sediment contributions from floodplains and legacy sediments to piedmont streams of baltimore county maryland abstract disparity between watershed erosion rates and downstream sediment delivery has remained an important theme in geomorphology for many decades with the role of floodplains in sediment storage as a common focus in the piedmont province of the eastern usa upland deforestation and agricultural land use following european settlement led to accumulation of thick packages of overbank sediment in valley bottoms commonly referred to as legacy deposits previous authors have argued that legacy deposits represent a potentially important source of modern sediment loads following remobilization by lateral migration and progressive channel widening this paper seeks to quantify 1 rates of sediment remobilization from baltimore county floodplains by channel migration and bank erosion 2 proportions of streambank sediment derived from legacy deposits and 3 potential contribution of net streambank erosion and legacy sediments to downstream sediment yield within the mid atlantic piedmont we calculated measurable gross erosion and deposition rates within the fluvial corridor along 40 valley segments from 18 watersheds with drainage areas between 0 18 and 155 km2 in baltimore county maryland we compared stream channel and floodplain morphology from lidar based digital elevation data collected in 2005 with channel positions recorded on 1 2400 scale topographic maps from 1959 1961 in order to quantify 44 46 years of channel change sediment bulk density and particle size distributions were characterized from streambank and channel deposit samples and used for volume to mass conversions and for comparison with other sediment sources average annual lateral migration rates ranged from 0 04 to 0 19 m y which represented an annual migration of 2 5 0 9 4 4 channel width across all study segments suggesting that channel dimensions may be used as reasonable predictors of bank erosion rates gross bank erosion rates varied from 43 to 310 mg km y median 114 and were positively correlated with drainage area measured deposition within channels accounted for an average of 46 28 75 of gross erosion with deposition increasingly important in larger drainages legacy sediments accounted for 6 90 of bank erosion at individual study segments represented about 60 of bank height at most exposures and accounted for 57 16 of the measured gross erosion extrapolated results indicated that first and second order streams accounted for 62 38 of total streambank erosion from 1005 km2 of northern baltimore county after accounting for estimated redeposition extrapolated net streambank sediment yields 72 mg km2 y constituted 70 of estimated average piedmont watershed yields 104 mg km2 y the results suggest that streambank sediments are a relatively large source of sediment from piedmont tributaries to the chesapeake bay

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

sorting out abrasion in a gypsum dune field 1 grain size distributions in eolian settings are the result of both sorting and abrasion of grains by saltation the two are tightly coupled because mobility of particles determines abrasion rate while abrasion affects the mobility of particles by changing their mass and shape few field studies have examined this quantitatively we measured grain size and shape over a 9 km transect downwind of a line sediment source at white sands national monument a gypsum dune field the sediment source is composed of rodlike elongate coarse particles whose shapes appear to reflect the crystalline structure of gypsum dispersion in grain size decreases rapidly from the source coarse particles gradually become less elongate while an enrichment of smaller more elongate grains is observed along the transect transport calculations confirm that white sands is a threshold sand sea in which 1 the predominant particle diameter reflects grains transported in saltation under the dune forming wind velocity and 2 smaller elongate grains move in suspension under this dominant wind size selective transport explains first order trends in grain size however abrasion changes the shape of saltating grains and produces elongate smaller grains that are spallation and breaking products of larger particles both shape and size changes saturate 5 6 km downwind of the source as large particles become more equant abrasion rates slow down because protruding regions have been removed such asymptotic behavior of shape and abrasion rate has been observed in theory and experiment and is likely a generic result of the abrasion process in any environment

evolution of channel morphology and hydrologic response in an urbanizing drainage basin the dead run catchment in baltimore county maryland has undergone intense urbanization since the late 1950s reconstruction of the channel planform from topographic maps dating back to the 1890s and aerial photographs dating back to the 1930s indicates that the channel has remained stable in planform since at least the 1930s the relative stability of dead run contrasts with the alterations in channel morphology reported for other urbanizing streams in the piedmont physiographic province of the eastern united states trend analyses of discharge records in dead run show that urban development and stormwater control measures have had significant impacts on the hydrologic response of the catchment the flood hydraulics of the dead run catchment are examined for the event that occurred on 22 june 1972 in association with hurricane agnes a two dimensional hydraulic model telemac 2d was used with a finite element mesh constructed from a combination of high resolution lidar topographic data and detailed field survey data to analyse the distribution of boundary shear stress and unit stream power along the channel and floodplain during flooding from hurricane agnes the spatial and temporal distributions of these parameters relative to channel gradient and channel valley bottom geometry provide valuable insights on the stability of the dean run channel the stability of dead run s channel planform in spite of extreme flooding and decades of urban development is most likely linked to geological controls of channel and floodplain morphology copyright 2006 john wiley sons ltd

the distribution and residence time of suspended sediment stored within the channel margins of a gravel bed bedrock river previously undocumented deposits are described that store suspended sediment in gravel bedded rivers termed fine grained channel margin fgcm deposits fgcm deposits consist of sand silt clay and organic matter that accumulate behind large woody debris lwd along the margins of the wetted perimeter of the single thread gravel bed south river in virginia these deposits store a total mass equivalent to 17 to 43 of the annual suspended sediment load radiocarbon 210pb and 137c dating indicate that sediment in fgcm deposits ranges in age from 1 to more than 60 years reservoir theory suggests an average turnover time of 1 75 years and an annual exchange with the water column of a mass of sediment equivalent to 10 to 25 of the annual sediment load the distribution of ages in the deposits can be fitted by a power function suggesting that sediment stored in the deposits has a wide variety of transit times most sediment in storage is reworked quickly but a small portion may remain in place for many decades the presence of fgcm deposits indicates that suspended sediment is not simply transported downstream in gravel bed rivers in agricultural watersheds significant storage can occur over decadal timescales south river has a history of mercury contamination and identifying sediment sources and sinks is critical for documenting the extent of contamination and for developing remediation plans fgcm deposits should be considered in future sediment budget and sediment transport modeling studies of gravel bed rivers in agricultural watersheds copyright 2010 john wiley sons ltd

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

new constraints on the late cenozoic incision history of the new river virginia abstract the new river crosses three physiogeologic provinces of the ancient tectonically quiescent appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late cenozoic active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level however thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation we used cosmogenic 10be exposure dating aided by mapping and sedimentological examination of terrace deposits to constrain the timing of events in this history 10be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation fill cut and strath terraces at elevations 10 20 and 50 m above the modern river yield model cosmogenic exposure ages of 130 600 and 600 950 ka respectively but uncertainties on these ages are not well constrained these results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by new river terrace deposits the exposure ages yield a long term average incision rate of 43 m my which is comparable to rates measured elsewhere in the appalachians during specific intervals over the last 1 ma however the new river s incision rate reached 100 m my modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m my in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m my rates of surface erosion from ancient terrace remnants fluctuations between aggradation and rapid incision operate on timescales of 104 105 year similar to those of late cenozoic climate variations though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events these second order fluctuations appear within a longer term signal of dominant aggradation until 2 ma followed by dominant incision a similar signal is observed on other appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late cenozoic

stratigraphy petrography and correlation of late cenozoic middle atlantic coastal plain deposits implications for late stage passive margin geologic evolution regional chronostratigraphic correlation of middle atlantic upper coastal plain and fall zone fluvial deposits long thought to represent the updip proximal facies of a well dated post oligocene marine sequence in the salisbury embayment remains incomplete in part because of the paucity of biostratigraphic data petrography based lithostratigraphic correlations provide an opportunity to extend age control present in downdip marine deposits updip to fluvial deposits thus establishing the means for a regional chronostratigraphic correlation previous stratigraphic studies in new jersey delmarva southern maryland virginia and the central appalachian piedmont are linked by a crucial sequence of fluvial deposits at the head of chesapeake bay here petrography based lithostratigraphic correlations demonstrate 1 late oligocene late miocene susquehanna river dominated polygenetic quartzose fluvial aggradation bryn mawr formation 2 pliocene heterolithic but quartz dominated fluvial terrace and alluvial fan deposition perryville formation 3 pliocene early pleistocene paleo delaware hudson river dominated lithic and feldspathic rich fluvial aggradation pensauken formation and 4 the initiation of estuarine conditions turkey point beds by early pleistocene time these results are synthesized to propose a new late cenozoic regional chronostratigraphic correlation and depositional history for the salisbury embayment the correlation elucidates a complex interaction between passive margin isostasy tectonics eustasy and sediment source

increased mid twentieth century riverbank erosion rates related to the demise of mill dams south river virginia a recent hypothesis suggests that fluvial processes in areas of the eastern united states are strongly influenced by the demise of colonial mill dams rather than reflecting a quasi equilibrium adjustment to the current hydrologic and sediment regime we evaluated the control of colonial mill dams on twentieth and twenty first century bank erosion rates on the south river virginia through studies of historical aerial photographs historical documents hydrologic and climatic records and hydraulic modeling historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century all but one of these dams disappeared in the 1950s and the last was breached by 1976 from initially low values between 1937 and 1957 mean bank erosion rates increased by more than a factor of 2 after 1957 remaining high through 2005 accelerated bank erosion rates cannot be explained by changes in storm intensity the frequency of freeze thaw cycles or by changes in the density of riparian trees hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through 80 of our study reach by considering the timing of mill dam loss the spatial extent of backwater influence and the locations of our study sites we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 and possibly 10 of our 14 monitoring sites these results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region

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

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

on the retreat of forested cohesive riverbanks abstract data from the brandywine creek in pennsylvania and the south river in virginia document spatial and temporal patterns describing the retreat of gently curving bends with forested riparian zones containing a mix of very large and smaller trees cohesive banks and slow rates of migration 0 1 m yr field data include 2 5 years of bank profile surveys measurements of tree morphology and ground based laser scanner lidar surveys repeated biannually these banks retreat through a cycle of erosion on decadal timescales initially small volumes mean 0 173 m3 per bank erosion event of soil are removed between large trees growing on the bank near trees the rate of bank retreat is initially negligible creating a scalloped bank morphology buttressed by large trees as the trees are increasingly exposed to the flow they become slowly undercut gradually leaning into the channel and sliding down to the toe of the bank eventually toppling into the river after a few years which restarts the cycle a magnitude frequency analysis of monitoring data from the brandywine creek indicates that the effective bank erosion event only 0 11 m3 thus forested bank retreat is dominated by small soil failures large erosion events many associated with trees toppling into the river greatly increase the variance of time averaged rates of bank erosion such that almost 4 years of monitoring are needed to measure time averaged rates of erosion to within an accuracy of 10 because of the limited spatial extent of our survey data and the absence of large discharges these results are provisional improved process based models to predict the retreat of forested riverbanks should account for the spatial structure of riparian forests and the interactions between the flow the gradual removal of bank soils and the ongoing efforts of the trees to remain attached to the bank

role of debris flows in long term landscape denudation in the central appalachians of virginia four major storms that triggered debris flows in the virginia west virginia appalachians provide new insights into the role of high magnitude low frequency floods in longterm denudation and landscape evolution in mountainous terrain storm denudation in the blue ridge mountain drainage basins is approximately an order of magnitude greater compared to basins located in the mountains of the valley and ridge province this difference is probably the result of higher storm rainfall from the blue ridge storms radiocarbon dating of debris flow deposits in the blue ridge indicates a debris flow return interval of not more than 2 4 k y in mountainous river basins this finding combined with measurements of basin denudation suggests that approximately half of the long term denudation from mechanical load occurs episodically by debris flow processes although floods of moderate magnitude are largely responsible for mobilizing sediment in lowgradient streams our data suggest that high magnitude low frequency events are the most significant component in delivering coarse grained regolith from mountainous hollows and channels to the lowland floodplains

spatial discontinuity and temporal evolution of channel morphology along a mixed bedrock alluvial river upper drome river southeast france contingent responses to external and internal controls abstract the rehabilitation of degraded river channels is often guided by assumptions of continuity yet in response to spatial and temporal variations in controlling conditions rivers typically display discontinuous response in space and time this study examines the development of a 5 km reach of the drome river s e france characterised by alternating alluvial and bedrock zones that are separated by abrupt downstream transitions this reach is representative of the drome river as a whole and other rivers in the european alps where braided channel planforms have been replaced by more complex discontinuous morphologies the primary aims are to understand how this spatial complexity has developed on the drome evaluate how temporal channel changes have been affected by local factors particularly bedrock exposures and by long term catchment scale changes in sediment supply and the flood activity and consider the implications of this discontinuous geomorphology for reach management the development of geomorphological zonation is examined by documenting sequential changes in channel planform between seven periods using aerial photography 1948 2006 and by analysing change in bed elevation from profiles surveyed in 1928 2003 and 2005 between 1948 and 2001 bedrock exposed in the channel bed and along the floodplain margins defined discontinuities in sediment connectivity that were largely responsible for the configuration of channel zones the impact of floods on this system was not proportional to flood magnitude a modest flood in 1978 was an important event that by incision and avulsion at key locations defined a pattern of zonation that persisted until the end of the study in 2006 during the final 5 years of the study alluvial zones that previously responded to large floods by widening underwent narrowing despite the occurrence of a large flood and led to an overall reduction in width variance this resulted from progressive incision beneath and disconnection from formerly active channel areas in response to long term catchment scale reductions in sediment supply and flood frequency in 2006 the pattern of zonation remains distinct disguising this recent change in channel response and underlining the need for long term and sequential perspectives of channel development to fully understand the processes in operation contemporary snapshots of channel form may be misleading understanding interactions between inherent channel complexity and prevailing flow and sediment conditions and how this shapes channel response to individual floods is essential when interpreting future trajectories of channel change and likely response to management intervention

the impact of humans on continental erosion and sedimentation rock uplift and erosional denudation of orogenic belts have long been the most impor tant geologic processes that serve to shape continental surfaces but the rate of geomor phic change resulting from these natural phe nomena has now been outstripped by human activities associated with agriculture con struction and mining although humans are now the most important geomorphic agent on the planet s surface natural and anthro pogenic processes serve to modify quite dif ferent parts of earth s landscape in order to better understand the impact of humans on continental erosion we have examined both long term and short term data on rates of sediment transfer in response to glacio fl u vial and anthropogenic processes phanerozoic rates of subaerial denuda tion inferred from preserved volumes of sedimentary rock require a mean conti nental erosion rate on the order of 16 m per million years m m y resulting in the accumulation of 5 gigatons of sediment per year gt yr erosion irregularly increased over the 542 m y span of phanerozoic time to a pliocene value of 53 m m y 16 gt yr current estimates of large river sediment loads are similar to this late neogene value and require net denudation of ice free land surfaces at a rate of 62 m m y 21 gt yr consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters 83 of the global river sediment fl ux is derived from the high est 10 of earth s surface subaerial erosion as a result of human activity primarily through agricultural practices has resulted in a sharp increase in net rates of continental denudation although less well constrained than estimates based on surviving rock volumes or current river loads available data suggest that present farmland denudation is proceeding at a rate of 600 m m y 75 gt yr and is largely confi ned to the lower elevations of earth s land surface primarily along passive continental margins 83 of cropland erosion occurs over the lower 65 of earth s surface the conspicuous disparity between natu ral sediment fl uxes suggested by data on rock volumes and river loads 21 gt yr and anthropogenic fl uxes inferred from measured and modeled cropland soil losses 75 gt yr is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod ing croplands over the history of human agriculture accumulation of postsettlement alluvium on higher order tributary channels and fl oodplains mean rate 12 600 m m y is the most important geomorphic process in terms of the erosion and deposition of sedi ment that is currently shaping the landscape of earth it far exceeds even the impact of pleistocene continental glaciers or the cur rent impact of alpine erosion by glacial and or fl uvial processes conversely available data suggest that since 1961 global cropland area has increased by 11 while the global population has approximately doubled the net effect of both changes is that per capita cropland area has decreased by 44 over this same time interval 1 per year this is 25 times the rate of soil area loss antici pated from human denudation of cropland surfaces in a context of per capita food pro duction soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth

boulder streams debris fans and pleistocene climate change in the blue ridge mountains of central virginia the west slope of the blue ridge mountains in central virginia is a polygenetic landscape containing interglacial and periglacial features this paper proposes a general model relating the distribution and origin of hillslope and toeslope quaternary landforms to climatically influenced geomorphic processes two generations of interglacial debris fans in the study area differ in their degree of soil development and clast weathering boulder streams which clog debris flow chutes for the upper debris fans are interpreted as solifluction features formed during successive periglacial episodes growth of the boulder streams and associated talus slopes can influence the magnitude and frequency of debris flows and fan formation during interglacials

upland sediment supply and its relation to watershed sediment delivery in the contemporary mid atlantic piedmont u s a abstract we use sediment accumulation in ponds and reservoirs to examine upland sediment sources and sinks in the piedmont physiographic region of maryland usa in zero order and first order watersheds sediment yield is greatest from suburban land cover followed by agriculture and forest the idea that sediment yield is small from mature suburban development appears to not be correct first order channel enlargement is an important sediment source causing sediment yield to increase from zero order to first order watersheds nonchannel sources provide one third to two thirds of the upland sediment load long term sediment accumulation in a reservoir at fifth order indicates that cumulative sediment load from upland areas is reduced by one quarter by net valley bottom sedimentation if upland supply exceeds the load delivered from a watershed sediment must accumulate along valley bottoms in our study watershed net sedimentation rate sedimentation less erosion averaged over valley bottom area is 2 6 mm y a value that is similar to independent direct measurements of sedimentation and erosion in a nearby watershed evaluation of the relative contributions to sediment mass balance of upland supply valley bottom sedimentation and erosion and watershed delivery indicates that if valley bottom rates of sedimentation exceed erosion as indicated by recent studies then the proportion of watershed sediment delivery derived from stream banks is necessarily small although sediment yield estimated from stream gage records is similar in magnitude to that from ponds for watersheds smaller than 20 km 2 sediment yield from reservoir sedimentation is a factor of five larger than that estimated from gage records for watersheds larger than 140 km 2 this observation confirms that the different methods provide very different estimates of sediment yield this possibility is reinforced by a sediment yield of 14 mg km 2 y from a gage immediately above a reservoir with a yield of 142 mg km 2 y based on reservoir accumulation

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

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

examining the physical meaning of the bank erosion coefficient used in meander migration modeling widely used models of meander evolution relate migration rate to vertically averaged near bank velocity through the use of a coefficient of bank erosion e in applications to floodplain management problems e is typically determined through calibration to historical planform changes and thus its physical meaning remains unclear this study attempts to clarify the extent to which e depends on measurable physical characteristics of the channel boundary materials using data from the sacramento river california usa bend average values of e were calculated from measured long term migration rates and computed near bank velocities in the field unvegetated bank material resistance to fluvial shear k was measured for four cohesive and noncohesive bank types using a jet test device at a small set of bends for which both e and k were obtained we discovered that variability in k explains much of the variability in e the form of this relationship suggests that when modeling long term meander migration of large rivers e depends largely on bank material properties this finding opens up the possibility that e may be estimated directly from field data enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed another implication is that vegetation plays a limited role in affecting long term meander migration rates of large rivers like the sacramento river these hypotheses require further testing with data sets from other large rivers

natural streams and the legacy of water powered mills gravel bedded streams are thought to have a characteristic meandering form bordered by a self formed fine grained floodplain this ideal guides a multibillion dollar stream restoration industry we have mapped and dated many of the deposits along mid atlantic streams that formed the basis for this widely accepted model these data as well as historical maps and records show instead that before european settlement the streams were small anabranching channels within extensive vegetated wetlands that accumulated little sediment but stored substantial organic carbon subsequently 1 to 5 meters of slackwater sedimentation behind tens of thousands of 17th to 19th century milldams buried the presettlement wetlands with fine sediment these findings show that most floodplains along mid atlantic streams are actually fill terraces and historically incised channels are not natural archetypes for meandering streams

apparent increasing rates of stream incision in the eastern united states during the late cenozoic stream incision rates were obtained from 16 studies of terraces or stream associated cave levels in the eastern united states a log log regression of age against height above modern river level arl shows that age increases approximately as the square of height arl this relation may indicate an increasing rate of stream incision through time alternatively the relation may simply reflect an inherent effect of measured time interval on process rate as described by gardner et al who found that data from hundreds of studies showed a tendency for geologic process rates to decrease for increasing measurement intervals a regression equation for landscape denudation rates showed that the thickness removed increased as the 0 82 power of the time interval over which the denudation was measured gardner et al suggested that this bias could be removed by the use of statistically derived scaling functions however a similar regression equation for the data in this study shows a slope of 0 38 representing a much more rapid decrease in rate with increasing measurement interval than found by gardner et al even after applying the scaling function calculated by gardner et al my data show a 19 fold decrease in incision rate for a 103 increase in the measured time interval this result suggests that the observed decrease in stream incision rates with greater time spans may be at least partly real

late quaternary floodplain reworking and the preservation of alluvial sedimentary archives in unconfined and confined river valleys in the eastern interior of south africa abstract in this study geomorphological sedimentological and geochronological work was conducted on unconfined and confined reaches of three rivers in the eastern interior of south africa in order to quantify the relative rates of floodplain reworking and alluvial preservation along river courses with variable valley confinement and lithology using optically stimulated luminescence osl dating techniques new chronologies for the schoonspruit and mooi river were created and the existing klip river chronology was expanded the results suggest that floodplains in both unconfined and confined reaches preserve complex spatial and temporal patterns of alluviation although differences in boundary conditions lead to variation in the processes and rates of floodplain construction and reworking on the klip and mooi rivers where local base levels are stable channels in unconfined reaches rework floodplain sediments through slow lateral migration punctuated by local erosion during avulsion events on the schoonspruit where base level has lowered the channel in the unconfined reach is incised the floodplain is abandoned and a large gully has formed in the confined reaches of all three rivers the narrow floodplains are reworked by scour and fill activity and limited lateral migration the osl results suggest that unconfined reaches preserve relatively continuous alluvial records that extend into the pleistocene while the floodplains in the confined reaches preserve relatively discontinuous alluvial records biased toward the late holocene the alluvial geochronologic records in these systems preserve signals of changes in local base level controlled by variation in lithology and incision rather than climate change by defining the processes rates and patterns of floodplain reworking in reaches with different degrees of valley confinement and channel incision the findings contribute to understanding how rivers build modify and preserve alluvial sedimentary archives

patterns of floodplain sediment deposition along the regulated lower roanoke river north carolina annual decadal centennial scales abstract the lower roanoke river on the coastal plain of north carolina is not embayed and maintains a floodplain that is among the largest on the mid atlantic coast this floodplain has been impacted by substantial aggradation in response to upstream colonial and post colonial agriculture between the mid eighteenth and mid nineteenth centuries additionally since the mid twentieth century stream flow has been regulated by a series of high dams we used artificial markers clay pads tree ring dendrogeomorphic techniques and pollen analyses to document sedimentation rates amounts over short intermediate and long term temporal scales respectively these analyses occurred along 58 transects at 378 stations throughout the lower river floodplain from near the fall line to the albemarle sound present sediment deposition rates ranged from 0 5 to 3 4 mm y and 0 3 to 5 9 mm y from clay pad and dendrogeomorphic analyses respectively deposition rates systematically increased from upstream high banks and floodplain to downstream low banks reaches except the lowest reaches conversely legacy sediment deposition a d 1725 to 1850 ranged from 5 to about 40 mm y downstream to upstream respectively and is apparently responsible for high banks upstream and large wide levees along some of the middle stream reaches dam operations have selectively reduced levee deposition while facilitating continued backswamp deposition a gis based model predicts 453 000 mg of sediment is trapped annually on the floodplain and that little watershed derived sediment reaches the albemarle sound nearly all sediment in transport and deposited is derived from the channel bed and banks legacy deposits sources and regulated discharges affect most aspects of present fluvial sedimentation dynamics the lower river reflects complex relaxation conditions following both major human alterations yet continues to provide the ecosystem service of sediment trapping