facies analysis of a pliocene river dominated deltaic succession siena basin italy implications for the formation and infilling of terminal distributary channels the recent recognition and description of terminal distributary channels in ancient delta front deposits have led to important progress when it comes to understanding delta construction processes the plan view shapes of both modern and ancient river dominated deltas and sediment dispersal within deltas deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops approximately up to 300 m wide and 50 to 70 m thick in the central sector of the pliocene siena basin in tuscany italy stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings was not affected by tidal effects and was influenced only locally by wave action as a consequence these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex which was built up almost exclusively as the result of river related processes the studied deltaic succession is composed of vertically stacked parasequence like units which are invariably characterized by the lack of coarse grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition these deposits were interpreted previously as deep water turbidites this study provides new elements for the correct distinction between hyperpycnal dominated delta fronts and basinal turbidite deposits the terminal distributary channel facies documented in this study are more complex than those investigated in previous research the channels are often passively filled during abandonment phases and not just due to the aggradation and lateral upstream migration of mouth bars as classically proposed moreover the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision and before its infilling with subsequent implications for the formational and infilling mechanisms of terminal distributary channels furthermore the channel fills are often the result of alternating depositional and erosional phases leading to a complex stratigraphic arrangement

the use of landsat imagery to map fluvial sediment discharge into coastal waters abstract the sediment discharge of the river yare via breydon water estuary into the north sea has been studied sixty samples were taken the average of each sample in the band tm1 was used to estimate the concentrations of suspended solids sixty samples were used to extract information from the reflective bands tm1 tm4 the spatial distribution of suspended solids as well as the spatial and vertical extent of different turbidity levels were then detected estimated and mapped based on this information the water circulation pattern at the mouth of river yare was identified and mapped the influence of wind velocity and direction as well as the tidal current conditions prior to and during the image time on turbidity and water circulation patterns were examined the tidal influence was found to be the most significant factor in shaping and guiding water circulation near the mouth of river yare

ancient lacustrine hyperpycnites a depositional model from a case study in the rayoso formation cretaceous of west central argentina abstract hyperpycnal flows originate when sediment laden fluvial discharges enter standing lower density water because of their excess density the flows plunge near the river mouth and continue to travel basinward as a quasi steady and fully turbulent underflow the related deposits are hyperpycnites and constitute a particular type of turbidite with poorly known facies and facies tracts although hyperpycnal flows seem to be quite common in present times their occurrence in fossil strata is poorly documented this paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites on the basis of the field analysis of well exposed lower cretaceous strata rayoso formation in the neuquen basin of west central argentina the rayoso formation is composed of clastic and minor evaporitic red beds up to 1200 m thick deposited in a shallow perennial lake of variable salinity affected by long lived hyperpycnal flows main clastic facies are composed of fine grained sandstones with climbing ripples and plane beds other common sandstone facies include massive beds and low angle cross stratification most sandstone facies are related to traction plus fallout processes and often show a vertical fluctuation between sedimentary facies originated under different traction plus fallout conditions within single beds these fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows most beds internally show the existence of three depositional phases acceleration ap erosion plus bypass ep and deceleration dp which record the complete evolution of a single long lived hyperpycnal flow at a fixed point additionally the depositional evolution of a single long lived hyperpycnal flow with distance records initially the progressive basinward migration of the ap and ep phases and finally an overall deposition under the dp phase both in proximal and distal areas this evolution provides an adequate explanation for the basinward extension of channelized features and for the occurrence of fine grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system consequently facies analysis derived from application of the bouma sequence is not valid for deposits of quasi steady hyperpycnal flows

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

evolution of atchafalaya lacustrine deltas south central louisiana abstract the entrenched pleistocene holocene mississippi alluvial valley was cut during the last low sea level stand and alluviation of the entrenched valley occurred during the holocene sea level rise regional gulf coast structures salt domes and faults have strongly influenced the morphology of the valley valley paleotopography has controlled depositional patterns and locations of major river courses as well as partially isolated large interdistributary basins within the alluvial deltaic plain interdistributary basin formation is further enhanced by distributary channel progradation switching and abandonment in conjunction with compactional subsidence of the alluvial plain surface geomorphology and sediment cores from the atchafalaya basin louisiana revealed that this basin has alternately accumulated fine grained organic rich sediments and terrigenous clastic dominated meander belt and lacustrine deposits basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary fill prograding lacustrine deltas of the atchafalaya river are the primary mechanism for filling the atchafalaya basin these upward coarsening deltas are relatively thin 3 m but are areally extensive tens to hundreds of kilometers and because they occur so frequently during the infilling process they account for a significant volume of the basin sequence the atchafalaya basin sequence documents lacustrine deltaic episodes dating from the early holocene maringouin and teche deltas to the present atchafalaya delta continued development of the atchafalaya river will rework portions of the backswamp and lacustrine deposits into meander belt deposits upon complete filling of the atchafalaya basin sediment will mostly bypass the basin and become deposited in the marine delta in atchafalaya bay

multi thread rivers in the polish carpathians occurrence decline and possibilities of restoration multi thread rivers are typified by the presence of interconnected channels separated by mid channel bars or the islands forming as a result of vegetation encroachment on mid channel bars high gradients of channels and valley floors high discharge variability relatively low content of fine cohesive sediments in the alluvial plain coarse bed material and high sediment supply are environmental conditions that favour the development of braided channel pattern under natural conditions of the holocene island braided channel pattern was typical of the rivers in the foreland of the tatra mountains and most likely also those draining the parts of the flysch outer carpathians underlain by thick bedded sandstones complexes these braided channels formed in the rivers flowing through mostly non cohesive floodplains and fed with coarse grained material exceeding competence of the rivers progressive deforestation of mountain catchments with the increasing area of cultivated hillslopes resulted in higher flood peaks and substantially increased sediment supply to river channels coupled with greater humidity of the final stages of the little ice age this led to the transformation of the island braided rivers into bar braided ones throughout the nineteenth century the braided channel morphology expanded downstream to the foreland river reaches in turn over the twentieth century the braided morphology nearly disappeared from polish carpathian rivers mostly as a result of their channelization the effectiveness of these works was however strengthened by a decrease in sediment supply to the rivers caused by land use changes and by the reduced availability of bed material for fluvial transport resulting from in channel gravel mining and partitioning of rivers with dam reservoirs environmental changes which have since occurred in the river catchments and disruption of the continuity of sediment transport by dams limit the feasibility of the restoration of braided channel pattern in polish carpathian rivers to their reaches supporting such morphology under natural conditions of the holocene

discussion of cyclic tests on high quality undisturbed block samples of soft marine norwegian clay the article by wichtmann et al 2013 presents a comprehensive set of laboratory test data on the cyclic undrained shear strength of soft marine norwegian onsoy clay the tests were performed on specimen quality designation sqd b specimens andresen and kolstad 1979 terzaghi et al 1996 cut from block samples taken by the sherbrooke sampler lefebvre and poulin 1979 this article is a valuable contribution because compared to data and interpretation for sands undrained cyclic strength data for soft clays are rare e g andersen and lauritzen 1988 andersen et al 1988 furthermore the previous norwegian geotechnical institute ngi tests on soft marine norwegian drammen clay were performed on specimens consolidated to the compression range beyond the pre consolidation pressure p whereas the tests on onsoy clay have been performed using high quality specimens subjected to the in situ effective stress condition ho vo before undrained shear a meaningful impression of undrained shear strength behavior of soft clay deposits is obtained by normalizing undrained shear strength with respect to pre consolidation pressure as both su and p represent points on the yield envelope skempton 1948 mesri 1975 mesri and huvaj 2007 the cyclic undrained shear strengths of sands have been commonly normalized with respect to effective overburden pressure vo because it is not readily possible to determine pre consolidation pressure of sand deposits this discussion concerns cyclic triaxial shear data on specimens cut from samples taken in the depth range of 10 50 to 11 37 m and consolidated before undrained shear to the in situ effective stress condition the onsoy clay samples had a pore water salt content of 32 5 g l natural water content of 63 to 65 liquid limit of 63 and plasticity index of 33 to 34 lunne et al 2006 at the depth range of the samples the end of primary eop pre consolidation pressure p corresponds to p vo 1 4 note that i reduced values of p from the constant rate of strain crs tests by a factor of 0 912 because the imposed strain rates i in the ngi tests were about 10 times the eop strain rate p mesri et al 1994 the in situ values of ko ho vo can be determined according to the following empirical equations mesri and hayat 1993

present day downcutting of the raba river channel western carpathians poland and its environmental effects abstract timing and causes of the present day channel downcutting of carpathian tributaries to the vistula river are exemplified by a case study of the raba river a gravel bed stream characterized by great variability of water stage and discharge up to 3 m of river incision has occurred since the beginning of the present century associated with an increase in stream power caused by river control works and reduction in bed material load linked to changes in basin management gravel extraction from the channel and modifications in flood flows have increased rates of incision over the last thirty years lowering of ground water levels on the valley floor increase in river bank susceptibility to erosion decrease or cessation of overbank sediment accretion and impoverishment of plant and animal communities of riverside biotopes are the principal environmental effects of channel downcutting

twentieth century floods recorded in the deep mediterranean sediments flood generated turbidity currents represent an important process of marine sedimentation however no deposit related to this process has yet been described at sea turbiditic sequences cored at 2000 m water depth in the mediterranean show tractive structures and superposition of reverse and normal grading they are related to floods triggered during the twentieth century deposits from flood generated turbidity currents have a great impact on the interpretation of deep ocean paleoclimate records obtained near continental margins by linking deep marine sedimentary records to continental climatic changes through flood frequency and magnitude implications of this study should help redefine the reservoir character of oil fields in fine grained turbidites

oblique reflection of turbidity currents turbidity currents meeting obstacles for example the margins of a confined basin are subject to reflection the consequent change in flow direction is expressed in the sequence of depositional structures of the resulting bed of sediment putative examples of orthogonal reflection have been described based on 180 opposed current directions we present field evidence for the more general case of oblique reflection of turbidites and we report the results of flume experiments indicating a mechanism involving generation of internal solitary waves at an oblique ramp these propagate normal to the ramp regardless of the angle of incidence flow directions in reflected turbidites may indicate the orientation of reflecting surfaces such as basin margin slopes and thus may be of considerable help in paleogeographic and tectonic reconstructions

bathymetry and sediment dispersal dynamics along the yallahs fan delta front jamaica abstract the shallow offshore yallahs fan delta serves as a model for the interaction of littoral processes and bathymetry and how they influence sedimentation on steep submarine slopes and in deep marine basins in tectonically active areas bottom reflection profiles show that the yallahs delta front is characterized by two bathymetric zones a narrow island shelf and a steep slope dissected by the heads of at least three submarine canyons the refraction and diffraction of waves across these canyons and shelf control sediment distribution and the alternation of erosional and depositional beaches a dominantly westward longshore drift transports coarse grained sediments until they are trapped in the heads of the submarine canyons or temporarily stored on steep unstable slopes between the canyons mass gravity processes presumably move these sediments downslope where they are deposited as turbidites in the yallahs basin and out onto the colombian abyssal plain

channel changes due to river regulation the case of the piave river italy this paper deals with the channel changes of the piave river in the eastern alps italy which have occurred during this century in response to human interventions in the fluvial system the flow regime and the sediment supply of the river have been considerably altered by hydroelectric dams flow diversions and gravel mining in addition river dynamics have been affected by the construction of streambank protection structures to document these changes a historical analysis was performed using maps and aerial photographs morphological features that were examined included planform configuration channel width braiding index and bed elevation the results indicate that as a consequence of decreases in the flows and sediment supply remarkable channel changes have occurred in the river during this century especially during recent decades the channel has undergone a general narrowing with a decrease in average width to 35 per cent of its initial value while the braiding index has decreased from about 3 to 1 5 in several reaches the planform pattern has changed from braided to wandering the observed trends of channel change suggest that the river has not yet reached a new equilibrium condition and it may therefore be predicted that reductions in width and braiding intensity are likely to continue in the immediate future copyright 1999 john wiley sons ltd

twentieth century channel change on the dunajec river southern poland patterns causes and controls abstract spatial and temporal patterns of the twentieth century channel changes of the dunajec the second largest river of the polish carpathians are analysed using data from six historical maps hand auger drillings in paleochannels and hydrometric data from 10 gauging stations in its lower course the river was considerably narrowed shortened and embanked between the 1880s and the 1920s here bed degradation commenced in the late 19th century and has so far resulted in 3 1 m of channel incision in the middle river course little change occurred in mountain reaches where the dunajec is confined by valley sides and where bedrock exposures prevent channel incision in the reaches within intramontane basins channelization works carried out in the 1950s 1970s considerably narrowed the river and transformed its multi thread channel into a single thread straight channel rapid bed degradation induced by the works resulted in up to 2 m of channel incision over the second half of the century upstream progression of bed degradation into the adjacent mountain reach was hindered by bedrock exposures at its downstream end in the upper river course reaches with the deeply incised channels up to 3 5 m prevail over a few past decades in response to gravel exploitation river channelization and a reduction in catchment sediment supply more scarce are vertically stable reaches which owe their stability to low unit stream power in wide unmanaged reaches and to fixing of the channel bed by drop structures in channelized reaches this study shows that despite the overall tendency to incision over the 20th century considerable differences in the timing and course of channel changes occurred between particular river reaches due to variable human impact and local geological and geomorphological conditions the main phase of channel incision took place progressively later in the upstream direction however upstream progression of bed degradation from the already incised downstream reaches was precluded by the roznow dam and bedrock controlled reaches where bed material flushed out from an incising upstream reach had been introduced into an artificially narrowed channel of high transport capacity it was easily transferred downstream and could not restore the downstream reach to its pre channelization vertical position

land use floods and channel changes upper middle fork willamette river oregon 1936 1980 flow trends and channel characteristics from 1936 to 1980 were evaluated for the middle fork willamette river which drains a 668 km2 forested watershed in the cascade mountains of western oregon an inventory of aerial photographs from 1959 to 1972 shows that landslides associated with roads and in clearcuts were 27 and 23 times more frequent respectively than in forested areas numerous landslides unloaded sediments directly into the drainage system most landslides appear to have been initiated during a large flood return period greater than or equal to 100 years of december 22 1964 analysis of precipitation and peak flows greater than 100m3 s 1 from 1958 to 1980 by means of power function models suggests a trend of increasing flows as timber harvesting and road building expanded in the basin changes in channel pattern documented from aerial photographs show major increases in channel width from 1959 to 1967 and a trend of decreasing width from 1967 to 1980 during summer low flows in 1979 and 1980 65 cross sections of the channel were surveyed to provide detailed measurements of existing channel conditions channel widths of 62 of the aggraded reaches were significantly greater 0 05 than those for nonaggraded reaches

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

the response of braided planform configuration to flow variations bed reworking and vegetation the case of the tagliamento river italy morphological features of braided rivers bars channels and pools experience major changes in area shape and spatial distribution as a response to i the pulsation of discharge during a flood and ii the bed evolution induced by floods in this work at a station relationships between water level and planform configuration were investigated on the tagliamento river a large gravel bed braided river in northeast italy over a 2 year study period comprising three bankfull events and several small to medium floods the analysis was performed on two 1 km long reaches characterized by different riparian vegetation cover ground based images with an hourly temporal resolution were acquired using software controlled digital cameras bars channels pools and vegetated patches were manually digitized on more than 100 rectified images sequences of constant level images spanning the study period were used to quantify the impact of floods on the stability of at a station relationships and on the turnover rate of water bodies the analysis shows that wetted area increased almost linearly with water level in both reaches the average number of branches per cross section peaked at intermediate flow levels increasing from 2 at low flow up to 6 7 the number of branches displayed the largest fluctuations over time with significant changes produced also by moderate floods turnover rates were high in both reaches with more than 30 of wetted areas at low flow converting into bare gravel in less than 2 months vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer deeper anabranches the number of channels per cross section was 30 40 less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3 to 14 copyright 2012 john wiley sons ltd

anatomy of shelf deltas at the edge of a prograding eocene shelf margin spitsbergen although shelf edge deltas are well imaged seismic features of holocene and pleistocene shelf margins documented outcrop analogues of these important sand prone reservoirs are rare the facies and stratigraphic architecture of an outcropping shelf edge delta system in the eocene battfjellet formation spitsbergen is presented here as well as the implications of this delta system for the generation of sand prone shelf margin clinoforms the shelf edge deltas of the battfjellet formation on litledalsfjellet and hogsnyta produced a 3 5 15 km shelf edge attached slope apron 70 m of sandstones proximally tapering to zero on the lower slope the slope apron consists of distributary channel and mouth bar deposits in its shelf edge reaches passing downslope to slope channels chutes that fed turbiditic lobes and spillover sheets in the transgressive phase of the slope apron estuaries developed at the shelf edge and these also produced minor lobes on the slope the short headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf and the shelf edge position of the discharging deltas made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin the abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane parallel laminae within turbidite beds in the slope channels and thick spillover lobes with repetitive alternations of massive and flatlaminated intervals the examined shelf edge to slope system now preserved mainly below the shelf break and dominated by sediment gravity flow deposits has a threefold stratigraphic architecture a lower progradational part in which the clinoforms have a slight downward directed trajectory a thin aggradational zone and an upper part in which clinoforms backstep up onto the shelf edge a greatly increased density of erosional channels and chutes marks the regressive to transgressive turnaround within the slope apron and this zone becomes an angular unconformity up near the shelf edge this unconformity with both subaerial and subaqueous components is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level the studied shelf margin clinoforms accreted mostly during falling stage sea level below the shelf edge but the outer shelf later became estuarine as sea level became re established above the shelf edge

interplay between river discharge and topography of the basin floor in a hyperpycnal lacustrine delta basin floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the red river delta in lake texoma usa the red river discharge is typically a hyperpycnal plume due to elevated total dissolved solids because the river plume is a bottom hugging hyperpycnal flow lake bathymetry and topography strongly influence deposition and subsequent delta morphology in addition to elevated total dissolved solid concentrations compared with lake texoma water the density contrast of the red river outflow is increased by high suspended sediment concentrations during high discharge events steep lateral slopes in the lake texoma basin deflect hyperpycnal river plumes and subsequently change the delta progradation direction before the delta reaches the opposite bank of the lake analysis of multi temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake bottom gradients corresponding to the pre impoundment river thalweg i e bypassing shallow parts of the lake an analytical model for the hyperpycnal plume trajectory indicates plume deflection during low discharge or high discharge events towards the deepest part of the basin the magnitude of plume deflection is a function of river discharge and basin margin gradients plume deflection can vary between 10 and 80 from the channel axis towards the old river thalweg the high deflection appears in the case of maximum basin side gradients of 12 8 and in conditions of low river discharge during low discharge periods the red river delta builds a lobate shape with multiple terminal distributary channels whereas during high discharge periods the red river delta builds an elongate shape with a single large distributary channel the elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries therefore the lobate shaped delta is expected to be preserved in the rock record

geomorphic diversity in the braided wandering bela river slovak carpathians as a response to flood variability and environmental changes abstract changes in the morphological pattern of the gravel bed bela river slovak carpathians were investigated to infer about the impact of flood events and environmental changes on the evolutionary trend of the river since the mid twentieth century the post flood period serial geomorphic analysis popsega approach coupled with gis analysis was used to identify morphological changes that occurred during particular flood periods the study focused on assessing how six flood periods fp with differing flood characteristics have influenced the evolutionary trend of the river several sets of aerial photos 1949 1961 1973 1986 1992 2003 2009 taken after the periods with the occurrence or lack of major floods provided data about channel landform changes in 227 river segments of 100 m length a decreasing trend of the area of the river s active zone from 2 5 km2 in 1949 to 1 4 km2 in 2009 was identified the first fp 1949 1961 with a very high magnitude flood in 1958 ri50 was characterized by increasing total channel length island number area and the number density of confluence diffluence pairs nodes and by decreasing bar area during this fp the frequency of cores of channel dynamics as well as the shannon s index of landforms diversity reached the highest values the following fp 1962 1973 with four 5 to 10 year floods exhibited the inverse pattern of changes in channel form parameters although the core frequency of channel dynamics and shannon s diversity index were still high the river system seems to have been relatively stable between 1974 and 1986 third fp with some decrease in the area of channel forms and in pattern metrics values during the fourth fp 1987 1992 several 1 5 to 3 year floods occurred and were associated with a decrease in some channel forms and a further decrease of the pattern metrics values an increase in all analysed channel parameters especially bar area and pattern metrics values occurred in the years 1993 2002 fifth fp as a consequence of two floods of moderate magnitude in 1997 ri7 and 2001 ri3 5 the lowest values of bar area and pattern metrics during the whole study period were recorded during the last fp 2002 2008 whereas island area increased at that time the study shows that the occurrence of floods during the last six decades has not returned the bela to its former state because of the overall decrease in flood magnitudes changes in catchment sediment supply induced by the increasing forest cover in the catchment from 22 7 in 1957 to 34 8 in 2008 and changes in channel boundary conditions channelization channel incision small hydropower plant constructions

splitting rivers at their seams bifurcations and avulsion river bifurcations are critical but poorly understood elements of many geomorphological systems they are integralelements of alluvial fans braided rivers fluvial lowland plains and deltas and control the partitioning of water and sediment throughthese systems bifurcations are commonly unstable but their lifespan varies greatly in braided rivers bars and channels migrate splitand merge at annual or shorter timescales thereby creating and abandoning bifurcations this behaviour has been studied mainly bygeomorphologists and fluid dynamicists bifurcations also exist during avulsion the process of a river changing course on afloodplain or in a delta which may take 102 103years and has been studied mainly by sedimentologists this review synthesizesour current understanding of bifurcations and brings together insights from different research communities and different environmen tal settings we consider the causes and initiation of bifurcations and avulsion the physical mechanisms controlling bifurcation andavulsion evolution mathematical and numerical modelling of these processes and the possibility of stable bifurcations we end thereview with some open questions copyright 2012 john wiley sons ltd

deltaic infill of a deglaciated arctic fjord east greenland sedimentary facies and sequence stratigraphy abstract the deltaic fill of a deglaciated side entry fjord in western jameson land east greenland shows the record of an interplay between regional and local controls including climate and relative sea level change onshore and offshore topography and sediment yield a four phase model is suggested for the delta development and fjord filling at the deglaciation stage phase 1 the newly flooded fjord floor primarily accumulated sediment from reworked glacial material these deposits were relatively thin and draped the uneven sea floor topography the subsequent onset of paraglacial delta sedimentation phase 2 was characterized by a high meltwater runoff and extreme sediment yield and corresponded to the highstand and gradual fall of relative sea level a gilbert type delta with a short distributary plain formed at the head of the tributary fjord which received a very high amount of fine grained sediment the stage of enhanced delta progradation phase 3 was characterized by decreased sediment yield and runoff and a rapid fall of relative sea level delta progradation was fast because of rapidly decreasing accommodation space and a solitary river channel deeply incised the delta the equilibrium stage phase 4 spanned most of holocene time and involved a lowered rate of relative sea level fall seasonal runoff supplied limited amounts of sediment to the river mouth and the delta front became increasingly reworked by wave action and longshore drift the side entry fjord a small deglaciated valley invaded by the sea was filled chiefly by highstand and regressive systems tracts whereas relatively little sea floor aggradation was associated with the transgressive systems tract the stratigraphic pattern documented in this paper contrasts with the pattern usually reported from nonglaciated incised valley systems which are filled mainly by the transgressive systems tract typically involving a retreating estuarine system the pattern also differs from some large gradually deglaciated fjord valleys containing thick transgressive glaciomarine deposits accumulated during glacier retreat the study demonstrates the large variability of valley fills and illustrates the importance of topography sediment supply and climate as well as the style of deglaciation on the sediment filling of a fjord valley

morpho dynamic evolution patterns of subcarpathian prahova river romania abstract the prahova river romania at the southern carpathian mountains was affected by a number of measureable changes in its planform pattern and longitudinal profile over the past 200 years the mountain river has been incising into bedrocks narrowing its valley abandoning its lateral channels and straightening its path while crossing the subcarpathian study section the purpose of this study is to analyze using gis the relationship between short term less than 200 years lateral and vertical morphological adjustments observed over the 18 km length subcarpathian prahova river and specific long term evolutionary patterns more than 200 years human disturbances and hydrodynamic characteristics during high intensity flood periods are assessed in the context of existing neotectonics and lithology controls on channel and valley morphology surveyed cross section profiles were used to construct the geometry of a 1d hydraulic model simulations were performed using the hec ras software usace under steady flow conditions for flow values between bankfull and the peak discharge value of typical flood events with a return period of 20 and 50 years three main areas with high values of computed shear stresses and velocities and corresponding low values of width to depth ratio w d were identified these areas were found to match the observed erosion areas where the river has incised into bedrock and eroded its banks other potentially aggradation areas of low shear stress values and high w d were confirmed by field observations the evolutional patterns of prahova river channel are tectonic structurally determined developed and maintained during the quaternary mainly by the principles of river morphodynamics the evolution over shorter timescale bears the mark of the man induced impact changes in sediment and hydrologic regimes in a long term perspective the overall channel evolution of this river is expected to be strongly influenced by human activity

river response to channel regulation case study of the raba river carpathians poland processes induced by the channelization of the raba river in the present century are examined to illustrate the response of a gravel bed stream following narrowing and straightening of its channel up to 3 m of incision has occurred the change from a slow and relatively steady degradation in the lower reaches to separate degradation events in the higher reaches is attributed to the differential rate of headcut retreat and to the control exerted by mid channel bars upon the rate of river energy dissipation progressive outwashing of finer grains from bed material has followed the diminishing sediment yield of the basin and the increase in stream power the ensuing growth in mean grain size and changes in sediment fabric have increased boundary resistance to flow and reduced particle susceptibility to entrainment downstream magnification of peak discharges has become increasingly pronounced with the advancing incision the decrease in flood plain storage and self acceleration of flows passing the relatively deep and straight channel has caused flood waves to become progressively more flashy in nature an increase in channel depth and reduction in gradient caused by downward and backward erosion as well as bed material coarsening has promoted the re establishment of an equilibrium conversely flow velocity increases due to flow concentration in the deepened channel reduction in grain mobility allows the river to attain a new equilibrium at flow velocity and stream power levels higher than before the channelization numerous disadvantages of the applied regulation scheme and its failure to reduce flood hazard raise the question of its maintenance to be successful any regulation design must take into account changes in sediment supply and flood hydrographs resulting from the simultaneous alterations in basin management

isolated highstand shelf sandstone body of turbiditic origin lower kenilworth member cretaceous western interior book cliffs utah usa abstract an isolated highstand shelf sandstone body of turbiditic origin has been identified in the lower kenilworth member hatch mesa area book cliffs utah relative to most other marine mudstone encased isolated sandstone bodies the hatch mesa succession has an abundance of bouma like turbidite beds a greater proportion of mudstone and siltstone is thinner 6 20 m and is weakly coarsening upward these attributes in combination with locally abundant hummocky cross stratified sandstones a paucity of wave ripples in the background fair weather deposits carbonaceous matter in all facies a lack of clinoforms and a low diversity abundance trace fossil suite suggest deposition in a prodelta environment between fair weather and storm wave base a three component shoreface to shelf model consisting of delta front subaqueous channel and prodelta turbidite deposits is proposed to explain the depositional environment and setting of the hatch mesa succession regional correlation has demonstrated that the hatch mesa succession is time equivalent with the second parasequence in the kenilworth kps2 which is characterized by inner shelf turbidite bodies at a number of other localities paleogeographic reconstructions reveal that the hatch mesa succession was deposited at least 16 km basinward of the kps2 lower shoreface in approximately 20 m of water the kps2 hatch mesa sandstone body package is part of the early highstand systems tract indicating that isolated or stray sandstone bodies are not solely restricted to the falling stage or lowstand systems tracts

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

hyperpycnal plume derived fans in the santa barbara channel california 1 hyperpycnal gravity currents rapidly transport sediment across shore from rivers to the continental shelf and deep sea although these geophysical processes are important sediment dispersal mechanisms few distinct geomorphic features on the continental shelf can be attributed to hyperpycnal flows here we provide evidence of large depositional features derived from hyperpycnal plumes on the continental shelf of the northern santa barbara channel california from the combination of new sonar lidar and seismic reflection data these data reveal lobate fans directly offshore of the mouths of several watersheds known to produce hyperpycnal concentrations of suspended sediment the fans occur on an upwardly concave section of the shelf where slopes decrease from 0 04 to 0 01 and the location of these fans is consistent with wave and auto suspending sediment gravity current theories thus we provide the first documentation that the morphology of sediment deposits on the continental shelf can be dictated by river generated hyperpycnal flows

channel adjustments and vegetation cover dynamics in a large gravel bed river over the last 200 years article i nfo the timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river the piave river eastern italian alps that was heavily impacted by human activities training structures hydropower schemes and gravel mining have been analyzed by historical maps aerial photos repeated topographic measurements and geomorphological surveys results show that the channel underwent a strong narrowing during the twentieth century but with a faster pace during the 1970s 1990s and with an associated shift from a dominant braided pattern to a wandering morphology bed incision up to 2 m mostly from gravel mining has been documented for this period large areas of the former active channel were colonized by riparian forests both as islands and as marginal woodlands the ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend with evidence of vegetation erosion channel widening even though a significant aggradation phase is not present we conclude that alteration of sediment regime has played a major role on the long term channel evolution however only relevant flood events rin10 15 years appear to determine substantial island erosion and therefore the proportion of island vs channel area fluctuates depending on flood history

spillage sedimentation on large river floodplains active deposition across the floodplains of large rivers arises through a variety of processes collectively these are here termed spillage sedimentation three groups of 11 spillage sedimentation styles are identified and their formative processes described form presences on large river floodplains show different combinations of active spillage styles only some large floodplains have prominent levees some have coarse splays many have accessory channel dispersion and reworking while still water sedimentation in lacustrine environments dominates some lower reaches infills are also commonly funnelled into prior and often linear negative relief forms relating to former migration within the mainstream channel belt 2016 the authors earth surface processes and landforms published by john wiley sons ltd shuttle radar topography mission srtm and landsat 8 data are used to map spillage form types and coverage along a 1700 km reach of the amazon that has an active floodplain width of up to 110 km with a systematic character transformation down valley spillage forms associated directly with mainstream processes rarely account for more than 5 of the floodplain deposits there is a marked decrease in floodplain point bar complexes pbc over 1700 km downstream from 34 to 5 and an increase in the prevalence of large water bodies 2 to 37 and accompanying internal crevasses and deltas 0 to 5 spillage sedimentation is likely within the negative relief associated with these forms depending on mainstream sediment laden floodwater inputs 2016 the authors earth surface processes and landforms published by john wiley sons ltd spillage style dominance depends on the balance between sediment loadings hydrological sequencing and morphological opportunity down river form sequences are likely to follow gradient change prior up river sediment sequestration and the altered nature of spilled loads but also crucially local floodplain relief and incident water levels and velocities at spillage times considering style distribution quantitatively as a spatially distributed set of identifiable forms emphasizes the global variety to spillage phenomena along and between large rivers 2016 the authors earth surface processes and landforms published by john wiley sons ltd

hydro geomorphic hazards and impact of man made structures during the catastrophic flood of june 2000 in the upper guil catchment queyras southern french alps the guil river valley queyras southern french alps is prone to catastrophic floods as the long historical archives and holocene sedimentary records demonstrate in june 2000 the upper part of this valley was affected by a b30 yearq recurrence interval r i flood although of lower magnitude and somewhat different nature from that of 1957 n100 year r i flood the 2000 event induced serious damage to infrastructure and buildings on the valley floor use of methods including high resolution aerial photography multi date mapping hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the guil river and its tributaries the total rainfall 260 mm in four days and maximum hourly intensity 17 3 mm h 1 aggravated by pre existing saturated soils explain the immediate response of the fluvial system and the subsequent destabilisation of slopes abundant water and sediment supply landsliding bank erosion particularly from small catchment basins cut into slaty schist bedrock resulted in destructive pulses of debris flow and hyperconcentrated flows the specific stream power of the guil and its tributaries was greater than the critical stream power thus explaining the abundant sediment transport the guil discharge was estimated as 180 m 3 s 1 at aiguilles compared to the annual mean discharge of 6 m 3 s 1 and a june mean discharge of 18 m 3 s 1 the impacts on the guil valley floor flooding aggradation generalised bank erosion and changes in the river pattern were widespread and locally influenced by variations in the floodplain slope and or channel geometry the stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades as exemplified by the aiguilles case study where the worst damage took place a comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that despite their poor maintenance the flood control structures built after the 1957 event were relatively efficient in contrast to unprotected places the comparison also demonstrates the role of land use changes conversion from traditional agro pastoral life to a ski hikingbased economy construction of various structures in reducing the guil channel capacity and more generally in increasing the vulnerability of the human installations the efficiency of the measures taken after the 2000 flood narrowing and digging out of the channel is also assessed final evaluation suggests that in such high mountainous environments there is a need to keep

logistic analysis of channel pattern thresholds meandering braiding and incising a large and geographically diverse data set consisting of meandering braiding incising and post incision equilibrium streams was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel pattern and instability an energy based index was developed for estimating the risk of channel instability associated with specific stream power relative to sedimentary characteristics the strong significance of the 74 statistical models examined suggests that logistic regression analysis is an appropriate and effective technique for associating basic hydraulic data with various channel forms the probabilistic diagrams resulting from these analyses depict a more realistic assessment of the uncertainty associated with previously identified thresholds of channel form and instability and provide a means of gauging channel sensitivity to changes in controlling variables

hyperpycnal sediment discharge from semiarid southern california rivers implications for coastal sediment budgets southern california rivers discharge hyperpycnal river density greater than ocean density concentrations of suspended sediment 40 g l according to buoyancy theory during flood events mostly during el nino southern oscillation enso conditions because hyperpycnal river discharge commonly occurs during brief periods hours to occasionally days mean daily flow statistics often do not reveal the magnitude of these events hyperpycnal events are particularly important in rivers draining the transverse range and account for 75 of the cumulative sediment load discharged by the santa clara river over the past 50 yr these events are highly pulsed totaling only 30 days 0 15 of the total 50 yr period observations of the fate of sediment discharge although rare are consistent with hyperpycnal river dynamics and the high likelihood of turbidity currents during these events we suggest that much of the sediment load initially bypasses the littoral circulation cells and is directly deposited on the adjacent continental shelf thus potentially representing a loss of immediate beach sand supply during particularly exceptional events 100 yr recurrence intervals flood underflows may extend past the shelf and escape to offshore basins

reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors a review of complementary approaches abstract until recently one way relationships between flow dynamics geomorphology and plant ecology were considered dominantly when studying the functioning of river systems whereby fluvial landforms and hydrogeomorphic processes drive the evolution of riparian plant communities however biological communities may significantly control geomorphic processes and have strong impacts on landform dynamics in order to fully identify the processes linked to river dynamics changes in time and space of fluvial landforms and associated plant communities conceptual multidisciplinary progress is clearly needed to understand the mutual interactions and feedbacks between fluvial landforms and vegetation community dynamics this paper presents a detailed literature review of fluvial geomorphology riparian plant ecology and hydraulic engineering knowledge the historical and recent development of ecological plant succession theory toward the integration of hydrogeomorphic disturbances is discussed as well as the integration of vegetation within geomorphology as a significant landform control factor incorporating both hydrogeomorphic controls on riparian vegetation dynamics and mechanical impacts of vegetation structures on flow properties and sediment dynamics recent progress in ecology hydraulic engineering and fluvial geomorphology emphasises interdependence between biological and physical forms and processes based on this literature review a fluvial biogeomorphic succession concept is proposed to link fluvial landform and riparian vegetation community evolution within a bi directional model the succession of fluvial landforms and associated vegetation communities is composed of four main critical phases that represent a shift in the relative dominance of hydrogeomorphic and ecological processes as a response to biostabilisation and passive bioconstruction processes the positive feedbacks associated with this shift lead to the development of characteristic biogeomorphic structures such as vegetated banks islands or floodplains which are moderated by the biogeomorphic functional roles of ecosystem engineers that induce or reinforce the positive feedbacks this fluvial biogeomorphic succession concept relates the natural darwinian selection and ecological succession theories to fluvial geomorphology

evidence of across shelf transport of fine grained sediments turbidite filled shelf channels in the campanian aberdeen member book cliffs utah usa the shore normal transport of fine grained sediments by shelf turbidity currents has been the focus of intense debate over the last 20 years many have argued that turbidity currents are unlikely to be a major depositional agent on the shelf however sedimentological architectural stratigraphic and palaeogeographic data from the campanian aberdeen member book cliffs eastern utah suggests otherwise and clearly demonstrates that storm generated and river flood generated underflows can transport a significant volume of fine grained sediments across the shelf these across shelf flowing turbidity currents cut large subaqueous channel complexes up to 7 m deep tens of kilometres basinward of their time equivalent shoreface the shelf channels were filled with organic rich siltstones mudstones and very fine to fine grained bouma like sandstone beds including wave modified turbidites hyperpycnites and classical turbidites deposition was above storm wave base palaeocurrent data reveal an overwhelmingly dominant across shelf east south east offshore directed transport trend tectonic activity and or concomitant palaeogeographic reorganization of the basin may favour the generation of these turbidite rich shelf deposits by altering the relative balance of wave versus fluvial energy increased erosion and sediment supply rates because of tectonic uplift of the hinterland may have increased the probability of fluvial dominance along the coastline and hence the possibility of submarine channelization in front of the river mouths additionally the coastline may have become more sheltered from direct wave energy thus allowing the fluvial processes to dominate seasonal increases in rainfall and storm activity may also favour the generation of across shelf underflows on wave dominated shorelines isolated shelf channels and lobes are most likely to be found down dip of fluvial feeder systems in relatively high sediment supply settings these features are also most likely to occur in systems tracts that straddle a sequence boundary especially those which are tectonically generated as these would enhance the potential for altering basin morphology and hence the balance of fluvial and wave energy isolated shelf channels are recognized in older and younger strata in the book cliffs region implying that wave supported gravity flows were a recurrent phenomena in the campanian of utah it is probable that isolated shelf bodies are preserved in other stratigraphic intervals in the cretaceous western interior of north america and other basins worldwide and are currently being overlooked or misidentified shoreface to shelf facies models should be revised to incorporate turbidite rich shelf deposits in some shelf settings

planform dynamics of braided streams the high dynamism and complexity of braided networks poses a series of open questions significant for river restoration and management the present work is aimed at the characterization of the morphology of braided streams in order to assess whether the system reaches a steady state under constant flow conditions and in that case to determine how it can be described and on which parameters it depends a series of 14 experimental runs were performed in a laboratory physical model with uniform sand varying the discharge and the longitudinal slope planimetric and altimetric configurations were monitored in order to assess the occurrence of a steady state a set of parameters was considered such as the braid plain width and the number and typology of branches and nodes results point out that a relationship exists between braiding morphology and two dimensionless parameters related to total water discharge and stream power we found that network complexity increases at higher values of water discharge and a larger portion of branches exhibits morphological activity results are then compared to the outputs of a simple one dimensional model that allows to easily predict the average network complexity once the bed topography is known model computations permit also the investigation of the effect of water discharge variations and to compare different width definitions the at a station variability of planimetric parameters shows a peculiar behaviour both regarding number of branches and wetted width in particular the analysis of the relationship between width and discharge highlighted relevant differences in comparison to single thread channel copyright 2009 john wiley sons ltd

geomorphic and sedimentological controls on the effectiveness of an extreme flood the 1993 flood on the upper mississippi river was a rare large magnitude hydrological event field and aerial survey analyses and landsat 5 thematic mapper data were used to appraise the thickness of overbank deposits on leveed and unleveed reaches results indicate that minimal 5 mm overbank sedimentation occurred except in the immediate vicinity of a levee break unleveed sections also lacked overbank sedimentation little geomorphological or sedimentological evidence of this extreme event is likely to be preserved this raises questions about the completeness of the stratigraphic record in situations where wide floodplains with cohesive soils provide effective resistance and dissipate energy so that erosion is minimized and or sediment supply is limited by event timing or sequencing a large flood may leave little or no substantive evidence of its occurrence

flood stratigraphies in lake sediments a review abstract records of the frequency and magnitude of floods are needed on centennial or millennial timescales to place increases in their occurrence and intensity into a longer term context than is available from gauged river flow and historical records recent research has highlighted the potential for lake sediment sequences to act as a relatively untapped archive of high magnitude floods over these longer timescales abyssal lake sediments can record past floods in the form of coarser grained laminations that reflect the capacity for river flows with greater hydrodynamic energy to transport larger particles into the lake this paper presents a framework for investigating flood stratigraphies in lakes by reviewing the conditioning mechanisms in the lake and catchment outlining the key analytical techniques used to recover flood records and highlighting the importance of appropriate field site and methodology selection the processes of sediment movement from watershed to lake bed are complex meaning relationships between measureable sedimentary characteristics and associated river discharge are not always clear stratigraphical palaeoflood records are all affected to some degree by catchment conditioning fluvial connectivity sequencing of high flows delta dynamics as well as within lake processes including river plume dispersal sediment focussing re suspension and trapping efficiency with regard to analytical techniques the potential for direct e g laser granulometry and indirect e g geochemical elemental ratios measurements of particle size to reflect variations in river discharge is confirmed we recommend care when interpreting fine resolution geochemical data acquired via micro scale x ray fluorescence xrf core scanning due to variable down core water and organic matter content altering x ray attenuation we also recommend accounting for changes in sediment supply through time as new or differing sources of sediment release may affect the hydrodynamic relationship between particle size and or geochemistry with stream power where these processes are considered and suitable dating control is obtained discrete historical floods can be identified and characterised using palaeolimnological evidence we outline a protocol for selecting suitable lakes and coring sites that integrates environmental setting sediment transfer processes and depositional mechanisms to act as a rapid reference for future research into lacustrine palaeoflood records we also present an interpretational protocol illustrating the analytical techniques available to palaeoflood researchers to demonstrate their utility we review five case studies of palaeoflood reconstructions from lakes in geographically varied regions these show how lakes of different sizes and geomorphological contexts can produce comprehensive palaeoflood records these were achieved by consistently applying site validated direct and proxy grain size measurements well established chronologies validation of the proxy process interpretation and calibration of the palaeoflood record against instrumental or historical records