geomorphic and hydrologic aspects of monsoon floods on the narmada and tapi rivers in central india abstract high magnitude floods during the monsoon season are considered to be india s recurring and leading natural disaster such large flood are extremely important events not only in terms of human impact but also from the standpoint of geomorphic effectiveness and geomorphic work the narmada and tapi rivers in central india are characterized by one of the most intense flood regimes in the seasonal tropics modern historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the narmada and tapi rivers a catastrophic flood on tapi in july 1991 that occurred from heavy rains and a dam break represents one of the highest unit discharges yet recorded in any part of central india flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world the rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium and in choral river a 5000 year old record has been located examination of the synoptic conditions associated with the flood generating low pressure systems reveals that all but a few of them are the result of bay of bengal depressions the flooding in such large rivers profoundly challenges flood hazard management because of limited instrumental records and large spatio temporal variation it is therefore imperative that the design engineering community in india incorporates the geomorphic sedimentologic and botanic information in the planning and design of water resource development projects

gradational thresholds and landform singularity significance for quaternary studies abstract geomorphic threshold conditions have been identifed at which stream patterns change and gully initiation occurs for both the threshold conditions are defined by the parameter of relative shear stress which is a measure of the energy state of the system and is based on known values of stream slope and mean annual discharge for patterns or drainage area for gullies the probability of passing from a stream pattern to another or from stable to gullied valley floors is a smooth function of relative shear stress and so the thresholds separating the different states of the geomorphic systems are gradational the singularity of landforms prevents the identification of a sharp threshold and as a result landform sensitivity will differ within the same area and under the same conditions therefore geomorphic predictions and postdictions will be uncertain and quaternary correlations will lack precision

channel hydraulics and geomorphic effects of an extreme flood event on the sabie river south africa the effects of the february 2000 extreme flood 6000 7000 m3 s 1 200 year return period flow on channel morphology are examined over 108 km of the semiarid bedrock influenced sabie river south africa the river is incised into bedrock forming a macrochannel with a planform that is fixed over the timescale of change being investigated within the confines of the macrochannel the river may be zoned into distinct channel types pool rapid bedrock anastomosing etc between 1 and 20 km long each characterised by a distinctive assemblage of smaller morphologic units ranging from large lateral bars filling most of the macrochannel to metre scale lee bars and sand sheets changes in morphologic unit composition and spatial coverage are presented for each channel type along with reconstructed flood hydraulics this appraisal is based on 1 10 000 scale aerial photographs taken 5 months before and 7 months after the event the latter verified by post flood field observations some channel types e g cohesive mixed anastomosing displayed widespread sedimentary stripping while others e g bedrock anastomosing remained essentially unchanged at the morphologic unit scale large cohesive lateral deposits were largely unaffected by the flood whereas smaller active channel unconsolidated and consolidated features associated with perennially flowing sub channels were stripped at the scale of channel type change appears to be controlled by a complex combination of factors including i spatial and temporal flow energy level change affecting sediment transport and ii tributary location susceptibility to erosion at the scale of the morphologic unit appears to be linked to elevation above the low flow active channel with features at lower elevations in the channel being stripped field observations suggest that unconsolidated active channel units are re forming at stripped locations

channel form and processes of the flood dominated narmada river india the 1300 km long narmada river flows along a structural lineament alternating between constricting rocky gorges and rapids and meandering wide alluvial reaches channel forms and processes were studied in a 120 km long section of an alluvial reach channel size shape and bedforms in the narmada river are related to very large floods which have occurred three times in this century during such floods the entire 400 m wide channel is utilized and 10 15 m high cliffs on both sides operate as riverbanks normally even the high flows of the south westem monsoon are insufficient to fill the whole channel and hence their effects are limited to building of discontinuous floodplains between the cliffs and modifying bedforms and bars a channel in channel topography is thus created the very large floods are also responsible for erosion of the rocky stretches and building of point bars the river meanders but its movement is restricted because of 1 rocky gorges and scablands operating as anchor points at intervals and 2 the presence of high alluvial cliffs which are topped on extremely rare occasions in spite of being located in a tectonically active zone in a monsoon setting it is the exceptional high magnitude floods at irregular intervals which control the form and behaviour of the narmada river

middle late pleistocene landscape evolution of the dover strait inferred from buried and submerged erosional landforms abstract prominent landforms either buried or preserved at the seafloor provide important constraints on the processes that led to the opening and present day configuration of the dover strait here we extend previous investigations on two distinct landform features the fosse dangeard and lobourg channel to better understand the poly phase history of their formation and inferences for the opening and pleistocene evolution of the dover strait the fosse dangeard consist of several interconnected palaeo depressions their morphology and spatial distribution are interpreted to be the result of plunge pool erosion generated at the base of north eastward retreating waterfalls their infills comprise internal erosional surfaces that provide evidence for the occurrence of several erosional episodes following their initial incision the lobourg channel comprises various sets of erosional features attesting to the occurrence of several phases of intense fluvial and or flood erosion the last one of these carved a prominent inner channel which truncates the uppermost infill of the fosse dangeard the morphology of the lobourg inner channel and the erosional features associated with its incision strongly resemble landforms found in megaflood eroded terrains indicating that this valley was likely eroded by one or several megafloods our study therefore corroborates the existence of waterfalls in the dover strait at least once during the pleistocene epoch it also provides evidence of the occurrence of multiple episodes of fluvial and flood erosion including megafloods finally this study allows us to establish a relative chronology of the erosional depositional episodes that resulted in the present day morphology of this region

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

reconstruction of holocene high altitude vegetation cover in the french southern alps evidence from soil charcoal the study presented here from the southern french alps demonstrates the reliability of soil charcoal analysis for the study of holocene past treeline positions the occurrence of charcoal in mineral soils along transects from 1950 m up to 2920 m demonstrates the role of fire in the establishment of the present vegetation patterns the abrupt decrease of charcoal concentration at about 240 0 2 600 m which varies across the study sites corresponds to the modern transition between subalpine forest and alpine tundra charcoal particles formedinsitu providehighspatial resolutiondataforthereconstructionofpastforestandtreelinechanges soil charcoal analysis indicated that 1 treeline was 300 m higher around 6800 cal bp than today and 2 the uppermost forest belt up to 2810 m was colonized by larch larix decidua mill and arolla pine pinus cem bra l thispineispresenttodaybutpatchilydistributed itisabsentfromthethreeareasstudied radiocarbon dates ranging from c 6800 cal bp to the modern period along with historical and archaeological data sug gest that the present pattern of the uppermost forest belt and the patchy distribution of arolla pine in the stud ied area are the results of anthropogenic fire mainly agropastoral activities the question of global warming consequences on treelines in this part of the french alps is discussed

dynamic evolution of riverbed potholes in the granitic bedrock of chota nagpur plateau middle part of subarnarekha river basin india abstract potholes are an extraordinary abiotic micro geomorphic feature on the granite and quartzite topography of the subarnarekha river basin these erosional features are carved by the running water actions of river subarnarekha and its tributaries the study has been conducted on 17 sites n 1707 which are exemplary for the development of this micro fluvial feature located on the rejuvenated middle part of subarnarekha river basin hydrological and geological factors are imperative for their formation joint length joint density joint orientation rock hardness variations of fluvial hydraulic bed roughness river bed slope and hydraulic gradient are conducive factors for the development of this geomorphic feature monsoon is the ideal season for carving these features by river turbulence when abrasive work of river is predominant on the river bed monsoon 3000 6500 k g m 2 s 3 non monsoon 0 50 3 0 k g m 2 s 3 the evolutionary sequence of potholes is from round to oval and then elongated with vertical scouring the relation between pothole radius and depth has been established to explain the process of its expansion joint length shows positive relation r2 0 71 with pothole area while joint density has a negative relation r2 0 29 as these are structurally weaker zones characterised by destruction of these features suspended load is also a hindrance in their formation sites with high suspended load are delineated as pothole destructive zones the mean dynamic evolution of circular oval and elongated shaped potholes occurs at spatio temporal level o e 0 005 0 60 m 3 c e 0 01 0 25 m 3 e e 0 005 0 30 m 3 and the initiation of this development depends on inner depth and radius of individual pothole

controlling factors in the distribution and development of incised meanders in the central colorado plateau a study of the distribution and geometry of incised meanders in 64 reaches encompassed approximately 600 km of the green colorado and san juan rivers in the central colorado plateau the sinuosity average planform size and average cross sectional symmetry of each reach were determined by map measurements and by spectral analysis of the curvature series for each reach as determined from interpolations of the digitized traces of the channels possible controlling variables examined including average channel gradient drainage area average bedrock erodibility and bedrock structure were compiled for each reach using available maps gradients in the studied reaches are significantly correlated with bedrock type sinuous incised channels are generally found in low gradient reaches in the san juan river the channel is sinuous where it flows against the bedrock dip and generally straight in reaches where flow is downdip this correlation is weak in the green and colorado rivers the average meander size of the sinuous reaches as described by the median curvature value for each reach is generally less in steeper reaches than in low gradient reaches although the relation of bend size to controlling variables is much less clear than for sinuosity or cross section shape most meander cross sections in the area are relatively symmetrical but highly ingrown forms are also present in general symmetric bends are associated with resistant bedrock units whereas ingrown forms develop in massive sandstone and in highly erodible bedrock gradient significantly influences the distribution of ingrown bends with asymmetric meanders concentrated in reaches of low average gradient this correlation is stronger than that between cross section shape and lithology itself incised meanders of the central colorado plateau are probably at least partly inherited from ancestral streams of unknown age that flowed across the area before the present canyons were cut correlation between meander distribution and regional structures suggests that the general location of low gradient sinuous reaches has probably not changed during the incision of the present canyons incised meanders however are clearly able to modify their geometry in response to changes in bedrock resistance as indicated by the strong correlation between bedrock type and cross section symmetry one mechanism of modification is abandonment of bends which is documented by 18 cutoff meanders in the study area

formation of inner gorges abstract under certain conditions a distinctive type of landform the inner gorge evolves in drainage basins undergoing persistent base level lowering inner gorges are typical of basins in north coastal california u s a and one such basin was selected for detailed study based on analysis of along channel distribution of rock types streamside land sliding and stored sediment as well as computation of stream power the following set of conditions appears to accompany the formation of an inner gorge 1 inner gorge parent materials are relatively competent homogenous rock types 2 in inner gorge reaches there is an absence of any base level control that would prevent downcutting 3 continued downcutting occurs due to continued base level fall and this fall is most likely induced by tectonic uplift 4 mass slope failures that form steep straight slopes debris slides are the main denudational hillslope process in the inner gorge 5 sufficient stream power is available to transport landslide derived material out of the inner gorge reach such that alluvial sediment does accumulate in the reach over time and 6 the initial condition of the landscape is a low relief erosionally mature land surface into which the inner gorge is cut this initial condition implies that inner gorges develop in tectonically rejuvenated landscapes the processes of denudation on inner gorge slopes acting over a time scale of 105 years result in a style of landform evolution that approximates parallel slope retreat this type of slope retreat maintains the distinct inner gorge morphology during sustained downcutting though the analysis is confined to one basin in northern california so as to provide a detailed framework to understand inner gorge landform inner gorges appear to be commonplace in areas of moderate to high uplift where lithologic and tectonic conditions are appropriate for their formation a global search for this landform yet to be undertaken is needed to confirm whether it indeed occurs where the above conditions are met

unusual vegetation stability in a lowland pine forest area doksy region czech republic although the holocene is characterized by prominent vegetation changes some vegetation types can reveal surprising stability we studied holocene woodland history in a sandstone pseudokarst area

gis analysis of fluvial knickzone distribution in japanese mountain watersheds although a knickzone a location at which stream gradient is locally large and intense erosion occurs has been regarded as an important geomorphic feature in bedrock river morphology the distribution of knickzones has not been well investigated especially for broad area this study examines the distribution of fluvial knickzones along mountain rivers for the entire japanese archipelago whereas conventional manual methods of identifying knickzones based on map readings or field observations tend to be subjective and are impractical for a broad scale analysis this study employs a semi automated method of knickzone extraction using dems and gis in a recent study by the authors this method has been shown to enable efficient examination of knickzone distribution over a broad area investigations on major mountain rivers revealed that knickzones are generally abundant in upstream steep river reaches suggesting hydraulic origins for the knickzones the broad presence of such knickzones in the steep japanese mountain rivers indicates that rivers subjected to active erosion show complex morphology induced by natural irregularities of water flow hydraulics as well as various environmental perturbations such as climatic changes there also seems to be a characteristic frequency of knickzone distribution common to moderately steep to very steep bedrock reaches in japan although volcanic products such as lavas and welded pyroclastic flow deposits in valleys can cause distinct knickzones substrate geology plays only a limited role in determining the distribution and form of knickzones

bedrock channel incision along piccaninny creek australia piccaninny creek in northwestern australia drains approximately 60 km 2 of high relief sandstone and conglomerate terrain the drainage basin lies within the seasonal tropics and is characterized by ephemeral high magnitude flows flood flows of 50 to 100 m 3 s 1 are probably primarily responsible for the spectacular erosional features along the bedrock channel segments of the creek these erosional features occur in two basic forms as parallel longitudinal grooves and as a downstream sequence that begins with shallow linear depressions parallel to flow and ends in a deep narrow inner channel the longitudinal grooves are assumed to result from longitudinal vortices while the erosional sequences which closely resemble von karman vortex streets are interpreted to result from turbulent vortices shed off irregularities of the channel bed the longitudinal grooves and the inner channel sequence alternate with each other and with gravel floored depositional reaches downstream along the channe

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

the evolution of potholes in granite bedrock w spain abstract spectacular bed forms eroded by flood flows are a feature of the bare rock terraces and channels of some rivers cut into granitic rock in the districts of salamanca ca ceres and badajoz w spain the bed forms comprise a range of pothole types which individually represent various stages in the evolution of mature potholes pothole evolution reflects the hydrodynamics of the erosion by high velocity flood flows of bedrock they evolve in time and in concert with the evolution of the landscape initial shallow depressions deepen gradually into cylindrical forms as vertical erosion increases as time passes lateral erosion increases and leads to wider cylindrical forms which evolve into asymmetrical and bulbous forms second order scouring leads to the development of minor decorative potholes tunnels and niche caves in the large mature potholes data on pothole geometry should complement the findings of other geomorphological studies on individual rivers and on regions

late holocene human induced changes to the extent of alpine areas in the east sudetes central europe the easternmost parts of the hercynian mid mountains of central europe namely hrub jesenik and kralick sn nik are characterised by extensive alpine areas at altitudes above approximately 1300 m a s l in order to determine the contribution of human activities to the extent of these summit grasslands we analysed charcoal assemblages and pollen profiles taken from high elevation sites the first burn was dated to the iron age about the first to second centuries bc with successive fire events recorded in the early mediaeval epoch from about ad 670 significant human influence as recorded in pollen diagrams was detected as late as during the high middle ages about the twelfth to thirteenth centuries ad charcoal assemblages reveal similar trends in species composition the oldest and or deepest samples are represented by charcoal fragments of picea abies and various broadleaf trees and shrubs such as betula sp sorbus sp juniperus sp and salix sp towards the surface picea abies gradually becom

pleistocene cataclysmic flooding along the big lost river east central idaho relationships between cataclysmic flood generated landforms and flood hydraulics were investigated along box canyon an 11 km long bedrock gorge of the lower big lost river geomorphic mapping along box canyon indicates that a cataclysmic flood completely inundated the gorge resulting in large scale erosional and depositional features on the adjacent basalt upland step backwater hydraulic modeling indicates that a discharge of 60 000 m 3 s was required to produce the geologic paleostage evidence maximum stream power per unit area of bed locally attained values of 26 000 w m 2 during the peak ranking the big lost river flood third in terms of power behind the famous missoula and bonneville floods the spatial distribution of unit stream power indicates that bedrock erosion and boulder deposition on the basalt upland adjacent to box canyon were governed primarily by decreasing unit stream power and or fluctuating unit stream power gradients a preliminary depositional threshold for the largest flood boulders defines a lower limit of flood power required to sustain boulder transport along this bedrock fluvial system ultimately hydrodynamic controls on box canyon flood erosion and deposition derive from the irregular volcanic rift topography of the eastern snake river plain outburst floods from a glacial lake in headwater regions during the late pleistocene may have induced the torrential discharges within box canyon

relationships between block quarrying bed shear stress and stream power a physical model of block quarrying of a jointed bedrock channel we examine interactions among substrate characteristics hydraulic forces and erosive processes acting on a jointed resistant substrate in a flume experiencing supercritical flow we conducted six experimental runs with no sediment in transport constant slope varying discharge varying channel width and varying geometry of concrete blocks forming the channel bed the initial downstream boundary was a vertical knickpoint lip perpendicular to flow observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat the knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side resulting in a large magnitude event in contrast to previous studies where blocks were vertically plucked blocks were removed by sliding from the knickpoint building on previous work we develop a theoretical force threshold for sliding block mobilization which suggests that increased block side length parallel to flow increases block stability whereas increased side length perpendicular to flow decreases block stability stream power was also linked to observed erosion rates as a function of block length consistent with the idea that block loosening rates play a key role in controlling average erosion rates inferred amplification of bed shear stress and pressure at the knickpoint lip stream power and joint spacing correlate with the rate of block erosion the results although limited in scope and scale suggest a linear relationship between bed shear stress and stream power with the rate of block erosion block sliding especially at near vertical knickpoints can play a key or even dominant role in the morphologic dynamics of the river channel

lithologic and hydrologic controls of mixed alluvial bedrock channels in flood prone fluvial systems bankfull and macrochannels in the llano river watershed central texas usa abstract the rural and unregulated llano river watershed located in central texas usa has a highly variable flow regime and a wide range of instantaneous peak flows abrupt transitions in surface lithology exist along the main stem channel course both of these characteristics afford an opportunity to examine hydrologic lithologic and sedimentary controls on downstream changes in channel morphology field surveys of channel topography and boundary composition are coupled with sediment analyses hydraulic computations flood frequency analyses and geographic information system mapping to discern controls on channel geometry profile pattern and shape and dimensions along the mixed alluvial bedrock llano river and key tributaries four categories of channel classification in a downstream direction include i uppermost ephemeral reaches ii straight or sinuous gravel bed channels in cretaceous carbonate sedimentary zones iii straight or sinuous gravel bed or bedrock channels in paleozoic sedimentary zones and iv straight braided or multithread mixed alluvial bedrock channels with sandy beds in precambrian igneous and metamorphic zones principal findings include i a nearly linear channel profile attributed to resistant bedrock incision checkpoints ii statistically significant correlations of both alluvial sinuosity and valley confinement to relatively high f mean depth hydraulic geometry values iii relatively high b width hydraulic geometry values in partly confined settings with sinuous channels upstream from a prominent incision checkpoint iv different functional flow categories including frequently occurring events

controls on bedrock channel incision along nahal paran israel nahal paran drains 3600 km2 of egypt s sinai peninsula and israel s negev desert much of the channel is alluvial but a canyon 10 5 km long has been incised into late cretaceous chert and dolomite in the lower portion of the basin slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3 s 1 over a period of at least 350 years step backwater simulations of flood flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon and associated variability in energy expenditure and sediment transport these variations reflect channel cross sectional morphology the greatest values of stream power occur along the lower half of the study reach in association with three pronounced knickpoints and an inner channel the locations of these features reflect the exposure of thick resistant chert layers along the channel the presence of several similar but buried and inactive knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time probably in response to baselevel changes associated with tectonic activity along the dead sea rift thus the rate and manner of channel incision along the canyon of nahal paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient which in turn influence hydraulics and sediment transport

regional and local controls on the spatial distribution of bedrock reaches in the upper guadalupe river texas while studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types the objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the upper guadalupe river we compare reach length channel and floodplain width sinuosity bar length and spacing bar surface grain size and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types we find that local disturbances interaction of the channel and valley sides variation in lithology and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel alluvial and bedrock channel reaches in the upper guadalupe river are similar particularly with respect to the distribution of gravel bars surface grain size distributions of bars and channel slope and width our observations suggest that the fluvial system has adjusted to changes in base level associated with the balcones escarpment fault zone by phased incision into alluvial sediment and the underlying bedrock essentially shifting from a fully alluvial river to a mixed alluvial bedrock river

energy time and channel evolution in catastrophically disturbed fluvial systems abstract two diverse fluvial systems show that with time channels adjust such that the rate of energy dissipation is minimized one fluvial system characterized by high relief and coarse grained sediment was subjected to an explosive volcanic eruption the other system characterized by low relief and fine grained sediment was subjected to dredging and straightening study of the expenditure of kinetic and potential energy components of total mechanical energy provide an energy based rationale of the interdependency between processes and forms during channel evolution spatial and temporal trends of aggradation and degradation are similar although relative amounts of aggradation in the high energy system are greatly enhanced by the deposition of large amounts of eroded bank material from upstream reaches degradation accompanied by widening is the most efficient means of energy dissipation because all components of total mechanical energy decrease with time widening dominates energy dissipation in the coarse grained system to offset increases in hydraulic depth caused by incision in the low energy fine grained system channel adjustment and energy dissipation are dominated by vertical processes because of low relative values of kinetic energy and because eroded bank sediment is transported out of the drainage basin and does not aid in downstream aggradation energy dissipation or channel recovery specific energy is shown to decrease nonlinearly with time during channel evolution and provides a measure of reductions in available energy at the channel bed data from two sites show convergence towards a minimum specific energy with time time dependent reductions in specific energy at a point act in concert with minimization of the rate of energy dissipation over a reach during channel evolution as the fluvial systems adjust to a new equilibrium

lithologic control of bedrock meander dimensions in the appalachian valley and ridge province marked differences in bedrock meander dimensions in the appalachian valley and ridge province at times in adjacent reaches of a single stream are related to differences in relative erodability of the bedrock meanders cut in thick bedded to massive lithologies typically carbonates are distinctly smaller overall than meanders cut into shaly lithologies other factors that could affect bedrock meander dimensions were considered and none appeared to offer any help in explaining the dimensional differences observed above in plateau regions underlain by essentially horizontal strata the meander form characteristic of one lithology may become superimposed upon another as incision progresses producing anomalous relationships between bedrock meander dimensions and lithology empirical relations developed from meander geometry measurements and estimated bedrock erodability for 78 bedrock meander reaches containing a total of 1089 individual meander loops show that meanders cut in shaly lithologies ml 105 q0 50f where ml meander length and qf most probable annual flood are about twice the length of meanders cut in non shaly lithologies ml 39 30 56f the valley floor width of the meanders cut in shaly bedrock vf 28 q0 43f is two to three times wider than the valley floor width of the meanders cut in the more resistant non shaly bedrock vf 8 q0 43f the mean and median meander length values for individual reaches typically differ by less than 10 per cent

bedrock benches and boulder bars floods in the burdekin gorge of australia the burdekin gorge of northeastern australia lies within the seasonal tropics and is characterized by high discharge variability slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1 200 yrs these floods range in magnitude from 11 000 to 30 000 m 3 s 1 and are characterized by large downstream variations in hydraulics the floods generate high values of velocity boundary shear stress and stream power per unit area downstream fluctuations in these variables help to explain the location of boulder bars high flood levees small scale erosional features in the bedrock and the formation of inner channels boulder bars and flood levees form where shear stress and stream power decrease due to channel widening small scale erosional features such as potholes and troughs are best developed at sites of channel constriction and associated increases in shear stress and stream power the development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure and by flow hydraulics all of the channel features are related to the hydraulics of large floods which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge

river patterns and their meaning abstract rivers are largely responsible for shaping the earth s continental landscapes river patterns the spatial arrangements of channels in the landscape are determined by slope and structure at site and sector scale channel morphology varies spatially and in time but river patterns and drainage texture or the frequency of stream lines per unit area together determine the intricacy or otherwise of topography most river patterns evolve through natural selection slope induces the formation of such patterns as parallel radial and distributary while structure produces straight angular trellis and annular arrangements once established patterns tend to persist nevertheless at many sites the usual patterns have been disturbed and patterns that are anomalous in terms of slope and structure have been produced by diversion tectonism volcanism glaciation mass movements and human activities by antecedence superimposition inheritance or underprinting by the persistence of deeply eroding rivers which encounter alien structures and by climatic change river patterns provide clues to underlying structure and to the chronology of events they have also proved significant in the search for minerals

development of anastomosing channels in south central indiana abstract numerous streams in south central indiana tributary to the ohio river exhibit anastomosed channel patterns the anastomosing patterns are characterized by a primary channel and one or more auxiliary channels anabranches that are often abandoned or receive flow only during flood development of the anastomosed pattern involves avulsion and proceeds in a sequential manner where 1 aggradation significantly reduces channel depth 2 overbank flow occurs more frequently as channel capacity is reduced 3 floodplain deposits are incised by sediment deficient overbank flow producing a new channel gully having a lower gradient and elevation than the primary channel and 4 the gully created by overbank flow extends upstream and captures the drainage system converting the previous trunk channel into an anabranch avulsion is typically associated with aggrading fluvial systems but the investigated streams are undergoing an episode of degradation avulsion occurs because of localized deposition along concave up channel segments the concave up channel reaches are spatially controlled by the occurrence of non resistant strata that separate beds of resistant sandstone thus the location of avulsion and anastomosing channels is predetermined by the lithology and stratigraphy of the underlying bedrock

response of alluvial rivers to slow active tectonic movement alluvial rivers respond to valley slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river on the basis of experimental results and field examples hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers an experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift with subsidence aggradation in the central reach was the main response transverse bars developed downstream of the subsidence axis an experimental meandering channel responded to slope steepening with a sinuosity increase bank erosion and point bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis upstream of the uplift axis and downstream of the subsidence axis where the slope was flattened water flooded over bars local convexity in longitudinal profiles of the middle rio grande new mexico is considered to be formed by a domal uplift local aggradation and degradation could be explained by the effect of uplift the san joaquin river california which is now highly controlled does not show clear adjustment to the rapid subsidence due to ground water withdrawal it shows however a channel pattern adjustment to active tectonic subsidence that has been occurring for a long time the san antonio and guadalupe rivers in texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes

alluvial river response to neotectonic deformation in louisiana and mississippi repeat geodetic surveys show uplift of the monroe and wiggins anticlines in louisiana and mississippi there are deformed quaternary terraces which indicate long term deformation in the valleys of the alluvial rivers that cross these structures and there are floodplain and channel convexities that provide evidence of modern deformation in addition the channels show significant variations of morphology sinuosity gradient and depth and behavior appropriate to reaches of increased and decreased valley slope these alluvial rivers are adjusting to modern deformation and their adjustment confirms two geodetic leveling anomalies

multi channel patterns of bedrock rivers an example from the central narmada basin india abstract an anomalous multiple channel pattern in bedrock is observed on a predominantly downcutting reach of the narmada river the multi channel reach 800 2750 m in width and 8500 m in length is bounded by major faults and is underlain by granite and gneiss bedrock geomorphological investigations reveal differences among the upstream middle and downstream sub reaches of the multi channel study area whereas the upstream sub reach is dominated by deep flows and fine sediments the lower sub reach is characterized by a steep gradient and rapids the middle sub reach is the widest and is marked by thickly forested islands and boulder berms the characteristics of the three different sub reaches suggest control by the interactions of lithology flood processes and tectonics estimations of hack s 1973 stream gradient index values indicate considerable variations for the sl values along the length of narmada river the highest value of gradient index sl 797 is associated with the multi channel reach implying lithologic or tectonic control given the dimensions of the reach and its channels it appears that the present hydrological regime is inadequate to produce the feature we hypothesize that the multi channel pattern development in bedrock was initiated by block or domal uplift enhanced gradients and extreme floods permitted the system to exploit linear weaknesses in the bedrock leading to the development of anabranches and establishment of multiple channels in bedrock abrupt changes in the channel planform and morphology at the study site indicate that the river is adjusting its channel geometry width depth gradient and plan configuration to a new equilibrium channel morphology through the action of the extreme floods characteristic of this fluvial environment

patterns of alluvial rivers the pattern planform of a river can be considered at vastly different scales depending upon both the size of the river and the part of the fluvial system that is under consideration figure 1 for example in the broadest sense river patterns comprise a drainage network dendritic parallel trellis etc figure la the type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs at another scale a river reach which in figure lb is meandering is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior and to the engineer who is charged with maintaining navigation and preventing major instability when a single meander is examined figure 1 c the hydraulics offtow the sediment transport and the potential for bank erosion are of concern in addition the sedimentologist is interested in the distribution of sediment within the bend bed forms within the channel figure ld and sedimentary structures figure ie which also establish a component of roughness for the hydraulic engineer finally the individual grains figure if provide geologic information on the sediment sources the nature of sediment loads and the feasibility of dredging for gravel there is an interaction of hydrology hydraulics geology and geomorphology at all scales which emphasizes the point that the fluvial system as a whole cannot be ignored even though only a component of the system is to be studied in this review only the patterns or planforms of alluvial rivers are discussed although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin figure la as well as its geologic history cannot be ignored in the explanation of the pattern of any river

constraints on rift propagation history at the cobb offset juan de fuca ridge from numerical modeling of tectonic fabric abstract rift propagation has been proposed as the mechanism for the reorganization of the juan de fuca ridge during the cenozoic in particular magnetic anomalies in the cobb offset region of this ridge have been interpreted in terms of a complex dueling propagator history in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments although the overall propagation history has been inferred from the magnetic data these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant we thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery by modeling lineations as isochrons synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically for the cobb we used a propagation history model with a given timing and direction of propagation and generated synthetic fabric for different propagation rates the fabric is best fit for propagation rates between 400 and 900 mm yr slower 900 mm yr propagation rates yield slightly poorer fits the fabric results are thus in accord with the interpretation based on magnetics alone that the rates of rift propagation significantly exceeded the spreading rates an interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale 1 km by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics

adjustments by the charwell river new zealand to uplift and climatic changes major climatic changes and rapid local and regional tectonic movements were common in new zealand during the late quaternary and caused a diversity of adjustments in the drainage basin and piedmont reaches of the charwell river which are separated by the hope fault the onset of semi arid frigid climates during the latest pleistocene probably greatly increased hillslope sediment yields in a periglacial environment and the piedmont reach aggraded as much as 42 m on top of a broad strath with the return of humid mesic climates in the holocene sediment yields decreased as dense forests again mantled the slopes and the piedmont reach degraded as mush as 81 m dating of eleven cut and strath terraces by radiocarbon calibrated weathering rind measurements on greyawake cobbles shows the degradation rates varied greatly during the last 14 ka 1 ka 1000 yr initial degradation rates of 4 m ka 1 increased to 30 m ka 1 by 6 ka ago during a mid holocene climatic optimum since 4 ka ago degradation rates have been only 1 2 m ka 1 comparable to uplift rates in the piedmont reach inferred from marine terrace studies and the river is again cutting a broad strath each broad strath represents equilibrium conditions attained by this powerful stream during interglacial times despite episodes of being overwhelmed by climatically induced sediment yield increases during full glacial climates and having to maintain a long term degradation rate equal to the uplift rate the 75 81 m of degradation since formation of the latest pleistocene fill terrace tread is the sum of the amount of late pleistocene valley floor aggradation and the amount of regional uplift that occurred between the estimated times of major strath formation at about 30 and 0 ka the 39 m of tectonically induced degradation below the pre aggradation strath is sufficiently large that post 30 ka uplift may have doubled holocene degradation rates each of the eleven degradation terraces represents pauses of a few centuries in holocene downcutting brief equilibrium conditions were attained by streambed armoring and concurrent growth of riparian plants both processes progressively increased hydraulic roughness and the shear stresses needed to entrain streambed materials occasional floods possibly from rare cyclones derived from tropical moisture sources destroyed streambed armor and channel downcutting was renewed thus the formation of eleven equilibrium terraces can be accounted for without postulating additional tectonic perturbations or secular climatic changes

flood geomorphology of the katherine gorge northern territory australia the katherine gorge is a narrow deep canyon system developed in resistant sandstone the tropical monsoonal climate results in rare flood events of very large magnitude these floods flush sand sized sediment as washload through the gorge but local accumulations are preserved as slack water deposits at tributary mouths paleoflood hydrologic analysis of the deposits employing step backwater flow modeling allows quantitative estimates to be made of geomorphically significant flows for an annual exceedence probability of 10 2 the gorge may experience discharges of 6 000 m 3 s mean velocities of 7 5 m s flow depths of 15 to 45 m stream powers per unit area as great as 1 10 4 watts m 2 and bed shear stresses of 1 5 10 3 n m 2 such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large scale riffles at maximum stage pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock probably through intense hydraulic action pool and riffle development thus reflects the channel boundary characteristics for extreme flow conditions additional indicators of especially intense flow phenomena include potholes flutes abraded facets on rock bar surfaces and scabland development on upland bedrock surfaces

channel form and processes in bedrock and alluvial reaches of the raritan river new jersey fluvial geomorphologists have generally considered alluvial and bedrock channels to be fundamentally different in character alluvial channels being shaped by sediment transport processes and bedrock channels being shaped by structural and lithologic controls this study examines a 12 km bedrock floored reach of the raritan river that separates two gravel bed reaches above and below the bedrock channel has a patchy veneer of sandy gravel that covers the bed in areas of low slope but elsewhere consists of lateral bars and discrete two dimensional gravel bed forms perched on bedrock mean b axes of the 25 largest clasts on the crests of these bed forms range from 10 4 to 13 8 cm shear stresses estimated from measured velocity profiles and depth slope calculations indicate that most clasts on the bed forms are entrained by the bankfull discharge 290 m s which is equaled or exceeded about 1 of the time the thalweg of the 80 m wide channel is incised about 0 5 to 0 7 m and it meanders with a wavelength of about 750 m or approximately 10 times channel width this thalweg meander wavelength is consistent with reference to drainage area when compared to channel meander wavelengths in other locations within the basin and is similar to that in other rivers of comparable dimensions and discharge the bedrock alluvial reach is similar to alluvial channels with reference to meander geometry and sediment distribution it occupies an intermediate position on a continuum of channels of varying sediment supply in relation to transport capacity ranging from alluvial channels to those completely formed in bedrock

anabranching in mixed bedrock alluvial rivers the example of the orange river above augrabies falls northern cape province south africa abstract anabranching is characteristic of a number of rivers in diverse environmental settings worldwide but has only infrequently been described from bedrock influenced rivers a prime example of a mixed bedrock alluvial anabranching river is provided by a 150 km long reach of the orange river above augrabies falls northern cape province south africa here the perennial orange flows through arid terrain consisting mainly of precambrian granites and gneisses and the river has preferentially eroded bedrock joints fractures and foliations to form multiple channels which divide around numerous large up to 15 km long and 2 km wide stable islands formed of alluvium and or bedrock significant local variations in channel bed gradient occur along the river which strongly control anabranching style through an influence on local sediment budgets in relatively long 10 km lower gradient reaches 0 0013 within the anabranching reach local transport capacity exceeds sediment supply bedrock crops out extensively and channels flow over an irregular bedrock pavement or divide around rocky islands channel incision into bedrock probably occurs mainly by abrasion with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting mixed bedrock alluvial anabranching also occurs in a number of other rivers worldwide and appears to be a stable and often long lived river pattern adjusted to a number of factors commonly acting in combination 1 jointed fractured granitoid rock outcrop 2 erosion resistant banks and islands 3 locally variable channel bed gradients 4 variable flow regimes

modelling the bulk flow of a bedrock constrained multi channel reach of the mekong river siphandone southern laos the general nature of bulk flow within bedrock single channel reaches has been considered by several studies recently however the flow structure of a bedrock constrained large river with a multiple channel network has not been investigated previously the multiple channel network of the siphandone wetlands in laos a section of the mekong river was modelled using a steady one dimensional hydraulic model the river network is characterized by a spatially varying channel form leading to significant changes in the bulk flow properties between and along the channels the challenge to model the bulk flow in such a remote region was the lack of ideal boundary conditions the flow models considered both low flow high inbank and overbank flows and were calibrated using spot satellite sensor imagery and limited field data concerning water levels the application of the model highlighted flow characteristics of a large multi channel network and also further indicated the field data that would be required to properly characterize the flow field empirically important results included the observation that adjacent channels within the network had different water surface slopes for the same moments in time thus calibration data for modelling similar systems needs to account for these significant local differences further the in channel hydraulic roughness coefficient strongly varied from one cross section to the next manning s n range 0 01 to 0 10 these differences were amplified during low flow but persisted in muted form during high discharges

topographic hydraulic and vegetative controls on bar and island development in mixed bedrock alluvial multi channeled dryland rivers we investigate processes of bedrock core bar and island development in a bedrock influenced anastomosed reach of the sabie river kruger national park knp eastern south africa for sites subject to alluvial stripping during an extreme flood event 4470 5630 m3 s 1 in 2012 pre and post flood aerial photographs and lidar data 2d morphodynamic simulations and field observations reveal that the thickest surviving alluvial deposits tend to be located over bedrock topographic lows at a simulated peak discharge 4500 m3 s 1 most sediment sand fine gravel is mobile but localized deposition on bedrock topographic highs is possible at lower simulated discharges 1 000 m3 s 1 topographic highs are not submerged and deposition occurs in lower elevation areas particularly in areas disconnected from the main channels during falling stage field observations suggest that in addition to discharge rainwash between floods may redistribute sediments from bedrock topographic highs to lower elevation areas and also highlight the critical role of vegetation colonization in bar stability and in trapping of additional sediment and organics these findings challenge the assumptions of preferential deposition on topographic highs that underpin previous analyses of kruger national park river dynamics and are synthesized in a new conceptual model that demonstrates how initial bedrock topographic lows become topographic highs bedrock core bars and islands in the latter stages of sediment accumulation the model provides particular insight into the development of mixed bedrock alluvial anastomosing along the kruger national park rivers but similar processes of bar island development likely occur along numerous other bedrock influenced rivers across dryland southern africa and farther afield

charcoal fragments in surface samples following a fire and the implications for interpretation of subfossil charcoal data abstract following an uncontrolled fire in an area of heathland in sw england surface samples were taken from selected positions charcoal concentrations charcoal pollen ratio and size class frequencies were measured and the data from burned unburned and forest areas are compared charcoal concentrations show distinct variability between closely spaced samples but despite this differences are shown between the burned and unburned areas selected size classes are suggested as being appropriate for distinguishing between burned and unburned areas in palaeoecological studies while in need of replication these data should provide a basis for further studies of this type