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

stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

lateglacial climate development in nw romania comparative results from three quantitative pollen based methods abstract this paper provides quantitative reconstructions of the lateglacial changes in four climate parameters from two fine resolution pollen profiles in the gutaiului mountains nw romania climate estimates are based on two modern analogue techniques with and without considering vegetation types and weighted averaging partial least squares regression wa pls giving evidence for several climatic fluctuations during the period from 14 700 to 11 500 cal yr bp the comparative results of the two modern analogue techniques show consistent trends of climate changes that are also coherent at both sites but these results appear to largely disagree compared with climate reconstruction provided by wa pls the modern analogue techniques revealed four intervals with low temperatures prior to 14 700 cal yr bp between 13 950 and 13 800 13 400 and 13 200 and 12 700 and 11 700 cal yr bp the temperature declines were more pronounced for winter than for summer suggesting an intensification of seasonality which together with a drop in precipitation indicates an increase in continentality the younger dryas is the most pronounced cooling phase with winter temperatures 14 16 c colder than modern conditions annual and summer temperatures 2 5 c and 2 c respectively below present ones precipitation was 400 500 mm half that of present during the bolling and allerod summer temperatures were close to modern values 13 to 17 c whereas winter 6 to 12 c and annual temperatures 0 5 to 6 c as well as precipitation were 550 to 700 mm lower indicating more continental conditions compared to the present day climate

channel bed steps along nahal yael negev desert israel abstract the height and spacing of channel bed steps formed in boulder and bedrock in the ephermeral channels of the nahal yael watershed show a strong correlation with the channel slope the distance between steps is inversely proportional to slope at slopes of 0 to 20 but then remains fairly constant at higher slopes step height increases proportionally with channel slope these bed steps are similar to those described on perennial channels elsewhere where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps however the ephemeral flows in the tributary channels of the nahal yael system are not sufficient to submerge the clasts forming the boulder steps using rouse s criteria channel bed steps are creating maximum flow resistance at slopes of 12 to 50 along the channels of nahal yael using davies criteria bed steps are creating maximum flow resistance at slopes of 3 to 10 the bed steps also relate to sediment transport nahal yael has a low sediment yield by world standards and the increase in flow resistance due to step formation may serve to increase sediment movement both the boulder and bedrock steps at nahal yael are hypothesized to reflect the scales of flow turbulence along the channels we were unable to evaluate the role of flow volume or boundary roughness in controlling these scales but channel slope appears to exert some control on the scale of turbulence based on the correlation between slope and bed steps the hypotheses developed from field evidence at nahal yael should be testable in flume simulations

hydraulics morphology and energy dissipation in an alpine step pool channel 1 to investigate the relationship between hydraulics and channel morphology in step pool channels we combined three dimensional velocity measurements with an acoustic doppler velocimeter and topographic surveys in a steep step pool channel the rio cordon italy measurements were organized around step pool and tread units and occurred within a range of 36 57 of bankfull discharges as flow moved from steps to their downstream pools in our study reach an average of approximately two thirds of the total energy was dissipated as measured by relative head loss through step pool sequences much of this head loss was achieved by elevation potential energy loss rather than velocity reductions although an overall expected pattern of flow acceleration toward step crests and deceleration in pools was present pool velocities were high especially where upstream step crests were irregular and where residual pool depths were low many steps were porous or leaky with irregular cross channel bed and water surface topography producing high velocity jets and less flow resistance than channel spanning dammed steps longitudinal variations in hydraulics are thus often overshadowed by lateral variations arising from morphologic complexities velocity and turbulence characteristics in the rio cordon show marked differences from data we have collected in a more stable and wood rich channel in the colorado rockies in which ponded steps are more prevalent and pools are slower and more turbulent comparison of these channels illustrates that step pool structure and hydraulics are strongly influenced by flow regime sediment supply lithology time since the last step forming flood and availability of in stream wood

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

the geomorphic significance of step pools in mountain streams abstract this paper develops a conceptual model to define the changing role and significance of step pools as energy dissipators in steep mountain channels although energy dissipation is a significant function of step pools this role changes over time with variations in discharge steps are effective in reducing stream energy at low flows but their effectiveness diminishes with increasing stage accordingly the manner in which energy dissipation occurs also varies with increasing flows spill resistance gives way to a dominance in form and grain resistance the conceptual model for the changing role of step pools is illustrated with data from hydraulic analysis and modeling of step pools in the santa monica mountains of california the model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics the model offers a new articulation for the geomorphic significance of step pools in mountain streams and it serves as a useful template for a more complete understanding of step pools over a longer time scale

influence of provenance and hydrodynamic sorting on the magnetic properties and geochemistry of sediments of the oujiang river china abstract we present the results of environmental magnetic granulometric and geochemical analyses of sediments of the oujiang river china the aim of the study was to assess the influence of hydrological sorting and sediment source on sediment composition from upstream down to the river mouth the results reveal that the sedimentary magnetic properties are dominated by ferrimagnetic minerals in the fluvial reach the proportion of imperfect antiferromagnetic minerals is relatively high due to strong soil chemical weathering in a subtropical climate from upstream down to the river mouth the proportion of ferrimagnetic minerals increases consistently and the magnetite becomes finer grained as a result of hydrological sorting in the tidal reach the sediments consist of a mixture of fluvial and marine sourced sediments derived from the yangtze river as a result of the strong tidal influence a combination of the demagnetization parameter s 100 mt and element ratios fe k and al mg enables the discrimination of the sediments of the fluvial reach from those of the tidal reach due to the rock types and subtropical weathering conditions in the catchment the sediments of the oujiang river are significantly different to those of the yangtze river our results potentially contribute to the study of the provenance of the muddy sedimentary deposits in the inner east china sea they demonstrate that sediment sorting has a significant effect on bulk magnetic properties and geochemical compositions during transport from source to sink in addition magnetic and geochemical characterization of sized fractions of sediment can provide more reliable information regarding sediment source tracing

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

rheology of the lower mantle abstract the rheology of the lower mantle of the earth is examined from the viewpoint of solid state physics recent developments in high pressure research suggest that the lower mantle contains a considerable amount of mg fe o with fe mg fe 0 2 0 3 the pressure and temperature dependences of diffusion in mg fe o are estimated by the theory of diffusion in ionic solids of the materials composing the lower mantle mg fe o may be the softest and therefore the rheology of the lower mantle may be that of mg fe o unless the framework effect is important temperatures in the lower mantle are inferred from the depths of phase transitions and the melting temperatures of the core materials a thermal boundary layer at the base of the mantle is suggested the physical mechanisms of creep are examined based on a grain size stress relation and non newtonian flow is shown to be the dominant flow mechanism in the earth s mantle the effective viscosity for the temperature models with and without the thermal boundary layer is calculated for constant stress and constant strain rate with depth for constant strain rate which may be appropriate for discussing the mechanics of descending slabs the increase in effective viscosity with depth is smaller than for the constant stress case which may be appropriate for discussing the flow induced by the surface motion of plates the relatively small depth gradient of viscosity for constant strain rate suggests that the lower mantle could also participate in convection the effective viscosity increases with depth however by at least 10 2 to 10 3 from the top to the bottom of the lower mantle for a reasonable range of activation volumes and temperatures there will be a low viscosity layer at the base of the mantle in contrast to the high viscosity layer at the top of the mantle plates if a thermal boundary layer is present the constant newtonian viscosity inferred from rebound data may be an apparent feature resulting from the difference in deformation mechanisms between isostatic rebound and large scale flow

the dynamical equations of polar wander and the global characteristics of the lithosphere as extracted from rotational data abstract the initial value problem describing the linear responses of the spin axis of a layered viscoelastic planet from surface loading is studied by means of laplace transform techniques a complete solution of polar motion requires the usage of two classes of eigenspectra one arising from viscoelastic relaxation of the mantle due to surface forcing the other involving the gradual readjustment of the spin axis as a consequence of mantle viscoelasticity our analytical 4 layer model comprising an elastic lithosphere a two layer adiabatically stratified viscoelastic mantle and an inviscid core has been incorporated into this formalism in which rotational deformation and isostatic relaxation are taken into account for all times from employing both sets of rotational data polar variations from the 70 years of data from the international latitude service i l s and the non tidal deceleration of the length of the day l o d an estimate of the globally averaged lithosphere of between 130 and 200 km is obtained from the long wavelength flexural mode due to the degree two harmonic this range of values may have strong implications on the mode of continental evolution

the periodic nature of step pool mountain streams rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers but whether periodicities exist in the occurrence of step pools has not been investigated in a comprehensive way and the implications for such periodicities have not been explored spectral analysis was applied to bed elevation data from streams in the santa monica mountains of southern california to investigate periodicity in the occurrence of step pools results indicate significant step pool periodicities confirmed by independent field measurements that vary from 0 43 channel widths to 2 40 channel widths spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step pool spacing underlying periodicities are nevertheless de tected suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms periodicity in the occurrence of step pools suggests that step pools are bedforms that represent a fundamental mutual adjustment between flow channel morphology and energy expenditure a rhythmic channel thalweg supports the notion that step pools are analogous to meandering in the vertical dimension and implies that because step pool channels are not able to adjust energy expenditure in the plane dimension they instead adjust boundary roughness in the vertical dimension a continuum of rhythmic gravel bedforms that displays the results of such adjustments and that shows how step pools merge into pools and riffles in the stream channel system is discussed and illustrated with empirical data from the santa monica mountains although many differences exist between steep mountain streams and broad alluvial rivers the findings of this study suggest that generalized principles of uniform energy expendi ture may also apply to rhythmic step pool streams these findings are potentially valuable in the management and design of steep channels

direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils 1 variations in magnetic properties of buried soils can be used to reconstruct past climatic conditions during paleosol formation most methods however are based on comparisons between the magnetically enriched upper soil horizons and the magnetically unaltered parent material in thin loess paleosol sequences such a comparison can be problematic because all horizons soil and underlying loess may be affected to varying degrees by pedogenesis we propose two direct estimates of pedogenic magnetite based on the analysis of anhysteretic remanent magnetization ratios carm isothermal remanent magnetization and coercivity distributions these estimates are independent of any information regarding the parent material and are possible if pedogenic minerals have similar magnetic properties throughout the study region this condition seems to be met throughout the midwestern united states and a few loessic soils elsewhere the remanence carrying part of pedogenic magnetite is composed of single domain particles with consistent well constrained magnetic properties these particles are extremely well dispersed in the soil matrix as indicated by the absence of noticeable magnetostatic interaction effects our analyses of over 70 modern loessic soil profiles demonstrate that the abundance of pedogenic magnetite correlates well with modern climate and that the method is suited for reconstruction of past climates from paleosols

steady state secondary convection beneath lithospheric plates with temperature and pressure dependent viscosity we have constructed a thermomechanical model for upper mantle convection such that the thickness and structure of the lithosphere are determined self consistently by the heat transported by convection in this study of the interaction between the lithosphere and upper mantle strongly temperature and pressure dependent rheologies for both newtonian and non newtonian creep mechanisms are employed for a strictly temperature dependent rheology an insignificant amount of heat less than 12 5 mw m2 can be transported convectively for an interior viscosity 0 1021 pas compatible with postglacial rebound on the other hand for similar values of the interior viscosity steady heat fluxes between 20 and 40 mw m2 are produced by introducing pressure dependence into the rheology for the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated once all of the rheological parameters are fixed this finding may have important ramifications for parameterized convection we employ both the single mode mean field approximation and the complete two dimensional equations using finite elements to obtain solutions for the various types of rheologies from evaluating the geophysically relevant observables such as topography free air gravity anomalies surface heat flow and stress fields in the lithosphere we find that the lateral variations of these quantities predicted by a non newtonian rheology are much smaller than those derived from a linear rheology these results suggest that surface variations of geophysical observables are more compatible with a non newtonian rheology in the upper mantle

convection in a variable viscosity fluid newtonian versus power law rheology abstract a number of finite element calculations of convection in a variable viscosity fluid have been carried out to investigate the effects of non newtonian flow when rheology is also subject to a strong temperature and pressure influence a variety of cases has been studied in the range of effective rayleigh numbers between 104 and 106 including different modes of heating and a range of values for activation energy and activation volume power law creep with a stress exponent of 3 turns out to lead to considerably different flow pattern and heat transfer properties than newtonian rheology in general the effect is to reduce viscosity contrasts imposed by p t dependence which can lead in some circumstances to the mobilisation of otherwise stagnant regions within the cell the properties of non newtonian flow can be closely imitated by a newtonian fluid with a reduced value of the activation enthalpy bh with b 0 3 0 5 it appears possible that non newtonian rheology plays a key role in determining the convective style in a planetary mantle

pool spacing in forest channels field surveys of stream channels in forested mountain drainage basins in southeast alaska and washington reveal that pool spacing depends on large woody debris lwd loading and channel type slope and width mean pool spacing in pool riffle plane bed and forced pool riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing lwd loading whereas pool spacing in generally steeper step pool channels is independent of lwd loading although plane bed and pool riffle channels occur at similar low lwd loading they exhibit typical pool spacings of greater than 9 and 2 4 channel widths respectively forced pool riffle channels have high lwd loading typical pool spacing of 2 channel widths and slopes that overlap the ranges of free formed pool riffle and plane bed channel types while a forced pool riffle morphology may mask either of these low lwd loading morphologies channel slope provides an indicator of probable morphologic response to wood loss in forced pool riffle reaches at all study sites less than 40 of the lwd pieces force the formation of a pool we also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear cut forests have lower lwd loading and hence fewer pools than reaches in pristine forests

shear velocity and shear attenuation models inverted from the world wide and pure path average data of mantle rayleigh waves 0s25 to 0s80 and fundamental spheroidal modes 0s2 to 0s24 summary global and regional structures of the earth s mantle are studied original phase velocities and q 1 of rayleigh waves are converted to equivalent free oscillation periods and q 1 global average upper mantle shear velocity models are derived from the inversions of the world wide average eigenperiods corrected for attenuative dispersion resolution analysis shows that the thickness and velocity of the lithosphere cannot be determined from our data set inverted models have a well developed low velocity zone in the depth range between 70 and 280 km global average q 1 models for the whole mantle are derived from the world wide average q 1 of mantle rayleigh waves and fundamental spheroidal modes the resolution analysis indicates that q 1 at depths shallower than about 100 km cannot be determined by our data set and that the depth resolution for q 1 of the lower mantle is poor all inverted q 1 models are characterized by a well developed high q 1 zone in the depth range between about 100 and 300 km in the upper mantle q has a minimum value of about 90 at depths around 200 km begins to increase at a depth of about 300 km rapidly increases in the depth range from 300 to 400 km and gradually increases at depths between 400 and 800 km q of the lower mantle is about 330 the boundary between the high q 1 upper mantle and low q 1 lower mantle is sharper than has been suggested in the past inverted q 1 models predict qscs 226 to 250 the earth s surface is divided into mountainous oceanic ridge shield and trench and marginal sea regions and a regionalized shear velocity model of the upper mantle is derived the inversions of the pure path eigenperiods without correction for attenuative dispersion show that strong lateral variations in shear velocity exist at depths shallower than 250 km in the upper mantle a low velocity zone for shear waves exists in the upper mantle beneath all the tectonic regions except perhaps the shield region if the sn velocity of the starting model is equal to or higher than 4 7 km s 1 the shield data require a moderate low velocity zone data from the mountainous region require a much lower shear velocity than the shield region at depths shallower than 250 km the oceanic ridge and trench and marginal sea models have a well developed low velocity zone the shear velocity model for the trench and marginal sea region is characterized by a velocity increase at depths between 250 and 400 km this increase in velocity is interpreted as a consequence of the subducted lithospheric slabs beneath many of our trench and marginal sea regions regional differences in rayleigh wave q 1 are found for modes from 0s25 to 0s45 since the resolving power of this data set is limited we could not locate the depths at which regional variations of q 1 exist since the oceanic q 1 for modes from 0s45 to 0s78 appear to be stable the oceanic q 1 model is derived this model has a well developed high q 1 zone q has a minimum value of about 110 at depths between 200 and 300 km and rapidly increases to a high value of about 250 at a depth of about 400 km

an experimental study on the ripple dune transition flume experiments were conducted on different bed stages across the ripple dune transition as flow velocity increases an initially flat bed surface made of fairly uniform sandy material is gradually transformed into a two dimensional rippled bed with further increase in velocity two dimensional ripples are replaced by irregular linguoid ripples as the average velocity necessary for the ripple dune transition to occur is imposed on the bed surface these non equilibrium linguoid ripples are further transformed into larger two dimensional dunes for each of these stages across the transition a concrete mould of the bed was created and the flow structure above each fixed bed surface investigated an acoustic doppler velocimeter was used to study the flow characteristics above each bed surface detailed profiles were used along a transect located in the middle of the channel results are presented in the form of spatially averaged profiles of various flow characteristics and of contour maps of flow fields section view they clearly illustrate some important distinctions in the flow structure above the different bedform types associated with different stages during the transition turbulence intensity and reynolds stresses gradually increase throughout the transition two dimensional ripples present a fairly uniform spatial distribution of turbulent flow characteristics above the bed linguoid ripples induce three dimensional turbulence structure at greater heights above the bed surface and turbulence intensity tends to increase steadily with height above bed surface in the wake region a very significant increase in turbulence intensity and momentum exchange occurs during the transition from linguoid ripples to dunes the turbulent flow field properties above dunes are highly dependent on the position along and above the bed surface and these fields present a very high degree of spatial variability when compared with the rippled beds further investigations under natural conditions emphasizing sediment transport mechanisms and rates during the transition should represent the next step of analysis together with an emphasis on quadrant analysis copyright 2001 john wiley sons ltd

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

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

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

a numerical model of coupled subcontinental and oceanic convection abstract at least two different types of convection should exist in the earth s mantle a large scale flow which is revealed by the creation and subduction of oceanic crust and a circulation of unknown scale under the continents which does not approach the surface the effects of these coexisting and interacting modes on mantle dynamics and thermal regime are investigated by means of a finite element model system of two adjacent cells which are coupled thermally and mechanically at their interface an a priori viscosity distribution is applied in such a way that the surface layer of the longer cell can move with almost uniform velocity while it is immobile on the shorter cell the results suggest that the pattern of sub oceanic convection is stabilized to some extent by the frame of the highly viscous but moving lithosphere whereas convection under continents is more complex and less stable most of the sub continental heat is transferred into the oceanic cell this causes an imbalance of surface heat flow which is well in accordance with recent determinations of global heat loss distribution furthermore the model reflects at least semiquantitatively the continent ocean differences in lithospheric thickness mean topographic level and upper mantle temperatures while the average geotherms below ocean and continent diverge by up to 400 k at moderate depth they nearly coincide below 250 km depth thus giving no support for deep reaching continent ocean differences these results refer to convection penetrating the lower mantle if the circulation is confined to moderate depth the continental upper mantle is too hot compared with the oceanic however this may be due to the strict separation of the flow on either side the model results stimulate some speculations on the possible form of whole mantle convection

a model for lateral hyporheic flow based on valley slope and channel sinuosity 1 the meandering of rivers results in head gradients across point bar deposits leading to hyporheic exchange the flow of groundwater from and back to the stream two models for predicting horizontal fluid flux through the hyporheic zone one based solely on valley slope and another that uses both valley slope and channel sinuosity are presented in this article both models require knowledge of aquifer hydraulic conductivity the models are developed via a suite of groundwater flow simulations with boundary conditions taking a variety of channel geometries idealized as sinusoidal and valley slopes that we assume are similar to that of the water table functional predictive models are developed by fitting a line or surface to discrete model output which covers the spectrum of most natural meandering rivers since the fitted models only require channel slope and or sinuosity plus hydraulic conductivity they are amenable for regional estimation of hyporheic flux using digital terrain data

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

the geoid small scale convection and differential travel time anomalies of shear waves in the central indian ocean we have measured 37 differential travel times for the phase pairs ss s sss ss scs2 scs scs2 s sscs2 sscs and sscs2 ss each pair included a surface bounce point in the central indian ocean between 4 n and 21 s and 69 and 85 e residuals were calculated relative to both the j b travel time tables and the pem c earth model and were corrected for the elevation of the bounce points the mean j b residual associated with bounce points in the central indian basin is 2 8 0 7 s fast this contrasts with 4 2 s slow reported for the western pacific by sipkin and jordan and is similar to the 4 0 s found for old continental nucleii after correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique we find that travel times in the mantle beneath the central indian basin are midway in character between those beneath pre cambrian shields and those beneath the western pacific ocean basin the fastest differential travel times occur along a line passing close to the center of the large indian ocean geoidal low there is a clear increase in travel time with distance from the center of the geoid low suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the central indian and pacific basins the chagos laccadive ridge seems to coincide with a transition from the fast differential travel times of the central indian basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers there are also variations within the central indian basin that are not related to surface geological features or to the age of the seafloor these variations are periodic with a wavelength of about 640 km and with a nne trend which agrees approximately with the trend that would be expected for linear convective rolls in the upper mantle

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

some consequences of the differences in seismic properties in old and young continental plates for isostatic compensation abstract we present a short review of seismological data and show that a difference of upper mantle structure is clearly resolved between old and young continental plates an archean plate is approximately 1 second faster than a paleozoic plate for p waves such a change in plate structure implies that the density of an archean lithosphere should be greater than the density of a paleozoic lithosphere if they have the same chemical composition applying isostasy we would expect a precambrian plate to sink and its surface to stand at a level 8 km beneath that of a paleozoic plate we first check that the weight of a crustal column is not smaller for a precambrian crust than for a paleozoic crust explosion seismology data show that isostatic equilibrium is nearly achieved in the crust therefore a compensating mechanism has to be acting in the subcrustal lithosphere two hypotheses are presented provided that the chemical composition of the lower lithosphere does not vary with time a simple condition for a continental plate not to sink when aging would be that the density of the lower lithosphere be 0 1 g cm 3 smaller than the density of the asthenosphere another condition that would prevent plates to sink would be a lowering of the density of the lower lithosphere with age due for example to a variation of its chemical composition and to a reduction in the mean atomic weight both hypotheses imply that isostatic equilibrium of old continental plates is maintained by a geochemical mechanism although thermal evolution is a key factor for explaining the evolution of continents it has to be complemented by a hypothesis on the change of the chemistry of the lower continental lithosphere with respect to the asthenosphere and or with respect to age

quantifying grain size distribution of pedogenic magnetic particles in chinese loess and its significance for pedogenesis quaternary glacial interglacial cycles have been imprinted on the chinese loess paleosol sequences through pedogenesis in order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution gsd to date the gsd has not been accurately determined because of the dearth of available means for analyzing extremely fine grained nanometer scale pedogenic magnetic particles using low temperature techniques we systematically investigated the temperature dependency of fd defined as 1hz 10hz where 1hz and 10hz are ac magnetic susceptibility measured at 1 and 10 hz respectively from two characteristic loess profiles one located at the western chinese loess plateau and the other in the central plateau on the basis of neel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures a quantitative gsd for pedogenic particles in chinese loess paleosols was constructed we found that the dominant magnetic grain size lies just above the superparamagnetic single domain threshold 20 25 nm and that the gsd is almost independent of the degree of pedogenesis this observation agrees well with other constraints from previous studies this new gsd model improves our understanding of the pedogenic processes in chinese loess enabling further explicit linkage of environmental magnetism to paleoclimate changes

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

wood controls on pool spacing step characteristics and sediment storage in headwater streams of the northwestern cascade mountains abstract wood influences channel morphology in headwater streams by creating steps and pools in the longitudinal profile and trapping sediment we used field data from 32 headwater stream reaches in the northwestern cascade mountains to test the hypotheses that 1 larger diameter classes of large wood lw defined as pieces with diameter 10 cm and length 0 5 m are critical step keying materials in headwater channels despite narrow channel widths 1 4 m 2 wood keyed steps trap more sediment than clast and root keyed steps and 3 the negative relationship between lw frequency and the distance between pools observed elsewhere in large streams extends to headwater streams we found that the frequency of step key pieces peaked in the 20 40 cm diameter class similarly 40 100 cm diameter pieces were disproportionately associated with key piece function compared to their overall association with step formation wood with diameter

convection in a two layer mantle with a strongly temperature dependent viscosity recent geochemical and isotopic data support the hypothesis that the earth s mantle convects in two separate layers with an interface at the 650 km seismic discontinuity this paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers we are particularly concerned with a possible decrease in the mantle viscosity across this interface in order to study the structure of the thermal boundary layers in a fluid with a strongly temperature dependent viscosity we utilize an approximate method developed by howard 1966 if the upper and lower mantle have the same viscosity law we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity this is not consistent with studies of postglacial rebound it is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures however the required law is not consistent with the currently available experimental data on lower mantle materials therefore we conclude that thermal arguments favor whole mantle convection

effect of storms during drought on post wildfire recovery of channel sediment dynamics and habitat in the southern california chaparral usa author s florsheim joan l chin anne kinoshita alicia m nourbakhshbeidokhti samira abstract current global warming projections suggest a possible increase in wildfire and drought augmenting the need to understand how drought following wildfire affects the recovery of stream channels in relation to sediment dynamics we investigated post wildfire geomorphic responses caused by storms during a prolonged drought following the 2013 springs fire in southern california usa using multi temporal terrestrial laser scanning and detailed field measurements after the fire a dry season dry ravel sediment pulse contributed sand and small gravel to hillslope channel margins in big sycamore creek and its tributaries a small storm in wy 2014 generated sufficient flow to mobilize a portion of the sediment derived from the dry ravel pulse and deposited the fine sediment in the channel totaling 0 60 m3 m of volume per unit length of channel the sediment deposit buried step pool habitat structure and reduced roughness by over 90 these changes altered sediment transport characteristics of the bed material present before and after the storm the ratio of available to critical shear stress t o t c increased by five times storms during wy 2015 contributed additional fine sediment from tributaries and lower hillslopes and hyperconcentrated flow transported and deposited additional sediment in the channel together these sources delivered sediment on the order of six times that in 2014 further increasing t o t c these storms during multi year drought following wildfire transformed channel dynamics the increased sediment transport capacity persisted during the drought period characterized by the longer residence time of relatively fine grained post fire channel sedimentation this contrasts with wetter years when post fire sediment is transported from the fluvial system during the same season as the post fire sediment pulse results of this short term study highlight the complex and substantial effects of multi year drought on geomorphic responses following wildfire these responses influence pool habitat that is critical to longer term post wildfire riparian ecosystem recovery

retracted and replaced a modelling study of hyporheic exchange pattern and the sequence size and spacing of stream bedforms in mountain stream networks oregon usa this article has been retracted and replaced see retraction and replacement notice doi 10 1002 hyp 6350 studies of hyporheic exchange flows have identified physical features of channels that control exchange flow at the channel unit scale namely slope breaks in the longitudinal profile of streams that generate subsurface head distributions we recently completed a field study that suggested channel unit spacing in stream longitudinal profiles can be used to predict the spacing between zones of upwelling flux of hyporheic water into the stream and downwelling flux of stream water into the hyporheic zone in the beds of mountain streams here we use two dimensional groundwater flow and particle tracking models to simulate vertical and longitudinal hyporheic exchange along the longitudinal axis of stream flow in second third and fourth order mountain stream reaches modelling allowed us to 1 represent visually the effect that the shape of the longitudinal profile has on the flow net beneath streambeds 2 isolate channel unit sequence and spacing as individual factors controlling the depth that stream water penetrates the hyporheic zone and the length of upwelling and downwelling zones 3 evaluate the degree to which the effects of regular patterns in bedform size and sequence are masked by irregularities in real streams we simulated hyporheic exchange in two sets of idealized stream reaches and one set of observed stream reaches idealized profiles were constructed using regression equations relating channel form to basin area the size and length of channel units step size pool length etc increased with increasing stream order simulations of hyporheic exchange flows in these reaches suggested that upwelling lengths increased from 2 7 m to 7 6 m and downwelling lengths increased from 2 9 m to 6 0 m with increase in stream order from second to fourth order step spacing in the idealized reaches increased from 5 3 m to 13 7 m as stream size increased from second to fourth order simulated upwelling lengths increased from 4 3 m in second order streams to 9 7 m in fourth order streams with a pool riffle step channel unit sequence and increased from 2 5 m to 6 1 m from second to fourth order streams with a pool step riffle channel unit sequence downwelling lengths also increased with stream order in these idealized channels our results suggest that channel unit spacing size and sequence are all important in determining hyporheic exchange patterns of upwelling and downwelling though irregularities in the size and spacing of bedforms caused flow nets to be much more complex in surveyed stream reaches than in idealized stream reaches similar trends emerged relating the average geomorphic wavelength to the average hyporheic wavelength in both surveyed and idealized reaches copyright 2005 john wiley sons ltd

on the stability of step pool mountain streams although step pools are generally considered to be stable bedforms stability is not absolute but depends on size scale and perspective hydraulic analysis of the stability of step pool sequences in the santa monica mountains california indicates that they are active channel features that are generally restructured within 5 to 100 years the degree of mobility depends on step particle size steps are stable within small temporal and spatial scales where they function as independent variables that dissipate stream energy and regulate channel hydraulics but stability decreases at larger scales where step pools become dependent variables that respond to discharge and sediment load hence the role of step pools changes from energy dissipation to one of channel adjustment these results underscore the need to consider larger spatial and temporal scales in order to reveal the complete function and significance of step pools in mountain fluvial systems

canyons with undulating walls channels incised into cohesive substrates commonly develop undulating walls characterized by regular repetitive variations in channel width canyons with undulating walls are found in a variety of lithologic climatic and tectonic regimes and are most commonly present downstream from a knick zone of active incision flume experiments and natural channel morphologic patterns lead us to infer that wall undulations are the remnants of breached offset potholes or of sinuous longitudinal grooves formed during incision of the knick zone substrate characteristics seem to have little influence on these undulations using measurements from eight channels incised into the navajo sandstone of southern utah we found that intact rock strength rock mass strength joint orientation and spacing bedding and porosity did not differ significantly between channel reaches with straight walls undulating walls and meanders the wavelength and amplitude of wall undulations also did not correlate with any substrate characteristic these results lead us to conclude that the primary control on formation of undulating walls is hydraulic processes along the study channels deep narrow reaches with undulating walls alternate with wider straight or meandering reaches using flume simulations and one and two dimensional hydraulic modeling we found that the wall undulations act to reduce interreach spatial variability in energy expenditure and to minimize energy expenditure within a reach we propose that the wall undulations and flow hydraulics create a feedback whereby the wall undulations are preserved following knickpoint incision the wall undulations then act to regulate downstream energy expenditure in a manner analogous to bedforms this implies that generalized principles of uniform energy expenditure developed for alluvial channels may also apply at the reach scale to bedrock channels with relatively homogeneous substrates

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

using geostatistics and spectral analysis to study spatial patterns in the topography of southeastern washington state u s a topography for four areas in the palouse region of southeastern washington state having different patterns and encompassing areas of about 900 ha each were quantitatively compared and described using two dimensional semi variograms and periodograms the four areas studied were from the garfield la crosse thornton and wilcox quadrangle topographic maps semivariance of elevation residuals were modelled using a combination of spherical periodic or linear semivariogram models the range of the spherical component was interpreted as a relatively short range scale of correlation which was not periodic for each of the study areas a model was developed to describe the variation in range with orientation values for the range from this model reached maximum values of from about 700 m to 800 m at an orientation of from 35 to 55 approximately northeast in each study area this orientation was interpreted as the dominant direction of non periodic small scale landscape features the wavelength and amplitude of the periodic semivariogram component which were highly correlated were interpreted in terms of parallel northeast trending ridges having relatively long scales of periodic correlation varying in size from about 1350 m to 2100 m this attempt to identify the dominant orientation of periodic landscape features using models for the wavelength and amplitude was however not completely definitive two dimensional spectral analysis provided significantly more detail concerning orientation and wavelengths of the periodic topographic patterns than the semivariogram analysis in the garfield study area spectral analysis identified north trending ridge systems separated by a wavelength of 1494 m and northeast trending systems with a wavelength of 747 m in the la crosse study area both north and northeast trending patterns were identified having periodic spacings of 980 and 735 m north trending ridges separated by wavelengths of 996 m and northeast trending ridges separated by wavelengths of 747 or 996 m were the predominant periodic features of topography in the thornton study area in the wilcox study area northeast trending ridge systems separated by wavelengths of 373 498 or 996 m were detected a comparison of the results from geostatistical and spectral analysis of these complex topographic surfaces shows that each approach had significant strengths and weaknesses two dimensional analysis with semivariograms was the only method which could be used for identifying the correlation scale and orientation of relatively small non periodic landscape features two dimensional spectral analysis accurately identified the predominant orientation of relatively large periodic features in topography whereas semivariogram analysis was somewhat inconclusive also semivariograms were generally unable to detect the presence of multiple or harmonic periodicities operating at different wavelengths along a given orientation

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

short term post wildfire dry ravel processes in a chaparral fluvial system abstract dry ravel the transport of sediment by gravity transfers material from steep hillslopes to valley bottoms during dry conditions following wildfire dry ravel greatly increases in the absence of vegetation on hillslopes thereby contributing to sediment supply at the landscape scale dry ravel has been documented as a dominant hillslope erosion mechanism following wildfire in chaparral environments in southern california however alteration after initial deposition is not well understood making prediction of post fire flood hazards challenging the majority of big sycamore canyon burned during the may 2013 springs fire leaving ash and a charred layer that covered hillslopes and ephemeral channels dry ravel processes following the fire produced numerous deposits in the hillslope channel transition zone field data focus on 1 deposition from an initial post wildfire dry ravel pulse and 2 subsequent alteration of dry ravel deposits over a seven month period between september 2013 and april 2014 we quantify geomorphic responses in dry ravel deposits including responses during the one small winter storm that generated runoff following the fire field measurements document volumetric changes after initial post wildfire deposition of sediment derived from dry ravel erosion and deposition mechanisms that occurred within dry ravel deposits situated in the hillslope channel transition zone included 1 mobilization and transport of a portion or the entire deposit by fluvial erosion 2 rilling on the surface of the unconsolidated deposits 3 deposition on deposits via continued hillslope sediment supply and 4 mass wasting that transfers sediment within deposits where surface profiles are near the angle of repose terrestrial lidar scanning point clouds were analyzed to generate profiles quantifying depth of sediment erosion or deposition over remaining dry ravel deposits after the first storm season this study contributes to the understanding of potential effects of wildfire on fine sediment delivery to fluvial systems in chaparral ecosystems

bed material size and the riffle pool sequence relationships between the bed material size distribution and riffle pool geometry of a small coarse bedload upland stream channel are described downstream sequences of bed material size are highly variable although series analysis reveals some persistence in the data over short distances cross correlation of bed material series with bed elevation series indicates riffles tend to coincide with coarser bed material sequences comparison of the bed material size distributions of individual riffles pools and bars is however more revealing mean particle size was found to be the most useful criterion for distinguishing between bed forms and in these terms bars are the most distinctive features moreover riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser grained sedimentary unit coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased however conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters plan geometry and coarse sediment supply in the upland environment are also shown to influence bed material size distributions tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments size of material in pools is also negatively related to an index of cross section asymmetry

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

frequency and magnitude of bedload transport in a mountain river magnitude and frequency of bedload transport was examined in the lainbach river bavaria using magnetically tagged particles during the study 18 small to moderate events and one major event occurred about 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles the step pool pattern which characterized the study site prior to the large event was obliterated however the channel recovered quickly and has returned to a new step pool pattern the event changed the boundary conditions by increasing the availability of loose sediment and creating higher river bed slopes in reaches between breached check dams as a result movement of individual particles measured for events of both the same magnitude and duration occurring before and after the large event yielded different values for events which occurred after the large event the range and the mean distance of movement were about ten times higher

rapid vegetation response to lateglacial and early holocene climatic fluctuation in the south carpathian mountains romania abstract high resolution pollen conifer stomata and plant macrofossil analyses of two glacial lake sediments 1740 and 1990 m a s l are used to reconstruct lateglacial lg and early holocene eh vegetation and tree line changes in the retezat mountains our results show that during the lg tree line was between 1750 and 1800 m a s l formed by larix decidua pinus mugo and picea abies early lg spread to high altitudes suggests refugia of these tree species in the mountain the younger dryas cooling resulted in regional steppe tundra expansion but tree line position and composition showed little change the abundance of trees and shrubs decreased at 1740 m a s l but species richness increased with the arrival of pinus cembra our data support climate model hindcasts for only modest decrease in accumulated growing season heat at mid high altitudes regionally the pollen records suggest enhanced aridity and seasonality in the eh tree line reached 2000 m a s l higher than today by 11 100 cal yr bp p mugo p cembra p abies established around the upper lake suggesting rapid increase in summer temperatures the eh maximum of l decidua between 11 200 10 600 cal yr bp was connected to high summer insolation high altitude expansion of abies alba between 10 600 10 300 cal yr bp suggested summer mean temperatures 2 8 c higher then today in comparison with other mountain sites in europe lg interstadial tree line was at similar altitude in the s alps and 350 m higher in the pirin mountains lg tree line fluctuation had similar low amplitude in the se alps retezat and pirin mts suggesting relatively weak influence of the north atlantic thermohaline circulation changes on growing season temperatures

the evolution of a colluvial hollow to a fluvial channel with periodic steps following two transformational disturbances a wildfire and a historic flood abstract the transition of a colluvial hollow to a fluvial channel with discrete steps was observed after two landscape scale disturbances the first disturbance a high severity wildfire changed the catchment hydrology to favor overland flow which incised a colluvial hollow creating a channel in the same location this incised channel became armored with cobbles and boulders following repeated post wildfire overland flow events three years after the fire a record rainstorm produced regional flooding and generated sufficient fluvial erosion and sorting to produce a fluvial channel with periodically spaced steps an analysis of the step spacing shows that after the flood newly formed steps retained a similar spacing to the topographic roughness spacing in the original colluvial hollow prior to channelization this suggests that despite a distinct change in channel form roughness and bedform morphology the endogenous roughness periodicity was conserved variations in sediment erodibility helped to create the emergent steps as the largest particles d84 remained immobile becoming step features and downstream soil was easily winnowed away

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

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

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

modelling gravel bed topography from sediment transport a simple queueing model which generates bed topography consistent with the mechanics of gravel motion is presented the equations on which it is based are derived from the application of simple theoretical ideas and an analysis of published flume data meland and norrman 1966 francis 1973 abbott and francis 1977 the simulation produces an extremely variable bed topography in which at least two scales of bedform may be identified features of 5 10 grain wavelengths similar to pebble clusters are superimposed on other forms whose wavelength is up to 30 40 grains and which resemble step pools systems and antidunes sediment transport shows many of the characteristics found in the field and the model is therefore thought to provide a useful starting point from which to examine the interaction between flow sediment transport and bedforms in gravel bed and cobble streams

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

human impacts to mountain streams mountain streams are here defined as channel networks within mountainous regions of the world this definition encompasses tremendous diversity of physical and biological conditions as well as history of land use human effects on mountain streams may result from activities undertaken within the stream channel that directly alter channel geometry the dynamics of water and sediment movement contaminants in the stream or aquatic and riparian communities examples include channelization construction of grade control structures or check dams removal of beavers and placer mining human effects can also result from activities within the watershed that indirectly affect streams by altering the movement of water sediment and contaminants into the channel deforestation cropping grazing land drainage and urbanization are among the land uses that indirectly alter stream processes an overview of the relative intensity of human impacts to mountain streams is provided by a table summarizing human effects on each of the major mountainous regions with respect to five categories flow regulation biotic integrity water pollution channel alteration and land use this table indicates that very few mountains have streams not at least moderately affected by land use the least affected mountainous regions are those at very high or very low latitudes although our scientific ignorance of conditions in low latitude mountains in particular means that streams in these mountains might be more altered than is widely recognized four case studies from northern sweden arctic region colorado front range semiarid temperate region swiss alps humid temperate region and papua new guinea humid tropics are also used to explore in detail the history and effects on rivers of human activities in mountainous regions the overview and case studies indicate that mountain streams must be managed with particular attention to upstream downstream connections hillslope channel connections process domains physical and ecological roles of disturbance and stream resilience

the role of fire in landscape dynamics an example of two sediment records from the rodna mountains northern romanian carpathians fire is an important disturbance agent worldwide and one of the major causes of geomorphological change dependent on its characteristics size duration temperature fire induces changes in soil stability modifying infiltration and promoting soil erosion here we employ a suite of physical mineral magnetic loss on ignition geochemical xrf derived elemental data biological macro charcoal pollen proxies and dating measurements ams 14c 210pb and 137cs to examine the relationship between fire activity sedimentary characteristics and geomorphological change over the holocene our results highlight that fire events imprint a magnetic signature characterised by secondary ferrimagnetic assemblage particularly very fine superparamagnetic sp magnetite and maghemite in lake buh iescumare the early holocene is characterised bymoderate fire activity and coarser sediment particle size indicating a relatively unstable catchment affected by burning events which activated erosional processes subsequently the late holocene 4 000 0 cal yr bp is marked by intermittently variable fire activity with sporadic increases between 3 400 and 3 200 cal yr bp and around 2 000 cal yr bp when burning was strong enough to lead to the magnetic enrichment of topsoil and detrital inputs to the lake in our high resolution record lake tiol the interval 2 500 700 cal yr bp and also the last 100 years are characterised by intense burning that coincides with sustained surface soil erosion and magnetic enrichment the fire erosion landscape relationship varies over time being highly dependent on fire behaviour fire temporal and spatial distribution and landscape features slope soil type vegetation cover

a modelling study of hyporheic exchange pattern and the sequence size and spacing of stream bedforms in mountain stream networks oregon usa studies of hyporheic exchange flows have identified physical features of channels that control exchange flow at the channel unit scale namely slope breaks in the longitudinal profile of streams that generate subsurface head distributions we recently completed a field study that suggested channel unit spacing in stream longitudinal profiles can be used to predict the spacing between zones of upwelling flux of hyporheic water into the stream and downwelling flux of stream water into the hyporheic zone in the beds of mountain streams here we use two dimensional groundwater flow and particle tracking models to simulate vertical and longitudinal hyporheic exchange along the longitudinal axis of stream flow in second third and fourth order mountain stream reaches modelling allowed us to 1 represent visually the effect that the shape of the longitudinal profile has on the flow net beneath streambeds 2 isolate channel unit sequence and spacing as individual factors controlling the depth that stream water penetrates the hyporheic zone and the length of upwelling and downwelling zones 3 evaluate the degree to which the effects of regular patterns in bedform size and sequence are masked by irregularities in real streams we simulated hyporheic exchange in two sets of idealized stream reaches and one set of observed stream reaches idealized profiles were constructed using regression equations relating channel form to basin area the size and length of channel units step size pool length etc increased with increasing stream order simulations of hyporheic exchange flows in these reaches suggested that upwelling lengths increased from 2 7mt o 7 6 m and downwelling lengths increased from 2 9mt o 6 0 m with increase in stream order from second to fourth order step spacing in the idealized reaches increased from 5 3mt o 13 7 m as stream size increased from second to fourth order simulated downwelling lengths increased from 4 3m in second order streams to 9 7 m in fourth order streams with a pool riffle step channel unit sequence and increased from 2 5m to 6 1 m from second to fourth order streams with a pool step riffle channel unit sequence upwelling lengths also increased with stream order in these idealized channels our results suggest that channel unit spacing size and sequence are all important in determining hyporheic exchange patterns of upwelling and downwelling though irregularities in the size and spacing of bedforms caused flow nets to be much more complex in surveyed stream reaches than in idealized stream reaches similar trends emerged relating the average geomorphic wavelength to the average hyporheic wavelength in both surveyed and idealized reaches this article replaces a previously published version hydrological processes 19 17 2915 2929 2005 doi 10 1002 hyp 5790 copyright 2006 john wiley sons ltd

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

laboratory convection experiments effect of lateral cooling and generation of instabilities in the horizontal boundary layers convection experiments are carried out in a tank with two isothermal heat sinks the top plate and one of the sidewalls heating is supplied either by an isothermal bottom plate or by generation within the fluid this situation is similar to that of the earth s subcontinental mantle in the presence of a neighboring subducting oceanic lithosphere there the mantle material loses heat not only to the base of the continental lithosphere but also to the cold dipping oceanic slab the thermal structure of the convective fluid is observed by a variety of techniques including differential interferometry and strioscopy two parameters characterize the observations the usual vertical rayleigh number and a newly defined lateral rayleigh number the lateral cooling induces a large roll with axis parallel to the cold wall the variation of its width relative to the values of the two rayleigh numbers has been determined an application to the earth s upper mantle would predict rolls 5 times wider than high the circulation in the large roll remains two dimensional for rayleigh numbers up to about 3 104 beyond this value boundary layer instabilities are observed within the persisting large roll their period has been determined the interferometric method is shown to be very useful for visualizing other time dependent processes such as the growth of the induced large rolls this growth is rapid enough to allow us to propose the existence of such large rolls in the earth and argue that their action could have led to continental break up

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

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

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

younger dryas to mid holocene environmental history of the lowlands of nw transylvania romania pollen micro charcoal and total carbon analyses on sediments from the turbuta palaeolake in the transylvanian basin of nw romania reveal younger dryas to mid holocene environmental changes the chronostratigraphy relies on ams 14c measurements on organic matter and u th tims datings of snail shells results indicate the presence of pinus and betula open woodlands with small populations of picea ulmus alnus and salix before 12 000 cal yr bp a fairly abrupt replacement of pinus and betula by ulmus dominated woodlands at ca 11 900 cal yr bp likely represents competition effects of vegetation driven by climate warming at the onset of the holocene by 11 000 cal yr bp the woodlands were increasingly diverse and dense with the expansion of quercus fraxinus and tilia the establishment of corylus and the decline of upland herbaceous and shrubs taxa the marked expansion of quercus accompanied by tilia between 10 500 and 8000 cal yr bp could be the result of low effective moisture associated with both low elevation of the site and with regional change towards a drier climate at 10 000 cal yr bp corylus spread across the region and by 8000 cal yr bp it replaced quercus as a dominant forest constituent with only little representation of picea abies carpinus became established around 5500 cal yr bp but it was only a minor constituent in local woodlands until ca 5000 cal yr bp results from this study also indicate that the woodlands in the lowlands of turbuta were never closed

stability of the oceanic lithosphere with variable viscosity an initial value approach abstract we have studied the problem concerning the onset of convective instabilities below the oceanic lithosphere a system of linear partial differential equations in which the background temperature field is time dependent is integrated in time to monitor the evolution of incipient disturbances two types of rheologies have been examined one depends strongly on temperature the other involves a viscosity which is both temperature and pressure dependent the results from this initial value approach in which the viscosity profiles migrate downward with time reveal the importance of considering temperature and pressure dependent rheology in issues regarding the development of local instabilities in upper mantle convection for temperature dependent viscosity viscosities of 0 1020p are required to produce instabilities with growth rates of 0 1 ma in contrast these same growth rates can be attained for a temperature and pressure dependent viscosity profile with a mean value close to 0 1020p in the upper mantle owing to the presence of a low viscosity zone 0 1020p existing right below the lithosphere unlike the results of temperature dependent viscosity whose growth rates increase with time the amplification of disturbances in a fluid medium with temperature and pressure dependent rheology reaches a maximum at an early age

on the origin of step pool sequences in mountain streams this paper reports a field test of the prevailing antidune theory to explain the mechanisms of step pool formation in steep mountain streams the theory is supported by laboratory data but it has not received extensive field testing that would enable wide applicability to natural conditions data from the santa monica mountains of california are used to test the antidune model where step wavelength and reconstructed hydraulic conditions are compared with the formation domain predicted by experimental and theoretical studies results show general agreement between field and laboratory data step wavelength and hydraulic requirements are consistent with those of the antidune process of formation but results also suggest that true antidunes may be difficult to achieve in the steep headwaters where channels may be dominated by large roughness elements these findings strengthen the link between theory and observation in clarifying how step pools form they contribute to the basis for elucidating why step pools form in mountain streams

synchronisation of palaeoenvironmental records over the last 60 000 years and an extended intimate event stratigraphy to 48 000 b2k the intimate protocol for the correlation of marine ice core and terrestrial records is here updated reflecting recent developments in palaeoclimate research originally focussed on last glacial to interglacial transition lgit 22 000 11 500 years b p the intimate group has now extended its study period back to 60 000 years as a first step the intimate event stratigraphy has now been extended to include 8000 48 000 b2k based on a combined ngrip and grip isotope profile against a gicc05 chronology and key tephra horizons from iceland and continental european volcanic sources in this lead article of this intimate special issue we introduce some of the recent achievements of the intimate community and discuss the challenges associated with extending the intimate approach back in time focussing in particular on the chronological problems inherent in comparing different proxy records the intimate approach has thus far been limited predominantly to the north atlantic and australasian regions and we also discuss the potential to extend the intimate protocol beyond these regions

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

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

reconstruction of climatic and environmental changes in nw romania during the early part of the last deglaciation 15 000 13 600 cal yr bp high resolution pollen plant macrofossil charcoal mineral magnetic and sedimentary analyses combined with ams 14 c measurements were performed on multiple sediment sequences alonga transect through the former crater lake preluca tiganului in

old continental geotherms constraints on heat production and thickness of continental plates summary absolute and mutual constraints on radioactive heat productivity plate thickness and steady state continental geotherms are demonstrated for a model in which heat productivity comprises a near surface exponentially decreasing component and a component independent of depth for a given surface heat flux there are separate upper limits on each component and a limiting trade off curve between them below these limits there are linear trade offs imposed if either the plate thickness or the temperature at a given depth are known and in principle tighter bounds are imposed on each component preliminary application of the model shows that a kimberlite pyroxene geotherm requires a plate at least 200km thick and that lower plate upper continental mantle heat productivities may be but are not required to be as great as about 0 1 yw m3 which is about ten times the heat productivity of chrondrites the estimation of continental geotherms depends strongly on assumptions about the vertical distribution of radioactive heat sources in the continental plates and on whether or not a steady state is assumed to have been achieved the thickness of the plate is also directly involved since the base of the plate can be defined as the depth at which a characteristic temperature is reached it is the purpose of this paper to demonstrate using a simple but plausible model the relationships between temperature heat production and plate thickness including some absolute limits on heat production the relationships can be used to evaluate the implications of various independent estimates of these quantities and some examples are discussed here an obvious limit on heat production is that in the steady state no more heat can be generated in the plate than emerges at the surface a constraint with less obvious implications is that the geotherm must reach the mantle temperature at a reasonable depth these constraints will be demonstrated here using a simple model of heat source distribution which is nevertheless plausible enough to give quantitatively useful results attention will be confined here to steady state models although the possibility of indefinitely continued cooling and thickening of the continental plates has been discussed pollack chapman 1977 a severe problem with this hypothesis is that the associated thermal subsidence should be accompanied by lokm or more of crustal thickening a

a late glacial paleolake record from an up dammed river valley in northern transylvania romania lake sediments from the magherus valley in the lowlands of northern transylvania provide new evidence of paleoenvironmental development in romania during the late glacial the studied sediments were deposited as a result of the damming of a small river valley following a series of mass wasting events that occurred during the deglaciation period a continuous sedimentary sequence belonging to the former lake is preserved in the banks of the stream and contains evidence of paleoenvironmental changes associated with the late glacial interstadial bolling allerod or greenland interstadial 1 gi 1 and the late glacial stadial younger dryas or greenland stadial 1 gs 1 implying that the hallmark climatic episodes of northwestern europe are also expressed in eastern europe we employ a multiproxy approach based on the analysis of sediment composition and texture mineral magnetism organic macrofossils and radiocarbon dating the reconstructed paleoenvironmental evolution at the site captures the warm and humid conditions associated with gi 1 as well as the subsequent cooling concomitant with the onset of gs 1 these climatic events are paralleled by vegetation shifts in the region as deduced from comparisons with pollen sequences from magherus and neighboring locations in transylvania and the carpathians the late glacial interstadial was warmer and wetter as evidenced by increased organic matter content in the lake decreased erosion in the catchment development of palustrine plant communities in the proximity of the lake and expansion of spruce during the allerod gi 1c a the late glacial stadial was colder and dryer as indicated by low sedimentary organic matter content an increase in erosion markers and the decline of spruce and its replacement by birch the landscape was more open as herbs and grasses also expanded during this time these fluctuations imply that late glacial climatic events are well expressed not only in upland areas of romania but also in lowland regions such as the transylvanian basin where climatic effects are expected to be more muted c 2014 elsevier ltd and inqua all rights reserved

regional dispersion of first order overtone rayleigh waves summary records of the first rayleigh overtone in the 70 110s period range following deep subduction zone events are analysed through standard surface wave techniques to retrieve dispersion characteristics in the oceanic environment our data are in general agreement with published models obtained from regionalization of fundamental rayleigh wave paths a general increase in phase velocity with age is observed with a somewhat smaller effect on group velocities continental and trench area data are processed through a regionalization scheme indicating that the difference between average continental and oceanic velocities both phase and group is about 0 2 km s l at periods of the order of 95 s an investigation of a set of simple models allowing structural heterogeneities between oceans and continents down to variable depth suggests that they probably do not extend past the bottom of the asthenosphere 250 km

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

controls on the distribution of channel reach morphology in selectively glaciated catchments abstract to assess the controls on the distribution of channel reach morphology in a selectively glaciated landscape we used field mapping and a geographical information system gis in the river dee catchment northeast scotland controls on channel morphology were investigated using 1 continuous longitudinal assessment of channel morphology distribution in relation to geology glacial history topography and total stream power in two subcatchments and 2 slope s and a slope drainage area s a framework to understand the occurrence of 173 widely distributed bedrock mixed bedrock alluvial and alluvial three different types reaches the s a framework used indicators of transport capacity qc and sediment supply qs to differentiate channel types the study highlights the disjointed nature of channel reach distribution at the river scale that reflects variable lithology and glacial modification because of the subdued topography in contrast to other regions colluvial forcing of channel morphology in the headwaters was lacking however in common with other glaciated landscapes repeated sequences of channel reach type progression determined by valley steps were evident the s a analysis successfully discriminated 87 2 of alluvial and 91 4 of bedrock reaches despite the variable land use and glacial modification discrimination of the full range of channel types using s or the s a framework was poor however notably a third of the transport alluvial reaches were located in the bedrock s a domain and the majority of mixed reaches were widely distributed mostly within the bedrock domain and not close to the critical slope sc in comparison to other regions the sc above which qc qs and bedrock reaches dominate was notably higher we hypothesise that a drier climate and the higher entrainment threshold of coarse granite dominated bed materials create a higher sc

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

effects of coarse woody debris on morphology and sediment storage of a mountain stream system in western oregon effects of coarse woody debris cwd on channel morphology and sediment storage were investigated at five sites representative of first order to fifth order streams in the steep and bedrock confined stream first second order interaction between the channel and cwd was limited except where breakage upon falling produced cwd pieces shorter than channel width channel widening steepening and sediment storage associated with cwd were observed predominantly in third to fifth order streams variation in channel width and gradient was regulated by cwd in the fifth order stream most of the cwd pieces derived from the riparian forest interacted directly with the channel without being suspended by sideslopes in this system cwd promoted lateral channel migration but sediment storage was temporary with annual release and capture

irregular bed forms in steep rough channels 2 field observations a linear stability analysis using a flow model appropriate for steep rough channels suggests that initial wavelength selection of alternate bars is strong for extant flow conditions in north boulder creek colorado this is due in part to effects of velocity correlation terms in the streamwise momentum balance that involve deviations in velocity components about depth averaged values the analysis also suggests that alternate bars and midchannel bars can compete during initial bed form growth at low sediment transport rates these points are consistent with the observation that alternate and midchannel bar forms in north boulder creek have comparable topographic relief as revealed by spectral analysis of the bed topography moreover the dominant wavelengths of these bar forms match the fastest growing wavelengths predicted by the stability analysis however whereas the alternate bar topography can be attributed to free bar growth much of the midchannel bar topography probably is a forced bar topography that is related to flow accelerations associated with variations in channel width the complex bed topography of north boulder creek thus consists of a decipherable mixture of bed forms that have evolved as free alternate bars together with forced bars whose positions and geometries are fixed in relation to variations in channel width or to large immobile boulders

the influence of debris flows on channels and valley floors in the oregon coast range u s a debris flows are one of the most important processes which influence the morphology of channels and valley floors in the oregon coast range debris flows that initiate in bedrock hollows at heads of first order basins erode the long accumulated sediment and organic debris from the floors of headwater first and second order channels this material is deposited on valley floors in the form of fans levees and terraces in channels deposits of debris flows control the distribution of boulders the stochastic nature of sediment supply to alluvial channels by debris flows promotes cycling between channel aggradation which results in a gravel bed morphology and channel degradation which results in a mixed bedrock and boulder bed morphology temporal and spatial variability of channel bed morphology is expected in other landscapes where debris flows are an important process

tectonic stresses in the lithosphere various types of observables earthquake focal mechanisms in situ measurements and geological deformations give information about the large scale lithospheric stress field the latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates this approach ignores the role of mass heterogeneities within the lithosphere here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude the presence of internal sources makes the problem three dimensional we show however that it can be reduced to a two dimensional plane stress formulation whereby the edge forces are expressed by the non hydrostatic stresses and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress for the oceanic lithosphere we derive a compression that increases with age the comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag for the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd