spatial organization in the step pool structure of a steep mountain stream vogelbach switzerland 1 spatial organization in the step pool structure of a 1 58 km long section of the vogelbach a steep mountain stream average channel bed slope 0 17 m m in the alpthal basin switzerland is analyzed to evaluate the extent of randomness in the occurrence of steps and to isolate the potential hydraulic and topographic controls on step placement statistics of the observed step sequence are compared with those of random sequences generated by permutation from the observed step height distribution hydraulic and topographic controls on step placement are analyzed from step length and steepness distributions and relations between mean step properties and step height and channel gradient for both observed and random sequences results show that 1 observed step length distributions are statistically significantly different from randomly generated sequences 2 step steepness is significantly different in observed data because of a positive correlation between mean step length and height and it remains fairly constant for all step sizes 3 spatial organization in steps does not extend far beyond the nearest step and 4 the influence of channel gradient on step properties is insignificant and highly variable indicating that hydraulic rather than topographic controls are dominant for step placement in this stream although the vogelbach is a steep stream on the boundary between a cascading and step pool morphology where we would expect randomness to dominate spatial organization was nevertheless detected in many important aspects of the step pool geometry

high grade metamorphism and partial melting of basic and intermediate rocks rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust however the former lack of appropriate thermodynamic models for silicate melt amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite and granulite facies metamorphism in this work we present phase diagrams calculated in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o2 nckfmashto compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2 12 kbar and 600 1050 c using newly parameterized activity composition relationships detailed in a companion paper by green et al in this issue particular attention is given to mid ocean ridge basalt morb and diorite protolith bulk compositions calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase with variable proportions of epidote clinopyroxene garnet biotite muscovite quartz titanite or ilmenite present at different pressures the h2o saturated wet solidus has a negative p t slope and occurs between 620 690 c at mid to lower crustal pressures of 5 10 kbar the lowest t melts generated close to the wet solidus are calculated to have granitic major element oxide compositions melting at higher temperature is attributed primarily to multivariate hydrate breakdown reactions involving biotite and or hornblende partial melt compositions calculated at 800 1050 c for morb show good correlation with analysed compositions of experimental glasses produced via hydrate breakdown melting of natural and synthetic basic protoliths with niggli norms indicating that they would crystallize to trondhjemite or tonalite diorite is shown to be significantly more fertile than morb and is calculated to produce high t melts 800 c of granodioritic composition subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family although the p t conditions at which orthopyroxene stabilizes thus defining the prograde amphibolite granulite transition is strongly dependent on bulk rock oxidation state and water content the petrological effects of open and closed system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case study analysis of a natural archean amphibolite from the lewisian complex northwest scotland

provenance and tectonic setting of the paleoproterozoic metasedimentary rocks from the liaohe group jiao liao ji belt north china craton insights from detrital zircon u pb geochronology whole rock sm nd isotopes and geochemistry abstract geochronological geochemical and isotopic studies were carried out on the paleoproterozoic metasedimentary rocks from the jiao liao ji belt north china craton to constrain their provenance and tectonic setting as well as to evaluate the effects of weathering and sedimentary processes on the source rock signature u pb isotopic dating using the la icp ms method on zircons from the paleoproterozoic metasedimentary rocks reveals that the depositional age can be constrained in the period between 1 98 and 1 9 ga geochemical data reveal low intermediate degrees of weathering of the source compositionally immature and poorly sorted detailed analysis of detrital zircon u pb geochronology whole rock sm nd isotope and geochemistry show that these metasedimentary rocks are derived from a mixed provenance the predominant derivation is from metavolcanic rocks of the liaohe group and paleoproterozoic gneissic granitoids there is also an input of archean continental crust the metasedimentary rocks were probably deposited in a back arc basin within an arc system thus supporting the paleoproterozoic arc continent collision model as already proposed by some authors

unravelling the development of a spheroidally weathered diorite gabbro santa margarita ecological reserve peninsular ranges southern california usa abstract within santa margarita ecological reserve smer southern california an 108 ma diorite gabbro pluton has weathered spheroidally resulting in the formation of concentric rindlets i e a rindlet zone around an unaltered ellipsoidal core of the parent rock to our knowledge little is known about how spheroidal weathering is produced in mediterranean climates like that characteristic of portions of the peninsular ranges hence we undertook a detailed study aimed at determining the textural and chemical changes associated with the spheroidal weathering of the diorite gabbro pluton the parental spheroidally weathered corestone is characterized by a hypidomorphic granular texture and consists primarily of plagioclase amphibole and biotite along with minor amounts of pyroxene and quartz within the corestone sericite and uncommonly calcite and chlorite replace plagioclase the latter mineral also infrequently replaces portions of amphibole and biotite such alteration products are derived from sub solidus deuteric alteration of the pluton xrd clay mineral analysis of the 13 rindlets indicates the presence of mostly kaolinite and vermiculite and a far lesser volume of green smectite thin section study of rindlet samples suggests that vermiculite is derived from the weathering of biotite the most extensively altered mineral in the rindlet zone such alteration is paralleled by statistically significant losses of k mass across the rindlet zone in addition statistically significant losses of ca and na mass across the rindlet zone likely reflect conversion of sericite to kaolinite in contrast the absence of statistically significant losses of mg fe and mn over most of the rindlet zone implies that fluids were mostly oxidizing and that any mg leached from amphibole was likely fixed within smectite while leached fe and mn precipitated out as oxides or oxyhydroxides though calcite was present in the corestone its absence in the rindlets indicates that fluids were sufficiently acidic to dissolve and remove it previous studies have shown that spheroidal weathering occurs when volume expansion produced by a positive v of reaction builds up internal elastic strain energy in the rock for example this type of reaction occurs when iron oxidizes within biotite resulting in an expansion of d 001 from 10 a to 10 5 a or when biotite is transformed into vermiculite leading to an expansion of d 001 from 10 a to 14 a data presented here suggest that the conversion of biotite to vermiculite and the resulting positive v produced the spheroidal fracturing of the corestone at smer hence these and other data discussed in this paper suggest that the weathering of biotite is the primary driving force in the formation of spheroidally weathered corestone in the mediterranean climate of the peninsular ranges

on the association between veining and index mineral distributions in barrow s metamorphic zones glen esk scotland the concept of index mineral based metamorphic zones was first introduced by george barrow in 1912 and the barrovian metamorphic zones continue to be used as a framework for describing regional met

analysis of changes in the step pool morphology and channel profile of a steep mountain stream following a large flood abstract the step pool morphology of steep mountain streams is reorganized during large floods by the removal of steps scouring of the channel bed and the formation of a new step pool sequence by the re deposition of step forming grains in this paper we document these changes caused by the largest recorded flood return period 50 years in the steep mountain stream erlenbach in central switzerland in 2007 by comparing pre flood and post flood surveys of a 650 m long study reach we show that the flood led to statistically significant changes in the distributions of step height and spacing most notably an increase in the variance of both variables organization at the river reach scale is not apparent and the correlations between step properties both pre and post flood are generally weak the flood led to widespread erosion of the river bed locally up to 2 3 m of vertical channel change was observed at least 60 of the steps moved even the largest ones however they remained constrained to the parts of the study reach where landslides on the adjacent hillslopes are most active detailed changes in bed structure were analyzed from the pre and post flood profiles with a statistical scaling approach which identifies the complexity of the spatial organization of sediment in the longitudinal profile multiscaling found in both step and pool sequences reflects the complex spatial structure which is the result of the superposition of steps formed by the history of flood events

widespread fluid infiltration during metamorphism of the witwatersrand goldfields generation of chloritoid and pyrophyllite chloritoid and pyrophyllite occur together in all major goldfields of the witwatersrand basin and are widespread in virtually all rock types of the upper witwatersrand supergroup including metaconglomeratic reefs and altered mafic rocks both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages but they are also developed locally in the regionally persistent metapelites that have basin wide extent pyrophyllite is particularly common in foliated zones adjacent to quartz veins and near unconformably overlying auriferous conglomerates the wide distribution of chloritoid and pyrophyllite in metapelites of the witwatersrand basin is attributed to alteration of chlorite rich shales rather than to unusual premetamorphic starting materials this alteration event involved the redistribution of many elements with up to 40 volume loss mainly due to removal of silica removal of most of the mg and some fe accounts for the stabilization of chloritoid and pyrophyllite relatively immobile elements included al ti nb cr v p la and ce whereas si fe mn zn co ni cu mg and ca were lost and k rb and ba were introduced by an infiltrating fluid the alteration event is inferred to have been within the chloritoid and pyrophyllite stability field and thus syn metamorphic as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite rather than to lower grade minerals such as kaolinite muscovite chlorite chloritoid and muscovite chloritoid pyrophyllite assemblages are attributed to fluid buffering along appropriate curves as their production by metamorphism of lower grade mineral mixes is considered unlikely based on the present bulk rock compositional data a metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration the transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite quartz veining and textures fluid rock ratios calculated from a silica budget in one metapelitic horizon exceed 100 1 over many square kilometres these values need not imply multi pass fluid flow as much of the silica migration may be redistribution on a scale of a few metres from source rocks into veins although infiltration during metamorphism may have affected much of the upper witwatersrand succession channelized fluid flow within reef packages along faults and unconformities and in certain metaconglomerates and metapelites is inferred

bedload entrainment travel lengths step lengths rest periods studied with passive iron magnetic and active radio tracer techniques this paper reports results from bedload transport investigations with active radio and passive iron magnetic tracers in the lainbach a step pool mountain river in bavaria southern germany the spatial distributions of the iron tracers after flood events can be best described by exponential or gamma distributions there is some indication of a tendency of size selective transport of the iron tracers but there is also a considerable amount of scatter in the correlations between weight size and travel length owing to the masking influence of other variables such as the shape of the particles and different positions in the river bed the experiments with artificial magnetic tracers showed that elongated pebbles rods had the longest mean transport distance platy ones discs remained relatively close to the starting points the particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded the pebble transmitter system petsy consists of transmitters implanted into individual pebbles a computerized receiver a stationary antenna system with an antenna switchboard and a data logging system the particles do not move continuously but in a series of transport steps and non movement intervals a single value for a given size class is not adequate to describe the critical conditions of entrainment under natural circumstances a probability approach is much more suitable the critical unit discharges total discharge divided by active channel width along the measuring reach are dependent on river bed morphology in the steps bedload needs higher unit discharges to be entrained once entrained the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods the distributions of both parameters can be approximated by exponential functions applying the stochastic concept proposed by einstein the mean values of the random variables step length and duration of rest period measured with the petsy technique were used for the simulation of spatial distributions of bedload particles from point sources more field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

the bed morphology of upland single thread channels in semi arid environments evidence of repeating bedforms and their wider implications for gravel bed rivers single thread gravel bed streams of moderate slope in the northern negev are characterized by three channel units bars exhibit steeper than average slopes and poorly sorted mixtures of small medium cobbles and coarse very coarse pebbles flats are associated with more gentle slopes and well sorted medium fine pebbles and granules and transitional units have intermediate slopes and grain size in general all three units are planar span the full channel width and have well defined boundaries bars and flats are more common than the transitional units and alternate downstream for distances of several hundred metres forming sequences that are reminiscent of the riffle pool structure commonly observed in humid temperate gravel bed rivers a notable contrast is the absence of significant bed relief bars lack crests and flats lack depressions the relative lack of bed relief in bar flat sequences is attributed to the high rate of sediment supply from the sparsely vegetated hillslopes which promotes the infilling of depressions and to the erosion of crests under conditions of intense transport this reduction of bed relief lowers channel roughness which in turn increases flow velocity and therefore the ability of the channel to transmit the large sediment loads it receives although our analyses pertain to a semi arid river system the results have wider implications for understanding the adjustment of channel bedform to high sediment loads in other fluvial environments copyright 2012 john wiley sons ltd

the significance of fixed ammonium in palaeozoic sediments for the generation of nitrogen rich natural gases in the north german basin in contrast to predominantly hydrocarbon rich natural gases in the western part of the central european basin ceb accumulations of natural gases from the eastern part of the north german basin ngb are nitrogen rich with up to 90 n2 this study is focused on the behaviour of fixed ammonium in clay minerals of organic rich palaeozoic sediments in the eastern part of the ngb as a major source of nitrogen rich natural gases carboniferous shales have been investigated for a better understanding of nitrogen fixing during diagenesis storage during burial and release during devolatilization processes or fluid rock interactions the total nitrogen contents in the studied carboniferous shales of the ngb reach up to 2700 ppm with an inorganic fixed portion in the form of nh4 n of more than 60 the results of this study indicate an increasing proportion of the mineralogically fixed ammonium with increasing thermal maturity and storage up to catagenetic conditions the isotopic composition of fixed nh4 is relatively homogeneous in the majority of the shales and ranges from 1 to 3 5 in contrast samples from the basin centre show a significant decrease in ammonium contents down to 460 ppm coupled with a shift in 15n up to 5 6 suggesting a release of nitrogen on a large scale calculation of nitrogen loss and isotopic fractionation indicate that more than 30 of nitrogen was released as ammonium probably as a consequence of fluid rock interaction with highly saline brines

geochemistry and nd isotope signature of the collserola range palaeozoic succesion ne iberia gondwana heritage and pre mesozoic geodynamic evolution the collserola range includes a representative stratigraphic succession of the palaeozoic of the central part of the catalan coastal ranges ranging from cambro ordovician to carboniferous times in this paper we present an up to date review of the stratigraphy and structure of the palaeozoic of the collserola range and provide geochemical and sm nd isotope data to constrain the pre mesozoic crustal evolution of this sector of the variscan belt geochemical compositions indicate that the palaeozoic siliciclastic rocks of the collserola range were fed by a relatively mature heterogeneous source of sediment comprising quartz rich sediments to intermediate igneous rocks the siliciclastic rocks of the collserola range show great geochemical affinity with the turbidites of passive margins the sm nd signature of the siliciclastic rocks is compatible with those of the palaeozoic and late proterozoic fine grained siliciclastic rocks of the neighbouring terrains of sw europe there is a small decrease of the end t with decreasing age of sedimentation from the cambro ordovician to the carboniferous suggesting an increase of the amount of more juvenile material the presence of small volumes of alkaline basaltic rocks provides evidence for the input of juvenile material in the early palaeozoic basin and suggests that an extensional tectonic regime prevailed during the cambro ordovician sedimentation from a geodynamic point of view the overall analysis of the data implies that the palaeozoic rocks of the catalan coastal ranges were part of the northern gondwana passive margin before the closure of the rheic ocean and the subsequent variscan orogeny

nitrogen isotope fractionations during progressive metamorphism a case study from the paleozoic cooma metasedimentary complex southeastern australia abstract the well studied paleozoic cooma metamorphic complex in southeastern australia is characterized by a uniform siliciclastic protolith of uniform age with a continuous range of metamorphic grade from subgreenschist to upper amphibolite facies and migmatite grade in an annular pattern around the cooma granodiorite those conditions are optimal for investigating variations of n concentrations and 15 n values during progressive metamorphism nitrogen concentrations decrease and 15 n increases with increasing metamorphic grade sub chlorite zone 120 ppm n 15 n 2 3 chlorite zone 110 ppm n 15 n 3 0 biotite and andalusite zone 85 ppm n 15 n 3 8 sillimanite and migmatite zones 40 ppm n 15 n 10 7 covariation of k and n contents is consistent with n substituting for k as nh 4 in micas observed trends of increasing 15 n values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in 15 n with progressive metamorphism which causes an enrichment of 15 n in the residual nitrogen of the rock equilibrium models for rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by n 2 nh 4 exchange at temperatures of 300 600 c whereas observed large fractionations for the upper amphibolite facies and melt products in the migmatite grade samples may be interpreted as nh 3 nh 4 exchanges at temperature of 650 730 c lower values in the highest grade zones may also stem in part from input of 15 n depleted fluids from the granodiorite the magnitude of isotope fractionation of nitrogen is about 1 2 during progressive metamorphism of metasedimentary rocks from sub chlorite zone to biotite andalusite zone which is consistent with previous studies consequently the large spread of 15 n values in archean greenschist facies metasedimentary rocks of 6 to 30 can be accounted for by variable mixtures of mantle plume dominated volatiles with a 15 n of 5 and a 15 n enriched marine sedimentary kerogen component inherited from a ci chondrite veneer having 15 n of 30 to 42

morphology of step pools in a wilderness headwater stream the importance of standardizing geomorphic measurements the morphology of step pools and cascades reflect the geological and climatic factors affecting channels in mountain watersheds this study uses longitudinal and cross section surveys to describe a headwater stream in the boston mountains of the ozarks plateau region in arkansas and develop morphological relationships for comparisons with other regions in the bowers hollow creek watershed 3 5 km 2 located within the boundaries of the upper buffalo wilderness area step height and wavelength relationships are generally similar to those reported from other regions step pool reaches were widely distributed in a discontinuous manner throughout the watershed average values of the sampled reaches are reach slopes 0 105 m m width 6 10 m crest particle sizes 440 mm step heights 0 87 m and step wavelengths 6 62 m the mean step steepness for the watershed was 0 13 whereas the mean length of a reach step to height ratio was 9 1 a comparison of morphological definitions found that the values of step height and steepness can vary by 30 according to how step parameters are defined step height is particularly sensitive thus comparison of step height based relationships from published data requires great care

surface sampling in gravel streams the accuracy of studies concerning the characteristics of gravel bed waterways is often dependent upon the techniques used to sample and quantify the material found on the channel boundary due to the vertical stratification present in the riverbed the use of sampling techniques that remove only the particles within a thin surface layer is often necessary attributes of clay and grid sampling are considered in the present work criteria for determining the minimum sample size for a desirable level of accuracy are also presented many of the current surface sampling techniques truncate either the upper or lower size range of particles truncation of part of a size distribution not only limits the available information but can also bias the rest of the distribution the present work rectifies this problem by proposing the use of a hybrid technique that can sample the entire size range of either dry or submerged bed material results of field tests that utilize this new method are described in detail

characteristics of a debris flow drainage channel with a step pool configuration abstracttheoretical analysis and experimental testing of a debris flow drainage channel with a step pool configuration are conducted in this paper the design conditions of the drainage channel wit

step pool and cascade morphology mosquito creek british columbia a test of four analytical techniques the identification and geometric definition of individual cascade and step pool bedforms are investigated in a steep coarse grained mountain stream mosquito creek by testing four analytical techniques visual identification zero crossing bedform differencing and power spectral analysis the test is the first use of these techniques in a headwater stream and the analysis of two bed profiles showed that visual identification was able to i identify ii determine the geometry of and iii classify the type of individual bedforms better than the other methods the other techniques were not able to differentiate step pools from cascades and the large range of grain sizes and bedform heights hampered their ability to consistently identify stepped bedforms the step pool pronounced channel spanning steps that alternate with channel spanning pools and cascade multi tiered partially channel spanning structures morphology in mosquito creek has formed in the last 20 years as fluvial action has restructured its previously engineered revetment lined planar bed the channel bed exhibits a morphologic regularity that power spectral analysis captured as periodic fluctuations in the bed profiles with mean wavelengths slightly greater than those identified by the other methods further the active reorganization of revetment has formed stepped structures with geometries similar i e height to wavelength ratios to stepped features found in natural mountain streams channel slope partially controlled bedform geometry wavelength and height and bedform height weakly controlled individual step spacing but there was no relation between wavelength and grain size d 90

behaviour of apatite during partial melting of metapelites and consequences for prograde suprasolidus monazite growth abstract the suprasolidus behaviour of apatite and monazite is examined for an average metapelite composition using phase equilibria modelling coupled with solubility equations of these minerals both closed and open system scenarios are considered partial melting above the solidus requires apatite and monazite breakdown in order to saturate the anatectic melt in phosphorus and the light rare earth elements in general melt loss is predicted to increase the stability of apatite and monazite at high temperature most apatite is predicted to survive up to ultrahigh temperature conditions except for rocks with low bulk phosphorus concentrations by contrast most monazite is expected to be consumed by uht conditions thorium substitution in monazite is expected to increase the stability of monazite to higher temperatures the presence of lree rich apatite decreases the stability of monazite above the solidus but the breakdown of this apatite during anatexis may generate prograde monazite at the apatite melt interface in local pockets of melt oversaturation however prograde suprasolidus monazite along grain boundaries is expected to be consumed during further partial melting or during melt homogenization when an interconnected melt network develops anatectic melts are predicted to be saturated with respect to apatite except at uht conditions and for rocks with low initial p2o5 bulk concentrations

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

identification of steps and pools from stream longitudinal profile data abstract field research on step pool channels has largely focused on the dimensions and sequence of steps and pools and how these features vary with slope grain sizes and other governing variables measurements by different investigators are frequently compared yet no means to identify steps and pools objectively have been used automated surveying instruments record the morphology of streams in unprecedented detail making it possible to objectively identify steps and pools provided an appropriate classification procedure can be developed to achieve objective identification of steps and pools from long profile survey data we applied a series of scale free geometric rules that include minimum step length 2 25 of bankfull width w b minimum pool length 10 of w b minimum residual depth 0 23 of w b minimum drop height 3 3 of w b and minimum step slope 10 greater than the mean slope the rules perform as well as the mean response of 11 step pool researchers who were asked to classify four long profiles and the results correspond well with the channel morphologies identified during the field surveys from which the long profiles were generated the method outperforms four other techniques that have been proposed sensitivity analysis shows that the method is most sensitive to the choice of minimum pool length and minimum drop height detailed bed scans of a step pool channel created in a flume reveal that a single long profile with a fixed sampling interval poorly characterizes the steps and pools five or more long profiles spread across the channel are required if a fixed sampling interval is used and the data suggest that survey points should be located more frequently than the diameter of the step forming material a single long profile collected by a surveyor who chooses breaks in slope and representative survey points was found to adequately characterize the mean bed profile

a field experiment on the development of sedimentary structures in a gravel bed river in gravel bed rivers bed sedimentary structures exert a significant influence on the stability of the bed particle motion and stream dynamics a detailed description of the development of sedimentary structures is essential to define the overall activity of sediment transport this paper aims to improve our understanding of the dynamics of sedimentary structures at the reach scale using a field approach the objectives of the study are 1 to describe the processes of sedimentary structures development from a manipulated bed surface and 2 to show the effect of keystones on the development of the sedimentary structures by monitoring the displacement of individual tagged particles we investigated the dynamics of sedimentary structures in three 15 m long reaches of a small gravel bed river characterized by moderate slope and high relative roughness the bed material is composed of a range of gravel sizes with sparse boulders that remain immobile during most flows we have disrupted the bed surface material of the first reach to create a disorganized bed surface two reaches where the bed material remained undisturbed were used as control sections detailed topographic maps and passive transponders were used to describe the formation processes of sedimentary structures the initial manipulations of the bed led to a more fragmented surface in the experimental section however sedimentary structures developed very quickly with an equilibrium bed surface developing after only two sediment transport events of low magnitude discharge this rapid adjustment of the bed is associated with the development of key sedimentary structures anchored by the large boulders at the reach scale these structures increased the probability of congested areas in the displacement of bed material in both control sections sedimentary structures were maintained during the experiment copyright 2007 john wiley sons ltd

a field investigation of the influence of sediment supply on step pool morphology and stability abstract the effect of the sediment supply on step pool morphology and stability was studied through the investigation of 42 step pool reaches eight in the vosges and 34 in the alps france step pool geometry step wavelength l and height h was measured for each reach and the data set split into two distinct groups comprising 25 connected to a sediment source step pool reaches and 17 disconnected step pool reaches the geometry analysis showed differences between the two groups with the shape factors k l hr and the h hr ratios hr is the residual pool depth being lower for disconnected step pools this was interpreted to result from progressively changing geometry over time with disconnected step pool geometry defining the probable asymptotic characteristic associated with this long term process the measured step pool dimensions were in good agreement with equations based on maximum flow resistance abrahams et al 1995 energy of a falling jet comiti et al 2005 and channel spanning deposition chartrand et al 2011 however the average shape factor for the comiti et al equation was k 17 instead of 8 as the authors had proposed the data set was extended to 255 values with data from the literature which confirmed this average value for k field evidence moss cover and stone imbrication indicate that connected step pools are mobile whereas disconnected step pools are stable this was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are 20 50 years for connected step pools and even less when considering scouring effects and greater than 50 years for disconnected step pools this difference in step mobility was ascribed to the fact that with sediment supply the channel can freely adjust its local slope allowing a relatively efficient transfer downstream of large boulders by scouring effects

residual regolith derived from the biotite controlled weathering of cretaceous tonalite quartz diorite peninsular ranges southern california usa a case study abstract though k feldspar free tonalite and quartz diorite are common plutonic rocks in exhumed continental margin magmatic arc rocks around the globe how they weather has not been extensively studied we therefore undertook an investigation aimed at understanding how an approximately 6 m thick section of residual regolith saprock and overlying alfisol developed from cretaceous k feldspar free tonalite and quartz diorite at santa margarita ecological reserve southern california usa we analyzed three samples of soil 10 of saprock and 10 of corestone for their major trace and rare earth element compositions thin sections of each major textural group also were studied and point counted common xrd techniques were used to determine clay mineralogy three samples of soil and five of saprock were analyzed for their ph within the saprock an extensive network of cracks was likely produced by the volumetric expansion that accompanied the conversion of biotite to vermiculite migrating near neutral fluids responsible for this conversion resulted in a loss of k mass and affected several trace elements that are commonly found in biotite but with the exception of an interval between 177 and 270 cm left most other major trace and rare earth elements ree little affected within the 177 to 270 cm interval at a depth of 208 cm relative to the immobile framework incongruent leaching of plagioclase by acidic solutions resulted in the losses of si al ca na pb sr and ga mass in contrast more neutral solutions added heavy ree hree and y mass at 177 to 237 cm depths while acidic solutions removed ree and y mass at 270 cm depth in the soil acidic solutions attacked plagioclase biotite and apatite leaching and removing relative to the immobile framework ca na mg k and p mass in addition the dissolution of apatite resulted in depletions in hree and y mass none of the elemental mass removed from the soil accumulated in the directly underlying saprock and must have been removed from the near vertical plane of sampling within the saprock the above changes are mostly the result of near neutral ph water rock interactions occurring within a warm and dry holocene climate identified changes in elemental mass other than those associated with the conversion of biotite to vermiculite are localized and are likely the result of the paths taken by fluids with differing ph moving through the complex network of cracks any model purporting to describe the evolution of residual regolith must explain such natural complexities

objective identification of pools and riffles this paper presents a new technique for objectively identifying pools and riffles the technique is termed the bed form differencing technique and as its data base has bed elevations surveyed at a fixed interval along the channel these elevations are differenced and the difference or elevation change values are used to identify pools and riffles where the cumulative elevation change since the last bed form exceeds a certain tolerance value a new bed form is considered to exist the type of bed form being determined by the direction of the elevation change analysis of a small stream in western new york suggests that the most realistic pool riffle sequence is obtained where the tolerance value is 0 75 times the standard deviation of the difference values the bed form differencing technique can be used to identify bed forms other than pools and riffles such as ripples and dunes by simply modifying the surveying interval to reflect the scale of the bed forms

recent 1999 2003 canadian research on contemporary processes of river erosion and sedimentation and river mechanics contributions by canadian fluvial geomorphologists between 1999 and 2003 are discussed under four major themes sediment yield and sediment dynamics of large rivers cohesive sediment transport turbulent flow structure and sediment transport and bed material transport and channel morphology the paper concludes with a section on recent technical advances during the review period substantial progress has been made in investigating the details of fluvial processes at relatively small scales examples of this emphasis are the studies of flow structure turbulence characteristics and bedload transport which continue to form central themes in fluvial research in canada translating the knowledge of small scale process related research to an understanding of the behaviour of large scale fluvial systems however continues to be a formidable challenge models play a prominent role in elucidating the link between small scale processes and large scale fluvial geomorphology and as a result a number of papers describing models and modelling results have been published during the review period in addition a number of investigators are now approaching the problem by directly investigating changes in the system of interest at larger scales e g a channel reach over tens of years and attempting to infer what processes may have led to the result it is to be expected that these complementary approaches will contribute to an increased understanding of fluvial systems at a variety of spatial and temporal scales

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

evidence for major mass transfer and volume strain during regional metamorphism of pelites systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism statistical analysis of the data shows that barrovian zone metamorphism of pelitic schist generally causes increases in the whole rock concentrations of the low solubility elements ti and all the observed increases in ti and al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species application of mass balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30 of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions the bulk of the lost mass is silica not volatiles in addition other elements particularly ca na and k appear to be highly mobile in deep crustal metamorphism contrary to conventional interpretations it is concluded that the regional metamorphism of pelites is not an isochemical process

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

morphological organization of a steep tropical headwater stream the aspect of channel bifurcation abstract channel characterization and instream wood surveys demonstrate the spatial complexity of an ungauged tropical headwater stream hws in the tilaran mountains of costa rica throughout the stream course we characterized 29 individual reaches based on their morphological organization and instream wood occurrences bifurcated reaches were the most numerous and consistently distributed they also had the highest values of wood abundance and loading and contained the majority of unattached instream wood we selected two bifurcated reaches for ultrahigh resolution terrestrial laser scanning tls surveys orthographic tls produced millimeter resolution point cloud data sets from which 2 cm planimetric resolution digital elevation models dems of the channel surface were derived for both reaches cross sectional surveys of these dems using the river bathymetry toolkit rbt produced hydraulic geometries at two flow levels high and low for every meter length of channel reach we input these geometries into a one dimensional analytical model to demonstrate the downstream evolution of mean stream power through varying morphology elements mean stream power variation displays a regular pattern through the vertical modulation of step pool sequences but this regular pattern is lost through channel widening and bifurcation reach geometric data indicates that increasing width to depth ratio is the most notable feature of channel bifurcation and corresponds to a reduction in slope and roughness variation this primary understanding of channel dynamics denotes the complex organization of a tropical hws and promotes further research into tropical hws systems

strain accommodation about strike slip fault discontinuities in granitic rock under brittle to ductile conditions abstract brittle and ductile structures adjacent to fault terminations and echelon fault steps of left lateral strike slip faults in the lake edison granodiorite of the central sierra nevada california are related to stress perturbations caused by fault slip the structures associated with fault terminations change from dilatant splay fractures only to splay fractures and ductile fabrics on opposing sides of the fault to ductile fabrics ahead of the fault with decreasing distance from a younger neighboring pluton and presumably with increased temperatures the distributions of deformation mechanisms correlate with the inferred local magnitudes and spatial distributions of the maximum tensile stress the mean stress and the maximum shear stress displacements along the faults are transferred across echelon stepovers by mineralized dilatant fractures in extensional steps to accommodate slip transfer across contractional steps rock apparently was squeezed vertically out of the step by ductile flow concentrations of mobile and immobile elements indicate that diffusive mass transfer was not significant increased mean stress in contractional steps apparently enhanced crystal plastic flow of quartz a flow law that includes a pressure effect through correlation with pressure dependent solidus temperatures is more successful than other experimentally derived flow laws in predicting the observed distribution of ductile fabrics

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

the self organization of step pools in mountain streams abstract spontaneous autogenic self organization has been described in numerous geomorphic systems but it has not been investigated in detail with respect to coarse bedforms in general or step pools in particular in this paper we review the spatial organization of step pool systems and present example evidence of step pool development as an autogenic self organization process we then outline the mathematical language for defining spatially divergent self organization and test these ideas using two unique field examples from oregon andrews experimental forest and california baxter creek where step pools developed from planar beds in artificially manipulated channels results show that step pool development is consistent with a spatially divergent self organization phenomenon entropy increases as initially undifferentiated planar channels diverge into steps and pools then declines when a series of steps and pools of consistent size and spacing is established signifying stability in the system the self organization process is accompanied by increasing flow resistance and decreasing slope through increasing the vertical sinuosity of the step pool profile and creation of low or negative gradient pool areas suggesting a minimization of stream power the self adjustment of the step pool bed profile over time represents another manifestation of a general process that results in rhythmic patterns on the surface of earth

oxygen isotope zoning in garnet a record of volatile transport abstract this study examines oxygen isotope zoning in garnets from a barrovian metamorphic terrane in eastern vermont using a co 2 laser extraction system previous strontium isotopic and structural studies of these garnets have shown that they grew over an approximately 10 ma interval during thrusting and nappe emplacement rosenfeld 1970 christensen et al 1989 our studies show that the garnets are strongly zoned in 18 o this zoning is the result of equilibration of garnet with water derived from dehydration of subjacent pelites during nappe stage deformation the magnitude and nature of 18 o zoning depends upon the garnet s location in the outcrop studied the garnets examined in this study come from an isotopically low 18 o paragonitic schist 18 o whole rock 9 that is adjacent to a relatively high 18 o schist 18 o whole rock 12 5 garnets from the paragonitic schists within 10 m of the contact with the isotopically heavier schists have relatively homogenous 18 o values varying from 9 5 in the core to 10 5 at the rim garnet in the paragonitic schists 85 m from the contact are more strongly zoned with 18 o ranging from a low of 6 0 in the cores to a high of 9 0 at the rims of the garnet these zoning patterns were produced by continuous infiltration of relatively high 18 o waters derived from the subjacent schists into the paragonitic schists during garnet grade metamorphism it is possible to determine the time integrated fluid fluxes by comparison of observed 18 o zoning profiles in garnet with those calculated from the equation describing combined advection diffusion of a tracer using this method we calculate time integrated fluid fluxes of 1 5 10 4 cm 3 cm 2 fluxes of this magnitude could have been produced by dewatering of 1 5 km of schist during garnet grade metamorphism

volume loss and metasomatism during cleavage formation in carbonate rocks abstract controversy exists regarding the amount of volume loss associated with cleavage development a combination of geometrical and geochemical techniques provides evidence for contemporaneous passive concentration volume loss and metasomatism leading to the formation of cleavage in carbonate lithologies and emphasizes the relationship between observational scales and textural heterogeneity comparison of protoliths and weakly to strongly cleaved rocks along steep layer parallel strain gradients within the doublespring duplex lost river range idaho enables quantitative assessments of geometric transformation and elemental mass transfer accompanying progressive deformation geometric finite strain analysis documents shortening in the z direction at low strains es 0 15 unbalanced shortening in z and y with extension in x results in volume loss geometrically derived volume strain estimates at the 1 4 cm3 scale indicate volumetric dilations of from 2 to 12 in uncleaved to weakly cleaved rocks and from 20 to 50 in strongly to very strongly cleaved rocks small scale significant volume losses and gains contrast with the estimated volume loss of sampling scale is also a factor in evaluating the major and trace element and o and c isotope compositions of the variably deformed rocks samples collected at the 4 15 cm3 scale reflect mixtures of chemically and mineralogically distinct selvage and microlithon domains microdrilling of samples at scales of a few cubic millimeters to 1 cm3 affords examination of chemical and isotopic differences among individual selvages and microlithons the data for microdrilled samples demonstrate depletion of ca and 18o and enrichment of k al si ti fe na mg and p in selvages relative to microlithons and nearby undeformed carbonate layers the enrichments of some elements mg p na and possibly fe are more compatible with a passive concentration mechanism whereas the concentrations of other elements particularly k al si and ti require significant metasomatic addition during deformation shifts in o isotope compositions of deformed samples relative to the protolith 18o values unequivocally demonstrate open system behavior during deformation trends of deformed samples toward lower 18o with increasing strain and the 18o values of veins are compatible with infiltration by h2o rich fluids with relatively low 18ov smow near 0 the formation of disjunctive cleavage at the doublespring duplex occurred by passive concentration resulting from calcite dissolution and related volume loss and neo crystallization of illite kaolinite quartz anatase in selvages as a result of metasomatic additions chemical and geometric strain softening within selvages likely led to enhanced and preferential fluid flow in selvages

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

grampian migmatites in the buchan block ne scotland rocks exposed along the scottish coast between fraserburgh and inzie head contain information critical to understanding the evolution of the buchan block the type locality for low p high t regional metamorphism and its relationship with the rest of the grampian terrane one of the major tectonostratigraphic components of the scottish caledonides the 8 km long section traverses a regional network of shear zones and at the highest grades around inzie head passes into the core of the buchan anticline a large scale open fold that is commonly regarded as a late structure post dating metamorphism the metasedimentary rocks increase in grade from upper amphibolite to granulite facies and preserve unequivocal evidence for partial melting the diatexite migmatites around inzie head along with other gneissose units within the buchan block have been regarded as allochthonous precambrian basement rocks that were thrust into their current position during the grampian orogenesis however field observations show that the onset of in situ partial melting in metapelitic rocks which was associated with the formation of garnet bearing aplites and associated pegmatites occurred around fraserburgh where shear fabrics are absent thus the rocks preserve a continuous metamorphic field gradient that straddles the shear zone network this observation supports an alternative interpretation that anatexis was the result of mid ordovician grampian metamorphism rather than an older tectonothermal event and that the inzie head gneisses are autochthonous using an average mid dalradian pelite as a plausible representative protolith phase equilibria modelling satisfactorily reproduces the observed appearance and disappearance of key minerals providing that peritectic garnet produced with the first formed melts represented by the garnet bearing aplites depleted the source rocks in mn the modelled metamorphic field gradient records a temperature increase of at least 150 c from 650 c near fraserburgh to in excess of 800 c at inzie head but is isobaric at pressures of 2 7 2 8 kbar suggesting the buchan anticline developed synchronous with partial melting the buchan anticline is likely an expression of crustal thinning and asthenospheric upwelling which produced voluminous gabbroic intrusions that supplied the heat for buchan metamorphism

geometric scaling of step pools in mountain streams observations and implications understanding the geometric structuring of alluvial step pools is an enduring problem in mountain stream geomorphology many previous studies propose that the key control parameter governing the step pool form is mean bed slope as explored through several examples however we find that slope control formulations are inconsistently demonstrated for broad data sets on this basis we reexamine the bed form architecture of step pools with a new descriptive parameter termed the aspect ratio active stream width divided by step drop height we test with data collected from three different mountain settings for step pools observed over mean bed slopes ranging from 2 to 22 we find that step wavelength and height are well described by when these dimensions are normalized by the step drop height furthermore we observe that varies inversely with mean bed slope context for our findings is provided by recent field and laboratory work which highlights that the cross stream component to the overall flow structure in step pools and scour pools is significant this is important because it expands upon and relates our work to formative models which are based on jet scour and the associated three dimensional flow structure of step pools our work has practical implications for the design of step pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry we observe however that under certain conditions this approach may yield constructed step pools that are unstable as an alternative approach we propose that the geometric relationships developed here can be applied to identify artificial step pool geometries that will be more fully reflective of natural analogs and thus possibly more stable

morphology of riffle pool sequences in the river severn england despite the occurrence of pool riffle sequences in many rivers there are few data concerning pool riffle unit morphology of many criteria proposed to identify pool riffle units only two methods can be regarded as objective and robust these are the zero crossing and the control point methods in this paper statistics are developed from the zero crossing method to describe the streamwise morphology of 275 riffles and 285 pools which form a near continuous 32 1km of the bed of the river severn in shropshire england yalin s theoretical relationship between pool riffle unit length i p and channel width w i p 3w applies to the river severn reach average riffle height h is a constant proportion of bankfull depth h typically h i 0 16h riffle height is a positive function of riffle length pool depth is a positive function of pool length however both riffle length and pool length increase more rapidly than the bed level amplitude such that long riffles or pools are relatively flat as channel gradient reduces bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes the statistical relationships between riffle height length and water depth are similar to those for equilibrium subaqueous dunes the severn data are partially consistent with yalin s theoretical analysis relating riffle bedform length lr to water depth i e lr i i2i h wherein i i i 1 for steep near equilibrium bedforms but i i i 2 to 3 as the relative depth decreases and riffles become long low features a consideration of turbulence data indicates that the frequency of coherent turbulent flow structures associated with the riffle pool mixing length is of the order of 60 seconds the morphological and dynamic similarity of the river severn riffles with dunes raises intriguing questions with respect to self similar convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly

alluvial architecture in headwater streams with special emphasis on step pool topography alluvial mountain streams exhibit a range of channel forms pool riffle plane bed step pool and cascades previous work suggested that these forms exist within discrete and progressively steeper slope classes measurements conducted at over 100 sites in west central and central idaho confirm that slope steepens progressively as one moves from pool riffle to plane bed to step pool and finally to cascades median slope for pool riffle topography is 0 0060 for plane beds 0 013 for step pools 0 044 and for cascades 0 068 there is substantial overlap in the slopes associated with these channel forms pool riffle topography was found at slopes between 0 0010 and 0 015 plane beds between 0 0010 and 0 035 step pools between 0 015 and 0 134 and cascades between 0 050 and 0 12 step pools are particularly striking features in headwater streams they are characterized by alternating steep and gentle channel segments the steep segments step risers are transverse accumulations of boulder and cobbles while the gentle segments pools contain finer material step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers this result differs from past studies that have reported channel slope to be the dominant control on step wavelength the presumed geometry and froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step pools are formed by antidunes copyright 2000 john wiley sons ltd

rapid quench hydrothermal experiments at mantle pressures and temperatures new experimental methods for the encapsulation extraction and analysis of large volumes of aqueous fluids enable studies of mineral solubilities and dissolution kinetics with the piston cylinder apparatus quench rates and volumes of experimental fluids with these methods are comparable with those used in modern hydrothermal experiments employing cold seal vessels analyses of fluid compositions are reproducible and consistent with independent results the techniques permit experimental investigations that will better characterize fluid rock interaction in such environments as subducting slabs the mantle wedge barrovian metamorphic belts and middle to lower crustal granulites

the origin and fate of 4 methyl steroid hydrocarbons i diagenesis of 4 methyl sterenes abstract treatment of 4 methylcholest 4 ene under mild acid conditions at low temperatures gives chemical evidence for certain features seen in the distributions of sedimentary 4 methyl steroid hydrocarbons and further indicates that many low temperature diagenetic reactions of steroids are explicable in terms of acid catalysed rearrangements specifically the results provide i indirect evidence that the 4 ene skeleton is a key intermediate in the dehydration of 4 methyl stanols in sediments ii an explanation for the distribution of 4 methyl sterenes and a nor sterenes in the lacustrine messel shale eocene iii an explanation for the presence of 4 methyl steranes in relatively immature sedimentary rocks despite the precursor stanols having the 4 methyl configuration with increasing maturity in the paris basin shales lower toarcian the less stable 4 methyl steranes decrease gradually in abundance relative to their 4 methyl counterparts at a rate fairly similar to the change in pristane stereochemistry