inception of sediment transport on steep slopes using dimensional and theoretical analysis together with available experimental data it has been shown how flow conditions and stream bed slope affect the beginning of sediment transport the dimensional analysis shows that in general the shields parameter is a function of the stream slope and the reynolds number the traditional shields diagram is a special case of this relationship for a zero of near zero slope the theoretical treatment is an adaptation of work from earlier authors which is used in conjunction with dimensional analysis to show the interdependence of flow conditions and forces contributing to the critical point of particle motion additional experiments are recommended

identification of sulfur sources and isotopic equilibria in submarine hot springs using multiple sulfur isotopes abstract multiple sulfur isotopes were measured in metal sulfide deposits elemental sulfur and aqueous hydrogen sulfide to constrain sulfur sources and the isotopic systematics of precipitation in seafloor hydrothermal vents areas studied include the eastern manus basin and lau basin back arc spreading centers and the unsedimented basalt hosted southern east pacific rise sepr and sediment hosted guaymas basin mid ocean ridge spreading centers chalcopyrite and dissolved hydrogen sulfide h 2 s 34 s values range from 5 5 to 5 6 in manus basin samples 2 4 to 6 1 in lau basin samples and 3 7 to 5 7 in sepr samples values of 34 s for cubic cubanite and h 2 s range from 1 4 to 4 7 in guaymas basin samples multiple sulfur isotope systematics in fluid mineral pairs from the sepr and lau basin show that crustal host rock and thermochemical reduction of seawater derived dissolved sulfate so 4 are the primary sources of sulfur in mid ocean ridge and some back arc systems at pacmanus and susu knolls hydrothermal systems in the eastern manus basin a significant contribution of sulfur is derived from disproportionation of magmatic sulfur dioxide so 2 while the remaining sulfur is derived from crustal host rocks and so 4 reduction at the sedimented guaymas basin hydrothermal system sulfur sources include crustal host rock reduced seawater so 4 and biogenic sulfide vent fluid flow through fresher less mature sediment supplies an increased quantity of reactant organic compounds that may reduce 34 s enriched so 4 while fluid interaction with more highly altered sediments results in h 2 s characterized by a small but isotopically significant input of 34 s depleted biogenic sulfides near zero 33 s values in all samples implicate the abiotic processes of so 4 reduction and leaching of host rock as the major contributors to sulfur content at a high temperature unsedimented mid ocean ridge and at a back arc system 33 s values indicate that so 2 disproportionation is an additional process that contributes sulfur to a different back arc system and to acid spring type hydrothermal fluid circulation at the sedimented guaymus basin near zero 33 s values are also observed despite negative 34 s values that indicate inputs of biogenic pyrite for some samples in contrast with previous studies reporting isotope disequilibrium between h 2 s and chalcopyrite the 34 s values of chalcopyrite sampled from the inner 1 2 mm of a chimney wall are within 1 of 34 s values for h 2 s in the paired vent fluid suggesting equilibrium fluid mineral sulfur isotope exchange at 300 400 c isotopic equilibrium between hydrothermal fluid h 2 s and precipitating chalcopyrite implies that sulfur isotopes in the chalcopyrite lining across a chimney wall may accurately record past hydrothermal activity

predicting incipient motion including the effect of turbulent pressure fluctuations in the bed 1 incipient motion of particles on a river bed can in principle be understood and predicted from a balance of the forces acting on the particles on a natural river bed the exposure of particles to the flow is variable the effect of exposure on the initiation of motion including the case of deep embedding is studied in this paper so far the turbulence induced lifting force has been derived exclusively from examinations of the surface flow the understanding of destabilization processes of the riverbed is improved when turbulence induced vertical pressure gradients in the upper sediment layer are included this is particularly important for particles that lie in the lee of larger ones an analytical solution for the critical shields parameter of spherical particles is found including the effects of pressure fluctuations in the bed macroscale flow structures average bed slope and shallow flow existing laboratory data designed to test the effect of exposure and roughness conditions are in excellent agreement with the new model

microbial sulfate reduction within the iheya north subseafloor hydrothermal system constrained by quadruple sulfur isotopes abstract subseafloor hydrothermal systems may host spatially extended and numerically abundant microbial communities sustained by sulfate reduction as one of the important terminal electron accepting metabolisms in order to estimate microbial sulfate reduction in a subseafloor hydrothermal regime we analyzed sulfur isotopes s 32 s 33 s 34 s 36 of pore water sulfate and mineralized sulfide in the upper 100 m of sedimentary sequences at the iheya north hydrothermal field in the okinawa trough recovered in integrated ocean drilling program expedition 331 iodp exp 331 on the basis of the pore water chemistry and temperature profiles the subseafloor environment is divided into three hydrogeologic units in the topmost unit 1 relatively fresh seawater is recharged and the bottommost unit 3 is characterized by predominance of endmember like high temperature hydrothermal fluid 300 c underlying the impermeable cap rock layers intermediate unit 2 is subject to mixing between the hydrothermal fluid and seawater the 34 s values of sulfate in the unit 2 mixing zone were found to be more 34s enriched than the values expected from simple mixing model of seawater sulfate in the unit 1 with the hydrothermal fluid in the unit 3 the observed s so 4 34 enrichment and sulfate concentration so2 4 depletion suggest sulfate reduction is taking place below the seafloor based on our model calculation the isotope discrimination e 34 is estimated to be 21 this large isotope discrimination together with slight 33 s enrichment and 36 s depletion reveals that sulfate reduction is caused by microbial processes but not by thermochemical processes in addition our numerical simulation points out that sulfate may be reduced prior to presently undergoing mixing with high temperature fluid probably within the seawater recharge zone despite the abundant input of hydrothermal h2s mineralized sulfide below 10 m seafloor mbsf shows characteristic sulfur isotopic signatures that support the significant contribution of subseafloor microbial sulfate reduction potentially corresponding to approximately 20 of the total sulfide mineral formation active microbial sulfate reduction below the seafloor may be promoted by significant input of seawater to the habitats through the vigorous hydrothermal circulation in vicinity of iheya north field

spatially averaged open channel flow over rough bed in this paper it is suggested that the double averaged in temporal and in spatial domains momentum equations should be used as a natural basis for the hydraulics of rough bed open channel flows especially with small relative submergence the relationships for the vertical distribution of the total stress for the simplest case of 2d steady uniform spatially averaged flow over a rough bed with flat free surface are derived these relationships explicitly include the form induced stresses and form drag as components of the total stress using this approach we define three types of rough bed flows 1 flow with high relative submergence 2 flow with small relative submergence and 3 flow over a partially inundated rough bed the relationships for the double averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible the double averaged turbulent and form induced intensities and stresses for the case of regular spherical segment type roughness show the dominant role of the double averaged turbulence stresses and form drag in momentum transfer in the near bed region

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

inception sediment transport relationships at high slopes this study proposes relationships between mean streamflow depth and cohesionless particle bed characteristics at the beginning of the sediment transport at high slopes in low submergence conditions the inception of sediment transport is theoretically approached by the incipient motion condition a previous theoretical analysis of the equilibrium of a gravel particle based on the exposure of the particle itself to the streamflow is generalized by introducing the relative position of the exposed part of the particle with respect to the bottom and by considering the case of a partially submerged particle using a low submergence flow velocity profile in the particle balance of forces at the limit of equilibrium leads to a relationship between the mean flow depth and the sediment size another velocity profile of low submergence flow is added to that used in the previous study and two analytical expressions of flow depths corresponding to the beginning of sediment transport are obtained for both fully submerged particles and partially submerged particles the theoretical flow depths given by the analytical expressions are then compared with those from previous experimental tests at the beginning of sediment transport at high slopes with fully submerged particles the comparison shows a satisfactory agreement between theoretical and experimental values of flow depth corresponding to the degree of exposure and the relative position of the exposed part of the surface layer bed particles

river channel slope flow resistance and gravel entrainment thresholds 1 river beds are traditionally assumed to become mobile at a fixed value of nondimensional shear stress but several flume and field studies have found that the critical value is higher in steep shallow flows explanations for this have been proposed in terms of the force balance on individual grains the trend can also be understood in bulk flow terms if total flow resistance has base and additional components the latter due to protruding immobile grains as well as any bedforms and the stress corresponding to additional resistance is not available for grain movement in threshold conditions a quantitative model based on these assumptions predicts that critical shields stress increases with slope critical stream power is near invariant with slope and each has a secondary dependence on bed sorting the proposed slope dependence is similar to what force balance models predict and consistent with flume data and most field data possible explanations are considered for the inability of this and other models to match the very low critical values of width averaged stress and power reported for some low gradient gravel bed rivers

provenance discrimination of siliciclastic sediments in the middle okinawa trough since 30 ka constraints from rare earth element compositions the late quaternary paleoceanography and paleoenvironment in the okinawa trough east china sea have been well reconstructed over the last decade while in contrast the provenance of terrigenous sediments that have accumulated there remains enigmatic in this study rare earth elements ree were used to investigate provenance changes in sediments from core dgks9604 taken from the middle okinawa trough discrimination plots based on ree fractionation parameters suggest that the cored sediments have variable provenances over the last 30 ka with the lower part ca 31 8 2 ka ultimately originating mostly from the changjiang yangtze river and the upper part 7 1 0 ka primarily from taiwan during the last glacial maximum and early deglacial period sea level was low and the main stream of the kuroshio current was deflected to the east of the ryukyu islands as a result the changjiang derived sediments might have dominated sedimentation of the middle okinawa trough however since about 7 ka the main stream of the kuroshio current strengthened in the area of the trough as sea level approximated the modern position this caused near bottom transport of fine grained sediments from the continental margin to the trough to become weak and instead taiwan derived terrigenous sediments dominated in the middle trough the changing provenances of terrigenous sediments into the middle okinawa trough are closely related to the evolution of oceanic circulation and sea level in the east china sea two tephra layers in the core have distinct ree compositions and correlate well with two volcanic eruptions at 7 6 and 25 8 ka in southern japan

antidunes on steep slopes 1 when rates of subcritical flow are increased on gentle slopes the bed successively produces ripples dunes and flat beds antidunes defined here as all bed undulations for which the surface gravity waves are in phase with the bed profile appear only in high flow rates and may be found in some extreme natural flow events inversely on steep slopes flume experiment s 1 approximately flows are supercritical and antidunes are observed to appear just after the beginning of sediment motion and to disappear for high flow rates using data from new experiments we seek to improve the prediction of antidune geometry on steep slopes an equation for antidune wavelength was deduced from dimensional analysis and fitted to new experimental data the equation was successfully evaluated using a data set extended to 167 values with data from the literature obtained on both steep and gentle slopes this equation gives results very similar to the usual analytical equation from kennedy when tested on the results from gentle slope experiments but proved to be better adapted for antidune wavelength on steep slopes

incipient sediment motion at high slopes in uniform flow condition 1 this paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream a theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow the parameter relative degree of exposure relates the positioning of a particle with the upstream one the problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence the results of the theory are compared with a series of ad hoc experiments performed by c gregoretti

entrainment threshold of cohesionless sediment grains under steady flow of air and water existing formulations for bed sediment entrainment under steady flow are incapable of explaining two well documented observational facts i water flow requires considerably higher dimensionless shear stresses to move the bed grains than air flow and ii under open channel flow steep granular beds are more stable than beds with milder slopes these two facts together with recent direct measurements of forces acting on bed grains giving time mean negative drags schmeeckle et al 2007 question the conventional models of forces used so far here fluid forces acting on bed particles are treated in a new way in order to take into consideration the fundamental interference effects thus obtaining appropriate magnitude estimates that exhibit good agreement with direct force measurements by schmeeckle et al 2007 impulsive pressure fluctuations generated by turbulence are shown to be capable of dislodging the bed grains by saltation under air flow whereas they can only produce a rocking effect under water flow thus explaining the first anomaly on the other hand previous work by the authors allows a direct estimate of space averaged time mean drag and lift forces exerted on bed grains both components have the same order of magnitude but contrary to the common belief the mean lift is downward which provides an explanation for the second anomaly finally spatial disturbances of pressure both positive and negative appear to generate maximum persistent local forces considerably greater than mean forces thus allowing an explanation for the observed negative time mean drag a new formula for predicting incipient motion of sediment under open channel flow is derived which incorporates all dynamically significant effects and gives very good agreement with observation for the entire range of bed slopes

the groundwater budget a tool for preliminary estimation of the hydraulic connection between neighboring aquifers abstract groundwater management authorities usually use groundwater budget calculations to evaluate the sustainability of withdrawals for different purposes the groundwater budget calculation does not always provide reliable information and it must often be supported by further aquifer monitoring in the case of hydraulic connections between neighboring aquifers the riardo plain aquifer is a strategic drinking resource for more than 100 000 people water storage for 60 km2 of irrigated land and the source of a mineral water bottling plant over a long period the comparison between the direct recharge and the estimated natural outflow and withdrawals highlights a severe water deficit of approximately 40 of the total groundwater outflow a groundwater budget deficit should be a clue to the aquifer depletion but the results of long term water level monitoring allowed the observation of the good condition of this aquifer in fact in the riardo plain the calculated deficit is not comparable to the aquifer monitoring data acquired in the same period 1992 2014 the small oscillations of the groundwater level and the almost stable streambed spring discharge allows the presumption of an additional aquifer recharge source the confined carbonate aquifer locally mixes with the above volcanic aquifer providing an externally stable recharge that reduces the effects of the local rainfall variability the combined approach of the groundwater budget results and long term aquifer monitoring spring discharge and or hydraulic head oscillation provides information about significant external groundwater exchanges even if unidentified by field measurements and supports the stakeholders in groundwater resource management

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

acidic and sulfate rich hydrothermal fluids from the manus back arc basin papua new guinea hot 88 120 c and acidic ph 2 1 hydrothermal fluids rich in sulfate were discovered venting in the desmos caldera depth 1926 m eastern manus back arc basin bismarck sea surrounded by papua new guinea the abundant sulfate 32 8 m m higher than the seawater value of 28 m m with elemental sulfur deposition around the vents and remarkably low d h 2 o and 34 s h 2 s values 8 1 and 5 6 respectively are suggestive of the incorporation of a magmatic fluid and the disproportionation of the exsolved so 2 from a magma body the desmos fluid may be similar in origin to the acidic sulfate chloride hot springs associated with subaerial volcanic activity in contrast to the typical hydrothermal end member mg concentration of 0 the desmos fluids are rich in mg 46 52 m m probably because of mg dissolution by acid attack upon magnesium silicate minerals

revisiting slope influence in turbulent bedload transport consequences for vertical flow structure and transport rate scaling gravity driven turbulent bedload transport has been extensively studied over the past century in regard to its importance for earth surface processes such as natural riverbed morphological evolution in the present contribution the influence of the longitudinal channel inclination angle on gravity driven turbulent bedload transport is studied in an idealised framework considering steady and uniform flow conditions from an analytical analysis based on the two phase continuous equations it is shown that i the classical slope correction of the critical shields number is based on an erroneous formulation of the buoyancy force ii the influence of the slope is not restricted to the critical shields number but affects the whole transport formula and iii pressure driven and gravity driven turbulent bedload transport are not equivalent from the slope influence standpoint analysing further the granular flow driving mechanisms the longitudinal slope is shown to not only influence the fluid bed shear stress and the resistance of the granular bed but also to affect the fluid flow inside the granular bed responsible for the transition from bedload transport to debris flow the relative influence of these coupled mechanisms allows us to understand the evolution of the vertical structure of the granular flow and to predict the transport rate scaling law as a function of a rescaled shields number the theoretical analysis is validated with coupled fluid discrete element simulations of idealised gravity driven turbulent bedload transport performed over a wide range of shields number values density ratios and channel inclination angles in particular all the data are shown to collapse onto a master curve when considering the sediment transport rate as a function of the proposed rescaled shields number

runoff generated debris flows observations and modeling of surge initiation magnitude and frequency 1 runoff during intense rainstorms plays a major role in generating debris flows in many alpine areas and burned steeplands yet compared to debris flow initiation from shallow landslides the mechanics by which runoff generates a debris flow are less understood to better understand debris flow initiation by surface water runoff we monitored flow stage and rainfall associated with debris flows in the headwaters of two small catchments a bedrock dominated alpine basin in central colorado 0 06 km2 and a recently burned area in southern california 0 01 km2 we also obtained video footage of debris flow initiation and flow dynamics from three cameras at the colorado site stage observations at both sites display distinct patterns in debris flow surge characteristics relative to rainfall intensity i we observe small quasiperiodic surges at low i large quasiperiodic surges at intermediate i and a single large surge followed by small amplitude fluctuations about a more steady high flow at high i video observations of surge formation lead us to the hypothesis that these flow patterns are controlled by upstream variations in channel slope in which low gradient sections act as sediment capacitors temporarily storing incoming bed load transported by water flow and periodically releasing the accumulated sediment as a debris flow surge to explore this hypothesis we develop a simple one dimensional morphodynamic model of a sediment capacitor that consists of a system of coupled equations for water flow bed load transport slope stability and mass flow this model reproduces the essential patterns in surge magnitude and frequency with rainfall intensity observed at the two field sites and provides a new framework for predicting the runoff threshold for debris flow initiation in a burned or alpine setting

post drilling changes in fluid discharge pattern mineral deposition and fluid chemistry in the iheya north hydrothermal field okinawa trough 1 the integrated ocean drilling program iodp expedition 331 investigated the iheya north hydrothermal field in the okinawa trough several post drilling underwater vehicle investigations were conducted over 2 years to identify post drilling changes in fluid discharge pattern mineral deposition and fluid chemistry drilling induced high temperature hydrothermal fluid vents were identified at deep holes not only near the naturally occurring nbc hydrothermal fluid vent site c0016 but also at the seafloor 450 m distal to the nbc vent site c0014 where no hydrothermal fluid discharge was observed prior to drilling a chimney structure at hole c0016a grew rapidly at the nbc mound crest where only small chimneys had been found before drilling a drilling induced diffuse hydrothermal flow region spread at site c0014 and this area was newly colonized by the galatheid crab from a fluid chemistry perspective the post drilling hydrothermal fluids were enriched in cl relative to seawater although this fluid chemistry was not observed during the 12 years prior to drilling the cl enriched fluid reservoir underlying the subseafloor impermeable layers observed by iodp expedition 331 is likely source for the cl enriched fluids discharging from the post drilling vents the drilling induced physical disturbance of subseafloor hydrogeological structures would release such fluids to the seafloor in turn the rapid chimney growth at the nbc mound crest may also be attributed to highly turbulent fluid flow with the enlarged artificial vent of hole c0016a which can contribute to the retention of the fluid seawater mixture for a sufficiently long period to precipitate sulfide sulfate minerals on the seafloor

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

the spectrum of ore deposit types volcanic environments alteration halos and related exploration vectors in submarine volcanic successions some examples from australia variations in shape metal content alteration mineralogy and volcanic host rocks of the ore deposits in the two major volcanic hosted massive sulfide vhms districts of eastern australia the cambrian mount read volcanics and the cambro ordovician mount windsor subprovince strongly reflect their volcanic environment conditions of ore formation and hydrothermal alteration processes lens and sheet style polymetallic zinc rich deposits such as rosebery hellyer que river and thalanga are considered to have formed in moderate to relatively deep water environments 500 1 000 m these deposits probably formed either on the sea floor e g hellyer que river or by replacement of porous volcaniclastic units directly below the sea floor e g rosebery the footwall alteration associated with these polymetallic vhms deposits was controlled by host rock permeability and porosity which are in turn related to volcanic facies type degree of fracturing and synvolcanic structural architecture focusing of hydrothermal fluids along synvolcanic structures has resulted in well zoned chlorite sericite footwall alteration pipes within footwall lavas at hellyer on the other hand diffuse fluid flow through very thick pumice breccia at rosebery and hercules has resulted in strata bound sericite dominated footwall alteration zones parallel to the paleosea floor and the ore lenses massive and disseminated pyritic cu au deposits such as those in the mount lyell field and at highway reward formed by subsea floor replacement and are associated with only minor zinc lead massive sulfide ore these deposits formed from higher temperature fluids 300 degrees c in which copper transport is enhanced and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions e g highway reward the cu au ore lenses may be strata bound e g mount lyell or crosscutting pipes e g highway reward depending on the structure and permeability characteristics of the felsic volcanic host rocks the presence of high sulfidation alteration minerals e g pyrophyllite zunyite in some of the cu au deposits e g mount lyell field indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system alteration zonation associated with the cu au vhms deposits is more symmetrical than that of the zn rich deposits with sericite rich alteration extending into the hanging wall in keeping with the subsurface replacement origin of these deposits synvolcanic gold rich deposits with high gold base metal ratios are less common than the cu au and zn rich vhms ore types the gold rich ores e g henty south hercules are strata bound in nature have low sulfide contents and are associated with central zones of intense silicification surrounded by envelopes of sericite pyrite and carbonate alteration volcanological and geochemical studies at henty indicate the gold rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies lavas and limestones this spectrum of cu au zn rich and au only deposits in the mount read volcanics and the mount windsor subprovince is interpreted to represent a continuum from classic sea floor vhms ores toward those with features more akin to porphyry cu au and epithermal au ag deposits this spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows 1 proportions of volcaniclastic lava and subvolcanic intrusive facies 2 depth of seawater 3 permeability and porosity of volcanic host rocks 4 balance between magmatic components and seawater components in the ore fluid and 5 temperature and acidity of the ore fluid mineralogical lithogeochemical and isotopic studies have revealed a range of alteration vectors useful in exploration for both the zn rich and cu au vhms deposits carbonate and white mica compositional variations are highlighted as important mineralogical vectors thallium and antimony halos may be useful trace element vectors and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system

multiple sulfur isotope constraints on the modern sulfur cycle we present 28 multiple sulfur isotope measurements of seawater sulfate 34sso4 34sso4 and 33sso4 33sso4 from the modern ocean over a range of water depths and sites along the eastern margin of the pacific ocean the average measured 34sso4 34sso4 is 21 24 0 88 2 0 88 2 with a calculated 33sso4 33sso4 of 0 050 0 050 0 014 2 0 014 2 with these values we use a box model to place constraints on the gross fraction of pyrite burial in modern sediments this model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of 34spyrite 34spyrite which is poorly constrained but instead uses new information about the relationship between 34s 34s and 33s 33s in global marine sulfate our calculations indicate that the pyrite burial flux from the modern ocean is between 10 and 45 of the total sulfur lost from the oceans with a more probable range between 20 and 35

multiple sulfur isotopes and the evolution of earth s surface sulfur cycle abstract the distribution of sulfur isotopes in geological materials reveals information about earth history and biogeochemical processes research during the last several decades has used sulfur isotope geochemistry as a tool to better understand microbial processes harrison and thode 1958 kaplan 1975 monster et al 1979 peck 1959 peck 1962 rees 1973 and sediment diagenesis berner 1969 berner 1982 canfield et al 1993b earth historians also realized this potential as there exists a rich record of environmental change within the sedimentary records canfield and teske 1996 claypool et al 1980 goodwin et al 1976 habicht et al 2002 kah et al 2004 monster et al 1979 shen et al 2001 strauss 1993 thode and goodwin 1983 these applications have championed the use of the two most abundant sulfur isotopes 32s and 34s and provide a rich introduction to what the sulfur isotope record has to offer see canfield 2004 canfield and raiswell 1999 within the last decade this information has been supplemented by new data derived from the less abundant isotopes 33s and 36s the measurement of all four stable sulfur isotopes multiple sulfur isotope geochemistry has expanded our understanding of biological evolution and activity atmospheric chemistry and transport crustal recycling and many more fields related to earth surface processes see farquhar and wing 2003 here i present a review of recent works in multiple sulfur isotope geochemistry with a focus on results that inform our understanding of biogeochemical processes and earth surface evolution

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

lead and sulfur isotopes and a model for the origin of the ducktown deposit tennessee strata bound sulfide deposits at ducktown tennessee have many characteristics of volcanogenie exhalative deposits but occur in an overwhelmingly sedimentary environment lead isotope ratios of galenas and massive pyritic and pyrrhotitic ores fall into three distinct groups that are not related to wall rock type type of ore or to orebody a positive correlation between the lead and sulfur isotope ratios of galena indicates a relation between sulfur and lead that does not seem to be an effect of metamorphism but is rather an original feature of the ore the groupings of lead isotope ratios and a possible bimodality of sulfur isotope compositions as well as the above mentioned positive correlation suggest that ducktown lead and sulfur represent the mixing of two sources the dominant source of lead was probably u pb enriched clastic sediments seawater sulfate may have dominated sulfur contributions from this source the second source could have been a deep seated mafic igneous source tapped by a deep fault associated with late precambrian rift basin development the deep source could have provided heat to drive the hydrothermal cell in the sediments a relatively nonradiogenic component of lead sulfur with delta 34 s sub sigma asymp 0 per mil a considerable amount of cu and zn as well as the small amphibolite pods at ducktown and along strike lead isotope studies of contemporaneous rift volcanics the mount rogers formation indicate that lead in ducktown ores could not have been derived from a similar source mount rogers felsic volcanics contain lead derived from a relatively u pb poor th pb enriched source that appears to have been granulitized grenville age basement lead isotope ratios of galenas from the gossan lead are similar to those at ducktown other blue ridge massive sulfide deposits have less radiogenic 206 pb 204 pb values but none contain any discernable basement component similar to that of mount rogers reconnaissance sulfur isotope ratios from other blue ridge massive sulfides suggest a tendency toward heavier values than those at ducktown

hydraulic resistance in overland flow during partial and marginal surface inundation experimental observations and modeling hydraulic resistance as a function of surface roughness inundation was evaluated in a set of experiments designed to simulate overland flow on a rough granular surface the data are compared with an additive drag model based on the contribution of individual elements to flow resistance and a mixing length model for estimating bulk flow resistance during partial inundation of the surface roughness the observed coefficient of drag per element is much higher than for an isolated element and a model based on element form drag alone underestimates the observed friction these high resistance values are strongly correlated with the hydrostatic wave drag estimated from the free surface deformation around elements the mixing length model incorporating a multiplier setting the hydraulically effective roughness height reliably reproduces the trends in resistance during marginal inundation this multiplier is shown to have a value similar to the root mean square of the surface height

three dimensional spatially constrained sulfur isotopes highlight processes controlling sulfur cycling in the near surface of the iheya north hydrothermal system okinawa trough

discussion of understanding the influence of slope on the threshold of coarse grain motion revisiting critical stream power by c parker n j clifford and c r thorne geomorphology volume 126 march 2011 pages 51 65 abstract this paper presents a discussion of parker et al 2011 in particular some theoretical developments are presented that give additional material to apprehend the effects of grain size water depth and slope on the critical shields parameter and critical stream power

seepage induced mass wasting in coarse soil slopes an expression is derived for the critical shear stress of surface flow on a noncohesive soil slope subjected to seepage induced mass wasting i e erosion caused by a successive mobilization of flake aggregates of surficial grains the expression is supported with data from experiments with stone fill subjected to throughflow in current theories for hill slope erosion of riprap protection on slopes buoyancy is assumed to be a constant vector directed vertically or the seepage force is omitted as a variable however the present analysis shows that for many conditions buoyancy should be described in accordance with the dynamic pressure distribution and the seepage force should be included as a variable in the theory furthermore a parameter denoted minimum stable volume per unit width of stone fill is introduced as a measure of global stability of the stone fill along with seepage height on the downstream face and grain size experimental results indicate that the minimum stable volume decreases with increase in grain size and for the materials investigated is about 20 lower for crushed rock than for gravel

entrainment and motion of coarse particles in a shallow water stream down a steep slope we investigate the entrainment deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope this is an idealization of bed load transport in mountain streams earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics the intermittent character of particle transport at low water discharges led us to consider it as a random process sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles we develop a birth death immigration emigration markov process to describe the particle exchanges between the bed and the water stream a key feature of the model is its long autocorrelation times and wide frequent fluctuations in the solid discharge a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments we present experimental data obtained using a nearly two dimensional channel and glass beads as a substitute for sediment entrainment trajectories and deposition were monitored using a high speed digital camera the empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model experiments confirmed the existence of wide and frequent fluctuations of the solid discharge and revealed the existence of long autocorrelation time but theory overestimates the autocorrelation times by a factor of around three particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model which performs well when a single particle is moving for our experiments the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity we also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased the more vigorous the stream was the more continuous the solid discharge became moreover although 90 of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 particles dissipate more and more energy when the bed slope is increased but surprisingly the dissipation rate is nearly independent of fluid velocity a movie is available with the online version of the paper

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

hydrothermalism in the tyrrhenian sea inorganic and microbial sulfur cycling as revealed by geochemical and multiple sulfur isotope data abstract the palinuro volcanic complex and the panarea hydrothermal field both located in the tyrrhenian sea italy are associated with island arc magmatism and characterized by polymetallic sulfide mineralization dissolved sulfide concentrations ph and eh measured in porewaters at both sites reveal a variable hydrothermal influence on porewater chemistry multiple sulfur isotopic measurements for disseminated sulfides crs chromium reducible sulfur extracted from sediments at palinuro yielded a broad range in 34s range between 29 8 and 10 2 and 33s values between 0 015 and 0 134 in contrast sediments at panarea exhibit a much smaller range in 34scrs with less negative values between 11 3 and 1 8 the sulfur isotope signatures are interpreted to reflect a mixture between hydrothermal and biogenic sulfide with a more substantial biogenic contribution at panarea multiple sulfur isotope measurements were performed on sulfides and elemental sulfur from drill core material from the palinuro massive sulfide complex 34s and 33s values for pyrite between 32 8 and 1 1 and between 0 012 to 0 042 respectively as well as for elemental sulfur with 34s and 33s values between 26 7 and 2 1 and between 0 035 and 0 109 respectively point to a microbial origin for much of the sulfide and elemental sulfur studied moreover data suggest a coupling of bacterial sulfate reduction sulfide oxidation and sulfur disproportionation in addition 34s values for barite between 25 0 and 63 6 are also in agreement with high microbial turnover of sulfate at palinuro although a magmatic so2 contribution towards the formation of the palinuro massive sulfide complex is very likely the activity of different sulfur utilizing microorganisms played a fundamental role during its formation thus porewater and multiple sulfur isotope data reveal differences in the hydrothermal activity at palinuro and panarea drill sites and underline the importance of microbial communities for the origin of massive sulfide mineralizations in the hydrothermal subsurface

the triggering of debris flow due to channel bed failure in some alpine headwater basins of the dolomites analyses of critical runoff the debris deposits at the bottom of very steep natural channels and streams in high mountain areas can be mobilized by runoff triggering a water sediment mixture flow known as debris flow the routing of debris flow through human settlements can cause damage to civil structures and loss of human lives the prediction of such an event or the runoff discharge that triggers it assumes an interest in risk analyses and the planning of defence measures the object of this study is to find a method to determine the critical runoff value that triggers debris flow as a result of channel bed failure historical and rainfall data on 30 debris flows that occurred in six watersheds of the dolomites north eastern italian alps were collected from different sources field investigations at the six sites together with the hydrologic response to the rainfalls that triggered the events were performed to obtain a realistic scenario of the formation of the debris flow there occurred field observations include a survey along the channel of the triggering reach of debris flow with measurements of the channel slope and cross section and sampling of debris deposits for grain size distribution simulated runoff discharge values based on the rainfall recorded by pluviometers were then compared with values obtained through experimental criteria on the initiation and formation of debris flow by bed failure the results are discussed to provide a plausible physical based method for the prediction of the triggering of debris flow by channel bed failure copyright 2007 john wiley sons ltd

field measurement of boulder flow drag a field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested drag is determined from the momentum deficit in the boulder wake obtained by measuring the flow velocity in a traverse across the wake a field test yielded a mean drag coefficient of 0 36 which is low compared with flume measurements of 1 1 1 5 for cubes this may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment

friction angle measurements on a naturally formed gravel streambed implications for critical boundary shear stress we report the first measurements of friction angles for a naturally formed gravel streambed for a given test grain size placed on a bed surface friction angles varied from 10o to over 100o friction angle distributions can be expressed as a function of test grain size median bed grain size and bed sorting parameter friction angles decrease with increasing grain size relative to the median bed grain size and are a systematic function of sorting with lower friction angles associated with poorer sorting the probability distributions of critical shear stress for different grain sizes on a given bed surface as calculated from our friction angle data show a common origin but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses the potential mobility of a grain as defined by its probability distribution of critical shear stress may be overestimated for larger grains in this analysis because our calculations do not take into account the effects of grain burial and altered near bed flow fields

laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain a series of experiments has been undertaken to determine how this angle depends on grain shape rollability and angularity on the ratio of the size of the pivoting grain to those beneath and on factors such as imbrication the experiments involved gravel sized spheres ball bearings and marbles natural pebbles selected for their approximately triaxial ellipsoid shapes and angular crushed basalt pebbles the pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined the angle of the board being equal to the pivoting angle at the instant of grain movement the pivoting angles of spheres showed reasonable agreement with a theoretically derived equation showing much better agreement than in previous studies which utilized sand sized spheres a series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size our results are therefore consistent with the earlier studies limited to sand size spheres the cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out similar size dependencies are also found for the ellipsoidal pebbles and angular gravel the experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle being a function of the ratio of the pebble s smallest to intermediate axial diameters this ratio controls the grain s ability to roll and pivot with small ratios of these diameters the pebbles tended to slide out of position whereas with ratios closer to unity circular cross section true pivoting took place and the angles were smaller experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position the pivoting angle being increased approximately by the imbrication angle the angular crushed gravel also required high pivoting angles apparently due to interlocking of the grains resulting from their angularity other factors being equal the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres ellipsoidal grains angular grains and imbricated grains the results obtained here make possible the quantitative evaluation of these shape effects on grain threshold as well as evaluation of the selective entrainment of grains from a bed of mixed sizes

stability of armour layers in rivers the importance of both the overpassing and the embedding of river bed particles is demonstrated by experiments when a single layer thick armouring layer of uniform d d1 size particles on a bed of uniform finer d d2 size particles is broken at one location the stability of the remainder of the armour layer depends on the tendency of the d d1 particles to embed into the surface of d d2 particles if a d d1 size particle is seated in the exposed position on the surface of the d d2 sized bed sediment it may by virtue of its exposure to the flow be preferentially entrained and it may then overpass the finer bed sediment alternatively the d d1 particle may be too large for overpassing and it will become embedded armour layers composed of particles which embed tend to be more stable

multiple sulfur isotopes and the evolution of the atmosphere interest in multiple sulfur isotope analyses has been fueled by recent reports of mass independent sulfur isotope signatures in the geologic record a non zero multiple isotopic signature of sulfur 33s and 36s is produced primarily through photochemical reactions and it is an almost perfect tracer of the source of sulfur once the signature is passed on to a given sulfur reservoir it will be preserved unless there is addition of sulfur with a different 33s or 36s in the geological record this signature has been used to study the evolution of the earth s atmosphere and to trace movement of sulfur through geological systems early in earth s history recently small but significant mass independent signatures have been reported for some younger samples raising the possibility of additional applications of multiple isotope studies the purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous but not uncommon isotopic signatures

generation and evolution of subduction related batholiths from the central urals constraints on the p t history of the uralian orogen abstract batholiths from the accreted terranes in the urals were generated by repeated episodes of melting and intrusion verkhisetsk the largest and one of the most complex subduction related batholiths from the urals comprises an outer envelope of older tonalites trondhjemites and granodiorites dated at 315 320 ma and equilibrated at 6 kbar intruded by an inner core of younger granodiorites adamellites and granites dated at 275 290 ma and equilibrated at 4 kbar older rocks have a high a1 ttd adakite chemistry 320machur nd 2 5 and initial 87 sr 86 sr 0 7043 with pronounced lateral zoning marked by the increase of lree hree cr ni and mg eastwards they were originated by melting of metabasalts in a subducted slab of young hot oceanic lithosphere with a temperature depth trajectory that intersected the garnet in univariant at 1050 c and 13 kbar thus causing the lateral zoning we believe that such a warm trajectory can be attributed to the oblique subduction of a young lithosphere younger rocks range from na rich metaluminous granodiorites to k rich peraluminous two mica granites with relict epidote crystals and have almost the same isotopic signature as older rocks from which they were generated by anatexis as a consequence of a melting event that occurred throughout the urals at 275 290 ma and produced most of the huge continental type batholiths on the eastern side this melting event is tentatively supposed to be the result of underplating by mafic magmas which also caused the growth of the urals crust from the moho downwards until it reached the increased thickness revealed by seismological studies

development of a vector based 3d grain entrainment model with application to x ray computed tomography scanned riverbed sediment sediment transport equations typically produce transport rates that are biased by orders of magnitude a causal component of this inaccuracy is the inability to represent complex grain scale interactions controlling entrainment grain scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two dimensional 2d force or moment balances however this approach neglects many complexities of real grains including grain shape cohesion and the angle of entrainment relative to flow direction to better represent this complexity we develop the first vector based fully three dimensional 3d grain rotation entrainment model that can be used to resolve any entrainment formulation in 3d and which also includes the effect of matrix cohesion to apply this model we use x ray computed tomography to quantify the 3d structure of water worked river grains we compare our 3d model results with those derived from application of a 2d entrainment model we find that the 2d approach produces estimates of dimensionless critical shear stress urn x wiley mma media esp4608 math 0001 that are an order of magnitude lower than our 3d model we demonstrate that it is more appropriate to use the c axis when calculating 2d projections which increases values of urn x wiley mma media esp4608 math 0002 to more closely match our 3d estimates the 3d model reveals that the main controls on critical shear stress in our samples are projection of grains cohesive effects from a fine grained matrix and bearing angle for the plane of rotation the lateral angle of departure from downstream flow that in part defines the grain s direction of pivot about an axis formed by two contact points in 3d the structural precision of our 3d model demonstrates sources of geometric error inherent in 2d models by improving flow properties to better replicate local hydraulics in our 3d model entrainment modelling of scanned riverbed grains has the potential for benchmarking 2d model enhancements

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small