the role of feedback mechanisms in historic channel changes of the lower rio grande in the big bend region abstract over the last century large scale water development of the upper rio grande in the u s and mexico and of the rio conchos in mexico has resulted in progressive channel narrowing of the lower rio grande in the big bend region we used methods operating at multiple spatial and temporal scales to analyze the rate magnitude and processes responsible for channel narrowing these methods included hydrologic analysis of historic stream gage data analysis of notes of measured discharges historic oblique and aerial photograph analysis and stratigraphic and dendrogeomorphic analysis of inset floodplain deposits our analyses indicate that frequent large floods between 1900 and the mid 1940s acted as a negative feedback mechanism and maintained a wide sandy multi threaded river declines in mean and peak flow in the mid 1940s resulted in progressive channel narrowing channel narrowing has been temporarily interrupted by occasional large floods that widened the channel however channel narrowing has always resumed after large floods in 1990 and 1991 the active channel width of the lower rio grande has narrowed by 36 52 narrowing has occurred by the vertical accretion of fine grained deposits on top of sand and gravel bars inset within natural levees channel narrowing by vertical accretion occurred simultaneously with a rapid invasion of non native riparian vegetation tamarix spp arundo donax which created a positive feedback and exacerbated the processes of channel narrowing and vertical accretion in two floodplain trenches we measured 2 75 and 3 5 m of vertical accretion between 1993 and 2008 in some localities nearly 90 of bare active channel bars were converted to vegetated floodplain during the same period upward shifts of stage discharge relations occurred resulting in over bank flooding at lower discharges and continued vertical accretion despite a progressive reduction in stream flow thus although the magnitude of the average annual flood was reduced between 40 and 50 over bank flooding continued these changes reflect a shift in the geomorphic nature of the rio grande from a wide laterally unstable multi thread river to a laterally stable single thread channel with cohesive vertical banks and few active in channel bars

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

geomorphic responses of lower bega river to catchment disturbance 1851 1926 abstract prior to significant european settlement of the area in the 1850s lower bega river on the south coast of nsw had a narrow relatively deep channel lined by river oaks the river had a suspended or mixed load with platypus habitat available in pools banks were fine grained and relatively cohesive silts and clays as was the floodplain which graded to a series of valley marginal swamps and lakes extensive evidence from maps and portion plans archival photographs bridge surveys and anecdotal sources complemented by field analysis of floodplain sedimentology including radiocarbon dated samples and vegetation remnants are used to document the dramatic metamorphosis in the character and behaviour of lower bega river in the latter half of the nineteenth century by 1926 the channel had widened extensively up to 340 and shallowed in association with bed aggradation by coarse sandy bedload floodplain accretion was dominated by fine to medium sands with some coarse sand splays in contrast with most other studies of channel metamorphosis in australia which have emphasised river responses to climatically induced flood histories relegating human impacts to a secondary role the profound changes to the geomorphic condition and behaviour of bega river reflect indirect human disturbance of bega catchment and direct but non point source disturbance to the channel extensive clearance of catchment floodplain and channel marginal vegetation occurred within a few decades of european settlement altering the hydrologic and sediment regime of the river and transforming the geomorphic effectiveness of floods although this study is situated in a relatively sensitive granitic catchment catchment clearance is likely to have induced equally significant responses in many other river systems in eastern australia in some instances the diffuse aspects of human disturbance on landscapes induce impacts on river character that are just as profound as major direct disturbances of river channels this may have profound implications in understanding and hence managing the geomorphic consequences of river behaviour in australia and elsewhere

morphological and historical resilience to catastrophic flooding the case of lockyer creek se queensland australia this study aimed to determine the extent of geomorphic change resulting from the catastrophic flood of 2011 in the lockyer valley in southeast queensland and to place these impacts within a history of geomorphic adjustment aerial photographs dated from 1933 to 2011 and parish maps and historical on ground photographs dating from 1865 to 1966 were examined for evidence of geomorphic adjustment since european settlement in the first half of the nineteenth century eleven forms of geomorphic adjustment were identified in three categories erosional depositional and reorganisational only 26 of the lockyer creek channel length has been affected by some form of geomorphic adjustment since european settlement most of this adjustment was localised and dominated by reorganisation of geomorphic unit assemblages within the macrochannel and sediment deposition on floodplains no wholesale river change in the form of lateral migration or avulsion has occurred and the river s morphology has remained relatively characteristic over time i e morphology remains relatively uniform in a reach averaged sense geomorphic responses to extreme flooding have been minor and the geomorphic effectiveness of floods in this system including the 2011 flood has been limited over the last several hundred years the system is likely still adjusting to past flooding events that set the morphology of the current system i e the macrochannel a form of event resilience has resulted in this system such that it is less prone to geomorphic adjustment during events than would normally be considered geomorphically effective as a result antecedent controls on macrochannel presence and capacity are considered to be first order controls on contemporary forms and processes in this system work is required to test whether the resilience of this system will hold in the future with more extreme episodes of flooding predicted to occur in this region under future climate change

geomorphic constraints on landscape sensitivity to climate in tectonically active areas abstract the geomorphology of fluvial landscapes is known to record information about uplift rate spatial patterns of faulting and tectonic history data is far less available when addressing the sensitivity of common geomorphological metrics such as channel steepness to climatic boundary conditions we test the relationship between channel steepness and precipitation rate by measuring a large number of channels in different mountainous areas these regions exhibit a tenfold variation in precipitation rate between them 100 1000 mm y 1 but have similar uplift rates allowing the tectonic variable to be controlled by accounting for the orographic coupling of rainfall with uplifted topography we find that channel steepness is significantly suppressed by higher precipitation rates in a measurable way that conforms to simple stream power erosion laws and empirical constraints on their parameters we demonstrate this using modern and estimated glacial precipitation rates and climate emerges as an important quantifiable control on channel geometry these findings help to explain why highly variable measurements of channel steepness are reported from different locations and provide important empirical constraints on how climate shapes tectonically active landscapes

effect of precessional insolation changes on cretaceous climate and cyclic sedimentation the often striking presence of cyclical beddings has frequently been noted in cretaceous marine sediment deposits in many instances these beddings appear to have periodicities similar to those of the earth s orbit i e its precession obliquity and eccentricity variations in the primary deposition of ocean sediments are strongly linked to atmosphere land ocean interactions we report a set of numerical experiments using an atmospheric general circulation model that are designed to examine the sensitivity of mid cretaceous climate to insolation changes due to the precessional cycle we restrict attention to a single orbital parameter because most limestone black shale bedded sequences were deposited at low latitudes where precessional insolation variations predominate we varied systematically the precessional index between the maximum positive and negative values calculated over the last 2 m y and used mid cretaceous paleogeographic and paleotopographic reconstructions and zonally symmetric sea surface temperatures based on earlier work by barron and washington surface wetness and albedo values appropriate for grassland were prescribed uniformly over all land regions to capture the seasonality of precession we employed both perpetual january and perpetual july simulations changes in surface heating due to the imposed insolation anomalies resulted in a strong basically linear response of monsoonal circulations dominated by changes over the northern hemisphere the hydrologic cycle shows a significant response over many regions the low latitude proto south atlantic region varies from a strong net sink for water vapor excess precipitation to a strong source excess evaporation over the precession cycle this variation is consistent with an alternation between a stagnant stratified ocean basin with anoxic bottom water and an evaporative basin that produces bottom water that is warm saline and oxygenated the portion of the tethys ocean that overlays the limestone black shale sequences now found in present day italy does not exhibit comparable behavior but the reconstruction of orography and basin boundaries in this region is less known the simulations show however that as the monsoonal flow over south proto asia intensifies water vapor evaporated in this part of tethys is prevented from crossing the proto alps into the interior of the continent thereby remaining in the tethian drainage basin as a result central proto asia shifts between arid and moist conditions with precessional variations the water vapor budget over northeast africa varies systematically from zero seasonality to a marked wet dry seasonal cycle the deposition of eolian material can be inferred from a combination of continental aridity favorable mean wind directions and significant wind variances in our experiments the mean surface and upper winds often show a systematic response to precession but surface wind variances show a systematic response only in isolated regions our simulations do not provide a simple mechanism to explain the cyclicity in eolian deposition that has been reported for north central tethys

geomorphic effects flood power and channel competence of a catastrophic flood in confined and unconfined reaches of the upper lockyer valley southeast queensland australia flooding is a persistent natural hazard and even modest changes in future climate are believed to lead to large increases in flood magnitude previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change this paper provides an assessment of the geomorphic effects of a rare high magnitude event that occurred in the lockyer valley southeast queensland in january 2011 the average return interval of the resulting flood was similar to 2000 years in the upper catchment and decreased to similar to 30 years downstream a multitemporal lidar derived dem of difference dod is used to quantify morphological change in two study reaches with contrasting valley settings confined and unconfined differences in geomorphic response between reaches are examined in the context of changes in flood power channel competence and degree of valley confinement using a combination of one dimensional 1 d and two dimensional 2 d hydraulic modelling flood power peaked at 9800 w m 2 along the confined reach and was 2 3 times lower along the unconfined reach results from the dod confirm that the confined reach was net erosional exporting similar to 287 000 m 3 of sediment whilst the unconfined reach was net depositional gaining similar to 209 000 m 3 of sediment 70 of the amount exported from the upstream confined reach the major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width in the unconfined reach the benches and floodplains were the major loci for deposition whilst the inner channel exhibited minor width increases the presence of high stream power values and resultant high erosion rates within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration the confined reach experienced large scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study c 2013 elsevier b v all rights reserved

theoretical and methodological premises of geomorphological forecasting abstract there are a number of yet unsolved problems with regard to geomorphological prognosis the theoretical and methodological foundations of geomorphic prediction are outlined it is shown that there is a conceptual link between field studies of spatial structures and landscape evolution and delimiting trajectories of geomorphic systems several basic issues in applying this principle to forecasting are discussed these include the role of characteristic and relaxation times of geomorphic systems and the concept of geomorphological memory complex responses and the relative role of conceptual and statistical models it is suggested that instability in geomorphic systems can be a source of order at higher heirarchical levels and that this property can be exploited in practical forecasting

quantifying different riverbank erosion processes during an extreme flood event riverbank erosion is a major contributor to catchment sediment budgets at large spatial scales data is often restricted to planform channel change with little information on process distributions and their sediment contribution this study demonstrates how multi temporal lidar and high resolution aerial imagery can be used to determine processes and volumes of riverbank erosion at a catchment scale remotely sensed data captured before and after an extreme flood event enabled a digital elevation model of difference dod to be constructed for the channel and floodplain this meant that the spatial area that could be assessed was extensive three dimensional forms of bank failures could be mapped at a resolution that enabled process inference and the volume and rates of different bank erosion processes over time could be assessed a classification of riverbank mass failures integrating form and process identified a total of 437 mass failure polygons throughout the study area these were interpreted as wet flow mass failures based on the presence of a well defined scarp wall and the absence of failed blocks on the failure floor the failures appeared to be the result of bank exfiltration antecedent moisture conditions preceding the event and the historic development of the channel using one dimensional hydraulic modelling to delineate geomorphic features within the main boundary of the macrochannel an estimated 1 466 322m2 of erosion was interpreted as fluvial entrainment occurring across catchment areas from 30 to 1668 km2 only 8 of the whole riverbank planform area was occupied by mass failures whilst fluvial entrainment covered 33 a third of the volume of material eroded came from mass failures even though they occupied 19 of the eroded bank area the availability of repeat lidar surveys combined with high resolution aerial photography was very effective in erosion process determination and quantification at a large spatial scale

the character and age structure of valley fills in upper wolumla creek catchment south coast new south wales australia extensive valley fills at the base of the escarpment in upper wolumla creek on the south coast of new south wales australia have formed from a combination of cut and fill processes the valley fills comprise series of alternating horizontally bedded sand and mud units reflecting reworking of detritus from deeply weathered granites of the bega batholith sand units are deposited as sand sheets or splays on floodplain surfaces or in floodouts that form atop intact valley fill surfaces downstream of discontinuous gullies alternatively sands are deposited from bedload and form bars or part of the valley floor within channel fills organic rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts within 5 km of the escarpment valley deposits grade downstream from sand sheet and splay deposition in floodouts to mud deposition in swamp and seepage zones radiocarbon dates indicate that virtually the entire valley fill of upper wolumla creek was excavated prior to 6000 years bp remnant terraces are evident at valley margins the valley subsequently filled between 6000 years bp and 1000 years bp producing valley fills around 12 m deep but no greater than 300 m wide reincision into the valley fill on a scale smaller than the present incision phase is indicated at around 1000 years bp following which the channel refilled portion plans dated from 1865 refer to the study area as wolumla big flat and show large areas of swampy terrain suggesting that the valley fill had re established by this time within a few decades of european settlement the valley fill incised once more upper wolumla creek now has a channel over 10 m deep and 100 m wide in places draining a catchment area of less than 20 km2 1998 john wiley sons ltd

impact of reach geometry on stream channel sensitivity to extreme floods predicting spatial and temporal variations in bank erosion due to extreme floods presents a long standing challenge in geomorphology we develop two methodologies for rapid regional scale assessments of stream reaches susceptible to channel widening the first proposes that channel widening occurs when unit stream power exceeds a critical threshold 300 w m2 the second is motivated by the observation that widening often occurs at channel bends we introduce a new metric the bend stress parameter which is proportional to the centripetal force exerted on a concave bank we propose that high centripetal forces generate locally high bank shear forces and enhance channel bank erosion we test both metrics using the geomorphic signature of tropical storm irene 2011 on the white and the saxtons rivers vermont specifically we test if reaches where significant channel widening occurred during irene required one or both metrics to exceed threshold values we observe two distinct styles of channel widening where unit stream power and bend stress parameter are high widening is usually due to bank retreat elsewhere widening is usually due to the stripping of the upstream end of mid channel islands excluding widening associated with the stripping of the heads of mid channel islands almost all the widening 98 occurred along reaches identified as susceptible to widening the combined metrics identify up to one quarter of the reaches lacking susceptibility to channel widening copyright 2014 john wiley sons ltd

riparian vegetation and fluvial geomorphic processes abstract riparian vegetation and fluvial geomorphic processes and landforms are intimately connected parts of the bottomland landscape relations among vegetation processes and landforms are described here for representative streams of four areas of the united states high gradient streams of the humid east coastal plain streams great plains streams and stream channels of the southwestern united states vegetation patterns suggest that species distributions in the humid east are largely controlled by frequency duration and intensity of floods along channelized streams vegetation distribution is largely controlled by variation in fluvial geomorphic processes cycles of degradation and aggradation in response to increases in channel gradient associated with channelization similarly riparian vegetation of great plains streams may be controlled by fluxes in sediment deposition and erosion along braided streams patterns of riparian vegetation in semi arid regions may be most closely related to patterns of water availability unlike most other streams in more humid environments channel equilibrium conditions control stability of the coincident fluvial landform and attendant vegetation pattern throughout the continent in most situations riparian vegetation patterns are indicative of specific landforms and thus of ambient hydrogeomorphic conditions

antecedent controls on river character and behaviour in partly confined valley settings upper hunter catchment nsw australia abstract a partly confined river is one along which the contemporary channel abuts the valley margin along 10 90 of its length they occur along sections of longitudinal profiles that are transitional from fully bedrock confined to fully alluvial rivers at this position in the landscape there is sufficient accommodation space for discontinuous floodplain pockets to form segregation of the river s load and dissipation of flow energy result in sediments being stored outside the channel bedrock and planform controlled variants of these rivers and associated floodplains are differentiated terraces fans and piedmont zones act as secondary confining features on channel planform and floodplain formation in these valleys the influence of these antecedent features on contemporary river character and behaviour in partly confined valleys of the upper hunter catchment new south wales australia is appraised the balance of formative and reworking processes and associated capacity for geomorphic adjustment is influenced by the site specific configuration of antecedent controls at any floodplain pocket downstream patterns of these river types are analysed along all major tributaries of the upper hunter catchment to highlight the extent of antecedent control on contemporary river forms and processes in this landscape setting

the timing and magnitude of coarse sediment transport events within an upland temperate gravel bed river this paper describes the application of a new instrument to continuously measure bedload transport an impact sensor to a 72 km2 test catchment in the yorkshire dales northern england data from a network of impact sensors are linked to repeat surveys of channel morphological response to get a better understanding of the conditions that lead to sediment generation and transfer results suggest certain areas of the catchment act as key sediment sources at the annual time scale with material being quickly delivered to the lower parts of the catchment along the steep bedrock channel sediment transfer within the tributaries occurs in significantly smaller magnitudes than within the main channel but it moves more frequently and at different times of the year with transfer rates being strongly conditioned by larger scale valley geomorphology the lower 5 6 km reach sees a significant reduction in gradient and a widening of the valley this permits significant accumulation within the channel which has persisted for many years this lower reach is very sensitive to changes in sediment supply and there is good agreement between changes in bedload transport data and the surveyed channel response these observations have major implications for how river management projects should be developed in upland environments especially those where large scale geomorphological controls have a major impact upon the sediment transfer process evidence suggests that where river management restricts lateral movement of the channel and transfer of sediment into floodplain storage changes in sediment supply can lead to areas of severe accumulation acceleration of bank erosion and exacerbated flood risk

reach scale channel sensitivity to multiple human activities and natural events lower santa clara river california usa abstract understanding the cumulative impact of natural and human influences on the sensitivity of channel morphodynamics a relative measure between the drivers for change and the magnitude of channel response requires an approach that accommodates spatial and temporal variability in the suite of primary stressors multiple historical data sources were assembled to provide a reach scale analysis of the lower santa clara river lscr in ventura county california usa sediment supply is naturally high due to tectonic activity earthquake generated landslides wildfires and high magnitude flow events during el nino years somewhat typically for the region the catchment has been subject to four reasonably distinct land use and resource management combinations since european american settlement when combined with analysis of channel morphological response quantifiable since ca 1930 reach scale and temporal differences in channel sensitivity become apparent downstream reaches have incised on average 2 4 m and become narrower by almost 50 with changes focused in a period of highly sensitive response after about 1950 followed by forced insensitivity caused by structural flood embankments and a significant grade control structure in contrast the middle reaches have been responsive but are morphologically resilient and the upstream reaches show a mildly sensitive aggradational trend superimposing the natural and human drivers for change reveals that large scale stressors related to ranching and irrigation have been replaced over time by a suite of stressors operating at multiple spatial scales lower reaches have been sensitive primarily to local scale impacts urban growth flood control and aggregate mining whereas upstream catchment scale influences still prevail including flow regulation and climate driven sediment supply factors these factors illustrate the complexity inherent to cumulative impact assessment in fluvial systems provide evidence for a distinct anthropocene fluvial response and underpin the enormity of the challenge faced in trying to sustainably manage and restore rivers

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

spatial variability in the timing nature and extent of channel response to typical human disturbance along the upper hunter river new south wales australia prior to european settlement the upper hunter river near muswellbrook new south wales was a passively meandering gravel bed river of moderate sinuosity and relatively uniform channel width analyses of floodplain sedimentology archival records parish maps and aerial photographs document marked spatial variability in the pattern of channel change since european settlement in the 1820s different types rates and extents of change are reported for seven zones of adjustment along an 8 km study reach this variable adjustment reflects imposed antecedent controls buried terrace material and bedrock which have significantly influenced local variability in river sensitivity to change as well as contemporary morphodynamics and geomorphic complexity local variability in system responses to disturbance has important implications for future river management and rehabilitation

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

influence of spatial variability of soil strength parameters on probabilistic seismic slope displacement hazard analysis abstract permanent sliding displacement analysis is commonly used to evaluate the seismic performance of slopes specifically the fully probabilistic seismic slope displacement hazard analysis pssdha has attracted increasing attention recently since it appropriately accounts for various sources of uncertainties associated with the estimation of sliding displacement within this analysis the uncertainty of soil strength parameters e g effective cohesion c and effective internal friction angle is commonly quantified by a logic tree method in which the spatial variability of c and is simply ignored this study aims at proposing an extended pssdha framework by rigorously accounting for the spatial variability of c and and then investigating its influence on the resultant slope displacement hazard curves based on the newmark rigid block model two different approaches namely an analytical based and a simulation based method are proposed separately the results of the analytical based method are found to be consistent with those obtained by the simulation based one it is also shown that the displacement hazard would be underestimated if the spatial variability of c and is ignored such underestimation is more significant for slopes with soil parameters exhibiting weak c correlation and strong spatial variability besides it suggests that the proper configuration of logic tree framework plays an important role in developing displacement hazard curves and a relatively large range of discrete strength values is recommended to better account for the effect of the spatial variability of soil strength parameters

channel change following european settlement gilmore creek southeastern australia european settlement in southeastern australia led to rapid changes in the morphology of many upland streams however our knowledge of the nature of these changes is limited as historical records and preserved palaeochannels are rare in this study we compare a well preserved section of the late holocene palaeochannel of gilmore creek to its present channel we used a combination of map and aerial photograph interpretation field survey osl dating and discharge analysis to describe and compare the modern and palaeochannels and establish a firm date for the timing of channel change in common with many other streams in southeastern australia gilmore creek s late holocene channel meandered across a stable well vegetated and frequently inundated floodplain after about 1830 european settlers quickly modified the catchment by clearing riparian and hillslope vegetation introducing grazing animals and other exotic species and mining for alluvial gold in the headwaters the osl dates show that between about 1850 and 1880 the small meandering channel aggraded with coarse sands and then up to about 1 m of silty sand was deposited over the floodplain declining sediment input from upstream channel avulsion before 1890 resulted in the establishment of a straighter larger capacity channel that incised to the level of basal cobbles and in places to bedrock the dramatic change in channel pattern resembles that described on the cann river in eastern victoria following the removal of riparian vegetation and within channel coarse woody debris at gilmore creek increased channel capacity has greatly reduced the average frequency of floodplain inundation high values of specific stream power suggest that channel morphology is now well adjusted to the present flow regime copyright 2007 john wiley sons ltd

a channel evolution model for subtropical macrochannel systems a channel evolution model cem represents stages of channel development in response to specific types of disturbance in recent years classic incised disturbed cems have provided process based understanding of channel adjustment and formed the cornerstone for river restoration and rehabilitation while broadly applicable to alluvial systems in temperate and semi arid regions these models cannot be assumed to be universally applicable lockyer creek in south east queensland australia has notable macrochannel morphology and is subject to high hydrological variability typical of many subtropical climates the aim of this paper is to present a case study of channel adjustment and evolution in lower lockyer creek to determine if existing cems adequately describe processes of channel adjustment and the associated trajectories of change typical of river systems in subtropical settings lockyer creek has recently been subjected to a spate of flooding resulting in significant channel erosion this offers an ideal opportunity to investigate the nature and rate of channel adjustment processes and place them in context of longer term geomorphic adjustments in these systems specifically we address two questions firstly do the classic incised disturbed cems adequately represent the observed macrochannel adjustment secondly if current cems are inadequate what is the channel evolution model for these systems of which lower lockyer creek is an example results show that these are non incising systems where wet flow bank mass failures wbmfs are the dominant process of channel adjustment they occur within the channel bank top boundary resulting in no change to overall bank top width furthermore subsequent floods deposit sediment in the failure scars and failure headwalls generally do not retreat beyond channel bank top channel adjustment has not followed the evolutionary stages for incised disturbed channels and a new four stage macrochannel cem is outlined for these subtropical systems the proposed cem illustrates a cyclical pattern of erosion by channel bank wbmf followed by re aggradation through deposition and oblique processes contributing to bank rebuilding this cem provides sufficient information to determine the stage of macrochannel adjustment enabling decisions to be made over whether intervention is required or will be successful

the long term control of vegetation and woody debris on channel and flood plain evolution insights from a paired catchment study in southeastern australia abstract numerous case studies have demonstrated that alluvial and semi alluvial rivers in se australia have undergone dramatic metamorphosis in historical times however very few studies place these changes within a long term evolutionary context as a consequence the magnitude of and ultimate controls on the changes to river form and processes are not fully appreciated in this study a paired catchment analysis is undertaken between two moderate sized sand bed rivers in east gippsland australia from the thurra river direct insight is gained into the predisturbance control exerted by riparian vegetation and wood in a lowland alluvial river this river is effectively in the same condition today as it was at the time of the arrival of europeans in australia in contrast the adjacent cann river which has been settled by europeans for 150 years but was previously very similar to the thurra river exhibits stark differences today channel morphodynamics observed within the thurra river when coupled with historical and geomorphic evidence for the former condition of the cann river provide a detailed reference by which the recent changes to the cann river are measured chronostratigraphic evidence from both flood plains places recent channel behavior within an evolutionary context extending well into the pleistocene since european settlement the study reach of the cann river has experienced a 360 increase in channel depth a 240 increase in channel slope a 700 increase in channel capacity and up to a 150 fold increase in the rate of lateral channel migration the contemporary condition of the cann river channel differs profoundly to that which has prevailed over at least the previous 27 ka the first order control on the historical channel metamorphosis is the removal of riparian vegetation and woody debris wd numerous thresholds have been crossed as a result of historical channel changes particularly the relationship between average length of woody debris pieces and channel width consequently channel recovery will not simply be achieved by reintroducing the pre existing riparian vegetation and woody debris load devising sound management strategies and realistic management targets for the cann river or any similarly altered river requires an understanding of the long term channel and flood plain evolutionary history as well as the underlying controls on historical changes

the human role in changing river channels abstract direct consequences of the human role where human activity affects river channels through engineering works including channelization dam construction diversion and culverting have been long recognised marsh 1864 thomas 1956 the less obvious indirect effects of point and reach changes occurring downstream and throughout the basin however are much more recently appreciated dating from key contributions by strahler strahler a n 1956 the nature of induced erosion and aggradation in w l thomas ed man s role in changing the face of the earth university of chicago press chicago 621 638 wolman wolman m g 1967 a cycle of sedimentation and erosion in urban river channels geografiska annaler 49a 385 95 schumm schumm s a 1969 river metamorphosis proceedings american society of civil engineers journal hydraulics division 95 255 73 and graf graf w l 1977 the rate law in fluvial geomorphology american journal of science 277 178 191 these are complemented by effects of alterations of land use such as deforestation intensive agriculture and incidence of fire with the most extreme effects produced by building activity and urbanisation changing river channels are most evident in the channel cross section where changes of size shape and composition are now well established with up to tenfold increases or decreases illustrated by results from more than 200 world studies in addition the overall channel planform the network and the ecology have changed specific terms have become associated with changing river channels including enlargement shrinkage and metamorphosis although the scope of adjustment has been established it has not always been possible to predict what will happen in a particular location because of complex response and contingency the ways in which changes in cross section relate to reach and network changes are less clear despite investigations showing the distribution of changes along segmented channels when considering the human role in relation to changing river channels at least five challenges persist first because prediction of the nature and amount of likely change at a particular location is not certain and because the contrasting responses of humid and arid systems needs to be considered modelling is required to reduce uncertainty as was first emphasised by burkham burkham d e 1981 uncertainties resulting from changes in river form american society civil engineers proceedings journal hydraulics division 107 593 610 second feedback effects incorporated within the relationship between changes at channel reach and network scales can have considerable implications especially because changes now evident may have occurred or have been initiated under different environmental conditions third consideration of global climate change is imperative when considering channel sensitivity and responses to threshold conditions fourth channel design involving geomorphology should now be an integral part of restoration procedures this requires fifthly greater awareness of different cultures as a basis for understanding constraints imposed by legislative frameworks better understanding of the ways in which the perception of the human role in changing river channels varies with culture as well as varying over time should enhance application of design for river channel landscapes

the use of multi temporal lidar to assess basin scale erosion and deposition following the catastrophic january 2011 lockyer flood se queensland australia advances in remote sensing and digital terrain processing now allow for a sophisticated analysis of spatial and temporal changes in erosion and deposition digital elevation models dems can now be constructed and differenced to produce dems of difference dod which are used to assess net landscape change for morphological budgeting to date this has been most effectively achieved in gravel bed rivers over relatively small spatial scales if the full potential of the technology is to be realised additional studies are required at larger scales and across a wider range of geomorphic features this study presents an assessment of the basin scale spatial patterns of erosion deposition and net morphological change that resulted from a catastrophic flood event in the lockyer creek catchment of se queensland seq in january 2011 multitemporal light detection and ranging lidar dems were used to construct a dod that was then combined with a one dimensional flow hydraulic model hec ras to delineate five major geomorphic landforms including inner channel area within channel benches macrochannel banks and floodplain the lidar uncertainties were quantified and applied together with a probabilistic representation of uncertainty thresholded at a conservative 95 confidence interval the elevation change distribution ecd for the 100 km2 study area indicates a magnitude of elevation change spanning almost 10 m but the mean elevation change of 0 04 m confirms that a large part of the landscape was characterised by relatively low magnitude changes over a large spatial area mean elevation changes varied by geomorphic feature and only two the within channel benches and macrochannel banks were net erosional with an estimated combined loss of 1 815 149 m3 of sediment the floodplain was the zone of major net deposition but mean elevation changes approached the defined critical limit of uncertainty areal and volumetric ecds for this extreme event provide a representative expression of the balance between erosion and deposition and importantly sediment redistribution which is extremely difficult to quantify using more traditional channel planform or cross sectional surveys the ability of lidar to make a rapid and accurate assessment of key geomorphic processes over large spatial scales contributes to our understanding of key processes and as demonstrated here to the assessment of major geomorphological hazards such as extreme flood events

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

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

evolutionary trajectory of channel morphology and controlling factors in a large gravel bed river abstract interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology a quantitative analysis of controlling factors and finally identification of links between evolutionary trajectories and controlling factors the aim of this paper is to explain channel adjustments and controlling factors in the tagliamento river a large gravel bed river in northeastern italy traditional methods for studying historical channel changes i e use of aerial photos topographic data were employed but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution river channel underwent three main phases of adjustment over the last 200 years the first two phases from the end of the nineteenth century to the early 1990s were characterized by narrowing channel width decreased from 1250 to 540 m and incision about 1 m and the third phase from the 1990s to present day by widening from 540 to 600 m and slight aggradation about 0 2 m combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long term channel evolution of the tagliamento river was driven mainly by human intervention at the reach scale i e sediment mining and channelization changes in sediment supply in the catchment area had no or minor effects in the study reach the most recent changes i e widening and slight aggradation are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes while this was not the case during the previous phases of adjustment

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