stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

strengthening of north american dust sources during the late pliocene 2 7 ma here we present orbitally resolved records of terrestrial higher plant leaf wax input to the north atlantic over the last 3 5 ma based on the accumulation of long chain n alkanes and n alkanl 1 ols at iodp site u1313 these lipids are a major component of dust even in remote ocean areas and have a predominantly aeolian origin in distal marine sediments our results demonstrate that around 2 7 million years ago ma coinciding with the intensification of the northern hemisphere glaciation nhg the aeolian input of terrestrial material to the north atlantic increased drastically since then during every glacial the aeolian input of higher plant material was up to 30 times higher than during interglacials the close correspondence between aeolian input to the north atlantic and other dust records indicates a globally uniform response of dust sources to quaternary climate variability although the amplitude of variation differs among areas we argue that the increased aeolian input at site u1313 during glacials is predominantly related to the episodic appearance of continental ice sheets in north america and the associated strengthening of glaciogenic dust sources evolutional spectral analyses of the n alkane records were therefore used to determine the dominant astronomical forcing in north american ice sheet advances these results demonstrate that during the early pleistocene north american ice sheet dynamics responded predominantly to variations in obliquity 41 ka which argues against previous suggestions of precession related variations in northern hemisphere ice sheets during the early pleistocene

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

distribution of bedrock and alluvial channels in forested mountain drainage basins mountain river networks often consist of both bedrock and alluvial channels1 5 the spatial distribution of which controls several fundamental geomorphological and ecological processes6 7 the nature of river channels can influence the rates of river incision and landscape evolution1 2 as well as the stream habitat characteristics affecting species abundance and aquatic ecosystem structure8 11 studies of the factors controlling the distribution of bedrock and alluvial channels have hitherto been limited to anthropogenic badlands12 here we investigate the distribution of channel types in forested mountain drainage basins and show that the occurrence of bedrock and alluvial channels can be described by a threshold model relating local sediment transport capacity to sediment supply in addition we find that valley spanning log jams create alluvial channels hospitable to aquatic life in what would otherwise be bedrock reaches the formation of such jams depends critically on the stabilizing presence of logs derived from the largest trees in the riverside forests suggesting that management strategies that allow harvesting of such trees can have a devastating influence on alluvial habitats in mountain drainage basins

coarse sediment movement in the vicinity of a logjam in a neotropical gravel bed stream wood influences channel morphology by increasing channel roughness altering flow hydraulics and decreasing sediment transport in most fluvial systems where it is present in many locations the collection of wood into jams leads to even greater interruption of sediment transport by means of sediment impoundment and the creation of low velocity backwater flow zones we examined tracer clast movement near a wood jam in a neotropical gravel bed stream in order to evaluate the effect of wood mobility and piece transience on the ability of the jam to impede sediment transport we found that the jam had minimal influence on the passage of any individual clast with tracer clast mobilization and travel distance being best predicted by flow magnitude and grain size rather than location relative to the jam the rate of clast passage through the jam did not correlate with either the piece retention rate of the jam or with the density of the jam the jam did however temporarily trap a sediment wedge during a period when fluvially imported wood pieces blocked flow under a key wood piece diverting flow to a secondary flow path these results suggest that there is an upper bound on the flows in which wood is able to disrupt sediment transport and unlike for bedrock roughness elements this bound is frequently crossed at our study site

geomorphic response to flow regulation and channel and floodplain alteration in the gravel bedded cedar river washington usa article i nfo decadal to annual scale analyses of changes to the fluvial form and processes of the cedar river in washington state usa reveal the effects of flow regulation bank stabilization and log jam removal on a gravel bedded river in a temperate climate during the twentieth century revetments were built along 60 of the lower cedar river s length and the 2 year return period flow decreased by 47 following flow regulation beginning in 1914 the formerly wide anastomosing channel narrowed by over 50 from an average of 47 m in 1936 to 23 m in1989and became progressively single threaded subsequent high flows and localized revetment remov al contributed to an increase in mean channel width to about 34 m by 2011 channel migration rates between 1936 and 2011 were up to 8 m year in reaches not confined by revetments or valley walls and less than analysis uncertainty throughout most of the cedar river s length where bank armoring restricted channel movement in unconfined reaches where large wood and sediment can be recruited contemporary high flows though smaller in magnitude than preregulation high flows form and maintain geomorphic features such as pools gravel bars and side channels reaches confined by revetments remain mostly unmodified in the regulated flow regime while high flows are important for maintaining channel dynamics in the cedar river their effectiveness is currently reduced by revetments limited sediment supply the lack of large wood available for recruitment to the channel and decreased magnitude since flow regulation published by elsevier b v

lithologic and tectonic controls on bedrock channel form at the northwest himalayan front 1 recognition that channel form reflects a river s ability to erode rock and transport material has spawned stream power models that estimate incision patterns by approximating energy dissipation within a channel these models frequently assume that channel width scales as a power law with drainage area partly because drainage area is easily extracted from digital elevation models dems however this assumption is often confounded by local variations in rock strength and rock uplift rate that can cause channel constriction downstream here we investigate the morphological response to spatial changes in rock strength and rock uplift rate of 10 bedrock channels traversing the mohand range along the northwest himalayan front we present a new method to continuously measure and compare channel width slope and other hydraulic parameters that integrate satellite imagery and dem analysis our method corrects for an 13 overestimation of average channel gradient from a 90 m resolution dem that arises from short circuits of fine scale meanders we find that channels 1 narrow 1 km upstream from knickpoints formed by an increase in rock strength 2 adjust laterally more than vertically in response to downstream decreases rock erodibility and uplift rate and 3 meander where shear stresses are high and channel widths are low we attribute these results to a high ratio of sediment supply to transport capacity which enhances lateral erosion relative to vertical incision our results suggest that substrate strength and sediment supply substantially influence channel form and that channel width should be explicitly measured when interpreting tectonic signals from bedrock channel morphology

debris dams and the relief of headwater streams in forested mountain landscapes where debris flows are common their deposition commonly forms valley spanning dams of wood boulders or complex mixtures of both in headwater valleys sediment impoundment behind these dams causes alluviation in what would otherwise be bedrock channels in this paper the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined in the oregon coast range usa longitudinal profiles of three headwater channels approximately 2 km 2 maximum contributing area two of which are nominally bedrock and all debris dams were surveyed channel and valley widths were measured and surface bed material measurements pebble counts were taken at several locations along one stream cumulative relief in debris dams is highly variable within and among streams reaching a maximum of 58 of profile relief at one location the surveyed dams comprise 9 8 19 and 6 4 of total basin relief in the three corresponding drainage basins respectively a model of bedrock erosion and channel profile evolution is derived that accounts for a fractional coverage by the channel of total valley width at any given time and b shielding by sediment impounded behind debris dams the equation for incision rate assumed constant is solved for valley gradient as a function of contributing area and parameter values are estimated from or supplied by field data results are compared to stream gradient contributing area relationships obtained for the field sites these comparisons suggest a strong effect of network structure which varies significantly among the sites on profile shape and relief because of the different susceptibilities to debris flow deposition and therefore debris dam formation longitudinal profiles with and without the effects of debris dams are modeled for one forced alluvial i e nominally bedrock channel for these model profiles which extend only as far as the surveyed profile at this site 55 of the profile relief is due to debris dams and the concomitant shielding of the bed by impounded sediment the results suggest that a significant fraction of the relief of such forested mountain landscapes is due to the effects of relatively immobile wood and boulders deposited in the valleys by debris flows

comparative geomorphic analysis of surficial deposits at three central appalachian watersheds implications for controls on sediment transport efficiency abstract factors that control the routing and storage of sediments in the appalachian region are poorly understood this study involves a comparative geomorphic analysis of three watersheds underlain by sandstones and shales of the acadian clastic wedge these areas include the fernow experimental forest tucker county west virginia the north fork basin pocahontas county west virginia and the little river basin augusta county virginia gis based analyses of surficial map units allow first order approximation of sediment storage volumes in valley bottoms estimates of volumes are examined in tandem with morphometric analyses and the distribution of bedrock channels to make inferences regarding controls on sediment transport efficiency in the central appalachians the fernow and north fork areas are characterized by v shaped valleys with mixed reaches of alluvial bedrock channels distributed throughout the drainage network in contrast the little river valley is notably wider and gravelly alluvial fill is abundant comparator watershed parameters for the fernow north fork and little river areas include respectively 1 basin area 15 2 km 2 49 3 km 2 41 5 km 2 2 basin relief 0 586 km 0 533 km 0 828 km 3 drainage density 4 2 km 1 3 3 km 1 4 7 km 1 4 ruggedness 2 5 1 7 3 9 5 shreve magnitude 139 287 380 6 total valley bottom area km 2 0 76 km 2 1 86 km 2 3 09 km 2 7 average hillslope gradients 17 2 18 4 22 1 8 total debris fan surface area 0 113 km 2 0 165 km 2 0 486 km 2 and 9 debris fan frequency 2 0 km 2 1 0 km 2 2 8 km 2 the storage volumes in valley bottoms were estimated using map polygon areas and surface heights above channel grade the little river contains significantly higher sediment volumes in floodplain terrace and fan storage compartments total volumes of the valley bottoms are approximately twice that of the fernow and north fork areas combined unit storage volumes for the fernow north fork and little river are 5 2 10 4 m 3 km 2 5 5 10 4 m 3 km 2 and 1 6 10 5 m 3 km 2 respectively a conceptual model postulates that valley width morphometry and style of delivery from hillslopes are the primary factors controlling the efficiency of sediment transport steep debris flow prone hillslopes at the little river deliver high volumes of gravelly sediment at magnitudes greater than transport capacity of the channel patterns of stream power are complex as low order tributaries are under capacity and high order tributaries over capacity with respect to sediment load aggraded alluvial fill insulates valley floor bedrock from vertical erosion and valley widening dominates expansion of the valley width creates a positive response via increased storage capacity and lower unit stream power conversely the fernow and north fork are characterized by diffusive mass movement on hillslopes with incremental bedload transport to higher order tributaries rates of hillslope delivery are balanced by the rate of channel export mixed alluvial bedrock reaches provide the optimal channel configuration for active incision of the valley floor low expansion of valley width promotes high unit stream power and processes of vertical erosion the model implies that the fernow and north fork have been more effective at sediment transport during the late quaternary given similar climatic and tectonic settings variation in bedrock lithofacies is likely the primary factor modulating the efficiency of sediment transport

sex that moves mountains the influence of spawning fish on river profiles over geologic timescales abstract a key component of resilience is to understand feedbacks among components of biophysical systems such as physical drivers ecological responses and the subsequent feedbacks onto physical process while physically based explanations of biological speciation are common e g mountains separating a species can lead to speciation less common is the inverse process examined can a speciation event have significant influence on physical processes and patterns in a landscape when such processes are considered such as with ecosystem engineers many studies have focused on the short term physical and biological effects rather than the long term impacts here we formalized the physical influence of salmon spawning on stream beds into a model of channel profile evolution by altering the critical shear stress required to move stream bed particles we then asked if spawning and an adaptive radiation event similar to the one that occurred in pacific salmon species could have an effect on channel erosion processes and stream profiles over geological timescales we found that spawning can profoundly influence the longitudinal profiles of stream beds and thereby the evolution of entire watersheds the radiation of five pacific salmon from a common ancestor additionally could also cause significant geomorphic change by altering a wider section of the profile for a given distribution of grain sizes this modeling study suggests that biological evolution can impact landscape evolution by increasing the sediment transport and erosion efficiency of mountain streams moreover the physical effects of a species on its environment might be a complementary explanation for rapid radiation events in species through the creation of new habitat types this example provides an illustrative case for thinking about the long and short term coupling of biotic and abiotic systems

pool spacing in forest channels field surveys of stream channels in forested mountain drainage basins in southeast alaska and washington reveal that pool spacing depends on large woody debris lwd loading and channel type slope and width mean pool spacing in pool riffle plane bed and forced pool riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing lwd loading whereas pool spacing in generally steeper step pool channels is independent of lwd loading although plane bed and pool riffle channels occur at similar low lwd loading they exhibit typical pool spacings of greater than 9 and 2 4 channel widths respectively forced pool riffle channels have high lwd loading typical pool spacing of 2 channel widths and slopes that overlap the ranges of free formed pool riffle and plane bed channel types while a forced pool riffle morphology may mask either of these low lwd loading morphologies channel slope provides an indicator of probable morphologic response to wood loss in forced pool riffle reaches at all study sites less than 40 of the lwd pieces force the formation of a pool we also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear cut forests have lower lwd loading and hence fewer pools than reaches in pristine forests

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

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

channel bed adjustments following major aggradation in a steep headwater setting findings from oyabu creek kyushu japan abstract a typhoon in 1993 induced major aggradation along oyabu creek a steep gravel bed mountain stream in kyushu japan processes of sediment reworking are inferred from a 7 year monitoring program that measured adjustments to channel cross sections the longitudinal profile and the extent distribution of bedrock outcrops along a 3 km study reach over time the reach adopted a riffle and pool structure with notable increase in the area of exposed bedrock on the bed this adjustment process was characterised by progressive reduction in sediment storage change per unit flow the relaxation pathway following disturbance induced by the typhoon was shaped by the magnitude and frequency of subsequent rainfall events the capacity of these events to transport available sediments and physical linkages between reaches adjacent subreaches demonstrated differing relaxation pathways in response to these influences induced by spatial and temporal variability in threshold conditions along the channel longer term evidence indicates that responses to major disturbance such as the 1993 typhoon occur as cycles of around 20 year duration a relaxation period of 7 years is required to attain a quasi equilibrium bed configuration and rate of sediment flux the timeframe of cycles is considered to reflect changes to hillslope channel bed coupling marking the period required to generate sufficient sediment stores to reactivate phases of aggradation and subsequent degradation

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

bed load transport in an obstruction formed pool in a forest gravelbed stream abstract this paper examines channel dynamics and bed load transport relations through an obstruction forced pool in a forest gravel bed stream by comparing flow conditions sediment mobility and bed morphology among transects at the pool head centre and tail variable sediment supply from within and outside of the channel led to a complex pattern of scour and fill hysteresis despite the large flood magnitude large portions of the bed did not scour scour was observed at three distinct locations two of these were adjacent to large woody debris lwd and the third was along the flow path deflected by a major lwd obstruction bed material texture showed little change in size distribution of either surface or subsurface material suggesting lack of disruption of the pre flood bed fractions larger than the median size of the bed surface material were rarely mobile sediment rating relations were similar although temporal variation within and among stations was relatively high relations between bed load size distribution and discharge were complex showing coarsening with increasing discharge followed by fining as more sand was mobilized at high flow lack of local scour in the pool combined with bed load fining and net fill by relatively fine material implied that the dominant sources of mobile sediment were upstream storage sites and local bank collapse patterns of flow channel dynamics and sediment mobility were strongly affected by a lwd flow obstruction in the pool centre that created turbulent effects thereby enhancing entrainment and transport in a manner similar to scour at bridge piers

changes in woody debris and bed material texture in a gravel bed channel abstract this study investigated the response of bed material texture under two distinct periods of log jam conditions in a 900 m study reach of carnation creek vancouver island bc a field program extending over seven flood seasons coupled with existing streamflow records channel maps and low altitude aerial photography provides observations of jam stability and permeability surface and subsurface grain size as well as bed material fluxes sediment supply calibre streambed stability and sediment exchange near jams all jams functioned as partial dams throughout the field program but the overall perturbation of sediment transfers decreased when two jams were breached bed material texture exhibited the largest differences over the study reach during a period when log jams exerted a stronger influence over the passage of water and sediment at individual gravel bars most adjustments in surface and subsurface grain size constituted less than a 20 change in percentile values larger changes occurred primarily in subsurface sediment temporal patterns in bed material texture exhibited variation between bars but downstream of breached jams surface sediment conformed to the expected fining trend while the subsurface sediment increased in size surface and subsurface sediments evolved in the same direction only 64 of the time and the different magnitudes of adjustment caused local armor ratios to vary over time results highlight that the temporal dynamism of large woody debris lwd should be coupled with spatial considerations for effective stream restoration and management of streambed sediment

holocene strath terraces climate change and active tectonics the clearwater river basin olympic peninsula washington state the 400 km 2 clearwater river basin located on the pacific flank of the actively uplifting olympic mountains of western washington state contains a well preserved flight of holocene fluvial terraces we have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic fluvial active tectonic and climatic processes that operate at holocene spatial and temporal scales detailed field mapping reveals three prominent holocene straths and their overlying terrace deposits terrace ages fall into three broad ranges ca 9000 11 000 yr b p qt4 4000 8000 yr b p qt5 and 0 3000 yr b p qt6 terrace deposit stratigraphy sedimentology and age distributions allow us to consider two alternative models for their genesis the favored model states that the terrace ages are coincident with lateral incision of the clearwater channel emplacement of the terrace alluvium and the carving of the straths vertical incision of the clearwater channel was primarily relegated to the brief 1000 yr intervals when we have no record of terraces alternatively the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision in both models we envision a clearwater river channel at or near capacity with a temporally variable rate of both lateral and vertical incision small deviations from this at capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel we consider several causes for variable hillslope sediment flux in this tectonically active setting including holocene climate change and ground accelerations related to earthquakes holocene rates of vertical incision are reconstructed along nearly the entire clearwater valley from the wide distribution of dated terraces incision rates clearly increase upstream mimicking a pattern documented for pleistocene terraces in the same basin however the rates are 2 3 times those determined for the pleistocene terraces the faster holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift however we favor an alternative explanation in which the holocene rates represent a channel rapidly reacquiring its stable graded concavity following protracted periods of time in the pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation these results illustrate how even in tectonically active settings representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial interglacial cycle

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

observations on the role of lithology in strath terrace formation and bedrock channel width field observations from western washington and eastern tibet indicate a strong lithologic influence on strath terrace formation and highlight the response of bedrock channel width to spatial gradients in bedrock erodibility and or rock uplift rates measurements of local bedrock bed and bank erosion rates together with observations of the role of weathering processes on erosion of siltstones and sandstones illustrate a mechanism that underlies a conceptual model that predicts a strong lithological control on strath terrace formation direct measurements of bedrock erosion rates in lithologies susceptible to accelerated erosion upon sub aerial exposure show that lateral rates of bedrock bank erosion can substantially exceed vertical incision below the perennial flow level due to an asymmetry in erodibility between perennially submerged rock and rock exposed to cyclic wetting and drying on bedrock channel walls under such conditions the rapid weathering mediated lowering of bedrock exposed above baseflow promotes development of a beveled bedrock platform that could become a strath terrace upon abandonment in contrast to previous studies channel widths measured across a substantial gradient in lithology slope and long term erosion rates in eastern tibet do not follow conventional relations where channel width scales as a power law function of drainage area instead as they flow across a zone of rapid uplift these channels systematically narrow relative to the predicted width extrapolated from traditional power law relations field studies from a wide range of geological settings and lithologies support the interpretation that in general strath terraces tend to be more extensive in rivers flowing over weak sedimentary rock and tend to be poorly developed and or preserved where rivers flow over hard erosion resistant rock

experimental study of bedrock channel alluviation under varied sediment supply and hydraulic conditions 1 channel incision into bedrock plays a critical role in mountain landscape evolution a bed that is completely alluviated cannot undergo incision as a result incision driven by the collision of bedload grains and bedrock requires transport at below capacity conditions for example where the bed is partially covered by alluvium in a mechanistic model of bedrock incision it is important to have a formula describing the relationship between the bedload transport rate and the degree to which the bed is covered by sediment the purpose of this work is to understand the mechanisms underlying the degree of bed exposure in bedrock rivers by means of an experimental study a number of experiments with various hydraulic and morphological bed conditions were performed in order to characterize this model here we focus on planar bedrock beds with some roughness the results suggest that the sediment supply channel slope hydraulic bed roughness the degree of areal coverage and thickness of antecedent gravel in the channel as well as the presence of boulders in the channel are major factors controlling bedrock exposure for sufficiently low ratios of sediment supply to transport capacity it was found that bedrock roughness can play a role in determining the degree of bedrock exposure for higher sediment ratios on the other hand the composite roughness associated with grain roughness bars and or antidunes dominates so that the underlying bedrock roughness no longer affects the degree of exposure of the bed for lower bed slopes i e less than 0 015 based on our experimental setting bedrock exposure decreases more or less linearly with increasing values of the ratio of sediment supply rate to capacity rate for higher bed slopes i e more than 0 015 there is a range of lower values of the ratio of sediment supply rate to capacity rate where a bedrock surface becomes fully exposed without any alluvial deposit for a given bedrock roughness this range expands solely with increasing slope regardless of shear stress within the upper range of values of the ratio of sediment supply to capacity rate a linear relationship between the degree of bedrock exposure and this supply to capacity ratio still prevails the addition of model boulders can be viewed as very high hydraulic bed roughness to the channel has been found to suppress the overexposure of bedrock and so restore a linear relation between the degree of exposure and the ratio of sediment supply rate to capacity rate formulations for estimating bedrock exposure as a function of sediment supply to capacity ratio under different river channel characteristics are proposed a linear relation between the degree of alluvial cover and the ratio of sediment supply rate to capacity rate is a previously proposed model the present study expands the result for other cases including runaway alluviation some landscape evolution models assume an abrupt shift between fully exposed bedrock and complete alluviation whereas others assume the linear relation the present study shows that both formulations are valid under different settings of river characteristics

evaluating survey instruments and methods in a steep channel abstract methods for surveying and analyzing channel bed topography commonly lack a rigorous characterization of their appropriateness for project objectives we compare four survey methods a hand level two different methods of surveying with a laser rangefinder and a real time kinematic gnss rtk gnss to explore their accuracy in determining channel bed slope and roughness for a study reach in a small dry steep channel additionally we evaluate the variability among four operators for each survey technique two methods of calculating reach slope were computed a regression on the channel profile and a calculation using only survey endpoints using data from the rtk gnss as our accuracy reference the hand level and two person laser rangefinder surveying systems performed with high accuracy

transport slopes sediment cover and bedrock channel incision in the henry mountains utah 1 field data from channels in the henry mountains of utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds the cover effect we compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit navajo sandstone but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources bedrock channels that contain abundant clasts diorite rich have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment diorite poor the diorite poor tributaries have incised more deeply to lower average slopes and have more reach scale slope variability which may reflect bedrock properties longitudinal sediment sorting and incision at lower sediment supply diorite rich channels have less bedrock exposed and smoother longitudinal profiles than diorite poor channels we find that 1 coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision validating the hypothesized cover effect in fluvial incision models 2 the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock suggesting that the slope of incising bedrock channels can become adjusted to the sediment load 3 when abundant sediment is available transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope and 4 cover effects can be important even when moderate amounts of bedrock are exposed in channel beds

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

controls on the distribution of channel reach morphology in selectively glaciated catchments abstract to assess the controls on the distribution of channel reach morphology in a selectively glaciated landscape we used field mapping and a geographical information system gis in the river dee catchment northeast scotland controls on channel morphology were investigated using 1 continuous longitudinal assessment of channel morphology distribution in relation to geology glacial history topography and total stream power in two subcatchments and 2 slope s and a slope drainage area s a framework to understand the occurrence of 173 widely distributed bedrock mixed bedrock alluvial and alluvial three different types reaches the s a framework used indicators of transport capacity qc and sediment supply qs to differentiate channel types the study highlights the disjointed nature of channel reach distribution at the river scale that reflects variable lithology and glacial modification because of the subdued topography in contrast to other regions colluvial forcing of channel morphology in the headwaters was lacking however in common with other glaciated landscapes repeated sequences of channel reach type progression determined by valley steps were evident the s a analysis successfully discriminated 87 2 of alluvial and 91 4 of bedrock reaches despite the variable land use and glacial modification discrimination of the full range of channel types using s or the s a framework was poor however notably a third of the transport alluvial reaches were located in the bedrock s a domain and the majority of mixed reaches were widely distributed mostly within the bedrock domain and not close to the critical slope sc in comparison to other regions the sc above which qc qs and bedrock reaches dominate was notably higher we hypothesise that a drier climate and the higher entrainment threshold of coarse granite dominated bed materials create a higher sc

confronting hysteresis wood based river rehabilitation in highly altered riverine landscapes of south eastern australia abstract this study evaluates an experiment in river rehabilitation which uses large wood to stimulate and emulate natural system processes in an effort to reverse channel degradation excess sediment transport and habitat simplification that has resulted from two centuries of human induced disturbances particularly desnagging the experiment involved the reintroduction of 436 logs 350 t within 20 engineered log jams eljs over an 1100 m reach commencing in 1999 the experiment was set up as a standard baci design with a control reach 3 km upstream in the 5 years since implementing the rehabilitation strategy the study reach has experienced five floods greater than the mean annual and a further five events capable of mobilising the gravel bed five surveys of channel terrain have been completed since treatment implementation and the changes to net sediment storage and morphologic diversity assessed in comparison to the control reach seven surveys of the fish population in the reach have also been undertaken during the project to measure the ecological response to the introduced wood the experiment has demonstrated the effectiveness of elj technology in achieving engineering and geomorphic goals to date the treatment has halted further degradation of the river and increased sediment storage with the test reach now storing on average 40 m3 1000 m2 more sediment than in the control these values it would appear represent a new reach scale dynamic equilibrium storage level over decadal timescales additional sediment storage amounts to 3 5 m3 m3 of wood added at the reach scale this additional storage represents a reduction of just 2 or less of the post european expansion in channel capacity suggesting far greater efforts are required than those employed here to reverse the cumulative effects of 180 years of channel erosion and simplification pool and bar area in the test reach increased by around 5 and 3 5 respectively while the corresponding values in the control were around 1 5 and 1 respectively two indices of morphologic diversity were measured for each bed survey the standard deviation of 3d residuals of change compared with the baseline survey sdi 3d and the standard deviation of thalweg residuals from the line of best fit sditr the sdi 3d index shows both reaches increased in complexity through the study with the treatment increasing more than the control 0 37 and 0 29 respectively the sditr index does not detect clear changes because of the low signal to noise ratio however it does suggest the test reach was more complex than the control at the outset the observed increase in fish abundance after the first 12 months of monitoring reported previously is now far less distinct 4 years on a pattern seemingly reflecting the relatively minor increases in critical pool habitat and habitat diversity over the same period although no significant differences were detectable in fish species richness or total abundance from the reach aggregate data after 4 years analysis of individual structures show them to be high quality habitat for native fish compared to the rest of the reach and the upstream control these results highlight the challenges river managers face in achieving measurable improvements in the health of aquatic ecosystems in highly altered rivers managers must confront hysteresis in a biophysical and institutional sense when attempting to reverse the degradation of rivers the scale of treatment implemented in this experiment was at the upper end of the spectrum of rehabilitation efforts currently being undertaken in australia suggesting that far greater resources and longer timescales are required to achieve the levels of improvement in the diversity of stream habitat expected by the community the study also highlights problems with the strategy of attempting to meet multiple objectives within a reach scale rehabilitation project while this treatment successfully met some geomorphic study objectives maximising the benefits for fish habitat would require a strategy focused primarily on the creation of complex woody habitat within deeper pools particularly pools immediately below riffles

hyporheic temperature dynamics and heat exchange near channel spanning logs 1 the flow of river water around large woody debris lwd creates pressure gradients along the riverbed that drive river groundwater mixing or hyporheic exchange and heat transport within the hyporheic zone we quantify hyporheic fluid and heat exchange induced by current interaction with channel spanning logs using two approaches laboratory flume experiments and numerical simulations that link turbulent open channel fluid flow porous fluid flow and heat transport flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio a fraction of the channel depth blocked by the log channel froude number and sediment permeability upstream of a log downwelling water transports the river s diel thermal signal deep into the sediment downstream upwelling water from shallow flow paths has a thermal signal similar to the surface water while upwelling water from deep flow paths forms a wedge of buffered low amplitude temperatures since most hyporheic water emerges from shallow flow paths upwelling water has limited potential to buffer surface water temperature because of historic channel clearing practices modern rivers have unnaturally low densities of lwd a key implication is that lwd removal has contributed to thermal homogenization and potential degradation of hyporheic habitats lwd reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone however hyporheic exchange near lwd may not impact diel surface water temperatures at the reach scale

zircon u th pb he double dating of the merlin kimberlite field northern territory australia abstract this paper discusses the development of a combined u pb and u th he geochronology method for single zircon grains and the potential application of this technique in diamond exploration zircon entrained within kimberlite deposits should have distinctive u th pb he signatures compared to those found in the host terrane to investigate the application of zircon double dating to kimberlite diamond exploration we analysed zircon from the sacramore kimberlite pipe located in the merlin field in the northern territory of australia we also conducted a double dating study of detrital zircon from a regional sample of the bukalara sandstone the principal host rock for the merlin field repeated analysis of fish canyon zircon verified that the ion microprobe sputtering process used for u pb dating does not disturb the he content of the sample the zircon u pb age for the sacramore pipe n 14 ranged from 1541 2433 ma consistent with the mesoproterozoic formation of the north australian craton and indicating that the zircon obtained from the kimberlite is of xenocrystic origin u th he thermochronometry of these kimberlite zircon xenocrysts n 33 yielded a mean weighted average age of 368 4 ma 2 concordant with a previously determined phlogopite rb sr age of 367 4 ma for the merlin field the u pb age 1472 2939 ma n 41 of detrital zircon from the bukalara sandstone is statistically indistinguishable from that of the kimberlite zircon xenocrysts while the detrital u th he ages range from 459 to 1279 ma the double dating study indicates that the majority of the zircon grains in the sacramore kimberlite pipe had their helium clocks reset to the time of kimberlite emplacement thermal resetting of the zircon xenocrysts may have been caused by either one or a combination of these processes i the grains originated from lower to mid crustal depths 6 km where ambient geothermal temperatures allowed the helium to diffuse out of the zircon as soon as it was produced by radioactive decay ii the grains were heated by deuteric processes to temperatures in excess of 180 c following emplacement of the kimberlite pipe the consistently young u th he ages of the kimberlite zircon xenocrysts distinguish them from surficial detrital zircon and this geochemical feature could have application in regional diamond exploration where standard kimberlite indicator minerals e g cr pyrope cr diopside picroilmenite chromite are prone to destruction by chemical weathering in tropical and sub tropical climates