high resolution temperature logging in shallow drillholes for the determination of terrestrial heat flow field examples and analysis abstract the first 200 m of the earth s crust is the depth range most investigated by drilling temperature profiles in such shallow drillholes generally show severe influences of groundwater flow therefore they are rarely used for heat flow determinations but regardless how severely the temperature logs are influenced by advective heat transfer the background conductive heat flow signal is still present in the data it is shown how to handle distorted temperature profiles by means of simple one dimensional steady state models the undisturbed background heat flow is estimated by model calculations the comparison of the modelled temperature profiles with the measured logs is a test for the credibility of the modelling approach examples from northwestern switzerland and northwestern turkey with irregular temperature profiles are given for the following situations conductive convective regime with vertical groundwater movement over the entire observation depth vertical flow in a certain layer located between purely conductive layers and water flow inside of the drillhole measured temperature profiles can be approximated by modelling within 0 2 c repeated high precision digital temperature logging is needed to prove the temporal stability of the convective disturbances in order to use steady state models

sediment characteristics of an extreme flood 1993 upper mississippi river valley the 1993 mississippi river flood was notable for its high magnitude long duration summer occurrence and low sediment discharge a field survey of a 70 km long reach in the vicinity of quincy illinois revealed that the event was characterized by 100 yr flood had remarkably little sedimentological or geomorphological impact on the flood plain within the study reach because the transport effectiveness of floods in large drainage basins is influenced by event sequencing in the same manner as floods in small watersheds and the cohesive flood plain soils were not susceptible to erosion

differential recovery from the effects of a 100 year storm significance of long term hillslope channel coupling howgill fells northwest england a 100 year storm that occurred in 1982 caused major geomorphic changes in the main valleys of the northern howgill fells northwest england those changes which were documented at that time involved extensive hillslope gully erosion alluvial fan sedimentation and substantial sediment input to the stream systems the streams channels which had hitherto been dominantly single thread relatively stable channels responded in many reaches by switching to wide shallow unstable locally braided channels over the 20 years since the event there has been a partial recovery to channel geometries similar to the pre flood conditions however the degree of recovery contrasts between two neighbouring valleys bowderdale and langdale the channel of bowderdale beck has largely recovered flood sedimentation zones have largely stabilised and new single thread channels have cut through most of the former braided reaches in some places channel widths remain higher than the pre flood values and locally recovery has been modified by a lagged complex response in langdale recovery is only partial with many reaches demonstrating sustained instability over the 20 year post flood period furthermore the overall spatial patterns suggest some reach to reach transfer of coarse sediment shifting zones of instability downstream the contrasts between the two valleys appear to relate to different hillslope to channel coupling characteristics themselves inherited from late pleistocene conditions these contrasts are also evident in the longer term post 1949 history of channel change and stability in these two streams indicative of the higher intrinsic instability of the langdale system

effects of large floods on sediment transport and reach morphology in the cobble bed sainte marguerite river abstract sediment transport rates were estimated for two flood events on the cobble bed sainte marguerite river in the saguenay region canada morphologic methods were used to derive one set of estimates and a combination of the meyer peter and muller equation with a dimensionless sediment transport ratio after dietrich et al nature 340 1989 215 was used to derive another set of estimates both sets of estimates give consistent results for the first event which had a decade scale return period and for the second event which was the largest flood on record and had a century scale return period the transport occurring during the second event was an order of magnitude greater than that occurring during the first event despite this disparity in the transport intensity of the two events the channel morphology remained qualitatively similar the observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s

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

assessment of vegetation effects on hydraulics and of feedbacks on plant survival and zonation in ephemeral channels the interaction of vegetation and flow in channels is important for understanding the influences of forces in channels and effects on erosion sediment flux and deposition it has implications for channel habitats channel instability and restoration schemes methods are needed for calculating forces on plants and data are required on thresholds for plant destruction and survival a simple method of calculating the effect of hydraulics on vegetation and its zonation within ephemeral channels is described detailed cross section surveys of channel morphology vegetation and estimates of manning s n are input into the software program winxspro to calculate the hydraulics of flows across the channel for a given event or flow level incorporating subdivision into zones of differing morphology and vegetation across the section this was applied to a number of cross sections on ephemeral channels in se spain and typical roughness values for mediterranean vegetation types in channels were assessed the method is demonstrated with reference to two well documented floods in se spain in september 1997 on the torrealvilla and in october 2003 along the salada these flows led to the mortality of herbs reed and smaller shrub species some damage to larger shrubs and trees occurred but trees such as tamarisk tamarix canariensis were shown to withstand high forces some grasses were highly resistant to removal and induced sedimentation significant erosion was limited to areas with little vegetation covering the channel floor further quantification of resistance of vegetation to flows and upper threshold values for removal is continuing by relating calculated hydraulic conditions using the methods outlined to measurements of vegetation responses in events at monitoring sites copyright 2009 john wiley sons ltd

a simulation model of morphological vegetation and sediment changes in ephemeral streams a model to simulate channel changes in ephemeral river channels and to test the effects of hydrological changes due to climate change and sol or land use change was developed under the auspices of the eu funded medalus programme mediterranean desertification and land use the model changism channel change gis simulation model is designed to simulate the effect of channel flow events and of climate conditions on morphology sediment and vegetation through sequences of events and conditions over periods of up to several decades the modelling is based on cellular automata but with calculations for water and sediment continuity process rules have both deterministic and stochastic elements an important feature of the model is that it incorporates feedback elements between each event the main aim of the model is to indicate the likely outcomes of events and combinations of conditions it is linked to gis for both input and output the modelling is based on a channel reach and state is input as gis layers of morphology dem sediment and vegetation cover and state other initial conditions of soil moisture groundwater level and overall gradient are input parameters for processes are read from tables and can be easily changed for successive runs of the model the bases for decisions on process specifications are discussed in this paper initial tests of the operation and sensitivity of the model were made on idealized reaches the model was then tested using data from monitored sites in se spain simulations using clearwater flow worked well but initial simulations using events with sediment loads showed some tendency for excess deposition further tests and modifications are taking place overall the model is one of the most sophisticated that simulates the interaction of flows with sediment and vegetation and the outcomes in terms of erosion deposition morphology sediment cover vegetation cover and plant survival over periods of up to 30 years for the scale of a channel reach copyright 2005 john wiley sons ltd

variations in flood magnitude effect relations and the implications for flood risk assessment and river management abstract in spite of major physical impacts from large floods present river management rarely takes into account the possible dynamics and variation in magnitude impact relations over time in flood risk mapping and assessment nor incorporates feedback effects of changes into modelling using examples from the literature and from field measurements over several decades in two contrasting environments a semi arid region and a humid temperate region temporal variations in channel response to flood events are evaluated the evidence demonstrates how flood physical impacts can vary at a location over time the factors influencing that variation on differing timescales are examined the analysis indicates the importance of morphological changes and trajectory of adjustment in relation to thresholds and that trends in force or resistance can take place over various timescales altering those thresholds sediment supply can also change with altered connectivity upstream and changes in state of hillslope channel coupling it demonstrates that seasonal timing and sequence of events can affect response particularly deposition through sediment supply duration can also have a significant effect and modify the magnitude relation lack of response or deposits in some events can mean that flood frequency using such evidence is underestimated a framework for assessment of both past and possible future changes is provided which emphasises the uncertainty and the inconstancy of the magnitude impact relation and highlights the dynamic factors and nature of variability that should be considered in sustainable management of river channels

discontinuous ephemeral streams abstract many ephemeral streams in western north america flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos these inherently unstable streams of semiarid regions are sensitive to short term climatic changes and to human impacts because hillslopes supply abundant sediment to infrequent large streamflow events discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation either aggradation or degradation may become dominant attainment of equilibrium conditions is brief disequilibrium is promoted by channel entrenchment that causes the fall of local base level and by deposition of channel fans that causes the rise of local base level these opposing base level processes in adjacent reaches are maintained by self enhancing feedback mechanisms the threshold between erosion and deposition is crossed when aggradational or degradational reaches shift upstream or downstream extension of entrenched reaches into channel fans tends to create continuous arroyos upvalley migration of fan apexes tends to create depositional valley floors with few stream channels less than 100 years is required for arroyo cutting but more than 500 years is required for complete aggradation of entrenched stream channels and valley floors discontinuous ephemeral streams have a repetitive sequence of streamflow characteristics that is as distinctive as sequences of meander bends or braided gravel bars in perennial rivers the sequence changes from degradation to aggradation headcuts concentrate sheetflow a single trunk channel conveys flow to the apex of a channel fan braided distributary channels end in an area of diverging sheetflow and converging sheetflow drains to headcuts the sequence is repeated at intervals ranging from 15 m for small streams to more than 10 km for large streams lithologic controls on the response of discontinuous ephemeral streams include 1 amount and size of sediment yielded from hillslopes 2 infiltration capacity of valley floor alluvium that influences both the unit stream power available for upstream headcut migration and the attenuation of flashy streamflow events by riparian vegetation in sheetflow reaches and 3 cohesiveness of alluvium which affects headcut and streambank morphology and rates of arroyo extension downcutting and widening initiation of arroyo cutting may be too complex to be attributed to a single cause such as change in mean annual precipitation or grazing by livestock but is most likely associated with a decrease in density of protective plant cover on hillsides and along valley floors relatively larger unit stream power makes downstream reaches more susceptible to initial entrenchment during floods than headwaters reaches and favors persistent arroyos entrenchment continues until an equilibrium longitudinal profile is briefly attained then channel widening occurs streambanks are undercut and aggradation begins

the non layering of gravel streambeds under ephemeral flood regimes abstract the two layer format common to perennial streambeds in which a relatively coarse armour overlies a finer subarmour develops as a function of both the ingress and subsequent near surface winnowing of interstitial material and the selective non entrainment or slower transport velocity of coarse clasts ephemeral streams appear to lack such vertical layering or are characterized by weak layer development some of this may be due to the degree of mixing associated with the scour and fill process however continuous monitoring of bedload discharge in the nahal yatir in the northern negev desert reveals that sediment transport rates are extremely high so that the chance of armour layer development through selective non entrainment is much reduced indeed a comparison of the bedload and bed material size distributions confirms a high degree of similarity and hints at equal mobility regardless of clast size the monitoring programme also indicates that the bed becomes highly mobile at comparatively modest fluid shear so that practically all floods are associated with high transport rates consequently the winnowing that might be brought about by low transport rate events does not occur even within a single event winnowing is precluded by the rapid nature of flow recession that is so characteristic of flash floods the high degree of bed material mobility is attributable in part to the lack of strength that would otherwise be a corollary of armour development however it also highlights the divergent nature of the feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems

rates of sedimentation and stratigraphical completeness the most obvious result of sedimentation processes is the change in the thickness of accumulated sediment with time whenever the net change over a period of time is positive a record is left of a sedimentation system s existence a stratigraphical section for the stratigrapher the problem that such a section poses is to determine the detailed correspondence of thickness to time1 for the sedimentologist the problem is to infer the nature of the parent sedimentation system and to estimate its parameters a simple stochastic model is used here 1 to illustrate some basic stratigraphical results of clastic sedimentation processes 2 to offer an explanation for the commonly cited systematic decrease of mean sedimentation rate with increasing time span and 3 to assess the concept of stratigraphical completeness it will be assumed that all post depositional non erosional processes such as compaction can be identified and their effects on stratigraphical thickness allowed for

flow hydraulics and geomorphic effects of glacial lake outburst floods in the mount everest region nepal glacial lake outburst floods glofs on 3 september 1977 and 4 august 1985 dramatically modified channels and valleys in the mount everest region of nepal by eroding transporting and depositing large quantities of sediment for tens of kilometres along the flood routes the glof discharges were 7 to 60 times greater than normal floods derived from snowmelt runoff glacier meltwater and monsoonal precipitation referred to as seasonal high flow floods shffs specific stream power values ranged from as low as 1900 w m 2 in wide low gradient valley segments to as high as 51 700 w m 2 in narrow high gradient valley segments bounded by bedrock along the upper 16 km of the glof routes the reach averaged specific stream power of the glofs was 3 2 to 8 0 times greater than the reach averaged specific stream power of the shffs the greatest geomorphic change occurred along the upper 10 to 16 km of the glof routes where the ratio between the glof specific stream power and the shff specific stream power was the greatest there was an abundant supply of sediment and channel valley boundaries consisted primarily of unconsolidated sediment below 11 to 16 km from the source area the geomorphic effects of the glofs were reduced because of the lower specific stream power ratio between the glofs and shffs more resistant bedrock flow boundaries reduced sediment supply and the occurrence of past glofs copyright 2003 john wiley sons ltd

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

thresholds and the spatial variability of flood power during extreme floods abstract analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power this paper analyzes the variation in flood power both at a station and downstream in a basin of differing lithologies and discusses the geologic and geomorphic controls on flood power design floods ranging from the 2 year discharge tothe 500 year discharge are routed trough a drainage basin and channel boundary shear stress and unit stream power are calculated using hec 2 along 21 cross valley transects because of the strong lithologic control on valley width and channel slope an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases thus maximizing flood power in these zones the opposite occurs in anomalously wide valleys where flood power is minimized a downstream spatial pattern emerges reflecting both local and general controls for a given probability event flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section holding flood power constant produces an extremely wide range in flow probabilities in order to establish a link between critical flood power magnitude and flow frequency minimum flood powers are developed from an analysis of previous work on catastrophic flooding for humid alluvial channels these minimum thresholds tentatively correspond to 100 n m2 or 300 w m2 for shear stress and unit stream power respectively discharges required to attain these critical flood powers vary from events roughly twice the 100 year flood to a maximum of roughly 18 times the discharge of the 100 year flood

contrasting channel response to floods on the middle gila river arizona floods of january and february 1993 in arizona resulted in the most dramatic channel widening on the middle gila river since 1905 an earlier flood in october 1983 had a larger instantaneous discharge but resulted in little channel change the 1993 flood was of greater volume and duration factors important in destabilizing flood plain vegetation and eroding bank material the 1983 flood was produced by a dissipating eastern pacific tropical storm whereas the 1993 flood was produced by a series of cold fronts from the northern pacific ocean supplied with subtropical moisture from a split jet stream meridional global circulation patterns enhance the frequency of winter storms that produce sustained flooding in arizona and are more likely to result in channel widening and flood plain instability on main trunk streams like the gila river

can we distinguish flood frequency and magnitude in the sedimentological record of rivers consideration of the origin of alluvial deposits and their paleoenvironmental interpretation has traditionally involved two schools of thought that they are either the result of processes that on average have acted uniformly through time or that they are related to exceptional events that occur infrequently despite the long running debate of gradualism versus catastrophism within the earth sciences there are surprisingly few quantitative data to assess the magnitude of events that produce alluvial sedimentary successions this paper reports on a unique natural experiment where surface digital elevation model dem and subsurface ground penetrating radar gpr data were taken immediately prior to and after a large 1 in 40 yr flood event on the sandy braided south saskatchewan river canada results show that although this high magnitude flood reworked the entire braidplain the scale of scour and style of deposition was similar to that associated with lower magnitude annual floods the absence of a distinct imprint of this large flood within the deposits is related to the fact that as river discharge rises and begins to flow overbank channel width increases at a much faster rate than flow depth and thus the rate of increase in channel bed shear stress declines hence rather than being a product of either frequent or rare events alluvial deposits will likely be created by a range of different magnitude floods but discriminating between these different scale events in the rock record may be extremely difficult

channel floodplain connectivity during an extreme flood event implications for sediment erosion deposition and delivery the term connectivity has emerged as a powerful concept in hydrology and geomorphology and is emerging as an innovative component of catchment erosion modeling studies however considerable confusion remains regarding its definition and quantification especially as it relates to fluvial systems this confusion is exacerbated by a lack of detailed case studies and by the tendency to treat water and sediment separately extreme flood events provide a useful framework to assess variability in connectivity particularly the connection between channels and floodplains the catastrophic flood of january 2011 in the lockyer valley southeast queensland australia provides an opportunity to examine this dimension in some detail and to determine how these dynamics operate under high flow regimes high resolution aerial photographs and multi temporal lidar digital elevation models dems coupled with hydrological modeling are used to assess both the nature of hydrologic and sedimentological connectivity and their dominant controls longitudinal variations in flood inundation extent led to the identification of nine reaches which displayed varying channel floodplain connectivity the major control on connectivity was significant non linear changes in channel capacity due to the presence of notable macrochannels which contained a 3000 average recurrence interval ari event at mid catchment locations the spatial pattern of hydrological connectivity was not straight forward in spite of bankfull discharges for selected reaches exceeding 5600m 3 s 1 data indicate that the main channel boundary was the dominant source of sediment while the floodplains where inundated were the dominant sinks spatial variability in channel floodplain hydrological connectivity leads to dis connectivity in the downstream transfer of sediments between reaches and affected sediment storage on adjacent floodplains consideration of such variability for even the most extreme flood events highlights the need to carefully consider non linear changes in key variables such as channel capacity and flood conveyance in the development of a quantitative connectivity index copyright c 2013 john wiley sons ltd

the temporal record of magmatism at cerro uturuncu bolivian altiplano twenty six new 40ar 39ar plateau ages for 23 lavas and domes from the uturuncu volcano in the altiplano of sw bolivia reveal a protracted eruptive history from 1050 5 to 250 5 ka eruptions have been exclusively effusive producing some 50 km3 of high k dacites and silicic andesites bimodal mineral compositions complex mineral textures the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at uturuncu post 458 ka distinct high and low mgo cr magmas are resolved these magmas erupt during similar times suggesting that eruptions are tapping different parts of the magma system albeit from the same vent system volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath uturuncu and calculated buoyancy forces are sufficient to drive effusive eruptions eruptive activity is episodic with six eruptive periods separated by hiatuses of 50 kyr cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka the episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying altiplano puno magma body

sensitivity of a hydraulic model to channel erosion uncertainty during extreme flooding recent research into flood modelling has primarily concentrated on the simulation of inundation flow without considering the influences of channel morphology river channels are often represented by a simplified geometry that is implicitly assumed to remain unchanged during flood simulations however field evidence demonstrates that significant morphological changes can occur during floods to mobilize the boundary sediments despite this the effect of channel morphology on model results has been largely unexplored to address this issue the impact of channel cross section geometry and channel long profile variability on flood dynamics is examined using an ensemble of a 1d 2d hydraulic model lisflood fp of the 1 2000 year recurrence interval floods in cockermouth uk within an uncertainty framework a series of simulated scenarios of channel erosional changes were constructed on the basis of a simple velocity based model of critical entrainment a monte carlo simulation framework was used to quantify the effects of this channel morphology together with variations in the channel and floodplain roughness coefficients grain size characteristics and critical shear stress on measures of flood inundation the results showed that the bed elevation modifications generated by the simplistic equations reflected an approximation of the observed patterns of spatial erosion that enveloped observed erosion depths the effect of uncertainty on channel long profile variability only affected the local flood dynamics and did not significantly affect the friction sensitivity and flood inundation mapping the results imply that hydraulic models generally do not need to account for within event morphodynamic changes of the type and magnitude of event modelled as these have a negligible impact that is smaller than other uncertainties e g boundary conditions instead morphodynamic change needs to happen over a series of events to become large enough to change the hydrodynamics of floods in supply limited gravel bed rivers such as the one used in this research

spatio temporal changes in river bank mass failures in the lockyer valley queensland australia wet flow river bank failure processes are poorly understood relative to the more commonly studied processes of fluvial entrainment and gravity induced mass failures using high resolution topographic data lidar and near coincident aerial photography this study documents the downstream distribution of river bank mass failures which occurred as a result of a catastrophic flood in the lockyer valley in january 2011 in addition this distribution is compared with wet flow mass failure features from previous large floods the downstream analysis of these two temporal data sets indicated that they occur across a range of river lengths catchment areas bank heights and angles and do not appear to be scale dependent or spatially restricted to certain downstream zones the downstream trends of each bank failure distribution show limited spatial overlap with only 17 of wet flows common to both distributions the modification of these features during the catastrophic flood of january 2011 also indicated that such features tend to form at some optimum shape and show limited evidence of subsequent enlargement even when flow and energy conditions within the banks and channel were high elevation changes indicate that such features show evidence for infilling during subsequent floods the preservation of these features in the landscape for a period of at least 150 years suggests that the seepage processes dominant in their initial formation appear to have limited role in their continuing enlargement over time no evidence of gully extension or headwall retreat is evident it is estimated that at least 12 inundation events would be required to fill these failures based on the average net elevation change recorded for the 2011 event existing conceptual models of downstream bank erosion process zones may need to consider a wider array of mass failure processes to accommodate for wet flow failures

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 related channel change in an arid region river a review of 112 years of change in the channel of the salt river central arizona u s a shows that this arid region river has a main flow channel that has migrated laterally up to 1 6 km 1 mi in response to floor events maps showing locational probabilities indicate that along the channel zones of relative locational stability alternate with zones of relative instability at a 3 2 km 2 mi interval construction of upstream reservoirs has reduced sediment input into the main river but has not controlled floods the channel width has not changed except for moderate fluctuations around mean values the main flow channel has incised approximately 6 m 20 ft over most of the 48 km 30 mi study reach during six recent floods gradient has remained unchanged during floods bed material was mobilized to a depth below the original bed level that was greater than the height of the water surface above the original bed calculations based on tractive force indicate a threshold discharge of instability that is equal to the flow with a five year return interval the river exhibits remarkable stability with respect to gradient and sinuosity irrespective of water and sediment discharges but horizontal channel location exhibited selective instability over the record period of more than a century the channel appears not to have been in equilibrium considering geometry discharge and sediment

channels wetlands and islands in the okavango delta botswana and their relation to hydrological and sedimentological processes the okavango wetland in northern botswana is one of the world s largest inland deltas the delta is a dynamic environment with shifting channel routes causing growth and decay of flanking wetlands