how do geomorphic effects of rainfall vary with storm type and spatial scale in a post fire landscape abstract in post fire landscapes increased runoff and soil erosion can cause rapid geomorphic change we examined how different types of rainfall events in 2013 affected hillslope scale erosion and watershed scale channel change in two 14 16 km 2 watersheds within the 2012 high park fire burn area in northern colorado usa the first set of rainfall events was a sequence of 12 short spatially variable summer convective rain storms and the second was a 200 mm week long storm in september we compared rainfall characteristics hillslope sediment yields stream stage and channel geometry changes from the summer storms to those from the september storm the summer storms had a wide range of rainfall intensities and each storm produced erosion primarily in one study watershed the september storm rainfall had less spatial variability covered both watersheds and its total rainfall depth was 1 5 to 2 5 times greater than the total summer rainfall because rainfall intensities were highest during some summer storms average hillslope sediment yields were higher for summer storms 6 mg ha 1 than for the september storm 3 mg ha 1 maximum storm rainfall intensities were good predictors of hillslope sediment yield but sediment yield correlated most strongly with total depths of rainfall exceeding 10 30 mm h 1 intensity thresholds the combined summer storms produced relatively small changes in mean channel bed elevation and cross section area with no clear pattern of incision or aggradation in contrast the sustained rain across the entire study area during the september storm led to extensive upstream incision and downstream aggradation because of different spatial coverage of storms summer storms produced more total hillslope erosion whereas the september storm produced the greatest total channel changes at both scales high intensity rainfall above a threshold was responsible for inducing most of the geomorphic change

predicting the crest height of a gravel beach the beach crest is a common morphological feature formed by the deposition of sediment carried up slope by wave swash the elevation to which waves can pile gravel is a function of the size and density of the material relative to the hydraulic components of swash velocity wave frequency and runup height two equations 17 and 21 that predict the height of the beach crest to the wave forces and the beach material are derived the first derivation compares the wave force acting to move a stone up the beach face with a weight force acting to hold the stone in place the second derivation relates the potential energy per unit area of the beach crest to the total wave energy that lifted and deposited the material above a given sea level datum as a test of the derived equations the actual crest height of one natural gravel beach was accurately estimated by both equations derived further research and testing on other natural beaches are needed to fully verify eqs 17 and 21 a comparison of other equations eqs 2 3 and 4 that predict crest height is also made the poor fit of the other equations are a result of using values for the coefficients that were established in wave tank experiments another shortcoming of eqs 3 and 4 is that they do not include variables related to the beach material hence they predict that all particle sizes from sand to boulders would be piled to the same elevation by the same wave forcing eq 2 includes a particle size variable but predicts increasingly higher beach crests for larger particles more realistic results for all three equations eqs 2 3 and 4 would be expected with field tested values for the coefficients but a range in values for each different beach would be necessary the advantage of eqs 17 and 21 is that equations are not dependent on coefficients whose values come from correlation with other data sets

moments of catchment storm area the portion of a catchment covered by a stationary rainstorm is modeled by the common area of two overlapping circles given that rain occurs within the catchment and conditioned by fixed storm and catchment sizes the first two moments of the distribution of the common area are derived from purely geometrical considerations the variance of the wetted fraction is shown to peak when the catchment size is equal to the size of the predominant storm the conditioning on storm size is removed by assuming a probability distribution based upon the observed fractal behavior of cloud and rainstorm areas

long term bedload mobility in gravel bed rivers using iron slag as a tracer abstract bedload dispersion is evaluated in gravel bed rivers using slag elements from ironworks established along rivers in the ardenne region between the fourteenth and the nineteenth centuries large quantities of slag were dumped close to these rivers or even directly into the channels for centuries slag elements were dispersed in the bedload and transported by floods of varying importance consequently slag may be considered as a reliable tracer to analyze bedload dispersion over several centuries the size of slag elements was studied along 16 ardenne rivers the longitudinal size trend of the largest slag particles allows the effective competence of these rivers to be determined between 19 and 129 mm for rivers where specific stream power for the bankfull discharge ranges between 20 and 134 w m a direct relationship doesn t exist between these two parameters as the size of slag elements must be considered with regard to the d 50 of the bed selective transport was analyzed directly downstream of the input sites the sorting distance varies from river to river and depends on the velocity of the coarse elements introduced into the river since the inception of the iron industry downstream of two metallurgic sites the slag propagation fronts were located as the periods of activity at these sites are known from historical studies the virtual velocity of bedload movement in these rivers was estimated to be 2 4 km century

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

recycling of graphite during himalayan erosion a geological stabilization of carbon in the crust at geological time scales the role of continental erosion in the organic carbon oc cycle is determined by the balance between recent oc burial and petrogenic oc oxidation evaluating its net effect on the concentration of carbon dioxide and dioxygen in the atmosphere requires the fate of petrogenic oc to be assessed here we report a multiscale nanometer to micrometer structural characterization of petrogenic oc in the himalayan system we show that graphitic carbon is preserved and buried in marine sediments while the less graphitized forms are oxidized during fluvial transport radiocarbon dating indicates that 30 to 50 of the carbon initially present in the himalayan rocks is conserved during the erosion cycle graphitization during metamorphism thus stabilizes carbon in the crust over geological time scales

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

large grains matter contrasting bed stability and morphodynamics during two nearly identical experiments while the stabilizing function of large grains in step pool streams has long been recognized the role they play in gravel bed streams is less clear most researchers have ignored the role of large grains in gravel bed streams and have assumed that the median bed surface size controls the erodibility of alluvial boundaries the experiments presented herein challenge this convention two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material the two distributions had the same range of particle sizes and nearly identical bulk d50 values 1 6 mm however the d90 of experiment gsd1 was slightly finer 3 7 mm than that for experiment gsd2 3 9 mm transport rates during gsd1 were nearly four times greater than during gsd2 even though the dimensionless shear stress was slightly lower and the channel developed a sinuous pattern with well developed riffles pools and bars during gsd2 the initial rectangular channel remained virtually unchanged for the duration of the experiment the relative stability of gsd2 seems to be associated with a slightly larger proportion of stable large grains on the bed surface at the beginning of gsd1 3 5 of the bed was immobile while almost twice as much of it 6 1 was immobile at the beginning of gsd2 the results demonstrate that the largest grains not the median size exert first order control on channel stability copyright 2017 john wiley sons ltd

stage dependent variability in tractive force distribution through a riffle pool sequence high resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle pool sequences are required to improve understanding of how fluvial processes maintain these meso scale bedforms this paper addresses this issue by providing velocity and boundary shear stress data over a range of discharges from base flow 0 07 m3 s 1 to just over bankfull 8 52 m3 s 1 from a sequence of four pools and three riffles in the river rede northumberland the data supports the reversal hypothesis of keller geol soc am bull 87 1971 753 as the primary explanation for the maintenance of the riffle pool sequence although they also indicate that spatial variability in tractive force is highly stage dependent and complex section averaged velocity data indicate reversal to be evident at four out of six riffle pool units an equalisation in velocity was found for the other two riffle pool units close to bankfull stage the spatial patterns of tractive force exhibited in the study reach as a result of increased discharge demonstrate that riffle pool units operate independently of one another shear stress reversals were observed in individual riffle pool units at different river stages during a flood hydrograph and in some instances two occurred in the same riffle pool unit during a single flow event pools were characterised by coarser bed sediments and narrower channel widths in comparison to riffles increasing the likelihood of tractive force reversal in the river rede areas of predicted bed sediment entrainment obtained from o c matched observed channel changes in the upper part of the study reach but over estimated change in the middle portion of the reach

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

petrogenic organic carbon retention in terrestrial basins a case study from perialpine lake constance abstract inland waters play a major role in the global carbon cycle with particulate organic carbon poc burial in terrestrial wetlands surpassing that in ocean sediments lake constance the second largest lake at the periphery of the european alps receives poc sourced from both aquatic and terrestrial productivity as well as petrogenic oc ocpetro from bedrock erosion distinguishing poc inputs to lake sediments is key to assessing carbon flux and fate as reworked ocpetro represents neither a net sink of atmospheric co2 nor source of o2 new stable and radiocarbon isotopic data indicate that 11 9 12 gg yr of ocpetro is buried in lake constance with underlying sediments on average containing 0 3 0 25 0 33 wt ocpetro extrapolation of these results suggests that 27 tgocpetro yr 12 54 tgoc yr could be subject to temporary geological storage in lakes globally which is comparable to estimates of 43 25 61 tgocpetro yr delivered to the ocean by rivers galy et al 2015 more studies are needed to quantify ocpetro burial in inland sedimentary reservoirs in order to accurately account for atmospheric carbon sequestration in terrestrial basins

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

linking spatial patterns of bed surface texture bed mobility and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout abstract in this paper we examined spatial patterns of bed texture bed mobility and channel hydraulics in four reaches of a mountain stream with moderate gradient

feedback between bed load transport and flow resistance in gravel and cobble bed rivers 1 to calculate bed load engineers often use flow resistance equations that provide estimates of bed shear stress in these equations on the basis of the estimate of the appropriate hydraulic radius associated with the bed only the bed roughness ks is commonly set as a constant whatever the bed load intensity however several studies have confirmed the existence of feedback mechanisms between flow resistance and bed load suggesting that a flow dependent bed roughness should be used therefore using a data set composed of 2282 flume and field experimental values this study investigated the importance of these feedback effects new flow resistance equations were proposed for three flow domains domain 1 corresponds to no bed load and a constant bed roughness ks d where d is a representative grain diameter whereas domain 3 corresponds to a high bed load transport rate over a flat bed with a constant bed roughness ks 2 6d between these two domains a transitional domain 2 was identified for which the bed roughness evolved from d to 2 6d with increasing flow conditions in this domain the darcy weisbach resistance coefficient f can be approximated using a constant for a given slope the results using this new flow resistance equation proved to be more accurate than those using equations obtained from simple fittings of logarithmic laws to mean values the data set indicates that distinguishing domains 2 and 3 is still relevant for bed load in particular the data indicate a slope dependence in domain 2 but not in domain 3 a bed load model based on the tractive force concept is proposed finally flow resistance and bed load equations were used together to calculate both shear stress and bed load from the flow discharge the slope and the grain diameter for each run of the data set efficiency tests indicate that new equations implicitly taking a feedback mechanism into account can reduce the error by a factor of 2 when compared to other equations currently in use showing that feedback between flow resistance and bed load can improve field bed load modeling

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

ferromagnetic resonance of magnetite biominerals traces redox changes abstract redox variations govern a multitude of key geochemical and microbiological processes within lacustrine and marine systems yet the interpretation of these geological archives can be limited because redox sensitive microorganisms leave behind sparse fossil evidence here we assess a biologically controlled magnetic proxy through investigation of a well constrained sedimentary record covering a perturbation of redox conditions driven by a complete trophic cycle in lake constance ferromagnetic resonance spectroscopy of sediments reveals strong uniaxial anisotropy indicative of single domain magnetite particles in intact or fragmentary chain arrangements which are an unambiguous trait of magnetotactic bacteria mtb and their magnetofossil remains we show that biogenic magnetite formed intra cellularly in mtb faithfully records changing redox conditions at or close to the sediment water interface biogenic magnetite within sedimentary records points to the proliferation of mtb parallel to a decline in water column dissolved oxygen and the formation of sulfidic surface sediments in lake constance associated with an episode of eutrophication 1955 1991 we conclude that magnetofossils may serve as a sensitive geological proxy to reconstruct dynamic redox changes along the sediment water interface and bottom waters

the importance of bed sediment sand content for the structure of a static armor layer in a gravel bed river the surface structure of static armor layers generated from water worked gravel bed channels was investigated with primary focus on the influence of sand content and flow rate flume experiments were conducted in which four sediment mixtures with sand contents between 1 and 38 were armored under one of three different flow rates first and second order statistical analyses were applied to digital elevation models of unarmored armored and clustered bed surface areas to identify changes in surface structure results were combined with data from previous research to create an extended data set of armored bed surfaces water worked unarmored bed surfaces established under a dynamic equilibrium flow rate impacted the topographic variability and structure of the armored beds surface complexity decreased with armor formation as surface grains preferentially aligned with the flow direction the bed surface became smoother and where sediment mixture sand content was constant there was greater smoothing of the surface during higher armoring flows as grains rearranged more easily as bulk sand content increased statistical analyses of the expanded data set showed that beds with very little sand content developed static armor layers that remained rough and had greater topographic variability than armor layers from sediments with higher sand contents the bulk sediment sand content exerted a stronger influence over the change in surface roughness and structure upon armoring than that of the flow rate during armor formation when combined with the knowledge of the local flow regime the sand content may aid in predictions related to armored bed surface structure

the persistence of memory the fate of ancient sedimentary organic carbon in a modern sedimentary system the cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales the burial of organic carbon occurs predominantly at sea in association with clay sized particles derived from the erosion of uplifted continental rocks it follows that the history of the fine grained particles on land may bear on the nature of the organic carbon buried in this study the evolution of clay associated organic matter was followed from bedrock source to the seabed in the eel river sedimentary system of northern california using natural abundance 13c and 14c tracers approximately half of the fine grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted mesozoic tertiary franciscan complex of the watershed the short residence time of friable soils on steep hill slopes coupled with rapid sediment accumulation rates on the shelf slope act to preserve the ancient organic carbon a comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope the bimodal mixture of ancient and modern c in soils and sediments may be characteristic of many short mountainous rivers if the eel river chemistry is typical of such rivers more than 40 tg of ancient organic c may be delivered to the world s oceans each year a flux of that magnitude would have a significant influence on marine and global c cycles

organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source we compared the burial efficiency of organic carbon buried oc deposited oc in a diverse set of 27 different sediments from 11 lakes focusing on the potential effects of organic matter source oxygen exposure and protective sorption of oc onto mineral surfaces average oc burial efficiency was high mean 48 and it was particularly high in sediments receiving high input of allochthonous organic matter mean 67 further oc burial efficiency was strongly negatively related to the oxygen exposure time again particularly so in sediments receiving high allochthonous loads on the other hand oc burial efficiency was not related to the mineral surface area which was used as a proxy of the sorption capacity of the mineral phase for oc the high oc burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes as well as to a strong dependence of oc burial efficiency on oxygen exposure time this study demonstrates that the carbon sink in lake sediments alters the oc export from the continents to the sea and that the fate of oc in lake sediments burial vs mineralization to carbon dioxide and methane is highly sensitive to environmental conditions

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

the role of attached kelp seaweed in lowering threshold of coarse gravel entrainment in tidal flows lag deposits of cobble sized clasts found within the troughs of fine gravel dunes on a rock bed intertidal zone are explained by the process of enhanced fluid transport due to attached seaweed field determinations of the competence of the tidal currents demonstrated that the currents were not able to transport weed free clasts on the tidal flat controlled flume experiments were used to demonstrate that the presence of the fucoid alga fucus vesiculosus l attached to clasts reduces the critical velocities and shear stresses to levels that allow the tidal currents to entrain the clasts and move them over the backs of dunes to the lee sides subsequent burial of the weedy clasts in the dune troughs by dune progression kills the weed leaving the latterly exposed clasts as an accumulating weed free lag the weight of attached weed typically equals the submerged weight of the clast and the critical velocity and critical shear stress for initial motion are both roughly halved by the presence of the weed the drag induced by weed is four times that experienced by weed free clasts and mobile clasts exhibit near equal mobility

channel bed mobility response to extreme sediment loading at mount pinatubo since the 1991 eruption of mount pinatubo the specific sediment yields from watersheds draining its slopes have been the highest ever recorded in spite of this overwhelming sediment load rivers inundated by pyroclastic flow deposits delivered almost half of the initial deposits to downslope alluvial and or debris fans by 1996 although most of this transport occurred by hyperconcentrated flows and debris flows lahars very high sediment transport rates also characterize low flow measured flow velocities and depths channel properties and the size of both mobile and stable clasts in the sacobia and pasig potrero rivers indicate median bed surface grain sizes of 2 to 8 mm grain size independent dimensionless critical shear stresses c of 0 016 to 0 041 and manning s n values of 0 017 to 0 024 values well below those previously reported for steep mountain channels the dramatic bed surface fining bed mobility and selective transport in these extremely sediment rich channels indicate that changes in grain size c values and bed roughness can increase transport capacity in response to high sediment supply our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain size dependent selective transport as end member concepts that apply to channels with low or intermittent and high or continuous sediment supply respectively

area perimeter relation for rain and cloud areas following mandelbrot9s theory of fractals the area perimeter relation is used to investigate the geometry of satellite and radar determined cloud and rain areas between 1 and 1 2 x 106square kilometers the data are well fit by a formula in which the perimeter is given approximately by the square root of the area raised to the power d see equation in the pdf where d is interpreted as the fractal dimension of the perimeter it is concluded that rain and cloud perimeters are fractals they have no characteristic horizontal length scale between 1 and 1000 kilometers

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

incipient motion of sand gravel sediment mixtures experiments on the incipient motion of gravel and sand mixtures of 0 100 10 90 25 75 45 55 and 100 0 percent ratios of gravel and sand respectively were made in a laboratory flume a series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate the sand in each of the five bed mixtures began to move at nearly the same bed shear stress all gravel sizes began to move at nearly the same bed shear stress in the 100 gravel experiments but in the sand gravel experiments the gravel showed an increase in critical shear stress with increasing size reasons for this change in the initiation of motion of the gravel from the 100 gravel to the sand gravel mixtures may result from the abundant sand sized sediment inhibiting the formation of a coarse bed surface layer in the sand gravel experiments

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

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

on hydrologic similarity 1 derivation of the dimensionless flood frequency curve the problem of flood frequency similarity among river basins is analyzed by deriving a scale independent dimensionless flood frequency curve the model is formulated using dimensionless rainfall inputs and a dimensionless basin response function for the rainfall intensity the probability distribution for dimensionless areal rainfall is derived and depends upon a geoclimatic scaling factor which is a function of the basin size and the rainfall areal correlation structure rainfall duration is scaled by a characteristic basin time this same parameter is used to develop a dimensionless geomorphologic unit hydrograph the derived dimensionless flood frequency curve can be used to further understand the variability of flood statistics within a contiguous area and to group stations for regional flood frequency analysis using statistical techniques

analysis of bedload transport characteristics of idaho streams and rivers field data are essential in evaluating the adequacy of predictive equations for sediment transport each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment data collected for 33 sites on 31 mountain streams and rivers in central idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological topographic morphological hydrological hydraulic and sedimentological characteristics all of these streams and rivers have armored poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer the fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges generally bankfull or less and in some cases two times higher than bankfull discharge the bedload transport is size selective the transport rates are generally low since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges or the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material detailed analyses of bedload transport rates bedload and bed material characteristics were performed for each site the obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel bed rivers copyright 2008 john wiley sons ltd

a derived flood frequency distribution using horton order ratios a flood frequency distribution is derived from assumed climatic distributions and the geomorphologic unit hydrograph guh model of catchment response the derived distribution is tested on two appalachian mountain catchments and shows good agreement with historical data because the guh is rooted in catchment shape and geomorphology this derived frequency distribution may be a promising vehicle for studying the effect of catchment shape on flood frequency characteristics furthermore it is felt that the question of catchment homogeneity and similarity necessary for regional flood estimation can also be studied through the guh derived flood distribution

dam triggered organic carbon sequestration makes the changjiang yangtze river basin china a significant carbon sink worldwide dam building in large river basins has substantially altered the carbon cycle by trapping much of the riverine transported particulate organic carbon poc in terrestrial reservoirs here we take the changjiang yangtze river basin in which 50 000 dams were built over the past 50 years as an example to evaluate the effect of dam building on poc sequestration we report the characteristics elemental composition radiocarbon and stable carbon isotopic compositions and raman spectra of bulk poc in the lower changjiang from october 2007 to september 2008 and we estimate the poc sequestration induced by dam building since the 1950s for the changjiang basin using radiocarbon measurements we quantify the fraction of biospheric poc pocbio and petrogenic poc pocpetro in changjiang poc over the study period around 25 of the changjiang poc is radiocarbon dead pocpetro the remaining is pocbio with a mean radiocarbon age of 3 5 kyr studies on the east china sea ecs shelf along with an oxidation experiment suggest that prior to dam building the changjiang pocbio was significantly oxidized in the ecs margin in contrast high preservation of poc is observed in changjiang reservoirs combining our poc data with hydrometric data sets our study indicates that over the past five decades dam building may have largely shifted the changjiang poc burial site from the ecs margin to terrestrial reservoirs this shift in burial site preserved labile pocbio that would have been oxidized suggesting a new temporary carbon sink we estimate that dam building in the changjiang has sequestered 4 9 1 9 megatons pocbio every year since 2003 approximately 10 of the global riverine poc burial flux to the oceans

stable carbon isotopes of amorphous kerogens from phanerozoic sedimentary rocks amorphous kerogens from phanerozoic sedimentary rocks may be divided into two groups on the basis of their 13c values h amorphous kerogens have isotopic values in the range of 24 to 20 per mil and l amorphous kerogens have isotopic values in the range of 35 to 26 per mil the occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups phytoplankton and to a lesser extent zooplankton and bacteria are considered to be the precursors for both kerogen types a plausible explanation for this dichotomy may be found in the source of co2 utilized by phytoplankton which is considered to be controlled by environmental factors h amorphous kerogens are expected to occur in silled basins that are overlain by expansive well circulated deep 500 m water where the prevailing source of carbon in the photic zone is atmospheric derived co2 conversely l amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow 200 m water where the prevailing source of carbon in the photic zone may be organic derived co2

particle comminution defines megaflood and superflood energetics abstract the concept of megafloods and superfloods was introduced at the end of the last century to define exceptionally large discharge floods primarily those associated with the failure of quaternary ice dams these floods exceeded by one or two magnitudes historically recorded floods megafloods having peak discharges equalling or exceeding 1 m m3 s 1 herein the sediment associations of exceptional floods are found to be distinctive being dominated by comminuted smashed grain size distributions which contrast to the sediment deposits of more moderate floods a consideration of the energetics of moderate floods superfloods and megafloods with respect to the entrainment dis entrainment suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits a threshold of energy expenditure 20 kj s 1 m 2 is proposed to separate moderate floods from superfloods the latter by this definition can be 0 1 m m3 s 1 this threshold separates floods competent to entrain cobbles and boulders with limited comminution of the load from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension the granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods the energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux significance exceptional floods occurred in prehistoric times the largest are referred to as megafloods or superfloods to distinguish such exceptional floods from other large floods megafloods were defined as having a peak discharge exceeding 1 m m3 s 1 however many reports of megafloods are flows with a peak discharge smaller than 1 m m3 s 1 recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel which is not the case for smaller floods the gravel is smashed by exceptionally high energy expenditures consideration of the energetics of floods to transport gravel allows a robust quantitative definition of superflood based on the energy needed to smash a range of gravel sizes

geological respiration of a mountain belt revealed by the trace element rhenium oxidation of rock derived petrogenic organic carbon ocpetro during weathering of sedimentary rocks is a major source of carbon dioxide co2 to the atmosphere this geological respiration is thought to be enhanced by physical erosion suggesting that mountain belts could release large amounts of co2 to counter the co2 sequestration achieved by the erosion riverine transfer and oceanic burial of organic carbon from the terrestrial biosphere however ocpetro oxidation rates in mountain belts have not been quantified here we use rhenium re as a proxy to track ocpetro oxidation in mountain river catchments of taiwan where existing measurements of physical erosion rate allow the controls on ocpetro oxidation to be assessed re has been shown to be closely associated with ocpetro in rocks and following oxidation during chemical weathering forms a soluble oxyanion reo4 which contributes to the dissolved load of rivers soils on meta sedimentary rocks in taiwan show that re loss is coupled to ocpetro loss during weathering confirming previous observations from soil profiles on sedimentary rocks elsewhere in taiwan rivers dissolved re flux increases with the catchment average sediment yield suggesting that physical erosion rate is a major control on ocpetro oxidation based on our current understanding of re mobility during weathering the dissolved re flux can be used to quantify an upper bound on the ocpetro oxidation rate and the associated co2 transfer the estimated co2 release from this mountain belt by ocpetro oxidation does not negate estimates of co2 sequestration by burial of biospheric oc offshore the findings are compared to oc transfers estimated for the himalaya where ocpetro oxidation in the mountain belt remains unconstrained together these cases suggest that mountain building in the tropics can result in a net sink of oc which sequesters atmospheric co2

release of biodegradable dissolved organic matter from ancient sedimentary rocks 1 sedimentary rocks contain the largest mass of organic carbon on earth yet these reservoirs are not well integrated into modern carbon budgets here we describe the release of dissolved organic matter dom from om rich sedimentary rocks under simulated weathering conditions results from column experiments demonstrate slow sustained release of dom from ancient sedimentary rocks under simulated weathering conditions 1h nmr analysis of shale derived dom reveals a highly aliphatic carbohydrate poor material distinct from other natural dom pools shale derived dom is rapidly assimilated and biodegraded by aerobic heterotrophic bacteria consequently no compositional signature of shale derived dom other than 14c depletion is likely to persist in rivers or other surface reservoirs combined these efforts show that dissolution provides a mechanism for the conversion of refractory kerogen into labile biomass linking rock weathering with sedimentary om oxidation and the delivery of aged om to rivers and ocean margins

oxidation of petrogenic organic carbon in the amazon floodplain as a source of atmospheric co2 the two long term sources of atmospheric carbon are co2 degassing from metamorphic and volcanic activity and oxidation of organic carbon oc contained in sedimentary rocks or petrogenic organic carbon ocpetro the latter fl ux is still poorly constrained in this study we report particulate organic carbon content and 14c activity measurements in amazon river sediments which allow for estimates of the ocpetro content of these sediments a large decrease of ocpetro content in riverine sediments is observed from the outlet of the andes to the mouth of the large tributaries this loss reveals oxidation of ocpetro during transfer of sediments in the floodplain and results in an escape of 0 25 mt c yr to the atmosphere which is on the same order of magnitude as the co2 consumption by silicate weathering in the same area raman microspectroscopy investigations show that graphite is the most stable phase with respect to this oxidation process these results emphasize the significance of ocpetrooxidation in large river floodplains in the global carbon cycle

transport of tracer gravels on a coastal california river abstract tracer gravels can provide information about the transport of coarse gravels not sampled by standard sampling methods white dolomite fragments emplaced as riprap on the carmel river monterey county california were entrained by progressively higher flows in 1981 1982 and 1983 and were transported progressively further following the 1981 flow tracer gravel concentration decreased exponentially downstream after the 1982 flows and again after the 1983 flows the distribution of gravels reflected channel morphology not an exponential downstream decrease peak concentrations corresponded to locations of major gravel bars while tracer concentration decreased overall in the downstream direction its distribution suggested movement by discrete jumps from bar to bar rather than continuous movement calculation of bedload transport rates for rocks 45 mm by spatial integration of tracer data yields bedload transport rates of about 3 5 of the total bedload flux measured by helley smith sampler 5 5 km downstream the spatial integration method is not strictly applicable here because injection of tracer was not instantaneous and channel form is quite irregular nonetheless it is notable that results by this method are compatible with sampled transport rates downstream the results of this study show the dominant influence of channel morphology on tracer gravel distribution reflecting the importance of in channel storage sites in coarse sediment movement this effect and the low recovery rate reported for tracer gravels in the literature are reason for caution in using short term tracer gravel studies to compute rates of coarse bedload flux however tracer studies can be profitably used to provide data on the movement of coarse particles unmeasured by helley smith samplers

partial area coverage distribution for flood frequency analysis in arid regions convective rainfalls of high intensity are largely responsible for most large floods occurring in arid regions convective storms are limited in size and rarely cover an entire basin therefore flood frequency is affected by the distribution of storm coverage c which is the fraction of basin area upon which rainfall occurs analytical probability density function pdf estimates of partial storm coverage are presented as a function of basin area sc and the probability distribution function for storm radius rs conditional pdfs for the covered area fraction c given the storm radius f c rs are obtained as a compound distribution with finite probability for maximum coverage and continuous density for smaller values the conditionality assumption is removed by assuming an exponential storm radius marginal distribution to obtain the catchment coverage pdf this pdf is compound with discrete probability for complete catchment coverage p c 1 and a continuous function for partial overlap the continuous function combines a weibull pdf and linear functions of c both p c 1 and f c depend on the average catchment to storm radius ratio the distribution explains the partial coverage data for the walnut gulch basin in arizona departures were observed however for slightly less than complete coverage namely for 0 9 c 1 a conceptual event classification and a method were designed to include partial coverage effects on flood frequency distribution

numerical simulation of widening and bed deformation of straight sand bed rivers ii model evaluation in this paper the numerical model presented in the companion paper is tested and applied assessment of model accuracy was based on two approaches first predictions of evolution of a 13 5 km reach of the south fork of the forked deer river in west tennessee were compared to observations over a 24 yr period results suggest that although the model was able to qualitatively predict trends of widening and deepening quantitative predictions were not reliable simulated widths and depths were within 15 of the corresponding observed values but observed change in these parameters at the study sites were also close to these values simulated rates of depth adjustment were within 15 of observed rates but observed rates of channel widening at the study sites were approximately three times those simulated by the model in the second approach the model was used to generate relationships between stable channel width and bank full discharge the model was able to successfully replicate the form of empirically

observations of downstream fining on the north fork toutle river near mount st helens washington we carried out a series of measurements to provide data for testing models of downstream fining in an actively braiding gravel bed river in particular the model developed in the previous paper we sampled a series of nine cross sections for bulk grain size distributions over a downstream distance of 5 4 km on the north fork toutle river near mount st helens washington in addition we sampled two of the nine sections for detailed analysis of surface grain size distributions in the cross stream direction by wolman counting within areas of distinct mean grain size patches our theory predicts that the rate of downstream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain size patches application of the theory to the study area shows that most of the observed downstream fining can be explained using patchiness as the dominant mechanism

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

coarse sediment transport in mountain streams in colorado and wyoming usa since the early 1990s us forest service researchers have made thousands of bedload measurements in steep coarse grained channels in colorado and wyoming usa in this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes including step pool plane bed pool riffle and near braided channels this effort builds upon previous work where we applied a piecewise regression model to 1 relate flow to rates of bedload transport and 2 define phases of transport in coarse grained channels earlier the model was tested using bedload data from eight sites on the fraser experimental forest near fraser colorado the analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied the earlier results were however derived from data collected at sites that for the most part have quite similar geology and runoff regimes in this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions the results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs the transition from primarily low rates of sand transport phase i to higher rates of sand and coarse gravel transport phase ii occurs on average at about 80 per cent of the bankfull 1 5 year return interval discharge a comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase i transport moreover the movement and capture of the d16 to d25 grain size of the bed surface seems to correspond with the onset of phase ii transport particularly in systems with largely static channel surfaces however while there were many similarities in observed patterns of bedload transport at the 19 studied sites each had its own bedload signal in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system published in 2005 by john wiley sons ltd