bed microtopography and entrainment thresholds in gravel bed rivers although increasingly acknowledged as an important factor in bedload transport few data on the effects of bed conditions on entrainment in gravel bed rivers exist this reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels using a sensor which allows initial movement of labeled particles to be detected during flood flow the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed open plane bed positions the threshold velocity of clasts from cluster bedforms the most prevalent type of bed microtopography is found to be above that of more exposed particles of like size and shape measurements indicate that the initial movement of 70 of particles in gravel bed rivers will be directly influenced by the presence of neighboring grains bed microtopography plays an important role in delaying incipient motion the influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events

probabilistic exner sediment continuity equation for mixtures with no active layer the exner equation of sediment continuity is the foundation of river morphodynamics generalization of this equation to mixtures of grain sizes has required the introduction of an active layer i e a buffer layer between the sediment moving in the water column and the immobile substrate below the active layer is defined to be a well mixed layer with no vertical structure that encompasses those grains available to exchange directly with the moving sediment the sediment in the substrate below exchanges with the active layer only as the bed aggrades or degrades the active layer concept is a useful one that has served the research community well for 3 decades however the division of the erodible bed into a discrete active layer and substrate must represent only an approximation of a more general formulation that contains no active layer and in which parameters pertaining to the entrainment from and deposition to the bed vary continuously with depth below the sediment water interface here the probability density function of bed elevation is used to derive a general exner equation of sediment continuity with no discrete layers the formulation is applicable to both sediment mixtures and tracers in uniform sediment although the treatment requires more information than that of the active layer approach it offers the prospect of a better understanding of how streams create a stratigraphic record of their activities through deposition

annual climate variability in the holocene interpreting the message of ancient trees abstract over vast areas of the world s landmasses where climate beats out a strong seasonal rhythm tree growth keeps unerring time in their rings trees record many climate melodies played in different places and different eras recent years have seen a consolidation and expansion of tree ring sample collections across the traditional research areas of north america and europe and the start of major developments in many new areas of eurasia south america and australasia from such collections are produced networks of precisely dated chronologies records of various aspects of tree growth registered continuously year by year across many centuries their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space with their extensive coverage high temporal resolution and rigid dating control dendroclimatic reconstructions contribute significantly to our knowledge of late holocene climates most importantly on timescales ranging from 1 to 100 years in special areas of the world where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved work building chronologies covering many millennia continues apace very recently trees have provided important new information about major modes of general circulation dynamics linked to the el nino southern oscillation and the north atlantic oscillation and about the effect of large volcanic eruptions as for assessing the significance of 20th century global warming the evidence from dendroclimatology in general supports the notion that the last 100 years have been unusually warm at least within a context of the last two millennia however this evidence should not be considered equivocal the activities of humans may well be impacting on the natural growth of trees in different ways making the task of isolating a clear climate message subtly difficult

dynamics of bedload transport in turkey brook a coarse grained alluvial channel automatic and continuously recording samplers are deployed in a hertfordshire gravel bed stream to show that bedload transport is related to stream power the pattern is similar to that already established for north american channels but because the record is so detailed it is possible to identify the cause of the considerable scatter that is normal in such relationships a major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables it is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow moving traction carpet and that they may be likened to kinematic waves the record also reveals that the threshold of sediment transport always presumed hithero to be associated with incipient motion is related to the cessation of bedload transport in a river flood indeed the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion because of this there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold these new data show that the general inverse relationship between bedload discharge and water depth grain size ratio proposed by bagnold 1977 1980 is not universal transport efficiency for this gravel bed stream is typically 0 05 per cent of available stream power which compares with 1 6 per cent for a river moving both gravel and sand and 5 per cent for another channel where bedload is composed predominantly of sand sized particles it is argued that coarse and fine grained alluvial channels may need to be considered separately by allowing for differences in traction threshold at the beginning and end of bedload events and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power

late holocene snow avalanche activity in southern norway interpreting lichen size frequency distributions using an alternative to simulation modelling recent work by mccarroll 1993 1994 suggests that a new approach to lichenometry can be used to date diachronous surfaces size frequency distributions of largest lichens obtained from the diachronous surfaces of avalanche boulder ramparts comprise two elements an equilibrium population growing on boulders deposited throughout the holocene and overlapping populations growing on groups of boulders deposited by events during the last 300 years in this study almost 10000 lichens were measured on two groups of closely similar ramparts the number of very large lichens is used to predict the form of the equilibrium distribution which is then removed leaving a corrected distribution which reflects the age frequency of surface boulders deposited during the little ice age the results confirm that maximum avalanche activity occurred during the 19th century but a minor peak at one site suggests that avalanche activity was enhanced during the mid 18th century

the non layering of gravel streambeds under ephemeral flood regimes abstract the two layer format common to perennial streambeds in which a relatively coarse armour overlies a finer subarmour develops as a function of both the ingress and subsequent near surface winnowing of interstitial material and the selective non entrainment or slower transport velocity of coarse clasts ephemeral streams appear to lack such vertical layering or are characterized by weak layer development some of this may be due to the degree of mixing associated with the scour and fill process however continuous monitoring of bedload discharge in the nahal yatir in the northern negev desert reveals that sediment transport rates are extremely high so that the chance of armour layer development through selective non entrainment is much reduced indeed a comparison of the bedload and bed material size distributions confirms a high degree of similarity and hints at equal mobility regardless of clast size the monitoring programme also indicates that the bed becomes highly mobile at comparatively modest fluid shear so that practically all floods are associated with high transport rates consequently the winnowing that might be brought about by low transport rate events does not occur even within a single event winnowing is precluded by the rapid nature of flow recession that is so characteristic of flash floods the high degree of bed material mobility is attributable in part to the lack of strength that would otherwise be a corollary of armour development however it also highlights the divergent nature of the feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems

clarification of capillary rise in dry sand abstract the extent of capillary rise into initially dry granular soil that is exposed at the surface and subjected to wetting at the base is not easy to predict simple rising head column experiments were performed using pure water in medium grained poorly graded and unconsolidated sand to shed light on the extent to which water rises in initially dry soil and its distribution rising head experiments have been conducted in initially dry sand to explore these phenomena in columns instrumented with time domain reflectometry tdr probes the data from which are compared with water contents ultimately found gravimetrically from sample slices water was found to reach much higher than theoretical estimates based on an equivalent capillary tube suggesting that capillary rise is responsible for fluid transport well into the zone of unsaturated soil above the saturated capillary zone this remained the case even when the sand was subjected to strong rates of evaporation at its surface this study challenges the conventional understanding of capillary rise in unsaturated soil and has particular relevance in desert soils subject to rapid evaporation

timing of surficial process changes down a mojave desert piedmont abstract we measured 10 be and 26 al in 29 sediment samples to infer the history and millennial scale rates of change down a low gradient piedmont a common but enigmatic landform that dominates the mojave desert nuclide data suggest that a large volume of sediment was deposited on the proximal east range road piedmont in fort irwin california 75 500 yr ago since then this material has been stable or eroding slowly in contrast on the distal piedmont 3 5 km from the upland source basins soil stratigraphy suggests that there have been alternating periods of surface stability erosion and deposition over the last 70 000 yr nuclide data from samples amalgamated along cross piedmont transects suggest that long term average down gradient sediment speeds range from 9 cm yr 1 near the uplands to 22 cm yr 1 6 km down piedmont these speeds are similar to 10 be estimated sediment speeds down three other piedmonts in the mojave desert suggesting that piedmont surface morphologies dominated by shallow migrating channels have similar sediment transport rates the timing of surface process change down the east range road piedmont is determined by a combination of sediment available in the source basins sediment transport rates and the size of the piedmont

scour fill and burial depth of coarse material in gravel bed streams scour fill and burial depths of coarse particles were examined in two ephemeral streams using magnetically tagged particles and scour chains the burial depth varied within a bedform and from one bedform to another the maximum burial depth was recorded in bars but differences in the mean burial depth in the pools bar and thalweg were insignificant mean burial depth was three to six times the median size of material on bar surfaces and ten to thirty times that in the pools and thalweg more than eighty per cent of the tagged particles which were located on or close to the bed surface moved in each event although previously buried particles were moved by flood events others which were initially exposed remained stationary on the surface or were buried in situ the data indicate that the mean burial depth was intermediate between the mean depths of scour and fill deeply buried particles probably reflect the development of scour holes during flood flows aggradation and degradation changed the recorded burial depth relative to the local bed surface of stationary tracer particles these observations indicate that scour and fill processes during a flow event are sporadic and change with water discharge

evolution of soil wetting patterns preceding a hydrologically induced landslide inferred from electrical resistivity survey and point measurements of volumetric water content and pore water pressure 1 the hydrological state of a hillslope prior to a sprinkling induced shallow landslide was monitored using electrical resistivity tomography ert along a 47 m long transect supplemented by local time domain reflectometry tdr and tensiometer measurements the spatial and temporal evolution of wetting patterns in the soil material indicated attainment of a stationary fully saturated profile in a slope region underlain by shallow sandstone bedrock the significant decrease in spatially averaged standard deviation of water saturation has not been observed during an earlier failed attempt to trigger a landslide by intense sprinkling while for the stable experiment no landslide was triggered water saturation and soil moisture variability were still increasing with time the unstable experiment reached a time invariant state of high pore water pressures and saturations until it finally failed the results indicate that when large and interconnected regions of hillslope are saturated as confirmed by high volumetric water content and low standard deviation of water saturation additional water cannot be redistributed to empty drier regions and may eventually enhance local pore water pressure and seepage force initiating large shear deformation and failure accordingly a transition to such a critical steady state of high average water saturation associated with low and constant spatial standard deviation may serve as additional hydro geophysical indicator for the imminence of a landslide release

five hundred years of gridded high resolution precipitation reconstructions over europe and the connection to large scale circulation we present seasonal precipitation reconstructions for european land areas 30 w to 40 e 30 71 n given on a 0 5 0 5 resolved grid covering the period 1500 1900 together with gridded reanalysis from 1901 to 2000 mitchell and jones 2005 principal component regression techniques were applied to develop this dataset a large variety of long instrumental precipitation series precipitation indices based on documentary evidence and natural proxies tree ring chronologies ice cores corals and a speleothem that are sensitive to precipitation signals were used as predictors transfer functions were derived over the 1901 1983 calibration period and applied to 1500 1900 in order to reconstruct the large scale precipitation fields over europe the performance quality estimation based on unresolved variance within the calibration period of the reconstructions varies over centuries seasons and space highest reconstructive skill was found for winter over central europe and the iberian peninsula precipitation variability over the last half millennium reveals both large interannual and decadal fluctuations applying running correlations we found major non stationarities in the relation between large scale circulation and regional precipitation for several periods during the last 500 years we identified key atmospheric modes for southern spain northern morocco and central europe as representations of two precipitation regimes using scaled composite analysis we show that precipitation extremes over central europe and southern spain are linked to distinct pressure patterns due to its high spatial and temporal resolution this dataset allows detailed studies of regional precipitation variability for all seasons impact studies on different time and space scales comparisons with high resolution climate models as well as analysis of connections with regional temperature reconstructions

late quaternary history of the chemehuevi mountain piedmont mojave desert deciphered using 10be and 26al we used the cosmogenic nuclides 10 be and 26 al as cosmic ray dosim eters to track sediment movement across the incised chemehuevi mountain piedmont surface the piedmont extends 12 km from highland source basins to an ephemeral wash at the toe of the piedmont nuclide activity in sediment from steep source basins and from the bedrock pediment allows us to estimate rates of sediment generation and thus the rate at which sediment is supplied to the piedmont from weathering rock analysis of amalgamated sediment samples collected along 4 km long transects spaced at 1 km intervals from the mountain front provide context for understanding nuclide activity in sediment samples collected from two soil pits and demonstrate that sediment steadily moves down the channel network samples from the two soil pits allow for estimates of sediment deposition rates and thus further illuminate piedmont history model results suggest that the chemehuevi mountains are eroding about 40 mm ky 1 whereas the mountain proximal bedrock pediment and the interpiedmont saw tooth range are eroding more slowly 10 to 21 mm ky 1 and 9 5 mm ky 1 respectively cosmogenic nuclide profiles in two soil pits suggest a complex history of deposition at rates between 19 to 39 mm ky 1 and stability over the past 60 ky a change in process from sediment deposition to sediment transport occurred distally on the piedmont approximately 8 ky ago while the mid piedmont piedmont surface has been stable since the mid holocene the incised chemehuevi mountain piedmont is changing at rates similar to planar and uniformly active piedmonts such as those adjacent to the iron and granite mountains all three piedmonts have average sediment grain velocities of decimeters per year long term source basin erosion rates between 35 to 40 mm ky depositional hiatuses around the pleistocene holocene climatic transition from moister to drier conditions and pleistocene deposition rates of 18 to 39 mm ky such similarity in piedmont behavior despite clear differences in morphology and appearance might suggest large scale geologic and climatic controls on the generation transport and deposition of sediment in the mojave desert

the hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field this has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows lift and drag forces are determined on clustered hemispherical particles of varying size in the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion on the stoss side of the cluster drag is uniform regardless of the separation of the component particles but lift is shown to increase when particle separation is small so affecting stability this mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape on average 46 of clustered particles are entrained by flood flow compared to 87 of particles in open plane beds the influence of clusters is a major determinant of sedimentary sorting

structures and textures in triassic braided stream conglomerates bunter pebble beds in the sherwood sandstone group north staffordshire england in the northern parts of the needwood and stafford eccleshall basins england the pebble beds of the sherwood sandstone group contain thick successions of texturally mature fluvial pebble cobble conglomerates which are organized into either horizontal or cross stratified sets the horizontally lying sets generally coarser grained and more poorly sorted than the cross bedded sets are usually disorganized and either matrix or clast supported although thin lenses of well sorted occasionally openwork units interpreted as falling stage phenomena are often present the cross stratified conglomerates have foresets exhibiting remarkable textural organization with a coarse bimodal sometimes matrix supported part grading upwards or being abruptly overlain by a finer well sorted occasionally openwork part and finally capped by sandstone these rhythmic textural changes are attributed partly to an avalanche process at high stage and partly to falling stage conditions the most common types of vertical association are thick successions of horizontally bedded conglomerates up to 20 m and sequences of an upwards coarsening nature 2 12 m in which cross stratified sets are overlain by flat lying sets the environment of deposition of the gravels is interpreted as one in which water depths at high stage were greater than depths in most modern braided stream plains proglacial or alluvial fan but shallower than depths associated with the pleistocene catastrophic floods from which texturally mature giant gravel bars have been recorded recent braided streams with relatively confined channels and considerable bar channel relief are better analogues in particular medial or mid channel bars with a two tier structure subaqueous and partly emergent portions may explain the upward coarsening sequences in which horizontally lying conglomerates overlie cross stratified conglomerates the thicker sequences of horizontally stratified conglomerates represent proximal longitudinal bar deposits sheets of pebbly sandstone and argillaceous sandstone lying between the conglomerates and commonly occurring towards the top of the succession largely represent deposition from sandwaves and dunes finer interbedded argillaceous sandstones siltstones and mudstones are interpreted as overbank and waning flood deposits basin forming tectonism of increasing intensity probably caused the initial coarsening upwards of the lower part of the succession whilst more stable tectonic conditions and decreasing relief on the margins of the basins and in the areas of provenance in the midlands and the hercynides account for the upwards fining of the upper part of the succession

experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers 1 desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below while gravel bed streams in humid and snowmelt areas typically have well armored surfaces the degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size a set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0 5 to 2 4 and with an average value of 1 2 the armor ratio of snowmelt fed gravel bed streams in the same set ranges from 2 to 7 with an average value of 3 4 the reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions the experiments have two limiting cases a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods all constant hydrograph experiments developed a well armored structured surface while short asymmetrical hydrographs did not result in substantial vertical sorting all symmetrical hydrographs show some degree of sorting and the sorting tended to become more pronounced with longer duration sediment supply appears to be a first order control on bed surface armoring while the shape of the hydrograph plays a secondary role

spatio temporal reconstruction of snow avalanche activity using tree rings pierres jean jeanne avalanche talus massif de l oisans france snow avalanches are a major threat in many parts of the alps where they periodically damage infrastructure disrupt transportation corridors or even cause loss of life nonetheless the spatial behavior of past avalanche activity and the analysis of areas affected during particular events remain often imprecise it was therefore the purpose of this study to reconstruct spatio temporal patterns of past avalanche activity on a forested avalanche talus in the french alps pierres jean jeanne talus massif de l oisans france a total of 232 european larches larix decidua mill with clear signs of snow wasting events was analyzed and growth disturbances gd related to avalanche activity was assessed such as tangential rows of traumatic resin ducts the onset of compression wood or abrupt growth suppression and release in total 901 gd were identified in the tree ring samples indicating that 20 high magnitude avalanches occurred between ad 1919 and 1994 the mean return period of snow avalanches was 4 years with a 26 probability that an avalanche occurs in any particular year interpolated maps allowed for explicit spatial estimates of return periods throughout the talus showing a rapid increase of return frequency from 2 5 to 50 years with increasing distance from the talus apex the distribution of avalanche years seems to be quite homogeneous in time with a gap between 1951 and 1959 snowfall from a nearby meteorological station saint christophe en oisans 10 km from the study site indicated that the five most recent high magnitude events on record occurred due to above average snowfall anomalies in december and january associated with abnormally low air temperatures findings suggest that a strong snow metamorphism under high temperature gradients in january could explain the occurrence of high magnitude snow avalanches 2010 elsevier b v all rights reserved

prediction of fingering in porous media immiscible displacement involving two fluids in a porous medium can be unstable and fingered under certain conditions in this paper the original linear instability criterion of chuoke et al 1959 is generalized considering wettability of two immiscible fluids to the porous medium this is then used to predict 24 specific flow and porous medium conditions for the onset of wetting front instability in the subsurface wetting front instability is shown to be a function of the driving fluid wettability to the medium differences in density and viscosity of the fluids the magnitude of the interfacial tension and the direction of flow with respect to gravity scenarios of water and nonaqueous phase liquid infiltration into the vadose zone are examined to predict preferential flow and contamination of groundwater the mechanisms of finger formation propagation and persistence in the vadose zone are reviewed and the existing equations for calculating the size the number and velocity of fingers are simplified for field applications the analyses indicate that fingers initiate and propagate according to spatial and temporal distribution of the dynamic breakthrough water or air entry pressures in the porous medium the predicted finger size and velocity are in close agreement with the experimental results

seven centuries of avalanche activity at echalp queyras massif southern french alps as inferred from tree rings the purpose of this study was to reconstruct spatiotemporal patterns of avalanche events in a forested avalanche path of the queyras massif echalp avalanche path southeast french alps analysis of past events was based on tree ring series from 163 heavily affected multicentennial larch trees larix decidua mill growing near or next to the avalanche path a total of 514 growth disturbances such as tangential rows of traumatic resin ducts the onset of compression wood as well as abrupt growth suppression or release were identified in the samples indicating 38 destructive snow avalanches between 1338 and 2010 the mean return period of snow avalanches was 22 years with a 4 probability that an avalanche occurs in a particular year on a temporal plan three maxima in snow avalanche frequency were reconstructed at the beginning of the 16th and 19th centuries and around 1850 correlating with below average winter temperatures and glacier advances analysis of the spatial distribution of disturbed trees contributed to the determination of four preferential patterns of avalanche events the comparison of dendrogeomorphic data with historical records demonstrate that at least 18 events six of which were undocumented reached the hamlet of echalp during the last seven centuries but no significant temporal trend was detected concerning the frequency of these extreme events

complex siliceous concretions in the jurassic morrison formation church rock new mexico usa implications of inorganic factors in ichnological interpretations abstract complex concretions in the jurassic morrison formation in new mexico usa were studied to elucidate their enigmatic origin the concretions mostly in the 0 1 2 m height range are characterized by a microcrystalline quartz matrix and many tiny 4 8 mm diameter cylindrical protrusions a common architecture revealed by weathering is a vertical cylindrical concentric casing around a homogenous core concretion morphologies include irregular bodies and larger horizontal cylinders with surficial protrusions sometimes cores and or protrusions are absent micromorphological data and open grain packing indicate concretions formed in unconsolidated sediments light and scanning electron microscopy shows gradational contacts between the siliceous concretions and host sandstones point counts of concretions and host rock show more cement and intergranular volume in the concretions but fewer grains and primary pores most concretions are cemented by microporous microcrystalline quartz chlorite kaolinite and illite smectite x ray fluorescence and instrumental neutron activation analyses indicate that silica was added to form the concretions previous models for the concretions include burrows termite nests and rhizocretion however these interpretations are challenged by geochemical macromorphological and micromorphological data presented in this study infiltration possibly due to hydrothermal activity may have added silica but explanations for several morphological details remain elusive these data imply that inorganic diagenetic factors can be important in ichnological interpretations

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

differentiating past events on a cone influenced by debris flow and snow avalanche activity a dendrogeomorphological approach dendrogeomorphology was used to investigate past events on a cone affected by both debris flows and snow avalanches we report on results of 520 cores from 251 injured larix decidua mill and picea abies l karst trees sampled on the birchbach cone swiss alps detailed analysis of tree ring sequences allowed dating of 561 growth disturbances in individual trees for a 252 yr period extending from 1750 to 2002 which could be attributed to 30 different event years we then localized the position of rows of traumatic resin ducts trds within the tree ring so as to assess the intra seasonal position of damage in agreement with data on the local growth period trds located at the beginning of the new growth ring were considered the result of avalanche impacts that occurred during the dormant season or in earliest earlywood between late october and early may in contrast trd found in late earlywood or within latewood were considered the result of periglacial debris flow activity as these layers of the tree ring are locally formed between july and early october for nine out of the 30 reconstructed event years the intra seasonal timing of trds indicated that reactions must be the result of past snow avalanche activity in 19 other event years trds showed that damage has been caused between july and early october and thus through debris flows in the birchbach torrent finally the spatial patterns of trees showing reactions as a result of particular events were assessed so as to approximate the extent of past debris flows and snow avalanches copyright 2006 john wiley sons ltd