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

instream wood loads in montane forest streams of the colorado front range usa abstract although several studies examine instream wood loads and associated geomorphic effects in streams of subalpine forests in the u s southern rocky mountains little is known of instream wood loads in lower elevation montane forests of the region we compare instream wood loads and geomorphic effects between streams draining montane forest stands of differing age old growth versus younger and disturbance history healthy versus infested by mountain pine beetles we examined forest stand characteristics instream wood load channel geometry pool volume and sediment storage in 33 pool riffle or plane bed stream reaches with objectives of determining whether i instream wood and geomorphic effects differed significantly among old growth younger healthy and beetle infested forest stands and ii wood loads correlated with valley and channel characteristics wood loads were standardized to drainage area stream gradient reach length bankfull width and floodplain area streams flowing through old growth forests had significantly larger wood loads and logjam volumes pairwise t tests as well as logjam frequencies kruskal wallis test residual pool volume and fine sediment storage around wood than streams flowing through younger forests wood loads in streams draining beetle infested forest did not differ significantly from those in healthy forest stands but best subset regression models indicated that elevation stand age and beetle infestation were the best predictors of wood loads in channels and on floodplains suggesting that beetle infestation is affecting instream wood characteristics wood loads are larger than values from subalpine streams in the same region and jams are larger and more closely spaced we interpret these differences to reflect greater wood piece mobility in subalpine zone streams stand age appears to exert the dominant influence on instream wood characteristics within pool riffle streams in the study area rather than beetle infestation although this may reflect the relatively recent nature

morphology of small discontinuous montane meadow streams in the sierra nevada various fluvial geomorphic models have been developed to characterize the relationships between planform and bedform features of large alluvial channels however little information exists for meadow channel morphology field investigation of seven narrow low energy meadow stream reaches in the northern sierra nevada range of california revealed similarities and differences to larger alluvial channels the average radius of curvature to channel width ratio 5 54 of the meadow streams was almost double that of larger alluvial streams 3 1 with a standard deviation of 4 66 average meander wavelength to channel width ratio 22 43 was almost triple that of typical alluvial streams 8 5 with a standard deviation of 16 80 bedform features occurred at an average of 6 72 channel widths similar to typical pool riffle spacing of 5 7 channel widths grass sod connected a series of scour pools providing the same energy drop function as riffles or steps results suggest that bedform regularity is similar to typical pool riffle systems especially as we move to larger watersheds and higher precipitation and runoff but planform features are less developed and highly influenced by vegetation restoration efforts can benefit from considering how planform and bedform channel patterns develop in these meadows

morphological and sedimentological responses of streams to human impact in the southern blue ridge mountains usa abstract morphological and sedimentological responses of streams to basin scale impact have been well documented for intensively agricultural or urban areas sensitivity thresholds of streams to modest levels of disturbance however are not well understood this study addresses the influence of forest conversion on streams of the southern blue ridge mountains a region that has received little attention with respect to human impact on stream channels basins were chosen for this study to represent the end members of the range of human impact in the area with the forest cover of the basin used as a proxy for level of impact ranging from about 70 100 regionally two pairs of lightly impacted 90 forest and moderately impacted 70 80 forest sub basins of the upper little tennessee river were identified for comparison reach characteristics e g slope drainage area and riparian cover were aligned in each pair to isolate contrasting forest cover as the primary driver of any detected differences in morphology and sedimentology a suite of standard cross sectional and longitudinal data was collected for each reach for characterization of the sedimentology and morphology of the streams difference of means tests were conducted to identify parameters significantly differing between the lightly and moderately impacted streams in both pairs consistent and significant differences within both pairs were demonstrated in bankfull width depth ratios baseflow wetted width and particle size on the stream bed both in the thalweg and throughout the channel bed the moderately impacted streams are narrower than the lightly impacted streams and the bed texture of the moderately impacted streams is finer than that of the lightly impacted streams the moderately impacted streams contain a higher percentage of

equilibrium pool dimensions and sediment sorting patterns in coarse grained new england channels abstract pools and riffles are natural bedforms that produce variations in width and depth along channels deemed critical for aquatic habitat an understanding of this morphology is necessary for the successful design of a reconstructed or restored channel to better understand pool geometry and sorting characteristics of new england channels characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions characterizations of channel bed sediments were also performed at 34 of these locations data from 120 pools were used in least squares multiple regression analysis with 19 different independent variables to develop prediction equations for pool depth length and exit slope gradient these equations were then tested using the 25 remaining pools eighteen different geometric variables were then used to predict sediment 50 values in pool centers pool exit slopes and riffles for the subset of 34 samples the regression analyses for pool geometry indicate that pool depth is significantly influenced to a decreasing degree by pool exit slope width constriction gradient constriction width drainage area upstream channel width and the exit slope expansion ratio with an r 2 value of 0 53 similarly pool length is influenced by channel gradient location of the channel constriction pool width drainage area and constriction length with an r 2 value of 0 72 the gradient of pool exit slope is influenced by drainage area channel gradient and constriction gradient with an r 2 value of 0 34 however r 2 values for exit slope increased to 0 65 when sediment d 50 data were utilized using the resulting regression equations to predict dimensions for the 25 verification pools yields average residuals near zero although moderate variations do exist a tukey kramer hsd comparison of the means for sediment 50 values indicates pool sediments are significantly coarser than pool exit slope and riffle sediments for the subset of 34 pools regression analyses also suggest significant relations between the size of sediments in pools and riffles and a combination of geometric variables with r 2 values that range from 0 45 to 0 62 the results suggest that pool dimensions are related to basin wide variability and local hydraulic conditions the findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout the pool riffle morphology

formation and maintenance of a forced pool riffle couplet following loading of large wood abstract pool riffle maintenance has been documented in numerous studies but it has been almost impossible to characterize detailed natural pool riffle formation mechanisms because of the lack of baseline data prior to pool establishment in 2013 a study was conducted on the blackledge river in connecticut to document the formation of a new pool riffle couplet on a section of river that had previously been studied from 1999 to 2001 in 2001 the study reach contained a scour hole with a residual depth of 0 08 0 09 m downstream of a 1930s paired deflector with no identifiable riffle immediately downstream at this time a large severely undercut hemlock tree was noted along the left bank sometime between fall 2001 and 2004 the tree fell perpendicular to flow across the channel and formed a large wood lw jam and new pool riffle couplet several meters downstream of the old scour hole pool spacing along the reach decreased from 4 47 bankfull widths bfw in 1999 to 3 83 bfw after the new pool riffle couplet formed the new pool has a residual depth the water depth of the streambed depression below the elevation of the immediate downstream hydraulic control of 1 36 0 075 to 1 59 0 075 m which resulted from a combination of 1 32 0 09 m or less of incision below the old scour hole 95 6 or less of the depth increase and up to 0 18 0 09 m of downstream deposition and associated backwater formation 13 2 or less of the depth increase to assess dynamic stability of the pool riffle couplet over several flood cycles surficial fine sediment and organic material along the reach were quantified the 23 m long pool stores 25 7 of the surficial fine grained sediments and 15 4 of organic material along a 214 m long reach that includes one additional artificially created pool an adjacent 50 m long secondary channel impacted by the lw jam stores 65 3 of the surficial fine grained sediments and 54 8 of organic material along the full reach

pool and riffle characteristics in relation to channel gradient abstract the depths of pools relative to the dephts of runs and riffles were correlated with reach scale channel gradient along three rivers in coastal northern california the sample included 122 pools formed in channels with gradients from 0 172 to 0 002 relative pool depth on these rivers and relative distance between pools increase as channel gradient decreases mean pool riffle depth is 2 8 1 at the highest channel gradient and 6 2 1 at the lowest gradient while mean pool riffle length is 1 0 8 at high channel gradient and 1 1 8 at low channel gradient we hypothesize that these trends reflect changes in energy expenditure with decreasing gradient as a result of the flow s ability to erode its channel boundaries channel reaches with high gradients are characterized by resistant channel boundaries coarse material and relatively low discharge and total stream power channel reaches with low gradients have less resistant channel boundaries finger grained bed material and higher values of discharge and total stream power these changes in channel and flow characteristics with decreasing gradient result in flows in high gradient reaches expending a greater proportion of their energy in overcoming boundary and internal resistance with less energy available for channel bed scour and the formation of pools in their relatively resistant channels in contrast with less energy available for channel bed scour the channel bed creating deeper pools because the channel boundaries are less resistant and the proportion of flow energy available for sediment entrainment and transport should be greater

sediment deposition and transport patterns following a reservoir sediment release a reservoir sediment release on the north fork poudre river supplied 7000 m3 of silt to pebble sized sediment to an originally boulder bed channel deposition along the 12 km of channel downstream from the reservoir occurred primarily in pools during the subsequent snowmelt hydrograph sediment was progressively scoured from the upstream and then the downstream pools initial sediment reworking in the pools created a deep narrow thalweg scoured to the original pool bed with additional sediment deposition in lateral eddies continued reworking reduced but did not completely remove these eddy deposits the channel became supply limited with respect to finer grain sized fractions clay to medium sand first at upstream and then at downstream sites and eventually became supply limited with respect to coarser grain sized fractions coarse sand to pebbles bedload transport rates at a site were strongly linked to the depletion of sediment stored in upstream pools magnitude duration and sequence of flows were all important controls on bedload transport and return of the channel to its prerelease state

bed stability variations after check dam construction in torrential channels south east spain the effects of check dams on the bed stability of torrential channels have been analysed in several tributary basins of the segura and guadalentin rivers south east spain in order to illustrate the large variability in channel bed forms and bed sediment sizes along the stream 52 reaches of 150 m in length were surveyed this variability is due to the behaviour of check dams which depends on bedrock control bed slope channel roughness lateral sediment input and a highly variable sediment transport capacity though the purpose of check dams is to diminish the boundary shear stress reducing the longitudinal slope and to stabilize the channel bed downstream they reduce the volume of channel stored material favouring local scour processes and upstream they can destabilize the sidewalls the results enable us to evaluate the impact of every check dam on the bed morphology distinguishing the structures installed in limy marl areas e g catchment of the carcavo rambla cieza and in schist and slate terrains e g catchment of the torrecilla rambla close to lorca in the first type bedrock and moderately thick granular beds predominate downstream from the check dams so that the length of bedrock reaches and increase of roughness due to scour processes are the best indicators to verify its geomorphological effectiveness on the other hand the metamorphic areas drained by ramblas and gullies produce great quantities of gravel that are retained by check dams creating more uniform and permeable beds where the balance between sedimentation and scouring and the ratio c84 0 rbs appear to be the parameters most frequently adopted to estimate the bed stability analysis of slope adjustments and the application of other indices to estimate the bed substrate stability lrbs sri and the structural influence of the dams sibs corroborate the differences in bed stability found in the corrected reaches in each catchment copyright 2007 john wiley sons ltd

a two dimensional discrete particle model of gravel bed river systems 1 the formation of bed forms in gravel bed rivers acts as a control on stream ecology and the response of rivers to floods available models do not reproduce the range of observed bed forms and do not consider interactions between the bed and flow hydraulics the model presented here considers a gravel bed river as a complex system in which sediment clasts are represented as discrete elements simple and local rules describe the sediment and flow dynamics using a trimodal sediment distribution irregular forms that scale with particle diameter develop without explicit feedback mechanisms because of the tendency of large particles to roll along the bed surface and collect into chains feedback mechanisms such as imbrication increase the effective entrainment threshold of groups of large particles and increase the stability of these imbricate forms a second type of bed form is associated with saltating grains and emerges where particles are transported at a preferred distance the development and maintenance of larger scale bed forms require feedback between the bed and flow properties by allowing mean velocity to adjust to bed morphology and considering the effect of acceleration on turbulence generation and mean velocity profiles we demonstrate the emergence of forms similar in morphology to gravel sheets dunes and riffle pools the model is best used to complement field based studies and is suitable for testing hypotheses of streambed behavior

post middle miocene accretion of franciscan rocks northwestern california deformed sedimentary rocks assigned to the franciscan assemblage in the king range south of cape mendocino northern california are dominantly deep water argillite and sandstone occurring as thick to thin bedded locally channelized marine turbidites of arkosic to andesitic volcani clastic composition these rocks structurally overlie late cretaceous coniacian to campanian basaltic pillow flows flow breccias diabase sills and arkosic sandstone turbidites that also are assignable to the franciscan assemblage and are well exposed along the coast at point delgada radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the king range may be early tertiary and that the youngest rocks are middle miocene or younger the youngest rocks yet reported from the franciscan assemblage partly coeval miocene relizian and luisian to quaternary rocks representing shallower sedimentary facies of the inner shelf to outer continental slope occupy the eel river basin and isolated fault bounded slivers adjacent to the king range on the north and east wide zones of penetrative shearing that may delineate a middle miocene or younger suture separate the deep water franciscan rocks of the king range from less severely deformed neogene rocks of the eel river basin the king range appears to be a displaced terrane of oceanic basement overlain by paleogene and neogene sedimentary and igneous rocks of continental and oceanic derivation this terrane may have been displaced north or eastward and obductively accreted to california 15 m y ago before or perhaps during northward passage of the pacific farallon north american plate junction

the influence of organic debris on channel morphology and bedload transport in a new zealand forest stream the behaviour and form of and bedload sediment transport through a 3 5 m wide forest stream have been monitored for nearly three years bedload transport is highly episodic and spatially variable and is controlled less by water discharge than by sediment availability organic debris in the channel creates temporary base levels and sites at which coarse sediment may remain stored for long periods collapse or disruption of log and debris jams makes sediment available for transport in only a small proportion of the runoff events that are actually competent to move the material even then sediment travels only a short distance before being redeposited frequently behind debris accumulations further downstream rates of sediment transport during a given runoff event can vary markedly over short distances along the stream again depending on whether sediment was made available for transport by log jam collapse upstream organic debris is therefore a major constraint on the application of physical laws and theories to explaining sediment movement in and the morphology of this stream

variability of rock erodibility in bedrock floored stream channels based on abrasion mill experiments we quantify variations in rock erodibility kr within channel cross sections using laboratory abrasion mill experiments on bedrock surfaces extracted from streams with sandstone bedrock in utah and basaltic bedrock in the hawaiian islands samples were taken from the thalweg and channel margins the latter at a height that is inundated annually for each sample a sequence of abrasion mill experiments was completed to quantify variations in erosion rate with erosion depth erosion rate data from these experiments shows two things first the erosion rate from channel margin samples is greater than for thalweg samples with the greatest difference observed for the rock surface that was exposed in the stream channel second erosion rate decreases with depth beneath the original rock surface by an order of magnitude in most cases the erosion rate becomes steady at depths of 1 3 mm for channel margin samples and 0 1 0 4 mm for thalweg samples because only rock properties and microtopography vary throughout the sequence of mill experiments these results suggest that kr of the bedrock surface exposed in stream channels is higher at the margins than near the channel center and that kr decreases over depths of 1 mm the simplest explanation for these patterns is that kr is enhanced at the bedrock surface and along the channel margins due to the effects of weathering on rock strength and surface roughness we hypothesize that a balance exists between weathering enhanced erodibility and episodic incision to allow channel margins to lower at rates similar to the thalweg

controls on pool characteristics along a resistant boundary channel abstract the north fork poudre river in phantom canyon has a pool riffle sequence formed in granitic bedrock we address two questions what controls downstream pool spacing within a pool what factors control pool dimensions we hypothesize that both pool spacing and pool geometry could be governed by available flow energy or by substrate characteristics these hypotheses were addressed using channel bed and water surface gradients surveyed under moderate flow selby rock mass strength at vertical outcrops forming lateral pool constrictions and at other vertical outcrops and joint density measured from high resolution digital images of the bedrock outcrop along each of 10 pools the downstream length between pools is quite variable documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fonten

long river profiles tectonism and eustasy a guide to interpreting fluvial terraces along three rivers at the mendocino triple junction northern california strath cut and fill terraces have formed in response to tectonic and eustatic processes detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level primarily aggradation and formation of fill terraces during sea level high stands alternating with deep incision during low stands a eustasy driven deposi tional wedge extends tens of kilometers upstream on all rivers tapering to zero thickness this distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere due in part to the large size of these rivers however the change in gradient is nearly identical to other base level rise studies the depositional gradient is about half that of the original channel middle to upper reaches of each fiver are dominated by the effects of long term uplift primarily lateral and vertical erosion and formation of steep unpaired strath terraces exposed only upstream of the depositional wedge vertical incision at a rate similar to that of uplift has occurred even during the present sea level high stand along rivers with highest uplift rates strath terraces have steeper gradients than the modem channel bed and do not merge with marine terraces at the river mouth consequently they cannot be used to determine altitudes of sea level high stands strath formation is a continuous process of response to long term uplift and its occurrence varies spatially along a river depending on stream power and hence position upstream strath terraces are found only along certain parts of a coastal stream upstream of the aggradational effects of oscillating sea level and far enough downstream that stream power is in excess of that needed to transport the prevailing sediment load for a given size fiver the greater the uplift rate the greater the rate of vertical incision and consequently the less the like lihood of strath terrace formation and preservation

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

are boundary conditions in surface productivity at the southern polar front reflected in benthic activity in austral summer 2012 during the expedition ant xxviii 3 on board rv polarstern two sites were sampled 1600 km apart in the south polar front area 52 s at the boundary of different productivity regimes for meio and macrobenthos using a multiple corer and an epibenthic sledge respectively patterns in density and abundance data were compared between different size classes of the benthos and interpreted in relation to surface primary productivity data and sediment oxygen consumption we tested the hypothesis that long term satellite derived surface phytoplankton biomass in situ real time biomass and productivity measurements at the surface and throughout the euphotic zone are reflected in abyssal benthos densities abundances and activity specifically we investigated the effect of boundary conditions for lower and higher surface productivity surface and integrated to 100 m depth biomass and primary productivity measurements vary stations with the lowest values at station 85 0 083 mg chl a m 3 at surface 9 mg chl a m 2 and 161 mg c m 2 d 1 integrated over the first 100 m depth and the highest values at station 86 2 231 mg chl a m 3 at surface 180 mg chl a m 2 and 2587 mg c m 2 d 1 integrated over first 100 m depth total meiofaunal densities varied between 102 and 335 individuals 10 cm densities were the highest at station 86 30 335 individuals and lowest at station 81 13 102 individuals total macrofaunal densities individuals 1000 m varied between 26 individuals at station 81 17 and 194 individuals at station 86 24 however three ebs hauls were taken at station 86 with a minimum of 80 and a maximum of 194 individuals sediment oxygen consumption did not vary significantly between stations from east to west bentho pelagic coupling of meio and macrobenthic communities could not be observed in the south polar front at the boundary conditions from low to high surface productivity between stations 81 and 86

the provenance of sediments in the barreme thrust top basin haute provence france abstract this paper records the results of an integrated analysis of heavy minerals sedimentary facies palaeocurrents and structural data the objective of the study was to evaluate the provenance of the sediments of the barreme thrust top basin this basin contains a late eocene to late oligocene succession of shallow marine alluvial and lacustrine sediments late eocene marine transgression resulted in starvation of clastic supply to the basin hence the calcaires nummulitiques received only a limited influx of heavy minerals derived from local sedimentary sources augmented by the first appearance of assemblages with andesitic affinities at c 42 41 ma clastic members associated with the overlying marnes bleues record a significant diversification of provenance and sediment transport paths clastic detritus supplied to the shallow marine gres de ville were derived from permo triassic sediments to the south conglomeratic submarine channels of the la poste member mark the introduction of alpine ophiolite blueschist derived minerals at c 37 35 ma eroded from the embrun ubaya thrust complex to the ne the deltaic st lions member is dominated by volcaniclastic assemblages completing the marine fill the shoreface gres de senez received detritus from the southerly maures esterel crystalline massif the succeeding oligocene continental sediments record an increasing supply of minerals from alpine lithologies the alluvial molasse rouge contains reworked sedimentary detritus including material cannibalized from the underlying marine tertiary and first cycle alpine minerals the succeeding lacustrine serie saumon serie grise show the increasing diversity of alpine suites the youngest sediments the fluvial gres verts signal a major influx of alpine derived assemblages related to reactivation of the embrun ubaye thrusts between c 28 27 and 25 24 ma

geometric adjustment of pools to changes in slope and discharge a flume experiment abstract to characterize the factors controlling pool shape 30 different forced pools were created utilizing a 50 triangular constriction in a 0 5 m wide 6 m long recirculating flume pools were scoured from an initial plane bed of sand with a d50 of 0 25 mm pool depth and length were measured and used as dependent variables in least squares multiple regression analyses discharge channel bed gradient and energy slope were the independent variables additional linear regression analyses were conducted with either pool depth or length and stream power results indicate that both pool depth and length are primarily a function of discharge channel bed and energy slopes are also significantly related to pool length but are not significantly related to pool depth stream power is significantly related to both pool depth and length but r2 values for pool depth versus discharge indicate stronger relations than those between pool depth and stream power observations on the type of geometric adjustment indicate that pools may minimize their rate of energy expenditure primarily through elongation in contrast pool depth appears to be more sensitive to the characteristics of the constrictions that create the forced pools the results suggest that many field studies may suffer from cross correlation problems in particular channel erodibility may exert a more dominant influence on pool geometry than hydraulic controls in many constriction influenced channels

selective gravel entrainment and the empirical evaluation of flow competence the concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported for the most part this evaluation has been empirical combining data from a number of separate flood events in different river systems those data are re examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes it is found that the competence relationships trend counter to those obtained for selective entrainment indicating that the competence evaluations are affected by varying degrees of selective size entrainment individual data sets which have been employed to establish the flow competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation instead being more compatible with the selective entrainment relationships in most instances the empirical competence equations greatly overestimate the hydraulics of flood flows and it is suggested that the better established selective entrainment equations be used for competence evaluations as well empirical equations are available for this purpose relating the dimensionless shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole

fluvial response a decade after wildfire in the northern yellowstone ecosystem a spatially explicit analysis forest fire is a vital ecological process capable of inducing complex fluvial response but the integration of these effects across entire watersheds remains poorly understood we collected downstream cross sectional and geomorphic data acquired geographic information on land cover and forest fire and performed spatially explicit statistical analyses to examine fire related impacts in catchments burned to varying degrees generalized least squares gls regression models suggested that channels with a greater percentage of burned drainage area were associated with markedly higher cross sectional stream power relatively smaller width depth ratios and lower bank failure rates 12 to 13 years after the fires these results implied that streams became more powerful in the aftermath of forest fire and that net incision had been the primary response in second to fourth order channels since the 1988 yellowstone fires the extensive geographic coverage of our data spanning multiple basins with measurements spaced every 100 m allowed us to hypothesize a process response model based on these results we suggest that a wave of fire related sediment propagates through burned catchments high runoff events or even moderate flows provide sufficient energy to evacuate the finer grained material delivered from burned hillslopes to the channel network over a period of 5 10 years the combination of elevated post fire discharges and decreased sediment supply then induces an episode of incision site specific channel changes are highly variable because streams can accommodate post fire increases in energy and sediment supply through multiple modes of adjustment characterizing the spatial distribution of stream power would provide a valuable management tool because this variable is strongly associated with percent burned drainage area and integrates several elements of complex fluvial response future research focused on the channel substrate and its evolution through time is needed but our results indicate a fundamental linkage between fire and fluvial processes d 2002 elsevier science b v all rights reserved

random controls on semi rhythmic spacing of pools and riffles in constriction dominated rivers average pool spacing between five and seven bankfull widths has been documented in environments throughout the world but has limited theoretical justification in coarse bedded and bedrock environments pool formation in coarse bedded and bedrock channels has been attributed to bedrock and boulder constrictions because the spacing of these constrictions may be irregular in nature it is difficult to reconcile pool formation processes with the supposedly rhythmic spacing of pools and riffles to address these issues a simulation model for pool and riffle formation is used to demonstrate that semi rhythmic spacing of pools with an approximate spacing of five to seven bankfull widths can be recreated from a random distribution of obstructions and minimum pool and riffle length criteria it is assumed that a pool riffle couplet will achieve a minimum length based on dominant discharge conditions values for the minimum length assumption are based on field data collected in new england and california while the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation results from the simulations show that the location of pools can be primarily random in character but still assume an average spacing between four and eight bankfull widths for a variety of conditions field verification data generally support the model but highlight a highly skewed distribution of pool forming elements and pool spacing the relation between pool spacing and bankfull widths is attributed to the common geometric response of these features to dominant discharge conditions copyright 2001 john wiley sons ltd

interpreting quaternary uplift rates at the mendocino triple junction northern california from uplifted marine terraces analysis of the altitudinal spacing of 14 flights of marine terraces indicates a spatial pattern of varying uplift rates that agrees with that determined from previously dated terraces for the past 3 81 ka and temporal changes in uplift rates from 1 m ka to 3 5 m ka that may reflect response to changes in tectonic regime during passage of the mendocino triple junction mtj a possible mechanism for regional uplift is growth of a slab window south of the mtj the region of most rapid uplift is 25 43 km south of the mtj immediately south of the northern boundary of the slab window the coastline is tilted upward to the south in the region directly above the southern edge of the subducted gorda plate at point delgada 55 km south of the present mtj where the northern edge of the slab window passes approximately 300 ka uplift rates have been 1 2 m ka for at least 330 ka more than 1 4 m y after passage of the southern edge of the subducted slab at the mendocino coast uplift rates have been less than 0 4 m ka for at least 330 ka

fluvial geomorphology of a boulder bed debris flow dominated channel in an active tectonic environment abstract numerous streams with a drainage area of a few square kilometers draining the south flank of the santa ynez range in southern california are characterized by steep boulder bed debris flow dominated channels rattlesnake creek is a good example approximately 90 of the stored sediment in the stream valley is stable debris flow deposits found in debris flow terraces and levees adjacent to the channel the remaining 10 of stored sediment resides in the active channel where the channel morphology is dominated by large boulders 1 to 4 m in diameter derived from debris flows or rock fall that along with bedrock outcrops form rock or boulder transverse ribs with frequent step pools the dominant channel pool morphology is the step pool which accounts for most of the pools and approximately 30 of relief drop in elevation through the study reaches less common pool types identified in the study reaches include forced pools that result from convergence of flow with scour between lbres large boulder roughness elements and bedrock outcrop along the channel at near bankfull discharge the lbres and boulders in step pools armor the channel and are primary controls on channel morphology and gradient rock strength is a secondary control on channel morphology location and abundance of pools we hypothesize that step pools when inundated with sand and fine gravel from a disturbance such as wildfire during a flow event fill near bankfull stage and scour on falling stage below bankfull when unit stream power is a maximum over the step thus as with forced pools step pool maintenance depends on a specific pattern of discharge and process the process of scour during falling stage reestablishes step pools during moderate flows following disturbance

riffle pool sequences and meander morphology abstract the study is aimed at investigating the response of the riffle pool sequence to increased energy expenditure in the horizontal plane through meander development it emphasizes field measurements of riffle pool height and spacing and their links with meander morphology river reaches in southern ontario canada were surveyed and statistical relations were established between parameters describing riffle pool morphology sequence length and amplitude and planform curvature the results indicate a lengthening of the sequence with increased total angular deflection along the measured reaches further analysis suggests that this lengthening occurs in the pool as a result of meander development apparently as the path length extends beyond a critical threshold for a given bed material size the transport capacity of the sequence is compromised this may promote an initial shortening of the downstream riffle with depositional processes ultimately forming a new riffle slightly upstream to achieve a new equilibrium additional extensive data sets provided by the toronto and region conservation authority on riffle pool morphology also suggest that bed material size plays a significant role in controlling riffle pool morphology

bar resistance in gravel bed streams at bankfull stage total flow resistance in gravel bed streams with high width depth ratios and low sinuosities is primarily a result of grain and bar roughnesses the relative importance of these two roughness elements has not been established the relative importance of grain and bar resistance is examined for 12 straight and divided gravel bed reaches at bankfull stage by using the keulegan equation to determine a resistance division of energy slope the remaining slope is assumed to represent the relative importance of bar resistance bar resistance accounts for 50 to 75 of the total resistance in these reaches calculated values of reach averaged bar slope correlate closely with field measurements of bar magnitude

thresholds in small rivers hypotheses developed from fluvial morphological research in western germany the objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics in north rhine westphalia germany based on a statistical research methodology such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised ruhr area following the approach of some classic empirical works in fluvial morphology of the last century several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables regressions between valley floor slope bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results the regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers if the specific properties and values of the underlying random sampling are taken into account in addition the relation between stream power and sinuosity shows the likely existence of a threshold exceeding a stream power of 100 w m the sinuosity decreases after an increase for lower stream power values comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness the thresholds could be explained by a change in the type of energy dissipation due to different physio geographical settings in highland rivers within forested v shaped valleys here large woody debris seems to increase the channel roughness and possibly replaces the significance of coarse grained bed material pool depth and step steepness as contributors to energy dissipation

the challenge of modeling pool riffle morphologies in channels with different densities of large woody debris and boulders studies on pool morphologies include reports of over 80 or 90 of pools being associated with structural controls and large obstructions that include boulders bedrock outcrops and large woody debris lwd a monte carlo simulation approach and developmental computer model was created to predict pool formation spacing and the percentage length covered by pools riffles scour holes and runs based on input data that include channel slope width the number of small and large boulders and the number of 10 30 cm 30 60 cm and 60 cm pieces of wood the statistical empirical model is founded on the idea that boulders bedrock outcrops and large woody debris provide a physical framework that then controls local water surface slopes velocity patterns and the locations of pools and riffles the spacing values of individual types and sizes of obstructions are modeled as log normal distributions with separate distributions for each obstruction type pools are assigned different probabilities of development depending on the obstruction type pool and riffle lengths used to create the subsequent morphology follow their own slope dependent log normal trends a minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation the total number and spacing of pools riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool riffle couplet the simulation model accurately captures the number of pools in the modeled data range at 65 of all the verification field sites and 86 of the verification field sites with a more limited range of width and slope characteristics lower levels of prediction capabilities are associated with modeled numbers of scour holes and log jams the model accurately mimics some statistical attributes of pool spacing and future versions of the model could be developed to improve overall predictive capabilities copyright 2011 john wiley sons ltd

the effect of wet dry weathering on the rate of bedrock river channel erosion by saltating gravel abstract previous work has shown that the bedrock erosion rate e because of collisions of saltating bedload can be expressed by e qb 1 pc where qb is the sediment transport rate pc is the extent of alluvial cover and is the abrasion coefficient however the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known and in particular the effects of wet dry weathering on the saltation abrasion bedrock incision has not been specifically characterized observation suggests that the typical wet dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment in this study flume experiments are performed to develop an understanding of how wet dry cycling affects the rock tensile strength and the bedrock erosion rate to represent the physical effects of weathering boring cores taken from natural bedrock channel are exposed to artificial wet dry cycles the experimental results suggest the following 1 the abrasion coefficient for fresh bedrock is estimated by 1 0 10 4 t 2 d ksb 0 5 where t is the tensile strength d is the diameter of colliding gravel and ksb is the hydraulic roughness height of bedrock 2 the tensile strength of the bedrock decreases exponentially as a result of repeated wet dry cycles t t0 exp ctnwa0 t0 where t0 is the initial tensile strength wa0 is the initial normalized rate of water absorption n is the number of wet dry cycles and ct is a constant 3 the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength but the erosion rate of weathered bedrock depends on the 1 5 power of tensile strength

synthetic river valleys creating prescribed topography for form process inquiry and river rehabilitation design abstract the synthesis of artificial landforms is complementary to geomorphic analysis because it affords a reflection on both the characteristics and intrinsic formative processes of real world conditions moreover the applied terminus of geomorphic theory is commonly manifested in the engineering and rehabilitation of riverine landforms where the goal is to create specific processes associated with specific morphology to date the synthesis of river topography has been explored outside of geomorphology through artistic renderings computer science applications and river rehabilitation design while within geomorphology it has been explored using morphodynamic modeling such as one dimensional simulation of river reach profiles two dimensional simulation of river networks and three dimensional simulation of subreach scale river morphology to date no approach allows geomorphologists engineers or river rehabilitation practitioners to create landforms of prescribed conditions in this paper a method for creating topography of synthetic river valleys is introduced that utilizes a theoretical framework that draws from fluvial geomorphology computer science and geometric modeling such a method would be valuable to geomorphologists in understanding form process linkages as well as to engineers and river rehabilitation practitioners in developing design surfaces that can be rapidly iterated the method introduced herein relies on the discretization of river valley topography into geometric elements associated with overlapping and orthogonal two dimensional planes such as the planform profile and cross section that are represented by mathematical functions termed geometric element equations topographic surfaces can be parameterized independently or dependently using a geomorphic covariance structure between the spatial series of geometric element equations to illustrate the approach and overall model flexibility examples are provided that are associated with mountain lowland and hybrid synthetic river valleys to conclude recommended advances such as multithread channels are discussed along with potential applications

relationships between land use and forced pool characteristics in the colorado front range abstract land use practices in colorado during the last two centuries altered the supply of sediment and water to many channels in the upper south platte basin as a result of increased supply of sediment and mobility and reduced peak flows the characteristics of pools associated with channel constrictions referred to as forced pools may have been altered increased supply of sediment and reduced transport capacity of high flows could lead to aggradation in forced pools channel confined by road corridors could lead to high velocities at normal flows increased energy dissipation from riprap or even increased pool frequency resulting from failed riprap to assess potential alterations four hypotheses were tested 1 impacted streams will show significantly different mean volume of pools than a control stream 2 mean volume of pools on streams where land use activities increased the supply of sediment will be significantly different from streams solely affected by flow regulation 3 the strongest change in characteristics of pools of impacted streams will be a reduced volume of pools 4 streams affected by road corridors will show statistically lower spacing of pools than streams unaffected by roads the downstream spacing and residual volume of twenty consecutive forced pools were surveyed on five streams in the colorado front range that varied from no contemporary impact to multiple historical and contemporary impacts ancova with stepwise model selection indicated that degree of land use categorical bankfull spacing of pools upstream riffle slope and expansion ratio were all significant 0 1 predictors of volume of pools r 2 0 73 simple linear regression of mean volume of pools and stream specific variables gradient drainage area and discharge and least square means comparison of mean volume of pools indicated a need to standardize volumes of pools by slope and discharge so that the volumes of pools could be compared among different levels of land use significant correlations between drainage area and volume of pools allowed volume of pools to be standardized by drainage area and thereby discharge this dimensionless variable was also significantly correlated with channel slope which permitted the construction of a new variable pv qs volume of pools standardized by discharge and slope least square means comparison of mean pv qs revealed that the control reach was significantly different from road impacted reaches mean volume of pools was significantly larger in the control reach compared to all but one road impacted stream this was likely a function of higher wood loading in the control reach and the competence of high flows in the road impacted reach streams affected by road corridors did not have significantly different bankfull spacing of pools from streams not impacted by roads the multiple interactions among control and response variables explored in this study indicate the need to identify the most constrained and sensitive response variables when attempting to assess channel response to land use

effects of aggradation and degradation on riffle pool morphology in natural gravel channels northwestern california after the flood of december 1964 12 gaging sections in northern california widened as much as 100 and aggraded as much as 4 m and then degraded to stable levels during a period of 5 years or more as channels aggraded bed material became finer and low to moderate flow through gaging sections in pools became shallower faster and steeper comparisons of longitudinal profiles also show the diminishment of pools as well as a decrease in bar relief accompanying the excessive sediment load as gaging sections degraded hydraulic geometries recovered to a limited degree full recovery probably depends on channel narrowing and further depletion of sediment supply the hydraulic changes with aggradation indicate an increase in the effectiveness of moderate discharges less than i to 2 year recurrence interval annual flood series to transport bed load and shape the bed bars become smaller pools preferentially fill and rimes armored with relatively small gravel tend to erode headward during falling stages and form a gentler gradient excess sediment can thus be more readily transported out of channels when additional contributions from watersheds are usuall slight

geomorphological explanation of the long profile of the lower mississippi river abstract concavity in the long profile of rivers has traditionally been explained through the concept of grade in which the slope declines downstream as a response to changing discharge bed material size and sediment transport applying this concept to particular river systems has however proved problematic the long profile reflects spatially distributed form process feedbacks between all aspects of channel morphology operating at a range of poorly defined time and space scales and in the presence of natural controls in many river systems process form dynamics are further complicated by engineering interventions which add additional extrinsic controls and constrain the range of intrinsic dynamics in this paper the 1974 75 long profile of the lower mississippi river is examined at three scales the regional the reach and the sub reach pool crossing scales a combination of curve fitting zonation algorithms and empirical classification techniques are used to show that although the long profile of the lower mississippi river is concave at the largest scale the profile is characterised by discontinuities shorter trends and zonal variations in the amplitude and wavelength of pool crossing morphology these characteristics are a response to morphological and bed material changes relating to a range of physical geological tectonic tributary input and engineering controls despite its apparent simplicity and correspondence to a graded condition the long profile of the lower mississippi river is actually a complex and scale dependent morphological property at best the concave river profile is therefore a property which emerges from several scales of process form interaction at worst it is no more than an artefact arising from the application of over simplified curve fitting techniques disclosure of the nature of the long profile thus requires the application of a variety of analytical techniques as well as geomorphological explanations which are themselves scale dependent and which consider the interaction of natural processes and the history of engineering intervention

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

fluvial erosion of physically modeled abrasion dominated slot canyons abstract abrasion dominated fluvial erosion generates slot canyons in massive bedrock with intricately undulating walls flows in slot canyons are unusual in that the walls comprise a significant portion of the wetted perimeter of the flow during geomorphically effective floods in wire pass utah the upper paria river incises through massive crossbedded navajo sandstone incision in wire pass and related slots occurs only during flash floods paleoflood debris indicates that the width depth ratios of these flows are at times as low as 1 1 submeter resolution field mapping of a 20 m length of wire pass shows that the wall morphology is a complicated combination of in phase meander like and out of phase pinch and swell undulations in order to investigate evolution of slot canyons and the influence of their wall shapes on flow dynamics we recorded the evolution of four distinct canyon wall morphologies in a 2 4 m flume box at the st anthony falls laboratory in a substrate consisting of 3 2 mixtures of f110 sand and plaster of paris we molded canyons with in phase and out of phase undulations and wide 6 5 cm and narrow 4 cm straight initial wall profiles discharges ranged from 1 4 l s to 2 9 l s and wall and bed morphology were measured at 5 h intervals at 0 5 cm resolution results show efficient back eddy erosion in the undulating canyon walls and related erosional bedforms in all channels created by vortices in the flow maximum filaments of velocity are depressed and asymmetric and the implied shear stress distribution varied in space and time on the channel beds flow width depth ratios strongly influence the flow structure and distribution of shear stress in a slot and appear to be a factor in dictating whether a bedrock channel widens its walls or incises its bed

pool fills a window to palaeoflood history and response in bedrock confined rivers channel scale sedimentary units associated with bedrock controlled riffle pool morphology are examined in detail along sandy creek gorge an ephemeral stream in arid south eastern central australia pool fills comprise cut and fill assemblages of poorly sorted sediments ranging in texture from muds to boulders five unit types are defined based on particle size sedimentary structures geometry and bounding surface character 1 coarse grained bar platform 2 fine grained bar supraplatform 3 fine grained pool fill 4 fine grained bench and 5 modern pool fill the last coarse grained unit currently lining the pools suggests an altered sedimentation style over the post settlement period post ad 1860s situated at bedrock valley constrictions pool fills are compared with other sedimentary units associated with recirculating currents eddy bars and slackwater deposits but only the fine grained bench units reflect eddy recirculation the pool fills are principally forced bars associated with bedrock controlled or forced riffle pool morphology a late holocene palaeoflood history is proposed based on radiocarbon ages from the pool fills multiple phases of cut and fill activity were preceded by a superflood 3400 1900 years ago that eroded the pool fills to bedrock the resilience of the pool fills was illustrated by the passage of a 1 in 100 year flood in 1992 which caused only minor erosion the presence of pool fills may provide a window to past phases of river activity that cannot be extracted from either historical records observations or palaeoflood slackwater sediment analyses the formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences but the occurrence of high magnitude floods exerts the dominant control

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern