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

magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

recent formation of arroyos in the little missouri badlands of southwestern north dakota abstract in the little missouri badlands of southwestern north dakota the channels of ephemeral streams are incised 2 to 10 m or more into mid to late holocene alluvium the objectives of this study were to determine the timing and cause s of the most recent episodes of fluvial incision and to develop a process response model that illustrates the formation and evolution of arroyos in this region the purpose was to distinguish natural from anthropogenic changes to the landscape and to discriminate allogenic from autogenic causes of incision thereby gaining a greater sense of how steep relatively small ephemeral streams evolve dendrochronologic and dendrogeomorphic analyses of riparian cottonwoods provide an inexpensive high resolution dating method to constrain the time of incision thereby permitting determination of the cause s of incision by evaluating environmental conditions prior to and at the onset of fluvial incision an examination of seven small 10 to 100 km2 drainage basins indicated ephemeral streams have undergone a four stage cycle of change within the past 200 years comprising i an initial period of relative geomorphic stability with pedogenesis on the flood plain and low rates of lateral channel migration ii a period of channel incision with subsequent widening of the flood plain through lateral corrasion along middle and upstream reaches iii a concomitant period of aggradation along downstream reaches and finally iv a period of downstream incision dendrochronologic data and dendrogeomorphic relations indicate there have been three distinct periods of fluvial incision in the past 200 years the first period of incision began in the 1860s and 1870s prior to the onset of european settlement and intensive grazing by domesticated cattle in the area this period of incision occurred along the middle reaches of all seven of the streams examined and coincided with a severe protracted drought suggesting an allogenic cause the second period of incision occurred during the end of the 19th century and into the 20th century along the upstream reaches of streams this period of incision was mentioned in some written accounts of early settlers and coincided with the introduction of cattle and historical overgrazing of the region however overgrazing may not have caused the incision of this period but instead exacerbated or accelerated the upstream migration of knickpoints formed earlier along middle reaches a third period of incision has occurred in the past 20 years along the lower reaches of two of the seven streams examined this incision was caused by the process of meander cutoff this autogenic process decreases sinuosity and increases channel slope stream power and the potential to incise along adjacent stream reaches

morphodynamics of bedrock influenced dryland rivers during extreme floods insights from the kruger national park south africa high magnitude flood events are among the world s most widespread and significant natural hazards and play a key role in shaping river channel floodplain morphology and riparian ecology development of conceptual and quantitative models for the response of bedrock influenced dryland rivers to such floods is of growing scientific and practical importance but in many instances modeling efforts are hampered by a paucity of relevant field data here we combined extensive aerial and field data with hydraulic modeling to document erosion deposition and vegetation changes that have occurred during two successive cyclonedriven extreme floods along a 50 km long reach of the bedrock influenced sabie river in the kruger national park eastern south africa aerial light detection and ranging lidar data and photography obtained after extreme floods in 2000 and 2012 discharges 4000 m3 s 1 were used to generate digital elevation models dems and provide the boundary conditions for hydraulic modeling flow shear stresses for three discharges up to 5000 m3 s 1 for the sabie river study reach as a whole dem differencing revealed that the 2012 floods resulted in net erosion of 1 219 000 m3 53 mm m 2 at the subreach scale however more complex spatial patterns of erosion deposition and vegetation change occurred as largely controlled by differences in channel type e g degree of bedrock and alluvial exposure and changing hydraulic conditions shear stresses widely 1000 n m 2 across the river around peak flow the impact of flood sequencing and relative flood magnitude is also evident in some subreaches remnant islands and vegetation that survived the 2000 floods were removed during the smaller 2012 floods owing to their wider exposure to flow these findings were synthesized to refine and extend a conceptual model of bedrock influenced dryland river response that incorporates flood sequencing channel type and sediment supply influences in particular with some climate change projections indicating the potential for future increases in the frequency of cyclone generated extreme floods in eastern southern africa the sabie and other kruger national park rivers may experience additional sediment stripping and vegetation removal over time such rivers may transition to a more bedrock dominated state with significant implications for ecological structure and function and associated ecosystem services these findings contribute to an improved analysis of the kruger national park rivers in particular but also to growing appreciation of the global diversity of dryland rivers and the relative and synergistic impacts of extreme floods

variability in sediment delivery and storage along river courses in bega catchment nsw australia implications for geomorphic river recovery in many catchments in southeastern australia alluvial stores have been the dominant source of sediments mobilised in the period since european settlement in bega catchment on the south coast of new south wales nsw this has been reflected by dramatic changes to river morphology extensive volumes of material have been released and efficiently flushed to the lowland plain with a sediment delivery ratio of almost 70 however only 16 of these alluvial sediments have been flushed through to the estuary as antecedent controls on valley width have resulted in the lowland plain acting as a large sediment sink the changing nature of sediment source transfer and accumulation zones has varied markedly from subcatchment to subcatchment the volume of material supplied to the lowland plain from differing subcatchments is not related to subcatchment area rather the pattern of river types dictates the spatial variability in storage and transfer over 67 of sediment released has been sourced from just 25 of the catchment from subcatchments characterised by large valley fills cut and fill river style that previously stored extensive volumes of material at the base of the escarpment these parts of bega catchment were especially sensitive to disturbance sediment exhaustion from these parts of the catchment and from river courses elsewhere has major implications for the geomorphic recovery potential of rivers constraining what can be realistically achieved in terms of river rehabilitation

lodestone natures only permanent magnet what it is and how it gets charged magnetite and titanomagnetite exhibit magnetic properties which are attributable to the micro structures developed during oxidation and exsolution all magnetite iron ores which are lodestones contain maghemite these lodestones have hc between 10 and 30 mt sirm between 8 and 18 am kg and ri between 0 10 and 0 26 magnetite titanomagnetite and metals have rem values ratio of nrm to sirm 0 05 samples called fulgarites obtained from the smithsonian institution have rem values ranging from 0 45 to 0 92 the rem value serves as a witness parameter to the magnetic fields associated with the lightning bolt if a high rem value say 0 1 can be verified as not to be due to contamination by man and does not contain md hematite then the rock has lrm lightning remanent magnetization the magnetic field associated with lightning can be revealed from an isothermal remanent acquisition ra curve

equilibrium or indeterminate where sediment budgets fail sediment mass balance and adjustment of channel form green river downstream from flaming gorge dam utah and colorado this study examines bed and bank adjustment in the 105 km reach of the green river immediately downstream from flaming gorge dam by the use of historical aerial and oblique photographs analysis of current and abandoned stream gaging records and field observations although this segment has been previously characterized as sediment deficient these data show that sediment is accumulating in all reaches and that the bed has not degraded at any location where historical data are available adjustment is occurring through a combination of deposition of post dam sediment and stabilization of pre dam deposits resulting in a 10 30 reduction in average width of the channel all post dam surfaces are colonized by woody riparian vegetation the style of channel adjustment varies between geomorphically defined reaches in canyons dominated by debris fans and gravel bedded restricted meandering reaches gravel bars have become inactive and accumulated fine sediment in the sand bedded meandering reaches existing islands have increased in size and new mid channel islands have formed in all of these types of reaches post dam deposits line the banks and sediment has accumulated in side channels that previously separated islands from the bank these findings demonstrate that sediment budgets that show a balance between inputs and outputs cannot necessarily be interpreted to indicate channel equilibrium a sediment mass balance for 150 km reach between the dam and the first long term gage indicates approximate balance of inputs and outputs for the pre and post dam periods when uncertainty in budget components is considered the mass balance is indeterminate although the green river may have been in approximate equilibrium in the pre dam period we have shown that channel width is decreasing in the post dam period the post dam deposits constitute a small but a significant component of the sediment budget upstream from the first major tributary sediment is supplied to this reach by small tributaries and to a lesser extent erosion of pre dam alluvium downstream from the study area the volume of the post dam deposits is tiny relative to the volume of sediment input from the first major tributary d 2005 elsevier b v all rights reserved

assessing channel changes of the ganges padma river system in bangladesh using landsat and hydrological data abstract the ganga ganges 1 is an important river system in south asia which supports the life and livelihoods of millions of people both in india and bangladesh the system has a number of names throughout its length below its confluence with the brahmaputra at aricha it is known as the padma which in turn merges with the upper meghna at chandpur below which the channel is known as the lower meghna there is a growing concern about this large river system because its channels are subject to frequent migration threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability land use change and agricultural intensification as the basin experiences rapid population growth concerns have been expressed that the construction of a barrage just upstream of the indo bangladesh border has adversely affected the ganges reach in bangladesh partly in order to investigate this the planform changes of the ganges and the padma within bangladesh was analysed over the period 1973 to 2011 using multitemporal landsat images and long term flow data in eight epochs with an average duration of 4 5 years the padma reach is less affected by the barrage and provides a useful control study areas of erosion and deposition were determined from sequential changes in the bankline positions mean channel width sinuosity and braiding index were analysed using a geographic information system gis flood frequency duration and magnitudes were studied using long term discharge records generally channel planform evaluation indicated that both the ganges and the padma experienced contraction expansion and readjustment in configuration over the last 38 years erosion and deposition statistics of the ganges indicate that 57 km2 of land was lost along the right bank whereas around 59 km2 has been gained along the left bank during the assessment period suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank the width of the ganges varied from a maximum of 5 36 to a minimum of 3 23 km during the observation period changes to sandbar area are in general much more radical than changes to the overall width and area of the channel measurement of areas of erosion and accretion showed that both banks of the padma experienced considerable loss of land the total net loss for left bank and right bank was 155 and 28 km2 respectively the padma is approximately twice the width of the ganges and the changes to its channel area are not as temporally dynamic as the ganges the relationship between bank curvature and erosion accretion of the river banks for both rivers was analysed and the results contradict established meander theory regression analysis between bank erosion rates annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the padma r2 0 6283 and that the ganges bank erosion rate is influenced by mean flood flow r2 0 6738 the flood frequency shows generally good stability across the first eight of the nine epochs for the ganges but for the padma the frequency showed even greater stability we were unable to support the widely held belief that the upstream barrage has a deleterious effect on the ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices

multidomain hematite a source of planetary magnetic anomalies thermoremanent magnetization trm in hematite is larger than trm in magnetite for grain sizes 10 m we show that hematite s weak spontaneous magnetization m s causes its strong trm since the self demagnetizing field h d opposing large domain wall displacements is proportional to m s in hematite h d is comparable to the earth s magnetic field but in magnetite h d is 1000 times larger as a result earth s field trm of md hematite 0 3 am 2 kg outweighs trm and induced magnetization of md magnetite 0 01 0 02 am 2 kg and rivals trm of single domain and psd magnetite as a source of magnetic anomalies on earth and perhaps on mars

magnetization of a dolomite bed in the monterey formation implications for diagenesis the effect of diagenesis upon natural remanent magnetization nrm has been studied at two sites within a single bed of dolomite from the monterey formation in santa barbara county california paleomagnetic direction demonstrates that the nrm is primary at one site and remagnetized at the other site magnetic separates from samples carrying a primary nrm yielded detrital magnetite separates from samples carrying secondary nrm yielded an unusual form of authigenic magnetite evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0 72 2 47 m y b p for both of these diagenetic events

lunar paleointensity measurements implications for lunar magnetic evolution abstract we analyze published and new paleointensity data from apollo samples to reexamine the hypothesis of an early 3 9 3 6 ga lunar dynamo our new paleointensity experiments on four samples use modern absolute and relative measurement techniques with ages ranging from 3 3 to 4 3 ga bracketing the putative period of an ancient lunar field samples 60015 anorthosite and 76535 troctolite failed during absolute paleointensity experiments samples 72215 and 62235 impact breccias recorded a complicated multi component magnetic history that includes a low temperature 500 c component associated with a low intensity 2 t similar multi component behavior has been observed in several published absolute intensity experiments on lunar samples additional material from 72215 and 62235 was subjected to a relative paleointensity experiment a saturation isothermal remanent magnetization or sirm experiment neither sample provided unambiguous evidence for a thermal origin of the recorded remanent magnetization we test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples specifically an overprint from exposure to a small magnetic field an isothermal remanent magnetization results in multi component behavior similar to absolute paleointensity results from which we could not recover the correct magnitude of the original thermal remanent magnetization in light of these new experiments and a thorough re evaluation of existing paleointensity measurements we conclude that although some samples with ages of 3 6 to 3 9 ga are strongly magnetized and sometimes exhibit stable directional behavior it has not been demonstrated that these observations indicate a primary thermal remanence particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock as relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable consequently current paleointensity measurements do not support the existence of a 3 9 3 6 ga lunar dynamo with 100 t surface fields a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models

β diopside a new ultrahigh pressure polymorph of camgsi 2 o 6 with six coordinated silicon minerals containing silicon in four fold coordination ivsi4 are common in crustal rocks while those involving six coordinated silicon visi4 dominate the earth s lower mantle and determine its properties here we show a new type of phase transition determined by single crystal high pressure x ray diffraction experiments in a diamond anvil cell dac using natural diopside camgsi2o6 the archetypic member of clinopyroxene family and one of the most abundant minerals of the earth s upper mantle above 50 gpa at ambient temperature diopside transforms to a previously unknown post clinopyroxene phase diopside with half of the tetrahedralivsi4 layers converted to octahedral visi4 coordination this phase is most probably a metastable state that is kinetically accessible at room temperature and the transformation is fully reversible on decompression this new type of phase transition provides important clues to the exact mechanisms of breakdown of clinopyroxene in the earth s mantle and may be expected to take place in other pyroxenes at pressures higher than previously explored

sediment supply versus local hydraulic controls on sediment transport and storage in a river with large sediment loads the rio grande in the big bend region of texas usa and chihuahua and coahuila mexico undergoes rapid geomorphic changes as a result of its large sediment supply and variable hydrology thus it is a useful natural laboratory to investigate the relative importance of flow strength and sediment supply in controlling alluvial channel change we analyzed a suite of sediment transport and geomorphic data to determine the cumulative influence of different flood types on changing channel form in this study physically based analyses suggest that channel change in the rio grande is controlled by both changes in flow strength and sediment supply over different spatial and temporal scales channel narrowing is primarily caused by substantial deposition of sediment supplied to the rio grande during tributary sourced flash floods tributary floods have large suspended sediment concentrations occur for short durations and attenuate rapidly downstream in the rio grande depositing much of their sediment in downstream reaches long duration floods on the mainstem have the capacity to enlarge the rio grande and these floods released from upstream dams can either erode or deposit sediment in the rio grande depending upon the antecedent in channel sediment supply and the magnitude and duration of the flood geomorphic and sediment transport analyses show that the locations and rates of sand erosion and deposition during long duration floods are most strongly controlled by spatial changes in flow strength largely through changes in channel slope however spatial differences in the in channel sediment supply regulate sediment evacuation or accumulation over time in long reaches greater than a kilometer

the influence of riparian vegetation on stream width eastern pennsylvania usa we surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches while deposition is localized on active floodplains on the insides of bends our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches two dimensionless parameters and e explain our observations represents the influence of grassy vegetation on rates of active floodplain deposition it is 5 times higher in nonforested reaches than in forested reaches e is proportional to rates of cutbank migration it is 3 times higher in nonforested reaches than in forested reaches differences in width between forested and nonforested reaches are proportional to e in forested reaches channels are wide with banks that are difficult to erode dense tree roots create a low value of e and the channel migrates slowly e is high however because is very low shade from trees inhibits the growth of grass on active floodplains in nonforested reaches channels are narrow with banks that are easy to erode e is high and the channel migrates rapidly e is low however due to a very large value of grass grows readily on nonforested convex bank floodplains thus differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation mediated rates of deposition on active flood plains these results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes rather than focusing on these processes in isolation from one another

recent channel adjustments in alluvial rivers of tuscany central italy drastic channel adjustments have affected the main alluvial rivers of tuscany central italy during the 20th century bed level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations changes in channel width were investigated by comparing available aerial photographs 1954 and 1993 98 bed incision represents the dominant type of vertical adjustment and is generalized along all the fluvial systems investigated the arno river system is the most affected by bed level lowering up to 9 m whereas lower incision generally less than 2 m is observed along the rivers of the southern part of the region human disturbances appear to be the dominant factors of adjustments the main phase of vertical change occurred during the period 1945 80 in concomitance with the phase of maximum sediment mining activity at the regional scale the second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel based on measurements of channel width conducted on aerial photographs 38 of the reaches analysed experienced a narrowing greater than 50 of the initial channel width the largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region a regional scheme of channel adjustments is derived based on initial channel morphology and on the amounts of incision and narrowing different styles of channel adjustments are described rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing in contrast sinuous meandering channels mainly adjusted vertically with a minor amount of narrowing copyright 2003 john wiley sons ltd

an experimental study of the intensity and stability of thermoremanent magnetization acquired by synthetic monodomain titanomagnetite substituted by aluminium summary titanomagnetites of composition fe2 4 al ti0 6o4 and fe2 6 al ti0 4o4 0 0 1 and 0 2 in both cases were prepared in the monodomain state by pulverization of sintered synthetic material in low fields the thermoremanence trm was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts the higher field trm acquisition curves follow the neel model curve for an assemblage of non interacting identical particles in a general way only the differences being due to interactions or the range of particle blocking temperatures and volumes or other features of the samples not included in the model the unblocking temperatures of low field trm lie in a narrow range below the curie point the low field trm is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the trm mechanism in preserving a stable record of a weak magnetic field the result of a lowrie fuller test on the material is consistent with the monodomain state

drainage basin scale geomorphic analysis to determine reference conditions for ecologic restoration kissimmee river florida major controls on the retention distribution and discharge of surface water in the historic precanal kissimmee drainage basin and river were investigated to determine reference conditions for ecosystem restoration precanal kissimmee drainage basin hydrology was largely controlled by landforms derived from relict coastal ridge lagoon and shallow shelf features widespread carbonate solution depressions and a poorly developed fluvial drainage network prior to channelization for flood control the kissimmee river was a very low gradient moderately meandering river that flowed from lake kissimmee to lake okeechobee through the lower drainage basin we infer that during normal wet seasons river discharge rapidly exceeded lake okeechobee outflow capacity and excess surface water backed up into the low gradient kissimmee river this backwater effect induced bankfull and peak discharge early in the flood cycle and transformed the flood plain into a shallow aquatic system with both lacustrine and riverine characteristics the large volumes of surface water retained in the lakes and wetlands of the upper basin maintained overbank flow conditions for several months after peak discharge analysis indicates that most of the geomorphic work on the channel and flood plain occurred during the frequently recurring extended periods of overbank discharge and that discharge volume may have been significant in determining channel dimensions comparison of hydrogeomorphic relationships with other river systems identified links between geomorphology and hydrology of the precanal kissimmee river however drainage basin and hydraulic geometry models derived solely from general populations of river systems may produce spurious reference conditions for restoration design criteria

streamflow regulation and multi level flood plain formation channel narrowing on the aggrading green river in the eastern uinta mountains colorado and utah abstract the style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river the magnitude of streamflow regulation and the post dam sediment transport regime we measured different magnitudes of channel adjustment on the green river downstream from flaming gorge dam ut usa that are related to these three factors bankfull channel width decreased by an average of about 20 in the study area in reaches with abundant debris fans and eddy deposited sand bars the amount of channel narrowing was proportional to the decrease in specific stream power the fan eddy dominated reach with the greatest decrease in stream power narrowed by 22 while the reach with the least decrease in stream power narrowed by 11 in reaches with the same magnitude of peak flow reduction meandering reaches narrowed by 15 to 22 and fan eddy dominated reaches narrowed by 11 to 12 specific stream power was not significantly affected by flow regulation in the meandering reaches in the diverse array of reach characteristics and deposit types found in the study area all pre and post dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre dam floods an intermediate bench that was inundated by rare post dam floods and a post dam floodplain that was inundated by the post dam mean annual flood analysis of historical photographs and tree ring dating of tamarix sp shows that the intermediate bench and post dam floodplain are post dam landforms in each reach type although these two surfaces occur at different levels they are forming simultaneously during flows of different magnitude and while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies both reach types contain deposits at all of these topographic levels the process of channel narrowing varied between fan eddy dominated and meandering reaches in the meandering reaches where stream power has not changed narrowing was accomplished by essentially the same depositional processes that operated prior to regulation in fan eddy dominated reaches where significant reductions in stream power have occurred channel narrowing has been accompanied by a change in dominant depositional processes mid channel sand deposits are aggrading on deposits that in the pre dam era were active gravel bars these deposits are creating new islands and decreasing the presence of open framework gravel bars in eddies bare sand bars are replaced with vegetated bars that have a simpler topography than the pre dam deposits

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

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

a review of lunar paleointensity data and implications for the origin of lunar magnetism the saturation remanence normalization method of paleointensity estimation has been applied to more than 50 lunar samples and has been calibrated in terms of standard intensity determinations done on 14 samples these results suggest that a lunar surface field comparable in intensity to the earth s magnetic field existed from about 3 6 to 3 8 ae absolute paleointensity estimates and lunar surface field strength determinations are consistent with this observation the data presented here differ from previous descriptions of the variation in the lunar field with time in that a the oldest samples 3 9 ae are among the most weakly magnetized and b an order of magnitude drop in paleointensity is suggested between the time of extrusion of the apollo 11 low and high potassium basalts the limited range in age of the strongly magnetized lunar samples and the absence of shocked samples in this set argues against a solar source for the lunar paleofield or for fields arising from transient phenomena such as impacts the coincidence of this high field era with the termination of basin forming impacts and the beginning of mare basalt extrusion suggests a common cause a close approach of the moon to the earth at about 3 8 ae might account for all three phenomena the confined duration of the strong lunar field may allow for the assignment of relative ages for certain lunar samples and surface features

causes of 20th century channel narrowing in mountain and piedmont rivers of southeastern france extensive channel narrowing in southeastern france provides an illustration of geomorphic response to land use changes the study region comprises a range of environments from large piedmont and intramountain gravel bed rivers to small mountain streams field measurements and analysis of historical data demonstrate two distinct periods of channel change from 1850 to 1950 channel narrowing is interpreted to be the result of a recovery process in response to widespread channel destabilization induced by major floods during the second half of the 19th century at the time the largely deforested basins were highly responsive to flooding whereas the recovery process was accelerated by floodplain and basin scale land use changes afforestation and torrential control works which in turn reduced sediment delivery and enabled vegetation development in channels from 1950 to 1970 channel narrowing accelerated in most of the studied rivers this recent phenomenon is considered as a human induced fluvial adjustment directly related to forest development on river margins and human abandonment of intensive floodplain land uses at the same time long profile degradation occurred as a result of long term bedload supply decrease in small mountain streams channel narrowing is mainly explained by channel incision which seems to progress from upstream where sediment sources are progressively stabilized by afforestation copyright 2002 john wiley sons ltd

effects of dam operation and land use on stream channel morphology and riparian vegetation dams are well known for influencing channel and vegetation dynamics downstream but little work has focused on distinguishing effects of land use and channel responses to the impoundment in this paper we examined interacting effects of a dam and land use on downstream changes in channel morphology and riparian vegetation along an agricultural stream system in northern california measurements of planform channel morphology vegetation area and land use were mapped along multiple stream segments based on a chronological sequence of historical aerial photographs over a 34 yr period prior to operation of the dam in 1983 and over a 17 yr period after dam operation and compared to a nearby undammed reference stream a two factor analysis of covariance ancova was used to examine the effect of the dam on changes in bankfull area stream length and riparian vegetation area while accounting for the effect of land use and distance downstream the dammed stream s bankfull area contracted 94 after dam operation prior to dam operation bankfull area decreased when land use area increased but not after operation of the dam stream length varied 64 less after dam operation as a consequence of less frequent episodic channel migration and entrenchment the area of riparian vegetation was decreasing during the pre dam period but then increased 72 after operation of the dam across time periods decreases in the area of riparian vegetation were also associated with increases in land use area in both the dammed and reference stream after operation of the dam reduced peak discharges and sediment reduction likely lead to channel incision and constrained channel migration which allowed vegetation to increase 50 on less accessible abandoned banks rating curve and hydraulic exponent analyses based on stream gauge measurements corroborate statistical analyses of the mapped changes in conclusion we found that operation of the dam and land use patterns together influenced spatial and temporal changes in channel morphology and riparian vegetation use of a nearby undammed reference stream in conjunction with multivariable analysis of spatially and temporally replicated observations provided an effective framework for unraveling interacting effects of dams and land use activities on stream channel and vegetation dynamics

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

matching martian crustal magnetization and magnetic properties of martian meteorites magnetic properties of 26 of 32 unpaired martian meteorites sncs are synthesized to further constrain the lithology carrying martian magnetic crustal sources magnetic properties of ultramafic cumulates i e chassigny allan hills alh 84001 and lherzolitic shergottites alh 77005 lewis cliff lew 88516 are one or two orders of magnitude too weak to account for the crustal magnetizations assuming magnetization in an earth like field nakhlites and some basaltic shergottites which are the most magnetic sncs show the right intensity titanomagnetite is the magnetic carrier in the nakhlites 7 meteorites whereas in most basaltic shergottites 11 meteorites it is pyrrhotite dhofar dho 378 los angeles and nwa 480 1460 and 2046 are anomalous basaltic shergottites as their magnetism is mainly due to titanomagnetite pyrrhotite should be among the candidate minerals for the magnetized noachian crust

influence of bank materials bed sediment and riparian vegetation on channel form along a gravel to sand transition reach of the upper tualatin river oregon usa we examined the role of bed sediment size bank materials riparian vegetation and discharge on channel form along a 10 km reach of the upper tualatin river encompassing both gravel and sand bed reaches statistical correlations and analyses of downstream changes of channel morphology reveal that bank materials and bed sediment are the dominant controls on channel cross section form a rapid downstream reduction in bed sediment size in the gravel bed channel is followed by an abrupt transition to a narrower deeper and less sinuous sand bed channel with more cohesive bank materials the simultaneous reductions in channel slope bed sediment size and width to depth ratio indicate the channel crosses a geomorphic threshold to maintain continuity in sediment transport this gravel to sand transition and associated change in channel form are induced by a break in valley slope and an increase in bank resistance from cohesive bank materials bank materials measured as the average percent silt and clay in banks are a primary influence on channel form within both subreaches but demonstrate a greater influence on channel width and vertical stability in the gravel bed channel and on channel depth and lateral stability in the sand bed channel riparian vegetation at current densities and compositions is not a significant control on contemporary channel cross section form and may be responding to the bank and channel stability provided by cohesive bank materials in the laterally dynamic gravel bed channel

millennial scale climatic fluctuation in the fluvial record during mis3 very high resolution seismic images from ne hungary abstract alluvial architectures of a meandering river existing in mis3 were observed on very high resolution vhr single channel waterborne seismic profiles 20 30 m below the tisza river in the pannonian basin hungary the study investigated the spatial and temporal variations of two more than 2 km long continuous series of inclined reflections interpreted as laterally accreted point bar complexes the phases of natural meander migration were reconstructed in 3d based on the changes in the geometry and dip of the inclined reflections a channel forming discharge curve extending over approximately 2 5 ky was calculated by using paleo width and depth data derived from the lateral accretion surfaces systematic analysis of the geometrical variations coupled with the discharge curve evaluation on each point bar complexes indicates millennial scale discharge fluctuations 40 50 ky ago to that the river responded principally by incision and infilling the primary periodicity comparable to the sub milankovitch cycles is superimposed by shorter periods ca 500 year cycles reflect the phases of unidirectional meander development while the smallest ones reflect the recurrence interval 150 200 years of the highest floods river bed incisions happened step by step related to extreme floods when the meander development also changed the smaller scale and more rapid fluctuations within a development unit were represented in variations of the channel width although the poor geochronology of the sandy fluvial deposits cannot allow any direct correlation to the climatostratigraphic events the millennial scale climate variations of mis3 were pronouncedly characteristic in the discharge fluctuations

magnetic characteristics of cv chondrules with paleointensity implications 1 we have conducted a detailed magnetic study on 45 chondrules from two carbonaceous chondrites of the cv type 1 mokoia and 2 allende allende has been previously extensively studied and is thought to have a high potential of retaining an extra terrestrial paleofield few paleomagnetic studies of mokoia have previously been undertaken we report a range of magnetic measurements including hysteresis first order reversal curve analysis forcs demagnetization characteristics and isothermal remanent irm acquisition behavior on both mokoia and allende chondrules the mokoia chondrules displayed more single domain like behavior than the allende chondrules suggesting smaller grain sizes and higher magnetic stability the mokoia chondrules also had higher average concentrations of magnetic minerals and a larger range of magnetic characteristics than the allende chondrules irm acquisition analysis found that both sets of chondrules have the same dominant magnetic mineral likely to be a feni phase taenite kamacite and or awaruite contributing to 48 of the mokoia chondrules and 42 of the allende chondrule characteristics forc analysis revealed that generally the allende chondrules displayed low field coercivity distributions with little interactions and the mokoia chondrules show clear single domain like distributions paleointensity estimates for the two meteorites using the remc and preisach methods yielded estimates between 13 and 60 t and 3 56 t respectively for allende and 3 140 t and 1 110 t respectively for mokoia from the data we suggest that mokoia chondrules carry a non primary remagnetization and while allende is more likely than mokoia to retain its primary magnetization it also displays signs of post accretionary magnetization

spatial and temporal variability in sedimentation rates associated with cutoff channel infill deposits ain river france floodplain development is associated with lateral accretion along stable channel geometry along shifting rivers the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence which exhibit specific overflow patterns in this contribution the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river ain river france the sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137cs and unsupported 210pb sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain sedimentation rates range between 0 65 and 2 4 cm a1 over a period of 10 to 40 years the data provide additional information on the role of distance from the bank overbank flow frequency and channel geometry in controlling the sedimentation rate channel infills lower than adjacent floodplains exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain additionally channel degradation aggradation and bank erosion which reduce or increase the distance between the main channel and the cutoff channel aquatic zone affect local overbank flow magnitude and frequency and therefore sedimentation rates thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills last the dendrogeomorphic and 137cs approaches are cross validated for estimating the sedimentation rate within a channel infill

management responses to pulses of bedload sediment in rivers abstract rivers can experience sudden pulses of sediment from human and natural erosion processes that can accumulate in the bed abundant studies have examined the sources and dynamics of sediment pulses and problems caused by these pulses particularly flooding avulsions and habitat simplification much less has been written about what managers can do about sediment pulses and that is the purpose of this review the first option for managers is to do nothing and this decision can be informed by many case studies and by theory on the propagation and character of sediment pulses their diffusion translation and celerity doing nothing should be informed by the secondary effects of sediment pulses on channels including widening avulsions and tributary interactions if managers decide that something needs to be done about the sediment they have four options 1 reducing the sediment supply at source 2 trapping sediment in the channel 3 accelerating sediment transport through a reach and 4 directly extracting sediment the most common of these actions is undoubtedly to reduce the supply at source but there are few examples of the consequences of this for sediment pulses there are even fewer examples of trapping accelerating and extracting sediment all of these options have great potential for managing sediment pulses however they also have the potential to trigger incision of tributaries and of the channel behind the passing sediment wave overall the literature equips managers to understand the dynamics of sediment pulses but it does not yet equip them to confidently manage these geomorphic events

ephemeral stream processes implications for studies of quaternary valley fills abstract three unstable ephemeral stream channels arroyos which drain source areas that have high sediment yields ranging from predominantly sand arroyo calabasas to a mixture of sand silt and clay sand creek to largely silt and clay sage creek were resurveyed to provide data on the rates and mechanics of erosion and sedimentation processes during periods ranging from 14 to 22 yr channel morphology changed significantly erosion occurred through nickpoint recession and bank collapse but erosional reaches are separated by aggrading or stable channel reaches in general sediment that is eroded as the nickpoint recedes upstream is trapped in the widened channel downstream in this manner sediment is transported episodically out of these basins during a series of cut and fill cycles the manner by which the channels aggrade and the morphology of the aggraded stable channels are controlled by the sediment type the wide and shallow channel of arroyo calabasas is filled by vertical accretion of sand size sediment the narrow and deep channels of sage creek and sand creek are created by the lateral accretion of cohesive fine grained sediment the channel modification and the cut and fill episodes are dependent on high sediment yields and therefore they are independent of subtle climatic shifts cut and fill deposits that have been created in this manner should not be equivalent in age from basin to basin and therefore channel trenching and filling in the semiarid western united states during the holocene need not be synchronous

alternate bar response to sediment supply termination 1 sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels yet quantitative linkages between sediment supply and bar topography are not well developed we explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments the first set of experiments was conducted in a 28 m long 0 86 m wide flume channel using a unimodal sand gravel mix the second set of experiments was conducted at field scale in a 55 m long 2 74 m wide channel using a unimodal gravel mixture in both experiments alternate bars and patchy surface grain size distributions developed under steady flow and sediment supply conditions the cessation of the sediment supply induced a reduction in the surface grain size heterogeneity and the bars were eliminated in both flumes mean boundary shear stress had declined but were capable of moving sediments after the bars disappeared albeit at relatively small rates compared to when the bars were present in the smaller flume the previously stationary bars migrated out of the flume and were not replaced with new bars a nearly featureless bed formed with limited surface grain size heterogeneity a slightly coarsened surface and a slightly reduced slope in the larger flume the formation of alternate bars was induced by an imposed upstream flow constriction and as such the bars did not migrate termination of sediment supply led to progressive erosion of bed topography and loss of the bars coarsening of the bed surface loss of bed texture patchiness and significant slope reduction the original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope this shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation the role of sediment supply in bar formation and stability is not often recognized in stream restoration our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars

floodplain dynamics of a wandering river dendrochronology of the morice river british columbia canada dendrochronological techniques are used to develop a history of flood occurrence and relative stage for the middle section of the morice river based on flood scars bank undercutting dates and tree establishment dates on fluvial features the morice river is the major tributary of the bulkley river skeena river system northwest british columbia canada reach 2 of the morice river drainage area 3499 km2 is forested and little disturbed by human activity it is presently a wandering multiple channel river and shows evidence of frequent channel changes which have reworked most of the floodplain several times in the last 180 years airphoto analysis and dendrochronology demonstrate that many channels show evidence of two or more periods of activity reoccupation and re excavation of pre existing channel segments is more frequent than development of new channels thirty one significant floods over the past 105 years were detected through dendrochronology of flood scarred trees on the morice river flood scars are predominantly caused by flood transported logs abundant log transport results in damage to channel margin trees during events only slightly above bankfull discharge the record of flood scars on channel margin trees and establishment dates of forest stands on abandoned channels and surfaces show that the morice river has maintained its present regimen for the past 115 years during the neoglacial the morice river was more active than in recent times and its channels occupied twice the present area tree establishment dates show that neoglacial surfaces and channels of the morice river were abandoned during the 1820 s the modern floodplain configuration was assumed by 1870

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

mapping the response of riparian vegetation to possible flow reductions in the snake river idaho abstract this study was initiated to determine the general effects of potential flow reductions in the middle snake river swan falls dam downstream to the idaho oregon border on its riparian vegetation considerable water from the river is currently used to irrigate the adjacent snake river plain and increased demand for water in the future is likely the problem was subdivided into several research components including field investigation of the existing riparian vegetation and river environment hydrological modeling to calculate the effects of one flow scenario on hydrological regime and integration of vegetation and hydrological modeling results with a geographic information system gis to map the riverbed island and bank conditions under the scenario flow field work was conducted in summer 1990 riparian vegetation along 40 u s geological survey cross sections was sampled at approximately 1 25 mile intervals within the 50 mile long study area cross section and flow data were provided by the u s geological survey gis mapping of land water cover using arc info was based on 1987 aerial photographs riverbed contour maps were produced by linking cross section data topographic contouring software anudem and gis the maps were used to spatially display shallow areas in the channel likely to become vegetated under reduced flow conditions the scenario would reduce flow by approximately 20 160 maf and lower the river an average of 0 5 ft the scenario flow could cause a drop in the elevation of the riparian zone comparable to the drop in mean river level and expansion of the lower riparian zone into shallow areas of the channel the gis maps showed that the shallow areas of the channel more likely to become vegetated under the scenario flow are located in wide reaches near islands some possible ecological consequences of the scenario flow include a greater area of riparian habitat reduced flow velocity and sedimentation in shallow channels leading to channel deactivation increased island visitation and nest predation by predatory mammals due to loss of a water barrier between some islands and banks and larger populations of alien plant species in the new riparian vegetation

fluvial processes and vegetation glimpses of the past the present and perhaps the future abstract most research before 1960 into interactions among fluvial processes resulting landforms and vegetation was descriptive since then however research has become more detailed and quantitative permitting numerical modeling and applications including agricultural erosion abatement and rehabilitation of altered bottomlands although progress was largely observational the empiricism increasingly yielded to objective recognition of how vegetation interacts with and influences geomorphic process a review of advances relating fluvial processes and vegetation during the last 50 years centers on hydrologic reconstructions from tree rings plant indicators of flow and flood frequency parameters hydrologic controls on plant species regulation of sediment movement by vegetation vegetative controls on mass movement and relations between plant cover and sediment movement extension of present studies of vegetation as a regulator of bottomland hydrologic and geomorphic processes may become markedly more sophisticated and widespread than at present research emphases that are likely to continue include vegetative considerations for erosion modeling response of riparian zone forests to disturbance such as dams and water diversion the effect of vegetation on channel and bottomland dynamics and rehabilitation of stream corridors research topics that presently are receiving attention are the effect of woody vegetation on the roughness of stream corridors and hence processes of flood conveyance and flood plain sedimentation the development of a theoretical basis for rehabilitation projects as opposed to fully empirical approaches the effect of invasive plant species on the dynamics of bottomland vegetation the quantification of below surface biomass and related soil stability factors for use in erosion prediction models and the effect of impoundments on downstream narrowing of channels and accompanying encroachment of vegetation bottomland vegetation partially controls and is controlled by fluvial geomorphic processes the purposes of this paper are to identify and review investigations that have related vegetation to bottomland features and processes to distinguish the present status of these investigations and to anticipate future research into how hydrologic and fluvial geomorphic processes of bottomlands interact with vegetation

channel instability in a forested catchment a case study from jones creek east gippsland australia abstract jones creek a forested sub catchment of the genoa river victoria australia has experienced channel metamorphosis induced by a series of floods since 1971 in the flood of record in 1971 the genoa river widened by up to two times at the confluence with jones creek this effectively shortened the course of jones creek resulting in a lagged tributary response incision and bed steepening during floods in 1975 and 1978 triggered significant changes in channel form along jones creek channel changes between 1972 1997 resulted in a four fold increase in cross sectional area channel depth increased by up to 1 5 m but has subsequently refilled by around 0 6 m initially incision resulted in increased stream power as a result of increased mean depth while sinuosity was maintained this was followed by channel widening a reduction in sinuosity and a continued increase in slope estimated stream power remains high as the channel continues to laterally adjust this study highlights the dynamic nature of tributary trunk stream relationships in a cut and fill landscape demonstrating how trunk stream adjustments can induce profound tributary instability in a forested subcatchment channel widening of the trunk stream primed the tributary for change bedlevel incision and increases in bedslope breached the threshold of landscape stability in this steep alluvial tributary three periods of channel change can be identified reflecting the complex response of the system to channel incision rather than developing simple cutoffs jones creek now exhibits a range of lateral adjustment and realignment features responses to external disturbance provide insights into natural recovery mechanisms in this forested setting lateral adjustment and associated sediment deposition have been accompanied by rapid rates of vegetation colonisation and stabilisation of realignment features effectively reducing the volume of sediment that is available to be reworked through the channel network

assessment of channel changes due to long term bedload supply decrease roubion river france this paper presents the observations made in the watershed of the upper roubion river which drains a submediterranean mountainous environment and in two of its main tributaries the soubrion and bine creeks our aim was to analyze contemporary morphological and sedimentary changes in response to a progressive decrease in bedload supply this decrease is linked to an increase of forest cover in the watershed during the 20th century following rural depopulation and to a correlated decrease in peak flows several variables were systematically measured width slope bed level change sediment size along 23 km of river whereas dendrochronological data floodplain cross sections and estimated recurrence intervals of sediment entrainment based on critical shear stress calculations were also produced for specific sites to complete our analysis evidence of three major phenomena was obtained on both the roubion and its tributaries i bed degradation ii development of stable pavement in incised reaches and iii channel narrowing associated with the development of alluvial forest on recently abandoned portions of the pre existing active channel forest development began in the 1950s these three phenomena show positively covariant relationships that lead us to propose a descriptive model of mountain fluvial system adjustments to progressive decrease of bedload

quantitative model of the growth of floodplains by vertical accretion a simple one dimensional model is developed to quantitatively predict the change in elevation over a period of decades for vertically accreting floodplains this unsteady model approximates the monotonic growth of a floodplain as an incremental but constant increase of net sediment deposition per flood for those floods of a partial duration series that exceed a threshold discharge corresponding to the elevation of the floodplain sediment deposition from each flood increases the elevation of the floodplain and consequently the magnitude of the threshold discharge resulting in a decrease in the number of floods and growth rate of the floodplain floodplain growth curves predicted by this model are compared to empirical growth curves based on dendrochronology and to direct field measurements at five floodplain sites the model was used to predict the value of net sediment deposition per flood which best fits in a least squares sense the empirical and field measurements these values fall within the range of independent estimates of the net sediment deposition per flood based on empirical equations these empirical equations permit the application of the model to estimate of floodplain growth for other floodplains throughout the world which do not have detailed data of sediment deposition during individual floods copyright 2000 john wiley sons ltd

unique thermoremanent magnetization of multidomain sized hematite implications for magnetic anomalies intense magnetic remanence 100 1000 a m associated with md hematite and or titanohematite and associated with high koenigsberger ratios 40 1000 indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant titanohematite behaves similarly to hematite and thus the grain size dependence of trm acquisition in hematite is considered as a generalization the transition between truly md behavior and tendency towards sd behavior in hematite has been established to be between grain sizes of 0 1 and 0 05 mm in contrast to magnetite and titanomagnetite hematite exhibits inverse grain size dependence with md hematite acquiring a relatively intense trm in the geomagnetic field comparable to sub micrometer sized magnetite and only an order of magnitude less than sd magnetite consequently md hematite and by analogy titanohematite remanence may be of significance as a source of magnetic anomalies at all scales md hematite exhibits trm weak field acquisition behavior that is different from all other magnetic minerals being the only magnetic mineral having an rem trm sirm value e0 1 for trm acquisition in the geomagnetic field the very different trm behavior of md hematite in contrast to magnetite is due to two factors the first is the lesser influence of demagnetizing energy with respect to wall pinning energy at temperatures almost up to the curie temperature for hematite the second is the greater importance of the magnetostatic energy in the applied field which for hematite dominates the total energy at high temperatures fl 2000 published by elsevier science b v all rights reserved

experimental shock metamorphism of the l4 ordinary chondrite saratov induced by spherical shock waves up to 400 gpa abstract we carried out shock experiments on macroscopic spherical samples of the l4 ordinary chondrite saratov natural shock stages s2 s3 using explosively generated spherical shock waves with maximum peak pressures of 400 gpa and shock induced temperatures 800 c up to several thousands c the evolution of shock metamorphism within a radius of the spherical samples was investigated using optical and scanning electron microscopy microprobe and magnetic analyses as well as mossbauer spectroscopy and x ray diffraction techniques petrographic analyses revealed a shock induced formation of three different concentric petrographic zones within the shocked samples zone of total melting i zone of partial melting ii and zone of solid state shock features iii we found a progressive pressure induced oxidation of fe ni metal whose degree increased with increasing shock peak pressure the amount of feo within zone i increased the factor of 1 4 with respect to its amount in the unshocked saratov sample this suggests that within zone i about 70 wt of the initial metallic iron was oxidized whereas magnetic analyses showed that about 10 wt of it remained intact this strongly supports the hypothesis that in addition to oxidation a migration of metallic iron from the central heavily shocked zone i toward less shocked peripheral zone took place as well likely through shock veins where metallic droplets were observed magnetic analyses also showed a shock induced transformation of tetrataenite to taenite within all shocked subsamples resulting in magnetic softening of these subsamples decrease in remanent coercivity these results have important implications for extraterrestrial paleomagnetism suggesting that due to natural impact processes the buried crustal rocks of heavily cratered solid solar system bodies can have stronger remanent magnetism than the corresponding surface rocks

response and recovery of the eel river california and its tributaries to floods in 1955 1964 and 1997 abstract northwestern california is prone to regional high magnitude winter rainstorms which repeatedly produce catastrophic floods in the basins of the northern coast ranges major floods on the eel river in 1955 and 1964 resulted in substantial geomorphic changes to the channel adjacent terraces and tributaries this study evaluated the changes and the effects of a moderate flood in 1997 through field observations and examination of aerial photographs that spanned from 1954 to 1996 the purpose was to document the nature and magnitude of geomorphic responses to these three floods and assess the rates and controls on the recovery of the eel river and its tributaries channel widening from extensive bank erosion was the dominant geomorphic change along the lower eel river during major floods as a result of the 1964 flood the largest amount of widening was 195 m and represented an 80 change in channel width channel narrowing characterized the periods after the 1955 and 1964 floods more than 30 years after the 1964 flood however the river had not returned to pre flood width which suggests that channel recovery required decades to complete a long recovery time is unusual given that the eel river is located in an area with a superhumid climate and has an exceptionally high sediment yield this long recovery time may reflect highly seasonal precipitation and runoff which are concentrated in 3 5 months each winter in contrast to the main stem of the eel river the dominant effects of floods on the tributaries of the eel river were rapid aggradation of channel bed and valley floor followed by immediate downcutting dendrogeomorphic data aerial photographs and field observations indicate that thick wedges of gravel derived largely from hillslope failures in upper reaches of the tributaries are deposited at and immediately upstream of the mouths of tributaries as the stage of the eel river exceeded that of the tributaries during major floods in the waning stages of the flood the tributaries cut through the gravel at a rate equal to the lowering of the eel and generated unpaired terraces and nickpoints the complete process of deposition and incision can occur within a few days of peak discharge although reworking of some sediment on the valley floor may continue for years after large floods channel morphology in the tributaries appears to be a product of infrequent high magnitude events the morphology of the tributary channel also appears to be greatly influenced by the frequency and magnitude of mass wasting in headwater areas of small basins

an early solar system magnetic field recorded in cm chondrites abstract we present a paleomagnetic study of seven cm carbonaceous chondrites cm chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system as such cm chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism the ferromagnetic assemblage of cm chondrites is composed of a mixture of primary metallic iron pyrrhotite and magnetite the remanent properties are usually dominated by secondary pyrrhotite paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin trending component of magnetization in the seven studied cm chondrites in each meteorite this component is homogeneous in direction at least at the cm scale and is therefore post accretional we interpret this stable component as a pre terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few t 2 1 5 t considering the timescale and intensities of primordial magnetic fields both internally generated fields from a putative dynamo and external fields generated in the protoplanetary disk may have been recorded by cm chondrites it is presently difficult to discriminate between the two hypotheses regardless cm chondrites likely contain the oldest paleomagnetic record yet identified

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

braid scale channel geometry changes in a sand bedded river significance of low stages abstract this study focuses on the importance of different stages of bed formation and transportation processes along the partly managed lowland section of maros river south hungary the role of different stages was determined applying a series of cross sectional data besides major morphological zones of over widened braided sections were defined along with their role and function in sediment storage and transport processes during the rising stage of floods a great volume of sediment is eroded from and transported through the cross sections on the falling stage accumulation will be the dominant process however concerning the total amount of sediment deposited during falling stages braids store overwhelmingly more than straight narrow reaches during long lasting low stage periods the cross sectional area increases which means net erosion and an increase in mean depth therefore braids have a significant role in falling stage sediment storage and low stage release nevertheless the location of accumulation within braids is different influenced by their structure in braids which are characterised by islands the sediment is deposited at their upstream end islands create a transportation zone towards the downstream end of the braid by increasing stream power over widened sections without islands are more likely to experience deposition at their downstream end constant narrowing of the studied reach was also observed braids on unmanaged reaches proved to be the most prominent places in this sense changes in the regime of the river and altered land use help to explain this process as the frequency of floods has decreased while the length of extreme low stages has increased therefore the need for the sediment storing and releasing function of braids is less necessary which leads to their narrowing

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

planform channel dynamics of the lower colorado river 1976 2000 abstract in the past two decades major flood events have occurred on the lower colorado river a dramatic shift from the low flows and moderate floods associated with prior decades of river regulation this study uses repeat aerial photography and geographic information system analysis to examine the planform channel response of the upper colorado river delta limitrophe reach to this recently altered hydrology results indicate that channel contraction has been the dominant planform process in recent decades but periodic floods resulted in channel expansion 1981 1988 1997 2000 or likely reduced the channel contraction measured between successive aerial photographs 1976 1981 1988 1994 sinuosity adjustments were limited during the timeframe of the study range 1 31 to 1 38 but the channel did respond with large adjustments in channel width range 44 m to 355 m and changes in total mid channel bar area range 0 to 52 3 ha a channel probability model developed for this study provides a raster image of the channel changes and depicts areas modified to varying degrees by flood flows this historical study provides insight into how floods affect the channel system that provides the foundation for aquatic and riparian biodiversity this issue is of increasing relevance given growing international interest in rehabilitating the riparian and aquatic ecosystems of the colorado river delta through intentional flood flows

dynamics of nonmigrating mid channel bar and superimposed dunes in a sandy gravelly river loire river france abstract a field study was carried out to investigate the dynamics during floods of a nonmigrating mid channel bar of the loire river france forced by a riffle and renewed by fluvial management works interactions between the bar and superimposed dunes developed from an initial flat bed were analyzed during floods using frequent mono and multibeam echosoundings acoustic doppler profiler measurements and sediment grain size analysis when water left the bar terrestrial laser scanning and sediment sampling documented the effect of post flood sediment reworking during floods a significant bar front elongation spreading on margins and swelling was shown whereas a stable area no significant changes was present close to the riffle during low flows and falling limbs of floods intense sediment reworking on the top of the bar and lateral scouring occurred hydrological variations controlled the sediment supply in terms of phasing quantity and grain size delivered by surrounding channels during floods and thus superimposed dune development their development was also linked to the sediment availability armor layers riffle proximity their relatively constant height highlights a preferential adaptation on dune length during floods the role of each morphological forcing parameters riffle vs channel widening and curvature on the bar dynamics and evolution is stage dependent the shape dynamics and long term morphological evolution of the bar and of the river reach surrounding islands channel translation mainly depends on the presence of the natural riffle

river channel response to climate and human induced hydrological changes case study on the meandering hernad river hungary abstract the increasing extent of human activities and climate change may have notable effects on river systems therefore a comprehensive knowledge of these effects is essential to mitigate their unfavourable consequences the aim of the present study is to analyse the hydromorphological changes caused by climate change and human impacts on the hungarian section of the hernad river and to evaluate the morphological responses of this river from a flood prevention perspective over the past 60 years considerable morphological changes were measured in the studied river sections caused by the alteration of their hydrology the main morphological change was the evolution of secondary bends on large meanders this process was driven by extrinsic factors as the morphometric changes were determined by an altered water regime caused by precipitation change and engineering works further mean discharge decreases caused the propagation of this process on smaller meanders as well meanwhile the cross sectional area of the channel also decreased primarily as a result of narrowing this altered morphology enhances the flood risk in the short run as the narrow channel is not sufficient to drain the rapid flood waves in the twenty first century climate change may cause decreasing mean discharge while also increasing the frequency of rapid floods further increasing flood risk however the projection of the geomorphological effects of future precipitation changes is very uncertain

longitudinal variability in hydraulic geometry and substrate characteristics of a great plains sand bed river article i nfo downstream trends in hydraulic geometry and substrate characteristics were investigated along a 200 km reach of the ninnescah river in south central kansas usa the ninnescah river is a large sand bed perennial braided river located in the central plains physiographic province and is a tributary of the arkansas river hydraulic ge ometry characteristics were measured at eleven reaches and included slope sinuosity bankfull channel width and bankfull channel depth results indicated that the ninnescah river followed a predicted trend of decreasing slope and increasing depth and width downstream there were localized divergences in the central tendency most notability downstream of a substantial tributary that is impounded and at the end of the surveying reach where the ninnescah river approaches the arkansas river surface grain size samples were taken from the top 10 cm of the bed at five points across the wetted cross section within each of the 11 reaches sediment analyses demonstrated a significant trend in downstream fining of surface grain sizes d90 and d50 but unlike previous studies of sand bedded rivers we observed coarsening of substrates downstream of the major tributary conflu ence we propose that the overall low discharge from the tributary was the primary reason for coarsening of the beddownstreamof thetributary results of this studyprovidevaluable baseline information thatcanprovide insight in to how great plains sand bed systems may be conserved managed and restored in the future

fluvial process and the establishment of bottomland trees the effects of river regulation on bottomland tree communities in western north america have generated substantial concern because of the important habitat and aesthetic values of these communities consideration of such effects in water management decisions has been hampered by the apparent variability of responses of bottomland tree communities to flow alteration when the relation between streamflow and tree establishment is placed in a geomorphic context however much of that variability is explained and prediction of changes in the tree community is improved the relation between streamflow and establishment of bottomland trees is conditioned by the dominant fluvial process or processes acting along a stream for successful establishment cottonwoods poplars and willows require bare moist surfaces protected from disturbance channel narrowing channel meandering and flood deposition promote different spatial and temporal patterns of establishment during channel narrowing the site requirements are met on portions of the bed abandoned by the stream and establishment is associated with a period of low flow lasting one to several years during channel meandering the requirements are met on point bars following moderate or higher peak flows following flood deposition the requirements are met on flood deposits high above the channel bed flood deposition can occur along most streams but where a channel is constrained by a narrow valley this process may be the only mechanism that can produce a bare moist surface high enough to be safe from future disturbance because of differences in local bedrock tributary influence or geologic history two nearby reaches of the same stream may be dominated by different fluvial processes and have different spatial and temporal patterns of trees we illustrate this phenomenon with examples from forests of plains cottonwood populus deltoides ssp monilifera along meandering and constrained reaches of the missouri river in montana

in channel woody vegetation controls on sedimentary processes and the sedimentary record within alluvial environments a modern example of an anabranch of the river loire france a multidisciplinary approach was used to analyse the role of pioneer trees populus nigra on bar formation in the modern fluvial system of the river loire france data were collected on seven plots located on a secondary channel chosen for their vegetation cover and their sedimentary context physical features of vegetation topographical evolution sediment grain size and flow velocity were investigated during and after floods between 2001 and 2003 woody vegetation responds morphologically to fast flowing waters and sediment deposition these adaptations influence the role played by trees on local sedimentary processes some morphological types of trees exert a strong influence during flood events by deflecting reducing or increasing the flow energy as a consequence the evolution of bedforms distribution of sediment grain size and slopes are controlled by the flood type the morphological context i e high or low energy zones the morphology of the vegetated bar and the characteristics of vegetation conceptual models are proposed to account for local processes and evolution of tree groves the first model which describes the behaviour of groves during a single flood underlines the sediment deposition downstream of the trees and the trapping of bedload sediments during the falling stage of the flood the reorientation of current streams constrained by both local bed topography and woody vegetation induces transverse sediment fluxes for these water levels the control exerted by trees on small scale sedimentary processes becomes significant the second model shows the evolution of vegetation bands over several flood events in particular it describes the influence of vegetation on the fixing and reworking of bedload sediments during the falling limb of the hydrograph the coalescence of vegetation induced bedforms and the resulting morphological changes increase the deflection power of the vegetated bar inducing the deposition of finer sediments

the geomorphic effectiveness of a large flood on the rio grande in the big bend region insights on geomorphic controls and post flood geomorphic response abstract since the 1940s the rio grande in the big bend region has undergone long periods of channel narrowing which have been occasionally interrupted by rare large floods that widen the channel termed a channel reset the most recent channel reset occurred in 2008 following a 17 year period of extremely low stream flow and rapid channel narrowing flooding was caused by precipitation associated with the remnants of tropical depression lowell in the rio conchos watershed the largest tributary to the rio grande floodwaters approached 1500 m 3 s between a 13 and 15 year recurrence interval and breached levees inundated communities and flooded the alluvial valley of the rio grande the wetted width exceeding 2 5 km in some locations the 2008 flood had the 7th largest magnitude of record however conveyed the largest volume of water than any other flood because of the narrow pre flood channel conditions record flood stages occurred we used pre and post flood aerial photographs channel and floodplain surveys and 1 dimensional hydraulic models to quantify the magnitude of channel change investigate the controls of flood induced geomorphic changes and measure the post flood response of the widened channel these analyses show that geomorphic changes included channel widening meander migration avulsions extensive bar formation and vertical floodplain accretion reach averaged channel widening between 26 and 52 occurred but in some localities exceeded 500 the degree and style of channel response was related but not limited to three factors 1 bed load supply and transport 2 pre flood channel plan form and 3 rapid declines in specific stream power downstream of constrictions and areas of high channel bed slope the post flood channel response has consisted of channel contraction through the aggradation of the channel bed and the formation of fine grained benches inset within the widened channel margins the most significant post flood geomorphic changes have occurred at and downstream from ephemeral tributaries that contribute large volumes of sediment

the nahal oz reservoir dam break flood geomorphic impact on a small ephemeral loess channel in the semi arid negev desert israel abstract the nahal oz reservoir in the coastal semi arid southwestern israel was designed to enhance local irrigation of crops using reclaimed sewage water during the dry summer months on march 2001 part of the western dike of the reservoir was breached and generated a flow release of 3 5 106 m3 of secondary irrigation water that was channeled down the 1st order ephemeral loess stream nahal yare akh the consequent 12 hour flood surge with an estimated peak discharge of 1000 m3s 1 inflicted severe loess erosion agricultural property and infrastructural damage downstream 6 million post flood mapping documented the geomorphic response to the flood which included channel scour and widening along the initial 2 km downstream of the reservoir where a spillway channel was formed the increase in the cross sectional area was about 60 and had an estimated 170 000 m3 of sediment bed bank and floodplain erosion calculated maximum shear stress and stream power along this section are estimated at 300 nm 2 and 900 wm 2 respectively the peak discharge at the end of this segment was estimated at 800 m3s 1 indicating only minor attenuation along this segment two km downstream of the breach a wide braided fan indicated deposition of the eroded sediments at the end of this segment the floodwater diverged into several watercourses and inundated tilled agricultural fields and neighborhoods downstream 9 km from the reservoir the discharge attenuated to 100 m3s 1 slightly above bankfull further downstream and upon reaching the large shikma stream the flow was already very low this reduction in discharge is attributed to the anthropogenic infrastructure roads neighborhoods and agricultural fields and the large transmission losses typical of sandy ephemeral streams the study shows that channels within erodible materials respond to high peak discharges very locally erosional thresholds that severely incised the channel are only maintained for 2 km below the breached reservoir and as distance from the source area increases available energy is substantially reduced due to high transmission losses gentler valley gradients very wide shallow flow and the ability of the drainage network to accommodate large discharges consequently the geomorphic impact was limited to the first 2 6 km with only minor erosional or depositional evidence downstream the current natural flow regime has only minor impact on the newly formed channel geometry below the breached dam accordingly the channel geometry may be preserved for a relatively long time however channel recovery in this segment was rapid and within a few years all bare loess surfaces exposed during the flood have been covered by dense vegetation the analysis also shows that nahal oz reservoir s dam break flood transmission losses and flood attenuation rates were extreme in comparison to other case studies this artificial flood event in a desert environment offers a rare opportunity to quantify channel and bank erosion evolution processes during an extremely high magnitude flood within highly erodible fine sediment as well as to detect the recovery processes a few years later

flow and sediment dynamics in the vegetated secondary channels of an anabranching river the loire river france this study investigates the hydrological and sedimentological mechanisms occurring in the vegetated secondary channels of an anabranching river affected by incision the loire river france during and after flood events that occurred between 2000 and 2003 observations and measurements were performed on a vegetated secondary channel located in the study site of brehemont 790 km downstream the source morphological changes and sediment dynamics were analysed using low elevation airborne photographs topographic and bathymetric surveys and scour chains the hydraulic behaviour of the channel was also analysed by measurements performed on flow velocity and direction during different flood stages in order to quantify the influence of woody vegetation on flow resistance the roughness of bands of trees was determined from measurements performed on the field the impact of the disruption of armour layers on bedload pulses the variation of sedimentary processes during a single flood event and the fixation of bedforms by vegetation are all identified as key processes influencing the behaviour of the study channel topographic surveys demonstrate that sediment dynamics is substantial in the upstream part of the channel and that sediment budgets are different according to the temporal scale considered moreover an asymmetrical behaviour of the secondary channel is demonstrated reduced quantities of sediment deposited and preserved in the vegetated zones contrast with material by passing observed in the third order channels flow velocity and direction measurements indicate that these parameters vary according to the water level and to the morphological units of the channel pools riffles vegetated areas during low flows scouring and export of particles from the secondary channel are a consequence of reduced sediment supply from the main channel of the loire river for these water levels sedimentation occurs in pools where velocity and turbulence decrease whereas third order channels are subjected to erosion during high discharges large quantities of sediment available in the main channel feed the temporary stores formed by riffles and bars in the secondary channel the vegetated area located in the downstream part of the secondary channel deflects current trickles at low discharges and decreases flow velocity during high water levels the sedimentary accretion observed in this area exerts a feedback on flow and sedimentary processes d 2005 elsevier b v all rights reserved

thalweg variability at bridges along a large karst river the suwannee river florida abstract geomorphologists and engineers have different perspectives and approaches for examining river channels and the changes that occur during floods the field oriented approach typically adopted by geomorphologists has little predictive ability and design usefulness in contrast the empirical approach adopted by engineers is based on predictive equations or models that often differ greatly from reality such equations are not based on comprehensive field data and often fail to consider a number of site conditions especially geology and geomorphology yet in order for geomorphic techniques to be useful to the design and planning of engineering structures such as bridges it is important that sufficient observations exist in order to characterize long term and short term changes in bottom topography and scour potential six gaging stations on the suwannee river a large river draining karst terrain in the southeastern us were used to examine the temporal variability in thalweg elevation the deepest point in a given cross section the cross sections have maximum thalweg variability of just a few meters despite the occurrence of several large floods suggesting that the bottoms are fairly stable historical approaches can be applied to design the length and depth placement of pilings by providing information on site conditions not considered in engineering equations such as response of bottom materials to various flow conditions and thus have potential benefits to public safety and cost effectiveness

the efficacy of stream power and flow duration on geomorphic responses to catastrophic flooding abstract geomorphologists have long studied the impacts of extreme floods yet the association between the magnitude of flow parameters discharge velocity shear stress or stream power and resulting geomorphic effectiveness remains vague and non deterministic attempts have been made to include flow duration and total expenditure of stream power in combination with peak unit stream power as important variables but there has been minimal exploration of this hydraulic combination taking advantage of tropical storm irene s rapid track through eastern vermont usa in late summer 2011 this paper presents the array of geomorphic responses to a short duration time to peak of

contribution to understanding the historical evolution of meandering rivers using dendrochronological methods example of the mala panew river in southern poland the mala panew is a meandering river that flows 20 km through a closed forest during times of high discharge the riverbed and floodplain are transformed under the influence of riparian trees the changes provide the opportunity to measure the intensity of erosion and sediment accumulation based on tree ages the dating of coarse woody debris cwd in the riverbed and the dating of eccentric growth of tilting trees and exposed roots the bed and floodplain in reaches of the mala panew river with low banks were greatly altered as a result of long periods of flooding between 1960 and 1975 banks were undercut during these floods and black alders tilted those parts of alder crowns or stems which tilt and sink generate small sand shadows when erosion is intensive alder clumps are undercut from concave banks and become mid channel islands while on the other side of the channel meandering bar levels are created the reaches with higher banks were altered by large floods especially in 1985 and 1997 the concave banks are undercut and sediment with cwd is deposited within the riverbed forming sand shadows behind the cwd copyright 2006 john wiley sons ltd

patterns of floodplain sediment deposition along the regulated lower roanoke river north carolina annual decadal centennial scales abstract the lower roanoke river on the coastal plain of north carolina is not embayed and maintains a floodplain that is among the largest on the mid atlantic coast this floodplain has been impacted by substantial aggradation in response to upstream colonial and post colonial agriculture between the mid eighteenth and mid nineteenth centuries additionally since the mid twentieth century stream flow has been regulated by a series of high dams we used artificial markers clay pads tree ring dendrogeomorphic techniques and pollen analyses to document sedimentation rates amounts over short intermediate and long term temporal scales respectively these analyses occurred along 58 transects at 378 stations throughout the lower river floodplain from near the fall line to the albemarle sound present sediment deposition rates ranged from 0 5 to 3 4 mm y and 0 3 to 5 9 mm y from clay pad and dendrogeomorphic analyses respectively deposition rates systematically increased from upstream high banks and floodplain to downstream low banks reaches except the lowest reaches conversely legacy sediment deposition a d 1725 to 1850 ranged from 5 to about 40 mm y downstream to upstream respectively and is apparently responsible for high banks upstream and large wide levees along some of the middle stream reaches dam operations have selectively reduced levee deposition while facilitating continued backswamp deposition a gis based model predicts 453 000 mg of sediment is trapped annually on the floodplain and that little watershed derived sediment reaches the albemarle sound nearly all sediment in transport and deposited is derived from the channel bed and banks legacy deposits sources and regulated discharges affect most aspects of present fluvial sedimentation dynamics the lower river reflects complex relaxation conditions following both major human alterations yet continues to provide the ecosystem service of sediment trapping

reconstruction of a pleistocene meandering river in east hungary by vhr seismic images and its climatic implications abstract the upper 20 30 m of the pleistocene alluvial sequence on the tisza river hungary is investigated by 200 km of vhr single channel seismic sections frequent internal configurations such as 1 bundles of inclined reflections 2 trough shaped converging and 3 horizontal parallel reflections are interpreted respectively as lateral accretion of point bars abandoned channel fills and floodplain deposits the series of 5 8 m thick inclined strata sets occurring at the same depth throughout the area suggest the presence of a meandering river dated to about 43 47 ka in the late pleistocene according to osl data newly recorded quasi 3d seismic profiles reveal that each 1 3 km long inclined series correlates with an individual point bar continuously migrating in the same direction small deviations in the direction of migration are caused by natural wandering of the meander bend morphometric parameters of point bars are used to calculate ancient channel parameters width and depth of 200 500 m and 5 8 m respectively values imply a channel forming discharge of q 400 900 m 3 s these results imply a greater amount of runoff and supply in the basin during mis3 compared to the present day long lasting meandering indicates the role of local factors and a different fluvial environment compared to the commonly anastomosing systems in nw europe our data support the thesis of the three axis drainage system for the basin new evidence is provided for the antecedent existence of a temporally active third trunk river in addition to the ancestors of danube and tisza rivers contrary to the present day two axis drainage pattern