holocene floodplain soils along the rio mamore northern bolivia and their implications for understanding inundation and depositional patterns in seasonal wetland settings abstract the llanos de moxos lm in the bolivian amazon basin host one of the largest seasonally inundated savannah landscapes on earth very little is known of this area with regard to sedimentary dynamics soil formation or their relationship to longer term climatic and hydrological variability in this setting here we present a detailed study of three floodplain depositional sequences building up the inundation savannah along the mamore river in the central lm pleistocene sands and silts unit 1 are overlain by thick overbank deposits of mostly holocene age unit 2 on both sides of the mamore river and underline the importance of extensive flooding processes for the late quaternary sedimentary and geomorphic evolution of the lm the fine grained distal overbank sediments show signs of strong modification by hydromorphic processes and reflect spatial and temporal variations in flood inundation patterns in addition widespread dark gray to blackish soil and paleosol horizons are intercalated with the overbank sediments but lack evidence for significant weathering these horizons are likely the result of cumulative incorporation of organic material under conditions of particularly low sedimentation rates therefore their formation should be linked to processes such as channel migration longer term meanderbelt evolution avulsive drainage reorganization changes in sediment supply or climatically induced variations in flooding type frequency or magnitude the common occurrence of these floodplain soils in the early to mid holocene may thus reflect the combined effects of hydrological geomorphic and sedimentary changes during a drier mid holocene

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

the changing geomorphology of the humid tropics abstract it is debatable whether tropical geomorphology is intrinsically different from the geomorphology of other places the traditional geomorphology of the humid tropics had certain expectations with regard to landforms and processes a number of case studies over the last three decades however using new concepts and field and laboratory techniques have changed such expectations this paper primarily examines the research themes of the last ten years or so identifies the major trends and attempts to predict areas where interest is expected to concentrate in near future the following topics among the current research trends are highlighted geochemical erosion movement on tropical slopes rivers in the tropics tropical landforms hydrology and sedimentation in the rainforest denudation efficiency in the humid tropics the quaternary evolution and changes and the applied side of tropical geomorphology clearly the landforms and geomorphological processes in the tropics are dependent only partially on climate but to greater extent on neotectonism lithology episodic and catastrophic events such as tropical cyclones and above all on anthropogenic alterations of the landscape a wide variety of landforms and processes are found in the humid tropics and generalisations are extremely difficult in the next century geomorphological studies in the tropics are likely to develop along certain lines environmental investigations will form a major component along with the study of postulated sea level rise current major research trends will undoubtedly continue but increasingly through a multidisciplinary and multipeople approach large scale investigations involving pantropic correlation of past events such as synchronised large floods will grow use of new research tools such as satellite imagery various dating facilities etc will become common and expected it seems likely that the arrival of so much information will modify mainstream geomorphological and sedimentological theories or will provide these with a tropical version

processes and controls involved in the transfer of fluviatile sediments to the deep ocean sediment transport processes are described in relation to land surfaces river systems estuaries deltas and continental shelves complex interrelationships between variables controlling transport within each of the environments are summarized extreme rates of mechanical denudation are presented and can be as high as 44 103 tonnes km2 a for taiwan they compare with mean european rates of around 30 tonnes km2 a comparison is made between some representative rates of mechanical and chemical denudation river inputs are examined within the context of deltaic sedimentation patterns it is suggested that fluviatile sediments can be transported over continental shelf areas and out into the oceans as hyperpycnal discharges organic material contained within the sediments is important geologically in terms of its burial within marine sediments

the fluvial sediment budget of a dammed river upper muga southern pyrenees abstract many rivers in the mediterranean region are regulated for urban and agricultural purposes reservoir presence and operation results in flow alteration and sediment discontinuity altering the longitudinal structure of the fluvial system this study presents a 3 year sediment budget of a highly dammed mediterranean river the muga southern pyrenees which has experienced flow regulation since the 1969 owing to a 61 hm 3 reservoir flow discharge and suspended sediment concentration were monitored immediately upstream and downstream from the reservoir whereas bedload transport was estimated by means of bedload formulae and estimated from regional data results show how the dam modifies river flow reducing the magnitude of floods and shortening its duration at the same time duration of low flows increases the downstream flow regime follows reservoir releases that are mostly driven by the irrigation needs in the lowlands likewise suspended sediment and bedload transport are shown to be notably affected by the dam sediment transport upstream was mainly associated with floods and was therefore concentrated in short periods of time i e 90 of the sediment load occurred in

cybertectonic earth and gaia s weak hand sedimentary geology sediment cycling and the earth system tectonics climate and sea level are dominant controls on the nature and distribution of sedimentary environments by inverse logic recognition of influences on sedimentation from the stratigraphic record as a result of perturbations in these variables leads to clarification of past environmental and structural change the succession of sedimentary environments in time and place is determined by a wider combination of variables namely sediment flux biological evolution atmosphere and water body productivity and chemistry the feedbacks and interrelationships between all variables cause the complexity seen in sedimentation processes over geological time the processes of global tectonics that cause widespread mountain belt and continental plateau uplift have produced numerous severe events during earth history these often random workings out of the plate tectonic cycle define the state of cybertectonic earth from cyber after the greek kubernan to steer or govern this has worked within a usually zonally arranged series of climatic belts and within the framework provided by biological evolution at certain times in the geological past a combination of factors arose as a response to continental uplift and have acted to cause certain earth surface conditions and variables notably mean global surface temperature atmospheric p co 2 p o 2 to have varied by very large amounts 2 10 compared with present values these large fluctuations such as those responsible for the neoproterozoic late palaeozoic and late tertiary glaciations lead one to doubt the reality of homeostatic control of surface conditions as proposed in lovelock9s gaia hypothesis the gaian kuberne0te0s steersman had a weak hand on the helm frequently unable to prevent vast areas of the globe experiencing rapid fluctuations in environmental conditions and inimical conditions to life for very long periods during neoproterozoic and phanerozoic times at the same time biogenic and abiogenic processes have proved capable of returning earth to states of mean stability although biogeochemical cycling models seem alarmingly ad hoc and largely untestable as scientific hypotheses in any true geological sense

holocene sedimentation patterns and bedforms in the wave current dominated nearshore waters of eastern mersin bay eastern mediterranean abstract the distribution and bedforms of holocene sediments in relatively high energy wave dominated nearshore waters of the eastern mersin bay eastern mediterranean were investigated by high resolution seismic profiles sidescan sonar records and surface sediment samples high resolution seismic reflection profiles show that the sedimentary column consists of two main sequences separated by a mid reflector r showing irregular and erosional surface features which is interpreted as the late pleistocene holocene boundary the upper sequence which is seen as parallel to slightly divergent reflection configurations in a sheet like to wedge shaped geometry approximates to holocene deposits reaching up to 10 m in thickness about 2 km seaward of the shoreline with a slight tendency to increase offshore seismic data indicate that the unconformity between the late glacial subaerial surface and the overlying post glacial transgressive deposits occurs at depths of 1 10 m below the seafloor holocene sedimentation rates are estimated to be 10 100 cm per 1000 yr typical ridge furrow systems cutting into beachrocks are apparently products of mechanical erosion at very shallow water depths the lower seismic sequence underlying the reflector r may represent pre holocene deposits and is characterized by complex stratified reflection patterns commonly showing hummocky to mounded and chaotic cut and fill facies sidescan sonar records indicate prominent features of these nearshore areas such as gravel waves between sand patches and beachrocks the former are thought to have been produced as a result of the interaction of waves and currents on the sediment surface with an apparently westerly migration direction for sand whereas the latter were formed during the lowstand of sea level in the late holocene muddy sand is the dominant sediment type in the nearshore waters of mersin bay in depths less than 15 m with local sand and gravel patches and beachrock outcrops

holocene paleohydrology of quistococha lake peru in the upper amazon basin influence on carbon accumulation abstract in order to study the impact of hydrological changes of the amazon river on sedimentation process and organic matter om accumulation in an amazonian floodplain lake during the holocene three sediment cores were collected from quistococha lake peru the cores were dated with 14c accelerator mass spectrometry ams radiographed and described bulk density granulometry loss on ignition loi total organic carbon toc and total nitrogen tn contents were measured and stable isotopic composition of toc 13c and tn 15n and carbon accumulation rates were determined two distinctive sedimentary depositional phases were identified based on the lithology granulometry and the geochemical characteristics of sedimentary om between about 6100 and 4900 cal years bp the lake was under strong influence of the amazon river the river induced highest sedimentation rates mean 0 5 cm yr 1 a predominant deposition of relatively coarse particles coarse silt low loi 1 low contents of organic carbon 0 5 low c n ratios 10 relatively low 13c values 26 0 very laminated sediments and high carbon accumulation rates between 14 and 29 g c m 2 yr 1 a gap in the record between about 4900 and 2600 cal years bp corresponds to a sedimentation hiatus during the dry mid holocene this gap is probably due to an avulsion of the main stem that induced significant changes in the lacustrine sedimentation after 2600 cal years bp sedimentation resumed but now the lake was isolated far from the amazon influence the resumption of sedimentation corresponds to wetter conditions during the late holocene and an increase in water levels the lake was now characterized by very low sedimentation rates 0 02 cm yr 1 fine organic rich sediments with high loi between 20 and 80 toc between 10 and 40 of toc high c n ratio 20 and lower 13c and 15n indicating a predominant deposition of c3 plant derived om carbon accumulation in this sedimentary depositional phase was about 8 g c m 2 yr 1 these results combined with those of studies from other isolated lakes cordeiro et al 1997 2008 turcq et al 2002 and from other floodplain lakes moreira et al 2012 moreira et al 2013 point out that floodplain lakes with strong influences from the amazon river act as important carbon sinks in the amazon basin despite their low carbon concentrations

human influence and the changing geomorphology of mediterranean deltas and coasts over the last 6000 years from progradation to destruction phase the present geomorphology of the mediterranean s coasts is largely a product of an intricate long term relationship between nature and human societies a cradle of ancient civilisations the mediterranean has seen its shores occupied by humans since prehistory and is therefore a particularly pertinent unit of analysis the morphotectonic context and other forcing agents e g climate shaped out a highly diversified coastal morphology and generated a sediment supply regime potentially favourable to the formation of numerous open coast deltas and bay head deltas in infilled rias as sea level stabilised during the mid holocene this supply of riverine sediment has also been the key agent in mediating human occupation of the mediterranean s clastic coasts expressions of this relationship have been extensively archived in clastic coastal deposits including base level deltaic and estuarine sedimentary sinks which comprise records to explore the interactions between geosystems and the human environment the stratigraphic sequences in these coastal sedimentary archives comprise in many places a clearly identified anthropogenic signature notably in ancient harbours some of which underwent extremely rapid silting up due to massive sediment sourcing generated by new agricultural practices from the neolithic onwards increasing human influence especially over the last 3000 years has been in turn an important driver of changes in sediment supply strongly modulating deltaic development pulses of sediment supply from catchments rendered vulnerable by human perturbations during the roman period resulted in a new cycle of inception of many other deltas and in rapid delta growth e g the ebro the po the arno and the ombrone another progradation dynamic during the little ice age at a time of strong rural population growth river discharge increases technological developments and urbanisation further consolidated delta growth understanding the life cycle of these deltas since their initial formation is in turn key to unravelling the relative role of natural and anthropogenic forcing agents rapid climate changes are deemed to have contributed through both the stripping of landscapes rendered fragile by human activities and active fluvial sediment transport to the coast but disentangling climate change effects from human impacts in the mediterranean remains a challenge the patterns of subsequent deltaic growth and delta morphodynamics reflect adaptations to pulsed sediment supply river discharge variations the microtidal fetch limited context of the mediterranean and direct engineering interventions the progradation dynamic of the roman period and little ice age contrasts markedly with the situation of common coastal destabilisation over the last two centuries particularly well documented for the last 50 years this period has been characterised by reduced sediment flux to base level geosystems due to catchment reforestation retenion within reservoirs fluvial regulation and dredging resulting in the erosion of deltas and barrier lagoon and beach dune systems large stretches of shoreline and narrow coastal plains have been massively engineered for coastal defence and protection against erosion but also for the construction of marinas leisure harbours and artificial beaches resulting in the emergence of veritable artificial seafronts these interventions have collectively and progressively raised societies to a pervasive and overarching position in the geomorphic stability instability of the mediterranean s coasts a situation that will be exacerbated by pressures from sea level rise paving the way for rampant coastal erosion and delta destruction

the geomorphic evolution and sediment balance of the lower rhone river southern france over the last 130 years hydropower dams versus other control factors article i nfo atthe end of thelittleice age lia agriculturalpractices and successive phasesof channel management involv ing embankments and dams however the respective roles of these various control factors in modifying sedi ment storage and channel stability are often difficult to identify the aim of this paper is to determine the main factors that have driven morphological changes over the past 130 years on the 120 km long downstream reach of the rhone river a gis based analysis of maps and aerial photographs available since 1860 coupled with chrono stratigraphic data on the river s alluvial deposits has enabled quantification of sediment storage and erosion in different morphological units changes measured include riverbed incision narrowing of the main channel in relation with accumulation of fine sediments along the channel margins and reduction of sed iment input at the mouth on the basis of local and reach scale sediment balances we show that these changes are synchronous with engineering works carried out at the end of the nineteenth century and beginning of the twentieth century a period that coincided with a decline in river sediment inputs largely generated by human activities the early stabilisation of mountain slopesresulting from a decline inrural agriculture rural exodus re forestation and engineered torrent control has had a much greater impact in geomorphically transforming the rhone than dams and gravel mining the major identified causes of river destabilisation in the world this calls for caution in the all too common and wholesale attribution of declining sediment budgets of european rivers

human impacts on fluvial systems in the mediterranean region the long history of substantial human impacts on the landscape of the mediterranean region and their effects on fluvial systems is documented these effects have included impacts of deforestation and other land use changes agricultural terracing on a wide scale water transfers and irrigation schemes during the 20th century major changes were made directly to channels through channelisation construction of dams of various sizes and extraction of gravel and indirectly by reforestation these changes have caused a major phase of incision on some rivers runoff and soil erosion have been affected by types of crops and agricultural practices as well as by the varying extent of cultivation and grazing some recent agricultural practices involve wholescale relandscaping of the topography and alteration of surface properties of material the importance of analysing the connectivity within different land units and of the spatial position of human activity within a catchment is illustrated the analysis of connectivity is the key to understanding the variability of impact and the extent of propagation of effects

remote sensing of floodplain geomorphology as a surrogate for biodiversity in a tropical river system madre de dios peru the complex floodplains of large rivers offer a striking example of how geomorphology by dictating patterns in the frequency and duration of soil saturation and surface flooding influences ecosystem structure and function this study draws on multiple sources of data from remote sensing together with ground observations and water sampling to distinguish floodplain ecosystems in the madre de dios river a tributary of the amazon river in peru this remote tropical river meanders across sub andean alluvial deposits in a tectonically active region and creates floodplain surfaces of varying ages including terraces that are above the reach of present day river floods data from landsat etm optical multispectral jers 1 l band radar and the shuttle radar topography mission c band interferometric digital elevation models were integrated in an object oriented image analysis approach to distinguish five classes of floodplain vegetation vegetation classes generally correspond with successional age and reflect the activity of the riverine meander belt stage data for the river show erratic fluctuations and an annual range exceeding 8 m but the maximum depth of floodplain inundation varied from 1 m close to the river to approximately 0 1 m on more elevated terraces the major ion composition of standing waters on the floodplain during the dry season indicated the importance of emergence of local groundwater in maintaining saturated soils particularly further from the river where backswamp vegetation is distinct and includes palm swamps thus a hydrological continuum exists from deep but sporadic river inundation near the river to constant soil saturation by groundwater emergence in distal backswamps reflecting the geomorphological origin and age of the floodplain deposits this hydrogeomorphic continuum results in fundamental ecological differences the exceptionally rich biodiversity of the sub andean region may be ascribed in part to the enhanced biodiversity associated with fluvial geomorphological features and thus conservation planning must account for the diverse landforms created by fluvial dynamics

distribution and dispersal of suspended particulate matter on the ebro continental shelf northwestern mediterranean sea abstract hydrographic data water and bottom sediment samples and a geoprobe tripod experiment were used to examine the distribution and dynamics of suspended particulate matter on the ebro shelf in the northwestern mediterranean sea in the absence of strong winds and storms primary sediment supply from the ebro river is dispersed along the shelf by a general southward flow in such calm conditions suspended matter concentrations on the shelf are lower than 3 mg l and transfer of material from the shelf to the slope takes place principally over the shelf edge north of the columbretes islands very fine sediment deposited in a mid shelf mud belt 30 80 m deep is cohesive and resistant to erosion only relatively rare strong storms are able to resuspend particles from the deeper central region of this cohesive deposit when resuspension takes place suspended particulate matter concentration increases and the general dispersal pattern of suspended matter is altered near the seafloor distribution of suspended matter is greatly influenced by the distribution of the mid shelf muds from which particles are resuspended resuspension occurs more intensively and frequently along the shallower 20 40 m edge of the cohesive deposit and near the delta

a multi proxy palaeoecological record of late holocene forest expansion in lowland bolivia abstract pollen charcoal and bulk sediment stable carbon isotope analysis of a 2 4 m sediment core from laguna yaguaru in eastern bolivia located near the modern ecotone between the humid evergreen amazon rainforest and the seasonally dry semi deciduous chiquitano dry forest provides a 5600 year record of environmental change in an area sensitive to changes in the strength of the south american monsoon the core consists of organic sediment interrupted by a 15 cm mineral facies deposited 5270 yr bp in a low energy flood event in the yaguaru basin pollen data indicate that the basin contained a cattail typha marsh prior to the flood some dry forest elements were present probably growing within small patches of forest within a matrix of generally open vegetation including marsh savanna and savanna woodland above the flood layer typha pollen is absent indicating open water conditions and pollen assemblages are similar to modern pollen spectra from bolivian dry forests around 1200 bp charcoal concentrations decrease carbon isotope ratios become more negative and celtis pollen rises dramatically signaling forest expansion and the development of the modern closed canopy dry forest at laguna yaguaru the wetter conditions implied by this forest expansion suggest the region was affected by the strengthening south american monsoon at this time the timing of the arrival of monsoon moisture at yaguaru is consistent with nearby records allowing a 700 year delay for the migration of the monsoon to reach the more southerly location of yaguaru

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

low sediment accumulation rates and erosion on the middle and outer nile delta shelf off egypt abstract the nile delta and its contiguous continental margin seaward of egypt form the major sediment repository on the north african margin together they constitute an integral depositional system man has increasingly modified the delta since pre dynastic time but most extensively this century by the emplacement of two dams across the nile at aswan in upper egypt sediment once transported by the nile river to the coast is now almost cut off and it is postulated that changes induced by man are likely to impact on sedimentation in distal sectors beyond the coast and inner continental shelf continued anthropogenic modification of the delta is expected to result in the increased erosion of the surficial sediment cover of the middle and outer nile delta shelf eddy flow patterns mapped by others show the interrelation of bottom currents on the mid and inner shelf thus in designing engineering projects on the nile delta and sinai margin it would be useful to monitor depositional changes seaward of the inner shelf

20th century sediment budget trends on the western gulf of lions shoreface france an application of an integrated method for the study of sediment coastal reservoirs abstract this paper presents a shoreface sediment budget established for the 20th century 1895 1984 2009 along the microtidal wave dominated coast of the western gulf of lions languedoc roussillon mediterranean sea se france the implementation of a diachronic bathymetric approach coupled with the definition of sand reservoirs upper sand unit usu by very high resolution seismic surveys and the results of lidar investigations offers a new means of defining precisely the magnitude and change trends of the sediment budget the aim of this study is to link the large scale coastal behaviour lscb of the littoral prism expressed in terms of shoreface sediment budget shoreface sediment volume and spatial distribution pattern of cells to climatic change river sediment input to the coast longshore sediment transport distribution impact of hard coastal defence structures and artificial beach nourishment the results show a significant reduction of the volume of the western gulf of lions littoral prism over 114 years 26 1 4 6 10 6 m 3 from 1895 to 1984 the overall budget is slightly positive with a volume estimated at 4 1 3 5 10 6 m 3 for 1984 2009 however the estimated sediment budgets clearly indicate that erosion is dominant over the last 25 years with a volume loss of 30 2 4 2 10 6 m 3 in relation to the long term sediment budget and longshore drift pattern the long term trend of the usu volume distribution displays strong spatio temporal contrasts linked to longshore sediment drift spatial distribution of fluvial sediment inputs and hard engineering structures locally the sedimentary reservoir is significantly eroded within a century 80 of usu since the initial amount present was low and not sustainable the emphasis is on the importance of considering the volume changes of available sediment reservoirs rather than their losses and gains erosion of the languedoc roussillon shoreface is likely to continue in the future due to the natural decrease of river sediment input and the sand removal for human purposes consequently the littoral sand prism results in sedimentary reservoirs that are gradually being used up

growth increments in gomphotherium tusks and implications for late miocene climate change in north america abstract changes in mammalian faunas in north america during the late miocene are thought to have been caused by the replacement of woodland habitats with grassland or steppe the proposed cause of this habitat shift is the transition from the relatively low seasonality early miocene climate to a more highly seasonal climate regime by the end of the miocene tusks which are highly modified incisor teeth of late miocene gomphotherium mammalia proboscidea were sectioned to document the nature of tusk growth in this genus and to test for patterns of seasonal growth both the dentin and enamel of gomphotherium tusks preserve incremental growth lines dentin has increments on three scales inferred to represent annual first order weekly second order and daily third order periodicities luminance and second order increment thickness profiles were measured on transverse thin sections of tusk dentin viewed at low magnification and the data were examined with bivariate plots and autocorrelation autocorrelograms of luminance data support the consistent identification and measurement of second order increments within and among the tusks sampled profiles of second order increment thickness in three tusks from the barstovian north america land mammal age nalma show no indication of seasonal patterns of growth one tusk from the late clarendonian and one from the early hemphillian nalma have patterns of growth consistent with an increase in seasonality during the late miocene an interpretation supported by autocorrelation of the increment thickness data from these tusks the growth patterns are consistent with the hypothesized changes in north american climate during the late miocene and are suggestive of an increase in aridity and development of a wet season tests of this hypothesis are suggested

oxygen isotope variation in the tusks of extinct proboscideans a measure of season of death and seasonality centimetre scale laminae in tusk and molar dentine of late pleistocene mastodonts and mammoths have been interpreted as annual growth bands produced in part by seasonal variation in growth rate to test this interpretation we measured the oxygen isotope composition 18 o of the co 3 fraction of dentinal hydroxyapatite from samples covering consecutive inferred years of growth in tusks in mammals changes in the 18 o value of dental tissues within individuals predominantly reflect variation in the 18 o value of body fluids which is controlled mainly by the isotopic composition of ingested water in northern hemisphere continental regions winter precipitation has substantially lower 18 o values than does precipitation in other seasons if ingested water tracks focal precipitation then seasonal variations in dentinal isotope composition should result the lowest 18 o values representing winter growth we demonstrate that there are substantial variations in the oxygen isotope composition of proboscidean dentinal apatite and that isotopic identifications of winter i e low 18 o values coincide with those based on growth rate i e slow growth zones finally the potential of oxygen isotope analyses of terrestrial mammals for assessing the seasonality of paleoclimates is considered

the hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field this has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows lift and drag forces are determined on clustered hemispherical particles of varying size in the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion on the stoss side of the cluster drag is uniform regardless of the separation of the component particles but lift is shown to increase when particle separation is small so affecting stability this mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape on average 46 of clustered particles are entrained by flood flow compared to 87 of particles in open plane beds the influence of clusters is a major determinant of sedimentary sorting

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

microbial biomarkers support organic carbon transport from methane rich amazon wetlands to the shelf and deep sea fan during recent and glacial climate conditions abstract we have investigated the delivery of terrestrial organic carbon oc to the amazon shelf and deep sea fan based on soil marker bacteriohopanepolyols bhps adenosylhopane and related compounds and branched glycerol dialkyl glycerol tetraethers gdgts as well as on 14c dating of bulk organic matter the microbial biomarker records show persistent burial of terrestrial oc evidenced by almost constant and high bit values 0 6 and soil marker bhp concentration 80 230 g g toc total oc on the late holocene shelf and even higher bit values 0 8 0 9 but lower and more variable soil marker bhp concentration 40 100 g g toc on the past glacial deep sea fan radiocarbon data show that oc on the shelf is 3 4 kyr older than corresponding bivalve shells emphasizing the presence of old carbon in this setting we observe comparable and unexpectedly invariant bhp composition in both marine sediment records with a remarkably high relative abundance of c 35 amino bhps including compounds specific for aerobic methane oxidation on the shelf avg 50 of all bhps and the fan avg 40 notably these marine bhp signatures are strikingly similar to those of a methane producing floodplain area in one of the amazonian wetland varzea regions the observation indicates that bhps in the marine sediments may have initially been produced within wetland regions of the amazon basin and may therefore document persistent export from terrestrial wetland regions with subsequent re working in the marine environment both during recent and past glacial climate conditions

discovery of permanent amazon lakes and hydraulic disturbance in the upper amazon basin we describe here the first autonomous permanent deep lakes found in the amazon lowlands permanent amazon lakes have hitherto been considered unlikely because the ubiquitous rivers meandering over flat terrain should obliterate ancient basins resulting in lakes subject to annual flooding the so called varzeas 1 4 the lakes described here are in the rain forests of ecuador in western amazonia within 100 km of the andes mountains but at elevations below 700 m fig 1 they occupy either deep abandoned channels of high energy rivers or closed volcanic basins sedimentary histories and pollen data over several millennia from these lakes record disturbance of the amazonian ecosystem by flooding encouraging the view that high amazonian species richness is maintained by non equilibrium processes5 6

geodynamic environment of quaternary morphostructures of the subandean foreland basins of peru and bolivia characteristics and study methods abstract the quaternary evolution of western amazon basins is considered using morphostructural analysis of fluvial belts and lakes meandering rivers from the basin surface are analyzed using three scales of elements the ridge and swale pattern the mosaic pattern and the fluvial belt lake patterns are differentiated according to their shape the hydrological environment and the structural context influences of basement structures and neotectonic deformations when known are considered the morphostructural patterns are discussed in the context of a balance of internal tectonic and external hydraulic and sedimentologic processes special attention is focussed on the effect of neotectonics which is generally neglected on the formation and the location of quaternary morphostructures the adaptation of study methods and sampling to the characteristics of the different morphostructures is considered

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

beach morphology and change along the mixed grain size delta of the dammed elwha river washington abstract sediment supply provides a fundamental control on the morphology of river deltas and humans have significantly modified these supplies for centuries here we examine the effects of almost a century of sediment supply reduction from the damming of the elwha river in washington on shoreline position and beach morphology of its wave dominated delta the mean rate of shoreline erosion during 1939 2006 is 0 6 m yr which is equivalent to 24 000 m3 yr of sediment divergence in the littoral cell a rate approximately equal to 25 50 of the littoral grade sediment trapped by the dams semi annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer shoreline change and morphology also differ spatially negligible shoreline change has occurred updrift west of the river mouth where the beach is mixed sand to cobble cuspate and reflective the beach downdrift east of the river mouth has had significant and persistent erosion but this beach differs in that it has a reflective foreshore with a dissipative low tide terrace downdrift beach erosion results from foreshore retreat which broadens the low tide terrace with time and the rate of this kind of erosion has increased significantly from 0 8 m yr during 1939 1990 to 1 4 m yr during 1990 2006 erosion rates for the downdrift beach derived from the 2004 2007 topographic surveys vary between 0 and 13 m yr with an average of 3 8 m yr we note that the low tide terrace is significantly coarser mean grain size 100 mm than the foreshore mean grain size 30 mm a pattern contrary to the typical observation of fining low tide terraces in the region and worldwide because this cobble low tide terrace is created by foreshore erosion has been steady over intervals of at least years is predicted to have negligible longshore transport compared to the foreshore portion of the beach and is inconsistent with oral history of abundant shellfish collections from the low tide beach we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta thus the cobble low tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta which in turn is related to the impacts of the dams on the elwha river to sediment fluxes to the coast

vegetation development in an amazonian peatland recent investigations of wetlands in western amazonia have revealed the presence of extensive peat deposits up to 7 5 m thick developing under a variety of vegetation types we report the first attempt to establish the long term centennial to millennial scale vegetation history of one of these peatland sites quistococha a palm swamp close to iquitos in northern peru pollen and sedimentological analyses show that peat formation began at the core site under sedge fen or floating mat vegetation c 2200 calendar years before present cal yr bp a seasonally flooded woodland developed c 1880 cal yr bp the permanently waterlogged palm swamp which persists today began to form c 1000 cal yr bp with the present vegetation community established by c 400 years ago the vegetation at this site has undergone continuous change throughout the period of peat formation with several abrupt transitions and reversals and repetitions in the apparent trajectory of change the pollen data combined with sedimentary evidence suggest that the dominant control on ecosystem functioning and development is the flooding regime there appears to have been a decrease in fluvial influence over time there is no clear evidence of direct climatic or anthropogenic influence although we cannot rule out the possibility of climatically driven hydrological changes our results caution against adopting a simple model of peatland vegetation succession in this region

carbon sedimentation at lago grande de curuai a floodplain lake in the low amazon region insights into sedimentation rates we studied recent organic carbon sedimentation in an amazonian floodplain using sediment trap samples and one core sediment traps were placed in several lakes of the lago grande de curuai floodplain during different water level phases the end of the period of water level reduction falling water and the start of the period of water level increase rising water vertical seasonal fluxes of settling particles sp are very difficult to estimate and interpret especially during falling water periods when the concentration of suspended matter in the water column is greater than during rising water periods probably due to resuspension processes vertical fluxes of settling particles sp varied between ca 4 and 2000 g m 2 day 1 fluxes of organic carbon ranged between 5 50 g c m 2 day 1 during the falling water and between 0 041 4 g c m 2 day 1 during the rising water the core is characterised by two laminated sedimentological units sediments in the upper unit which generally coarsens upwards are interpreted as a progradation sequence of the sand and silt flat at the entrance of an inflow channel where the core was collected this unit is composed of three sub units a superficial layer reworked by bioturbation a lower sub unit characterised by horizontal and sub horizontal laminations of fine sands and silts and a basal sub unit composed of horizontal laminations of fine silts and clays the lower unit is characterised by horizontal and sub horizontal laminations of fine sands and silts and corresponds to the top of an older accumulating sequence these changes show that the sedimentary environment is very dynamic at the centennial scale sedimentation rates generally ranged between 0 42 and 1 34 cm year 1 and were estimated to range from 0 56 to 1 35 cm year 1 after normalisation for clay sized particles 4 m floodplain sediment accumulation generally varies between 0 42 1 16 and 1 10 g m 2 year 1 within the first second and third sub units respectively in the second unit the non supported 210pb date is close to the detection limit carbon content is relatively low around 1 sediment weight through the core although mean carbon accumulation rates are very high generally 100 g m 2 year 1 and peaking at 250 g m 2 year 1 these results suggest that amazon floodplain lakes may constitute an important sink for organic carbon

mediterranean river systems of andalusia southern spain and associated deltas a source to sink approach abstract the northern shores of the semi isolated mediterranean sea with its low tidal range and a relatively voluminous fluvial supply of sediments constitute an ideal delta forming environment in this paper we examine the present day and multi decadal behaviour of 26 river systems from andalusia southern spain forming deltas in the alboran sea the westernmost basin in the mediterranean sea eastern central and western andalusian watersheds have been defined based on geomorphological climatological and hydrological characteristics a comprehensive data set has been compiled including satellite images aerial photographs a digital elevation model thematic maps time series of precipitation temperature and water discharge and the damming history of individual river basins this data set has been used to analyse basin morphology and hydrology and anthropogenic impact several modelling approaches have been applied to obtain the water budget and mean annual sediment yield of 12 of the 26 studied river systems in addition the periodicities of water discharge events and their possible link with north atlantic oscillation nao fluctuations have been also studied a decreasing trend has been observed in most water discharge time series during the last decades which has been attributed to natural factors although it could have been expected that the diminution of water discharge may have caused a reduction in sediment load calculated sediment discharge time series do not show any significant tendency in general sediment yield shows an opposite relationship with basin area a comparative analysis of spanish mediterranean deltas indicates that in terms of sediment transport andalusian river systems are quite efficient despite the small size of their catchments repetitive flood events and the consequent suspension plumes off river mouths play a major role in the development of deltaic and prodeltaic bodies nowadays 42 of the study area is regulated although to date the effect of dam building is hardly noticeable on river mouths

a model of two phase bedload transport in an oregon coast range stream a descriptive model for bed material routing in small sand and gravel bedded channels with sequences of pools and armoured riffles was developed aspects of the model are demonstrated at flynn creek a 202 ha third order drainage in the oregon coast range assuming that channel geometry sediment transport competence and the availability of sediments for transport are all non uniform in the downstream direction the model described bedload transport as occurring in two phases phase i involves the transport of fine predominantly sand sized bed materials over stable gravel armoured riffles phase ii occurs at higher flows that can entrain riffle armour and transport riffle sediments in addition to phase i sediments phase i bedload transport was sampled during three moderate flow events a power function related phase i bedload transport to water discharge the relationship was consistent between storms and between two successive years phase ii bedload transport sampled during a 1 8 yr return period streamflow event in february 1979 proved non uniform in a downstream direction and unsteady over time at a given stream discharge transport rates closely correlated with the rapid scour and redeposition of the riffle at the bedload sampling cross section transport peaks corresponded to scour and transport decreased greatly during deposition the transport of large 12 5 mm diam bed material increased up to 12 fold during riffle scour

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

dating modern deltas progress problems and prognostics 1 abstract radiocarbon dating is the method most frequently used to date holocene deltaic sequences but less than one quarter of 14c dates are within 500 years of predicted age such dates tend to be unreliable in other words often too old and commonly inverted upsection and core sample dates obtained near deltaic plain surfaces may be as old as mid to late holocene stratigraphic irregularities result primarily from downslope reworking of upland alluvial sediment with displacement of old carbon in the sediment that accumulates in lower valleys and deltaic plains use of dates that are too old results in inaccurately calculated rates most often too low of relative sea level rise and or land subsidence more reliable timing of deltaic sediment requires a multiple method dating approach including where possible identification of associated archaeological material developing an accurate dating strategy is a critical step for implementing reliable coastal protection measures needed for the rapi

tropical south america during the last glacial maximum evidence from glacial periglacial and fluvial records abstract the records of the lgm climates of tropical south america are still sparse compared to the vast region under consideration nevertheless a number of records hold up to critical evaluation early attempts to reconstruct the lgm temperature and precipitation conditions for the lgm around 21 14 c ka bp have suggested that relative to present lowland temperatures were on average only a few c cooler whereas in the andes the temperature depression was in the range of 6 8 c these reconstructions further suggested that tropical precipitation was more limited in both the lowlands and in the andes although recent pollen evidence supports significantly cooler tropical lowland temperatures during the lgm additional alkenone evidence from sea surface temperatures and model analysis are contradictory the data from tropical south america suggest that the late quaternary climatic changes of the lowlands as well as of the andean mountains reflect a response 1 to environmental changes in the source area of the moisture and 2 to global temperature fluctuations based on glacial periglacial and pollen evidence a lgm temperature depression of 5 6 c can be assumed for both tropical lowland south america and the andes these lgm values for the cooling correspond with recent coral records from barbados and the southwestern pacific ice core records from peru noble gas measurements in brazil and ocean core records from the western equatorial atlantic no difference in the value of lgm cooling can be recognised between the tropical lowlands and the high mountains of south america furthermore in all regions the lgm climates were more arid than the present day climates terrestrial records have not yet yielded records of younger dryas age apart from the tropical andes of ecuador where the younger dryas time is characterised by a cooler and drier climate pollen evidence for cooler younger dryas conditions comes from the bolivian andes

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

amazon diversity in light of the paleoecological record abstract disjunct distributions of amazonian species have been explained previously by a refugial theory which postulates that amazonian rain forest was preserved in large highland regions throughout the pleistocene no direct radiocarbon dated evidence exists for the last glacial maximum with which to test this theory the only radiocarbon dates of pleistocene age from the amazon basin are of fossiliferous deposits in the proposed napo refugium of the west where both pollen assemblages and wood samples indicate that forest with cool andean elements existed there at two intervals in the last cycle of northern hemisphere glaciation implying a temperature depression of at least 4 c in the amazon lowlands under modern climatic conditions lateral erosion by river meander together with surface erosion serves as a rejuvenating mechanism for the rain forests of peru and ecuador the instability of late holocene amazonian climates is demonstrated by documenting a precipitation event in the eastern andean cordillera that caused widespread flooding of western amazonian forests 800 1300 bp late holocene pollen histories from widely dispersed parts of central amazonia distinguish between vegetation histories in the drainage of northern south western and western watersheds but all show histories of fluctuating intensities of dry seasons radiocarbon dating of charcoal layers in soils of venezuelan amazonia demonstrates the apparently random incidence of wild fires at wide intervals over at least the last 6 ka the high species richness of amazonia is a result of numerous opportunities for vicariance because of a very large total area wide variety of habitats and intermediate levels of disturbance particularly by hydrological processes that has varied on timescales from years to millennia amazonian disjunct distributions probably reflect regional environmental discontinuities in both interglacial and glacial times

recent evolution of a mediterranean deltaic coastal zone human impacts on the inner thermaikos gulf nw aegean sea the inner thermaikos gulf is located in the northwestern aegean sea receiving water and sediment fluxes from the axios aliakmon gallikos and loudias rivers the geomorphological and sedimentological evolution of the system is reconstructed for the last 150 years 1850 2000 on the basis of detailed analysis of historical bathymetric charts late holocene history is considered within the context of changing riverine sediment supply and human activities three evolutionary stages are identified stage i 1850 1916 corresponds to a natural phase of rapid deltaic progradation and sea floor deposition with an average sediment accumulation rate of 6 5 10 6 m 3 a 1 during stage ii 1916 1956 human interference e g artificial changes in river delta plains realignment of channels and land reclamation schemes to the deltaic system increased sediment delivery to the coastal waters by a factor of three this in turn enhanced the progradation of the active river mouth areas in contrast stage iii 1956 2000 is characterized by significant coastline deltaic retreat and erosion of the adjacent sea floor net loss of 2 5 10 6 m 3 a 1 this was as a result of extensive river damming which trapped a significant part of the sediment moving seaward furthermore these human impacts have affected the character of the surficial sea bed sediments of the gulf by reducing the proportion of mud the response of the deltaic margin of the inner thermaikos gulf to various anthropogenic interventions seems to be analogous to that of other deltas in the mediterranean region where large drainage projects the development of irrigation networks and dam construction have taken place within their river basins

late quaternary vegetation and climate change in the amazon basin based on a 50 000 year pollen record from the amazon fan odp site 932 hemipelagic sediments from the amazon deep sea fan odp site 932 5 12 7 n 47 1 8 w and continental shelf provide a 50 000 yr long pollen record of amazon basin vegetation the age model for hole 932a is constrained by eight magnetic remanence intensity features one paleomagnetic excursion and three ams14c dates alchornea melastomataceae myrtaceae and moraceae urticaceae are dominant taxa in the pollen record between 40 200 and 19 800 cal yr b p andean taxa such aspodocarpusandhedyosmum increase in abundance between 19 800 and 11 000 cal yr b p and prior to 40 200 cal yr b p the holocene pollen assemblage derived from amazon river and continental shelf sediments is dominated by secondary growth taxa such ascecropia climatic factors influencing the development of glacial and interglacial tropical vegetation are considered by comparing marine with terrestrial records of vegetation change this comparison shows that the amazon basin forests were not extensively replaced by savanna vegetation during the glacial period contradicting the refugia hypothesis

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

environmental seasonality and incremental growth rates of beaver castor canadensis incisors implications for palaeobiology abstract periradicular bands are incremental structures seen on the external dentin surfaces of rodent incisors they correspond to circadian developmental rhythms in dentin growth and provide a record of the growth rate of the incisor along its longitudinal axis in the present study we investigate the relationship between daily environmental temperatures and longitudinal growth rates of incisors in 40 recent beavers castor canadensis we also look for trends in growth rates that reflect precipitation or photoperiod the goal is to explore the potential of using longitudinal growth data as indicators of seasonality in past environments the sample consists of upper and lower incisors of fall harvested beavers and upper incisors of winter harvested beavers with known dates of death all distances between sequential periradicular bands were measured using a dissecting microscope fitted with an ocular micrometer all increments for each individual were matched to corresponding dates of formation records of daily temperatures precipitation and photoperiod corresponding to the locality and period of growth represented by the teeth were obtained from the national weather service time series for both dental increments and temperature were smoothed using nonparametric regression to partition primary seasonal or long term and secondary daily or short term signals upper and lower incisors differ in their primary signals in that uppers exhibit a marked decline in growth rate during the late summer and early fall leveling off in the late fall this trend roughly parallels seasonal decreases in temperature precipitation and photoperiod although the decline in growth rate begins while the weather is still warm in late summer in contrast lower incisors show only a very weak seasonal trend in growth rate analysis of residuals from the trend lines reveals only a weak association between daily growth rate and both daily temperature changes and precipitation the results suggest that longitudinal growth rates in upper incisors may be used to estimate season of death in fossil rodents

dating of fluvial sediments from the amazon lowland in peru abstract tl dating was attempted on sediments from surficial alluvial formations of the peruvian amazon a huge area for which there are few pleistocene age determinations the samples were collected from several locations of erosional banks carved in older alluvial deposits in order to date the quaternary stratigraphy the major part of the time scale falls beyond the range of the radiocarbon method a complex resedimentation also makes the use of this method difficult here tl dating can provide an approximate idea of the age and history of sedimentation some preliminary results are presented and problems due to sample characteristics are discussed

recent subsidence and northeast tilting of the nile delta egypt abstract sediment borings collected in the nile delta of egypt many of them radiocarbon dated indicate that the entire northern sector of this major depositional center in the eastern mediterranean is presently subsiding mapping of the base of the holocene deltaic facies which is dated at about 8000 6500 yrs b p reveals that differential lowering of the northern delta plain is preferentially accentuated toward the northeast long term subsidence rates at or near the coast averaged for the mid to upper holocene range from about 0 1 to 0 25 cm yr between alexandria and the north central delta margin rates increase markedly eastward to a maximum of about 0 5 cm yr in the port said manzala lagoon region and this rapid lowering explains the presence of thick marine delta lobe sequences of holocene age in cores in the northeastern delta in contrast only reworked remnants of holocene marine delta lobe deposits of the earlier river nile branches which once flowed to the north and northwest are preserved seaward of the present north delta coast subsidence has induced marked environmental changes particularly with respect to coastal erosion and salt water incursion the asymmetric pattern of saline ground water here attributed in part to the northeast tilt of the delta plain has serious implications for agricultural development in this intensely cultivated region

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel