basement control of structure in the gettysburg rift basin pennsylvania and maryland abstract jurassic faulting formed the 93 km long gettysburg basin as an extensional half graben paralleling the basement structural grain preserved in the basin are rift related carnian to rhaetian strata that were tilted 20 30 nw into a se dipping listric normal fault at the northwest border of the basin vertical displacement on the border fault approaches 10 km the border fault developed parallel to the trend of the terminal paleozoic alleghenian south mountain cleavage of the blue ridge basement along 80 of its extent however it is only roughly parallel to discordant to dip of the cleavage relationship of cleavage and later border faulting may be the result of persistent reactivation of the original appalachian continental margin local complex structures in the half graben are related to reactivation of two subvertical pre mesozoic faults that transect basement structural grain cleavage at a large angle the northern shippensburg fault was reactivated during basin normal faulting offsetting the border fault in a right lateral sense by 3 5 km and forming within the basin a fold and a fault sliver of basement the southern carbaugh marsh creek fault was not reactivated but is the locus of a 20 30 change of trend of both the basement cleavage and later border fault however two large nw trending left lateral wrench faults antithetic to the carbaugh march creek fault developed here offsetting the border fault and forming en echelon folds and horst blocks of basement rock within the basin

evolution of the western amazon lowland relief impact of andean foreland dynamics the congruency in the depositional origin and age of the uppermost sedimentary strata forming non flooded rainforest ground terra firme in the western and central amazon lowlands is a much debated subject here we conclude from the study of remote sensing imagery that active andean foreland dynamics have played a major role in the evolution of the plio pleistocene fluvial landscape in the western amazon foreland dynamics have resulted in a terra firme composed of late tertiary alluvium and younger alluvial terraces and plains in peru thermoluminescence and 14c dating show local aggradation of this younger alluvium between 180 and 30 ka the documented high age heterogeneity of the terra firme has implications for considerations of the biogeography of the amazon forest

sediment supply as a driver of river meandering and floodplain evolution in the amazon basin the role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood despite analytical advances in our physical understanding of river meandering1 2 the amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads allowing us to explore the influence that sediment supply has on river evolution here we calculate average annual rates of meander migration within 20 reaches in the amazon basin from landsat imagery spanning 1985 2013 we find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads meander cutoff also occurs more frequently along rivers with higher sediment loads differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge because faster meander migration and higher cutoff rates lead to increased sediment storage space in the resulting oxbows we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers we conclude that imposed sediment loads influence planform changes in lowland rivers across the amazon

particle dynamics the continuum of bedrock to alluvial river segments abstract particle dynamics refers to production erosion transport and storage of particulate material including mineral sediment and organic matter particle dynamics differ significantly between the end members of bedrock and alluvial river segments and between alluvial river segments with different grain size distributions bedrock segments are supply limited and resistant to change with relatively slow linear adjustments and predominantly erosion and transport particle dynamics in alluvial segments in contrast are transport limited and dominated by storage of mineral sediment and production of organic matter alluvial segments are resilient to change with relatively rapid multidirectional adjustments and stronger internal influences because of feedbacks between particles and biota bedrock segments are the governors of erosion within a river network whereas alluvial segments are the biogeochemical reactors fundamental research questions for both types of river segments center on particle dynamics which limit network scale incision in response to base level fall bedrock segments and habitat biogeochemical reactions and biomass production alluvial segments these characterizations illuminate how the spatial arrangement of bedrock and alluvial segments within a river network influence network scale resistance and resilience to external changes in relative base level climate and human activities

modeling framework for sediment deposition storage and evacuation in the floodplain of a meandering river theory 1 a theory for the movement of suspendible sediment through the floodplain of a simple actively migrating river is developed the floodplain is represented as a series of sediment storage reservoirs each incorporating multiple bends of a river independent models for floodplain sediment deposition and for subsequent removal of sediment by lateral channel migration allow the volume of sediment stored in the floodplain and the associated average floodplain elevation to change over time and down the valley at a given quasi steady discharge deposition depends on a simple one dimensional partition between channel and floodplain discharge and on suspended sediment concentration one size class which varies along the reach depending on lateral supply and floodplain history sediment supply is specified at the upstream end of the reach using a suspended sediment rating curve long term average overbank deposition rates are approximated by weighting multiple steady state solutions using a flow duration curve the channel bed elevation of the system is assumed to be fixed and tributaries are handled using simple lateral source terms the interaction between the submodels is such that under constant boundary conditions the channel evolves toward a state of geomorphic equilibrium where the volume of sediment added to the floodplain by overbank deposition just balances the volume removed by channel migration the model is easily adapted to track conservative suspended sediment tracer material concentration as such material moves through the channel floodplain system

stream networks and long term surface uplift in the new madrid seismic zone stream networks are sensitive to low rates of surface uplift and can be used to decipher the history of large earthquakes even where faults do not rupture the surface as in intraplate seismic zones statistical analysis of alluvial network data from topographic maps in the new madrid seismic zone in the central united states shows that stream segment gradients deviate the most from an estimated natural stream profile where surface uplift is greatest evidence of cumulative deformation distilled from stream network patterns represents at least several meters of differential surface uplift during holocene time which suggests that more than one cycle of surface deformation occurred

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

inherited discontinuities and neogene structure the gulf of suez and the northwestern edge of the red sea the gulf of suez and the northern red sea rifts are the result of intracontinental deformation during neogene times the initiation and the development of the rift are controlled by i four main trends offaults n1400 n1500 ns to n200 sub e w and n40 60 ii a zigzag faulting pattern iii two main tectonic events the first is characterized by strike slip displacements inducing the formation of antithetic tilted blocks the geometry of the blocks changes according to their orientation complex structures can result from the combination of several trends vertical movements are weak the second is characterized by synthetic normal movements forming a horst and graben pattern vertical displacements are important and induce the generation of the axial trough and uplift of the shoulders of the rift the zigzag pattern of the faults that govern the rift and the initial strike slip displacements cannot result from a simple extensional model but imply a reactivation of inherited discontinuities in the miocene stress system induced by the northern convergent boundaries of the african and arabic plates the major trends of faults that control the neogene structure have been active in this area since palaeozoic or even proterozoic times

low angle extensional faulting reactivated mylonites and seismic reflection geometry of the newark basin margin in eastern pennsylvania low angle 25 to 35 dips have been determined for the border fault of the newark basin near riegelsville pennsylvania on the basis of a vibroseis profile and two continuously cored drill holes across faults at the basin margin a group of moderately strong planar reflections in a zone 0 5 km thick in gneiss and carbonate rocks of the footwall block coincides with the updip projection of imbricate fault slices and mylonites associated with the musconetcong thrust system of drake et al 1967 contrasts in acoustic impedance among mylonitic dolostone and mylonitic gneiss and their protoliths determined from measurements on samples from a third cored hole are sufficiently large to account for reflections seen in the footwall block analysis of drill core and surface outcrops supports the conclusion that low angle extensional faulting in the early mesozoic was localized by reactivation of paleozoic imbricate thrust faults in the basement rocks extension in the northwest southeast quadrant was approximately perpendicular to the strike of the ancient thrust faults in eastern pennsylvania the data presented here are the most explicit three dimensional information obtained thus far in the eastern united states in support of the concept of fault reactivation in controlling formation of early mesozoic extensional basins

delineation of delta ecozones using interferometric sar phase coherence mackenzie river delta n w t canada abstract interferometric processing of six c band synthetic aperture radar sar image pairs for the mackenzie river delta shows that phase coherence is more effective than radar backscatter for differentiating willow alder spruce ice and open water similar radar backscattering from vegetation and wind roughened water surfaces can make such distinctions difficult in sar backscatter images in contrast coherence is preserved over surfaces with rigid scattering elements such as leafless woody alder but is lost over temporally unstable surfaces such as open water we find that willow alder display highest coherence followed by spruce frozen lakes and channels and open water respectively this general hierarchy is preserved even when overall image coherence is decreased by increasing temporal and spatial baselines evaluation of spatial thermal noise and temporal components of total observed coherence indicates that temporal decorrelation is a primary control short temporal baselines such as those obtained during the european remote sensing satellite ers 1 day tandem missions are therefore optimal for obtaining maximum differentiation of surface classes

controls on the occurrence and prevalence of floodplain channels in meandering rivers the process of channelization on river floodplains plays an essential role in regulating river sinuosity and creating river avulsions most channelization occurs within the channel belt e g chute channels but growing evidence suggests some channels originate outside of the channel belt in the floodplain to understand the occurrence and prevalence of these floodplain channels we mapped 3064 km2 of floodplain in indiana usa using 1 5 m resolution digital elevation models dems derived from airborne light detection and ranging lidar data we find the following range of channelization types on floodplains in indiana 6 8 of floodplain area has no evidence of channelization 55 9 of floodplains show evidence e g oxbow lakes of chute channel activity in the channel belt and 37 3 of floodplains contain floodplain channels that form long coherent down valley pathways with bifurcations and confluences and they are active only during overbank discharge whereas the first two types of floodplains are relatively well studied only a few studies have recognized the existence of floodplain channels to understand why floodplain channels occur we compared the presence of channelization types with measured floodplain width floodplain slope river width river meander rate sinuosity flooding frequency soil composition and land cover results show floodplain channels occur when the fluvial systems are characterized by large floodplain to river widths relatively higher meandering rates and are dominantly used for agriculture more detailed reach scale mapping reveals that up to 75 of channel reaches within floodplain channels are likely paleo meander cutoffs the meander cutoffs are connected by secondary channels to form floodplain channels we suggest that secondary channels within floodplains form by differential erosion across the floodplain linking together pre existing topographic lows such as meander cutoffs copyright 2016 john wiley sons ltd

recycling of amazon floodplain sediment quantified by cosmogenic 26al and 10be the modern amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1 3 m y in floodplains by reworking of floodplain deposits this sediment is being incorporated into amazonian streams we conclude this from using the ratio of in situ produced cosmogenic 26 al to 10 be in detrital sediment from rivers draining the sediment producing andean guyana and brazilian shield areas as well as the largest amazonian lowland rivers samples from the modern amazon river yield 26 al 10 be ratios between 3 8 and 5 5 and thus represent mixing between a non buried sediment end member and a formerly buried sediment end member the non buried end member is sourced in the andes with a ratio of 6 5 reflecting recent erosion of hillslopes exposed to cosmic rays and the buried end member is potentially represented by deeply stored floodplain sediment as old as miocene age with a ratio of 3 that was partially shielded from cosmic rays this simple picture of binary mixing is complicated by grain size effects coarser grained sediment from non andean settings lowlands cratonic shields contains slightly more formerly buried material whereas finer grained lowland sediment mostly records non buried 26 al 10 be ratios and nuclide concentrations that are indicative of an andean source our assessment shows an incorporation of formerly buried old sediment of central amazonian floodplains to the fresh andean derived sediment mainly carried in the amazon trunk stream we interpret our results to mean that the amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current holocene channel

floodplain channel morphology and networks of the middle amazon river 1 floodplain channels are important components of river floodplain systems and are known to play a key role in hydrodynamic exchange and sediment transport the amazon floodplain exhibits complex networks of these channels and despite their potential importance to this globally important wetland system these floodplain channels are relatively unstudied the research presented here is the first systematic and detailed study of the network and morphologic characteristics of a large number of these channels in the middle reach of the central amazon river using analysis of data derived from landsat enhanced thematic mapper plus etm mosaic and field survey our findings show that the channels are ubiquitous their width varies widely and some of their characteristics can be fitted using power laws potentially much like the self similar or fractal like behavior hypothesized for other types of fluvial networks in all 96 of the floodplain channels are not wide enough to be represented well or at all in the 90 m shuttle radar topography mission data channel depths are tied closely to the local amplitude of the passing main river flood wave p value of 0 75 except where there are local runoff inputs which results in substantially deeper channels which provide preferential flow paths across the floodplain channel networks imply that areas of the floodplain function for large parts of the flood cycle as separate hydrogeomorphic land units here termed floodplain hydrological units fhus these hypothesized fhus also have distinct spatial and pattern characteristics and it is suggested here that their differences could provide the beginnings of a framework for understanding the detailed hydrodynamics of the floodplain in particular different types of fhus have differences in flood water source which will have important implications for biogeochemical studies of the wetlands

the auriferous late archaean sedimentation systems of south africa unique palaeo environmental conditions abstract studies of the auriferous witwatersrand placers and associated rocks have revealed that certain palaeo environments especially braided fluvial are particularly well represented in this part of the rock record however there is a paucity of lithofacies indicating certain other palaeo environments possible reasons for their scarceness or absence are suggested in this paper it is generally assumed that pre vegetational fluvial systems would have been characteristically braided because of the absence of land plants necessary to stabilise river banks a question arising is what kind of downstream changes such pre vegetational braided streams underwent a recent study of a braided stream placer revealed that the depositing system retained its braided character right down to the palaeo shoreline however gravel did not reach the palaeo beach as with many modern examples beach conglomerates are rare in the witwatersrand rocks the paucity of conglomeratic beach placers is ascribed to the low probability of gravel being transported across coastal plains because of the relatively low slopes or depths of rivers here the witwatersrand fluvial channels are generally considered to have had high width to depth ratios because of the absence of land vegetation to stabilise channel banks however two examples of deep relatively narrow scour features with predominantly fine grained fills occur near the base of the central rand group the low width to depth ratios of these scour features which probably represent palaeo valleys are ascribed to severe incision during a rapid sea level fall the auriferous fluvial systems of the witwatersrand are generally considered to have been entirely braided due to the lack of bank stability however the fluvial b placer of the welkom goldfield is confined to discrete channels their banks are thought to have been stable due to the cohesive nature of the lutite into which the gravelly streams incised in addition braiding sensu stricto was probably inhibited by initial incision and a low sediment supply no deposits of specifically deltaic sub environments have been found in the central rand group their absence is attributed to the following factors a the paucity absence of well defined palaeo river mouths b low concentrations of suspended sediment c intermittent sediment supply to the palaeo coastline and d reworking by tidal and longshore currents alluvial fan deposits are also apparently absent in the witwatersrand rocks the absence of fan deposits is attributed to the poor potential for development and preservation of fan deposits in the compressive tectonic setting proposed for the witwatersrand basin although ventifacts have been found in the witwatersrand rocks no aeolianites have been reported their apparent absence is probably due to a reworking in a predominantly humid climate and b transport of sand by dominant winds to areas unfavourable for the preservation of aeolianites

characterization of complex fluvial systems using remote sensing of spatial and temporal water level variations in the amazon congo and brahmaputra rivers the surface water and ocean topography swot satellite mission will provide global space based estimates of water elevation its temporal change and its spatial slope in fluvial environments as well as across lakes reservoirs wetlands and floodplains this paper illustrates the utility of existing remote sensing measurements of water temporal changes and spatial slope to characterize two complex fluvial environments first repeat pass interferometric sar measurements from the japanese earth resources satellite are used to compare and contrast floodplain processes in the amazon and congo river basins measurements of temporal water level changes over the two areas reveal clearly different hydraulic processes at work the amazon is highly interconnected by floodplain channels resulting in complex flow patterns in contrast the congo does not show similar floodplain channels and the flow patterns are not well defined and have diffuse boundaries during inundation the amazon floodplain often shows sharp hydraulic changes across floodplain channels the congo however does not show similar sharp changes during either infilling or evacuation second shuttle radar topography mission measurements of water elevation are used to derive water slope over the braided brahmaputra river system in combination with in situ bathymetry measurements water elevation and slope allow one to calculate discharge estimates within 2 3 accuracy these two studies illustrate the utility of satellite based measurements of water elevation for characterizing complex fluvial environments and highlight the potential of swot measurements for fluvial hydrology copyright 2010 john wiley sons ltd

formation and maintenance of single thread tie channels entering floodplain lakes observations from three diverse river systems 1 tie channels connect rivers to floodplain lakes on many lowland rivers and thereby play a central role in floodplain sedimentology and ecology yet they are generally unrecognized and little studied here we report the results of field studies focused on tie channel origin and morphodynamics in the following three contrasting systems the middle fly river papua new guinea the lower mississippi river and birch creek in alaska across these river systems tie channels vary by an order of magnitude in size but exhibit the same characteristic morphology and appear to develop and evolve by a similar set of processes in all three systems the channels are characterized by a narrow leveed single thread morphology with maximum width approximately one tenth the width of the mainstem river the channels typically have a v shaped cross section unlike most fluvial channels these channels develop as lakes become isolated from the river by sedimentation narrowing of the connection between river and lake causes a sediment laden jet to develop levees develop along the margins of the jet leading to channel emergence and eventual levee aggradation to the height of the mainstem levees bidirectional flow in these channels is common outflows from the lake scour sediment and prevent channel blockage we propose that channel geometry and size are then controlled by a dynamic balance between channel narrowing by suspended sediment deposition and incision and widening by mass failure of banks during outflows tie channels are laterally stable and may convey flow for hundreds to a few thousand of years

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

cosmogenic nuclide budgeting of floodplain sediment transfer abstract cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin a change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain we have modeled depth and time dependent cosmogenic nuclide concentration changes for 10 be 26 al and 14 c during sediment storage and mixing in various active floodplain settings ranging from confined shallow rivers with small floodplains to foreland basin scale floodplains traversed by deep rivers floodplain storage time estimated from channel migration rates ranges from 0 4 kyr for the beni river basin bolivia to 7 kyr for the amazon river basin while floodplain storage depth estimated from channel depth ranges from 1 to 25 m for all modeled active floodplain settings the long lived nuclides 10 be and 26 al show neither significant increase in nuclide concentration from irradiation nor decrease from decay we predict a hypothetical response time after which changes in 10 be or 26 al concentrations become analytically resolvable this interval ranges from 0 07 to 2 myr and exceeds in all cases the typical residence time of sediment in a floodplain due to the much shorter half life of 14 c nuclide concentrations modeled for the in situ produced variety of this nuclide are however sensitive to floodplain storage on residence times of the cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them is predicted to either increase or decrease in 10 be and 26 al concentration depending on the depositional depth these conditions can be evaluated using the 26 al 10 be ratio that readily discloses the depth and duration of storage we illustrate these models with examples from the amazon basin as predicted modern bedload collected from an amazon tributary the bolivian beni river shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering in contrast in the central amazon floodplain currently untouched by the modern river system low 26 al 10 be ratios account for minimum burial depths of 5 to 10 m for a duration of 5 myr the important result of this analysis is that in all likely cases of active floodplains cosmogenic 10 be and 26 al concentrations remain virtually unchanged over the interval sediment usually spends in the basin thus spatially averaged denudation rates of the sediment producing area can be inferred throughout the entire basin provided that nuclide production rates are scaled for the altitudes of the sediment producing area only because floodplain storage does not modify nuclide concentrations introduced from the sediment source area

characterizing large river history with shallow geophysics middle yukon river yukon territory and alaska abstract changes in longitudinal valley fill thickness can provide inferences about fluvial behaviour over millennia increased thickness of channel deposits may reveal climate induced accelerated sediment supply basin subsidence tectonic displacements or rising downstream base level decreases in thickness expressed as terraces may suggest active uplift decreased sediment flux or falling base level within a reach to date methods to characterize valley fills have depended largely on borehole data however recent developments in shallow geophysics make such investigations possible and provide a practical methodology to investigate alluvial rivers we carried out a longitudinal geophysical survey coupled with floodplain chronologies to characterize a 1000 km reach of the middle yukon river between western yukon territory and central alaska ground penetrating radar gpr and electrical resistivity ground imaging ergi profiles along the middle yukon river between central yukon and alaska coupled with channel bottom echo profiles demonstrates that there is little variation in thickness of gravel fill and depth of active scour holes gpr surveys near the white river the largest sediment source in the upper yukon show no significant longitudinal variation in gravel thickness 8 16 m up to near circle alaska 530 km of wandering channels geophysical profiles across the yukon flats sedimentary basin 430 km of braided and wandering channels show an increase in gravel thickness 25 30 m but this is consistent with compaction of underlying lacustrine sediment and maximum depths of contemporary scour holes from the western margin of the yukon flats to the dalton highway bridge 30 km confined valley with no floodplain or braid bars borehole data at the bridge indicates 6 10 m of gravel on bedrock beneath 3 6 m of water floodplain stratigraphy indicates there is little evidence of fluvial aggradation of the middle yukon during the holocene radiocarbon ages from the gravel overbank silt contact near present river level demonstrate a general vertical stability of the river with a period of rapid eolian accumulation during the late pleistocene and early holocene the consistency of gravel fill and scour hole depth and lack of holocene terraces along the 1000 km reach of the yukon indicate that the river has been in a state of mass balance equilibrium of sediment transport input output over the holocene despite massive sediment influxes from the glacial fed white river and crossing major faults variation in valley fill depth is not significant and further the yukon flats reach shows no evidence of bed aggradation during the pleistocene and has probably existed in equilibrium since the late pliocene

the hydro geomorphologic complexity of the lower amazon river floodplain and hydrological connectivity assessed by remote sensing and field control abstract hydrological connectivity processes along the lower amazon river floodplain at lago miratuba floodplain lmf were analyzed based on remote sensing and field measurements connectivity processes are related to 12 geomorphic sub units of the floodplain that are interconnected at different stage thresholds through channelized and overbank flow paths while lakes act as reservoirs for flood waters local rainfall and water table saturation seepage hydrological connectivity processes along the floodplain happen through complex internal routings of water flows over space and time mainly along floodplain channels and breached levees the contribution of local recharge mainly rainfall was decoupled from river recharge river hydrological connection to floodplain because precipitations and local runoff also recharge the floodplain the hydrological connectivity of the 12 geomorphic units varied significantly from each other because the inundation and the floodplain connections with the river were initiated at specific river stage thresholds for each sub unit an interesting finding is that connections with the river for all units were initiated through channelized flows below the bankfull stage of the amazon river the connectivity processes over different geomorphic units are also not correlated with their minimum distance shortest lines from the river this implies that a flood pulse model largely concerned with lateral connectivity mostly through overbank diffusion only partially explains the complex hydrological connectivity processes at a regional scale along the complex lower amazon river floodplain thus for hydrological connectivity assessments in the amazon river floodplain reach by reach scale studies have to be carried out relating the hydro geomorphologic processes channelized flow routing paths overbank diffusion and local recharge to the internal geomorphic variability and complexity of the floodplain

lowermost amazon river morphology and shallow seismic characteristics abstract the riverbed morphology of the lowermost amazon river was characterized by high resolution acoustic profiling in 21 areas this technique has been used for the first time in this area and allowed the discovery of a variety of reflectors which delineate areas either of sediment transport or of deposition on an older river bottom records obtained with the pinger system made it possible to distinguish three main kinds of reflectors both surface and sub bottom 1 flat 2 wavy and 3 irregular reflection patterns which can be interpreted as strata deposited in a variable mostly high energy setting or reworked structures of initially flat or wavy strata that have been changed mapping of these different reflectors shows that they are not randomly distributed the constant association of a given reflector with a particular morphological setting is indicative of its lithological significance supported by riverbed sampling and erosional depositional processes depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub bottom related to old substrates alter do chao or barreiras formation this may result from inundation and incision caused by quaternary sea level changes previously eroded areas are covered by a wavy layer consisting of very fine to fine sand unit fs which can change locally to medium coarse sand unit ms reflectors are more abundant downstream a minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge moreover the detection of fault scarps interpreted to result from neotectonic activity shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability the dynamic setting of the lowermost amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes the effect of the gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part

chemical weathering of fluvial sediments during alluvial storage the macuapanim island point bar solimoes river brazil abstract modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration at macuapanim island solimboes river amazon main channel brazil a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time in the very fine sand fraction the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments feldspars and accessory minerals with a concomitant enrichment in quartz this process produces first cycle sands whose detrital component is of quartz arenite composition among the salts quartz is enriched over time relative to more nstable phases particularly plagioclase this enrichment is probably largely caused by the destruction of plagioclase through chemical weathering but silt size quartz may also be produced through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction in the clay fraction an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction and probably reflects the degradation of volcanic glass the clay fractions of the oldest samples examined contain less smectite somewhat more kaolinite and markedly more vermiculite as compared to younger samples but an extensively leached cation poor clay fraction analogous to the quartz dominated silts and sands is never attained the compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands perhaps because of a continual influx into the clay fraction of immature cation rich clays derived from the physical breakdown of lithic fragments in coarser size fractions this effect is enhanced by the translocation of fines whereby younger less weathered fine material is juxtaposed with older more mature coarse material in the lower levels of the alluvial stratigraphy reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load if sediments have been intensely weathered during storage on the floodplain then the result will be a net dilution and replacement of younger relatively immature sands transported by the river by older more mature material from the floodplain the relative proportion of the older sands in the bedload will increase downstream resulting in a progressive downstream increase in the compositional maturity of the bedload of the river

long river profiles tectonism and eustasy a guide to interpreting fluvial terraces along three rivers at the mendocino triple junction northern california strath cut and fill terraces have formed in response to tectonic and eustatic processes detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level primarily aggradation and formation of fill terraces during sea level high stands alternating with deep incision during low stands a eustasy driven deposi tional wedge extends tens of kilometers upstream on all rivers tapering to zero thickness this distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere due in part to the large size of these rivers however the change in gradient is nearly identical to other base level rise studies the depositional gradient is about half that of the original channel middle to upper reaches of each fiver are dominated by the effects of long term uplift primarily lateral and vertical erosion and formation of steep unpaired strath terraces exposed only upstream of the depositional wedge vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates strath terraces have steeper gradients than the modem channel bed and do not merge with marine terraces at the river mouth consequently they cannot be used to determine altitudes of sea level high stands strath formation is a continuous process of response to long term uplift and its occurrence varies spatially along a river depending on stream power and hence position upstream strath terraces are found only along certain parts of a coastal stream upstream of the aggradational effects of oscillating sea level and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load for a given size fiver the greater the uplift rate the greater the rate of vertical incision and consequently the less the like lihood of strath terrace formation and preservation

re development of fluvial islands along the lower mississippi river over five decades 1965 2015 abstract the development and evolution of islands along the lower mississippi river were examined over a 50 year period between 1965 and 2015 fluvial islands were historically a fundamental component of the riparian corridor serving as important ecological habitat for aquatic and terrestrial species their degradation was associated with channel engineering leading to a pronounced single thread channel continued channel engineering in the latter twentieth century however is associated with a rapid increase in channel islands along the lower mississippi river the study utilizes a gis framework to examine and analyze a range of geospatial data sets including i lidar dems ii historic aerial photos iii historic and recent satellite imagery iv historic topographic maps and vi hydrologic data from gauging stations along the lower mississippi river islands were digitized in google earth pro and arcgis by identification of vegetation lines spatial data polygons were used to extract clip the lidar dem data for subsequent analysis the island data was statistically analyzed using a manova and two way t test to examine the association of dikes with the morphologic characteristics of islands the number of islands 1 ha significantly increased over the five decade study period from 105 in 1965 206 in 1995 to 295 in 2015 the average size of islands decreased from 101 ha hectare to 63 ha because so many new small islands formed within the dike fields the total area of islands within the river channel increased from 103 km2 in 1965 169 km2 in 1995 to 206 km2 in 2015 the total sediment storage within fluvial islands above the average low water surface is 1 81 billion m3 the lower mississippi has sufficient coarse sediment to construct new riparian lands despite the large historic decline in sediment load and overall channel bed degradation that occurred in response to large scale engineering projects over much of the twentieth century the new rapid growth and development of fluvial lands within the riparian corridor of the lower mississippi is a positive outcome of an unintended geomorphic consequence but one that requires additional management considerations

an introduction to the geological significance of sediment transport and accumulation on the amazon continental shelf in order to understand the formative processes and resulting stratigraphy on the amazon shelf and adjacent shoreline of amapa geological investigations were undertaken as part of a multidisciplinary amazon shelf sediment study amasseds the design of the study provided results of a multifaceted nature integrated with observations in other disciplines focused on sedimentary processes representative of fluctuating conditions on several time scales and broadly distributed on the shelf including regions not investigated in the past on short time scales most muddy sediment is transported northwest of the river mouth from there it moves seaward as fluid mud to cause rapid accumulation of foreset beds and moves northwestward to prograde the northernmost amapa shoreline and to supply sediment to the guianas fluid muds cover the shelf as far northward as 3 5 n and allow strong tides to propagate to shore where tidal currents cause most of the amapa shoreline to undergo erosion today averaged over decades and centuries about half of the amazon sediment discharge accumulates on the adjacent shelf and another significant fraction one sixth leads to accretion of the northernmost amapa shoreline and northwestward bypassing of sediment the remaining sediment is hypothesized to be trapped in deltaplain deposits of the lower amazon river system the foreset region and shoreline represent the two common loci of sediment accumulation that alternate their predominance on time scales of millennia and lead to a two stage progradation of coastal plain and subaqueous deltaic deposits this muddy regressive sedimentation during high sea level is replaced by formation of erosional sand layers during low sea level and transgressive conditions future research in the study area should address important considerations that were delineated by the present study including mechanisms of shoreline accretion the holocene history recorded in topset and coastal plain strata the role in local sedimentation played by the large shoal extending from cabo norte and the entrapment of amazon sediment by the delta plain

holocene floodplain lake sediments in the amazon 14c dating and palaeoecological use abstract the holocene biostratigraphic record of amazon lowland rain forests is fragmentary and is based on restricted number of cores the majority representing sedimentation in a floodplain environment these biostratigraphic data have often been interpreted as demonstrating climatic oscillation between wetter and drier conditions and the subsequent vegetation changes here we survey and 14c date a further six western amazon floodplain lakes and examine their fluvially controlled sedimentation processes the lakes which are of the cutoff type are located along the rivers manu tambopata and amazon solimoes in the peruvian amazon lowlands all the lakes cored are under the influence of current overbank sedimentation of the parent suspension rich whitewater rivers and receive an annual fill of allochthonous inorganic suspension during the flood most of the cores collected were rich in minerogenic sediments gyttja clay loss on ignition we think that the observed bias towards older 14c ages documented in the gyttja clays is caused by the high proportion of old fine particulate organic matter being recirculated and resedimented within the suspension load the strong fluvial control of lake sedimentation and the unreliability of the 14c dates from the amazon suspension rich floodplains call for caution in palaeoecological interpretation furthermore owing to the low number of within lake replicate cores in the amazon holocene stratigraphy analyses we consider the postulated holocene climatic oscillation schemes to be tentative only

structural stratigraphic models for extensional basins of half graben type abstract the equations describing subsidence in isostatically compensated basins formed by lithospheric stretching can be modified to define a notional depth to decollement which relates subsidence to the stretching factor in much the same way as the depth to a physical sole fault relates subsidence to extension in uncompensated thin skinned basins for a constant density basin fill this notional depth to decollement corresponds to the level of no vertical motion within the lithosphere in general the physical sole to a system of detached normal faults will not lie at the notional depth to decollement and must move vertically during extension this vertical movement must be implicitly corrected for before area bed length balancing is used to determine the depth to a physical sole fault the notional depth to decollement has been calculated numerically for an extensional basin loaded by sediments which follow a north sea shaly sand compaction law and increases with from 5 to 9 km a set of model half grabens has been constructed by combining subsidence curves generated by this shaly sand model with geometric relationships inherent in the rotated planar domino mechanism of upper crustal faulting for comparison a similar set of half grabens was constructed for an uncompensated basin with a sole fault at 7 5 km depth in both cases large fault to fault spacings give rise to large basement relief and consequent footwall uplift and erosion small fault spacings give rise to simple listric growth faults intermediate values generate more complex uplift subsidence histories which are capable of giving rise to such commonly observed features as rounded eroded fault block shoulders local regression followed by onlap or exponentially increasing subsidence at the crest of the fault block

structure and tectonics of central and southern appalachian valley and ridge and plateau provinces west virginian and virginia the valley and ridge and eastern allegheny plateau from pennsylvania into tennessee is subdivided into distinct allochthonous sheets emplaced over an essentially featureless precambrian basement surface sheet development is related to variations in stratigraphic thickness and lithology as well as regional basement configuration three sheets control central appalachian structural style the massanutten blue ridge sheet is easternmost and is bounded on the west by the little north mountain pulaski fault system continuing westward into the plateau the martinsburg sheet contains anticlinoria and synclinoria uncut by major thrust faults both massanutten blue ridge and martinsburg sheets are seated in upper ordovician martinsburg reedsville shales the underlying waynesboro sheet is rooted in lower cambrian waynesboro rome shales and its imbrication has controlled the locations of major anticlinoria and synclinoria in the upper sheet within the southern appalachians greater imbrication of the lower sheet and increased thrust fault displacement at longitudinal ramps have destroyed the lateral continuity along strike of the central appalachian waynesboro and overlying martinsburg sheets there imbrication to the surface has formed the saltville sheet continuous into alabama as well as the st clair narrows copper creek cove mountain and st paul honaker sheets southwest of the trend change the holston stone mountain sheet adjacent to the blue ridge is rooted primarily in lower and middle cambrian strata and is underlain by the pulaski sheet in the westernmost sector of this region the pine mountain and richlands sheets are seated in devonian shales the pulaski and massanutten blue ridge sheets rooted primarily in upper ordovician shales form a continuous structural unit and include blue ridge and piedmont rocks

distribution of fluid muds on the amazon continental shelf abstract a comprehensive study of suspended sediment transport on the amazon shelf was conducted as part of the amasseds project a multidisciplinary amazon shelf sediment study to understand the link between physical processes and the fate of amazon river sediment a small instrumented tripod was used to measure the fluid flow and suspended sediment characteristics throughout the water column on varying time scales e g semidiurnal fortnightly seasonal dense nearbed suspensions or fluid muds suspended sediment concentration 10 g l on the order of a few meters in thickness were found on the inner and middle shelf covering an area ranging from 5 700 km2 during falling and low river discharge to 10 000 km2 during rising and high discharge fluid muds generally observed in the region of bottom salinity fronts extend across the topset beds to the foreset beds during rising and high discharge salinity and temperature anomalies frequently observed in fluid muds imply that often the muds do not result from erosion and resuspension from the seabed but from trapping processes at the bottom salinity front and transport across the shelf the vertical structure of fluid mud varies in space and tune variability close to the river mouth is predominantly fortnightly related to the location and structure of the bottom salinity front on the open shelf the variability appears to be related to seasonal differences in river and sediment discharge and the location of the bottom salinity front the total suspended sediment inventory on the shelf during the season of high discharge is approximately equal to the annual sediment discharge from the river 1 2 109 tons yr and 90 of that inventory is within fluid mud suspended sediment transport on the shelf is dominated by fluid mud

high precision u pb zircon geochronology of the late triassic chinle formation petrified forest national park arizona usa temporal constraints on the early evolution of dinosaurs the triassic successions of the colorado plateau preserve an important record of vertebrate evolution and climate change but correlations to a global triassic framework are hampered by a lack of geochronological control tuffaceous sandstones and siltstones were collected from the upper triassic chinle formation exposed in the petrified forest national park arizona usa within a refined stratigraphic context of 31 detailed measured sections u pb analyses by the isotope dilution thermal ionization mass spectrometry id tims method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the chinle fluvial system the base of the blue mesa member of the chinle formation is placed at ca 225 ma and the top of the petrified forest member is placed at 208 ma or younger bracketing an 280 m thick section that spans nearly the entire norian stage of the late triassic estimated sediment accumulation rates throughout the section reflect extensive hiatuses and or sediment removal by channel erosion the new geochronology for the chinle formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria a mid norian age ca 219 213 ma for the distinctive sonsela conglomeratic sandstone bed constrains the adamanian revueltian land vertebrate faunachron boundary our new data permit a significant time overlap between the lower chinle sequence and the dinosauromorph rich ischigualasto formation of northwestern argentina near contemporaneity of the trans american deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the americas

river tributaries as sediment sinks processes operating where the tapajos and xingu rivers meet the amazon tidal river the amazon river is the largest fluvial source of freshwater and sediment to the global ocean and has the longest tidally influenced reach in the world two major rivers the tapajos and xingu enter the amazon along its tidal reach however unlike most fluvial confluences these are not one way conduits through which water and sediment flow downstream toward the sea the drowned river valleys rias at the confluences of the tapajos and xingu with the amazon river experience water level fluctuations associated not only with the seasonal rise and fall of the river network but also with semidiurnal tides that propagate as far as 800 km up the amazon river superimposed seasonal and tidal forcing distinct sediment and temperature signatures of amazon and tributary waters and antecedent geomorphology combine to create mainstem tributary confluences that act as sediment traps rather than sources of sediment hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary derived and amazon derived sediment within the ria basins characterize the sediment transport mechanisms within the confluence areas and estimate rates of sediment accumulation within both rias the tapajos and xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ca 20 mt yr 1 of sediment sourced from the amazon river these findings have implications for the interpretation of stratigraphy associated with incised valley systems such as those that dominated the transfer of sediment to the oceans during lowstands in sea level this article is protected by copyright all rights reserved

relaxation of continental lithosphere an explanation for late cretaceous reactivation of the sabine uplift of louisiana texas with few unconstrained assumptions a simple quantitative model for flexural isostasy between two crustal blocks of different thickness that subsequently undergoes partial relaxation of accumulated stress can explain reactivation of the sabine uplift in the mid cretaceous thin salt over the sabine uplift indicates that it was a positive area in the middle jurassic but began to subside during the late jurassic during the late jurassic and early cretaceous the sabine uplift had no topographic expression and was the center of a large flat bottomed basin covering most of east texas and north louisiana reactivation of the sabine uplift in the mid cretaceous caused a minimum of 150 m of uplift and extensive erosion of lower cretaceous rocks during the late cretaceous and tertiary the sabine uplift resumed subsiding but at a slower rate than the adjacent basins since the mid cretaceous the sabine uplift has risen more than 1 km relative to the east texas basin we suggest that this structural history is consistent with the following simple quantitative model for flexural isostasy during formation of the gulf of mexico in the late triassic early jurassic by rifting and extension of the lithosphere brittle deformation of the crust created small scale wavelength of tens of kilometers lateral variations in crustal thickness initially the newly formed margin was in point wise airy isostatic equilibrium thus synrift and or early postrift subsidence was characterized by a series of rift valleys or half grabens separated by uplifts however as the margin cooled and contracted the lithosphere became stronger and subsequent loading was regionally compensated thus postrift subsidence was characterized by a broad regional downwarp if at some later point in time the rigid portion of the lithosphere was weakened or relaxed then lateral density variations would have been recompensated at shorter wavelengths thus areas of little or no crustal extension would have been uplifted and areas of high extension would have experienced additional subsidence gravity and total tectonic subsidence interpretations agree that the sabine uplift region is underlain by thicker crust than the adjacent east texas and north louisiana basins possible mechanisms for relaxation of continental lithosphere are thermal rejuvenation horizontal in plane stress viscous relaxation and extensional faulting

channel floodplain geomorphology along the solimoes amazon river brazil across the cratonic landscape of brazil the solimoes amazon river transports to its delta plain 1240 mt of suspended sediment derived from andean erosion and reworks another 3200 mt of floodplain sediments distribution of these sediments has resulted in a variable along stream pattern of geomorphology the upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll bar topography that forms the boundaries of hundreds of long narrow lakes in contrast downstream reaches are characterized by channels restricted by stabilizing long term levee building and floodplain construction dominated by overbank deposition overbank deposition buries the scroll bar topography resulting in a flat floodplain covered by a patchwork of large more equant shallow lakes on the basis of estimated rates of recycling of floodplain sediments the modern floodplain of the brazilian amazon could have been recycled in 5000 yr and is recycled more rapidly in the upstream than the downstream reaches the cratonic interior is interrupted by structural arches that bound intracratonic basins four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel floodplain geomorphology at this spatial scale structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation thus restricting channel movement for example as the river crosses the purus arch the valley narrows to 20 km compared to an average of 45 km the water surface gradient decreases sediment is deposited and yet the rate of channel migration is negligible hence the effect of the arches is to create a landscape where on the spatial scale of hundreds of kilometres the river is confined and entrenched in its valley is straight and is relatively immobile local valley tilting apparently unrelated to the arch structures also imprints the geomorphology in particular a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large scale oxbow type features on the brazilian amazon river floodplain

meander cutoff and the controls on the production of oxbow lakes using satellite images archived by google earth we measured channel and oxbow lake characteristics of 30 large meandering rivers to identify the controls on the production of oxbow lakes by meander cutoff cutoff produced lognormal distributions of lake lengths within the studied reaches and the geometric mean lake length of each population correlated positively and exponentially with sinuosity due to more highly sinuous reaches being comprised of longer meanders and to cutoff removing longer segments of more sinuous channels we successfully predicted the size frequency distributions of lakes stored within the flood plains of five freely meandering reaches using only channel sinuosity and an assumption of the variance about the geometric mean lake length a variable that did not significantly vary between the studied reaches while the river s sinuosity remains steady the temporal rate of cutoff can be estimated using channel sinuosity the fraction by which cutoff reduces channel length and the rate at which the reach lengthens by meander growth

exchanges of sediment between the flood plain and channel of the amazon river in brazil sediment transport through the brazilian sector of the amazon river valley a distance of 2010 km involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at obidos 1200 mt yr 1 the exchanges occur through bank erosion bar deposition settling from diffuse overbank flow and sedimentation in flood plain channels we estimated the magnitude of these exchanges for each of 10 reaches of the valley and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel the annual supply of sediment entering the channel from bank erosion was estimated to average 1570 mt yr 1 1 3 the obidos flux and the amount transferred from channel transport to the bars 380 mt yr 1 and the flood plain 460 mt yr 1 in channelized flow 1230 mt yr 1 in diffuse overbank flow totaled 2070 mt yr 1 1 7 the obidos flux thus deposition on the bars and flood plain exceeded bank erosion by 500 mt yr 1 over a 10 16 yr period sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 mt yr 1 over 16 yr crudely validating the process based calculations of the sediment budget which in turn illuminate the physical controls on each exchange process another 300 400 mt yr 1 are deposited in a delta plain downstream of obidos the components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain which in turn are influenced by tectonic features of the amazon structural trough

sediment dynamics in the lower mekong river transition from tidal river to estuary a better understanding of flow and sediment dynamics in the lowermost portions of large tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean flow velocity salinity and suspended sediment concentration were measured for 25 h at three cross sections in the tidal song hau distributary of the mekong river vietnam two campaigns took place during comparatively high seasonal and low seasonal discharge and estuarine conditions varied dramatically between them the system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow the changing freshwater input sediment sources and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow the dinh an channel of the song hau distributary exported sediment to the coast at a rate of about 1 t s 1 during high flow and imported sediment in a spatially varying manner at approximately 0 3 t s 1 during low flow scaling these values results in a yearly mekong sediment discharge estimate about 65 smaller than a generally accepted estimate of 110 mt yr 1 although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate fluvial advection of sediment was primarily responsible for the high flow sediment export exchange flow and tidal processes including local resuspension were principally responsible for the low flow import the resulting bed sediment grain size was coarser and more variable during high flow and finer during low and the residual flow patterns support the maintenance of mid channel islands

sedimentation pattern in a trans himalayan quaternary lake at lamayuru ladakh india abstract located on the indo tsangpo suture zone lamayuru lake in western ladakh was created by neotectonic instability around 35 40 ka the valley fill sequence about 110 m thick reveals an interplay of lacustrine to fluvio deltaic to colluvial processes operating in response to changing climate and tectonic conditions based on integrated approach of sedimentology and palaeontological analysis we propose a sedimentation model for lamayuru lake with reference to changing climatic conditions and neotectonics the coarsening upward succession was divided into five lithofacies associations associations a e each association comprising one or more lithofacies represents a specific depositional environment and marks an independent episode of sedimentation a dominantly muddy sequence of facies association a a product of a lacustrine environment is followed by coarsening upward cycles association b composed of silt to sand that implies laterally shifting deltas thereafter thick sand and gravel deposits constituting fining upward fu unit cycles associations c and d represent deposition in a fluvial domain the upper part of association d composed of matrix supported gravel horizons was deposited as debris flows and marks the termination of sedimentation in the valley the clastic lithofacies facies associations a to d with plant fragments and charcoal presumably indicate warm humid climatic conditions in contrast association e comprising marl lithofacies occurring at four different levels is biogenic and indicates arid to semi arid climatic phases five main types of trace fossils including rhizoliths seem to be representatives of scoyenia ichnofacies and animal burrows indicating a thin population of soft bodied annelid and arthropod communities living under low energy condition nine fossiliferous horizons have yielded characteristic ostracods charophytes and gastropods indicating shallow low energy and cold water ecological conditions coarsening up lake succession extensive synsedimentary deformation tilting of lake beds and the entrenchment of the lamayuru drainage indicate basin evolution under an effective role of neotectonics in the area

fluvial perturbance in the western amazon basin regulation by long term sub andean tectonics haffer s refuge theory proposes that during the arid climatic phases of the late pleistocene tropical lowland forests of amazonia were reduced to isolated patches contributing to the high species richness of the present day forest the theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in amazonia analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active andes the geological setting causes long term fluvial perturbance this leads to a temporally structured highly complex mosaic of fossil and present floodplains these dynamics have been present with varying activity and geographic range during the tertiary and quaternary providing site turnover that has not been recognized by the biogeographic tradition of the amazon basin

geodynamic scenario and structural styles of mesozoic and cenozoic basins in china the continental lithosphere of china is situated at the junction of the marginal pacific and tethys himalayan tectonic domains the most manifest tectonic movements responsible for the deformation of the continental lithosphere of china are taphrogenic collisional and shearing these movements played an important role in the mesozoic and cenozoic tectonic evolution of china and formed various types of mesozoic and cenozoic basins in china the marginal pacific crust in eastern china was attenuated by rifting and developed north northeast trending mesozoic and cenozoic basins e g the songliao bohai bay and jianghan basins rifting mainly followed preexisting lineaments or shear zones in the basement then basin range graben horst structures and large fault bounded basins developed extensional structural styles have prevailed in mesozoic and cenozoic basins in eastern china with listric faults and tilted blocks in the basement and detached faults growth faults and rollover anticlines or drape folds in the cover rocks these structures are favorable for hydrocarbon accumulation thus the famous daqing and zhongyuan oil fields formed in songliao basin and bohai bay basin respectively in western china the continental crust thickened by collision and formed a series of mesozoic and cenozoic intermontane and foreland basins such as the junggar dzungaria tarim and qaidam basins contraction structural styles are predominant in mesozoic and cenozoic basins in western china with shortened fault blocks and ramps in basement and thrust faults and folds in the cover rocks the kelamayi and laojunmiao oil fields formed in these compressional basins in addition many great wrench faults played an important role in the evolution of china such as the tan lu fault in eastern china and the altunshan and talaso fergana faults in western china many mesozoic and cenozoic basins of china were characterized by transtension and transpression the mesozoic and cenozoic basins of china have undergone numerous tectonic events the extensional structures have been changed into contractional structures or conversely thus tectonic inversion occurred in successive events and different structural styles may be found in different tectonic levels of these basins these superposed deformations and structures have complicated petroleum exploration but they also enlarge prospective exploration targets and potential hydrocarbon reserves in china

control on sediment and organic carbon delivery to the arctic ocean revealed with space borne synthetic aperture radar ob river siberia an important control on river biogeochemistry and sediment load is the process of water exchange between primary channels and the flood plain particularly in low r elief areas containing lakes ephemeral channels and other aquatic ecosystems flood plain exchange may be a dominant process on the lowland rivers of arctic russia which are among the world s largest in water discharge yet are strikingly deficient in their delivery of sediment to the arctic shelf temporal synthetic aperture radar sar amplitude and interferometric images of the ob river siberia reveal a time varying limnological network controlling water sediment and nutrient exchange between flood plain wetlands and the main channel the amount of hydrologic exchange decreases by one order of magnitude from june to september enhancing sedimentation over as much as 90 of the flood plain and enriching channel waters with colloidal organic carbon this observation combined with russian field measurements of water discharge and sediment load indicates that a major sediment sink on the lower ob flood plain may be responsible for the low amount of sediment delivery by the ob river to its estuary and the kara sea

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

planform evolution of two anabranching structures in the upper peruvian amazon river we present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures for this purpose two locations of the peruvian amazon river were selected 1 a site with a medium to high sinuosity main channel ms site muyuy peru and 2 a site with a low sinuosity main channel ls site at the triple boundary between brazil colombia and peru the main channels for both the ms and ls anabranching structures have freedom to migrate in the lateral direction while at least one of their secondary channels is adjacent to the geological valley for ms and ls sites temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width sinuosity and annual maximum migration rates of main and secondary channels were calculated additionally detailed hydrodynamic and bed morphology field measurements were carried out and a two dimensional shallow water numerical model was developed for a medium to high sinuosity main channel anabranching structure the secondary channels present a dominant mechanism for reworking the floodplain while for the low sinuosity anabranching structure the main channel planform dynamics is dominant flow velocities along the main and secondary channels for low transition and high flow discharges describe that for ms ls site the velocities are much higher along the secondary main channels

solimoes megashear intraplate tectonics in northwestern brazil a belt of deformation in the solimoes basin of northwestern brazil extends east northeast from near the peruvian border for about 1300 km the belt is characterized by the en echelon arrangement of folds and faults interpreted as the result of right slip displacements in a transpressive regime the structures were formed during late jurassic time probably due to collision of south america with allochthonous terranes as the opening of the south atlantic ocean began the iquitos arch near the western end of the megashear separates the solimoes basin from subandean basins the development of the iquitos arch is interpreted to be related to orogenic loading along western south america by the andean mountain belt in brazil folds and faults associated with the solimoes megashear hold hydrocarbons within paleozoic strata and 15 gas and oil fields have so far been discovered

tributary distributary and other fluvial patterns what really represents the norm in the continental rock record abstract this paper evaluates the recent claim made by weissmann et al 2010 that deposits of distributive fluvial systems dfs may form the bulk of the continental fluvial record weissmann et al 2010 define dfs as a pattern of channel and floodplain deposits that radiate outward from an apex that is located where the river enters the sedimentary basin as such dfs are fan like systems on which multiple channels coexist and distribute water and sediment across the fan published criteria for the recognition of dfs weissmann et al 2010 are 1 a radial pattern of channels from the dfs apex 2 downslope decreases in channel size 3 down dfs grain size decreases and 4 a lack of lateral channel confinement however in weissmann et al 2010 and subsequent papers only the first of these criteria is applied rigorously thus allowing a variety of types of fluvial system including avulsive incised anabranching and coastal plain distributary systems to potentially be classified uncritically as dfs an ancient succession formed by dfs should preserve mostly small channel bodies with a limited range of dimensions and no outsized channel bodies channel bodies in dfs dominated successions should also decrease in size in a down palaeoslope direction and show palaeocurrent relationships indicative of radial dispersal in order for the term dfs to be applied these characteristics should also be established within stratigraphic bodies that formed coevally however very few examples have yet been published that satisfy these criteria if the claim that dfs dominate the alluvial stratigraphic record is valid then few if any sedimentary bodies formed by large channels should be preserved a review of published research demonstrates however that the alluvial stratigraphic record contains a broad diversity of fluvial network styles including abundant representatives of tributary avulsive anabranching and incised in addition to distributary types it is also clear that the deposits of large main stem rivers that are comparable to the big rivers of the modern world are abundantly preserved in the stratigraphic record an analysis of the planimetric area of modern tributary and distributary fluvial systems is presented demonstrating that tributary systems are also likely to represent a far larger proportion of the ancient sedimentary record than dfs the body of evidence presented herein from modern and ancient alluvial systems thus suggests that the hypothesis of weissmann et al 2010 should be wholly rejected dfs do not dominate the continental stratigraphic record

suspended sediments of the modern amazon and orinoco rivers abstract the amazon and orinoco rivers are massive transcontinental conveyance systems for suspended sediment they derive about 90 of their sediment from the andes that support their western headwaters transport it for thousands of kilometers across the breadth of the continent and deposit it in the coastal zones of the atlantic at their points of maximum suspended sediment discharge the amazon transports an average of 1100 1300 10 6 tons per year and the orinoco transports about 150 10 6 tons per year relations of sediment discharge to water discharge are complicated by unusual patterns of seasonal storage and remobilization increased storage and reduced transport of sediment in the middle orinoco during periods of peak water discharge and storage of suspended sediment in the lower amazon during rising discharge and resuspension during falling discharge spatial distributions of suspended sediment in cross sections of both rivers are typically heterogeneous not only in the vertical sense but also in the lateral the cross channel mixing of tributary inputs into the mainstem waters is a slow process that requires several hundred kilometers of downriver transport to complete considerable fine grained sediment is exchanged between rivers and floodplains by the combination of overbank deposition and bank erosion

pollen and spores date origin of rift basins from texas to nova scotia as early late triassic palynological studies of the nonmarine newark supergroup of eastern north america and of rift basins in the northern gulf of mexico facilitate correlation with well dated marine sections of europe new information emphasizes the chronological link between the newark basins and a gulf of mexico basin and their common history in the rifting of north america from pangea shales from the subsurface south georgia basin are shown to be of late karnian age early late triassic the known time of earliest sedimentation in the culpeper basin is extended from norian late late triassic to mid karnian and the date of earliest sedimentation in the richmond and deep river basins is moved to at least earliest karnian perhaps ladinian the subsurface eagle mills formation in texas and arkansas has been dated palynologically as mid to late karnian the oldest parts of the newark supergroup and the eagle mills formation mostly began deposition in precursor rift basins that formed in ladinian to early karnian time in the southern newark basins sedimentation apparently ceased in late karnian but continued in the northern basins well into the jurassic until genesis of the atlantic ended basin sedimentation

predicting natural channel patterns based on landscape and geomorphic controls in the columbia river basin usa 1 based on known relationships of slope discharge valley confinement sediment supply and sediment caliber in controlling channel patterns we developed multivariate models to predict natural channel patterns across the 674 500 km2 columbia river basin usa we used readily available geospatial data sets to calculate reach slopes 2 year flood discharge and valley confinement as well as to develop hypothesized landscape level surrogates for sediment load and caliber relative slope percent of drainage area in alpine terrain and percent of drainage area in erosive fine grained lithologies using a support vector machine svm classifier we found that the four channel patterns were best distinguished by a model including all variables except valley confinement 82 overall accuracy we then used that model to predict channel pattern for the entire basin and found that the spatial distribution of straight meandering anabranching and braided patterns were consistent with regional topography and geology a simple slope discharge model distinguished meandering channels from all other channel patterns but did not clearly distinguish braided from straight channels 68 overall accuracy addition of one or more of the hypothesized sediment supply surrogates improved prediction accuracy by 4 14 over slope and discharge alone braided and straight channels were most clearly distinguished on an axis of relative slope whereas braided and anabranching channels were most clearly distinguished by adding percent alpine area to the model

sedimentary facies and geomorphic evolution of a blocked valley lake lake futululu northern kwazulu natal south africa blocked valley lakes are formed when tributaries are impounded by the relatively rapid aggradation of a large river and its floodplain these features are common in the landscape and have been identified in the floodplains of the solimoes amazon brazil and fly strickland rivers papua new guinea for example but their inaccessibility has resulted in studies being limited to remotely sensed image analysis this paper documents the sedimentology and geomorphic evolution of a blocked valley lake lake futululu on the mfolozi river floodplain margin in south africa while also offering a context for the formation of lakes and wetlands at tributary junctions the study combines aerial photography elevation data from orthophotographs and field survey and longitudinal sedimentology determined from a series of cores which were sub sampled for organic content and particle size analysis radiocarbon dating was used to gauge the rate and timing of peat accumulation results indicate that following the last glacial maximum rising sea levels caused aggradation of the mfolozi river floodplain by 3980 years bp aggradation on the floodplain had impounded the futululu drainage line creating conditions suitable for peat formation which has since occurred at a constant average rate of 0 13 cm year 1 continued aggradation on the mfolozi river floodplain has raised the base level of the futululu drainage line resulting in a series of back stepping sedimentary facies with fluvially derived sand and silt episodically prograding over lacustrine peat deposits blocked valley lakes form where the trunk river has a much larger sediment load and catchment than the tributary stream similarly when the relative difference in sediment loads is less palustrine wetlands rather than lakes may be the result in contrast where tributaries drain a steep well connected catchment they may impound much larger trunk rivers creating lakes or wetlands upstream

modern pollen signatures of amazonian rivers and new insights for environmental reconstructions abstract paleoenvironmental reconstructions based on amazonian fluvial and marine pollen data are often hindered due to the scarcity of modern calibration data in amazonia rivers are the main pathways for pollen from upland and flooded forests to continental and marine sediment sinks linking pollen assemblages transported by rivers to watershed vegetation is crucial to interpret palynological archives in order to constrain the interpretation of palynological archives we analysed the pollen content of 36 riverbed sediment samples distributed from the mouth of the ica river ca 68ow to the amazon estuary ca 49ow with support of grain size and major element analyses of the inorganic sedimentary content the palynological analysis shows a strong imprint of the lowland floodplain vegetation as the main source of the riverborne pollen the upper reaches of the amazon river present abundant cecropia reflecting early successional stages induced by intense floodplain disturbance the middle and lower reaches of the amazon river exhibit a progressive increase in herbs mainly represented by poaceae and cyperaceae related to wider floodplains covered by seasonally inundated vegetation floodplain forests of whitewater rivers varzeas are palynologically characterized by iriartea mauritia ilex pseudobombax acalypha and luehea laetia with high frequency of cecropia or herbs and montane taxa at the black and clearwater rivers the igapo floodplain forests provide high percentages of symmeria myrtaceae and alchornea and can be distinguished by sapium sebastiana piranhea amanoa pouteria ferdinandusa genipa attalea astrocaryum and eschweilera which suggest conditions of higher stability and mature forests contribution from the non flooded uplands andean mountains and savannas cerrado occur but their signals are blurred by the high pollen production of the lowland floodplains our data provide a calibration set for environmental reconstructions of river influenced deposits across the amazon including offshore areas of the most iconic riverine dominated landscape in the world

anastomosis and the continuum of channel pattern anastomosing rivers are characterized by multiple channels separated by islands excised from the floodplain their status relative to the continuum concept of channel pattern is assessed with channel pattern defined in terms of three variables flow strength bank erodibility and relative sediment supply using an ordinal scaling l ow m oderate h igh the traditional forms of straight meandering and braided have respective representations of l l l m l m l m and h h m h in terms of those variables the anastomosing pattern is on average represented by l l m h but not so definitively as other forms specification of the third element sediment supply is particularly hampered by the paucity of data but aggradation a characteristic of many anastomosing rivers can be thought of as symptomatic of a moderately high rate of supply relative to the ability for onward transport a sufficiently high rate of supply to a channel with low flow strength and resistant banks would induce shoaling and or lateral constriction that locally forces flow out of the main channel and ultimately leads to the cutting of anabranches a flow regime characterized by concentrated floods of relatively large magnitude is also regarded as highly conducive to the formation of new channels where low bank erodibility constrains channel capacity anastomosis may in certain cases represent a transitional form of channel pattern but there is no denying the longevity of some anastomosing systems

the planform mobility of river channel confluences insights from analysis of remotely sensed imagery river channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment and which are locations for the preservation of thick fluvial deposits overlying a basal scour despite their importance there has been little study of the stratigraphic characteristics of river junctions or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential as a result although it is known that confluences can migrate through time models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location this is problematic for a number of reasons not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes such as confluence scour and that driven by allocyclic controls such as sea level change this paper investigates the spatial mobility of river confluences by using the 40 year record of landsat imagery to elucidate the styles rates of change and areal extent over which large river confluence scours may migrate on the basis of these observations a new classification of the types of confluence scour is proposed and applied to the amazon and ganges brahmaputra meghna gbm basins this analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally thus in the gbm basin a high sediment supply large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80 of large confluences are mobile over this 40 year window conversely this figure is 40 for the amazon basin these results highlight that i the potential areal extent of confluence scours is much greater than previously assumed with the location of some confluences on the jamuna brahmaputra river migrating over a distance of 20 times the tributary channel width ii extensive migration in the confluence location is more common than currently assumed and iii confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

inherited structural control from repeated cretaceous to recent extension in the north mernoo fault zone western chatham rise new zealand abstract the active north mernoo fault zone nmfz is the youngest of three discrete phases of basement involved extensional faulting to deform the northwestern corner of the continental chatham rise since the cretaceous the first two phases inferred to be of late cretaceous and eocene age respectively were both part of widespread extensional systems that developed over most of the chatham rise region the third phase the late miocene to recent nmfz developed on the northwestern corner of the rise as a consequence of the interaction between the chatham rise plateau and the southern end of the hikurangi subduction zone a study of seismic reflection data from the northwestern chatham rise has revealed 1 1 the detailed structure that relates to each phase of normal faulting 2 2 the patterns of fault reactivation during successive episodes of extension 3 3 the significant structural control that older deformational fabrics have had on the development of the nmfz during the late cretaceous phase of extension two major fault bounded half grabens together with other smaller basins developed beneath the upper continental slope these e w trending s dipping faults are inferred to have accommodated about 2 7 km of horizontal extension across a 30 km wide region the early cenozoic extensional system reactivated some cretaceous half graben structures but mainly formed new faults with similar geometry in crust north of the cretaceous structures the early cenozoic faults are inferred to have accommodated about 1 4 km of horizontal extension across a 60 km wide region the late cenozoic nmfz also reactivated some of the cretaceous normal faults and almost all of the major early cenozoic normal faults in this region in addition many new extensional structures developed and produced an e w to wnw ese trending southward dipping domino style array of faults up to 100 km in width although the density of faults in the nmfz is greater than that of the two previous extensional systems to include this region the total horizontal extension since late early pliocene times is estimated to be no more than 3 km the common reactivation of old normal faults during the late cenozoic development of the nmfz indicates a significant level of structural inheritance from the previous episodes of extension to effect the region furthermore the parallelism between the newly developed faults and the older two extensional systems suggests that the normal faults could be reactivating structures in the torlesse terrane basement which in turn relate to an early mesozoic phase 100 ma of accretionary tectonics the kinematics of active extension within the nmfz is therefore likely to be influenced by the pre existing structural discontinuities that resulted from a history of poly phase deformation

amazon flood wave hydraulics summary a bathymetric survey of 575 km of the central amazon river and one of its tributaries the purus are combined with gauged data to characterise the amazon flood wave and for hydraulic modelling of the main channel for the period june 1995 march 1997 with the lisflood fp and hec ras hydraulic models our investigations show that the amazon flood wave is subcritical and diffusive in character and due to shallow bed slopes backwater conditions control significant reach lengths and are present for low and high water states comparison of the different models shows that it is necessary to include at least the diffusion term in any model and the rmse error in predicted water elevation at all cross sections introduced by ignoring the acceleration and advection terms is of the order of 0 02 0 03 m the use of a wide rectangular channel approximation introduces an error of 0 10 0 15 m on the predicted water levels reducing the bathymetry to a simple bed slope and with mean cross section only introduces an error in the order of 0 5 m these results show that when compared to the mean annual amplitude of the amazon flood wave of 11 12 m water levels are relatively insensitive to the bathymetry of the channel model the implication for remote sensing studies of the central amazon channel such as those proposed with the surface water and ocean topography mission swot is that even relatively crude assumptions regarding the channel bathymetry will be valid in order to derive discharge from water surface slope of the main channel as long as the mean channel area is approximately correct

defining large river channel patterns alluvial exchange and plurality abstract large rivers have anabranching channels with components that may be defined as braided meandering or straight this paper shows that application of such holistic terminologies is complicated by recognition of within type and transitional type variety a confusingly varied use of terms and a coverage of pattern characteristics that for many large rivers is incomplete in natural states big rivers can be plural systems in which main accessory tributary and floodplain channels and lakes differ functionally and vary in terms of morphological dynamics a distinction is drawn between the hydrological and geomorphological connectivity of components in big river plural systems at any one time even at flood stage only some channels are geomorphologically active six types of geomorphological connectivity are described that range from coupled through to partially coupled and decoupled the interplay between geomorphological and hydrological connectivity in large rivers is shown to determine habitat status and therefore ecological diversity for improved understanding of the dynamics as well as the forms of these large composite systems it is helpful to 1 adopt element level specification not only for sediment bodies but also for functioning channels 2 track the sediment transfer processes and exchanges that produce channel forms over the highly varied timescales operating within large rivers and 3 recognise the ways in which partially coupled and connected geomorphological systems produce naturally a composite set of forms at different rates such augmenting information will provide an improved platform for both river management and ecological understanding

inland variability of carbon nitrogen concentrations and δ13c in amazon floodplain varzea vegetation and sediment we analysed carbon nitrogen and 13c values in sediment sampled in the amazon floodplains the surface sediment samples were collected along inland transects divided according to their vegetation cover a forest where the dominant vegetation was inundation forests b grass where the dominant vegetation was grasses mainly from echnochloa polystachya c mixed where the vegetation changed from grasses near the river bank to forest inland the average sediment carbon and nitrogen concentrations were significantly higher in forest samples c 0 99 n 0 12 than in grass samples c 0 84 n 0 10 the average sediment 13c was significantly heavier in grass 26 7 than in forest samples 28 6 reflecting the isotopically heavier c4 grasses carbon in the forest and mixed transects inland increases in the carbon and nitrogen concentrations were observed at approximately 50 to 60 m from the riverbank the concentration of carbon and nitrogen became similar to the river particulate organic carbon poc concentration which is the primary floodplain sediment source we hypothesized that this trend is caused by the deposition of carbon and nitrogen depleted sand size particles in the first few metres of the floodplain whereas the more enriched silt clay particles are preferentially deposited far inland in contrast to inland trends no significant downriver trend was observed in the carbon and nitrogen concentrations however a downriver 13c increase was observed for sediment in grass covered areas based on 13c values in different varzea environments we estimated the relative contribution of riverine poc and vegetation material present in the varzea sediments under different vegetation covers for forest sediment samples we estimated that 60 of the carbon was derived from the riverine poc and the forests provided the remaining 40 for open areas covered with grasses we estimated that only 10 was derived from these plants and 90 was derived from riverine poc copyright 2003 john wiley sons ltd

geomorphic response of coastal streams to low intermediate and high rates of uplift medocino triple junction region northern california analysis of three dimensional morphological properties of 24 coastal drainage basins that have evolved in areas of low 3 m 1 000 yr rates of uplift near the mendocino triple junction mtj coastal northern california identified channel gradients as the best indicator of tectonism in the landscape lower order tributaries best reflect tectonically controlled differences the largest streams examined of third order are able to adjust to most base level change and maintain their profile form whereas lower order streams farther upstream tend to accumulate the effects of net base level fall and have steepest profiles in the areas of highest uplift rates variations in steepness of first order channel gradients indicate that 1 high uplift rates in the wake of the mtj have existed for at least 73 000 yr and 2 differential tilt of the region to the north and south of the current locus of highest uplift rate is occurring in association with regional uplift although first order streams are excellent indicators of areas of both high uplift rates and regional differential tilting they are less useful in distinguishing between areas of low and intermediate uplift rate analysis of the longitudinal profile of the main trunk stream of ten of the 24 drainage basins using the stream gradient index was more useful to categorize broadly the uplift rates and to distinguish between streams in low and intermediate uplift rate areas although the hypotheses tested herein do not identify the exact mechanism off uplift in the mtj region they do indicate that the nature of deformation is most likely regional tilt rather than crustal shortening with localized compressional folding and thrust faulting they also indicate that geomorphic responses trail in the wake of the uplift rate response caused by development of a slab window beneath the north american plate 1 maximum uplift rates occur 9 km south of the northern boundary of the slab window 2 maximum mean first order channel gradient occurs 16 km south of the northern edge of the slab window and 3 maximum drainage basin relief occurs 20 km south of the northern edge of the slab window associated lag times between passage of the southern edge of the subducted gorda plate slab and geomorphic responses are 160 000 yr for maximum uplift rates 290 000 yr for maximum channel gradients and 370 000 yr for maximum relief