u pb detrital zircon and 39ar 40ar muscovite ages from the eastern parts of the karagwe ankole belt tracking paleoproterozoic basin formation and mesoproterozoic crustal amalgamation along the western margin of the tanzania craton abstract the karagwe ankole belt forms part of a complex system of orogenic belts in central east africa that record the amalgamation of archaean and or paleoproterozoic continental kernels in the proterozoic the actual timing of tectonic processes including initial basin formation and subsequent deformation is only poorly constrained so that the geodynamic significance of the karagwe ankole belt within the broader system of proterozoic orogens in central east africa is not well understood this paper presents u pb ages of detrital zircons constraining the age of sediments of the eastern parts of the karagwe ankole fold belt overlying the western tanzania craton and 39ar 40ar muscovite ages from mylonites and phyllonites associated with the main phase of subsequent shortening and fold and thrust tectonics d2 39ar 40ar muscovite ages from the main detachment between archaean basement gneisses and imbricated cover rocks of the kagera supergroup in the eastern frontal parts of the karagwe ankole belt constrain the timing of the main fabric forming event and thrusting d2 to 1326 10 ma this age is at the young end of the age range for the main phase of granite plutonism and mafic dyke emplacement that have previously been related to crustal extension and associated intracontinental rifting if correct and representative for the main phase of deformation in the rest of the karagwe ankole belt the ages may rather point to a collisional event between the congo and tanzania cratons west of the karagwe ankole belt in mesoproterozoic times indicating a closer correlation with rocks of the kibara belt sensu stricto to the immediate south u pb detrital zircon ages for the older muyaga and overlying bukoba group of the kagera supergroup confirm the paleoproterozoic timing of sedimentation from ca 1780 ma but before ca 1568 ma the presence of a distinct group of ca 1750 1790 ma zircons in the bukoba group indicate uplift erosion and subsequent reworking of sediments and volcanic units contained in the lower muyaga group to the west the ages also suggest a correlation between the bwezigoro group of sw uganda and the bukoba group indicating the presence of a far more extensive paleoproterozoic and or early mesoproterozoic foreland basin overlying the western margin of the archaean tanzania craton and paleoproterozoic uganda block

changes in a large regulated tropical river the parana river downstream from the porto primavera dam brazil abstract dams disturb in the fluvial dynamics by changing the natural cycle of the rivers interfering with the transport deposition processes and affecting river ecology the brazilian part of the parana basin has the largest hydrometrical power potential of the country with more than 150 hydroelectric power dams that transformed the parana river and its tributaries in a succession of lakes this research aims to analyze the changes in certain fluvial variables in the last natural downstream reach of the upper parana river after closure of the porto primavera dam data acquired before and after dam construction showed changes in water discharge bank erosion flood pulse bed load grain size discharge concentration of suspended load and bedform morphology and size those alterations generated changes in river ecology especially on fish reproduction benthic community and rotifers variety and density a review of the obscure regional literature generally in portuguese is also presented

gravel bed channel changes upstream of a reservoir the case of the dunajec river upstream of the czorsztyn reservoir southern poland abstract despite a significant increase in the number of dam reservoirs in the world in the last century the effects of these structures on the evolution of river channels above them are poorly understood constructed in 1997 in the polish carpathians the czorsztyn reservoir cr has raised the base level of the single thread gravel bed dunajec river an analysis of five sets of aerial photographs and orthophotos from the period before 1977 1982 1994 and after 2003 2009 the construction of the cr allowed me to investigate the temporal and spatial impact of the raised base level of the river upstream of the reservoir on the evolution of the river channel comparison of channel width changes in the backwater river section with changes observed in the control section unaffected by the presence of the reservoir has shown that in the pre dam period differences in the evolution of the river channel in the backwater section and in the control section were not statistically different in the periods without major floods both channel sections were dominated by channel narrowing or a relative stability of channel width in the post dam period a major flood in 1997 caused more than two times greater channel widening in the backwater section than in the control section the extent of channel widening in the longitudinal channel profile reached farther upstream of the reservoir than the extent of the backwater itself in the later part of the post dam period without major floods channel narrowing were similar in the backwater section and the control section as had been the case in the period before the reservoir construction the comparison of changes in the width of the channel in the backwater and control sections indicates that channel widening in the backwater interpreted as an early stage of backwater channel adjustment was reinforced by the flood of 1997 this study shows that the use of a control section makes it possible to isolate reservoir effects from flood effects in the context of channel dynamics

channel variations of the different channel pattern reaches in the lower yellow river from 1950 to 1999 abstract the yellow river is widely known as a hanging river due to its high sedimentation rate the current channel bed is about 2 74 m higher than the overbank plains on average in the lower reach understanding the variations in channel patterns and their influencing factors is crucial to regulate potential flooding risk in this critical region changes in discharge suspended sediment delivery rate median grain size sedimentation rate channel planform and cross section and channel width depth ratio during 1950 1999 from gauging stations in huayuankou gaocun sunkou and lijin that represent the braided meandering and straight channel sections respectively were compared during this period frequent internal channel variations dominated in the braided channel section the channel bed was continuously raised as a whole but experienced strong erosion in certain times mainly influenced by the operation and closure of the sanmenxia dam the straight channel section was relatively stable due to restricted lateral migration by the solid artificial levees in both banks instead channel aggradation was the dominant process and erosion only happened when channel avulsions occurred in the river mouth the meandering channel section between the upstream braided and downstream straight channel sections was formed as a result of stream self adjustment rapid lateral migrations were observed during this period but the extent of the migration was restricted by artificial levees sediment aggradation on the channel bed and floodplain was apparent since 1970 the channel shrank remarkably in all sections due to the increased use of water resources and the decreased precipitation in the yellow river basin recently migration of the channel is limited and the lower portion of the braided section seems to be a meandering channel

evolution of mid channel bars in a braided river and complex response to reservoir construction an example from the middle hanjiang river china using the middle hanjiang river as an example this paper deals with the development of mid channel bars in a braided river as controlled by channel boundary conditions runoff and sediment load as well as reservoir construction relations have been established between indices describing the mid channel bars and the controlling variables such as the channel width the percentage silt clay content in the channel boundary and the energy expenditure of flowing water bank erosion provides space and material for mid channel bar formation so bank erosion rate can be closely related to mid channel bar indices the middle channel bars exhibit a complex evolutionary response to the reservoir regulated hydrological regime in the middle hanjiang river a descriptive model showing the complex response has been established 1997 john wiley sons ltd

flood driven topographic changes in a gravel cobble river over segment reach and morphological unit scales regulated rivers generally incise below dams that cut off sediment supply but how that happens and what the consequences are at different spatial scales is poorly understood modern topographic mapping at meter scale resolution now enables investigation of the details of spatial processes in this study spatial segregation was applied to a meter scale raster map of topographic change from 1999 to 2008 on the gravel cobble regulated lower yuba river in california to answer specific scientific questions about how a decadal hydrograph that included a flood peak of 22 times bankfull discharge affected the river at segment reach and morphological unit scales the results show that the river preferentially eroded sediment from floodplains compared to the channel and this not only promoted valley wide sediment evacuation but also facilitated the renewal and differentiation of morphological units especially in the channel at the reach scale area of fill and mean net rate of elevational change were directly correlated with better connectivity between the channel and floodplain while the mean rate of scour in scour areas was influenced by the ratio of slope to bankfull froude number a ratio indicative of lateral migration versus vertical downcutting hierarchical segregation of topographic change rasters proved useful for understanding multiscalar geomorphic dynamics this article is protected by copyright all rights reserved

hydrologic variation with land use across the contiguous united states geomorphic and ecological consequences for stream ecosystems using daily discharge data from the usgs we analyzed how hydrologic regimes vary with land use in four large hydrologic regions that span a gradient of natural land cover and precipitation across the continental united states in each region we identified small streams contributing areab282 km 2 that have continuous daily streamflow data using a national database we characterized the composition of land cover of the watersheds in terms of aggregate measures of agriculture urbanization and least disturbed natural we calculated hydrologic alteration using 10 ecologically relevant hydrologic metrics that describe magnitude frequency and duration of flow for 158 watersheds within the southeast se central ce pacific northwest nw and southwest sw hydrologic regions of the united states within each watershed we calculated percent cover for agriculture urbanized land and least disturbed land to elucidate how components of the natural flow regime inherent to a hydrologic region is modified by different types and proportions of land cover we also evaluated how dams in these regions altered the hydrologic regimes of the 43 streams that have pre and post dam daily streamflow data in an analysis of flow alteration along gradients of increasing proportion of the three land cover types we found many regional differences in hydrologic responses in response to increasing urban land cover peak flows increased se and ce minimum flows increased ce or decreased nw duration of near bankfull flows declined se nw and flow variability increased se ce and nw responses to increasing agricultural land cover were less pronounced as minimum flows decreased ce near bankfull flow durations increased se and sw and flow variability declined ce in a second analysis for three of the regions we compared the difference between least disturbed watersheds and those having either n15 urban and n25 agricultural land cover relative to natural land cover in each region urbanization either increased se and nw or decreased sw peak flows decreased minimum flows se nw and sw decreased durations of near bankfull flows se nw and sw and increased flow variability se nw and sw agriculture had similar effects except in the se where near bankfull flow durations increased overall urbanization appeared to induce greater hydrologic responses than similar proportions of agricultural land cover in watersheds finally the effects of dams on hydrologic variation were largely consistent across regions with a decrease in peak flows an increase in minimum flows an increase in nearbankfull flow durations and a decrease in flow variability we use this analysis to evaluate the relative degree to which land use has altered flow regimes across regions in the us with naturally varying climate and natural land cover and we discuss the geomorphic and ecological implications of such flow modification we end with a consideration of what elements will ultimately be required to conduct a more comprehensive national assessment of the hydrologic responses of streams to land cover types and dams these include improved tools for modeling hydrologic metrics in ungauged watersheds incorporation of high resolution geospatial data

changes in hydrologic regime by dams dams have major impacts on river hydrology primarily through changes in the timing magnitude and frequency of low and high flows ultimately producing a hydrologic regime differing significantly from the pre impoundment natural flow regime this paper presents the analysis of pre and post dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the united states our overall goals are to document the type magnitude and direction of hydrologic shifts because of impoundment using the entire database for the national inventory of dams nid for dams possessing longstanding u s geological survey usgs gages downstream we identified 21 gage stations that met length of record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area to assess hydrologic changes associated with dams we applied a hydrologic model the indicators of hydrologic alteration iha supplemented with orientation statistics for certain hydrograph parameters dams had significant impacts on the entire range of hydrologic characteristics measured by iha for many characteristics the direction and significance of effects were highly consistent across the 21 sites the most significant changes across these sites occurred in minimum and maximum flows over different durations for low flows the 1 day through 90 day minimum flows increased significantly following impoundment the 1 day through 7 day maximum flows decreased significantly across the sites at monthly scales mean flows in april and may tend to decline while mean flows in august and september increase other significant adjustments included changes in annual hydrograph conditions primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment the number of high pulses has increased following impoundment but the average length declines the mean rate of hydrograph rise and fall has declined significantly these results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale for large and small rivers

morphological effects of damming on lower zambezi river the zambezi river is the fourth largest river in africa after the congo nile and niger and it is the largest african river flowing into the indian ocean the lower zambezi in mozambique is influenced by the presence of two very large reservoirs kariba dam and cahora bassa dam that have modified the natural seasonal flows as well as the sediment balance and morphology of the river in particular downstream of the cahora bassa reservoir down to the delta no negligible effects are reported to take place such as local scour bank collapse and shoreline progressive erosion davies et al 2000 beilfuss and dos santos 2001 in order to quantify and possibly mitigate these effects a simplified numerical model of the sediment transport and erosion sedimentation phenomena along the lower zambezi has been developed capable to deal with the scanty and uncertain data available indeed besides the systematic flow records at the dam sites and few occasional measurements of turbidity and bottom granulometry only the digital elevation model dem of the river will be used for the model the objective of this paper is to predict the present and future effects of the presence of the kariba and cahora bassa dams on the downstream morphology integrating the few coarse and nonsimultaneous data somehow improving their overall quality the model reproduces the time and space propagation of the disturbances that confirm with more generality the qualitative response of the river to the constructions of dams anticipated by various geomorphologists in fact the reduction of waterflow seems to have an immediate effect downstream by initially fostering the sediment deposition subsequently the total interception of sediment by the dam slowly takes over and inverts this tendency a slightly smaller aggradation or slightly larger degradation rate with respect to the natural conditions no dams seems to represent the dominant effect of damming in the long term evolution of the lower zambezi

effects of check dams on bed load transport and steep slope stream morphodynamics abstract check dams are transversal structures built across morphologically active streams in mountainous regions these structures have been used widely in torrent hazard mitigation for over 150 years thousands of them are regularly maintained by stream managers and torrent control services the stabilization role of these structures is well known i e they durably constrain the stream bed through the creation of vertical and planar fixed points what is not yet clear is to what extent check dams influence bed load transport how do peak solid discharge or flood transported volume change when check dams are added to a reach to address these questions long lasting small scale experiments were conducted in a 4 8 m long flume with either one three or no structures the results show that the addition of structures creates independent compartments in the bed level which have a strong influence on bed surface armouring and stream morphodynamics the consequence is that instantaneous transport intensities are unchanged but peak solid discharge occur more often and for shorter duration this results in the same total transported volume over the long term but reduced volume for a single transport event it reaffirms the observation of pioneering authors of the mid 19th and early 20th century who conceptualized the possible sediment transport regulation function of check dams in addition to stabilizing the stream bed check dams influence bed load transport through a buffering effect releasing frequently and in small doses what in their absence would be transported abruptly en masse during rare extreme events

response of a gravel bed river to dam closure insights from sediment transport processes and channel morphodynamics this work deals with the impacts of dams on large gravel bed rivers in terms of altering coarse transport regimes and the relationship with river morphodynamics using data collected by a tracer based monitoring program carried out in a 4 km long study sector of the parma river italy located downstream from a relatively recently established dam we applied a virtual velocity approach to estimate the coarse bed material load at four river cross sections monitoring and calculation results provided new insights into the impacts of the dam on streambed material mobility and the sediment regime over the 17 month calculation period a longitudinal gradient of effects was observed along the study sector sections located closer to the dam are characterized by more evident impacts due to deficits in coarse sediment input from upstream sediment mobility here is strongly altered especially in the highly armored main channel and the overall bed material load is extremely low a partial recovery of sediment dynamics was observed at the sections located further from the dam where estimates indicate higher sediment yield the observed longitudinal trend in the coarse sediment transport regime matches the morphology as the river shifts downstream from a sinuous configuration with alternate bars to a wandering one the novel insights into alteration of coarse sediment dynamics and the relationship with river morphodynamics are potentially applicable to many other fluvial contexts affected by similar impoundments

channel aggradation below chew valley lake somerset u k variations of channel substrate composition are described for a regulated river which has experienced channel change since dam completion sediment samples were obtained using bulk grab sampling and freeze coring data derived from the two methods are shown to differ bulk sample data describe the bed surface layer its grain size variations are related to bedform freeze core data describe the sub surface structure and the fine size grades the gravel framework supported substrate of the control reaches constrasts with the sand dominated substrate of the aggraded reach although in both cases the substrate coarsens upwards in the former this relates to the development of a sand matrix within the gravel framework substrate and in the latter sand bed channel to the progressive addition of a gravel component towards the surface which is related to the process of channel change at the local scale bedform is shown to exert an important control on the character of substrate change induced by flow regulation

analysis of the effects of human activities on the hydromorphological evolution channel of the saint maurice river downstream from la gabelle dam quebec canada abstract during the first half of the twentieth century many hydroelectric facilities were built in the saint maurice river watershed followed by other human activities in the second half of the century pleasure boating boom dismantling urbanization etc the goal of the study is to constrain the effects of these various types of human activities particularly those of the many dams in the watershed on the hydromorphological evolution of the saint maurice river downstream from the la gabelle dam power plant 43 000 km2 comparison of specific discharge in this river with streamflow measured in a natural river setting reveals a significant decrease in seasonal maximum flows aside from winter when daily maximum flows increased significantly also unlike natural rivers the long term trend in spring flows is not characterized by a significant change in mean downstream from the la gabelle plant these hydrological changes are linked to the inversion type management mode of the reservoirs built downstream from the plant as for the morphological evolution the longitudinal variability of bankfull width downstream from the plant shows two significant shifts in mean the first which was quasi abrupt took place downstream of the des forges rapid and the second which was gradual occurred upstream from the confluence of the saint maurice river with the st lawrence river above the point where the saint maurice splits into two branches comparison of aerial photographs taken at various times 1948 1964 1975 1996 and 2008 reveals no significant change in the mean of bankfull width over time however a significant increase in the surface area of islets located at the confluence was observed which is caused by sediment accumulation these sediments were likely derived from local bank erosion resulting from anthropogenic changes

bed stability variations after check dam construction in torrential channels south east spain the effects of check dams on the bed stability of torrential channels have been analysed in several tributary basins of the segura and guadalentin rivers south east spain in order to illustrate the large variability in channel bed forms and bed sediment sizes along the stream 52 reaches of 150 m in length were surveyed this variability is due to the behaviour of check dams which depends on bedrock control bed slope channel roughness lateral sediment input and a highly variable sediment transport capacity though the purpose of check dams is to diminish the boundary shear stress reducing the longitudinal slope and to stabilize the channel bed downstream they reduce the volume of channel stored material favouring local scour processes and upstream they can destabilize the sidewalls the results enable us to evaluate the impact of every check dam on the bed morphology distinguishing the structures installed in limy marl areas e g catchment of the carcavo rambla cieza and in schist and slate terrains e g catchment of the torrecilla rambla close to lorca in the first type bedrock and moderately thick granular beds predominate downstream from the check dams so that the length of bedrock reaches and increase of roughness due to scour processes are the best indicators to verify its geomorphological effectiveness on the other hand the metamorphic areas drained by ramblas and gullies produce great quantities of gravel that are retained by check dams creating more uniform and permeable beds where the balance between sedimentation and scouring and the ratio c84 0 rbs appear to be the parameters most frequently adopted to estimate the bed stability analysis of slope adjustments and the application of other indices to estimate the bed substrate stability lrbs sri and the structural influence of the dams sibs corroborate the differences in bed stability found in the corrected reaches in each catchment copyright 2007 john wiley sons ltd

sediment and particulate organic carbon budgets of a subarctic estuarine fjard lake melville labrador abstract sediment and particulate organic carbon poc budgets constructed for oceanic regions provide a means to quantify the burial of poc providing a baseline against which future change can be measured here we developed the first mass balance budgets of sediment and poc terrestrial and marine for the lake melville system lms goose bay and lake melville in central labrador using previously published data 210pb based mass accumulation rates river discharge and total suspended solids together with measurements of oc and 13c signatures within dated sediment cores collected from the lms basins and water from main tributaries and groswater bay a two end member mixing model was used to establish fractions of terrigenous and marine poc overall our mass balance budgets show that the churchill river is the major supply of sediment 16 7 7 8 108 kg y 1 and pocterr 18 4 6 6 106 kg y 1 to the lms accounting for 47 and 56 of the total input to the system respectively although the churchill river drains directly into goose bay along with the smaller goose river the majority of sediment 12 9 7 8 108 kg y 1 and pocterr 11 8 7 0 106 kg y 1 is transported eastward through the surface water plume into lake melville inputs of pocmar from autochthonous primary production estimated by applying an empirical relationship between sediment surface flux of pocmar and water depth to goose bay 1 8 106 kg y 1 and lake melville 98 3 4 7 106 kg y 1 increase with increasing distance from the churchill river to investigate the potential changes to the sediment load of the churchill river to the lms following impoundment at muskrat falls we calculated the sediment increase using the median reservoir shoreline erosion potential this approach showed that the delivery of sediment and pocterr to the lms by the churchill river may double during the two years following impoundment of the lower churchill river

channel changes following reservoir construction on a lowland english river the effects of base flow augmentation upon cohesive boundary channels are assessed by the examination of the river ter essex u k channel changes are identified from the comparison of field data for fifteen cross sections surveyed before nine and fifteen years after the completion of an off line storage reservoir a ten fold increase of low flow discharge below the reservoir appears to be responsible for a two fold increase of channel capacity variations in the magnitude and direction of change observed between individual locations were directly related to the initial channel form

river channel change problems of interpretation illustrated by the river derwent north yorkshire some inherent limitations of the spatial interpolation method of identifying and interpreting channel adjustment are illustrated by a study of the river derwent in yorkshire here cross sections downstream from a river diversion appear to have contracted in size when compared with predictions based on upstream relationships between channel form variables and basin area however these sections are slightly larger than expected for their diminished discharge suggesting that they have not fully adjusted to altered environmental conditions and are still in a transient state

downstream effects of flaming gorge reservoir on the green river colorado and utah the green river is one of the principal tributaries in the colorado river basin and drains 44 700 m 2 in wyoming colorado and utah since october 1962 flows of the green river have been regulated by flaming gorge reservoir which is located 412 river miles upstream from its confluence with the colorado river mean annual runoff has not been affected by the reservoir the duration of the relatively large discharges that transport most of the annual sediment load however has decreased significantly as a result the mean annual sediment discharge has decreased by 54 to 3 21 10 6 tons from 6 92 10 6 tons at the jensen gage located 105 river miles downstream from the reservoir and by 48 to 8 83 10 6 tons from 17 0 10 6 tons at the green river utah gage located 290 river miles downstream from the reservoir sediment supply to i he channel equals the annual transport within a relatively short distance 68 river miles downstream from the reservoir downstream from river mile 166 the supply of sediment from upstream plus tributary inflow exceeds the transport of sediment by 5 4 10 6 tons per year on an average the quasi equilibrium that appears to have existed prior to the reservoir no longer occurs along a majority of the green river in response to the reduced peak discharges the bankfull channel width of the green river has decreased by 10 adjustment of the channel to decreased peak flows and altered sediment loads is nowhere complete at present it appears that a century or more will be required for the green river to adjust to the effects of flaming gorge reservoir

changes in river regime after the construction of upstream reservoirs this article presents and analyses many years of investigations in china on the fluvial processes downstream of impounding and detention reservoirs the study covers the change in hydrograph the recovering of sediment concentration along the river course the degradation of stream bed the adjustment of longitudinal profile the coarsening of bed material the change in channel width and the trend of channel pattern variation for alluvial streams downstream of impounding reservoirs without confluence of major tributaries the degradation may extend to a great distance below the dam in the process of reducing the sediment carrying capacity of the flow to match the diminished sediment supply the coarsening of bed material is a factor of equal if not greater importance as compared with the flattening of channel gradient in places where the flow has not been sufficiently cut down and the bank is erosive non resistant a receding of banklines may take place in concurrence with the deepening of the river bed below detention reservoirs even if the total runoff and sediment supply remain essentially unchanged the modification of the hydrograph is sufficient to enhance the deterioration of the downstream channel

significant decadal channel change 58 67 years post dam accounting for uncertainty in topographic change detection between contour maps and point cloud models abstract construction of digital elevation models dems and the subtraction of dems between different points in time as a method to determine temporal patterns of scour and fill is a highly valuable procedure emerging in geomorphology these dems of differences dods must be assessed for error in order to distinguish actual topographic change from uncertainty and surface error current methods include 1 uniformly excluding all values that fall below a minimum threshold 2 using a spatially variable approach such as the construction of minimum level of detection lod grids or 3 the creation of a fuzzy inference system although spatially variable methods for determining error have been more accurate in excluding noise without discarding large amounts of meaningful data a challenge remains in performing dods against preexisting contour based maps for which no original point data are available the goals of this study were to 1 develop a method that overcomes the unknown point density of contour and other historical data sets and allows for some assessment of dod uncertainty on the basis of information on topographic variability 2 perform comprehensive uncertainty analysis testing to understand the opportunities and constraints associated with this new method and 3 report and interpret the overall pattern and volume of decadal topographic change for a regulated river 67 years post dam in light of alternate conjectured mechanisms of post dam longitudinal profile adjustment the key feature of the new approach is the introduction of a high density artificial point grid that samples the topographic variability evident in the available historical data set the testbed used to develop and assess this new dod method was the 37 5 km lower yuba river california historical data consisted of 0 6 m contours from a 1999 survey while a more detailed point cloud was available for the most recent survey in 2006 2008 to evaluate uncertainty in the method this study applied seven different uncertainty metrics of varying strictness t 1 t 1 96 uniform 0 3 m exclusion t 1 plus uniform 0 3 m exclusion t 1 with lod minimum at 0 3 t 1 96 plus uniform 0 3 m exclusion and t 1 96 with lod minimum at 0 3 and two different dod adjustment methods exclusion and subtraction a stringent approach involving joint use of the contour half interval and the spatially distributed lod grid at a 95 confidence limit excluded 44 3 of the study area from spatial assessment of channel change and volumetric change computation this preferred approach yielded an estimated 2 518 million m 3 of total scour and 2 455 million m 3 of total fill in 7 9 years after considering different mapping epochs the net annual average export was 17 000 m 3 32 500 tons to the feather river and a river valley sediment yield of 2205 tons km 2 year this amount is 36 of the post dam annual yield of gravel and cobble to the upstream reservoir that blocks sediment conveyance into the study domain suggesting that the lowland system is still highly dynamic 67 years after the dam was built the scientific significance of this is that the response of rivers to dams can be far more long lasting and complex depending on the suite of cumulative societal impacts to rivers the ability to account for spatially explicit dod uncertainty i e data retention when uncertainty is low and data removal when uncertainty is high when comparing historic contour based and modern point cloud based dems will allow for more detailed and reliable dods and sediment budgets in these cases

sediment replenishment influence of the geometrical configuration on the morphological evolution of channel bed dams trap sediment in the upstream reservoir which may lead to river bed armoring streambank erosion and failure channel incision and reduction of the morphological diversity in the downstream river reaches the replenishment of sediment is a mitigation measure for this problem to be applied in river reaches downstream of dams previously performed field experiments always used one single volume of sediment replenishment to explore different alternatives the replenished volume was here divided in four deposits with the motivation to influence also the morphological evolution downstream six different geometrical configurations together with three submergence conditions of sediment replenishment were tested for the first time in a laboratory experiment and are herein discussed the results of the sediment replenishment mitigation technique are described in terms of occupied surface of the flume bed and the temporal evolution of erosion and transport of the introduced sediments it is shown that under our experimental conditions complete submersion of the replenishment volume results in complete erosion of the placed sediment with a high persistence of the added material along the channel length the geometrical configuration of the replenishment volume plays a key role for the evolution of bed forms downstream parallel configurations lead to a wider spread of material across the channel alternated configurations are suitable to produce sediment clustering and high persistence of placed material in the channel observed periodic mounds considered as the initiating condition for alternate bars follow a wavelength related to the length of the replenishment when the replenishment volumes are alternating

a 184 year record of river meander migration from tree rings aerial imagery and cross sections abstract channel migration is the primary mechanism of floodplain turnover in meandering rivers and is essential to the persistence of riparian ecosystems channel migration is driven by river flows but short term records cannot disentangle the effects of land use flow diversion past floods and climate change we used three data sets to quantify nearly two centuries of channel migration on the powder river in montana the most precise data set came from channel cross sections measured an average of 21 times from 1975 to 2014 we then extended spatial and temporal scales of analysis using aerial photographs 1939 2013 and by aging plains cottonwoods along transects 1830 2014 migration rates calculated from overlapping periods across data sets mostly revealed cross method consistency data set integration revealed that migration rates have declined since peaking at 5 m year in the two decades after the extreme 1923 flood 3000 m 3 s averaged over the duration of each data set cross section channel migration occurred at 0 81 m year compared to 1 52 m year for the medium length air photo record and 1 62 m year for the lengthy cottonwood record powder river peak annual flows decreased by 48 201 vs 104 m 3 s after the largest flood of the post 1930 gaged record 930 m 3 s in 1978 declining peak discharges led to a 53 reduction in channel width and a 29 increase in sinuosity over the 1939 2013 air photo record changes in planform geometry and reductions in channel migration make calculations of floodplain turnover rates dependent on the period of analysis we found that the intensively studied last four decades do not represent the past two centuries

channel bifurcation and adjustment on the upper yadkin river north carolina usa abstract the bifurcation of river flow around large stable islands also known as anabranching represents a distinctive form of river adjustment that is uncommon in the appalachian piedmont province of the eastern u s within this province highly localized river branching similar to classical anabranching forms exists in one area near the foot of the blue ridge escarpment this paper examines channel form and processes along a reach of the upper yadkin river in north carolina usa before and after a flood induced division of its flow into two subparallel branches both of which remain active 35 years later the research draws on aerial photograph analyses channel surveying and observations of inset channel benches flood frequency analysis and discharge monitoring and modeling to analyze planform history and to track and explain changes in newly excised and losing channels along the bifurcated reach the characterization of this branched reach resembles some descriptions of gravel dominated laterally active anabranching although the match is imperfect reductions in valley slope and confinement and the presence of local valley constrictions near the base of the escarpment steeplands have been conducive to long term sedimentation and a greater likelihood of branching a period of relatively rapid cross section adjustment occurred on both branches soon after bifurcation and this may have permitted the losing branch to remain open well stratified benches currently observed along the losing branch are interpreted to be largely the late product of a waning early major adjustment phase and moderate recent and perhaps ongoing variations in flow division and appear to be not fully adjusted to prevailing discharges coarse benches along the newly excised branch are better adjusted inset floodplain fragments created primarily by lateral migration and point accretion patterns of benchfull flow frequency variation are interpreted to indicate that the flow bifurcation ratio has been neither static nor monotonically changing in recent decades further exploration of branching in this environment otherwise dominated by single thread meandering patterns is of importance for a variety of management and ecological concerns

nature s complex flume using a diagnostic state and transition framework to understand post restoration channel adjustment of the clark fork river montana abstract there is an imperfect symmetry between the patterns of channel evolution observed during laboratory flume experiments and those which materialize in rivers exposed to ambient environmental conditions that produce hydrogeomorphic fluxes which are more complex contingent and unpredictable one strategy to improve our understanding of short to medium term channel evolution is to study landscapes that have undergone significant disturbance and have had their biogeomorphic templates reset to a known condition in effect creating a flume in nature this study adopts a diagnostic state and transition framework to narrate and document baseline hypotheses for the potential evolutionary trajectories clark fork river near milltown montana following dam removal and remediation a 5 km stretch of the clark fork river and its adjoining floodplain were reconstructed since flow was introduced to the newly constructed channel in december 2010 complex evolutionary trajectories have been observed on the clark fork s mainstem its secondary channels and floodplain focusing particularly on the river s secondary channels this paper develops a typology of channel states that have been observed and demonstrates that multiple adjustment trajectories have materialized sometimes within the same channel a diagnostic state and transition framework offers a parsimonious strategy to quantitatively or qualitatively anticipate the influence of water sediment and ecological fluxes on channel evolution at the basin reach or segment scale it provides environmental agencies with a robust method to devise spatially explicit scenario based management plans for rivers in a variety of geomorphic settings

the meandering braided river threshold a reappraisal abstract the concept of a threshold discharge slope q s combination that separates braided from meandering streams is critically re examined partly by review of previous data and partly by analysis of new data from south island new zealand it is argue d that discriminant functions that use mean annual discharge provide a poor physical basis for interpretations based on natural processes while bankfull discharge is also deficient because it is not a fully independent variable the common use of channel rather than valley slope adds a further misleading component because channel slope is partly dependent on channel pattern sinuosity most previous studies have inadequately treated the role of bed particle size in pattern discrimination failing to recognize that active gravel streams must plot higher on a q s chart than sand bed channels irrespective of pattern because of the greater power requirements for bed material movement within any one size class of bed material there is no evidence to indicate a clear discrimination between braiding and meandering only a weak statistical association between pattern and slope discharge values while this association may be explicable in terms of the high shear stress and stream power that accompany high qs values promoting braiding the real prerequisite for braiding appears to be high loads of bed calibre material producing wide shallow channels a factor that is only partly controlled by stream power accordingly the search for a pattern threshold based on discharge and slope seems to be a futile exercise an interpretation of existing pattern discriminant functions from the perspective of the threshold shear stress for particle movement is made it is concluded that in effect these functions merely state that gravel bed streams are more likely to be braided than channels in finer sediment statistically this appears to be valid and reasons for that difference are offered based on the balance between imposed bed material load and channel capacity

quantifying downstream impacts of impoundment on flow regime and channel planform lower trinity river texas abstract as human population worldwide has grown so has interest in harnessing and manipulating the flow of water for the benefit of humans the trinity river of eastern texas is one such watershed greatly impacted by engineering and urbanization draining the dallas fort worth metroplex just under 30 reservoirs are in operation in the basin regulating flow while containing public supplies supporting recreation and providing flood control lake livingston is the lowest as well as largest reservoir in the basin a mere 95 km above the trinity s outlet near galveston bay this study seeks to describe and quantify channel activity and flow regime identifying effects of the 1968 closure of livingston dam using historic daily and peak discharge data from usgs gauging stations flow duration curves are constructed identifying pre and post dam flow conditions a digital historic photo archive was also constructed using six sets of aerial photographs spanning from 1938 to 1995 and three measures of channel activity applied using a gis results show no changes in high flow conditions following impoundment while low flows are elevated however the entire post dam period is characterized by significantly higher rainfall which may be obscuring the full impact of flow regulation channel activity rates do not indicate a more stabilized planform following dam closure rather they suggest that the trinity river is adjusting itself to the stress of livingston dam in a slow gradual process that may not be apparent in a modern time scale

natural levee deposition during the 2005 flood of the saskatchewan river abstract in summer 2005 a controlled flood of the saskatchewan river east central saskatchewan canada resulted in general floodplain inundation and extensive natural levee deposition along a 60 km reach extending from 40 km below the e b campbell dam to cumberland lake levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain new deposits on levee crests varied from 0 to 70 cm in thickness displaying large variations both along reach and in opposing sites across channels mean grain sizes mainly silt and very fine sand likewise varied considerably among sample sites pre and post flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour results show that both net deposition and net erosion occurred within the channel cross sections but that on average net channel enlargement of 4 2 prevailed over the entire survey reach when weighted by cross section size over the 60 km flooded reach zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation this bimodal distribution of flood deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel perimeter erosion two other factors 1 position of interfacial zones between clear floodbasin water and turbid channel water and 2 difference in pre flood levee heights contributed to the poor correspondence in thickness and grain size between opposing levees at some sites additional features of the new levee deposits including increases in transverse slopes abrupt basinward fining and paucity of deposition in distal areas due to clear floodbasin waters are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes

channel morphology and bedload pulses in braided rivers a laboratory study bedload pulses in gravel bed rivers have been widely reported in recent years and attempts have been made to relate them to channel morphology bedload transport and channel morphology were measured in a small scale generic model of braided gravel bed streams two experiments are described in which braided channels developed in a 14 m 3 m sand tray total bedload output from the tray was weighed every 15 minutes stream bed geometry was surveyed every four hours pulses were observed in the bedload output time series and were qualitatively related to the channel morphology immediately upstream of the measuring section the bagnold 1980 bedload equation generally overpredicts measured bedload transport rates when applied to channels that were in equilibrium or aggrading underprediction occurred when applied to degrading channels aggradation was associated with channel multiplication and bar deposition channel pattern simplification occurred when degradation took place and bars emerged from the water flow development of phases of aggradation and degradation is dependent upon the three dimensional geometry of the stream beds spatial and temporal feedback loops can be identified enabling links between channel morphology and bedload transport rate to be directly identified

particle size distribution of main channel bed sediments along the upper mississippi river usa abstract in this study we compared pre lock and dam ca 1925 with a modern longitudinal survey of main channel bed sediments along a 740 km segment of the upper mississippi river umr between davenport ia and cairo il this comparison was undertaken to gain a better understanding of how bed sediments are distributed longitudinally and to assess change since the completion of the umr lock and dam navigation system and missouri river dams i e mid twentieth century the comparison of the historic and modern longitudinal bed sediment surveys showed similar bed sediment sizes and distributions along the study segment with the majority 90 of bed sediment samples having a median diameter d50 of fine to coarse sand the fine tail d10 of the sediment size distributions was very fine to medium sand and the coarse tail d90 of sediment size distribution was coarse sand to gravel coarsest sediments in both surveys were found within or immediately downstream of bedrock floored reaches statistical analysis revealed that the particle size distributions between the survey samples were statistically identical suggesting no overall difference in main channel bed sediment size distribution between 1925 and present this was a surprising result given the magnitude of river engineering undertaken along the study segment over the past 90 years the absence of substantial differences in main channel bed sediment size suggests that flow competencies within the highly engineered navigation channel today are similar to conditions within the less engineered historic channel

impact of the magnitude and frequency of debris flow events on the evolution of an alpine alluvial fan during the last two centuries responses to natural and anthropogenic controls the dynamics and the surface evolution of a post lgm debris flow dominated alluvial fan tartano alluvial fan which lies on the floor of an alpine valley valtellina northern italy have been investigated by means of an integrated study comprising geomorphological field work a sedimentological study photointerpretation quantitative geomorphology analysis of ancient to modern cartography and consultation of historical documents and records the fan catchment meteoclimatic geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface 2 km2 the dynamics of the fan are determined by the alternation of low return period catastrophic alluvial events dominated by non cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel with periods of low to moderate streamflow discharge punctuated by low to intermediate magnitude flood events causing slower but steady topographic reworking the most ancient parts of the fan surface date back at least to the first half of the 19th century but most of the fan surface has been restructured after 1911 mainly during the debris flow dominated events of 1911 and 1987 phases of rapid fan toe incision and fan degradation have been recognized since the 1930s or 1940s the tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area post little ice age climate change and present day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace anthropogenic controls plausibly override the natural ones in particular the building of a dam in the late 1920s about 2 km upstream of the fan seems to have triggered fan dissection having altered the sediment discharge through sediment retention copyright 2011 john wiley sons ltd

braided channel pattern and palaeohydrology using an index of total sinuosity abstract channel patterns are commonly represented as discrete types e g straight meandering braided separated by threshold values of slope and discharge however for gravel bed river patterns an index of total sinuosity p can be defined which varies continuously with both stream power per unit channel belt length represented as the product of mean annual flood and valley gradient and median bed material diameter an empirical relationship between p and these controlling variables can be used to provide palaeo hydrological reconstructions for braided stream traces on terrace fragments as an alternative to methods based on bed material sizes and criteria for the threshold of grain motion

underlying gravel layers in a large sand bed river and their influence on downstream dam channel adjustment due to the changes in environmental factors during a river s historical development underlying gravel layers are found in many large plain rivers when the buried depth of this gravel layer is within the reach of down cutting by clear water scour after reservoir construction it may be exposed and exert a far reaching influence on channel adjustment in hanjiang river the longest tributary of the yangtze river in china down cutting has been greatly reduced and even stopped due to the total change of bed material composition the exposure of a gravel layer results in a dramatic increase in the bed s hydraulic roughness so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer moreover the exposed gravel layer increases the erosion resistance of bed material making a lower relative erosion resistance of bank to bed material where the bank s erosional resistance is weak this may lead to a tendency towards channel widening

transgressive systems tract of a ria type estuary the late holocene vilaine river drowned valley france the understanding of control parameters of estuarine infilling at different scales in time and space is important for near future projections and better management of these areas facing sea level change during the last 20 years the stratigraphy of late quaternary estuaries has been extensively investigated leading to the development of depositional models which describe the sedimentary facies distribution according to changes in sea level tidal range and wave activity but few of them describe in detail the influence of the estuarine geomorphology on the atlantic coast recent studies provide a detailed understanding of the sedimentary infill succession of incised valleys during the late holocene transgression with recent emphasis on the macrotidal bay of the vilaine river in south brittany the latter has shown the importance of the inherited morphology in controlling the preservation of the sedimentary deposits however until now there has been little information available on the geometry of the underlying bedrock of the estuary and its immediate approaches including the holocene history of the most proximal part of the vilaine river estuary the recent acquisition of very high resolution seismic reflection chirp and sediment core data in the vilaine estuary provides the opportunity to address several of these issues leading to the identification of the controlling factors that played a role in the sedimentary filling of the estuary the estuary of the vilaine river is one of the most sheltered estuaries of the atlantic coast and thus shows an excellent record of the successive transgressive phases since the last glacial period the valley of this ria type estuary is incised in the hercynian magmatic and metamorphic rocks of the armorican massif along the striking branches of the south armorican shear zone and filled with well stratified sediments the study of the morphology of the substratum in the estuary reveals narrow paleovalleys 1 km wide with a maximum depth of 20 m the shapes and dimensions of these paleovalleys are comparable with those observed upstream in the present day vilaine river valley facies analysis of seismic and sedimentary data complemented by 21 ams radiocarbon age dating enables the reconstruction of the infilling of the estuarine valley the valley is filled by pleistocene to early holocene alluvial coarse grained pebbly sands overlain by mid to late holocene marine sands silts and muds from 10 000 to 5000 cal bp the marine transgression shows a rapid initial phase of transgression followed by a progressive slowing down which can be recognised over more than 90 km in an upstream direction the rapid transgression developed homogeneously over the entire length of the estuary with a widespread expression of tidal currents tide dominated estuary the slow phase of the transgression gives rise to a differentiated expression of the hydrodynamic factors according to the geomorphological setting the sedimentary deposits of the innermost and narrow part of the estuary reflect a strong control of tidal and river currents when the influence of tidal currents increases at the expense of fluvial control in a downstream direction the sedimentary filling of the outer reaches of estuary shows a strong influence of tidal currents modulated by changes in sea level where from 5700 to 3000 cal bp the progressive decrease in accommodation space promotes the expression of wave action wave dominated estuary finally from 5000 years bp to present day the stabilisation of sea level gives birth to scour and lag sedimentary morphologies scour and lag estuary the sedimentary record appears thus strongly marked by the holocene transgression and the geomorphological setting where hydrodynamic parameters superimposed on the geomorphological and eustatic contexts expressed in different ways in time and space govern the evolution of the estuarine system

the impacts of the three gorges dam upon dynamic adjustment mode alterations in the jingjiang reach of the yangtze river china abstract dam operation profoundly modifies downstream flow sediment regimes resulting in a variety of geomorphologic responses without uniform laws the jingjiang reach of the middle yangtze river has undergone significant adjustment mode alterations with intensive channel erosion and major adjustment area changing from the bankfull channel to the medium flow channel at the time boundary of the three gorges dam tgd impoundment while these changes have received some attention little work has been undertaken to reveal the reason in this study based on 172 cross sections measured in the post flood period from 2003 to 2015 different river patterns longitude and reach averaged channel dimensions were assessed to reflect the channel adjustment characteristics of the jingjiang reach an index has been proposed to measure the cumulative water transport capacity of each discharge interval and to assess the relationship with reach scale medium flow channel dimensions the results indicate that the channel adjustment changes are directly driven by the alterations of flow duration and sediment transport capacity with the deficiency of floods medium flow channels respond quite well to small and medium discharges of under approximately 16 500 m3 s a discharge of 16 500 m3 s which can be regarded as a critical threshold corresponds to the average top levels of braided and meandering channel bars which are of morphological significance in constraining the passing flow in the medium flow channel small and medium discharges under the critical threshold associate with relatively larger reductions in sediment loads and hold increased geomorphic impacts qmjp eventually resulting in adjustment mode alterations the results present herein for the jingjiang reach of the yangtze river can provide a better understanding of dam operation and the corresponding impacts on downstream channel morphology

an experimental study of complex response in river channel adjustment downstream from a reservoir an experimental study is described which attempts to reveal the complex response process of river channel adjustment downstream from a reservoir a descriptive model has been established which identifies three adjustment stages in stage i following the change of water and sediment input the width depth ratio of river channel w d and the river channel gradient s both decrease but the river channel sinuosity r increases in stage ii because of feedback from the changing boundary conditions w d starts to increase r decreases and the rate of gradient decrease becomes very slow in stage iii equilibrium is reestablished w d r and s tend to constant values and the system recovers its stable state

statistical analysis of the evolution of a semialluvial stream channel upstream from an inversion type reservoir the case of the matawin river quebec canada abstract built in 1930 on the matawin river 5770 km to supply hydroelectric plants located downstream on the saint maurice river the taureau reservoir caused an inversion of the natural hydrological cycle maximum flows occur in winter and minimum flows occur in spring during snowmelt this inversion induces a very rapid rise of the reservoir level base level for the channel upstream from the reservoir in april may during the filling up process the goal of the study is to analyze the effect of the rapid rise of the base level on the morphological evolution of the 7 km long sandy section of the matawin river located immediately upstream from the reservoir and to compare it with jiongxin s general model comparison of aerial photos taken before 1928 and after 1937 1950 1965 1975 1983 and 1995 reservoir construction reveals no significant change in the bankfull mean width sinuosity and bankfull depth of the studied section it follows that the morphological evolution of the matawin river channel upstream from the dam is not consistent with the jiongxin model the effect of the inverted hydrological regime in fact produces high channel stability this stability will probably contribute to a slow but progressive reduction of the channel width and an increase of the sinuosity this morphological evolution seems typical of the semialluvial streams found upstream from the inverted type reservoir of the canadian shield

the relationship between flow shear stress and stream pattern based on simple hydraulic reasoning a discharge q slope s diagram for streams is given a physical interpretation permitting the assignment of a relative shear stress value to each stream a sample of 359 streams is used ranging in mean annual discharge from 900 000 to 0 05 ft 3s 1 varying in pattern from highly sinuous p 3 1 to straight and braided sorted according to their relative shear stress values 0 avg streams of high sinuosity p 1 5 are characterized by low relative shear stress median 0 56 which is significantly lower than that characterizing braided streams median 1 70 it is concluded that for natural stream populations the passage from one pattern to another is gradual and the geomorphic thresholds defined by relative shear stress are fuzzy ones

classification of geomorphological effects downstream of dams abstract the effects of dams on downstream geomorphology are reviewed and a typology is devised consisting of nine cases the classification can be seen as a further development of lane s balance between water discharge sediment load grain size and river slope depending on changes in released water flow and changes in released sediment load relative to the transport capacity of the flow it is possible to estimate resulting cross sectional geomorphology the longitudinal extent of changes and their variability with time and the tributary response to altered mainstream cross section changes are also discussed

water discharge and sediment load to sepetiba bay from an anthropogenically altered drainage basin se brazil summary empirical relationships coupled with the field measurements were used to assess water discharge and sediment load from an anthropogenically altered drainage basin into sepetiba bay in southeastern brazil discharge into sepetiba bay is modified by large scale water diversion from the paraiba do sul river which is one of the more important and impacted rivers in brazil the discharge into sepetiba bay has increased threefold from 41 to 129 m 3 s on average since the water redistribution began sediment budget calculations indicate retention of a large fraction of the sediment load by five reservoirs within the drainage system but with highly variable infilling rates as a consequence the water diversion scheme has lead to a 28 increase in sediment delivery to sepetiba bay or 270 10 3 t yr the paraiba do sul river contributes most of the freshwater input into sepetiba bay whereas the sediment load to the bay is mainly contributed by the local drainage basins

geomorphology of the colorado river in the grand canyon sediment supplied to the colorado river within the grand canyon has been sorted into distinct deposits of three grain size ranges the major rapids are formed by boulder deposits from side canyon tributaries as a result of a fourfold reduction in peak discharge when glen canyon dam was closed in 1963 new fan debris may increase the gradient through some of the rapids by a factor of 1 8 cobbles and gravel transported only during flood stages are preferentially deposited in the wider sections of the river as bars and riffles and are for the most part inactive during post dam discharges fine grain largely sandy terraces occur throughout the canyon especially along the banks of the large reverse eddies above and below the rapids the lower terraces are being reworked into beach like shores by diurnally varying post dam discharges a slight net lateral erosion of the terraces has resulted prior to construction of the dam sandy bed deposits underwent scour averaging about 1 m during spring floods

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

effect of reservoir construction on suspended sediment load in a large river system thresholds and complex response dam construction greatly alters the channel boundary of rivers making the dammed river system a human controlled system based on hydrometric data in the upper changjiang river basin the change in behaviour of sediment transport of some dammed rivers was studied as a result some phenomena of threshold and complex response were found when the coefficient c r a of actual runoff regulation by reservoirs defined as the ratio of total capacity of reservoirs to annual runoff input is smaller than 10 suspended sediment load at yichang station the control station of the changjiang river shows a mild decreasing trend when this coefficient becomes larger than 10 suspended sediment load decreases sharply the coefficient of 10 can be regarded as a threshold the c r a of 10 is also a threshold when the variation of suspended sediment concentration ssc with c r a at yichang station is considered the impacts of reservoir construction can be divided into several stages including road construction dam building and closure water storage and sediment trapping during these stages some complex response was identified at the station below the dam ssc increases and reaches a maximum and then declines sharply this phenomenon was found on the main stem and several major tributaries of the upper changjiang river in the minjiang river where a series of dams were built successively the response of ssc is more complicated copyright c 2010 john wiley sons ltd

the ebro river in the 20th century or the ecomorphological transformation of a large and dynamic mediterranean channel in the first decades of the 20th century the ebro river was the iberian channel with the most active fluvial dynamics and the most remarkable spatial temporal evolution its meandering typology the dimensions of its floodplain and the singularities of its flow regime produced an especially interesting set of river functions the largest dynamics of the ebro river are concentrated along the meandering profile of the central sector during the 20th century this sector experienced a large alteration of its geomorphological structure we present here an analysis of this evolution through the cartographic study of a long segment of the river 250 km in 1927 1956 and 2003 the results show a large reduction in bank sinuosity a progressive loss of fluvial territory and a large decrease in channel width these changes are especially clear in the areas previously most ecologically connected with the active channel the fluvial territory of the river in 2003 was approximately half that found during the first decades of the 20th century forest plantations which were non existent in 1927 occupied more than 1500 ha of the study area in the last decade this intense geomorphological transformation becomes ecologically visible in i a 35 reduction of the area occupied by riparian vegetation ii a loss of the heterogeneity of riparian forest spots which were formerly structured in an irregular mosaic far from the river thalweg and iii a modification of the riparian forest structure which is currently linear uniform thin and very close to the river axis the ecomorphological alteration was intensified by the remarkable reduction in bank length 13 and the reduced dynamism of the present river system indicated by an increase in the percentage of fluvial territory occupied by riparian forests and a reduction in the area occupied by the active channel copyright 2002 john wiley sons ltd

causes of river bed degradation river bed degradation can proceed downstream as well as upstream depending upon the cause of degradation the causes of downstream progressing degradation are primarily related to changes in independent river channel variables such as increase in water discharge decrease in size of bed material and decrease in bed material discharge the causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man made changes study of various case histories indicates that river slopes are increased by lowering a base level by decreasing the length of a river or by removal of a control point case histories also show that downstream and upstream progressing degradation can act in combination along a river system downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary

influence of four run of river dams on channel morphology and sediment characteristics in illinois usa dams are known to create discontinuities in river flow and sediment transport with ensuing effects on the fluvial geomorphology of dammed systems while the effects created by large impoundment dams are well documented less is known about the influence of small run of river dams common across the eastern united states on river geomorphology recent emphasis on dam removal has focused on run of river structures highlighting the need for improved understanding of the geomorphological effects of these types of dams this research examines spatial variation in channel morphology and bed sediment character upstream and downstream of four run of river dams in illinois results show that the four dams do not create major discontinuities in channel morphology or sediment character silt clay content of bed material at the four sites is higher upstream of the dams than downstream but this size fraction generally is a minor component by weight of the sediment samples collected although percentages of sand are generally higher upstream of the dams and gravel percentages are generally higher downstream not all of these differences are statistically significant longitudinal profiles through the dams and changes in channel depth upstream and downstream of the dams indicate that no major accumulations of sediment have occurred behind the dams analysis of 137cs in sediment cores at two sites shows no evidence of long term fine sediment storage apparently these dams are not acting as major sediment traps nor do these structures produce substantial downstream channel erosion variability in spatial patterns of channel morphology and sediment characteristics among the sites suggests that local site specific factors have an important influence on geomorphological responses because of this variability the findings of this study indicate that information on site specific conditions should be an important consideration in the removal planning process for run of river dams

bedload transport over run of river dams delaware u s a abstract we document the detailed morphology and bed sediment size distribution of a stream channel upstream and downstream of a 200 year old run of river dam on the red clay creek a fifth order stream in the piedmont of northern delaware and combine these data with hec ras modeling and bedload transport computations we hypothesize that coarse bed material can be carried through run of river impoundments before they completely fill with sediment and we explore mechanisms to facilitate this transport only 25 of the accommodation space in our study site is filled with sediment and maximum water depths are approximately equal to the dam height all grain size fractions present upstream of the impoundment are also present throughout the impoundment a characteristic coarse grained sloping ramp leads from the floor of the impoundment to the crest of the dam a 2 3 m deep plunge pool has been excavated below the dam followed immediately downstream by a mid channel bar composed of coarse bed material similar in size distribution to the bed material of the impoundment the mid channel bar stores 1472 m3 of sediment exceeding the volume excavated from the plunge pool by a factor of 2 8 these field observations are typical of five other sites nearby and suggest that all bed material grain size fractions supplied from upstream can be transported through the impoundment up the sloping ramp and over the top of the dam sediment transport computations suggest that all grain sizes are in transport upstream and within the impoundment at all discharges with return periods from 1 to 50 years our computations suggest that transport of coarse bed material through the impoundment is facilitated by its smooth sandy bed model results suggest that the impoundment is currently aggrading at 0 26 m year but bed elevations may be recovering after recent scour from a series of large floods during water year 2011 2012 we propose that impoundments upstream of these run of river dams behave as long pools that adjust their bed elevation and texture to transport the load supplied by the watershed rather than as impounded reservoirs with little bed material transport capacity scour may only occur during episodic high flows followed by aggradation during periods of low flow

sedimentation within a regulated river channel change induced by tributary sediment injection to a regulated gravel bed river is described for the river rheidol u k eighteen years after dam closure morphological and sedimentological data are presented for an aggrading reach a primary lobate bar appears to have initiated flow separation and the subsequent construction of channel side berms hydraulic sorting has operated to produce both longitudinal and lateral particle size reduction and selective particle size dispersal appears to be an important process influencing channel change the channel is adjusting to a lower width depth ratio form of smaller capacity and with a steeper slope