effects of hydraulic roughness on surface textures of gravel bed rivers field studies of forest gravel bed rivers in northwestern washington and southeastern alaska demonstrate that bed surface grain size is responsive to hydraulic roughness caused by bank irregularities bars and wood debris we evaluate textural response by comparing reach average median grain size d50 to that predicted from the total bank full boundary shear stress t0 bf representing a hypothetical reference condition of low hydraulic roughness for a given t0 bf channels with progressively greater hydraulic roughness have systematically finer bed surfaces presumably due to reduced bed shear stress resulting in lower channel competence and diminished bed load transport capacity both of which promote textural fining in channels with significant hydraulic roughness observed values of d50 can be up to 90 smaller than those predicted from t 0 bf we find that wood debris plays an important role at our study sites not only providing hydraulic roughness but also influencing pool spacing frequency of textural patches and the amplitude and wavelength of bank and bar topography and their consequent roughness our observations also have biological implications we find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse

germanium silicon fractionation in the weathering environment abstract we present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the ge si system as a tracer of silicate weathering processes in both modern and ancient environments intensely weathered soils developed on hawaiian basalts have bulk soil ge si ratios 2 to 10 times higher than fresh basalt e g 10 to 25 mol mol vs 2 5 mol mol soil ge concentrations increase with si and decrease with fe suggesting that ge sequestration is related to accumulation of secondary soil silicates rather than retention in soil fe oxy hydroxides sequential extractions of these soils suggest that ge si fractionation takes place by ge sequestration during the initial precipitation of secondary soil aluminosilicates principally allophane further si loss and changes in mineralogy as these soils age result in little additional ge si fractionation ge si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals kaolinite has a ge si ratio 5 9 mol mol higher than the plagioclase from which it forms 3 1 mol mol whereas accumulation of primary quartz ge si 0 5 mol mol prevents granitic soils from attaining high ge si ratios laboratory synthesis of allophane confirms that ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution

developments in monitoring and modelling small scale river bed topography recent research in fluvial geomorphology has emphasized the spatially distributed feedbacks amongst river channel topography flow hydraulics and sediment transport although understanding of the behaviour of dynamic river channels has been increased markedly through detailed within channel process studies less attention has been given to the accurate monitoring and terrain modelling of river channel form using three dimensional measurements however such information is useful in two distinct senses firstly it is one of the necessary boundary conditions for a physically based deterministic modelling approach in which three dimensional topography and river discharge drive within channel flow hydraulics and ultimately spatial patterns of erosion and deposition and therefore channel change secondly research has shown that an alternative means of estimating the medium term bedload transport rate can be based upon monitoring spatial patterns of erosion and deposition within the river channel this paper presents a detailed assessment of the distributed monitoring and terrain modelling of river bed topography using a technique that combines rigorous analytical photogrammetry with rapid ground survey the availability of increasingly sophisticated terrain modelling packages developed for civil engineering application allows the representation of topographic information as a landform surface intercomparison of landform surfaces allows visualization and quantification of spatial patterns of erosion and deposition a detailed assessment is undertaken of the quality of the morphological information acquired this allow some general comments to be made concerning the use of more traditional methods to monitor and represent small scale river channel morphology

tracing mechanisms controlling the release of dissolved silicon in forest soil solutions using si isotopes and ge si ratios the terrestrial biogenic si bsi pool in the soil plant system is ubiquitous and substantial likely impacting the land ocean transfer of dissolved si dsi here we consider the mechanisms controlling dsi in forest soil in a temperate granitic ecosystem that would differ from previous works mostly focused on tropical environments this study aims at tracing the source of dsi in forest floor leachates and in soil solutions under various tree species at homogeneous soil and climate conditions using stable si isotopes and ge si ratios relative to granitic bedrock clays minerals were enriched in si 28 and had high ge si ratios while bsi from phytoliths was also enriched in si 28 but had a low ge si ratio such a contrast is useful to infer the relative contribution of silicate weathering and bsi dissolution in the shallow soil on the release of dsi in forest floor leachate solutions the delta si 30 values in forest floor leachates 1 38 parts per thousand to 2 05 parts per thousand are the lightest ever found in natural waters and ge si ratios are higher in forest floor leachates relative to soil solution these results suggest dissolution of si 28 and ge enriched secondary clay minerals incorporated by bioturbation in organic rich horizons in combination with an isotopic fractionation releasing preferentially light si isotopes during this dissolution process ge si ratios in soil solutions are governed by incongruent weathering of primary minerals and neoformation of secondary clays minerals tree species influence si isotopic compositions and ge si ratios in forest floor leachates through differing incorporation of minerals in organic horizons by bioturbation and to a lesser extent through differing si recycling c 2010 elsevier ltd all rights reserved

glacially induced organization of channel reach morphology in mountain streams 1 we examine the spatial distribution of channel reach morphologies in formerly glaciated mountain drainage basins of coastal british columbia canada using field and geographic information systems derived data we show that the local channel slope and the degree of colluvial alluvial coupling imposed by the glacial valley morphology dictate the spatial organization of channel types in particular the complex glacially induced channel long profile produces characteristic sequences of channel reaches that depart from the downstream succession colluvial boulder cascade step pool rapids riffle pool distinctive of simple unglaciated mountain streams typically the presence of one hanging valley in the river long profile produces and separates two full successions of channel types a headmost one characterized by an ephemeral seasonal hydrologic regime and a downstream one where water runoff is perennial we document that channel types are well separated in plots of slope versus shear stress area versus shear stress and slope versus relative roughness in agreement with these outcomes multivariate discriminant analyses coupled with principal component analysis of 98 study reaches yield a highly successful channel type classification when slope shear stress and relative roughness are considered success rates depending on whether or not boulder cascade reaches are pooled together with step pools are 88 and 75 respectively previous work in unglaciated settings has suggested that mountain channels have distinct bed morphology states that vary primarily with slope our study reveals that even in formerly glaciated valleys where slope is largely inherited from glacial times these distinct bed states exist and vary mostly with slope adding considerable strength to this empirical knowledge

strontium isotopes as tracers of ecosystem processes theory and methods abstract the strontium sr isotope method can be a powerful tool in studies of chemical weathering and soil genesis cation provenance and mobility and the chronostratigraphic correlation of marine sediments it is a sensitive geochemical tracer applicable to large scale ecosystem studies as well as to centimeter scaled examination of cation mobility within a soil profile the 87sr 86sr ratios of natural materials reflect the sources of strontium available during their formation isotopically distinct inputs from precipitation dryfall soil parent material and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem the isotopic compositions of labile soil exchange complex and soil solution strontium and sr in vegetation reflect the sources of cations available to plants strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium the extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method in this paper we review the geochemistry and isotopic systematics of strontium and discuss the use of this method as a tracer of earth surface processes

weathering rates and 87sr 86sr ratios an isotopic approach the 87sr 86sr ratios of precipitation throughfall groundwater streamwater and mineral matter were analyzed precipitation strontium ratios are around 0 71 throughfall which is affected by strontium from the mineral soil has ratios of 0 72 0 73 and shallow groundwater in a drug well and runoff have ratios around 0 73 0 74 there is a tendency for this relationship to be reversed for discharge higher ratios are associated with lower discharges water from a fracture zone gave the highest value 0 78 approaching the value of the bedrock around 0 85 a calcium budget which reflects the mean conditions over a few years has been calculated this budget indicates that weathering should be of the order of 8 kg ca ha 1yr 1 to maintain the pool of exchangeable calcium at a constant level the major anion in the runoff is suphate which has entered the system in the form of acid rain sulphur deposition has been rather constant over the last two decades if acidification had an effect on weathering of silicates this would have shown up in a tendency of the strontium isotope ratios toward higher values in the soil pool and in runoff water thus its seems from the observed ratios that at least a major portion of the excess calcium export resulting from sulphur deposition is taken from the soil pool resulting in a gradual acidification of the soil

simulation of flow over pool riffle topography a consideration of the velocity reversal hypothesis computer flow simulations using the hec 2 step backwater routine are used to demonstrate the effect of systematically varying river channel width riffle spacing and channel roughness on the shear velocity section mean velocity and energy slope in fixed bed pool riffle sequences initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the river severn subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored the purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence reversal such that pools scour at high flow whilst deposition occurs on riffles it is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal for prescribed conditions the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool riffle topography

87sr 86sr ca sr and ge si ratios as tracers of solute sources and biogeochemical cycling at a temperate forested shale catchment central pennsylvania usa plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems hydrologic and other losses are offset by inputs from atmospheric deposition and weathering this multi tracer study investigates these effects using 87sr 86sr ca sr and ge si ratios from solid soil profiles and bedrock biological leaves and sap waters and water pore ground stream waters samples to study ca sr ge and si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central pennsylvania leaves and sap waters were found to have similar ge si ratios 1 mol mol which is consistent with biological fractionation occurring at the root soil water interface ge si ratios in soil porewaters were higher near the surface and increased over the growing season suggesting plant preferential uptake of si over ge ca sr ratios in oak leaves 1360 40 mol mol were significantly higher than those in maple leaves 650 20 mol mol ca sr ratios were also generally higher in leaves than in sap waters which are consistent with preferential sr adsorption or sr uptake during transpiration 87sr 86sr ratios in both leaves and sap waters were similar for a given site implying that trees access similar pools of sr and ca although there are site to site differences additionally 87sr 86sr ratios in plant tissues were most similar to shallow pore waters 0 50 cm suggesting that mineral nutrients are obtained by trees from the near sub surface 87sr 86sr ratios in soil solutions from ridgetop and swale sites are best explained by mixing sr derived from shale and atmospheric deposition valley bottom soil reservoirs and stream and groundwater samples include sr and ca derived from dissolution of two isotopically distinct generations of carbonates a preliminary estimate of the sr and ca stream fluxes and isotopic mass balances imply propagation of a carbonate weathering front of ca 300 m myr 1 a value larger than previously reported for regolith weathering advance rates calculated based on cosmogenic nuclides and u series isotopes the data for ca sr si and ge in soil soil solutions and stream waters reflect the interaction of slower weathering processes with rapid biologically driven cycling between soils and biomass

strontium isotopes as indicators of mineral weathering in catchments determination of the 87sr 86sr ratio in rain water stream water soils and rocks in two scottish catchments has provided useful information on mineral weathering over a period of 26 months the 87sr 86sr ratio in rain water had mean values of 0 7095 and 0 7098 n 12 whereas stream water had mean values of 0 7080 n 16 in catchment in devonian andesites with strontium isotope ratios of 0 7117 0 7232 in the rocks and soils and 0 7184 n 16 in catchment of cambrian metamorphic rocks with strontium isotope ratios of 0 7360 0 8218 in the rocks and soils respectively the mineralogy consists predominantly of varying proportions of quartz plagioclase feldspar and k feldspar with some mica and chlorite the fact that the 87sr 86sr ratios in the stream in the andesitic catchment are lower than for the rain water probably indicates preferential weathering of plagioclase feldspar releasing 86sr rather than k feldspar and mica which contain greater amounts of 87sr the 87sr 86sr ratio acts as a tracer for the origin of the solutes in the stream water long term monitoring of the strontium isotope ratio in stream water could be a potential means of detecting relative changes in soil weathering rates due to environmental change

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

germanium silicon fractionation in a tropical granitic weathering environment abstract germanium silicon ge si ratios were determined on quartz diorite bedrock saprolite soil primary and secondary minerals phytolith soil and saprolite pore waters and spring water and stream waters in an effort to understand ge si fractionation during weathering of quartz diorite in the rio icacos watershed puerto rico the ge si ratio of the bedrock is 2 mol mol with individual primary mineral phases ranging between 0 5 and 7 mol mol the ratios in the bulk saprolite are higher 3 mol mol than values measured in the bedrock the major saprolite secondary mineral kaolinite has ge si ratios ranging between 4 8 and 6 1 mol mol the high ge si ratios in the saprolite are consistent with preferential incorporation of ge during the precipitation of kaolinite bulk shallow soils have lower ratios 1 1 1 6 mol mol primarily due to the residual accumulation of ge poor quartz ge si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths spring water and baseflow stream waters have the lowest ge si ratios 0 27 0 47 mol mol reflecting deep initial weathering reactions resulting in the precipitation of ge enriched kaolinite at the saprolite bedrock interface mass balance calculations on saprolite require significant loss of si and al even within 1 m above the saprolite bedrock interface higher pore water ge si ratios 1 2 mol mol are consistent with partial dissolution of this ge enriched kaolinite pore water ge si ratios increase up through the saprolite and into the overlying soil but never reach the high values predicted by mass balance perhaps reflecting the influence of phytolith recycling in the shallow soil

soil mineralogy and slope effects on infiltration interrill erosion and slope factor 1 interactions between the effects of soil mineralogy and of slope gradient on seal formation infiltration rate ir runoff and soil loss were evaluated and the slope factor sf functions for various soils with differing mineralogy were examined three different soils on slopes of 9 15 20 and 25 were subjected to 80 mm of simulated rainfall the final ir was 20 5 mm h 1 in a clayey kaolinitic soil and 15 mm h 1 in smectitic soils the total runoff as a percentage of rainfall ranged from 24 to 18 65 to 43 and 50 to 35 from the kaolinitic clayey smectitic and sandy loam smectitic soils respectively the total soil loss ranged from 0 32 to 0 45 and from 1 14 to 3 93 kg m 2 for the kaolinitic and smectitic soils respectively in all three soils increasing the slope gradient increased the detachment of coarse particles 0 1 mm more sharply than that of small particles 0 1 mm especially in the smectitic soils combining these results with previous findings for six soils indicated that soils could be divided into two groups according to their sf values 1 soils that contained smectite and were therefore dispersive and 2 soils that did not contain smectite and were therefore stable for the former soils the regression sf 0 47exp 7 7 sin defined the sf values significantly whereas for the latter group a linear regression sf 0 81 1 77 sin was required it was suggested that the difference in the sf functions was mainly due to seal formation enhancement by the smectite

regolith production and transport in the susquehanna shale hills critical zone observatory part 1 insights from u series isotopes 1 to investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect we measured u series isotopes in regolith profiles from two hillslopes north facing versus south facing within the east west trending shale hills catchment in pennsylvania this catchment is developed entirely on the fe rich silurian rose hill gray shale hillslopes exhibit a topographic asymmetry the north facing hillslope has an average slope gradient of 20 slightly steeper than the south facing hillslope 15 the regolith samples display significant u series disequilibrium resulting from shale weathering based on the u series data the rates of regolith production on the two ridgetops are indistinguishable 40 22 versus 45 12 m ma however when downslope positions are compared the regolith profiles on the south facing hillslope are characterized by faster regolith production rates 50 15 to 52 15 m ma and shorter durations of chemical weathering 12 3 to 16 5 ka than those on the north facing hillslope 17 14 to 18 13 m ma and 39 20 to 43 20 ka the south facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry despite lower extents of chemical depletion these results are consistent with the influence of aspect on regolith formation at shale hills we hypothesize that aspect affects such variables as temperature moisture content and evapotranspiration in the regolith zone causing faster chemical weathering and regolith formation rates on the south facing side of the catchment the difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at shale hills at least 15 ka before present at that time the erosion rates may also have been different from those observed today perhaps denuding the south facing hillslope of regolith but not quite stripping the north facing hillslope an analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at shale hills is not in geomorphologic steady state i e so called dynamic equilibrium but rather is likely still responding to the climate shift from the holocene periglacial to the modern temperate conditions

flow resistance and bed form geometry in a wide alluvial channel 1 this paper explores the underlying mechanism of flow resistance in a wide alluvial channel with bed forms on the basis of published data it is shown that the grain roughness can be taken as equal to 2 times the median diameter of the bed sediment an empirical equation for the bed form roughness has been proposed and it depends on the bed form height and bed form steepness the influence of the bed form length and height on the total bed shear stress and energy slope is deliberated and empirical expressions for the length of the separation zone behind the bed forms are also proposed the study proposed an equation to compute the total bed shear stress as a function of the grain and bed form roughness as well as the important role of the bed form geometry in the overall flow resistance in alluvial channels the model is tested and verified against 670 flume measurements and 1540 field observations the computed and measured energy slopes are in good agreement with close to 71 of all data sets within the 20 error band

preliminary estimates of regolith generation and mobility in the susquehanna shale hills critical zone observatory pennsylvania using meteoric 10be abstract this study seeks to quantify the rate and timing of regolith generation in the critical zone at the susquehanna shale hills critical zone observatory ssho meteoric 10 be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10 be inventories the ssho is located in the temperate climate zone of central pennsylvania and comprises a first order watershed developed entirely on a fe rich organic poor silurian aged shale two major perturbations to the landscape have occurred at ssho in the geologically recent past including significant and sustained periglacial activity until after the retreat of the laurentide ice sheet 21 ka and deforestation during early colonial land use bulk soil samples n 16 were collected at three locations along a planar hillslope on the southern ridge of the catchment representing the ridge top mid slope and valley floor rock chip samples n 14 were also collected from a 24 m deep core drilled into the northern ridge top all meteoric 10 be concentration profiles show a declining trend with depth with most of the 10 be retained in the uppermost decimeters of the soil meteoric 10 be inventories are higher at the mid slope and valley floor sample sites at 3 71 0 02 10 10 at cm 2 and 3 69 0 02 10 10 at cm 2 than at the ridge top site 1 90 0 01 10 10 at cm 2 the 10 be inventory at the convex ridge top site implies a minimum residence time of 10 6 ka or if erosion is steady an erosion rate of 19 4 0 2 m my

reach scale channel geometry of mountain streams abstract the basic patterns and processes of steep channels remain poorly known relative to lower gradient channels in this analysis characteristics of step pool plane bed and pool riffle channels are examined using a data set of 335 channel reaches from the western united states nepal new zealand and panama we analyzed differences among the three channel types with respect to hydraulics channel geometry boundary roughness and bedforms step pool channels have significantly steeper gradients coarser substrate higher values of shear stress and stream power for a given discharge and larger ratios of bedform amplitude wavelength h l pool riffle channels have greater width depth ratios and relative grain submergence r d84 than the other channel types plane bed channels tend to have intermediate values for most variables examined relative form submergence r h is statistically similar for step pool and pool riffle channels despite the lesser relative grain submergence and greater bedform amplitude of step pool channels mean values of darcy weisbach friction factor do not change in response to changes in relative grain submergence these patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types

characteristics of velocity profiles along riffle pool sequences and estimates of bed shear stress abstract numerous velocity profiles were measured along a riffle pool sequence for three different flow conditions for each flow condition ranging from baseflow to about 2 3 of bankfull depth ten to fifteen velocity profiles were measured along the same transect the objectives of the study were i to differentiate between components of bed roughness and shear stress attributed to surface friction and large scale bed undulations and ii to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles no previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle pool sequences and to determine how near bed velocity gradients change with increasing flow discharge at low flows and as expected riffles are characterized by higher near bed velocity gradients than pools similarly average bed shear stresses as determined from measurements of flow depth and energy gradient indicate a greater resistance and bed shear stress in riffle sections invariably however local shear stresses as determined from near bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope the shape of spatially averaged velocity profiles changes as discharges increases the difference in terms of near bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull further away from the bed surface velocity differences between pools and riffles do remain significant finally the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate perhaps a result of increasing density of small scale sedimentary structures

tracing hydrologic flow paths in a small forested watershed using variations in 87sr 86sr ca sr ba sr and δ18o 1 natural variations in 87sr 86sr ca sr ba sr and 18o were used to investigate changing hydrologic flow paths during storm events in an 11 8 ha first order watershed of the hubbard brook experimental forest throughfall soil water and stream water were sampled during several storm events and a single snowmelt event throughfall had 87sr 86sr ca sr and 18o values that were distinct from other water types soil water was highly variable in composition but could be separated into three distinct elevation zones in the watershed with 87sr 86sr and ba sr both increasing with elevation in the watershed changes in 87sr 86sr ba sr and 18o of stream water during storm events yielded complementary information about watershed hydrologic flow paths traditional hydrograph separation using 18o values indicated that new water was only a small component of storm flow 9 18 whereas 87sr 86sr and ba sr ratios indicated that shallow subsurface flow soil water contributed significantly to storm flow in amounts up to 40 depending on soil moisture conditions

the influence of soil properties on the erodibility of belgian loamy soils a study based on rainfall simulation experiments twenty soils from the leuven region were tested in the laboratory with a rainfall simulator their texture varied from loam to loamy sand on the basis of the results obtained they were classified as a function of the runoff and splash erodibility for every soil several properties were determined and tentatively used to explain the classifications based on the runoff and splash erodibility significant negative correlations were found between silt content aggregate stability c5 10 index water content at saturation and cohesion on the one side and erodibility on the other a positive correlation was found between sand content and erodibility

the effect of streambed topography on surface subsurface water exchange in mountain catchments a numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography at st kevin gulch an effluent stream in the rocky mountains of colorado subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well defined flow paths through the alluvium stream water filled flow paths in the alluvium referred to as substream flow paths returned to the stream a short distance downstream 1 to 10 m recharge to the substream flow paths occurred where stream water slope increased at the transition from pools 1 to steeper channel units 5 20 return of substream flow paths to the stream occurred where stream water slope decreased at the transition from steeper channel units to pools a net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments along our study reach at st kevin gulch the net inflow of water from subsurface to stream 1 6 ml s 1 m 1 underestimated the gross inflow 2 7 ml s 1 m 1 by 40 the influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water sediment contact times these effects could have important consequences for solute transport retention and transformation in catchments

development of channel organization and roughness following sediment pulses in single thread gravel bed rivers large episodic inputs of coarse sediment sediment pulses in forested mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness a conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels channels exhibited self organizing behavior to various degrees based on channel gradient presence of large in channel wood or other forcing elements the size of the sediment pulse and the number of bed mobilizing flows since disturbance typical channel changes following a sediment pulse were initial decreases in water depth in variability of bed elevations and in the regularity of bed form spacing trajectories of change subsequently showed increased average water depth more variable and complex bed topography and increased uniformity of bed form spacing bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale two to five channel widths than in streams without such forcing mechanisms five to 10 channel widths channel roughness increased as bed forms developed

probing deep weathering in the shale hills critical zone observatory pennsylvania usa the hypothesis of nested chemical reaction fronts in the subsurface weathering is both an acid base and a redox reaction in which rocks are titrated by meteoric carbon dioxide co2 and oxygen o2 in general the depths of these weathering reactions are unknown to determine such depths cuttings of rose hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the susquehanna shale hills observatory ssho pyrite concentrations are insignificant to depths of 23 m under the ridge and 8 9 m under the valley likewise carbonate concentrations are insignificant to 22 and 2 m respectively in addition a 5 6 m thick fractured layer directly beneath the land surface shows evidence for loss of illite chlorite and feldspar under the valley secondary carbonates may have precipited the limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale however carbonate depletion coincides with the winter water table observed at 20 m ridge and 2 m depth valley it would be fortuitous if carbonate containing strata are found under ridge and valley only beneath the water table furthermore pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature we propose that deep transport of o2 initiates weathering at ssho and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity according to this hypothesis the mineral distributions at ssho are nested reaction fronts that overprint protolith stratigraphy the fronts are hypothesized to lie subparallel to the land surface because o2 diffuses to the water table and causes oxidative dissolution of pyrite pyrite derived sulfuric acid h2so4 plus co2 also dissolve carbonates above the water table to understand how reaction fronts record long term coupling between erosion and weathering will require intensive mapping of the subsurface copyright 2013 john wiley sons ltd

hyporheic exchange in gravel bed rivers with pool riffle morphology laboratory experiments and three dimensional modeling received 7 june 2005 revised 30 june 2006 accepted 25 august 2006 published 31 january 2007 1 we report the first laboratory simulations of hyporheic exchange in gravel pool riffle channels which are characterized by coarse sediment steep slopes and three dimensional bed forms that strongly influence surface flow these channels are particularly important habitat for salmonids many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange with surface subsurface mixing measured directly from the concentration decay of a conservative tracer fluorescein injected into the surface flow near bed pressure measurements were also used to predict hyporheic exchange from a three dimensional pumping transport model comparison of the predicted and observed hyporheic exchange shows good agreement indicating that the major mechanism for exchange is bed form induced advection however the effect of bed forms is modulated by discharge and the degree of topographic submergence we also tested the performance of the hydrostatic pressure as a proxy for the observed near bed pressure in driving hyporheic exchange which would facilitate field measurement and analysis of hyporheic flow in natural rivers we found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows

modelling bedload yield in braided gravel bed rivers abstract this paper outlines an approach for estimating the annual bedload yield of a braided channel this procedure is based on the extension of theory of flow and sediment transport in braided rivers recently presented by paola 1996 the revised approach accounts explicitly for the relationship between increasing discharge and varying channel hydraulics and is suitable for use in obtaining bedload transport rate estimates over a range of discharges integration of such estimates using flow duration data allows annual bedload yield to be determined model parameterisation is achieved using topographic survey data for the waimakariri river new zealand comparison of modelled bedload yield with values estimated from repeated topographic surveys indicates that the model is able to accurately predict both the medium term c 30 years mean annual bedload yield of the waimakariri at crossbank the section 17 8 km upstream of the river mouth and also short term fluctuations in bedload yield associated with varying annual flow statistics streamwise patterns of volumetric erosion and deposition determined for a 45 km length of the waimakariri using the model are also in broad agreement with trends identified in topographic survey data for the period 1961 1997 however significant deviations between modelled and surveyed volumes of cut and fill are evident at some locations comparison of model performance with conventional applications of bedload transport equations which tend to underestimate transport rates for braided channels suggests that the approach presented here may represent a significant improvement this is the case because the model quantifies the relationship between braid intensity and spatial variability in flow hydraulics at a cross section output from the model suggests that braided rivers may transport a significant proportion of their annual bedload at lower discharges than those indicated by earlier theoretical approaches results also provide quantitative support for the argument that increased intensity of braiding may promote higher rates of bedload transport in gravel bed rivers

distinct silicon and germanium pathways in the soil plant system evidence from banana and horsetail plants strongly impact the continental silicon cycle by taking up si and precipitating opal phytoliths which are recycled into the soil studying ge incorporation a chemical analog of si relative to si may provide a useful tracer of si pathways however ge uptake and transport through plants and the impact on ge si of phytoliths remain poorly understood here we report ge uptake and accumulation and ge si fractionation in all plant parts and solutions from 1 hydroponic banana 2 in situ sampled banana and 3 horsetails we further combine these data with delta si 29 from banana plants our data reconcile opposite conclusions drawn from previous studies on ge uptake and pathways no discrimination of ge occurred at the root solution interface banana and horsetails were shown to accumulate ge in roots a previous study provided evidence of low ge si ratios in root phytoliths which contrasts with high bulk ge si ratios in roots we report here this suggests that ge is organically trapped in roots consequently shoots display lower ge si ratios without fractionation between shoot parts since ge would follow transpiration stream as silicon and is not discriminated between shoot parts this contrasts with the two step discrimination against heavy si isotopes at the root solution interface and then within the shoots the soil composition clays versus fe oxides has a leading role on the ge si signatures of plants which may in turn impact on the si and ge fluxes to the global ocean

time and the rivers flowing fluvial geomorphology since 1960 fluvial geomorphology has been the largest single subdiscipline within geomorphology for many decades fluvial geomorphic expertise is integral to understanding and managing rivers and to developing strategies for sustainable development this paper provides an overview of some of the significant advances in fluvial geomorphology between 1960 and 2010 with respect to conceptual models fluvial features and environments being studied tools used by fluvial geomorphologists geomorphic specialty groups within professional societies journals in which fluvial geomorphic research is published and textbooks of fluvial geomorphology during this half century fluvial geomorphology broadened considerably in scope from a focus primarily on physical principles underlying process and form in lower gradient channels with limited grain size range to a more integrative view of rivers as ecosystems with nonlinear behavior and great diversity of gradient substrate composition and grain size the array of tools for making basic observations analyzing data and disseminating research results also expanded considerably during this period as did the diversity of the fluvial geomorphic community

large woody debris and flow resistance in step pool channels cascade range washington abstract total flow resistance measured as darcy weisbach f in 20 step pool channels with large woody debris lwd in washington ranged from 5 to 380 during summer low flows step risers in the study streams consist of either 1 large and relatively immobile woody debris bedrock or roots that form fixed or forced steps or 2 smaller and relatively mobile wood or clasts or a mixture of both arranged across the channel by the stream flow resistance in step pool channels may be partitioned into grain form and spill resistance grain resistance is calculated as a function of particle size and form resistance is calculated as large woody debris drag combined grain and form resistance account for less than 10 of the total flow resistance we initially assumed that the substantial remaining portion is spill resistance attributable to steps however measured step characteristics could not explain between reach variations in flow resistance this suggests that other factors may be significant the coefficient of variation of the hydraulic radius explained 43 of the variation in friction factors between streams for example large woody debris generates form resistance on step treads and spill resistance at step risers because the form resistance of step pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel hence the distribution and function not just abundance of large woody debris is critical in steep step pool channels

mineral weathering and elemental transport during hillslope evolution at the susquehanna shale hills critical zone observatory located in the uplands of the valley and ridge physiographic province of pennsylvania the susquehanna shale hills critical zone observatory ssho is a tectonically quiescent first order catchment developed on shales of the silurian rose hill formation we used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical chemical weathering fluxes about 25 m of bedrock was also drilled to estimate parent composition depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate weathering front overlying this extending from 6 m below the bedrock soil interface up into the soil is the feldspar dissolution front in the soils depletion profiles for k mg si fe and al relative to the bedrock define the illite and chlorite reaction fronts when combined with a cosmogenic nuclide derived erosion rate on watershed sediments these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite 1 5 10 17 mol m 2 s 1 and illite 2 9 10 17 mol m 2 s 1 than observed in the laboratory mineral reactions result in formation of vermiculite hydroxy interlayered vermiculite and minor kaolinite during weathering exchangeable divalent cations are replaced by al as soil ph decreases the losses of mg and k in the soils occur largely as solute fluxes in contrast losses of al and fe are mostly as downslope transport of fine particles physical erosion of bulk soils also occurs results from a steady state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions chemical weathering losses of mg na and k are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters chemical weathering slows down in the valley floor and al and si even show net accumulation the simplest model for the hillslope that is consistent with all observations is a steady state clay weathering limited system where soil production rates decrease with increasing soil thickness

headwater peatland channels in south eastern australia the attainment of equilibrium abstract many small headwater catchments 2 in temperate south eastern australia store sediment in valley fills while accumulation in some of these systems commenced up to 30 000 years ago most did not commence filling with peat or clastic material until at least the mid holocene in such headwater settings many clastic valley fills develop cut and fill channels which contrast to some peatland settings where sinuous equilibrium channels have evolved four peatland systems within this dataset demonstrate stable channel systems which span nearly the full spectrum of observed valley floor slopes we assess new and published longitudinal data from these four channels and demonstrate that each of these channels has achieved equilibrium profiles new and published flow and survey data are synthesised to demonstrate how these peatland systems have attained equilibrium low rates of sediment supply and exceptionally high bank strengths have resulted in low width to depth ratios which accommodate rapid changes in flow velocity and depth with changes in discharge in small peatland channels planform adjustments have been sufficient to counter the energy provided by these hydraulically efficient cross sections and have enabled the achievement of regime energy slopes in larger and higher energy peatland channels large armoured stable bedforms have developed these bedforms integrate with planform adjustments to maintain a condition of minimum variance in energy losses as represented by the slope profiles and therefore a uniform increase in downstream entropy

riffle pool morphometry and stage dependant morphodynamics of a large floodplain river vereinigte mulde sachsen anhalt germany riffle pool sequences are a common feature of gravel bed rivers however mechanisms of their generation and maintenance are still not fully understood in this study a monitoring approach is employed that focuses on analysing cross sectional and longitudinal channel geometry of a large floodplain river vereinigte mulde sachsen anhalt germany with a high temporal and spatial resolution in order to conclude from stage dependant morphometric changes to riffle and pool maintaining processes in accordance with previous authors pool cross sections of the mulde river are narrow and riffle cross sections are wide suggesting that they should rather be addressed as two general types of channel cross sections than solely as bedforms at high flows riffles and pools in the study reaches changed in length and height but not in position pools were scoured and riffles aggraded a development which was reversed during receding flows below the threshold of 0 4qbf 40 bankfull discharge an index for the longitudinal amplitude of riffle pool sequences the bed undulation intensity or bedform amplitude is introduced and proved to be highly significant as a form parameter its first derivative as a process parameter the process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation it is indicated by increasing longitudinal bed amplitudes according to the observed dynamics of bed amplitudes maintenance of riffle pool sequences lags behind discharge peaks maximum bed amplitudes may be reached with a delay of several days after peak discharges increasing bed undulation intensity is interpreted to indicate bed mobility post flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools this type of transport behaviour is referred to as disconnected mobility the comparison of two river reaches one with undisturbed sediment supply the other with sediment deficit suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions it is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload copyright 2011 john wiley sons ltd

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

germanium geochemistry and mineralogy abstract germanium is enriched in the following geologic environments 1 1 iron meteorites and terrestrial iron nickel 2 2 sulfide ore deposits particularly those hosted by sedimentary rocks 3 3 iron oxide deposits 4 4 oxidized zones of ge bearing sulfide deposits 5 5 pegmatites greisens and skarns and 6 6 coal and lignitized wood in silicate melts ge is highly siderophile in the presence of native iron nickel otherwise it is highly lithophile among silicate minerals ge is concentrated in those having less polymerized silicate tetrahedra such as olivine and topaz in deposits formed from hydrothermal solutions ge tends to be enriched mostly in either sulfides or in fluorine bearing phases it is thus concentrated both in some hydrothermal sulfide deposits and in pegmatites greisens and skarns in sulfide deposits that formed from solutions having low to moderate sulfur activity ge is concentrated in sphalerite in amounts up to 3000 ppm sulfide deposits that formed from solutions having higher sulfur activity allowed ge to either form its own sulfides particularly with cu or to substitute for as sn or other metals in sulfosalts the ge in hydrothermal fluids probably derives from enrichment during the fractional crystallization of igneous fluids or is due to the incorporation of ge from the country rocks particularly from those containing organic material germanium bonds to lignin derivative organic compounds that are found in peat and lignite accounting for its common concentration in coals and related organic material germanium is precipitated from water together with iron hydroxide accounting for its concentration in some sedimentary and supergene iron oxide deposits it also is able to substitute for fe in magnetite in a variety of geologic environments in the oxidized zone of ge bearing sulfide deposits ge is concentrated in oxides hydroxides and hydroxy sulfates sometimes forming its own minerals it is particularly enriched in some iron and manganese bearing oxides and hydroxides including goethite up to 5300 ppm and hematite up to 7000 ppm

soil chemistry and shale weathering on a hillslope influenced by convergent hydrologic flow regime at the susquehanna shale hills critical zone observatory soil chemistry data major and rees are presented from a swale transect for comparison to similar measurements on a planar transect published previously for the susquehanna shale hills critical zone observatory similar reaction fronts are observed plagioclase dissolution is indicated by na and ca depletion and a negative eu anomaly clay dissolution followed by particle loss is accompanied by depletion of mg k fe al and si however in contrast to the planar transect soils along the swale transect especially in the topographically depressed site do not show smooth elemental profiles this documents both residuum soils and accumulation of colluvium sediments the soils in the swale transect are thicker and on average wetter than those along the planar transect however the ce anomaly observed in the swale soils is consistent with a generally oxic environment thus preferential flowpaths are an important mechanism for water transport preventing swale soils from water saturation

neogene rejuvenation of central appalachian topography evidence for differential rock uplift from stream profiles and erosion rates abstract the persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution in the eastern north american appalachians this question is manifest in the outstanding problem of whether topographic relief is in a quasi equilibrium state decaying slowly over many millennia or whether relief has increased during the late cenozoic here we present quantitative geomorphic data from the nonglaciated portion of the susquehanna river drainage basin that provide insight into these end member models analysis of channel profiles draining upland catchments in the northern valley and ridge appalachian plateau blue ridge and piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300 600 m elevation but not systematically associated with transitions from weak to resistant substrate cosmogenic 10 be inventories of modern stream sediment indicate that erosion rates are spatially variable ranging from 5 30 m myr above knickpoints to 50 100 m myr below knickpoints overall channel gradients normalized for drainage area scale linearly with catchment averaged erosion rates collectively regionally consistent spatial relationships among erosion rate channel steepness and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with 100 150 m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization channel response timescales imply that the onset of relative base level change predates 3 5 ma and may have begun as early as 15 ma we suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of north america during the neogene

alluvial cover dynamics in response to floods of various magnitudes the effect of the release of glaciogenic material in a swiss alpine catchment abstract fluvial incision into formerly glaciated landscapes is commonly affected by the release of glaciogenic sediments from the hillslopes via landsliding hillslope failure and debris flow activity these deposits typically comprise abundant block size clasts here we determine the ability of the fluvial processes to evacuate glacially derived sediments in an alpine hanging valley blanketed by 100 m thick glacial till we assess along stream profiles of basal shear stress during annual decadal and centennial floods by means of the hydrological model hec ras we use bed surface grain size data to i evaluate the shields stress profiles for these floods ii determine at which timescale the bed undergoes significant changes and to iii predict transport capacity patterns for each flood in addition we present digital photogrammetry based estimates of bed elevation changes over two selected time intervals that experienced low and high magnitude floods respectively the results indicate that floods with return periods of 10 years are capable of mobilizing the coarse fraction of the bed material thereby substantially altering the channel geometry the predicted near critical shields stress values for these floods in the area where the glacial deposits are thickest suggest a strong hillslope channel coupling which may have resulted in the formation of the knickzone observed in this sector however decadal to centennial floods are required to transport the material over long distances and scour the bed in the upper part of the inner gorge in addition the observed downstream trend from bed scouring to deposition in the inner gorge for these floods and the opposite trend for low magnitude floods suggest that these patterns are not directly controlled by downstream variations of transport capacity we propose that the supply of sediment through hillslope failure and bank collapse significantly contributes to the sediment flux for high magnitude floods we conclude that the morphology and the timescale of sediment transport in this glacially conditioned landscape have been substantially controlled by the hillslope derived supply of coarse grained material

regolith production rates calculated with uranium series isotopes at susquehanna shale hills critical zone observatory abstract in the critical zone where rocks and life interact bedrock equilibrates to earth surface conditions transforming to regolith the factors that control the rates and mechanisms of formation of regolith defined here as material that can be augered are still not fully understood to quantify regolith formation rates on shale lithology we measured uranium series u series isotopes 238 u 234 u and 230 th in three weathering profiles along a planar hillslope at the susquehanna shale hills observatory ssho in central pennsylvania all regolith samples show significant u series disequilibrium 234 u 238 u and 230 th 238 u activity ratios range from 0 934 to 1 072 and from 0 903 to 1 096 respectively these values display depth trends that are consistent with fractionation of u series isotopes during chemical weathering and element transport i e the relative mobility decreases in the order 234 u 238 u 230 th the activity ratios observed in the regolith samples are explained by i loss of u series isotopes during water rock interactions and ii re deposition of u series isotopes downslope loss of u and th initiates in the meter thick zone of bedrock that cannot be augered but that nonetheless consists of up to 40 clay silt sand inferred to have lost k mg al and fe apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m myr to 17 m myr with increasing regolith thickness from the ridge top to the valley floor with increasing distance from the ridge top toward the valley apparent equivalent regolith residence times increase from 7 kyr to 40 kyr given that the ssho experienced peri glacial climate 15 kyr ago and has a catchment wide averaged erosion rate of 15 m myr as inferred from cosmogenic 10 be we conclude that the hillslope retains regolith formed before the peri glacial period and is not at geomorphologic steady state both chemical weathering reactions of clay minerals and translocation of fine particles colloids are shown to contribute to mass loss of u and th from the regolith consistent with major element data at ssho this research documents a case study where u series isotopes are used to constrain the time scales of chemical weathering and regolith production rates regolith production rates at the ssho should be useful as a reference value for future work at other weathering localities

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

flood magnitude frequency and lithologic control on bedrock river incision in post orogenic terrain abstract mixed bedrock alluvial rivers bedrock channels lined with a discontinuous alluvial cover are key agents in the shaping of mountain belt topography by bedrock fluvial incision whereas much research focuses upon the erosional dynamics of such rivers in the context of rapidly uplifting orogenic landscapes the present study investigates river incision processes in a post orogenic cratonic landscape undergoing extremely low rates of incision the mobility of coarse bed materials up to 2 m diameter during late holocene palaeofloods of known magnitude and age is evaluated using step backwater flow modelling in conjunction with two selective entrainment equations a new approach for quantifying the formative flood magnitude in mixed bedrock alluvial rivers is described here based on the mobility of a key coarse fraction of the bed materials in this case the d84 size fraction a 350 m3 s formative flood fully mobilises the coarse alluvial cover with b 200 300 n m2 across the upper and lower gorge riffles peaking over 500 n m2 in the knickzone such floods have an annual exceedance probability much less than 10 2 and possibly as low as 10 3 the role of coarse alluvial cover in the gorge is discussed at two scales 1 modulation of bedrock exposure at the reach scale coupled with adjustment to channel width and gradient accommodates uniform incision across rocks of different erodibility in steady state fashion and 2 at the sub reach scale where coarse boulder deposits corresponding to b minima cap topographic convexities in the rock floor thereby restricting bedrock incision to rare large floods while recent studies postulate that decreasing uplift rates during post orogenic topographic decay might drive a shift to transport limited conditions in river networks observations here and elsewhere in post orogenic settings suggest to the contrary that extremely low erosion rates are maintained with substantial bedrock channel exposure although bed material mobility is known to be rate limiting for bedrock river incision under low sediment flux conditions exactly how a partial alluvial cover might be spatially distributed to either optimise or impede the rate of bedrock incision is open to speculation observations here suggest that the small volume of very stable bed materials lining sandy creek gorge is distributed so as to minimise the rate of bedrock fluvial incision over time

form resistance in gravel channels with mobile beds form resistance which includes pressure drag on bed forms and pressure and viscous drag on bed particles in transport is shown to be a major component of hydraulic roughness in steady quasi uniform two dimensional turbulent flow over mobile gravel beds dimensionless form shear stress exceeds dimensionless particle shear stress for values of about three times critical shields stress in subcritical flow and five times in supercritical flow theoretical analysis provides a physical basis for expressing friction factor as a function of particle friction factor estimated by the keulegan equation and the ratio of form to particle shear stress termed relative form shear stress dimensional reasoning and empirical modeling of flume data demonstrate that relative form shear stress depends on shields stress and relative roughness in subcritical and upper supercritical flow regimes and on relative roughness alone in the lower supercritical regime in particular for given shields stress and increasing rela

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

a global ge isotope budget we present measurements of ge isotope composition and ancillary data for samples of river water low and high temperature hydrothermal fluids and seawater the dissolved 74ge composition of analyzed rivers ranges from 2 0 to 5 6 which is significantly heavier than previously determined values for silicate rocks 74ge 0 4 0 7 escoube et al geostand geoanal res 36 2 2012 from which dissolved ge is primarily derived an observed negative correlation between riverine ge si and 74ge signatures suggests that the primary 74ge fractionation mechanism during rock weathering is the preferential incorporation of light isotopes into secondary weathering products high temperature 150 c hydrothermal fluids analyzed in this study have 74ge of 0 7 1 6 most likely fractionated during fluid equilibration with quartz in the reaction zone low temperature 25 63 c hydrothermal fluids are heavier 74ge between 2 9 and 4 1 and most likely fractionated during ge precipitation with hydrothermal clays seawater from the open ocean has a 74gesw value of 3 2 0 4 and is indistinguishable among the different ocean basins at the current level of precision this value should be regulated over time by the isotopic balance of ge sources and sinks and a new compilation of these fluxes is presented along with their estimated isotopic compositions assuming steady state non opal ge sequestration during sediment authigenesis likely involves isotopic fractionation 74gesolid solution that is 0 6 1 8

bar form resistance in gravel bed rivers resistance to flow in unvegetated gravel bed rivers is basically dependent on the skin resistance due to surface bed material and bar form resistance due to accelerations and decelerations in the flow between pools and riffles although flow is generally nonuniform at riffle sections flow is locally uniform it is these sections which principally control the velocity depth characteristics of the reach since hydraulic conditions in the pool result from the backwater effect of the riffle isolating the effect of bar forms on flow resistance is achieved by first establishing the roughness height of the surface bed material on the riffle overall resistance to flow in terms of the total roughness height due to bed forms and grains for equivalent uniform flow is determined from the riffle and reach average flow geometry and the grain roughness height differences between total and grain roughness heights define the bar form effect field data from 62 sites in the united kingdom illustrate that at bankful

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

the co2 consumption potential during gray shale weathering insights from the evolution of carbon isotopes in the susquehanna shale hills critical zone observatory abstract shale covers about 25 of the land surface and is therefore an important rock type that consumes co 2 during weathering we evaluated the potential of gray shale to take up co 2 from the atmosphere by investigating the evolution of dissolved inorganic carbon dic concentrations and its carbon isotopic ratio 13 c dic along water flow paths in a well characterized critical zone observatory susquehanna shale hills catchment in this catchment chemical weathering in shallow soils is dominated by clay transformation as no carbonates are present and soil pore waters are characterized by low dic and ph in shallow soil porewaters the dic dominated by dissolved co 2 is in chemical and isotopic equilibrium with co 2 in the soil atmosphere where pco 2 varies seasonally to as high as 40 times that of the atmosphere the degradation of ancient organic matter is negligible in contributing to soil co 2 the chemistry of groundwater varies along different flowpaths as soil pore water recharges to the water table and then dissolves ankerite or secondary calcite under the valley floor weathering of carbonate leads to much higher concentrations of dic 2500 mol l and divalent cations ca 2 and mg 2 in groundwaters than soil waters the depth to the ankerite weathering front is hypothesized to be roughly coincident with the water table but it varies due to heterogeneities in the protolith composition groundwater chemistry therefore shows different saturation indices with respect to ankerite depending upon location along the valley the 13 c dic values of these groundwaters document mixing between the ankerite and soil co 2 the major element concentrations dic and 13 c dic in the first order stream incising the valley of the catchment are derived from groundwater and soil waters in proportions that vary both spatially and temporally the co 2 degassed slightly in the stream but little evidence of c isotopic equilibration with the atmosphere is observed due to the short length of the stream and short contact time with air the ankerite reaction front also lies close to the pyrite dissolution front pyrite oxidation in bedrock likely released sulfuric acid and played a minor role in the ankerite dissolution shifting groundwater 13 c dic slightly above the expected mixing values at the catchment scale the stream so 4 2 is also dominantly derived from wet deposition as stream has 34 s so4 values around 3 well within the range of acid deposition a mass balance calculation shows that silicate and ankerite dissolution of the rose hill shale at shale hills consumes co 2 at a rate of 44 and 42 48 mol m 2 ky 1 respectively while degradation of ancient organic matter releases co 2 at a rate of 1 3 mol m 2 ky 1 silicate dissolution at the shallow soils is facilitated by low ph and high soil pco 2 as ankerite dissolution and organic matter oxidation are shown to occur early during shale alteration co 2 consumption by shale weathering is thus limited by initiation of rock disintegration e g fractures and exposure of fresh surface area to infiltrating co 2 and o 2 rich water

germanium sequestration by soil targeting the roles of secondary clays and fe oxyhydroxides abstract germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes sequestration of ge by clay minerals is thought to be the dominant mechanism responsible for ge si fractionation during weathering in some environments ge may be sequestered by iron oxyhydroxides we examine ge behavior in soils along a well characterized climate gradient on the hawaiian island of maui to clarify the relationship between fe geochemistry and soil ge si ratios all of the sites are developed on basaltic lavas and tephras approximately 400 ky old are highly si depleted and have ge si ratios higher than parent material sites experiencing less than 3300 mm rain yr host oxidized highly weathered basaltic soils in which fe is present as secondary fe oxyhydroxide minerals in contrast the wetter sites 3340 mm yr show extensive fe reduction mobilization and loss we show that ge sequestration is independent of fe redox behavior along the gradient and thus precipitation of fe oxyhydroxides is not a major factor contributing to ge si fractionation during weathering instead elevated soil ge si ratios reflect partitioning of ge into secondary clay minerals plus additional retention of ge by refractory non silicates

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

a semidiscrete finite volume formulation for multiprocess watershed simulation 1 hydrological processes within the terrestrial water cycle operate over a wide range of time and space scales and with governing equations that may be a mixture of ordinary differential equations odes and partial differential equations pdes in this paper we propose a unified strategy for the formulation and solution of fully coupled process equations at the watershed and river basin scale the strategy shows how a system of mixed equations can be locally reduced to ordinary differential equations using the semidiscrete finite volume method fvm domain decomposition partitions the watershed surface onto an unstructured grid and vertical projection of each element forms a finite volume on which all physical process equations are formed the projected volume or prism is partitioned into surface and subsurface layers leading to a fully coupled local ode system referred to as the model kernel the global ode system is assembled by combining the local ode system over the domain and is then solved by a state of the art ode solver the unstructured grid based on delaunay triangulation is generated with constraints related to the river network watershed boundary elevation contours vegetation geology etc the underlying geometry and parameter fields are then projected onto the irregular network the kernel based formulation simplifies the process of adding or eliminating states constitutive laws or closure relations the strategy is demonstrated for the shale hills experimental watershed in central pennsylvania and several phenomena are observed 1 the enslaving principle is shown to be a useful approximation for soil moisture water table dynamics for shallow soils in upland watersheds 2 the coupling shows how antecedent moisture i e initial conditions can amplify peak flows 3 the coupled equations predict the onset or threshold for upland ephemeral channel flow and 4 the model shows how microtopographic information controls surface saturation and connectivity of overland flow paths for the shale hills site the open source code developed in this research is referred to as the penn state integrated hydrologic model pihm

do alternate bars affect sediment transport and flow resistance in gravel bed rivers in natural gravel bed streams the complex topography of the bed can cause variation of flow resistance and sediment transport previous studies have shown that in addition to grain resistance skin friction flow resistance is also caused by bank roughness channel bars bed undulations and channel curvature sediment transport is similarly influenced by the complex topography and the transport rate can vary spatially a three dimensional 3d numerical model was used to generate a detailed description of the flow and bedload transport fields in gravel bed rivers here we quantify the reach averaged hydraulic resistance and sediment transport regime that prevails when self formed alternate river bars are present and compare with the regime that would prevail were no bars are present we do this by comparing the results of i a 3d morphodynamic model in which bars form as a consequence of flow bed interaction and ii an equivalent one dimensional 1d case which refers to flat bed conditions but otherwise corresponding to identical average velocity and bed slope the 3d numerical model is applied to generate different bed topographies of alternate bars at regime morphological equilibrium then extended to non equilibrium conditions for decreasing shear stress within a sensitivity analysis context the contribution of grain resistance is estimated with the local values of the bed shear stress while bar resistance results from the overall deviation of the flow field from that occurring in the flat bed configuration the local sediment transport in both the longitudinal and transverse directions is computed with the local shields stress and local bed inclination the calculations result in a method for correcting 1d models to account for the total sediment transport and resistance in a cross section due to 3d effects of alternate bars we term the resulting relations morphologically averaged sediment transport and resistance equations copyright 2012 john wiley sons ltd

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

strontium isotopes trace biological activity in the critical zone along a climate and vegetation gradient abstract weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock the geogenic nutrient pathway however the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter the organic nutrient cycle isotope ratios of strontium sr an element that behaves similarly to ca in ecosystems confer two types of information radiogenic sr isotopes inform as to the sources of sr and the degree of weathering while stable sr isotopes constrain partitioning between compartments of the critical zone bedrock water secondary solids and plants to date however neither the reactions nor the mass balance between compartments that fractionate sr isotopes nor the fractionation factors involved are well understood here we present geochemical budgets of sr using radiogenic and stable sr isotopes and ca sr ratios at four sites along a substantial climate and primary production gradient in the coastal mountains of chile we found that sr release through weathering is isotopically congruent and released sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool despite this the 88sr 86sr ratio of bio available sr which should reflect the ratio of dissolved sr is higher than that of rock and regolith we propose that this offset is caused by plants while 88sr 86sr in plant organs at the four study sites systematically increased from roots towards their leaves whole plant sr isotope compositions indicate preferential uptake of light sr into plants with a fractionation of up to 0 3 relative to the bio available pool despite this strong biological fractionation 88sr 86sr ratios in bio available sr do not covary with biomass production across our study sites because with greater plant growth sr is recycled more times after release by weathering an isotope neutral process rather the loss of sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio available sr organic solids thus appear to constitute a significant export path of elements released during weathering with the removal of solid plant debris reducing the recycling factor of sr and possibly that of other mineral nutrients too

tropical pacific high latitude south atlantic teleconnections as seen in δ18o variability in antarctic coastal ice cores 1 we use a network of eight ice cores from coastal dronning maud land dml antarctica to examine the role of the tropical enso el nino southern oscillation in the temporal variability of 18o in annual accumulation the longest record from the s100 ice core covering the period 1737 1999 is used to analyze the teleconnections between the tropical pacific and coastal dml on decadal scales and longer a shorter stacked coastal dml 18o series spanning 1955 1999 is constructed to assess the variability of enso teleconnection on interannual scales results suggest that on typical enso timescales of 2 6 years the strength of the teleconnection varies in time being stronger for years with generally negative phase of the southern annular mode sam on the timescales of approximately two decades bidecadal positive isotope anomalies are associated with oceanic warming and a westward sea surface temperature sst gradient in the equatorial pacific bidecadal variability in sam forced by the tropical pacific is proposed as a critical element in the teleconnection our analysis suggests that a multidecadal positive trend in the annual mean 18o values from the analyzed cores can be indicative of the atmospheric warming that begun in this part of the dml already in the 1910s the trend in 18o quantified in terms of long term surface air temperature sat changes is consistent with the instrumental data yet we speculate that the accurate estimation of sat trends requires an assessment of the potential role of secular sam and sea ice extent changes in shaping the isotopic signal

erodibility and runoff infiltration characteristics of volcanic ash soils along an altitudinal climosequence in the ecuadorian andes abstract soil erosion is a widespread phenomenon in andean south america where many regions are covered with soils derived from volcanic parent materials climate induced differences in the genesis of these soils have been demonstrated along toposequences on volcanic slopes this research was conducted to study the impact of such differential pedogenesis on erodibility and runoff infiltration characteristics along an altitudinal entisols inceptisols andisols sequence in the andes of northern ecuador surface soils were packed into small pans and placed on a 9 slope and a simulated rainstorm with varying intensities was applied for a duration of 30 min the runoff erosion behaviour of the studied volcanic ash soils is strongly affected by their pedological development accumulation of organic matter and precipitation of active amorphous materials at high elevations have led to the formation of well developed andisols with very stable aggregate structure these soils remain wettable when air dried show very high infiltration capacity and consequently low potential for runoff generation and soil erosion low organic matter contents and absence of active amorphous materials at low elevations have led to the formation of weakly aggregated entisols and inceptisols these soils are susceptible to surface crusting which lowers their infiltration capacity and increases their erodibility however in comparison with other soils of different origin and composition the interrill erodibilities determined for these more erodible low elevation soils are classified as low the findings of this study suggest that upland soil erosion is not a major threat to sustainability in the studied volcanic landscape which is generally confirmed by field observations

ca sr and 87sr 86sr ratios as tracers of ca and sr cycling in the rio icacos watershed luquillo mountains puerto rico abstract we investigated ca and sr cycling in a humid tropical forest by analyzing ca sr ratios and 87sr 86sr ratios in soil minerals soil exchangeable cations soil porewater and plant roots wood and leaves and calculating the relative contributions of sr from atmospheric inputs and weathering of local bedrock an unexpectedly large contribution of bedrock derived sr and presumably ca is cycled through the vegetation reflecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil this is surprising because over 99 of the ca and sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site and because there are large atmospheric inputs to the site of both sea salt and saharan dust substantial differences in ca and sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site measured ca sr ratios reflect fractionation between these elements during biogeochemical cycling fractionation was particularly evident between wood and foliar tissue but fractionation during soil exchange processes is also likely in comparing the ca sr ratios of plants exchangeable cations and bulk soils we found that foliar ca sr ratios were greater than exchangeable cation ca sr ratios which in turn were greater than soil ca sr ratios similar to patterns observed at other highly weathered tropical sites

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

a mathematical model for steady state regolith production at constant erosion rate it has been hypothesized that many soil profiles reach a steady state thickness in this work such profiles were simulated using a one dimensional model of reaction with advective and diffusive solute transport a model rock is considered consisting of albite that weathers to kaolinite in the presence of chemically inert quartz the model yields three different steady state regimes of weathering at the lowest erosion rates a local equilibrium regime is established where albite is completely depleted in the weathering zone this regime is equivalent to the transport limited regime described in the literature with an increase in erosion rate transition and kinetic regimes are established in the transition regime both albite and kaolinite are present in the weathering zone but albite does not persist to the soil air interface in the weathering limited regime here called the kinetic regime albite persists to the soil air interface the steady state thickness of regolith decreases with increasing erosion rate in the local equilibrium and transition regimes but in the kinetic regime this thickness is independent of erosion rate analytical expressions derived from the model are used to show that regolith production rates decrease exponentially with regolith thickness the steady state regolith thickness increases with the darcy velocity of the pore fluid and in the local equilibrium regime may vary markedly with small variations in this velocity and erosion rate in the weathering limited regime the temperature dependences for chemical weathering rates are related to the activation energy for the rate constant for mineral reaction and to the h of dissolution while for local equilibrium regimes they are related to the h only the model illustrates how geochemical and geomorphological observations are related for a simple compositional system the insights provided will be useful in interpreting natural regolith profiles copyright 2010 john wiley sons ltd

estimation of flow resistance in gravel bedded rivers a physical explanation of the multiplier of roughness length the need to estimate velocity and discharge indirectly in gravel bedded rivers is a commonly encountered problem semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information although such equations have a semi theoretical basis in natural gravel bed channels an empirical constant 6 8 or 3 5 has to be introduced to scale up the characteristic grain size d50 or d84 to represent the effective roughness length in this paper two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small scale form resistance reflecting the occurrence of microtopographic bedforms in gravel bedded environments first spatial elevation dependence in short detailed bed profiles from a single gravel bedded river is investigated using semivariogram and zero crossing analyses this leads to objective identification of two discrete scales of bed roughness associated with grain and microtopographic roughness elements second the autocorrelation structure of the three dimensional near bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms apart from improving the capacity to determine friction factors for velocity and discharge estimation the findings have implications in general for the initial motion of gravelly bed material

a consistency index and rainwash mechanisms on belgian loamy soils the effects of soil crusting in belgium are particularly conspicuous on the loamy plateaus and hill regions of southern flanders brabant and hesbaye field observations made at the end of rainy winter seasons permitted the making of a simple distinction between a stable soils showing crusting on less than 50 per cent of their total surface without any evident slope wash and b unstable soils crusting over more than 50 per cent of the total surface with evident rainwash and basal colluviation the atterberg limits have been examined for both soil types it was found that the upslope part of the liquid limit curves in the range of low consistencies high water contents is steeper for stable than for unstable soils so a distinctive consistency index c2 10 was deduced from the liquid limit curves which may be used for prediction of the relative sensitivity of topsoils for crusting stable well structured clods are more hydrophobic than unstable material this was made clear by wettability tests under a rain simulator therefore more water must be added to remolded stable material to lower its consistency as is shown by the liquid limit curves crustability decreases and c5 10 values increase with increasing clay and organic matter contents as one would expect stable soils are well aggregated but rain simulator tests also showed that the detachment of this material by raindrop impact is high therefore crumbs and clods of stable soils are relatively more sensitive to splash erosion their erosion results from the ejection of stable aggregates clods of unstable topsoils are compacted and flattened by raindrop impact which causes internal liquefaction micromorphological analysis indicates that typical stable topsoils originate from well structured ap horizons and from bt horizons of grey brown podzolic soils alfisols laminated silt and mudcrust fragments characterize unstable topsoils which originate at least partly from slopewash deposits the external stability of both stable and unstable clods is increased by compaction and by surficial concentration of gravel coarse sands root and other plant fragments thin sections of unstable material show the internal collapse of structures due to liquefaction a clear distinction must be made between the structural stability and the erodability of topsoils

frequency distribution of river bed materials the size distribution of river bed material plays an important role in the study of fluvial hydraulic processes it is not always easy to sample the river bed material for the determination of its size distribution but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution therefore data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them the log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered the method of power transformation is found to normalize these mixtures relationships have been proposed between the parameters of the normalization and the stream slope these relations can be used to predict the size distribution of the river bed material

perennial flow through convergent hillslopes explains chemodynamic solute behavior in a shale headwater catchment abstract stream chemistry reflects the mixture of complex biogeochemical reactions that vary across space and time within watersheds for example streams experience changing hydrologic connectivity to heterogeneous water sources under different flow regimes however it remains unclear how seasonal flow paths link these different sources and regulate concentration discharge behavior i e changes in stream solute concentration as a function of discharge at the susquehanna shale hills critical zone observatory sshczo in central pennsylvania usa concentrations of chemostatic solutes k mg na si cl vary little across a wide range of discharge values while concentrations of chemodynamic solutes fe mn ca decrease sharply with increasing stream discharge to elucidate controls on chemodynamic solute behavior we investigated the chemistry of surface water and shallow subsurface water at the sshczo in early autumn when discharge was negligible and concentrations of chemodynamic solutes were high dissolved ions colloids and micron sized particles were extracted from hillslope soils and stream sediments to evaluate how elements were mobilized into pore waters and transported from hillslopes to the stream during the study period when flow was intermittent the stream consisted of isolated puddles that were chemically variable along the length of the channel inputs of subsurface water to the stream were limited to an area of upwelling near the stream headwaters and the water table remained over a meter below the stream bed along the rest of the channel chemodynamic elements fe and mn were preferentially mobilized from organic rich soils as a mixture of dissolved ions colloids and micron sized particles consequently subsurface water draining organic rich soils in the upper catchment was enriched in fe and mn conversely ca increased towards the catchment outlet and was primarily mobilized from stream sediments as ca2 concentrations of chemostatic solutes were relatively invariable throughout the catchment we conclude that chemodynamic behavior at sshczo is driven by seasonally variable connectivity between the stream and hillslope soils during the dry season stream water derives from a shallow perched water table interflow that upwells to generate metal rich stream headwaters high concentrations of soluble fe and mn at low discharge occur when metal rich headwaters are flushed to the catchment outlet during periodic rain events interflow during the dry season originates from water that infiltrates through organic rich swales thus metals in the stream at low flow are ultimately derived from convergent hillslopes where biological processes have concentrated and or mobilized these chemodynamic elements in contrast high concentrations of ca2 at low discharge are likely mobilized from stream sediments that contain secondary calcite precipitates we infer that chemodynamic solutes are diluted at high discharge primarily due to increased flow through planar hillslopes this study highlights how spatially heterogeneous biogeochemistry and seasonally variable flow paths regulate concentration discharge behavior within catchments

magnesium isotope fractionation during shale weathering in the shale hills critical zone observatory accumulation of light mg isotopes in soils by clay mineral transformation abstract magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments but few have focused on the mechanisms that control mg isotope fractionation during shale weathering in this study we focus on understanding mg isotope fractionation in the shale hills catchment in central pennsylvania mg isotope ratios were measured systematically in weathering products along geochemical pathways of mg during shale weathering from bedrock to soils and soil pore water on a planar hillslope and to sediments stream water and groundwater on a valley floor significant variations of mg isotopic values were observed 26mg values 0 6 to 0 1 of stream and soil pore waters are about 0 5 to 1 lighter than the shale bedrock 26mg values of 0 4 consistent with previous observations that lighter mg isotopes are preferentially released to water during silicate weathering dissolution of the carbonate mineral ankerite depleted in the shallow soils but present in bedrock at greater depths produced higher mg2 concentrations but lower 26mg values 1 1 in groundwater 1 5 lighter than the bedrock 26mg values 0 2 to 0 4 of soil samples on the planar hillslope are either similar or up to 0 2 lighter than the bedrock hence a heavy mg isotope reservoir complementary to the lighter mg isotopes in soil pore water and stream water is missing from the residual soils on the hillslope in addition soil samples show a slight but systematic decreasing trend in 26mg values with increasing weathering duration towards the surface we suggest that the accumulation of light mg isotopes in surface soils at shale hills is due to a combined effect of i sequestration of isotopically light mg from soil water during clay dissolution precipitation reactions and ii loss of isotopically heavy particulate mg in micron sized particles from the hillslope as suspended sediments this latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation stream sediments 26mg values of 0 3 to 0 5 accumulated on the valley floor are 0 2 heavier than the bedrock and are thus consistent with that mobile particulates are the heavy mg isotope reservoir our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter mg isotopes in residual bulk soils this example also demonstrates that transport of isotopically distinct fine particles from clay rich systems could be a new and important mechanism to drive the mg isotope compositions of silicate weathering residuals this mechanism drives fractionation in an opposite direction as might be expected from previous studies i e residual soils are driven to lighter mg values and sediments become isotopically heavier

a geostatistical framework for quantifying the reach scale spatial structure of river morphology 2 application to restored and natural channels abstract alluvial rivers are shaped by interactions between the bed topography the flow field and the movement of sediment to help refine our understanding of these connections between form and process i developed a geostatistical framework for quantifying the reach scale spatial structure of river morphology described in a companion paper in this study i applied this approach to a restored channel and three disparate reaches of a dynamic natural stream repeat topographic surveys of each site were used to examine relationships between channel change and the variability and organization of the topography for the restored river the development of point bars increased overall morphologic diversity primarily because of greater cross sectional asymmetry the three natural reaches experienced a variety of adjustments ranging from 1 gradual bar growth and bank erosion to 2 extensive deposition followed by channel abandonment and 3 chute cutoff and incision of a new channel in both the restored and natural streams geostatistical analysis which involved variogram modeling calculation of integral metrics and inspection of variogram maps provided an effective informative summary of the observed channel changes the use of dimensionless variables accounted for channel size highlighted differences in spatial structure and enabled a comparison among sites the restored reach had not yet achieved the same degree of heterogeneity as the more pristine channels emphasizing variability and spatial pattern via this geostatistical framework could yield insight on form process interactions and help to quantify geomorphic complexity and habitat heterogeneity in the applied context of river restoration

geochemical behaviors of different element groups during shale weathering at the susquehanna shale hills critical zone observatory major and trace element concentrations were measured in bedrock regolith and stream sediments from a first order catchment developed entirely on grey shale in central pennsylvania usa these elements can be classified into five major groups based on statistical data analysis the presence of different elemental groups is due to the mineralogical origin cycling processes and geochemical properties of these elements during soil formation a better understanding of the behaviors of these elements during chemical weathering would allow for their possible use as natural tracers in critical zone processes

changing sources of base cations during ecosystem development hawaiian islands 87 sr 86 sr evidence from a soil chronosequence in the hawaiian islands demonstrates that the atmosphere supplies 85 of putatively rock derived sr in older sites initially bedrock is the dominant source for sr and other lithophile elements such as ca but high rates of weathering and leaching of the substrate by 20 ka lead to a shift to atmospheric sources the loss of weathering inputs coincides with other physio chemical changes in the soil and results in a steep decline of base cations in the soil pool while these patterns imply the potential for limitation of biological productivity by low base cation supply the atmosphere provides a supply of base cations in excess of nutritional needs even after nearly all rock derived base cations have been leached from the soil this raises the possibility that p limitation in terrestrial ecosystems may develop at least as much because of low rates of atmospheric deposition of p relative to ca k and other rock derived elements as because of its chemical interaction in soil

how mineralogy and slope aspect affect ree release and fractionation during shale weathering in the susquehanna shale hills critical zone observatory abstract to understand the factors that control rare earth elements ree release and fractionation during shale weathering we investigate the ree contents in solid bedrock regolith stream sediments and natural waters stream and pore waters from a first order catchment developed entirely on gray shales in central pennsylvania usa up to 65 of the ree relative to parent bedrock is depleted from the weathering profiles in the acidic and organic rich soils due to chemical leaching in addition newly formed fine particles were also lost along with the down slope movement of soil waters weathering profiles on the south facing slope show less depletion of ree than those on the north facing slope 33 vs 45 on average we hypothesize that the different degrees of ree depletion on the two transects reflect a history of different chemical weathering rates and possible different surface erosion rates controlled by contrasting slope aspect induced microclimate conditions in addition weathering profiles natural waters and sequential extractions all show a preferential removal of middle ree up to 22 more relative to light ree and heavy ree during shale weathering due to preferential release of mree from rhabdophane furthermore the long term phosphate mineral dissolution rates e g rhabdophane were estimated at 10 15 to 10 14 mol m 2 s 1 under field conditions based on ree depletion profiles strong positive ce anomalies average ce ce n value 1 79 observed in the regolith stream sediments and regolith extractions point to the fractionation and preferential precipitation of ce as compared to other ree due to the generally oxidizing conditions during release transport and redistribution of ree in the surface and subsurface environments positive eu anomalies average eu eu n value 1 30 observed in the natural waters of the catchment are attributed to weathering of plagioclases in the shale bedrock this study highlights the use of ree as natural tracers for low temperature geochemical processes

variables influencing water surface slopes in gravel bed streams at bankfull stage the relationship between water surface slope and 12 hydraulic variables was examined for a reach scale and a more local scale using data from 12 reaches measurements of width depth velocity slope and roughness elements were made on 12 reaches of gravel bed streams at bankfull stage all of the reaches have fairly well developed riffle pool sequences and are either straight or braided in channel pattern measurements were made at numerous locations within each reach so that the influence of hydraulic and resistance variables on water surface slope could be examined both within and among reaches results show that roughness elements and resistance coefficients correlate most highly with water surface slope bed particle size correlates well with water surface slope at both reach and local scales the magnitude of bed undulations correlates well with slope at the scale of a reach in the type of stream channels examined the important components of resistance in the channels are also the most important influences on water surface slope

constraining soil mineral weathering 87sr 86sr for calcium apportionment studies of a deciduous forest growing on soils developed from granitoid igneous rocks abstracts we used 87 sr 86 sr as a proxy for ca to apportion the contribution of atmospheric deposition and soil mineral weathering sources to the ca pools in trees in a small forested watershed of southern quebec the effects of topography and forest stand composition were assessed by dividing the watershed into three study zones representing two elevations and differences in tree species apportionment calculations show that all tree species studied contained ca that is dominantly sourced from soil mineral weathering averaging 85 due to the presence in the soils of easily weathered ca bearing minerals such as calcite and apatite as well as the relatively deep rooting habits reaching the mineral soil of the tree species calcium pools in trees are shifted towards the atmospheric component on hilltops due to the filtering of dry aerosols of the tree canopies which are more exposed to atmospheric circulation than trees at the bottom of the slopes although some variability in soil bulk chemistry and mineralogy in the watershed obscures the full extent of this phenomenon the buildup and recycling of ca in the soil vegetation system with the forest floor acting as the major accessible ca pool for the tree roots must be considered when interpreting apportionment calculations this work supports other ca cycling models in forests suggesting that topography can influence the balance of ca captured from atmospheric sources by trees even in cases where mineral weathering is the dominant source of ca nutrition in trees

lateral sediment sources and knickzones as controls on spatio temporal variations of sediment transport in an alpine river modern mixed alluvial bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse grained material has been entrained and transported from their sources to the adjacent sedimentary sink where these deposits are preserved as conglomerates this article assesses the shear stress conditions needed for the entrainment of the coarse bed particles in the glogn river that drains the 400 km2 val lumnezia basin eastern swiss alps in addition quantitative data are presented on sediment transport patterns in this stream the longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0 02 to 0 2 m m 1 and where the valley bottom confined into a 10 m wide gorge downstream of these knickzones the stream is flat with gradients 0 01 m m 1 and widths 30 m measurements of the grain size distribution along the trunk stream yield a mean d84 value of ca 270 mm whereas the mean d50 is ca 100 mm the consequences of the channel morphology and the grain size distribution for the time scales of sediment transport were explored by using a one dimensional step backwater hydraulic model hydrologic engineering centre river analysis system the results reveal that along the entire trunk stream a two to 10 year return period flood event is capable of mobilizing both the d50 and d84 fractions where the shields stress exceeds the critical shields stress for the initiation of particle motion these return periods however varied substantially depending on the channel geometry and the pebble boulder size distribution of the supplied material accordingly the stream exhibits a highly dynamic boulder cover behaviour it is likely that these time scales might also have been at work when coarse grained conglomerates were constructed in the geological past