sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

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

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

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

an ice ocean coupled model an ice model an ocean model and a method of coupling the models are described the ice model is a synthesis with variations and extensions of previous modeling ideas ice thickness concentration velocity and internal energy are prognostic variables the ice thermodynamics are represented by temperatures at the snow surface ice surface the interior and the bottom surface melting and freezing rates are calculated at the ice atmosphere ice ocean and atmosphere ocean interfaces a prescribed portion of summer meltwater can be stored on the surface and refrozen in the fall the ocean model includes a second moment turbulence closure submodel and enables one to solve for oceanic heat flux the interfacial stress and subsurface properties in this paper the model is applied to one dimensional simulations but the equations are cited in a form for implementation by two and three dimensional models in a companion paper kantha and mellor this issue the model is used for two dimensional vertical plane simulations in the bering sea several one dimensional sensitivity studies are performed in the case where the ice model is decoupled from the ocean here the oceanic heat flux and sea surface temperature are prescribed constants the studies reveal the role and sensitivity of surface trapped meltwater ice concentration and ice divergence with the coupled ice ocean model the seasonally varying oceanic heat flux and mixed layer properties are determined by the model some comparisons with observations in the central arctic ocean are possible the role of the molecular sublayer immediately adjacent to the ice is examined frazi ice production is related to the large disparity in the molecular diffusivities for temperature and salinity the mixed layer model contains empirical constants which are known from turbulence data the molecular sublayer parameterization requires one empirical parameter b which is uncertain but from this study is assuredly greater than zero the value implicit in previous models the ice model requires the empirical parameters ci f and ci u to quantitatively account for freezing or melting processes in open leads their values are also uncertain but we present reasoning and sensitivity studies to suggest specific values finally an empirical parameter g is introduced it is the ratio of the value of the ice thickness used to represent average ice volume in the dynamic and thermodynamic equations to the value of the thickness needed in the heat conduction equation estimates of g are made from observed thickness distribution functions sensitivity studies show it to be an important parameter

a model for the thickness and salinity of the upper layer in the arctic ocean and the relationship between the ice thickness and some external parameters abstract this paper presents a dynamical model for the salinity and thickness of the upper layer in the arctic the parameters are the river runoff to the arctic the buoyancy supply through the bering strait the export of ice from the arctic and a parameter characterizing the vertical mixing an ice model is formulated having the following two important properties 1 the horizontal surface area of the exported ice is essentially determined by external parameters the wind field over the arctic and 2 there is a relationship between the ice thickness and the fraction of open water in the arctic the model for the upper layer and the ice model are used together with a heat budget for the arctic also including the effect of different albedo for ice and open water a relationship between the freshwater supply and the ice thickness is derived also investigated are the effects on the ice thickness of a changed export of ice area and changed properties of the flow through the bering strait it is found t

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

return of the cold halocline layer to the amundsen basin of the arctic ocean implications for the sea ice mass balance 1 ctd measurements from the arctic ocean 2001 expedition reveal that the cold halocline layer chl has returned to the amundsen basin at a position close to that found during the oden 91 expedition river water from the siberian shelves formed a strong freshwater front in the amundsen basin extending from the gakkel ridge to the lomonosov ridge furthermore we show from model computations that the presence of a chl may increase winter sea ice growth by 0 25 m over one season compared to a case with a non existing chl due to increased vertical heat flux from the warm atlantic water the difference in sea ice growth is due to a much shallower winter convection with a chl present which is not able to reach into the warm atlantic layer resulting in a considerably smaller oceanic heat flux

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

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

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 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

estimates of sea ice thickness distribution using observations and theory the thickness distribution of sea ice is maintained by a balance of thermal and mechanical processes observations now exist that make it possible to quantify this balance and to test models of the individual physical processes in particular the observed distributions used with fairly well established thermodynamic growth rates give an estimate of the redistribution process that models the formation of pressure ridges to highlight the main ideas the evolution of the thickness distribution is recast as a markov process forced by the mean velocity divergence and a measure of the random short term deformation this model reproduces features of the observed thickness distribution such as the peak near 3 m the mean thickness somewhat greater than 3 m the long tail and the variable shape of the thin side of the distribution analytical expressions for the mean and variance of the ice thickness are given showing how they depend on the growth rates for thick and thin ice the equilibrium thickness the ice deformation and the rule for building pressure ridges these are sensitive to the mean divergence and the melt rate for thick ice with large amounts of data now available for both the ice thickness and the ice motion further progress is anticipated toward understanding the processes which shape the thickness distribution

open water production in arctic sea ice satellite measurements and model parameterizations sequential synthetic aperture radar sar images from the ers 1 spacecraft were routinely used as input to the geophysical processor system at the alaska sar facility from 1992 through 1994 to produce sea ice displacement vectors on a 5 km grid we have combined some 5000 of these products consisting of nearly 700 000 displacement vectors into a set of 914 strips or mosaics of ice motion that span mostly 3 day intervals for each strip we compute the opening and closing of leads based on the change in area of the 5 km grid cells we also compute the area averaged deformation or strain invariants of the motion the small scale several kilometer lead activity can be parameterized fairly well in terms of the large scale several hundred kilometer strain invariants sea ice models implicitly contain such a parameterization through their constitutive equations the well known model of hibler 1979 that uses a viscous plastic rheology and an elliptical yield curve is in good agreement with our data the variance in the data about this theoretical relationship is less than that suggested by a random model of sea ice motion the data also indicate that shearing deformation contributes to the opening and closing of leads models with two categories or levels of ice thickness generally do not take shearing deformation into account in the evolution of the ice concentration nor do they redistribute ice as a result of ridging we show how to add these features to two level models however we argue that three level models consisting of open water thin ice and thick ice are much better suited to the proper treatment of open water production and we present such a model

ridging and strength in modeling the thickness distribution of arctic sea ice a theory describing evolution of the ice thickness distribution the probability density of ice thickness was proposed by thomdike et al 1975 and has been used in several sea ice models the advantage of this theory over the widely used two level formulation is that it treats ridging explicitly as a redistribution of ice thickness and ice strength as a function of energy losses incurred by ridge formation however the parameterization of these processes remains rather speculative and largely untested and so our purpose here is to explore these parameterizations using a numerical model based on this theory the model uses a 160 km resolution grid of the arctic and 7 years of observed atmospheric forcing data 1979 1985 monthly oceanic heat flux and current fields are obtained from a 40 km resolution coupled ice ocean model run separately with the same forcing by requiring the computed monthly mean ice drift to have the same magnitude as observed buoy drift we estimate the primary strength parameter the ratio of total to potential energy change during ridging this ratio depends on the value of other parameters however the standard case has a ratio of 17 which is within the range estimated by hopkins 1994 in simulations of individual ridging events the effects of ridge redistribution and shear ridging parameters are illustrated by a series of sensitivity studies and comparisons between observed and modeled ice thickness distributions and ridge statistics in addition these comparisons highlight the following shortcomings of the thickness distribution theory as it is presently implemented first the process of first year to multiyear ridge consolidation is ignored and second the observed preferential melt of thick ridged ice is not reproduced

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

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

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

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

the contribution of alaskan siberian and canadian coastal polynyas to the cold halocline layer of the arctic ocean numerous arctic ocean circulation and geochemical studies suggest that ice growth in polynyas over the alaskan siberian and canadian continental shelves is a source of cold saline water which contributes to the maintenance of the arctic ocean halocline the purpose of this study is to estimate for the 1978 1987 winters the contributions of arctic coastal polynyas to the cold halocline layer of the arctic ocean the study uses a combination of satellite oceanographic and weather data to calculate the brine fluxes from the polynyas then an oceanic box model is used to calculate their contributions to the cold halocline layer of the arctic ocean this study complements and corrects a previous study of dense water production by coastal polynyas in the barents kara and laptev seas recurrent polynyas form on the canadian and alaskan coasts from banks island to the bering strait and on the siberian coast from the bering strait to the new siberian islands in the bering sea polynyas form in norton sound south and west of st lawrence island and in the gulf of anadyr two regions that account for almost 50 of the total dense water production are the siberian coastal polynyas in the adjacent regions of the gulf of anadyr and anadyr strait and the alaskan coastal polynyas which occur along the coast from cape lisburne to point barrow for all of the western arctic coastal regions examined the mean annual total brine flux is 0 5 0 2 sv combination of this flux with the contribution from the barents kara and laptev seas which is recalculated from data in the earlier study shows that over the entire arctic coastal polynyas generate about 0 7 1 2 sv of dense water this compares well with the theoretical estimates of 1 1 5 sv because an unknown fraction of the barents kara and laptev brine flux must go to the eurasian basin deep water the coastal polynyas alone cannot renew the halocline layer other potential brine generation mechanisms include overall freezing on the shelves and the response of the ice to infrequent violent storms for example during february 1982 an intense storm generated a large region of low ice concentration in the eastern chukchi sea over barrow canyon the refreezing of the region was followed by the flow of a dense plume down barrow canyon although the ocean dynamical response to this refreezing needs to be established the possible response of a barrow canyon flow to this refreezing event suggests that the overall refreezing in response to infrequent violent storms may be a potential source of the additional brine needed to maintain the arctic ocean halocline

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

the under ice thickness distribution of the arctic basin as recorded in 1958 and 1970 the uss nautilus ssn 571 was the first vessel to cross the arctic basin via the north pole in early august 1958 during this expedition almost continuous acoustic under ice thickness profiles were recorded in august 1970 uss queenfish ssn 651 retraced nautilus route yielding the only duplicate transect of under ice topography across the arctic basin comparisons of the statistical analysis of the under ice draft measurements obtained through use of wide beam and narrow beam and narrow beam only acoustic profilers by nautilus and queenfish respectively along the coincidental route are presented geographic areas found to have distinct under ice characteristics and ice composition are identified the under ice statistics of both cruises are considered in relation to representative field observations and modeling results of other researchers principal findings are 1 nautilus recorded generally more severe ice conditions within the canada basin than did queenfish 12 years later overall mean drafts were 3 08 and 2 39 m respectively 2 the under ice topography becomes progressively more severe when proceeding from the canadian to the eurasian side of the arctic basin 3 the canada basin may contain the most moderate under ice topography and the greatest number of open water and refrozen polynyas and leads within the central arctic basin 4 the makarov and amundsen basins and the arctic mid ocean ridge may contain some of the most severe under ice topography within the arctic basin and 5 the present study indicates an overall arctic basin mean of 3 4 open water new ice less than 30 cm in summer

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

on the response of the equilibrium thickness distribution of sea ice to ice export mechanical deformation and thermal forcing with application to the arctic ocean the areal ice distribution in a pack ice field is described using a number of classes characterized by different ice thicknesses each class may have a snow layer on top and evolves according to the thermodynamic forcing same for all classes and is followed through time mechanical forcing leads to ice export and deformation the ice export is quantified by the parameter e describing the areal export of ice per unit of time the deformation is quantified by the parameter r describing the area of open water generated by deformation per unit of time the fraction of summer heat absorbed in leads and utilized for ice melting is quantified by the parameter gop the model is applied to the arctic ocean it is run to a steady state for different combinations of e r and gop and the response of the thickness distribution to these parameters may thereby be quantified it is shown that the ice generally gets thinner for increasing export the response to deformation is dependent on gop such that the mean thickness increases with r for small gop i e no effect or just a small melting effect of the heat absorbed in leads but decreases with r for large gop the overall effect of gop is that the mean thickness decreases with increasing gop for gop close to 1 and moderate values of e and r the ice sheet melts completely in the summer

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

contour mapping of arctic basin ice draft and roughness parameters a data base of ice draft and roughness parameters has been constructed for selected portions of the arctic ocean based upon analysis of under ice draft distribution data acquired by inverted echo sounder systems on submarines from the voyages of 12 submarines which traversed the alaskan canadian and central arctic regions of the arctic ocean during the summer and winter seasons a series of mean ice draft and deep draft keel statistics was calculated for 50 km segments along each submarine track contour maps of the mean ice draft its standard deviation the mean keel draft and the spatial frequency of ice keels were constructed they show that the greatest ice drafts the roughest ice and the greatest number of deep draft keels are found off the north coasts of the canadian archipelago and greenland due to ice convergence on these land barriers

the relation between ice deformation oceanic heat flux and the ice thickness distribution in the arctic ocean a one dimensional ocean model coupled with an ice model where the ice cover is partitioned into a number of thickness categories is used to investigate how the ice thickness distribution in the arctic ocean depends on the two deformation processes export net divergence and ridging the model standard case generates vertical profiles of temperature salinity and an ice thickness distribution similar to the observations the standard case oceanic heat flux agrees well with recently deduced heat fluxes from arctic ice dynamics joint experiment aidjex data the oceanic heat flux is about 18 w m 2 during summer and about 1 w m 2 during winter it is found that the mean ice thickness decreases with increasing ice export in close agreement with other investigations however the dependence of the mean ice thickness on ridging activity is found to be very weak the change of the mean ice thickness is only 12 cm when increasing the ridging activity in the model from 0 to 0 4 yr 1 where the larger value represents an estimate of the actual ridging activity in the arctic ocean in a sensitivity study different types of ridging and albedo parameterizations are tested and this weak dependence is found to be very robust other properties of the model ice cover are sensitive to ridging though the equilibrium thickness of undeformed ice decreases with both export and ridging activity the annual maximum open water fraction and the amplitude of the seasonal thickness variation increases with export and ridging

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

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

large scale patterns and variability of snowmelt and parameterized surface albedo in the arctic basin abstract visible band satellite imagery is used to manually map surface brightness changes over sea ice throughout the arctic basin from may to mid august over a 10 yr period these brightness changes are primarily due to snowmelt atop the ice cover using image processor techniques parameterized albedos are estimated for each brightness class snowmelt begins in may in the marginal seas progressing northward with time finally commencing near the pole in late june large year to year differences are found in the timing of melt exceeding one month in some regions parameterized albedo for most regions of the pack ice exceed 0 70 during may declines rapidly during june and reaches a seasonal low of between 0 40 and 0 50 by late july for august regional albedos which also include areas of open water beyond the southern pack ice limit are up to 0 16 lower than the corresponding values for pack ice areas only

on the ridging of intact lead ice the sea ice pressure ridging process is modeled using a two dimensional particle simulation technique in this model blocks are broken from an intact sheet of relatively thin lead ice driven against a thick multiyear floe at a constant speed the blocks of ice rubble accumulate to form the ridge sail and keel the energy consumed in ridge growth including dissipation is explicitly calculated a series of numerical experiments are performed to establish the dependence of the energetics on the thickness of the ice sheet and the friction between blocks the results suggest that the total energy required to create a pressure ridge is an order of magnitude greater than the potential energy in the ridge structure a typical sea ice cover in the polar regions contains a variety of ice thicknesses that evolve in response to both dynamic and thermodynamic forcing the variable thickness of the ice cover is created by deformation which simultaneously causes formation of thick ice through ridge building and thin ice through lead creation since the energy expended in deformation is largely determined by the ridging process an understanding of the energetics of pressure ridging is critical in the determination of ice strength on a geophysical scale

dependence of the arctic ocean ice thickness distribution on the poleward energy flux in the atmosphere 1 the sensitivity of the arctic ocean ice cover on the atmospheric poleward energy flux d is studied using a coupled column model of the ocean ice and atmosphere in the model the ice cover is described by a thickness distribution and the atmosphere is a simple two stream gray body in radiative equilibrium it is shown that the thickness distribution in combination with the albedo function gives a strong nonlinear response to positive perturbations of d the response on d is sensitive to the albedo parameterization and the shape of the thickness distribution controlled by ridging and divergence an increase of about 9 w m 2 from a standard value of d 103 w m 2 has a dramatic effect reducing the ice thickness by more than 2 m and generates a large open water fraction during summer the reduction of ice thickness is characterized by a clear transition between two regimes going from a regime where first year ice survives the next summer melt period to a seasonal ice regime where the first year ice melts completely it is shown that the existence of seasonal ice regime is dependent on the surface mixed layer thickness the model enters a completely ice free state if the thickness of the mixed layer is increased above a threshold value the adjustment timescale for the ice cover is 6 years for small positive and negative perturbations in d for larger positive perturbations of about 10 w m 2 the adjustment timescale is up to 20 years

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

thermohaline circulation in the arctic mediterranean seas the renewal of the deep north atlantic by the various overflows of the greenland scotland ridges is only one manifestation of the convective and mixing processes which occur in the various basins and shelf areas to the north the arctic ocean and the greenland iceland and norwegian seas collectively called the arctic mediterranean the traditional site of deep ventilation for these basins is the greenland sea but a growing body of evidence also points to the arctic ocean as a major source of deep water this deep water is relatively warm and saline and it appears to be a mixture of dense brine enriched shelf water with intermediate strata in the arctic ocean the deep water exits the arctic ocean along the greenland slope to mix with the greenland sea deep water conversely very cold low salinity deep water from the greenland sea enters the arctic ocean west of spitsbergen within the arctic ocean the lomonosov ridge excludes the greenland sea deep water from the canadian basin leaving the latter warm saline and rich in silica in general the entire deep water sphere of the arctic mediterranean is constrained by the greenland scotland ridges to circulate internally therefore it is certain of the intermediate waters formed in the greenland and iceland seas which ventilate the north atlantic these waters have a very short residence time in their formation areas and are therefore able to rapidly transmit surface induced signals into the deep north atlantic

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

on the arctic ocean ice thickness response to changes in the external forcing submarine and satellite observations show that the arctic ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades here the response of the arctic ocean ice cover to changes in the poleward atmospheric energy transport fwall is investigated using coupled atmosphere ice ocean column models two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness ice volume export feedback and the positive surface albedo feedback the steady state ice thickness as a function of fwall is determined for various model setups and defines what we call ice thickness response curves when a variable surface albedo and snow precipitation is included a complex response curve appears with two distinct regimes a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime the two regimes are separated by a steep transition associated with surface albedo feedback the associated hysteresis is however small indicating that the arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback the results are discussed in the context of the recent reduction of the arctic sea ice cover a new mechanism related to regional and temporal variations of the ice divergence within the arctic ocean is presented as an explanation for the observed regional variation of the ice thickness reduction our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i e large areas of the arctic ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness corresponding to the above mentioned transition and into a state dominated by seasonal ice

arctic ocean perennial sea ice breakdown during the early holocene insolation maximum arctic ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle holocene similar to 6000 10 000 years bp compared to present day conditions this sea ic

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

large scale drift of arctic sea ice retrieved from passive microwave satellite data a method of determining the large scale sea ice drift using 85 5 ghz special sensor microwave imager data are presented a cross correlation method is applied to sequential images of gridded data covering the entire arctic individual correlation results are validated with ice velocities derived from buoy data the satellite derived mean drift values and the variabilities of the ice drift correspond closely with the buoy data similarly time series of buoy data and associated satellite data are in good agreement even over large time periods an example of a satellite retrieved 3 day mean drift field demonstrates the potential of the method for providing large scale ice circulation patterns mean drift fields of the winter periods 1987 1988 and 1992 1993 indicate a considerable interannual variability of the sea ice drift pattern in the arctic ocean the arctic region is divided into seven larger areas and the area flux between these regions has been derived the kara sea and the laptev sea show the largest area ice export with 0 02 and 0 015 km 2 s 1 respectively the central arctic export through fram strait amounts to 0 12 sv during the winter of 1992 1993 with a maximum of 0 15 sv in january

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

sea ice deformation state from synthetic aperture radar imagery part i comparison of c and l band and different polarization in this paper we present a quantitative comparison of l and c band airborne synthetic aperture radar imagery acquired at like and cross polarizations over deformed sea ice under winter conditions the parameters characterizing the deformation state of the ice are determined at both radar bands and at different polarizations the separation of deformed and level ice is based on a target detection technique the threshold is set such that image pixels with intensities equal to or larger than the highest 2 of the level ice intensity distribution are classified as deformed ice independent of the radar configuration and ice conditions optical imagery of sufficient quality for comparison is available only in a very few cases to characterize the deformation state the areal fraction of deformation features and the average distance between these features are evaluated the values obtained for both parameters are very sensitive to the radar frequency aeral fractions are larger and average distances are smaller at l band than at c band because of the much higher intensity contrast between the deformed and level ice at l band the differences between polarizations at one radar band are smaller but not always negligible in comparison to optical images it was observed that deformed ice areas can be distinguished from level ice over their whole length and extension at l band whereas at c band often only prominent parts are visible

sea ice thickness variability in storfjorden svalbard results from electromagnetic induction surveys of sea ice thickness in storfjorden svalbard reveal large interannual ice thickness variations in a region which is typically characterized by a reoccurring polynya the surveys were performed in march 2003 may 2006 and march 2007 with helicopter and ship based sensors the thickness distributions are influenced by sea ice and atmospheric boundary conditions 2 months prior to the surveys which are assessed with synthetic aperture radar sar images regional quikscat backscatter maps and wind information from the european centre for medium range weather forecasts ecmwf reanalysis dataset locally formed thin ice from the storfjorden polynya was frequently observed in 2003 and 2007 mean thickness 0 55 and 0 37 m respectively because these years were characterized by prevailing northeasterly winds in contrast the entire fjord was covered with thick external sea ice in 2006 mean thickness 2 21 m when ice from the barents sea was driven into the fjord by predominantly southerly winds the modal thickness of this external ice in 2006 increased from 1 2 m in the northern fjord to 2 4 m in the southern fjord indicating stronger deformation in the southern part this dynamically thickened ice was even thicker than multi year ice advected from the central arctic ocean in 2003 mean thickness 1 83 m the thermodynamic ice thickness of fast ice as boundary condition is investigated with a one dimensional sea ice growth model 1dice forced with meteorological data from the weather station at the island of hopen southeast of storfjorden the model results are in good agreement with the modal thicknesses of fast ice measurements in all years

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

the underside topography of sea ice over the continental shelf of the beaufort sea in the winter of 1990 a spatial section of draft across 348 km of sea ice was obtained by moored subsea sonars in the beaufort sea in the winter of 1990 the ice field was a mixture of first year and multiyear types with mean draft of the order of 3 m level ice of 0 to 4 m draft occupied approximately 50 of the section within this fraction draft values of 0 and 1 5 m were most common while at 2 to 3 m draft a noticeable contribution was made by rafted first year ice deformed ice was observed to a maximum draft of nearly 27 m keels within the section were identified using both the rayleigh criterion and a new criterion based on level ice both criteria defined exponentially distributed populations but with different e folding scales the observations revealed an ice cover whose statistical properties are consistent with other data from the periphery of the polar pack in the beaufort sea but which are much more severe than those of ice found in the seasonal ice zone only a few tens of kilometres to the south the cross sectional shape of individual keels was highly variable but on average the distribution of ice within a keel was an exponential function of draft the topography of both first year and old ice subsections of the data was fractal in character with a dimension of about 1 5 for both ice types the average mass of sea ice per unit area was estimated as 2540 2970 kg m 2 with 68 73 of this mass contributed by deformed ice the large uncertainties in these estimates arise because the porosity of keels and the burden of snow or melt water upon the ice are not known

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

the melt freeze cycle of the arctic ocean ice cover and its dependence on ocean stratification 1 a time dependent 1 d coupled ice ocean model is used to quantify the impact of ocean stratification on the arctic ice cover the model results show that the ice growth during winter equals the ice melt in summer for areas with a well developed cold halocline layer chl provided that the initial ice thickness is around 3 m while thinner initial ice thickness results in net growth areas with weak salt stratification can have a negative annual thickness change irrespective of the initial ice thickness and are thus dependent on ice import in order to remain ice covered the model results also show that ocean stratification is mostly important for ice thickness development during the growing season areas with weak stratification have an ocean heat flux up to 8 w m 2 reaching the ice during the growing season while areas with a chl have an average of about 0 7 w m 2 in the extreme area north of svalbard the ocean heat fluxes are typically around 25 w m 2 but can be up to 400 w m 2 during the initial adjustment when the warm atlantic water has direct contact with the ice a general outcome of the study is that depending on ocean stratification the ice cover of arctic ocean can be divided into one part with net ice growth the major part and another part with net ice melt mainly in the nansen basin

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

a one dimensional time dependent model for the vertical stratification of the upper arctic ocean abstract a one dimensional time dependent model of the upper arctic ocean is presented it describes the circulation above a dynamically passive reservoir of atlantic water the model is driven by freshwater runoff from land ice production and export bering strait inflow and wind the mixed layer thickness is controlled by the shortest among the following three length scales the depth to the upper pycnocline the ekman length and the monin obukov length the outflow is assumed to occur as geostrophically controlled coastal currents four integral quantities are defined in order to make objective comparisons between model generated and observed salt and temperature profiles the model generates a seasonally varying mixed layer in qualitative agreement with measurements a halocline with water near the freezing temperature is also generated this structure is sustained by the water coming from the bering strait by comparing the model result with the observed freshwater content in the basin likely value

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

grease ice in basin scale sea ice ocean models the first stage of sea ice formation is often grease ice a mixture of sea water and frazil ice crystals over time grease ice typically congeals first to pancake ice floes and then to a solid sea ice cover grease ice is commonly not explicitly simulated in basin scale sea ice ocean models though it affects oceanic heat loss and ice growth and is expected to play a greater role in a more seasonally ice covered arctic ocean we present an approach to simulate the grease ice layer with as basic properties the surface being at the freezing point a frazil ice volume fraction of 25 and a negligible change in the surface heat flux compared to open water the latter governs grease ice production and a gradual transition to solid sea ice follows with 50 of the grease ice solidifying within 24 hours the new parameterization delays lead closing by solid ice formation enhances oceanic heat loss in fall and winter and produces a grease ice layer that is variable in space and time results indicate a 10 30 increase in mean winter arctic ocean heat loss compared to a standard simulation with instant lead closing leading to significantly enhanced ice growth

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

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