climate: The case of modern fluvial systems in the Indonesian islands

The present study investigates (1) the relationships between discharge parameters and bankfull channel width (Wb) based on statistical analyses of hydrological data obtained from 649 sites at rivers on the Indonesian islands of Java, Sumatra, and Kalimantan, and (2) the relationship between depth parameters and Wb using channel cross sections obtained from 86 sites at rivers on these three large islands. These analyses are used to describe geomorphological and hydrological features of fluvial systems developed in a convergent margin under the influence of a low-latitude tropical climate. The relationship between mean discharge (Qmean) and Wb shows distinct variation within the Indonesian rivers, and reflects regional variations in annual rainfall of a tropical climate. The relationship between bankfull discharge (Qb) and Wb shows only minor variations across the three islands, regardless of bed and bank material types. The Qb–Wb relationship in the Indonesian rivers has a pattern similar to that of fluvial systems in Europe and the North American continent, which are influenced by mid- and high-latitude climates. Minor regional variation in the Qb–Wb relationship, regardless of the tectonic and climatic settings, suggests that the development of fluvial morphology is controlled mainly by flood-related episodic discharge. In contrast, the empirical equations between maximum channel depth (db) and Wb and between mean channel depth (dm) and Wb show distinct regional variations not only within the Indonesian fluvial systems but also in fluvial systems of Europe and the North American continent. These regional variations are thought to reflect the fact that db–Wb and dm–Wb relationships are controlled by the tectonic and climatic settings of a drainage basin, and factors such as the interrelationships between precipitation, vegetation, and slope. Consequently, selection of an appropriate dm–Wb (or db–Wb) equation combined with an appropriate Qb–Wb equation permits us to reconstruct spatial and temporal variations in hydrological features in both modern and ancient fluvial systems more precisely than applying previously proposed equations to fluvial systems formed in active margin and low-latitude climatic settings.

Tectonic Topography Changes in Cenozoic East Asia: A Landscape Erosion‐Sediment Archive in the South China Sea

AbstractThe mode and tempo of Cenozoic fluvial input into the South China Sea by Asian rivers were strongly controlled by tectonically induced topographic changes and landscape development that resulted in repeated river captures and reversals. New Nd isotope and U‐Pb detrital zircon data from clastic sedimentary sequences in the northern South China Sea display secular variations reflecting significant changes in the compositions of fluvial sediment as well as changes in source rocks. The oldest, Eocene‐Oligocene (before ∼25 Ma) clastic rocks of the South China Sea contain sediments withɛNdvalues of −5 to −7.5 and detrital zircons of Cretaceous, Jurassic, and Permo‐Triassic, all derived from southern Cathaysia. The Oligocene‐Middle Miocene (∼25–11 Ma) sediments in the South China Sea strata displayɛNdvalues of −12 to −14 and Archean, Proterozoic, and Paleozoic detrital zircons, suggesting northern Cathaysia and/or the Yangtze Blocks as their provenance. The headward expansion of the Pearl River and reversal of the Middle Yangtze River correlates with the opening of the South China Sea and growth of the eastern Tibetan Plateau at ∼25 Ma. Further uplift of the eastern Tibetan Plateau and the Wuyi‐Nanling Mountains in SE China around 11 Ma reshaped the landscape and caused another phase of erosion—pattern adjustment and drainage—divide migration in East Asia, leading to the establishment of the modern fluvial systems of the Pearl, Minjiang, and Lower Yangtze Rivers.

Origin and sedimentary fate of plant-derived terpenoids in a small river catchment and implications for terpenoids as quantitative paleovegetation proxies

Tricyclic diterpenoids and non-steroid or non-hopanoid pentacyclic triterpenoids are almost exclusively taxon-specific terrestrial plant biomarkers produced by conifers and angiosperms, respectively. Due to this source specificity and their prevalence in the geologic record, these compounds are often used to reconstruct paleovegetation. However, the physical and chemical processes that influence the dispersal and fate of terpenoids in sedimentary archives are poorly constrained. Modern fluvial systems can be used as ancient river analogs to provide information on the utility of terrestrial plant terpenoids as paleovegetation proxies by defining their potential flux and identifying the processes that control their transport to, and deposition and degradation in, sediments. To determine if the contribution of terpenoids from vegetation is reflected in forested soil and river sediments and to constrain the dispersal of these compounds in fluvial systems, di- and triterpenoid concentrations in Miners River drainage basin (Upper Peninsula of Michigan, USA) were quantified. In the basin, evergreen conifers are less abundant than deciduous angiosperms, but yet contribute substantially more terpenoids to soils and river sediments when scaled for leaf litter production and present vegetation cover. The composition and relative concentration of di- and triterpenoids in source vegetation do not match those in soils and river sediments, suggesting that some process or processes result in the preferential removal of diterpenoids. While the soil and river sediment terpenoid concentration, corrected for differential terpenoid inputs, can closely predict the basin wide vegetation cover in Miners River drainage basin, the extent to which terpenoids can be used a paleovegetation proxy in other modern or geologic sediments remains unclear.

Classification of Vegetation over a Residual Megafan Landform in the Amazonian Lowland Based on Optical and SAR Imagery

The origin of large areas dominated by pristine open vegetation that is in sharp contrast with surrounding dense forest within the Amazonian lowland has generally been related to past arid climates, but this is still an issue open for debate. In this paper, we characterize a large open vegetation patch over a residual megafan located in the northern Amazonia. The main goal was to investigate the relationship between this paleolandform and vegetation classes mapped based on the integration of optical and SAR data using the decision tree. Our remote sensing dataset includes PALSAR and TM/Landsat images. Five classes were identified: rainforest; flooded forest; wooded open vegetation; grassy-shrubby open vegetation; and water body. The output map resulting from the integration of PALSAR and TM/Landsat images showed an overall accuracy of 94%. Narrow, elongated and sinuous belts of forest within the open vegetation areas progressively bifurcate into others revealing paleochannels arranged into distributary pattern. Such characteristics, integrated with pre-existing geological information, led us to propose that the distribution of vegetation classes highlight a morphology attributed to a Quaternary megafan developed previous to the modern fluvial tributary system. The characterization of such megafan is important for reconstructing landscape changes associated with the evolution of the Amazon drainage basin.

Relationships of bankfull channel width and discharge parameters for modern fluvial systems in the Japanese Islands

The relationships between the bankfull channel width and the mean, bankfull, and maximum discharges of Japanese rivers were examined using the hydrological and geomorphological data from 368 sites. The relationships between the bankfull channel width and the mean and bankfull discharges do not show any distinct regional variations. In contrast, the relationship between the maximum discharge and the bankfull channel width shows regional variations, and the lower and higher maximum discharges relative to bankfull channel widths are documented in the fluvial systems in Hokkaido and Southwest Japan, respectively. These variations are interpreted to reflect regional variations in precipitation intensity during the rainy season, and the magnitude and frequency of typhoon-related flooding. The relationship between the bankfull channel width and the bankfull discharge can be described by an empirical equation similar to that described from modern fluvial systems on the European and American continents. Consequently, this empirical equation may have wider applicability for the estimation of hydrological features of modern and ancient fluvial systems, not only in active margin settings influenced by mid-latitude temperate climates, but also in passive continental margins and continental interior basins.

Scale invariance in fluvial barforms: implications for interpretation of fluvial systems in the rock record

Understanding the controls on the size and shape of sandstone bodies deposited by fluvial systems is important in the reconstruction of ancient fluvial deposits and construction of quantitative reservoir models. Measurements and analyses of sandbodies from remotely sensed imagery have allowed quantification of width and length ratios of barforms in modern fluvial systems. For bank-attached bars the width:length ratios range between 0.12 and 0.47 (arithmetic mean: 0.25), for lateral bars between 0.19 and 0.42 (arithmetic mean: 0.30), for mid-channel bars between 0.09 and 0.49 (arithmetic mean: 0.28), and for point bars between 0.14 and 0.50 (arithmetic mean: 0.30). The majority of width:length ratios for all bar types range between 0.15 and 0.35. Examination of other parameters such as basin type, planform geometry, apparent stream width, river length, gradient over the investigated area, aggradational or degradational system, tectonic setting and climate do not significantly affect the width:length ratio. Therefore, the bar planform shape, the width:length ratio, can be considered to be scale invariant. The recognition that bar planform shape in fluvial systems is scale invariant will be useful in the construction of subsurface three-dimensional models of fluvial deposits with variable dimensions. Supplementary material: Data tables with information obtained for all of the rivers studied are available at http://www.geolsoc.org.uk/SUP18745 .

Palaeozoic co-evolution of rivers and vegetation: a synthesis of current knowledge

As vegetation evolved during the Palaeozoic Era, terrestrial landscapes were substantially transformed, especially during the ∼120 million year interval from the Devonian through the Carboniferous. Early Palaeozoic river systems were of sheet-braided style – broad, shallow, sandbed rivers with non-cohesive and readily eroded banks. Under the influence of evolving roots and trees that stabilised banks and added large woody debris to channels, a range of new fluvial planform and architectural styles came to prominence, including channelled- and island-braided systems, meandering and anabranching systems, and stable muddy floodplains. River systems co-evolved with plants and animals, generating new ecospace that we infer would have promoted biological evolution. By the end of the Carboniferous, most landforms characteristic of modern fluvial systems were in existence.

Provenance of North Atlantic ice-rafted debris during the last deglaciation—A new application of U-Pb rutile and zircon geochronology

Understanding the provenance of ice-rafted debris (IRD) provides a means to link the behavior of individual ice sheets to proxy records of climate change. Here we present a new approach to determining IRD provenance using U-Pb geochronology of detrital minerals rutile and zircon. We characterize potential source regions from Scotland using detrital rutile from modern fluvial systems, and demonstrate that their unimodal rutile U-Pb ages reflect the timing of the last amphibolite facies metamorphism of the source rocks, imparting a distinctive source signature. Contrasts between these spectra and the bimodal IRD (ca. 470 Ma and ca. 1800–2000 Ma) rutile age signatures rule out Scotland as the sole source and suggest a Laurentian contribution; IRD zircon ages further support this view. U-Pb mineral dating has the potential to provide new insight on IRD provenance, because it allows linkage between IRD and individual source terranes based on their differing magmatic and tectonothermal histories. The occurrence of Laurentian-sourced IRD proximal to Scotland demonstrates widespread and rapid dispersal of debris across the subpolar North Atlantic during the Older Dryas cold oscillation, and implicates the Atlantic meridional overturning circulation as a control. This highlights the sensitivity of some IRD records to rapid climate change during the last deglaciation and supports the interpretation of Heinrich events as time-parallel marker horizons.

Identification and provenance determination of distally transported, Vredefort-derived shocked minerals in the Vaal River, South Africa using SEM and SHRIMP-RG techniques

The record of meteorite impacts on Earth is incomplete due to the destruction of impact craters by erosion and burial. Shocked minerals residing in sediments may help further document the impact record. To evaluate the survival of shocked minerals in fluvial systems we investigated ∼10,000 sand grains from seven sites along the Vaal River in South Africa, ranging from 103 to 759km downriver from the 2.020Ga Vredefort impact structure, for the presence of detrital shocked minerals. Detailed observations were made of >7000 quartz grains by petrographic analysis, and >3300 zircon and monazite grains by scanning electron microscopy, using BSE, CL, and EBSD methods. Shock microstructures were identified in detrital quartz, monazite and zircon grains in all of the Vaal River sediment samples analyzed, from the Vredefort Dome to the confluence with the Orange River, 759km downstream. Analysis of U–Th–Pb and REE compositions of detrital shocked zircon and monazite by SHRIMP-RG allows correlation of the grains to Archean basement exposed in the Vredefort impact structure.Three distinct detrital shocked zircon age populations at 3104Ma, 3065Ma and 2867Ma were identified. Apparent ages of detrital shocked monazite range from 3044Ma to 2157Ma, including two groupings at 2640Ma and 2454Ma. The Vredefort impact event does not appear to have caused significant Pb-loss for the majority of the grains; Neoproterozoic lower concordia intercepts (1093–751Ma) in the zircon populations indicate most Pb-loss occurred during the Kibaran orogeny in southern Africa.This study demonstrates that shocked minerals survive long distances (at least 750km) of transport within fluvial environments, and can be correlated to known bedrock localities using U–Th–Pb dating. The implications of these results are: (1) detrital shocked minerals within modern sediments can be used to identify the existence of an impact structure; geochronology of shocked zircon and monazite can provide target bedrock ages and metamorphic history that can constrain the location of the structure. (2) The persistence of ancient shocked zircon in modern fluvial sedimentary systems suggests that a detrital shocked mineral record of early impact events on Earth may be preserved in siliciclastic sedimentary rocks.