the role of feedback mechanisms in historic channel changes of the lower rio grande in the big bend region abstract over the last century large scale water development of the upper rio grande in the u s and mexico and of the rio conchos in mexico has resulted in progressive channel narrowing of the lower rio grande in the big bend region we used methods operating at multiple spatial and temporal scales to analyze the rate magnitude and processes responsible for channel narrowing these methods included hydrologic analysis of historic stream gage data analysis of notes of measured discharges historic oblique and aerial photograph analysis and stratigraphic and dendrogeomorphic analysis of inset floodplain deposits our analyses indicate that frequent large floods between 1900 and the mid 1940s acted as a negative feedback mechanism and maintained a wide sandy multi threaded river declines in mean and peak flow in the mid 1940s resulted in progressive channel narrowing channel narrowing has been temporarily interrupted by occasional large floods that widened the channel however channel narrowing has always resumed after large floods in 1990 and 1991 the active channel width of the lower rio grande has narrowed by 36 52 narrowing has occurred by the vertical accretion of fine grained deposits on top of sand and gravel bars inset within natural levees channel narrowing by vertical accretion occurred simultaneously with a rapid invasion of non native riparian vegetation tamarix spp arundo donax which created a positive feedback and exacerbated the processes of channel narrowing and vertical accretion in two floodplain trenches we measured 2 75 and 3 5 m of vertical accretion between 1993 and 2008 in some localities nearly 90 of bare active channel bars were converted to vegetated floodplain during the same period upward shifts of stage discharge relations occurred resulting in over bank flooding at lower discharges and continued vertical accretion despite a progressive reduction in stream flow thus although the magnitude of the average annual flood was reduced between 40 and 50 over bank flooding continued these changes reflect a shift in the geomorphic nature of the rio grande from a wide laterally unstable multi thread river to a laterally stable single thread channel with cohesive vertical banks and few active in channel bars

present day stress field rotations associated with evaporites in the offshore nile delta evaporitic horizons are routinely interpreted to act as mechanical detachment sequences and thus signifi cantly infl uence the structural evolution of sedimentary basins and fold thrust belts however over 30 years of global in situ stress analysis have provided only poor evidence to support this widespread assumption this study examines present day stress orientations inferred from borehole breakout and drilling induced fractures in 44 boreholes in the offshore nile delta a total of 446 breakouts and 19 drilling induced fractures from 37 wells reveal sharply contrasting present day maximum horizontal stress shmax orientations in sequences above and below the extensive messinian evaporites of the eastern nile delta a typical deltaic margin parallel shmax e w in the central and ese wnw in the eastern nile delta is observed in parts of the nile delta that are below or do not contain evaporites however a scattered but largely margin normal nne ssw shmax is observed in sequences underlain by evaporites the 90 variation in present day shmax orientation above and below the messinian salts provides the fi rst convincing in situ evidence that evaporite sequences can act as major mechanical detachment horizons in addition the margin normal shmax orientation is subperpendicular to the strike of nearby active extensional faults indicating the existence of non andersonian faulting in the suprasalt region furthermore the evidence that the messinian evaporites act as an effective mechanical detachment suggests that suprasalt faulting in the eastern nile delta is not the result of basement related deformation and thus raises doubts about the often postulated extension of the suez fault zone into the eastern mediterranean

bank accretion and the development of vegetated depositional surfaces along modified alluvial channels this paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized west tennessee streams most perennial streams in west tennessee were straightened and dredged since the turn of the century patterns of fluvial ecological responses to channelization have previously been described by a six stage model dendrogeomorphic tree ring techniques allowed the determination of location timing amount and rate of bank sediment deposition channel cross sections and ecological analyses made at 101 locations along 12 streams encompassing bends and straight reaches show that channel and bank processes initially react vertically to channelization through downcutting a depositional surface forms on banks once bed degradation and heightened bank mass wasting processes have eased or slowed the formation of this depositional surface marks the beginning of bank recovery from channelization dominating lateral processes characteristic of stable or natural channels return during the formation and expansion of the depositional surface suggesting a relation with thalweg deflection point bar development and meanderloop extension characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks the depositional surface initially forms low on the bank and tends to maintain a slope of about 24 mean accretion rates ranges from 5 9 cm yr on inside bends to 0 cm yr on most outside bends straight reaches have a mean accretion rate of 4 2 cm yr the relatively stable convex upward depositional surface expands and ultimately attaches to the flood plain the time required for the recovery process to reach equilibrium averaged about 50 years indicative pioneer speccies of woody riparian vegetation include black willow river birch silver maple and boxelder stem densities generally decrease with time after and initial flush of about 160 stems per 100 m2 together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation

a combined analytic signal and euler method an eul for automatic interpretation of magnetic data we present a new automatic method of interpretation of magnetic data called an eul pronounced an oil the derivation is based on a combination of the analytic signal and the euler deconvolution methods with an eul both the location and the approximate geometry of a magnetic source can be deduced the method is tested using theoretical simulations with different magnetic models placed at different depths with respect to the observation height in all cases the method estimated the locations and the approximate geometries of the sources the method is tested further using ground magnetic data acquired above a shallow geological dike whose source parameters are known from drill logs and also from airborne magnetic data measured over a known ferrometallic object in both these cases the method correctly estimated the locations and the nature of these sources

climate and ephemeral stream processes twentieth century geomorphology and alluvial stratigraphy of the little colorado river arizona during the first 40 years of the twentieth century erosion was the dominant geomorphic process affecting the morphology of the little colorado river channel the discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation in the 1940s and early 1950s average annual precipitation declined reducing annual discharge to about 57 of that of the preceding period as well as reducing the frequency of large floods the channel adjusted to the new hydrologic regimen by reducing its width parts of the channel were frequently dry and riparian vegetation primarily nonnative salt cedar became established on the higher channel surfaces precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process as indicated by accretion of 2 to 5 m of flood plain alluvium between 1952 and 1978 events of 1980 however suggest that the flood plain has ceased to accrete although climate has not fluctuated the flood plain has probably reached a critical height above the channel beyond which further accretion is unlikely under the existing discharge regimen the recent history of the little colorado broadly suggests that flood plain development was initiated by climatically induced hydrologic fluctuations flood plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions

use of radiometric cs 137 pb 210 geomorphic and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage grants uranium region new mexico in the absence of historic geochemical baseline data for the grants uranium region environmental changes resulting from uranium mine mill activities can be determined only by indirect methods a methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic stratigraphic and radiometric dating techniques because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources sample sites which contain fine grained deposits that both predate and postdate mine mill activity were located in abandoned channel segments oxbows of major streams draining the eastern grants uranium region aerial photographs and derivative maps taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned channel segments pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide cs 137 and the natural radionuclide pb 210 as well as other u 238 and th 232 daughters because of the dynamic nature of the system absolute dating with cs 137 was not possible but samples could be dated as either pre or post 1950 the 1950 date is important because it marked the beginning of the uranium exploitation in the region the pb 210 dating was not possible because background pb 210 was very high relative to fallout pb 210

accuracy of post bomb 137cs and 14c in dating fluvial deposits abstract the accuracy and precision of 137cs and 14c for dating post 1950 alluvial deposits were evaluated for deposits from known floods on two rivers in arizona the presence of 137cs reliably indicates that deposition occurred after intensive above ground nuclear testing was initiated around 1950 there was a positive correlation between the measured level of 137cs activity and the clay content of the sediments although 137cs was detected even in sandy flood sediments with low clay content 137cs is a valuable dating tool in arid environments where organic materials for 14c or tree ring dating are scarce and observational records are limited the 14c activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit and the accuracy was species dependent however undifferentiated mixtures of fine organic materials from several post bomb deposits of various ages repeatedly yielded dates between 1958 and 1962 and detrital charcoal yielded a date range of 1676 1939 in semiarid environments the residence time of most types of organic debris precludes accurate annual resolution of post bomb 14c dates

processes of arroyo filling in northern new mexico usa we documented arroyo evolution at the tree trench and arroyo scales along the lower rio puerco and chaco wash in northern new mexico usa we excavated 29 buried living woody plants and used burial signatures in their annual rings to date stratigraphy in four trenches across the arroyos then we reconstructed the history of arroyo evolution by combining trench data with arroyo scale information from aerial imagery light detection and ranging lidar longitudinal profiles and repeat surveys of cross sections burial signatures in annual rings of salt cedar and willow dated sedimentary beds greater than 30 cm thick with annual precision along both arroyos incision occurred until the 1930s in association with extreme high flows and subsequent filling involved vegetation development channel narrowing increased sinuosity and finally vertical aggradation a strongly depositional sediment transport regime interacted with floodplain shrubs to produce a characteristic narrow trapezoidal channel the 55 km study reach along the rio puerco demonstrated upstream progression of arroyo widening and filling but not of arroyo incision channel narrowing or floodplain vegetation development we conclude that the occurrence of upstream progression within large basins like the rio puerco makes precise synchrony across basins impossible arroyo wall retreat is now mostly limited to locations where meanders impinge on the arroyo wall forming hairpin bends for which entry to and exit from the wall are stationary average annual sediment storage within the rio puerco study reach between 1955 and 2005 was 4 8 10 5 t yr 16 of the average annual suspended sediment yield and 24 of the long term bedrock denudation rate at this rate the arroyo would fill in 310 yr

dendrogeomorphology in badlands methods case studies and prospects soil and vegetation are interacting factors controlling erosion soil degradation processes may affect the normal tree and shrub development and inversely vegetation can modulate the velocity and intensity of soil development or denudation a dendrogeomorphological approach can be used to study these interactions allowing to obtain a date and estimate mean or specific erosion rates this is especially useful in an unrecorded badlands and gullied environments where the scarce vegetation may be the only proxy available to quantify the different geomorphic processes which have occurred this paper provides a fundamental review of the dendrogeomorphological methodology applied to erosion measurement in badlands focusing on the response of the vegetation to the geomorphic processes this paper a describes the methodology developed to estimate erosion rates with exposed roots b shows new advances through case studies and finally c discusses future lines of research to reduce methodological uncertainties and for making dendrogeomorphology more widely applicable

dating of geomorphological processes using dendrogeomorphological methods abstract the object of this study was to develop methods for investigating recent geomorphodynamics of aggradation processes by dendrogeomorphological analyses of the stems and the roots of partially buried spruces every accumulation process causes growth reactions in the affected trees which is reflected in the growth rings analysis of the growth rings enables us to determine the time and the magnitude of burial yet the critical burial depth which when exceeded causes the death of the tree is 1 6 to 1 9 m the burial of a spruce stem causes the sprouting of adventitious root horizons a single event may cause simultaneous sprouting of up to six root horizons of which the uppermost horizon usually is the strongest one the dating of the onset of sprouting of the oldest adventitious roots among each root system enables one to establish absolute dates of debris flow events with an accuracy 0 to 3 years yet this method is very time consuming the simplest dating method proved to be the determination of burial events by the onset of a pronounced reduction of increment in the stem wood of the affected trees with only one core to be taken of each tree this method therefore allows a spatiotemporal reconstruction of aggradation processes the information recorded on the stem cross section of stem by the growth ring pattern lessens with increasing burial depth though

seismic challenges of developing the pre pliocene akhen field offshore nile delta bp s recent drilling results in the akhen gas field offshore egypt provide an excellent case study into the challenges of drilling and developing the pre pliocene in the nile delta the challenges are significant and include pliocene overpressure cells variable thickness and rugosity of miocene aged anhydrite deep and complex structural elements and seismic quality challenges however the reservoirs in the pre pliocene are known to contain very condensate rich gas with high reservoir pressures and significant exploration potential making this basin and this play a significant asset in bp s global portfolio

interpretation of magnetic data using analytic signal derivatives this paper develops an automatic method for interpretation of magnetic data using derivatives of the analytic signal a linear equation is derived to provide source location parameters of a 2d magnetic body without a priori information about the nature of the source then using the source location parameters the nature of the source can be ascertained the method has been tested using theoretical simulations with random noise for two 2d magnetic models placed at different depths with respect to the observation height in both cases the method gave a good estimate for the location and shape of the sources good results were obtained on two field data sets

a new best estimate methodology for determining magnetic parameters related to field anomalies produced by buried thin dikes and horizontal cylinder like structures a new best estimate methodology is proposed and oriented towards the determination of parameters related to a magnetic field anomaly produced by a simple geometric shaped model or body such as a thin dike and horizontal cylinder this approach is mainly based on solving a system of algebraic linear equations for estimating the three model parameters e g the depth to the top center of the body z the index parameter or the effective magnetization angle and the amplitude coefficient or the effective magnetization intensity k the utility and validity of this method is demonstrated by analyzing two synthetic magnetic anomalies using simulated data generated from a known model with different random errors components and a known statistical distribution this approach was also examined and applied to two real field magnetic anomalies from the united states and brazil the agreement between the results obtained by the proposed method and those obtained by other interpretation methods is good and comparable moreover the depth obtained by such an approach is found to be in high accordance with that obtained from drilling information the advantages of such a proposed method over other existing interpretative techniques are clarified where it can be generalized to be automatically applicable for interpreting other geological structures described by mathematical formulations

2 d magnetic interpretation using the vertical integral a magnetic interpretation method is presented which numerically computes the vertical integral of the observed total field anomaly along a profile this transformed field is fitted by the vertical integral also computed numerically of the total field anomaly of an interpretation model using a standard nonlinear least squares estimation routine the vertical integral transformation used here is similar to using a spatial domain pseudogravity operator after reducing the magnetic anomaly to the pole the difference is that the reduction to the pole is not needed so that the knowledge of the magnetization direction is not required this method estimates the depth to a weakly magnetized basement that is intruded by strongly magnetized rocks this method is best suited for the case where sills cut the overlying sediments and interfere with the magnetic signal from basement attempts to interpret isolated anomalies of stocks in the basement should take into account this kind of interference the proposed method attenuates the effect of the shallow sources and produces a better lateral resolution than the upward continuation filter synthetic examples show that in interpreting the total field anomaly the presence of shallow sources leads to underestimated depths to the basement by using the vertical integral of the total field anomaly on the other hand depth estimates closer to the true values are obtained the method is applied to aeromagnetic data from amazon basin brazil and produces estimates for the basement depths that are consistent with the geological knowledge and with previous magnetic interpretations

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

effects of riparian vegetation on topographic change during a large flood event rio puerco new mexico usa 1 the spatial distribution of riparian vegetation can strongly influence the geomorphic evolution of dryland rivers during large floods we present the results of an airborne lidar differencing study that quantifies the topographic change that occurred along a 12 km reach of the lower rio puerco new mexico during an extreme event in 2006 extensive erosion of the channel banks took place immediately upstream of the study area where tamarisk and sandbar willow had been removed within the densely vegetated study reach we measure a net volumetric change of 578 050 490 000 m3 with 88 3 of the total aggradation occurring along the floodplain and channel and 76 7 of the erosion focusing on the vertical valley walls the sediment derived from the devegetated reach deposited within the first 3 6 km of the study area with depth decaying exponentially with distance downstream elsewhere floodplain sediments were primarily sourced from the erosion of valley walls superimposed on this pattern are the effects of vegetation and valley morphology on sediment transport sediment thickness is seen to be uniform among sandbar willows and highly variable within tamarisk groves these reach scale patterns of sedimentation observed in the lidar differencing likely reflect complex interactions of vegetation flow and sediment at the scale of patches to individual plants

spatiotemporal analysis of channel wall erosion in ephemeral torrents using tree roots an example from the patagonian andes this paper presents absolute ages for fl ash fl oods and related sediment entrainment in headwater catchments to construct a spatiotemporal framework of process dynamics and locations of major areas of channel wall erosion the most reliable method for dating erosion is through dendrogeomorphic studies of exposed tree roots based on the analysis of erosion signals in root ring records we documented a time series of channel wall erosion and successfully dated 21 erosive fl ash fl ood events since a d 1870 in an ephemeral gully in the patagonian andes the study was performed with roots from austrocedrus chilensis nothofagus dombeyi and pseudotsuga menziesii results demonstrate the potential of root analyses for the determination of major areas of sediment entrainment in addition we show that the position of damage within individual root rings allows inferences about the seasonal timing of fl ash fl ood effects and thus an assessment of possible meteorological triggers of erosive events short intense storms occurring primarily in austral fall and late winter in this case the approach presented adds signifi cantly to the documentation of sediment entrainment and facilitates identifi cation of areas of rapid erosion in small remote headwater catchments with ephemeral fl ash fl ood activity

dendrogeomorphic reconstruction of little ice age paraglacial activity in the vicinity of the homathko icefield british columbia coast mountains canada article i nfo article history moraine and glacier dams bordering the homathko icefield in the southern british columbia coast mountains failed in the 1980s and 1990s causing catastrophic downstream floods the largest of the floods occurred in august 1997 and was caused by overtopping and rapid breaching of the moraine dam that impounds queen bess lake the floodwaters from queen bess lake eroded holocene age sedimentary deposits along the west fork of nostetuko river and caused a steep rise in the hydrograph of homathko river at the head of bute inlet 115 km downstream a field investigation of the eroded valley fill in 2008 revealed multiple paraglacial valley fill units many of which are capped by in situ stumps and woody detritus dendrogeomorphological field techniques were employed to develop a chronology for the buried forests a regional tree ring chronology spanning the interval ce 1572 2007 was constructed from living subalpine fi r abies lasiocarpa trees at seven sites in the southern coast mountains in cases where subfossil stumps and boles predated the regional chronology relative death dates constrained by radiocarbon ages were assigned to floating chronologies by combining these dendrogeomorphological dating methods we identified six floodplain aggradation episodes within the past 1200 years comparison to local and regional glacial histories suggests that these events reflect climate induced little ice age changes in local glacier cover

effects of woody vegetation on overbank sand transport during a large flood rio puerco new mexico abstract distributions of woody vegetation on floodplain surfaces affect flood flow erosion and deposition processes a large flood along the lower rio puerco new mexico in august 2006 caused extensive erosion in a reach that had been sprayed with herbicide in september 2003 for the purpose of saltcedar tamarix spp control large volumes of sediment including a substantial fraction of sand were delivered to the reach downstream which had not been treated with herbicide we applied physically based one dimensional models of flow and suspended sediment transport to compute volume concentrations of sand in suspension in floodplain flow at a site within the sprayed reach and at a site downstream from the sprayed reach we computed the effects of drag on woody stems in reducing the skin friction shear stress velocity of flow and suspended sand transport from open paths into patches of dense stems total flow and suspended sand fluxes were computed for each site using well constrained flood flow depths water surface slopes and measured shrub characteristics results show that flow in open paths carried high concentrations of sand in suspension with nearly uniform vertical distributions drag on woody floodplain stems reduced skin friction shear stresses by two orders of magnitude yet sufficient velocities were maintained to transport sand more than 50 m into fields of dense free surface penetrating stems an increase in shrub canopy extent from 31 in the sprayed reach site to 49 in the downstream site was found to account for 69 of the computed decrease in discharge between the two sites the results demonstrate the need to compute the spatial distribution of skin friction shear stress in order to effectively compute suspended sand transport and to predict the fate of sediment and contaminants carried in suspension during large floods

modeling effects of bank friction and woody bank vegetation on channel flow and boundary shear stress in the rio puerco new mexico 1 we have applied a physically based model for steady horizontally uniform flow to calculate reach averaged velocity and boundary shear stress distributions in a natural stream with woody vegetation on the channel banks the model calculates explicitly the form drag on woody plant stems and includes the effects of vegetation on the boundary shear stress velocity and turbulence fields average channel shapes bed gradients and shrub characteristics were determined for four long internally similar reaches covering an 81 km segment of the lower rio puerco in north central new mexico hydraulic geometries for each of three flow events were determined from reach average channel shapes slopes and measured silt line heights above the bed model results show that friction on the lateral boundaries reduced the boundary shear stress in the center of the rio puerco channel by as much as 20 in reaches with moderate to dense bank shrubs perimeter averaged boundary shear stresses were reduced by almost 40 during near bankfull flows model calculated discharges for all three flow events indicate there was a loss of about 40 of the upstream discharge through the 81 km river segment infiltration rates estimated from discharge losses during the quasi steady peak flows and areas of the bed and banks suggest rates of flow loss into the silty sand of the upper banks are about 8 times greater than rates of loss into the clayey silt covered bed and lower banks

reservoir zonation rock typing and compartmentalization of the tortonian serravallian sequence temsah gas field offshore nile delta egypt abstract the present study aims to evaluate the petrophysical properties and to discriminate the tortonian wakar and the serravallian sidi salem formations in the offshore temsah gas field north egypt into distinctive rock types the measured conventional core data includes helium and summation fluids porosities grain and bulk densities horizontal and vertical permeabilities as well as the water saturation also the reservoir quality index rqi flow zone indicator fzi and reservoir potentiality index rpi were calculated to establish a successful reservoir zonation in the temsah field the tortonian serravallian sequence has been discriminated into four reservoir rock types rrts which are described based on its lithologic composition as 1 sandy to argillaceous carbonates rrt1 2 siliceous pebbly sandstones rrt2 and 3 argillaceous to calcareous sandstones rrt3 and 4 calcareous to dolomitic sandstones rrt4 based on the petrophysical interpretation and ranking for the studied samples samples of rrt2 have very good rank due to presence of very good porosity excellent permeability and good to very good rqi and fzi values which are assigned as the best reservoir rock type in the tortonian serravallian sequence the petrophysical behavior of rrt4 shows the lowest reservoir quality impervious rqi and poor fzi the examined sequence has been sliced into reservoir zones each zone has its hydraulic flow unit range that represents the studied rock types in each zone based on the applied pressure tools including repeat formation tester rft and the modular formation dynamic tester mdt both the wakar and sidi salem reservoirs represent completely isolated compartments therefore ranking both reservoirs and estimating their hydrocarbon potential must be done separately for each reservoir

logjam controls on channel floodplain interactions in wooded catchments and their role in the formation of multi channel patterns the role of wood in the formation and maintenance of complex dynamic floodplain surfaces is important and to date has received relatively limited attention compared to in channel habitat processes this paper explores the role of logjams as important agents of channel floodplain interaction we draw on a specific case study as well as examples from the literature to show that although the processes of interaction differ the resulting dynamic floodplain patchwork is a common feature of rivers with logjams in addition we contend that the presence of logjams is an important factor in the evolution and maintenance of multiple channel patterns in both montane and lowland river environments these observations have important implications for the definition of reference targets for river restoration the specific results of this research show 1 the presence of a range of types of multiple channel network dissecting the floodplains of low order channels that are strongly associated with the presence of logjams 2 the relatively rapid formation of floodplain channels following logjam formation 3 the dynamic nature of logjams within headwater channels on both seasonal and annual timescales that lead to a highly dynamic habitat mosaic on the floodplain surface 4 an increased frequency of overbank flooding and high rates of floodplain sediment accumulation upstream of logjams and along floodplain channel networks that create the complex topography observed in the case study forested floodplains

ontogeny of a flood plain the ontogeny of five flood plain segments is described for a period of 18 yr following a major flood in 1978 on the powder river in southeastern montana the flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood in cross section the flood plains resemble benches with well developed natural levees flood plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick resulting in some lateral but mostly vertical accretion annual and biannual measurements indicated that as the flood plain segments grew upward the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0 16 to 1 3 yr it is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood plain ontogeny these flood plains did not grow on migrating point bars and vertical accretion at least initially occurred within the channel rather than across the valley flat during extensive overbank flows sediments of these flood plains define narrow elongated stratigraphic units that border the active channel and onlap older flood plain deposits these characteristics are considerably different from those of many facies models for meandering river deposits facies similar to those described in this paper are likely to be preserved thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers

modeling of magnetic anomalies generated by simple geological structures through genetic price inversion algorithm abstract a new approach to the interpretation of magnetic anomalies generated by geological structures resembling simple geometrical bodies is presented the method is based on the genetic price hybrid algorithm gpa recently proposed by the authors for the inversion of potential field data specifically of self potential signals in this paper the effectiveness of the proposed algorithm is tested on magnetic data for retrieving the parameters of the anomaly causative sources first the testing is performed on synthetic noise free and noisy signals due to magnetized sphere dike and fault like models then the analysis is extended to field magnetic anomalies concerning the synthetic data analyses a very good agreement is obtained between assumed and retrieved source parameters specifically the error between true and inverted parameter sets was found to be no higher than 9 even by adding 15 of gaussian white random noise to the initial dataset as for the study of field data the values of depth horizontal position effective magnetization intensity and angle provided by the proposed gpa method compare well with those obtained by other interpretative approaches finally the results of the gpa application to the inversion of magnetic data measured in the mt somma vesuvius volcanic area are reported in particular the interpretation of the magnetic anomalies along a sw ne profile in terms of multiple dikes provides information about depth and location of buried volcanic structures that match well those from other geophysical and geological analyses

channel narrowing by vertical accretion along the green river near green river utah the green river is the longest tributary of the colorado river near the town of green river utah the green river narrowed in two discrete phases during the twentieth century the first phase of narrowing decreased average width by about 5 and occurred between about 1930 and 1940 when the magnitude of 2 yr flood mean annual discharge and effective discharge decreased by about 30 28 and 37 respectively during this first phase of narrowing saltcedar tamarisk spp an invading non native tree began to establish itself in the study area but botanists of that time did not describe the tree as abundant channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river9s banks further narrowing of an additional 14 occurred after 1959 this latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1 5 yr recurrence flood and when saltcedar were already abundant along the river the deposits that comprise the banks of the narrowing green river are composed of the suspended load of the river and these alluvial deposits are characterized by horizontal layers which indicate that they formed by vertical accretion a mechanism is proposed to explain the coarsening upward sequence of beds found in these vertically accreted deposits these changes in the channel of the green river are based on analysis of more than 2600 discharge measurements made by the u s geological survey resurvey of an abandoned measurement site matches of historical ground level photography and analysis of historical aerial photography within a geographic information system we have developed analytical techniques that permit analysis of width data from u s geological survey discharge measurements where gaging cross sections have adjustable beds and banks these techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means

dating floodplain sediments using tree ring response to burial floodplain sediments can be dated precisely based on the change in anatomy of tree rings upon burial when a stem of tamarisk tamarix ramosissima or sandbar willow salix exigua is buried subsequent annual rings in the buried section resemble the rings of roots rings become narrower vessels within the rings become larger and transitions between rings become less distinct we combined observations of these changes with tree ring counts to determine the year of deposition of sedimentary beds exposed in a 150 m long trench across the floodplain of the rio puerco a rapidly filling arroyo in new mexico this method reliably dated most beds thicker than about 30 cm to within a year of deposition floodplain aggradation rates varied dramatically through time and space sediment deposition was mostly limited to brief overbank flows occurring every few years the most rapid deposition occurred on channel margin levees which migrated laterally during channel narrowing at the decadal timescale the cross section average sediment deposition rate was steady but there was a shift in the spatial pattern of deposition in the 1980s from 1936 to 1986 sediment deposition occurred by channel narrowing with little change in elevation of the thalweg after 1986 sediment deposition occurred by vertical aggradation from 1936 to 2000 about 27 per cent of the arroyo cross section filled with sediment the rate of filling from 1962 to 2000 was 0 8 vertical m decade or 85 m2 decade published in 2005 by john wiley sons ltd

depth and structural index from normalized local wavenumber of 2d magnetic anomalies recent improvements in the local wavenumber approach have made it possible to estimate both the depth and model type of buried bodies from magnetic data however these improvements require calculation of third order derivatives of the magnetic field which greatly enhances noise as a result the improvements are restricted to data of high quality we present an alternative method to estimate both the depth and model type using the first order local wavenumber approach without the need for third order derivatives of the field our method is based on normalization of the first order local wavenumber anomalies and provides a generalized equation to estimate the depth of some 2d magnetic sources regardless of the source structure information about the nature of the sources is obtained after the source location has been estimated the method was tested using synthetic magnetic anomaly data with random noise and using three field examples

absence of caesium 137 from recent sediments in eastern australia indications of catchment processes summary sediment and soil containing the isotope caesium 137 137 cs are generally understood to have been sub aerially exposed within the last 35 to 40 years the horizon of first appearance of 137 cs in sedimentary deposits in australia is equated with the mid 1950s the time of first appearance of the isotope in australia some sediments however which are known to have been deposited in alluvial cutoffs since the mid 1950s do not contain the isotope this is interpreted as resulting from the high magnitude of the events which entrained and deposited the sediment the sediment was eroded from sub surface sites and deposited and buried rapidly thereby preventing the adsorption of the isotope the data indicate that 137 cs should be used very cautiously as a dating tool in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available

reconstructing the suspended sediment load of the yellow river since 1470 ce using the drought and flood index abstract the yellow river of china is well known around the world for its large sediment yield and suspended sediment load ssl to better understand the cause of ssl variability we need long term perspectives that go back 100 1000 yr however few long term reconstructions have been published especially with high temporal resolution in this study we show that the drought and flood index idf based on historical document analysis can be used as a proxy to reconstruct the ssl the high flow season june october stream flow and the annual precipitation of the middle yellow river from 1470 1919 the calibration equation explains 57 6 of the variance in the ssl produced from the drainage area between the hekouzhen and longmen stations on the yellow river variation in the idf reflects the variation in the summer monsoon in china that causes the variation in water vapor transport precipitation and soil erosion these links explain the mechanism of reconstructing the ssl based on the idf the reconstructed 450 yr series shows that the ssl responds sensitively to el nino events the reconstructed ssl may be used as a baseline to assess the effects of human activity on the ssl for the period after 1970 and the reduction in ssl for the period 1971 2013 was estimated as 4 09 108 t yr accounting for 48 9 of the annual ssl induced by only climate

rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic 10be and 26al using 10be and 26al measured in sediment and bedrock we quantify rates of upland erosion and sediment supply to a small basin in northwestern new mexico this and many other similar basins in the southwestern united states have been affected by cycles of arroyo incision and backfilling several times in the past few millennia the sediment generation 275 65 g m 2 yr 1 and bedrock equivalent lowering rates 102 24 m myr 1 we determine are sufficient to support at least three arroyo cycles in the past 3 000 years consistent with rates calculated from a physical sediment budget within the basin and regional rates determined using other techniques nuclide concentrations measured in different sediment sources and reservoirs suggest that the arroyo is a good spatial and temporal integrator of sediment and associated nuclide concentrations from throughout the basin that the basin is in steady state and that nuclide concentration is independent of sediment grain size differences between nuclide concentrations measured in sediment sources and reservoirs reflect sediment residence times and indicate that subcolluvial bedrock weathering on hillslopes supplies more sediment to the basin than erosion of exposed bedrock

episodic sediment accumulation on amazonian flood plains influenced by el nino southern oscillation continental scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains the spatial extent and depths of such deposits have been described and flood plain accumulation has been determined at decadal timescales but it has not been possible to identify discrete events or to resolve deposition on near annual timescales here we analyse 210 pb activity profiles from sediment cores taken in the pristine beni and mamore river basins which together comprise 720 000 km2 of the amazon basin to investigate sediment accumulation patterns in the andean amazonian foreland we find that in most locations sediment stratigraphy is dominated by discrete packages of sediments of uniform age which are typically 20 80 cm thick with system wide recurrence intervals of about 8 yr indicating relatively rare episodic deposition events ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with la nina events which debouch extraordinary volumes of sediments from the andes we conclude that transient processes driven by the el nino southern oscillation cycle control the formation of the bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon nutrients and pollutants to the amazon main stem