variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

holocene vegetation fire climate linkages in northern yellowstone national park usa abstract yellowstone national park has been an important location for paleoecologic studies that focus on the use of charcoal data to reconstruct past fire activity and on the role of climate variations in shaping past vegetation and fire regimes one hypothesis which has been explored in other parts of the western u s is the idea that present day summer dry and summer wet precipitation regimes were intensified during the early holocene as a result of greater than present summer insolation and its effect on atmospheric circulation patterns in yellowstone this hypothesis was previously examined at two sites one in summer wet and one in summer dry precipitation regions the records showed variation in both fire and vegetation history that supported the hypothesis we present a fire and vegetation history from blacktail pond located in pseudotsuga parkland in the transitional region the blacktail pond data indicate the following ecological history prior to 12 000 cal yr bp the site supported tundra vegetation and fire episodes were infrequent between 12 000 and 11 000 cal yr bp picea pinus parkland was established and fire activity increased these changes are consistent with increasing temperature as a result of rising summer insolation from 11 000 to 7600 cal yr bp the presence of a closed forest of pinus and some picea is attributed to high levels of winter moisture but high fire activity indicates that summers were drier than at present after 7600 cal yr bp the presence of forest and steppe vegetation in combination with high fire activity suggest that middle holocene conditions were warm and dry the decrease in picea and betula in the last 4000 cal yr indicates continued drying in the late holocene although fire episode frequency was relatively high until 2000 cal yr bp the pollen data at blacktail pond and other low elevation sites in the northern rocky mountains suggest a widespread vegetation response in summer wet regions to effectively wetter conditions in the early holocene and decreased moisture in the middle and late holocene in contrast the more variable fire history among the three sites implies either that 1 summer moisture stress and fire conditions are related to year round moisture balance and not well predicted by the hypothesis 2 the transitional area between summer wet and summer dry precipitation regimes experienced complicated shifts in effective moisture through time and or 3 fire episode data have a limited source area that makes it difficult to separate local influences from regional climate changes in understanding long term variations in fire episode frequency

effects of tectonic deformation and sea level on river path selection theory and application to the ganges brahmaputra meghna river delta the set of active rivers of the ganges brahmaputra meghna gbm delta in bangladesh overlies an active plate boundary that continually modifies the landscape of the delta by deformation the response of rivers to spatially variable subsidence from tectonic tilting or other causes has been thought to include preferred occupation of regions of higher subsidence in this paper we develop further the theoretical framework for analysis of the interplay of tectonics and river dynamics and apply this model to conditions in the gbm delta first we examine the overall competition between variable subsidence and channel dynamics and find that tilting in bangladesh should be strong enough to influence river path selection we then present new theory for the effect of subsidence that is spatially not temporally variable we find a constant residence timescale on different parts of the delta and differing frequencies of avulsion to these locations and describe the effects of incision or floodplain deposition on these quantities we present estimates of the channel residence timescale of the jamuna brahmaputra river reconstructed from the lithology provenance and dating of sediment cores we apply our framework to a map of regional subsidence to predict the effects on avulsion for the jamuna river comparison between our predicted 2150 years and our stratigraphically based estimates of avulsion timescale 1800 years shows encouraging consistency

precipitation development in a new mexico thunderstorm observations were made of a thunderstorm over langmuir laboratory for atmospheric research on 2 august 1984 four doppler radars were closely spaced allowing observations to be taken at high elevation angles as a result the vertical component of the precipitation particle velocity was accurately measured in conjunction with the vertical air velocity obtained by integration of the continuity equation the particle terminal velocity field was computed backward integration of particle trajectories showed that most precipitation embryos originated near the tops of thunderstorm precursor clouds when these cloud tops were 7 8 km above sea level early and mature phase precipitation grew by accretion at levels below 7 km and resulted in relatively high density graupel or hail late phase precipitation underwent growth mainly above 8 km and was sufficiently small in diameter and low in density to undergo rapid melting at the freezing level

rapid hydrologic shifts and prolonged droughts in rocky mountain headwaters during the holocene 1 rapid hydroclimatic shifts repeatedly generated centuries to millennia of extensive aridity across the headwaters of three of north america s largest river systems during the holocene evidence of past lake level changes at the headwaters of the snake columbia missouri mississippi and green colorado rivers in the rocky mountains shows that aridity as extensive and likely as severe as the ce 1930s dust bowl developed within centuries or less at ca 9 ka thousand years before ce 1950 and persisted across large areas of the watersheds until ca 3 ka regional water levels also shifted abruptly at 11 3 and 1 8 1 2 ka the record of low water levels during the mid holocene on the continental divide links similar evidence from the great basin and the midwestern u s and shows that extensive aridity was the holocene norm even though few gcms have simulated such a pattern

anthropogenic and geologic influences on subsidence in the vicinity of new orleans louisiana new measurements of ongoing subsidence of land proximal to the city of new orleans louisiana and including areas around the communities of norco and lutcher upriver along the mississippi are reported the rates of vertical motion are derived from interferometric synthetic aperture radar insar applied to uninhabited aerial vehicle synthetic aperture radar uavsar data acquired on 16 june 2009 and 2 july 2012 the subsidence trends are similar to those reported for 2002 2004 in parts of new orleans where observations overlap in particular in michoud the 9th ward and chalmette but are measured at much higher spatial resolution 6 m the spatial associations of cumulative surface movements suggest that the most likely drivers of subsidence are groundwater withdrawal and surficial drainage dewatering activities high subsidence rates are observed localized around some major industrial facilities and can affect nearby flood control infrastructure substantial subsidence is observed to occur rapidly from shallow compaction in highly localized areas which is why it could be missed in subsidence surveys relying on point measurements at limited locations

experimental measurement of the relative importance of controls on shoreline migration abstract shoreline position in sedimentary rocks is a sensitive recorder of the interplay of several controlling factors the most important of these are thought to be the stratigraphic trinity eustatic sea level subsidence and sediment supply in ancient rock sequences it is generally difficult to disentangle the effects of these variables here we analyze the relative influence of sea level subsidence sediment supply and other controlling variables on shoreline migration in an experimental basin equipped with a subsiding floor the experiment used a linear hinge type subsidence profile for which the rate was kept constant in time constant overall sediment supply and base level variation on two time scales that were first applied separately and then superimposed although base level was the only controlling variable that was externally varied in time the base level changes induced changes in other variables indirectly e g by changing the partitioning of sediment between the fluvial and offshore segments of the system we examine the relative importance of measured direct and indirect changes in all the governing variables through the use of a moving boundary equation for shoreline migration when measured values are used for all the variables in the equation shoreline migration rate throughout the run can be predicted with a maximum r2 of 0 92 starting with this optimal prediction of the observed shoreline behavior we successively replace variables in the equation with their run averaged values degrading the prediction the relative loss of prediction accuracy as each variable is replaced is a measure of the importance of that variable in accounting for the observed shoreline migration by this measure base level is the most important variable followed in turn by sediment supply to the foreset geometry of the foreset and the average subsidence rate over the foreset from the shoreline migration equation we also derive a quantitative version of the a s ratio often applied in sequence stratigraphy the new formulation reduces to a form comparable to the traditional descriptive a s ratio if changes in foreset slope gain or loss of sediment to the fluvial system and spatial variation in subsidence rate are all negligible

accelerated river avulsion frequency on lowland deltas due to sea level rise sea level rise subsidence and reduced fluvial sediment supply are causing river deltas to drown worldwide affecting ecosystems and billions of people abrupt changes in river course called avulsions naturally nourish sinking land with sediment however they also create catastrophic flood hazards existing observations and models conflict on whether the occurrence of avulsions will change due to relative sea level rise hampering the ability to forecast delta response to global climate change here we combined theory numerical modeling and field observations to develop a mechanistic framework to predict avulsion frequency on deltas with multiple self formed lobes that scale with backwater hydrodynamics results show that avulsion frequency is controlled by the competition between relative sea level rise and sediment supply that drives lobe progradation we find that most large deltas are experiencing sufficiently low progradation rates such that relative sea level rise enhances aggradation rates accelerating avulsion frequency and associated hazards compared to preindustrial conditions some deltas may face even greater risk if relative sea level rise significantly outpaces sediment supply then avulsion frequency is maximized delta plains drown and avulsion locations shift inland posing new hazards to upstream communities results indicate that managed deltas can support more frequent engineered avulsions to recover sinking land however there is a threshold beyond which coastal land will be lost and mitigation efforts should shift upstream

paleofire severity and vegetation change in the cascade range oregon usa paleoecological research has expanded our knowledge of the relationships between climate fire and vegetation fire can be a significant driver of forest composition and structure change but identifying and quantifying fire regimes has been elusive using high resolution charcoal analysis and pollen analysis we reconstructed a 13 200 year old fire and vegetation history from breitenbush lake oregon located in the central cascade range usa our objective was to examine if fire occurrence and severity may have been a driver of holocene forest composition change the data from this study suggests that while fire can create opportunities for successional process to occur fire events were not significant catalysts for forest change instead most major transitions at breitenbush lake occurred during prolonged fire free intervals our results reinforce the view that climate is the major control of vegetation composition change in the cascade range

variations in fire frequency and climate over the past 17 000 yr in central yellowstone national park a 17000 yr fire history from yellowstone national park demonstrates a strong link between changes in climate and variations in fire frequency on millennial time scales the fire history reconstruction is based on a detailed charcoal stratigraphy from cygnet lake in the rhyolite plateau region macroscopic charcoal particles were tallied from contiguous 1 cm samples of a 6 69 m long core and the data were converted to charcoal accumulation rates at evenly spaced time intervals intervals of high charcoal accumulation rates were interpreted as local fire events on the basis of information obtained from modern charcoal calibration studies in the yellowstone region the record indicates that fire frequency was moderate 4 fires 1000 yr during the late glacial period reached highest values in the early holocene 10 fires 1000 yr and decreased after 7000 calendar yr b p the present fire regime 2 3 fires 1000 yr was established in the past 2000 yr the charcoal stratigraphy correlates well with variations in july insolation through time which suggests that regional climate changes are responsible for the long term variations in fire frequency in the early holocene summer insolation was near its maximum which resulted in warmer effectively drier conditions throughout the northwestern united states at this time the fire frequency near cygnet lake was at its highest after 7000 calendar yr b p summer insolation decreased to present values the regional climate became cooler and wetter and fires were less frequent the cygnet lake record suggests that long term fire frequencies have varied continuously with climate change even when the vegetation has remained constant

complexity in a cellular model of river avulsion abstract we propose a new model of river avulsion that emphasizes simplicity self organization and unprogrammed behavior rather than detailed simulation the model runs on a fixed cellular grid and tracks two elevations in each cell a high elevation representing the channel levee top and a low one representing the channel bottom the channel aggrades in place until a superelevation threshold for avulsion is met after an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation flow path depends sensitively on floodplain topography particularly the presence of former abandoned channels several behavioral characteristics emerge consistently from this simple model 1 a tendency of the active flow to switch among a small number of channel paths which we term the active channel set over extended periods leading to clustering and formation of multistory sand bodies in the resulting deposits 2 a tendency for avulsed channels to return to their previous paths so that new channel length tends to be generated in relatively short segments and 3 avulsion related sediment storage and release leading to pulsed sediment output even for constant input each of these behaviors is consistent with observations from depositional river systems a single valued threshold produces a wide variety of avulsion sizes and styles larger nodal avulsions are rarer because pre existing floodplain topography acts to steer flow back to the active channel channel stacking pattern is very sensitive to floodplain deposition this work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution

principles of regression and transgression the nature of the interplay between accommodation and sediment supply the rate of change of accommodation at the shoreline a and the rate of sediment supply s are the primary factors controlling regression and transgression in the geological record or retreat and advance of a coastal depositional system the generally accepted notion here referred to as the a s ratio concept stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of a to s is insufficient and somewhat misleading partly owing to the problem of dimensional confusion the interplay of a and s inevitably results in autoretreat of the shoreline whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat provided there is a continuous rise of relative sea level how effective the autoretreat process is depends for a given period of relative sea level rising upon the length of potential time period for the seaward advance of shoreline and is proportional to a 2 s and a super 1 5 s super 0 5 in two and three dimensional sediment dispersal modes respectively autoretreat becomes more effective as a increases and or s decreases a functions more critically to the effectiveness than s does the a s ratio concept is only approximately applicable when autoretreat is less effective i e with very low a and or very high s in this sense the autoretreat theory and the a s ratio concept are complementary to each other the autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems e g barrier islands whose evolution has been considered in terms of the a s ratio concept

spatiotemporal variability of the precipitation dipole transition zone in the western united states 1 el nino southern oscillation related hydroclimatic variability in the western united states is characterized by a north south dipole pattern of precipitation anomalies with opposing signs here i use a high resolution dataset to analyze spatiotemporal patterns in the transition zone between the centers of opposite association results indicate that the transition zone is spatially limited west of the continental divide consisting of a narrow zone within the 40 42 n latitude band and portions of eastern washington eastern oregon and western colorado the transition zone was narrower when the southern oscillation index and the pacific decadal oscillation pdo were in a constructive phase and during the negative atlantic multidecadal oscillation phase although the transition zone has remained remarkably stationary across the great basin shifts likely pdo related have occurred over time in the west coast states implications for water resource planning in river basins underlying the transition zone vary by region

last glacial interglacial environments in the southern rocky mountains usa and implications for younger dryas age human occupation abstract the last glacial interglacial transition lgit 19 9 ka was characterized by rapid climate changes and significant ecosystem reorganizations worldwide in western colorado one of the coldest locations in the continental us today mountain environments during the late glacial period are poorly known yet archaeological evidence from the mountaineer site 2625 m elev indicates that folsom age paleoindians were over wintering in the gunnison basin during the younger dryas chronozone ydc 12 9 11 7 ka to determine the vegetation and fire history during the lgit and possible explanations for occupation during a period thought to be harsher than today a 17 ka old sediment core from lily pond 3208 m elev was analyzed for pollen and charcoal and compared with other high resolution records from the southern rocky mountains widespread tundra and picea parkland and low fire activity in the cold wet late glacial period transitioned to open subalpine forest and increased fire activity in the bolling allerod period as conditions became warmer and drier during the ydc greater winter snowpack than today and prolonged wet springs likely expanded subalpine forest to lower elevations than today providing construction material and fuel for the early inhabitants in the early to middle holocene arid conditions resulted in xerophytic vegetation and frequent fire

influence of water and sediment supply on the stratigraphic record of alluvial fans and deltas process controls on stratigraphic completeness 1 stratigraphy contains the most complete record of information necessary to quantitatively reconstruct paleolandscape dynamics but this record contains significant gaps over a range of time and space scales these gaps result from stasis on geomorphic surfaces and erosional events that remove previously deposited sediment building on earlier statistical studies we examine stratigraphic completeness in three laboratory experiments where the topography of aggrading deltas was monitored at high temporal and spatial scales the three experiments cover unique combinations in the absolute magnitudes of sediment and water discharge in addition to generation of accommodation space through base level rise this analysis centers on three time scales 1 the time at which a record is discretized t 2 the time necessary to build a deposit with mean thickness equivalent to the maximum roughness on a surface tc and 3 the time necessary for channelized flow to migrate over all locations in a basin tch these time scales incorporate information pertaining to the time variant topography of actively changing surfaces kinematics by which the surfaces are changing and net deposition rate we find that stratigraphic completeness increases as a function of t tc but decreases as a function of tc tch over the parameter space covered in the experiments our results suggest that environmental signals disconnected from a sediment routing system are best preserved in systems with low tc values nondimensionalizing t by tc however shows that preservation of information characterizing system morphodynamics is best preserved in stratigraphy constructed by systems with low water to sediment flux ratios

quantitative metrics that describe river deltas and their channel networks 1 densely populated river deltas are losing land at an alarming rate and to successfully restore these environments we must understand the details of their morphology toward this end we present a set of five metrics that describe delta morphology 1 the fractal dimension 2 the distribution of island sizes 3 the nearest edge distance 4 a synthetic distribution of sediment fluxes at the shoreline and 5 the nourishment area the nearest edge distance is the shortest distance to channelized or unchannelized water from a given location on the delta and is analogous to the inverse of drainage density in tributary networks the nourishment area is the downstream delta area supplied by the sediment coming through a given channel cross section and is analogous to catchment area in tributary networks as a first step we apply these metrics to four relatively simple fluvially dominated delta networks for all these deltas the average nearest edge distances are remarkably constant moving down delta suggesting that the network organizes itself to maintain a consistent distance to the nearest channel nourishment area distributions can be predicted from a river mouth bar model of delta growth and also scale with the width of the channel and with the length of the longest channel analogous to hack s law for drainage basins the four delta channel networks are fractal but power laws and scale invariance appear to be less pervasive than in tributary networks thus deltas may occupy an advantageous middle ground between complete similarity and complete dissimilarity where morphologic differences indicate different behavior

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

a model of water and sediment balance as determinants of relative sea level rise in contemporary and future deltas abstract modern deltas are dependent on human mediated freshwater and sediment fluxes changes to these fluxes impact delta biogeophysical functioning and affect the long term sustainability of these landscapes for human and for natural systems here we present contemporary estimates of long term mean sediment balance and relative sea level rise across 46 global deltas we model scenarios of contemporary and future water resource management schemes and hydropower infrastructure in upstream river basins to explore how changing sediment fluxes impact relative sea level rise in delta systems model results show that contemporary sediment fluxes anthropogenic drivers of land subsidence and sea level rise result in delta relative sea level rise rates that average 6 8 mm y assessment of impacts of planned and under construction dams on relative sea level rise rates suggests increases on the order of 1 mm y in deltas with new upstream construction sediment fluxes are estimated to decrease by up to 60 in the danube and 21 in the ganges brahmaputra meghna if all currently planned dams are constructed reduced sediment retention on deltas caused by increased river channelization and management has a larger impact increasing relative sea level rise on average by nearly 2 mm y long term delta sustainability requires a more complete understanding of how geophysical and anthropogenic change impact delta geomorphology local and regional strategies for sustainable delta management that focus on local and regional drivers of change especially groundwater and hydrocarbon extraction and upstream dam construction can be highly impactful even in the context of global climate induced sea level rise

how much subsidence is enough to change the morphology of river deltas understanding the effect of subsidence on fluviodeltaic morphology is important not only to maintain sustainable coastal cities and habitats but also to interpret the information contained in the stratigraphic record while tectonic steering in alluvial environments has been investigated similar studies in fluviodeltaic environments are limited to physical experiments and field observations we perform numerical experiments with a parcel based cellular model to analyze the deltaic surface and subsurface responses to regional subsidence we quantify model results using robust metrics and show that while sediment partitioning and shoreline pattern vary gradually with increasing subsidence rate channel mobility and stratigraphic connectivity of channel deposits show a threshold transition conditions for this transition are captured with a dimensionless filling index defined as the ratio between the rates of accommodation creation and sediment supply a channel locking mechanism activates when exceeds 0 6 and is responsible for the threshold transition

piping coarse grained sediment to a deep water fan through a shelf edge delta bypass channel tank experiments 1 it is now generally accepted that deltas that prograde to the shelf edge are able to transport coarse sediment to deep water either with or without sea level changes however it is still unclear how feeder rivers behave differently in the shelf edge delta case to rivers found in a delta that progrades over the shelf a series of nine shelf edge delta experiments are presented to investigate the lateral mobility of the feeder channel at the shelf edge and the associated deep water depositional system under a range of sediment supply rates and shelf front depths in the experiments constant sediment supply from an upstream point source under static sea level led the fluviodeltaic system to prograde over the shallow shelf surface and advance beyond the shelf edge into deep water the feeder river of the fluviodeltaic system became a bypass system once the toe of the delta front reached the shelf edge after the delta front was perched at the shelf edge a submarine fan developed in deep water although remaining disconnected from the delta in this bypass stage no regional avulsion or lateral migration of the feeder river occurred and all sediment from the upstream source bypassed the river delta front and shelf front slope the duration of the bypass stage is proportional to shelf front depth and inversely proportional to sediment discharge the combined duration of the shelf transit phase of the fluviodeltaic system and the bypass phase is the characteristic time scale for the continental margin to anneal transgression inducing perturbation due to high frequency and or high amplitude relative sea level rise the sequential evolution in the experiment compares favorably to the eocene sobrarbe formation a shelf edge delta in spain although natural variations are noted this comparison justifies the application of concepts proposed herein to natural systems and provides insight into interpreting processes from ancient shelf edge delta systems

dynamical microphysical evolution of a convective storm in a weakly sheared environment part i microphysical observations and interpretation abstract aircraft measurements of microphysical thermodynamic and vertical air motion properties supplemented by radar measurements of reflectivity structure are used to investigate precipitation development throughout much of the life cycle of a moderately intense convective storm in northeast colorado there was considerable variability of cloud properties such as updraft speed at scales of a few hundred meters early in the life of the storm greater organization was evident in the later more mature stage the earliest radar return from a major cell came from particles larger than 1 mm diameter in concentrations less than 10 m 3 it is suggested that these particles must have already followed complicated growth trajectories even at this early stage of the storm including more than one ascent in updraft in the more mature stage of the storm millimetric water drops and partially melted and refreezing ice particles were observed at 1 to 2 km above cloud base in both mixed and unmixed regions of the mai

regionalization of fire regimes in the central rocky mountains usa article i nfo fire is one of the most important natural disturbances in the coniferous forests of the us rocky mountains the rocky mountains are separated by a climatic boundary between 40 and 45 n which we refer to as the central rocky mountains crm to determine whether the fire regime from the crm was more similar to the northern rocky mountains nrm or southern rocky mountains srm during the holocene a 12 539 yr old sediment core from long lake wyoming located in the crm was analyzed for charcoal and pollen these data were then compared to charcoal records from the crm nrm and srm during the younger dryas chronozone the fire regime was characterized as frequent at long lake the early and middle holocene fire regime was charac terized as infrequent a brief interval from 4000 to 3000 cal yr bp termed the populus period had a frequent fire regime and remained frequent through the late holocene at long lake in comparison to sites from the nrm and srm the fire regime at long lake was most similar to the srm during the past 12 539 cal yr bp these results suggest the disturbance regime in the crm has a greater affinity with those of the srm

current subsidence rates due to compaction of holocene sediments in southern louisiana 1 relative contributions of geologic and anthropogenic processes to subsidence of southern louisiana are vigorously debated of these shallow sediment compaction is often considered dominant although this has never been directly observed or effectively demonstrated quantitative understanding of subsidence is important for predicting relative sea level rise storm surge flooding due to hurricanes and for successful wetland restoration despite many shallow borings few appropriate stratigraphic and geotechnical data are available for site specific calculations we overcome this by determining present compaction rates from monte carlo simulations of the incremental sedimentation and compaction of stratigraphies typical of the holocene of southern louisiana this approach generates distributions of present compaction rates that are not expected to exceed 5 mm yr but may locally locations with present subsidence rates greater than the predicted maximum probable shallow compaction rates are likely influenced by additional processes

out of the tropics the pacific great basin lakes and late pleistocene water cycle in the western united states the water cycle in the western united states changed dramatically over glacial cycles in the past 20 000 years higher precipitation caused desert lakes to form which have since dried out higher glacial precipitation has been hypothesized to result from a southward shift of pacific winter storm tracks we compared pacific ocean data to lake levels from the interior west and found that great basin lake high stands are older than coastal wet periods at the same latitude westerly storms were not the source of high precipitation instead air masses from the tropical pacific were transported northward bringing more precipitation into the great basin when coastal california was still dry the changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle

temporal density of pollen sampling affects age determination of the mid holocene hemlock tsuga decline abstract in establishing the age of a past event emphasis has been placed on dating accuracy precision calibration and age model development while the importance of adequate sampling and accurate reconstruction of the event itself is often neglected here we evaluate effects of temporal sampling density on stratigraphic and hence chronological placement of an event horizon using the mid holocene hemlock decline as an example results from simulations on theoretical and actual high resolution pollen sequences suggest that the pollen sampling density has a critical impact on the stratigraphic placement of the hemlock decline horizon and in turn on age estimation of the event also decline age frequency distributions from multiple sites were affected by sampling density therefore the hemlock decline should be used with caution as a precise chronostratigraphic marker and the current dataset of hemlock decline records may not be sufficient to analyze the spatiotemporal pattern of the hemlock decline finally we propose criteria for obtaining adequately sampled records in studying an event and estimated the adequate sampling density for age estimation of the mid holocene hemlock decline

holocene fire and vegetation along environmental gradients in the northern rocky mountains abstract holocene records of fire vegetation and climate were reconstructed from four sites in the bitterroot range region of the northern rocky mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by whitlock and bartlein 1993 regarding the effects of increased summer insolation on precipitation patterns vegetation history in the series of sites was broadly similar in the late glacial period the pollen data suggest open parkland dominated by picea or alpine meadow which reflect conditions cooler and drier than present these open forests were replaced in the early to middle holocene by forests composed mainly of pinus and pseudotsuga which suggest conditions warmer than present modern forest compositions were in place by ca 3000 cal yr bp and small variations in the timing of the vegetation shifts reflect local differences among sites the long term trends in fire occurrence support the hypothesis proposed by whitlock and bartlein 1993 that precipitation regimes were sharpened during the early holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints sites located in areas currently summer dry were drier than present during the early holocene and fires were more frequent conversely sites located in the areas that are summer wet at present were wetter than present in the early holocene and fires were less frequent on millennial time scales it appears that the climate boundary is controlled by topography and does not shift

a high resolution record of climate vegetation and fire in the mixed conifer forest of northern colorado usa high resolution pollen charcoal 13c total organic carbon toc and magnetic susceptibility data from sediment cores from a montane lake in northern colorado record variations in vegetation fire history and sedimentation since 14 5 ka 1 ka 1000 cal yr b p this record shows warm conditions during the bolling allerod and the coldest conditions in this area during the younger dryas event warming occurred throughout the early and middle holocene lasting until ca 5 ka when the warmest and wettest summer conditions were recorded progressive climate cooling and enhanced winter precipitation are then observed until present day these long term climatic trends correlate to changes in summer insolation charcoal accumulation rates char increased along with the arboreal vegetation from minima in the late glacial period to maxima during the early and middle holocene suggesting that charcoal influx was also controlled by climate and vegetation toc and 13c show a progressive increase and a decrease trend during the late pleistocene and holocene respectively related to changes in vegetation and productivity in the lake major peaks in the char record correspond with peaks in magnetic susceptibility indicating enhanced fire induced erosion and sedimentation millennial and centennial scale changes are also observed throughout the different proxy records they exhibit strong correlations with climate records from distant regions including greenland and the north atlantic providing evidence for global teleconnections among regional climates a solar climate connection is suggested by prominent ca 225 and 390 yr cycles which may correlate with the 208 yr suess and 400 yr solar cycles

determinants of fire activity during the last 3500 yr at a wildland urban interface alberta canada abstract long term records of wildfires and their controlling factors are important sources of information for informing land management practices here dendrochronology and lake sediment analyses are used to develop a 3500 yr fire and vegetation history for a montane forest in jasper national park alberta canada the tree ring record ad 1771 2012 indicates that this region historically experienced a mixed severity fire regime and that effective fire suppression excluded widespread fire events from the study area during the 20th century a sediment core collected from little trefoil lake located near the jasper townsite is analyzed for subfossil pollen and macroscopic charcoal 150 m when comparing the tree ring record to the 3500 yr record of sediment derived fire events only high severity fires are represented in the charcoal record comparisons between the charcoal record and historical climate and pollen data indicate that climate and vegetation composition have been important controls on the fire regime for most of the last 3500 yr although fire frequency is presently within the historical range of variability the fire return interval of the last 150 yr is longer than expected given modern climate and vegetation conditions indicating that humans have become the main control on fire activity around little trefoil lake

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

north south precipitation patterns in western north america on interannual to decadal timescales the overall amount of precipitation deposited along the west coast and western cordillera of north america from 258 to 558n varies from year to year and superimposed on this domain average variability are varying north south contrasts on timescales from at least interannual to interdecadal in order to better understand the north south precipitation contrasts their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large scale climatic patterns spatial empirical orthogonal functions eofs and spatial moments domain average central latitude and latitudinal spread of zonally averaged precipitation anomalies along the westernmost parts of north america are analyzed and each is correlated with global sea level pressure slp and sea surface temperature series on interannual defined here as 3 7 yr and decadal 7 yr timescales the interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much p

the 29 june 2000 supercell observed during steps part i kinematics and microphysics this is a two part study that addresses the kinematic microphysical and electrical aspects of a severe storm that occurred in western kansas on 29 june 2000 observed during the severe thunderstorm electrification and precipitation study steps field campaign in this first part polarimetric and doppler radar data are used along with a simple particle growth model to examine the evolution of the kinematic and microphysical properties of the storm from its earliest developing phase through its mature and dissipating phases during its severe stage the storm exhibited frequent positive cloud to ground lightning strikes very large 5 cm hail and a tornado doppler derived winds radar reflectivity and hydrometeor classifications from the polarimetric data over a nearly 4 h period are presented it is shown that updraft velocity and vertical vorticity had to reach magnitudes of at least 10 m s 1 and 10 2 s 1 and occupy major portions of the storm before it could produce most of the observed severe storm characteristics furthermore the establishment of cyclonic horizontal flow around the right flank of the updraft core was essential for hail production most of the largest hail grew from near millimeter sized particles that originated in the mid to upper level stagnation region that resulted from obstacle like flow of environmental air around the divergent outflow from the upper part of the updraft these recycling embryonic particles descended around the right flank of the updraft core and reentered the updraft intermingling with other smaller particles that had grown from cloud base along the main low level updraft stream

a multiproxy record of postglacial climate variability from a shallowing 12 m deep sub alpine bog in the southeastern san juan mountains of colorado usa pollen assemblages diatom assemblages and sedimentology from cumbres bog in the southeastern san juan mountains of colorado provide a record of climate and environmental change since the end of the last glacial maximum lgm cumbres bog is unusually deep basal sediments extend 12 m below the surface for its altitude 3050 m a s l and we extracted 7 m core of continuous sediment below 5 m of water and peat the resulting record provides strong evidence of a period of warming immediately after the lgm 18 13 cal kyr bp a cool interval coinciding with the younger dryas 12 8 11 5 cal kyr bp a warm stable period from 10 to 6 cal kyr bp and a cooler and highly variable climate interval after 6 cal kyr bp more specifically pollen ratios and fossil diatoms indicate that cold periods generally match with previously identified periods of rapid climate change that occurred at 10 6 8 7 7 9 7 0 6 9 5 4 5 2 3 3 3 0 2 3 2 0 and 1 5 cal kyr bp this record also adds resolution to previous

holocene vegetation fire and climate history of the sawtooth range central idaho usa the paucity of low and middle elevation paleoecologic records in the northern rocky mountains limits our ability to assess current environmental change in light of past conditions a 10 500 yr long vegetation fire and climate history from lower decker lake in the sawtooth range provides information from a new region initial forests dominated by pine and douglas fir were replaced by open douglas fir forest at 8420 cal yr bp marking the onset of warmer conditions than present presence of closed douglas fir forest between 6000 and 2650 cal yr bp suggests heightened summer drought in the middle holocene closed lodgepole pine forest developed at 2650 cal yr bp and fires became more frequent after 1450 cal yr bp this shift from douglas fir to lodgepole pine forest was probably facilitated by a combination of cooler summers cold winters and more severe fires than before five drought episodes including those at 8200 cal yr bp and during the medieval climate anomaly were registered by brief intervals of lodgepole pine decline an increase in fire activity and mistletoe infestation the importance of a holocene perspective when assessing the historical range of variability is illustrated by the striking difference between the modern forest and that which existed 3000 yr ago

stratigraphic charcoal analysis on petrographic thin sections application to fire history in northwestern minnesota results of stratigraphic charcoal analysis from thin sections of varved lake sediments have been compared with fire scars on red pine trees in northwestern minnesota to determine if charcoal data accurately reflect fire regimes pollen and opaque spherule analyses were completed from a short core to confirm that laminations were annual over the last 350 yr a good correspondence was found between fossil charcoal and fire scar data individual fires could be identified as specific peaks in the charcoal curves and times of reduced fire frequency were reflected in the charcoal data charcoal was absent during the fire suppression era from 1920 a d to the present distinct charcoal maxima from 1864 to 1920 occurred at times of fire within the lake catchment fire was less frequent during the 19th century and charcoal was substantially less abundant fire was frequent from 1760 to 1815 and charcoal was abundant continuously fire scars and fossil charcoal indicate that fires did not occur during 1730 1750 and 1670 1700 several fires occurred from 1640 to 1670 and 1700 to 1730 charcoal counted from pollen preparations in the area generally do not show this changing fire regime simulated sampling of the thin section data in a fashion comparable to pollen slide methods suggests that sampling alone is not sufficient to account for differences between the two methods integrating annual charcoal values in this fashion still produced much higher resolution than the pollen slide method and the postfire suppression decline of charcoal characteristic of my method but not of pollen slides is still evident consideration of the differences in size of fragments counted by the two methods is necessary to explain charcoal representation in lake sediments

holocene climate fire vegetation interactions at a subalpine watershed in southeastern british columbia canada abstract vegetation assemblages and associated disturbance regimes are spatially heterogeneous in mountain ecosystems throughout the world due to the complex terrain and strong environmental gradients given this complexity numerous sites describing postglacial vegetation and fire histories are needed to adequately understand forest development and ecosystem responses to varying climate and disturbance regimes to gain insight into long term historical climate fire vegetation interactions in southeastern british columbia canada sedimentological and paleoecological analyses were performed on a sediment core recovered from a small subalpine lake the pollen assemblages stomata and macroremains indicate that from 9500 to 7500 cal yr bp pinus dominated forests occurred within the catchment and alnus was also present climate was an important control of fire and fire frequency was highest at this time peaking at 8 fires 1000 yr 1 yet charcoal accumulation rates were low indicative of low terrestrial biomass abundance from 7500 to 4600 cal yr bp pinus decreased as picea abies and larix increased and fire frequencies decreased to 3 6 fires 1000 yr 1 since 7500 cal yr bp the fire regime varied at a millennial scale driven by forest biomass abundance and fuel accumulation changes local scale bottom up controls of fire increased in relative importance since at least 6000 cal yr bp

vegetation fire and climate history of the northwestern great basin during the last 14 000 years the northwestern great basin lies in the transition zone between the mesic pacific northwest and xeric intermountain west the paleoenvironmental history based on pollen macroscopic charcoal and plant macrofossils from three sites in the northwestern great basin was examined to understand the relationships among the modern vegetation fire disturbance and climate the vegetation history suggests that steppe and open forest communities were present at high elevations from ca 11 000 to 7000 cal yr bp and were replaced by forests composed of white fir western white pine and whitebark pine in the late holocene over the last 11 000 years fires were more frequent in mid elevation forests 10 25 fire episodes 1000 years and rare in high elevation forests 2 5 fire episodes 1000 years applying modern pollen climate relationships to the fossil pollen spectra provided a means to interpret past climate changes in this region in the past 9000 years summer temperatures decreased from 1 to 41c and annual precipitation has increased 7 15 these results indicate that the millennial scale climate forcing driving vegetation changes can be quantified within the intermountain west in general and northwestern great basin in particular in addition fire can be considered an important component of these ecosystems but it does not appear to be a forcing mechanism for vegetation change at the resolution of these records r 2007 elsevier ltd all rights reserved

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

a record of lateglacial holocene environmental change from a high elevation site in the intermountain west usa records of past vegetation and fire history can be complicated by changes in the depositional environment of a sampling location however these changes can alternatively be used as a measure of climate variability our study site ca 18 0 cal ka bp record from little brooklyn lake wyoming located near the crest of the snowy range records three moisture states initially the lake was likely a glacier fed pond indicated by the presence of pediastrum algae colonies around 13 0 cal ka bp this pond transitioned to a meadow environment suggested by the loss of pediastrum algae colonies and slow sedimentation rates meadow conditions were maintained until ca 5 0 cal ka bp when pediastrum algae colony abundance increased indicating the formation of a shallow lake from 18 0 to ca 5 0 cal ka bp the pollen record is suggestive of alpine vegetation conditions with relatively high spruce and herbaceous taxa low charcoal influx also characterizes the period between 18 0 and 5 0 cal ka bp after 5 0 cal ka bp the coincidence of the formation of shallow lake and pollen data indicating a shift to a spruce and fir forest suggests an increase in effective moisture fire remained rare in this basin over the entire record however once the lake established sedimentation rates and charcoal influx increased copyright 2013 john wiley sons ltd

spatial variations of holocene climatic change in the yellowstone region abstract the paleoecologic record of the northern rocky mountains shows considerable spatial variability which has been difficult to explain as a unified response to large scale changes in effective moisture or temperature through time comparison of past and present climatic patterns suggests that the spatial heterogeneity in the fossil record arises from a local response to the effects of greater summer solar radiation during the early holocene solar radiation directly determines evapotranspiration and indirectly controls the strength of the summer monsoon and the eastern pacific subtropical high pressure system the yellowstone region provides an example of an area where the indirect effects of solar radiation are manifest by the juxtaposition of two contrasting climatic regimes today and where these regimes were amplified during the early holocene fossil pollen data suggest that central and southern yellowstone the summer dry area became drier in the early holocene while northern yellowstone the summer wet area became wetter the boundary between regimes however is controlled by topography and has not changed significantly in the holocene the spatial variations of holocene vegetation and climatic changes in the northern rocky mountains may therefore be explained as a trade off between strengthened monsoonal circulation and a stronger subtropical high in the early holocene and the relative importance of these precipitation regimes on a topographically complex landscape

estimation of sediment compaction and its relationship with river channel distributions in the yellow river delta china abstract the geomechanical properties of the stratigraphic sequences strongly influence the land surface subsidence as the latter can be caused by the sediment compaction depositional units of transitional environments such as coastal plain and deltas are characterized by a high lithological variability due to the history and complexity of the sedimentary processes therefore detailed modeling of the stratigraphic architecture is necessary to better understand the land surface subsidence process and accurately estimate its rates the frequent avulsion and rearrangement of the channel network in the modern yellow river delta since 1855 has led to the rapid accumulation and deposition of large amounts of sediment in the delta plain the compaction of the substratum sediment due to the new sediment load and the consolidation of younger deposits caused land surface subsidence we used 108 sediment core data covering the delta and obtained detailed stratigraphic structural information a detailed distribution of clay and soft clayey soils which are the primary contributors to land subsidence was identified furthermore the compaction rates and the consolidation at time of the subaerial area and neritic area for the entire modern yellow river delta were estimated we also analyzed the characteristics of present land subsidence by combining a ground movement dataset obtained by high resolution topographic surveys with synthetic aperture radar sar interferometry insar of satellite images the results show that during the early stages of the development of the active deltaic lobe the compaction of neritic sediments was the primary contributor to initial subsidence furthermore combined with the relevant sedimentary age and environment determined by previous studies we found that the spatial variation in compaction induced subsidence in the subaerial deltaic area over time is closely related to the historical distributions of the river channels in the modern yellow river delta our findings providing a better characterization of the spatial and time heterogeneity of the land surface subsidence of both subaerial and submarine areas significantly improved the previous knowledge on land subsidence of the yellow river delta our results also demonstrate that the subaerial delta becomes increasingly stable over time with the gradual decrease in the average subsidence rate of each deltaic lobe

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

experimental steep braided flow application to flooding risk on fans flooding processes occurring on alluvial fans are considerably different from those occurring along single thread rivers with well defined floodplains active erosion rapid sedimentation and the uncertainty in flow path make the prediction of flood evolution and extent difficult based on a large scale experiment this study investigates the long term evolution of the flow on a steep noncohesive sediment surface resembling a complex of merged alluvial fans the results are pertinent to the assessment of flooding hazard on alluvial fans at any given time the average flow occupancy was 21 of the surface however the flow was characterized by active channel switching and overflow processes the percentage of the surface remaining dry was found to decay harmonically with time a reworking time was defined as the time at which half of the surface that was initially dry remained dry whereas the other half was inundated at least once an empirical expression was developed in which reworking time is proportional to the average cross sectional area of flow and inversely proportional to the sediment supply

experimental test of tectonic controls on three dimensional alluvial facies architecture abstract in an extended series of papers by m leeder j r l allen j s bridge j alexander and others the depositional stacking patterns of channel belt deposits were related to tectonic and other external controls we refer to these models collectively as lab models these models established a basic connection between subsidence channel avulsion and the mesoscale channel belt architecture of an alluvial deposit two fundamental predictions resulted from this work 1 channel belt stacking density and hence connectedness is inversely correlated to temporal vertical changes in sedimentation rate and 2 channel belt stacking density and hence connectedness is directly correlated to lateral changes in sedimentation rate in this paper we make use of the experimental earthscape xes facility at the st anthony falls laboratory to examine the effects of temporally and laterally variable subsidence rates on mesoscale alluvial architecture we then compare the experimental results with the main predictions of the early lab models regarding the predictions of the lab models under conditions of high sediment supply and a highly active alluvial system lateral and downstream variation in subsidence geometry and rate have little effect on the details of alluvial architecture we found that the principal architectural signature of changes in subsidence geometry i e laterally variable subsidence rate is stratal tilting and that channel belts are not attracted to the subsidence maximum we hypothesize that the effect of variable subsidence geometry is felt by the fluvial system only if subsidence proceeds faster than the river can adjust to the formation of a topographic low via the deposition of overbank material in the form of splays and sheet sands our results lend impetus to the development of more complete three dimensional numerical models of mesoscale alluvial architecture that couple architecture to broader allogenic forcing mechanisms