RIPARIAN VEGETATION AND SANDBAR MORPHOLOGY ALONG THE LOWER LITTLE COLORADO RIVER,ARIZONA

The distribution of riparian vegetation in relation to channel morphology is poorly understood in canyon rivers, which are characterized by in-channel fluvial sediment deposits rather than flood plains. This study focuses on vegetation and sandbar characteristics in two reaches of the lower Little Colorado River canyon in Arizona–one reach with ephemeral flow from the watershed, and another with perennial baseflow from a spring. Both reaches have been colonized by the exotic Tamarix chinensis, a riparian species known for its geomorphic influence on river channels. On the basis of a sampling of 18 bars, results show that vegetation frequency and density is significantly greater in the perennial study reach. However, sandbar morphology variables do not differ between reaches, despite a significantly narrower and deeper ephemeral channel. Hydraulic calculations of flood depths and Pearson correlations between bar and vegetation variables indicate reach-specific biogeomorphic relationships. In the ephemeral reach, higher bars are less affected by flood inundation, support older vegetation, and may be more stable habitat for vegetation. In the wider perennial reach where bars are lower and more expansive, vegetation patterns relate to bar size, Tamarix being most common on the largest bars. Overall results suggest that (1) vegetation variation relates to baseflow hydrology, (2) bar formation relates to high discharge events, and (3) vegetation patterns respond to, rather than influence, sandbar form in this canyon riparian system.     

Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Pánuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Pánuco basin (98,227 km²) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/m, whereas the texture (D₈₄) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed-scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature (R_c) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Pánuco. However, in the lower reaches of the Pánuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause–effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley.     

Channel Form, Bed Material and Sediment Sources of the Sprongdøla, Southern Norway: Evidence for a Distinct Periglacio-Fluvial System

A geomorphological and sedimentological case study is made of the Sprongdøla, a medium-sized river flowing in a predominantly bedrock channel in the mountains of southern Norway. Particular attention is given to bed material characteristics and potential sediment sources. Downstream patterns in clast size and shape, including sphericity and roundness, are poorly developed in the Sprongdøla. There is an apparent absence of down-stream fining and a large proportion of bedload in the lower channel remains in very angular and angular roundness categories. Covariance analysis of clast shape and roundness allows this to be attributed to the widespread input of coarse, angular rock particles derived from snow-avalanche activity on the valley sides and especially from frost-shattering of bedrock in the channel, which produces characteristically slabby clasts. The latter process is important in channel widening and in steepening the channel walls. Strictly fluvial processes, avalanche activity and frost-shattering are controlled in different ways by related annual environmental cycles. Their interaction in the context of the periglacial river channel produces a unique process-sediment-landform association that distinguishes alpine periglacio-fluvial systems from those associated with glacio-fluvial rivers and fluvial landscapes in the temperate zone.     

Caledonian foreland basin sedimentation: A new depositional model for the Upper Silurian‐Lower Devonian Lower Old Red Sandstone of the Midland Valley Basin,Scotland

Reconstruction of the geological history of orogenic events can be challenging where basins have limited and/or fragmentary preservation. Here, we apply understanding gained from modern analogues to the sedimentological analysis of the succession of Upper Silurian to Lower Devonian Lower Old Red Sandstone (LORS), northern Midland Valley, Scotland, in order to reconstruct the foreland to the Caledonian orogeny. A new depositional model is presented which differs significantly from current understanding. Using facies analysis, grain size distribution and palaeocurrent data a large distributive fluvial system is reconstructed. Three lithofacies and nine sublithofacies are identified, forming fluvial channel and floodplain facies associations. The system was derived from an emerging mountain range in the Caledonian foreland undergoing constant tectonic rejuvenation to produce 9 km of coarse‐grained sediment, exhibiting an overall decrease in thickness towards the west and a large‐scale downstream reduction in grain size. Conglomerate sublithofacies dominate proximal areas in the east where amalgamated fluvial channel facies association is abundant, with a downstream increase in the dominance of floodplain facies. Additionally, observed grain size cyclicity is attributed to a pulsatory tectonic influence. The LORS records the time‐period between the late phases of the Caledonian Orogeny and the onset of post‐orogenic collapse in the mid‐Devonian and the presented model allows improved understanding of the north‐Atlantic Caledonian foreland.     

Characteristics of freshly deposited sand and finer sediments along an island-braided, gravel-bed river: The roles of water, wind and trees

Over the past two decades there has been a growing interest in the geomorphological mosaic along large floodplain rivers where channel dynamics are seen to drive habitat-patch creation and turnover and to contribute to high biological diversity. This has required a new perspective on fluvial geomorphology that focuses on biological scales of space and time. This study examines the spatial pattern of surface fine sediment accumulations along a reach of a large gravel-bed river, the Tagliamento River in NE Italy; an area with a moist Mediterranean climate and seasonal flow regime. The study investigates changes in sediment characteristics during the summer low-flow period between April and September. Focussing on five areas representing a gradient from open, bar-braided to wooded island-braided morphologies, the paper demonstrates the importance of riparian vegetation and aeolian–fluvial interactions.Significant contrasts in particle size distributions and organic content of freshly deposited sand and finer sediments were found between sampling areas, geomorphological settings, and sampling dates. In particular, wooded floodplain and established islands supported consistently finer sediment deposits than both open bar surfaces and the lee of pioneer islands, and in September significantly finer sediments were also found in deposits located in the lee of pioneer islands than on open bar surfaces. Overall, the September samples had a greater variability in particle size characteristics than those obtained from the same sites in April, with a general coarsening of the D₅ (φ) (i.e., the coarse tail of the particle size distribution). Also in September, crusts of fine sediment (30 μm < D₅₀ < 64 μm) had formed on the surface of some of the open bar and pioneer island deposits within the more open sampling areas along the study reach. These crusts possessed similar particle size characteristics to aeolian crusts found in more arid environments. They were significantly finer than April samples and September subcrust samples obtained from the same sites and had similar particle size characteristics to some samples taken from wooded floodplain, established island surfaces and the lee of pioneer islands that were not crusted.Local climatological and river level data confirm significant wind and rainfall events during a period of consistently low river levels between the April and September sampling periods. These support deflation, deposition and rain wash of finer sediment during the summer, with windblown sediments being deposited on bar surfaces and in the lee of pioneer islands where wood and young trees provide foci for accelerated sedimentation and island growth as well as on marginal floodplains and established islands. We conclude that along braided rivers in moist settings but with a distinct dry season, aeolian reworking of sediment deposits may have a more important role in driving habitat dynamics than previously considered.     

Palaeohydraulics revisited: palaeoslope estimation in coarse-grained braided rivers

Methods for estimating palaeoslope from fluvial deposits have been available for some time, but new data and improved understanding of the relevant physical processes afford the possibility of improving existing methods, and the emerging field of quantitative stratigraphy provides a new context for the results. Here we focus on deriving palaeoslope estimates for coarse-grained fluvial deposits. These estimates can be used in basin analyses to constrain the magnitude of the slope change necessary for a given deflection of palaeocurrents, to constrain temporal and spatial variation in basin subsidence rate, and to provide a surface datum for use in sediment-backstripping calculations. The algorithm we derive to estimate palaeoslope applies to rivers that self-adjust through variations in channel width to maintain a temporally and spatially averaged bed shear stress equal to some constant multiple of the critical shear stress for initial motion of bed sediment. Data from modern coarse-grained rivers with minimal bank cohesion and form resistance suggest that this boundary shear stress is equal to about 1.4 times the critical shear stress for movement of the median-sized clast of the surface layer. The key sedimentological criteria for recognition of systems appropriate for this type of analysis are: (1) field relations suggesting that channel banks formed in effectively noncohesive gravel (i.e. free of clay-size sediment and plant roots); (2) absence of significant volumes of dune-derived cross-stratification; and (3) absence of indicators of extremely rapid, flash-flood-type deposition. The basic input data for a palaeoslope calculation are spatially averaged estimates of palaeodepth and median grain size. The most important aspect of data collection is that the depth and grain-size estimates should be determined independently by random sampling over the whole outcrop. Joint analysis of data from appropriate modern rivers and of errors associated with palaeodepth and grain-size estimates indicates that in coarse-grained braided-river deposits, palaeoslope can be estimated to within a factor of 2.     

Bank Erosion in Fifteen Tributaries in the Glaciated Upper Susquehanna Basin of New York and Pennsylvania

The proportional contributions of cultivated lands and stream banks as sources of fine sediment loads were quantified in 15 rural watersheds in the Glaciated Appalachian Plateau region of the Susquehanna River basin of New York and Pennsylvania. We utilized a relatively simple method of fingerprinting sediment sources by comparing the concentrations of the nuclear bomb-derived radionuclide ¹³⁷ Cs in fluvial sediment samples collected from channel margins with sediment from cultivated fields and stream banks. The proportion of fine sediment from bank erosion ranged from none to 100% in the study tributaries, with a median contribution of 53% across the 15 study streams. In one stream with no evidence of bank sediment, anomalously high ¹³⁷ Cs levels in the samples indicated that the sources were pasture or forest, probably scoured from marshy floodplains upstream of the sampling sites. In the 14 other streams, cultivated lands accounted for an average of 42% of the fine sediment. We discuss sources of eroded bank material and the processes driving stream bank erosion in this glaciated region, and examine the impact of historic mill-dam deposits on bank erosion.     

Trends of grain sizes on gravel bars in the Rio Chagres, Panama

We examined the trends of grain sizes along the upper 414 km² of the mountainous Rio Chagres drainage basin in Panama. Gravel bars were sampled along 40 km of the Rio Chagres and five major tributary streams using a transect pebble count of median diameter, lithology, and clast rounding. Although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages, we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium. Specifically, we tested the hypotheses that downstream fining would be present as a result of selective sorting, and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion. Statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the Rio Chagres, with a Sternberg diminution coefficient (α) for felsic and mafic grains of − 0.013 and − 0.017, respectively. Felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles, but tumbling-mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology, and downstream decreases in grain size are similar between lithologies. Dividing the study reach into six sub-reaches bounded by major tributary junctions, we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries, but that trends in grain sizes might be more visible at the reach scale between tributaries. We did not find any consistent trends in grain size between tributaries. Stream width appears to assert a local control on grain size; coarse particles are associated with narrow channel reaches, whereas smaller particles are associated with wide channel reaches.     

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.     

Analysis of controls upon channel planform at the First Great Bend of the Upper Yellow River, Qinghai-Tibet Plateau

The 270 km long section of the Upper Yellow River at the First Great Bend is comprised of single channel and multiple channel systems that alternate among anastomosing, anabranching, meandering and braided reaches. The sequence of downstream pattern changes is characterized as： anastomosing-anabranching, anabranching-meandering, meandering-braided and braided-meandering. Remote sensing images, DEM data and field investigations are used to assess ahd interpret controls on these reach transitions. Channel slope and bed sediment size are key determinants of transitions in channel planform. Anas- tomosing reaches have a relatively high bed slope （0.86‰） and coarser sediment bed material （d50 = 3.5 mm）. In contrast, meandering reaches have a low slope （0.30‰） and fine sediment bed material （d50 = 0.036 mm）. The transition from a meandering to braided pattern is characterized by an increase in channel width-depth ratio, indicating the important role of bank strength （i.e. cohesive versus non-cohesive versus channel boundaries）. Interestingly, the braided-meandering and meandering-braided transitions are coincident with variable flow inputs from tributary rivers （Baihe and Heihe rivers respectively）. Theoretical analysis of the meandering-braided transition highlights the key control of channel width-depth ratio as a determinant of channel planform.     

Sedimentological and Geomorphological Effects of Reservoir Flushing: The Cachí Reservoir, Costa Rica, 1996

The Cachí Reservoir on the Reventazón River, Costa Rica, is flushed on an almost yearly basis. A field investigation of the 1996 flushing was carried out in order to elucidate from where in the reservoir the material originates, where and to what extent the released sediments will deposit along the river from the dam to the sea, and finally how this can be explained. A reservoir survey was conducted by means of echo-sounding along certain cross-sections before and after the flushing. The material was found to both deposit in between flushings and to be eroded during flushing, mainly in the uppermost and lowermost parts of the old river channel. In the downstream reaches, the pulse of suspended sediment pertaining to the flushing was found to be clearly distinguishable by means of grain-size distribution, even in the lower parts. Surveys were conducted at eight sites along the river downstream from the dam both before and after the flushing such that the volumes of deposited or eroded material were found. By use of the dry bulk density, the results could be calculated in tonnes, thus facilitating comparison to the samplings of the hydrological stations along the river. A major factor in explaining the amounts and distribution of deposits was shown to be the phase lag between water discharge and suspended-sediment con-centration peaks. The influence of this phase lag on the deposition and erosion processes is illustrated by a schematic figure. A more detailed sediment budget is presented for the reach between the hydrological stations c . 10 and 30 km downstream from the dam. It was found that approximately 250 000 tonnes were deposited within the reach. Of these, 82% were channel-bed deposits while 18% were deposited on the river banks.     

Modern and ancient fluvial megafans in the foreland basin system of the central Andes, southern Bolivia: implications for drainage network evolution in fold-thrust belts

ABSTRACT Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland‐basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800–22 600 km²), fan‐shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low‐relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid‐channel and bank‐attached sand bars. Overbank areas are characterized by crevasse‐splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long‐term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening‐upward, > 2‐km‐thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold‐thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid‐Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse‐splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold‐thrust belt. In addition, the present‐day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid‐Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold‐thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 10⁴ km²– a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold‐thrust belt.     

Characterising the origin,nature and fate of sediment exported from catchments perturbed by active tectonics

Changes to the tectonic boundary conditions governing erosional dynamics in upland catchments have a significant effect on the nature and magnitude of sediment supply to neighbouring basins. While these links have been explored in detail by numerical models of landscape evolution, there has been relatively little work to quantify the timing, characteristics and locus of sediment release from upland catchments in response to changing tectonic boundary conditions that are well‐constrained independently. We address this challenge by quantifying the volume and granulometric characteristics of sediment exported from modern rivers draining across active normal faults in the Central Apennines in Italy. We demonstrate that catchments undergoing a transient response to tectonics are associated with significant volumetric export of material derived primarily from the zone upstream of the fault, producing bi‐modal grain‐size distributions with elevated D₈₄ values within the transient reach. This is in direct contrast to the headwaters, where the fluvial capacity to transport sediment is low and the grain‐size distribution of material in transit is fine and uni‐modal. The grain‐size response is driven by landslides feeding coarse material directly into the channel, and we show the amplitude of the signal is modulated by the degree of tectonic perturbation, once the threshold for bedrock landsliding is exceeded. Additionally, we evaluate the length‐scale over which this transient grain‐size signal propagates downstream into the basin. We show that the coarse‐fraction sediment released is retained in the proximal hanging‐wall if rates of tectonic subsidence are high and if the axial river system is small or far from the fault‐bounded mountain front. Our results therefore provide some of the first quantitative data to evaluate how transient landscape responses affect the locus, magnitude and calibre of sediment supply to basins.     

Disturbance, stream incision, and channel evolution: The roles of excess transport capacity and boundary materials in controlling channel response

Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non-incised surfaces. Large, anthropogenic disturbances, similar to large or catastrophic “natural” events, greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to antrhopogenic and “natural” disturbances are presented for fluvial systems in the mid continent and Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically-controlled sediment (sand) provided by bank erosion.The non-linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that effect available force, stream power or flow energy, or change erosional resistance such that an excess of flow energy occurs can result in incision. Channel incision, therefore, can be considered a quintessential feature of dis-equilibrated fluvial systems.     

Downstream geomorphic impacts of large dams: the case of Glenbawn Dam,NSW

Downstream hydrologic effects since the closure of Glenbawn Dam, a large dual-purpose storage for water conservation and flood mitigation, include: (i) a reduction in mean annual runoff of about 21 × 10⁶ m³; (ii) a change in the probability distribution of mean daily flows involving a truncation of flows >;8 × 10⁶ m³ d^?1, a much reduced frequency of flows >7 × 10⁵ m³ d^?1 and an increased frequency of flows <7 × 10⁵ m³ d^?1; and (iii) a reduction in flood magnitude of at least 80 per cent for all probabilities of exceedance. From suspended sediment samples collected before and after dam closure, sediment trap efficiency has been estimated at 99 per cent.An accommodation adjustment of the channel has occurred upstream of the first unregulated tributary because the bed is armoured, the banks are well vegetated, some bedrock and concrete controls are present and all regulated releases are incompetent to transport the bed material. Immediately downstream of the first unregulated tributary, the channel has contracted by up to 45 per cent and degraded by up to 69 per cent during lateral migration. Further downstream no channel changes were recorded although the bed material has progressively coarsened over time.     

Spatial and temporal variations in the grain-size characteristics of historical flood plain deposits, Blue River, Wisconsin, USA

This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.     

Rapid grain size coarsening at sandstone/conglomerate transition: similar expression in Himalayan modern rivers and Pliocene molasse deposits

Radical grain size changes between two main units of a sedimentary megacycle in a foreland basin are commonly interpreted to result from changes in tectonic activity or climate in the adjacent mountain range. In central Nepal, the Cenozoic Siwalik molasse deposits exposed in the frontal Himalayan folds are characterized by such a radical grain size transition. Locally gravel deposits completely replace sands in vertical succession over approximately a hundred metres, the median grain size (D₅₀) displaying a sharp increase by a factor of ca. 100. Such a rapid gravel‐sand transition (GST) is also observed in present‐day river channels about 8–20 km downstream from the outlet of the Siwalik Range. The passage from gravel‐bed channel reaches (proximal alluvial fans) to sand‐bed channel reaches (distal alluvial fans) occurs within a few kilometres on the Gangetic Plain in central Nepal, and the D₅₀ ratio between the two types of channels equals ca. 100. We propose that the dramatic and remarkably similar increase in grain size observed in the Neogene Siwalik series and along modern rivers in the Gangetic foreland basin, results from a similar hydraulic process, i.e. a grain sorting process during the selective deposition of the sediment load. The sudden appearance of gravels in the upper Siwalik series would be related to the crossing of this sorting transition during progressive southward migration of the gravel front, in response to continuous Himalayan orogen construction. And as a consequence, the GST would be diachronous by nature. This study demonstrates that an abrupt change in grain size does not necessarily relate to a change in tectonic or climatic forcing, but can simply arise from internal adjustment of the piedmont rivers to the deposition and run out of coarse bedload. It illustrates, in addition, the genesis of quartz‐rich conglomerates in the Himalayan foreland through gravel selective deposition associated with differential weathering, abrasion processes and sediment recycling during thrust wedge advance and shortening of the foreland basin.     

Spatial hydrogeomorphological influences on sediment and nutrient deposition in riparian zones: observations from the Garonne River,France

This paper investigates the influence of geomorphological setting on riparian zone sedimentation within a reach of the River Garonne, France, during three major floods. The sampling design was stratified to reflect landforms constructed by fluvial processes (e.g. floodplain, lateral benches, islands, side channels and point bars). Observed sedimentation varied significantly with flood event, planform context, landform type and associated vegetation cover and, in some cases, with sample location within the landform. Lowest sedimentation was associated with the flood with the smallest peak discharge, peak sediment concentration and sediment load. Sites under natural riparian vegetation experienced higher sedimentation than poplar plantations. Sites on concave (outer) banks received less sedimentation than those on convex (inner banks). Sedimentation on floodplain sites and higher benches was lower than on low benches, point bars and side channels. There was considerable interdependence among these patterns, reflecting the underlying geomorphological forms and processes. Meandering rivers tend to evolve through erosion of concave banks and deposition on convex banks. Point bar features tend to be built along convex banks, whilst concave banks are eroded into higher floodplain and bench features. As a result, concave banks tend to be bordered by higher riparian margins that are less frequently flooded than convex banks. Where river margins are developed for agriculture, the higher, less frequently flooded sites are preferentially selected.Analyses of the quantity, calibre, nutrient and carbon content of the deposited sediment reveal further significant relationships, which reflect the geomorphological structure of the riparian zone. Sediment particle size coarsens in locations with higher amounts of sedimentation. The quantities of total organic carbon (TOC), total organic nitrogen (TON) and total phosphorus (TP) all increase as the quantity of deposited sediment increases. The concentration of TOC and TON also increased significantly with an increase in the percentage of silt plus clay in the deposited sediments.Based upon the above observations, a conceptual model is proposed, which considers the spatial pattern in riparian zone sedimentation according to riparian morphology and flood magnitude. The implications of channel incision for the functioning of the model are also discussed.     

Magnetostratigraphy,age and depositional environment of the Lobo Formation,southwest New Mexico: implications for the Laramide orogeny in the southern Rocky Mountains

The Lobo Formation of southwestern New Mexico consists of spatially variable continental successions attributed to the Laramide orogeny (80–40 Myr), although its age and provenance are virtually undocumented. This study combines sedimentological, magnetostratigraphical and geochronological data to infer the timing and origin of the Lobo Formation. Measured sections of Lobo strata at two locations, Capitol Dome in the Florida Mountains and in the Victorio Mountains, indicate significant differences in depositional environments and sediment provenance. At Capitol Dome, where Lobo strata were deposited above a syncline developed in Palaeozoic strata, deposition took place in fluvial, palustrine and marginal lacustrine settings, with alluvial‐fan deposits only at the top of the formation. Combined magnetostratigraphy and a young U–Pb detrital zircon age suggest deposition of the section at Capitol Dome from ~60 to 52 Ma. The Lobo Formation in the Victorio Mountains was deposited in alluvial‐fan and fluvial settings; the age of deposition is poorly bracketed between 66 ± 2 Ma, the weighted‐mean age of two young zircons, and middle Eocene (~40 Ma), the approximate age of overlying volcanic rocks. U–Pb zircon ages from sandstones at the Victorio and Capitol Dome localities indicate that different source rocks provided sediment to the Lobo Formation. Local Proterozoic basement (~1.47–1.45 Ga) dominated the source of the Lobo Formation in the Victorio Mountains, consistent with abundant granitic clasts that are present in the proximal facies there; a diverse range of grain ages suggest that recycled Lower Cretaceous strata provided the dominant source for Lobo Formation sediment at the Capitol Dome locality. The U–Pb data suggest that the depositional systems at the two sites were not connected. Contrasts in depositional setting and detrital zircon provenance indicate that the Palaeogene Lobo Formation in southwest New Mexico was deposited in an assemblage of local depositional settings, possibly in separate structural basins, as a consequence of Laramide tectonics in the region.     

Sediments of Lake Vens (SW European Alps,France) record large-magnitude earthquake events

We studied sediment cores from Lake Vens (2,327 m asl), in the Tinée Valley of the SW Alps, to test the paleoseismic archive potential of the lake sediments in this particularly earthquake-sensitive area. The historical earthquake catalogue shows that moderate to strong earthquakes, with intensities of IX–X, have impacted the Southern Alps during the last millennium. Sedimentological (X-ray images, grain size distribution) and geochemical (major elements and organic matter) analyses show that Lake Vens sediments consist of a terrigenous, silty material (minerals and organic matter) sourced from the watershed and diatom frustules. A combination of X-ray images, grain-size distribution, major elements and magnetic properties shows the presence of six homogenite-type deposits interbedded in the sedimentary background. These sedimentological features are ascribed to sediment reworking and grain sorting caused by earthquake-generated seiches. The presence of microfaults that cross-cut the sediment supports the hypothesis of seismic deposits in this system. A preliminary sediment chronology is provided by ²¹⁰Pb measurement and AMS ¹⁴C ages. According to the chronology, the most recent homogenite events are attributable to damaging historic earthquakes in AD 1887 (Ligure) and 1564 (Roquebillière). Hence, the Lake Vens sediment recorded large-magnitude earthquakes in the region and permits a preliminary estimate of recurrence time for such events of ~400 years.