neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting pacific plate beneath the northeastern japan arc a triple planed deep seismic zone the northeastern japan arc is located in one of the most seismically active subduction zones in the world in this study we relocated hypocenters and determined focal mechanisms of small earthquakes m 5 beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate in the hypocenter relocation we adopted a source region station correction method in which station corrections vary with hypocenter locations we have developed a new focal mechanism determination technique named master solution method which is analogous to the master event method in hypocenter determination we applied the method to p and sh wave amplitude data to obtain 1106 focal mechanism solutions from the new mechanism solutions and relocated hypocenters we found that there occur both low angle thrust fault lt type and downdip compression dc type earthquakes at depths from 40 to 70 km near the aseismic front the dc type events are underlying the lt type events almost all the earthquake clusters are composed of lt type events the western limit of the region where lt type events have occurred is subparallel to the trench axis although it undulates considerably it delineates the westernend of the active region of interplate seismicity furthermore we found that normal fault nf type events also occur at depths from 70 km in the upper plane of the double planed deep seismic zone which is characterized mainly by dc type event these nf type events are distributed only in a thin uppermost portion of the slab close to the plate boundary below these in the lower plane are downdip extension de type events this result indicates that the deep seismic zone in the northeastern japan arc is not double planed but triple planed even beneath the land area which cannot be explained by any simple models

andean tectonics as a cause for changing drainage patterns in miocene northern south america newdatafromneogenestratainnorthernsouthamericasuggestthatmiocenetectonisminthenortheasternandeswasresponsible for the genesis of the amazon river and changes in the drainage patterns of other major rivers such as the magdalena and the orinoco here we present a new model for the paleogeographic evolution of northern south america during the miocene in the early miocene a large part of the drainage of northwest amazonia was directed northward along the paleo orinoco river system to a deltainlakemaracaibo upliftoftheeasterncordillerainthelate middle miocene caused thefirst development of the amazon river however no connection with the atlantic was established and the amazon fed the paleo orinoco river system which drained toward the caribbean substantial andean uplift in the late miocene resultedinmajorchangesinpaleogeography theorinocochangedits course the amazon established a connection to the atlantic causing the drowning of carbonate platforms and the amazon caribbeanconnectionwasclosed thusthedrainageandpaleogeography ofnorthernsouthamericainthemiocenewerestronglycontrolled by tectonic movements in the northeastern andes

amazonia during the last glacial abstract new data on the vegetational history and quaternary geology of amazonia permit an improved reconstruction of past environments in amazonia during the last glacial period although limited data from rondonia carajas and guyana show that in certain areas savanna type vegetation and savanna forest had replaced the rain forest during the late pleniglacial ca 22 000 13 000 yr b p the amazonian forest may have been split up into one major west amazonian and several other medium size forest areas this suggests a decline in rainfall of 500 to 1000 mm a reduction of 25 to 40 temperatures may also have been 2 to 6 c 4 2 c lower than today possibly substancially influencing amazonian vegetation during the humid middle pleniglacial 55 000 26 000 yr b p rivers carried a lot of water and sediment resulting in the deposition of lower terrace sediments with one dry interruption around 40 000 yr b p carajas and katira in carajas there is evidence of dry periods ocurring at about 40 000 yr b p and during the early pleniglacial ca 60 000 yr b p rivers carried little water and incised into the low terrace sediments during the dry late pleniglacial water levels rose during the late glacial 13 000 10 000 yr b p or at the beginning of the holocene 10 000 yr b p sedimentation in and of the present inundation valleys commenced after that

the illusion of diffusion field evidence for depth dependent sediment transport soil covered upland landscapes are common in much of the habitable world and our understanding of their evolution as a function of different climatic tectonic and geologic regimes is important across a wide range of disciplines erosion laws direct quantitative study of the processes shaping earth s surface and form the basis of landscape evolution modeling but are based on limited field data here we use in situ produced cosmogenic 1 0 be and 2 6 al concentrations from granitic saprolite to quantify an exponential decline in soil production with increasing soil thickness for a new field site in point reyes california results are similar to soil production functions from two different previously studied field sites and are used with extensive measurements of soil thickness to quantify depth integrated sediment transport flux plots of calculated sediment fluxes against the product of soil depth and hillslope gradient provide the first field based evidence that soil transport is a nonlinear depth dependent function data from all sites suggest that the widely used linear diffusion equation is only appropriate for shallow gradient convex up regions while the depth dependent transport law is more broadly applicable quantifying both the mobile soil thickness and landscape morphology is therefore critical to understanding how landscapes evolve

southern appalachian hillslope erosion rates measured by soil and detrital radiocarbon in hollows abstract understanding the dynamics of sediment generation and transport on hillslopes provides important constraints on the rate of sediment output from orogenic systems hillslope sediment fluxes are recorded by organic material found in the deposits infilling unchanneled convergent topographic features called hollows this study describes the first hollow infilling rates measured in the southern appalachian mountains infilling rates and bedrock erosion rates were calculated from the vertical distribution of radiocarbon ages at two sites in the coweeta drainage basin western north carolina at each site we dated paired charcoal and silt soil organic matter samples from five different horizons paired radiocarbon samples were used to bracket the age of the soil material in order to capture the range of complex soil forming processes and deposition within the hollows these dates constrain hillslope erosion rates of between 0 051 and 0 111 mm yr 1 these rates are up to 4 times higher than spatially averaged rates for the southern appalachian mountains making creep processes one of the most efficient erosional mechanisms in this mountain range our hillslope erosion rates are consistent with those of forested mountain ranges in the western united states suggesting that the mechanisms dominantly tree throw driving creep erosion in both the western united states and the southern appalachian mountains are equally effective

the great tumaco colombia earthquake of 12 december 1979 southwestern colombia and northern ecuador were shaken by a shal low focus earthquake on 12 december 1979 the magnitude 8 shock located near tumaco colombia was the largest in northwestern south america since 1942 and had been forecast to fill a seismic gap thrust faulting occurred on a 280 by 130 kilometer rectangular patch of a subduction zone that dips east beneath the pacific coast of colombia a 200 kilometer stretch of the coast tectonically subsided as much as 1 6 meters uplift occurred offshore on the continental slope a tsunami swept inland immediately after the earthquake ground shaking intensity vi to ix caused many buildings to collapse and generated liquefaction in sand fills and in holocene beach lagoonal and fluvial deposits

on the prediction of channel heads in a complex alpine terrain using gridded elevation data 1 threshold conditions for channel initiation are evaluated by using gridded elevation data derived from a lidar survey a reliable algorithm for the determination of surface flow paths and field observations of channel heads for a study area located in the eastern italian alps these threshold conditions are determined by considering the channel heads observed across a portion of the study area and computing the related values of 1 drainage area a 2 area slope function as2 with s being the local slope and 3 strahler order of surface flow paths extracted from gridded elevation data attention is focused on the dependence of the obtained threshold values on the size of grid cells involved and on the ability of the identified threshold conditions to provide reliable predictions of channel heads across the entire study area the results indicate that the threshold values of a as2 and are all significantly dependent on grid cell size and the uncertainty in the determination of threshold values of is significantly smaller than that affecting the determination of threshold values of a and as2 the comparison between predicted and observed channel heads indicates that the considered methods display variable reliability and sensitivity over different drainage basins and grid cell sizes with a general tendency to predict more channel heads than can be observed in the field acceptable predictions are normally obtained where channel heads are formed essentially by surface runoff more comprehensive methods seem however to be needed to predict channel heads affected by groundwater seeping upward

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

new physical evidence of the role of stream capture in active retreat of the blue ridge escarpment southern appalachians abstract the blue ridge escarpment of the southern appalachian mountains is a striking and rugged topographic feature of the ancient passive margin of eastern north america the crest of the escarpment generally coincides with an asymmetric regional drainage divide separating steep streams of the escarpment face from low gradient streams of the blue ridge upland recent exhumation and erosion rate studies suggest that the escarpment has evolved by inland erosional retreat but the mechanism timing and magnitude of retreat remain poorly understood longitudinal stream profiles and slope drainage area relationships of several upland basins draining the divide have led to the identification of 14 previously unknown fluvial terrace deposits preserved at the escarpment crest these relict terraces and the associated beheaded drainages indicate the role of large stream capture events in producing ongoing escarpment retreat through landward divide migration and subsequent topographic adjustment terrace location and preservation suggest that rectilinear drainage patterns and divide asymmetry generate discrete high order captures and episodes of rapid localized retreat that collectively produce slower evolution of the escarpment at large while overall retreat magnitude and rate remain unknown roundness of terrace alluvium suggests that the most recent captures have locally produced tens of kilometers of retreat within the limited preservation lifetime of the deposits in contrast with recent numerical modeling and cosmogenic studies these data show the potential for stream capture and divide migration to sustain passive margin escarpment evolution long after the cessation of rifting the fluvial record of divide retreat preserved atop the blue ridge escarpment suggests the potential for using field methods to better constrain the histories of younger taller and potentially more dynamic passive margin escarpments

palaeoenvironmental changes during the last ca 8590 calibrated yr 7800 radiocarbon yr in the dry forest ecosystem of the patia valley southern colombian andes a multiproxy approach two sediment cores were recovered from two swamps in the dry forest lowland ecosystem of the patia valley colombia the 500 cm long core el patia 1 was analysed using diatoms pollen lithostratigraphy geochemistry charcoal and radiocarbon dates the 270 cm long core patia 2 was analysed using pollen lithostratigraphy and radiocarbon dates the environmental reconstruction includes the local development and history of the patia 1 swamp and the regional vegetation history of the two swamps the environmental reconstruction shows a fluvial system in the area before 7830 14c yr bp ca 8590 cal yr bp after this date the river gradually moved away leading to the formation of the swamp patia 1 open vegetation with patches of dry forest surrounded the swamp between 7670 and 6860 14c yr bp 8350 7690 cal yr bp the swamp become partially isolated from the river and was a permanent water body which was alkaline and mesotrophic during this period the surroundings were covered by dry forest from 6860 to 5870 14c yr bp 7690 6690 cal yr bp the swamp was totally isolated from the river and suffered from frequent periods of desiccation and erosion from 5870 to 3570 14c yr bp 6690 3890 cal yr bp the swamp turned into a marsh terrestrial environment and in the surrounding area the dry forest alternated with open vegetation it is possible that there were human impacts on the environment during this period between 3570 and 1990 14c yr bp 3890 2000 cal yr bp unstable conditions prevailed after 1990 14c yr bp 2000 cal yr bp swamp conditions of permanent water were re established open vegetation surrounded the swamp agriculture and human activities were present and have continued since then the climatic reconstruction suggests an increase in humidity at ca 7670 14c yr bp 8350 cal yr bp and a dry climate between 6860 and 5870 14c yr bp 7690 6690 cal yr bp

glacial and periglacial erosion of the appalachians abstract the marine 18 o record indicates that the first ice advance into the appalachians equivalent to that of the late wisconsin occurred in the pliocene at about 2 4 ma only periodic localized glacial and periglacial activity occurred in the northern appalachians from 2 33 to 0 85 ma after 0 85 ma middle to late pleistocene eight out of ten glaciations covered the northern appalachians and brought periglacial conditions to the southern appalachians during the last 850 000 years glaciation extended to the late wisconsin limit within the appalachians for 15 of the time and extended to the early wisconsin limit just north of the appalachians for 25 of the time pleistocene glacial erosion of the appalachians of 120 200 m is necessary to account for the estimated volume of pleistocene ocean sediment derived from the appalachians and the estimated volume of pliocene pleistocene glaciogenic ocean sediment derived from the entire laurentide ice sheet this amount of glacial erosion occurring over the maximum expectable duration of pliocene pleistocene ice cover of 247 000 years yields glacial erosion rates from 0 5 0 8 m ka 1 such depths of erosion by thick warm based ice streams were localized in the lowlands and estuaries of the northern appalachians in the maritimes the presence of thin cold based ice caused minimal erosion of uplands periglacial landforms and tundra boreal forest were distributed throughout the appalachian highlands and the northern coastal plain during the glacial maxima periglacial erosion of 0 15 0 30 m ka 1 near the ice in pennsylvania decline to 0 1 0 2 m ka 1 in the southern piedmont each cold period almost completely renewed the regolith by eroding slopes and filling valleys the cumulative effect of the eight to ten major cold events could have produced as much as a few ten s of meters of periglacial ridge top lowering during the pleistocene both estimated glacial and periglacial erosion rates greatly exceed estimated current fluvial erosion rates therefore glacial and periglacial processes should be considered the dominant processes in shaping the present appalachian landscape the northern appalachians may be evolving towards a form where the landscape provides just the shape necessary to permit the least work conveyance of the ice across the region during glacial maxima the southern appalachians may be evolving towards a form where the landscape provides just the hillslope form necessary to transport the debris provided by periglacial conditions

late quaternary pollen records from the middle caqueta river basin in central colombian amazon abstract three pollen records are presented from the pantano de monica region 0 42 s 72 04 w 160 m elevation on the lower terrace of rio caqueta of the central colombian amazonas ten radiocarbon dates from three cores indicate that the deposits are of holocene age but the pollen data suggest that the record may also contain the late glacial the core pantano de monica 1 covers the time interval from 11 150 bp extrapolated to 4730 bp during the late glacial and early holocene this swamp was smaller in size and waters were more shallow than today with abundant mauritia palm trees this indicates that the lower terrace of the caqueta river was better drained than today which might be related to changes in the drainage system and or drier conditions during that time late glacial and early holocene vegetational changes in the rain forests surrounding the swamp pantano de monica indicate successional stages probably related to changes in the drainage system and or climate changes presence of podocarpus pollen grains up to 2 6 of the total sum and influx of 78 grains cm 2 yr 1 point to the regional presence of podocarpus at the beginning of the holocene evidence of podocarpus during glacial times in other pollen records from the amazon basin has been taken as indicative of cooling core pantano de monica 2 was taken in a small swamp 1000 m distance to pantano de monica 1 and starts at 4000 bp when this location was well drained several changes in the composition of the rain forest are documented such as increase in disturbance increase of cecropia followed by a quick recovery increase of psychotria protium and caryocar tree taxa were frequent suggesting that the lower terrace area was well drained until 3080 bp after which small swamps developed core pantano de monica 3 is from the wet forest in between the two swamps and represents the period since 3260 bp pollen data show a continuously increasing presence of palms probably euterpe suggesting either a transition from well drained to less drained conditions in the lower terrace during the late holocene and or a more intensive human impact of the rain forest on the lower terraces of rio caqueta the three records from the lower terrace of rio caqueta show different forest compositions in the past and indicate that the rain forest environments were not stable during late glacial and holocene times comparison with other data from lowland records of northwestern south america suggest that climate change is a major factor for environmental change in central colombian amazon

controls on zero order basin morphology zero order basins are common features of soil mantled landscapes defined as unchanneled basins at the head of a drainage network their geometry and volume control how quickly sediment may re accumulate after landslide evacuation and how water and subsequently pore pressure is delivered to hollow axes they also deliver water and sediment to the uppermost portions of the fluvial network despite this role as the moderator between hillslope and fluvial processes little analysis on their morphology has been conducted at the landscape scale we present a method to identify zero order basins in landscapes and subsequently quantify their geometric properties using elliptical fourier analysis we deploy this method across the coweeta hydrologic laboratory usa properties such as length relief width and concavity follow distinct probability distributions which may serve as a basis for testing predictions of future landscape evolution models surprisingly in a landscape with an orographic precipitation gradient and large hillslope to channel relief we observe no correlation between elevation or spatial location and basin geometry however we find that two physiographic units in coweeta have distinct zero order basin morphologies these are the steep thin soiled high elevation nantahala escarpment and the lower gradient lower elevation thick soiled remainder of the basin our results indicate that basin slope and area negatively covary producing the distinct forms observed between the two physiographic units which we suggest arise through competition between spatially variable soil creep and stochastic landsliding

removal of artifact depressions from digital elevation models towards a minimum impact approach artifact depressions in digital elevation models dems interrupt flow paths and alter drainage directions techniques for removing depressions should enforce continuous flow paths in a way that requires the least modification of the dem impacts on the spatial and statistical distributions of elevation and its derivatives were assessed for four methods of removing depressions 1 filling 2 breaching 3 a combination of filling and breaching with breaching constrained to a maximum of two grid cells 4 a combination of filling and breaching based on an impact reduction approach ira the ira removes each depression using either filling or breaching depending on which method has the least impact in terms of the number of modified cells and the mean absolute difference in the dem analysis of a lidar dem of a landscape on the canadian shield showed significant differences in the impacts among the four depression removal methods depression filling a removal method that is widely implemented in geographical information system software was found to impact terrain attributes most severely constrained breaching which relies heavily on filling for larger depressions also performed poorly both depression breaching and the ira impacted spatial and statistical distributions of terrain attributes less than depression filling and constrained breaching the most sensitive landscapes to depression removal were those that contained large i e 10 flat areas because of the occurrence of relatively large depressions in these areas copyright 2005 john wiley sons ltd

erosion of steepland valleys by debris flows episodic debris flows scour the rock beds of many steepland valleys along recent debris flow runout paths in the western united states we have observed evidence for bedrock lowering primarily by the impact of large particles entrained in debris flows this evidence may persist to the point at which debris flow deposition occurs commonly at slopes of less than 0 03 0 10 we find that debris flow scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river incision models much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows the degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions on the basis of these observations we hypothesize that valley slope is adjusted to the long term frequency of debris flows and that valleys scoured by debris flows should not be modeled using conventional bedrock river incision laws we use field observations to justify one possible debris flow incision model whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris flow sources the model predicts that increases in incision rate caused by increases in flow event frequency and length as flows gain material downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes and stronger less weathered bedrock these adjustments lead to a spatially uniform lowering rate although the proposed expression leads to equilibrium long profiles with the correct topographic signature the crudeness with which the debris flow dynamics are parameterized reveals that we are far from a validated debris flow incision law however the vast extent of steepland valley networks above slopes of 0 03 0 10 illustrates the need to understand debris flow incision if we hope to understand the evolution of steep topography around the world

potential for landsliding dependence on hyetograph characteristics 1 this paper examines the effect of hyetograph shape on the potential for landsliding in soil mantled landscapes an existing pore pressure response model iverson 2000 is used to study the effects of unsteady rainfall infiltration in hillslopes and the effect of slope and convergent topography is expressed using a steady state model of slope parallel subsurface flow slope stability is assessed using an infinite slope analysis this theoretical framework is coupled with simple hyetograph models and to intensity duration frequency functions to determine the return period of landslide triggering rainfall results also show that hyetographs with a peak at the end of a rainfall event have a stronger destabilizing effect than hyetographs with a constant rainfall or with a peak at the beginning of a storm thus the variability of hyetograph shapes adds uncertainty to the assessment of landsliding triggered by rainfall

the treatment of flat areas and depressions in automated drainage analysis of raster digital elevation models methods developed to process raster digital elevation models dem automatically in order to delineate and measure the properties of drainage networks and drainage basins are being recognized as potentially valuable tools for the topographic parameterization of hydrological models all of these methods ultimately rely on some form of overland flow simulation to define drainage courses and catchment areas and therefore have difficulty dealing with closed depressions and flat areas on digital land surface models some fundamental assumptions about the nature of these problem topographic features in dem are implicit in the various techniques developed to deal with them in automated drainage analysis the principal assumptions are 1 that closed depressions and flat areas are spurious features that arise from data errors and limitations of dem resolution 2 that flow directions across flat areas are determined solely by adjacent cells of lower elevation and 3 that closed depressions are caused exclusively by the underestimation of dem elevations it is argued that while the first of these assumptions is reasonable given the quality of dems generally available for hydrological analysis the others are not rather it seems more likely that depressions are caused by both under and overestimation errors and that flow directions across flat areas are determined by the distribution of both higher and lower elevations surrounding flat areas two new algorithms are introduced that are based on more reasonable assumptions about the nature of flat areas and depressions and produce more realistic results in application these algorithms allow breaching of depression outlets and consider the distribution of both higher and lower elevations in assigning flow directions on flat areas the results of applying these algorithms to some real and hypothetical landscapes are presented 1998 john wiley sons ltd

boulder deposits and the retreat of mountain slopes or gully gravure revisited observations on mountains composed chiefly of shale and capped with tuscarora sandstone in the valley and ridge province of southwest virginia suggest that slopes retreat by a process similar to but different from bryan s 1940 gully gravure the process appears to operate as follows bouldery alluvium protects the floors of hollows on the mountain flanks in each hollow the junction between the boulder armored floor and the base of the unprotected shale side slope is the locus of greatest erosion runoff from rainstorms that have recurrence intervals of the order of 10 to 10 years incises gullies along the margins of the bouldery deposits but lacks the competence to disturb the deposits periodically however runoff from catastrophic rainstorms that have recurrence intervals of the order of perhaps 10 years mobilizes the bouldery alluvium and deposits it in the gullies the armored floor thus shifts laterally subsequently new gullies are incised adjacent to the old ones the result is the later

significance of thick deposits of colluvium on hillslopes a case study involving the use of pollen analysis in the coastal mountains of northern california distinct bedrock hollows with a colluvial fill are common in soil mantled hillslopes it has been proposed that these hollows are sites of recurrent emptying by landsliding and subsequent slower refilling with colluvium on a hillslope in northern california we have quantified the time since emptying and the three dimensional configurations of the underlying bedrock surface correlation of the pollen record in the colluvium with a nearby pollen core from a lake with an accurate age control indicates that the oldest colluvium was deposited 11 000 to 13 500 years in the bedrock hollow since then deposition of colluvium in the hollow has been equivalent to 1 5 m of landscape lowering in the surrounding source area and to a local lowering rate of about 0 1 mm yr the longitudinal profile of the hollow is concave upward a form that causes the groundwater flow to rise toward the surface downslope leading to saturation and possibly to landsliding

analysis of hillslope erosion rates using dated colluvial deposits hollows are sites of long term deposition of colluvium and contain a valuable record of the timing and rates of hillslope erosion a model is presented that uses measured depositional rates of colluvium in hollows to calculate transport rates of colluvium and bedrock lowering rates on the surrounding slopes this model has been applied to eight dated colluvial deposits in the clearwater river basin of western washington the calculated average transport rate is about 34 cm 3 cm yr and the calculated average bedrock lowering rate is about 0 04 mm yr equivalent to an average sediment yield of 90 t km 2 yr for the holocene there is significant variability in erosion rates between sites which may be due to local lithologic variations although the average values are similar to an independent estimate of modern erosion rates for this area multiple stratigraphic levels have been dated in five deposits and document that the deposits are cumulative and that deposition has been active through the

how long is a hillslope hillslope length is a fundamental attribute of landscapes intrinsically linked to drainage density landslide hazard biogeochemical cycling and hillslope sediment transport existing methods to estimate catchment average hillslope lengths include inversion of drainage density or identification of a break in slope area scaling where the hillslope domain transitions into the fluvial domain here we implement a technique which models flow from point sources on hilltops across pixels in a digital elevation model dem based on flow directions calculated using pixel aspect until reaching the channel network defined using recently developed channel extraction algorithms through comparisons between these measurement techniques we show that estimating hillslope length from plots of topographic slope versus drainage area or by inverting measures of drainage density systematically underestimates hillslope length in addition hillslope lengths estimated by slope area scaling breaks show large variations between catchments of similar morphology and area we then use hillslope length relief structure of landscapes to explore nature of sediment flux operating on a landscape distinct topographic forms are predicted for end member sediment flux laws which constrain sediment transport on hillslopes as being linearly or nonlinearly dependent on hillslope gradient because our method extracts hillslope profiles originating from every ridgetop pixel in a dem we show that the resulting population of hillslope length relief measurements can be used to differentiate between linear and nonlinear sediment transport laws in soil mantled landscapes we find that across a broad range of sites across the continental united states topography is consistent with a sediment flux law in which transport is nonlinearly proportional to topographic gradient

hillslope to valley transition morphology new opportunities from high resolution dtms abstract the search for the optimal spatial scale for observing landforms to understand physical processes is a fundamental issue in geomorphology topographic attributes derived from digital terrain models dtms such as slope curvature and drainage area provide a basis for topographic analyses the slope area relationship has been used to distinguish diffusive hillslope from linear valley processes and to infer dominant sediment transport processes in addition curvature is also useful in distinguishing the dominant landform process recent topographic survey techniques such as lidar have permitted detailed topographic analysis by providing high quality dtms this study uses lidar derived dtms with a spatial scale between 1 and 30 m in order to find the optimal scale for observation of dominant landform processes in a headwater basin in the eastern italian alps where shallow landsliding and debris flows are dominant the analysis considered the scaling regimes of local slope versus drainage area the spatial distribution of curvature and field observations of channel head locations the results indicate that i hillslope to valley transitions in slope area diagrams become clearer as the dtm grid size decreases due to the better representation of hillslope morphology and the topographic signature of valley incision by debris flows and landslides is also best displayed with finer dtms ii regarding the channel head distribution in the slope area diagrams the scaling regimes of local slope versus drainage area obtained with grid sizes of 1 3 and 5 m are more consistent with field data and iii the use of thresholds of standard deviation of curvature particularly at the finest grid size were proven as a useful and objective methodology for recognizing hollows and related channel heads

slope control on the aspect ratio of river basins although it is traditionally thought that drainage basins are geometrically similar we present new results which indicate that the aspect ratio of weakly dissected river basins at large scales 10 103 km2 depends on the surface slope steeper surfaces develop narrower and lengthier basins whereas more gently dipping surfaces develop more equant and wider basins this relationship is interpreted to be related to the nature of water flow over rough surfaces with steeper slopes causing less flow convergence and lengthier and narrower basins we derive an empirical relationship that can be used to infer the slope of a surface on which a river basin acquired its geometry based solely on a measure of its basin form this relation provides a unique means of inferring the relative chronology of river basin development with respect to surface tilting and therefore provides a direct link between river basin morphology and tectonics

channel network extraction from high resolution topography using wavelets 1 the availability of high resolution topography from lidar offers new opportunities for objectively extracting the channels directly from a dem using local topographic information instead of inferring them indirectly based on global criteria such as area or area slope threshold relationships here we introduce the use of wavelet filtering to delineate threshold curvatures for defining valleys and threshold slope direction change for defining probable channeled portions of the valleys this approach exploits the topographic signatures uniquely found in high resolution topography and reveals the fuzzy topographic transition in which local weakly convergent areas lie at the transition between hillslopes and valleys

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

the relationship between drainage density erosion rate and hilltop curvature implications for sediment transport processes drainage density is a fundamental landscape metric describing the extent of the fluvial network we compare the relationship between drainage density dd and erosion rate e using the channel hillslope integrated landscape development child numerical model we find that varying the channel slope exponent n in detachment limited fluvial incision models controls the relationship between dd and e with n 1 resulting in increasing dd with e if all other parameters are held constant this result is consistent when modeling both linear and nonlinear hillslope sediment flux we also test the relationship between dd and e in five soil mantled landscapes throughout the usa feather river ca san gabriel mountains ca boulder creek co guadalupe mountains nm and bitterroot national forest id for two of these field sites we compare dd to cosmogenic radionuclide crn derived erosion rates and for each site we use mean hilltop curvature as a proxy for erosion rate where crn derived erosion rates are not available we find that there is a significant positive relationship between dd e and hilltop curvature across every site with the exception of the san gabriel mountains ca this relationship is consistent with an n exponent greater than 1 suggesting that at higher erosion rates the transition between advective and diffusive processes occurs at smaller contributing areas in soil mantled landscapes

topographic and ecologic controls on root reinforcement shallow landslides are a significant hazard in steep soil mantled landscapes during intense rainfall events the distribution of shallow landslides is controlled by variations in landscape gradient the frictional and cohesive properties of soil and roots and the subsurface hydrologic response while gradients can be estimated from digital elevation models information on soil and root properties remains sparse we investigated whether geomorphically controlled variations in ecology affect the spatial distribution of root cohesion by measuring the distribution and tensile strength of roots from soil pits dug downslope of 15 native trees in the southern appalachian mountains north carolina united states root tensile strengths from different hardwood tree species were similar and consistently higher than the only native shrub species measured rhododendron maximum roots were stronger in trees found on noses areas of divergent topography relative to those in hollows unchanneled convergent topography coincident with the variability in cellulose content this cellulose variability is likely related to topographic differences in soil water potential for all species roots were concentrated close to the soil surface with roots in hollows being more evenly distributed in the soil column than those on noses trees located on noses had higher mean root cohesion than those in hollows because of a higher root tensile force r maximum had the shallowest weakest roots suggesting that recent expansion of this species due to fire suppression has likely lowered the root cohesion of some hollows quantification of this feedback between physiologic controls on root growth and slope hydrology has allowed us to create a curvature based model of root cohesion that is a significant improvement on current models that assume a spatially averaged value

impact of flow routing on catchment area calculations slope estimates and numerical simulations of landscape development 1 flow routing across real or modeled topography determines the modeled discharge and wetness index and thus plays a central role in predicting surface lowering rate runoff generation likelihood of slope failure and transition from hillslope to channel forming processes in this contribution we compare commonly used flow routing rules as well as a new routing rule to commonly used benchmarks we also compare results for different routing rules using airborne laser swath mapping alsm topography to explore the impact of different flow routing schemes on inferring the generation of saturation overland flow and the transition between hillslope to channel forming processes as well as on location of saturation overland flow finally we examined the impact of flow routing and slope calculation rules on modeled topography produced by geomorphic transport law gtl based simulations we found that different rules produce substantive differences in the structure of the modeled topography and flow patterns over alsm data our results highlight the impact of flow routing and slope calculation rules on modeled topography as well as on calculated geomorphic metrics across real landscapes as such studies that use a variety of routing rules to analyze and simulate topography are necessary to determine those aspects that most strongly depend on a chosen routing rule

holocene history of the choco rain forest from laguna piusbi southern pacific lowlands of colombia abstract a high resolution pollen record from a 5 m long sediment core from the closed lake basin laguna piusbi in the southern colombian pacific lowlands of choco dated by 11 ams14c dates that range from ca 7670 to 22014c yr b p represents the first holocene record from the choco rain forest area the interval between 7600 and 610014c yr b p 500 265 cm composed of sandy clays that accumulated during the initial phase of lake formation is almost barren of pollen fungal spores and the presence of herbs and disturbance taxa suggest the basin was at least temporarily inundated and the vegetation was open the closed lake basin might have formed during an earthquake probably about 440014c yr b p from the interval of about 600014c yr b p onwards 200 different pollen and spore types were identified in the core illustrating a diverse floristic composition of the local rain forest main taxa are moraceae urticaceae cecropia melastomataceae combretaceae acalypha alchornea fabaceae mimosa piper protium sloanea euterpe geonoma socratea andwettinia little change took place during that time interval compared to the pollen records from the rain forests of the colombian amazon basin and adjacent savannas the choco rain forest ecosystem has been very stable during the late holocene paleoindians probably lived there at least since 346014c yr b p evidence of agricultural activity shown by cultivation ofzea maissurrounding the lake spans the last 1710 yr past and present very moist climate and little human influence are important factors in maintaining the stable ecosystem and high biodiversity of the choco rain forest

colombian dry moist forest transitions in the llanos orientales a comparison of model and pollen based biome reconstructions colombian vegetation at the ecological level of the biome is reconstructed at six sites using pollen data assigned a priori to plant functional types and biomes the chosen sites incorporate four savanna sites laguna sardinas laguna angel el pinal and laguna carimagua a site on the transition between savanna and amazon rainforest loma linda and a site within the amazon rainforest pantano de monica the areal extent of tropical moist forest tropical dry forest and steppe have been subject to significant change differential responses of the vegetation to climatic shifts are related to changes in plant available moisture duration of dry season and edaphic controls on the vegetation the record from el pinal shows that the present day savanna vegetation dominated by steppe poaceae with little occurrence of woody savanna taxa e g curatella byrsonima was present since the last glacial period of the northern hemisphere unfortunately el pinal is located on an edaphic savanna and is not particularly responsive to registering change most records cover the early holocene one site records the el abra stadial younger dryas equivalent when forest expansion reflects more humid climatic conditions and higher plant available moisture during the early and middle holocene the maximum expansion of steppe and tropical dry forest occurred indicating that dry climatic conditions continued to around 4000 14 c bp the following period from shortly before 4000 14 c bp is characterised by an increase in forest and gallery forests reflecting a wetter period probably with a shorter annual dry season anthropogenic influence on the vegetation is recorded by all the records over the last millennial particularly characterised by a reduction in forest cover and high amplitude changes in vegetation biome transitions from one type to another and the environmental controls on this shift are investigated by applying a vegetation model biome 3 the model uses climatic data from six meteorological stations that encompass a range of environments within lowland colombia which are similar to the pollen data the signals of vegetation change can be translated to the main environmental controls of temperature and moisture to indicate the degree of change needed in these parameters to record the vegetation change depicted by the pollen data moisture balance is the dominant control on driving vegetation change whether under seasonal or annual control the combined reconstruction from pollen data and model output of biome scale vegetation dynamics for lowland colombia allows an understanding of the environmental controls to be developed d 2005 elsevier b v all rights reserved

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

periglacial geomorphology of the appalachian highlands and interior highlands south of the glacial border a review abstract many features reported in the appalachian region have been assigned a paleoperiglacial origin based on field relationships and their similarities with analogs active in present day actuoperiglacial environments these forms include but are not limited to sorted and nonsorted varieties of patterned ground grezes litees block fields block slopes and block streams cryoplanation terraces and hillslope and river terrace landscapes although very small scale features generally less than 1 m in plan or section dimension and some larger forms on steep hillslopes are known to be at least sporadically active today the large scale features discussed here are interpreted as either inactive or truly fossil periglacial phenomena thus they hav implications for paleoclimatic reconstruction and significance as indicators of relative landscape stability since the time s of their development this paper briefly reviews the historical dimension of early researchs work present selected examples of recent results and gives authors conclusions

ecosystem processes at the watershed scale mapping and modeling ecohydrological controls of landslides mountain watersheds are sources of a set of valuable ecosystem services as well as potential hazards the former include high quality freshwater carbon sequestration nutrient retention and biodiversity whereas the latter include flash floods landslides and forest fires each of these ecosystem services and hazards represents different elements of the integrated and co evolved ecological hydrological and geomorphic subsystems of the watershed and should be approached analytically as a coupled land system forest structure and species are important influences on the partitioning of precipitation the lateral redistribution of water runoff and sediment production weathering and soil development forest regulation of hydrologic dynamics contributes to the development of patterns of soil pore pressure and slope instability during storms or snowmelt the spatial patterns of root depth structure and strength developed by the below ground allocation of carbon in the forest canopy in response to limiting resources of water and nutrients contributes to slope stability and drainage and the maintenance of stomatal conductance linking water and carbon cycling this in turn provides the photosynthate required to build leaf area stem and root biomass the linked ecological hydrologic and geomorphic systems are characterized by specific catenary patterns that should be captured in any coupled modeling approach in this paper we extend an ecohydrological modeling approach to include hydrologic and canopy structural pattern impacts on slope stability with explicit feedbacks between ecosystem water carbon and nutrient cycling and the transient development of landslide potential in steep forested catchments using measured distributions of canopy leaf area index and empirically modeled soil depth and root cohesion the integrated model is able to generate localized areas of past instability without specific calibration or training with mapped landslides as the model has previously been shown to simulate space time patterns of coupled water carbon and nutrient cycling the integration of slope stability as a function of hydrologic ecosystem and geomorphic processes provides the ability to closely link multiple ecosystem services with a unified approach

evidence of transient topographic disequilibrium in a landward passive margin river system knickpoints and paleo landscapes of the new river basin southern appalachians the upper new river basin of the southern appalachian mountains a major tributary of the modern ohio river represents the unglaciated headwaters of the tertiary teays river system of eastern north america dating of relict fluvial gravels have suggested that new river incision may be outpacing lowering of the surrounding uplands but physical evidence of transient topographic disequilibrium has yet to be identified we use focused topographic analysis of the upper new river basin to delineate a perched low relief paleo landscape that is experiencing transgressive dissection due to incision by the new river and its tributaries accelerated incision has decoupled hillslopes from the drainage network generating knickpoints which represent the boundary between remnants of the paleo landscape and actively adjusting topography downstream steepening of hillslopes downstream of knickpoints suggests dynamic headward migration which along with knickpoint occurrence throughout the drainage network is inconsistent with the development of fixed stream profile convexities atop strike extensive geologic contacts in the absence of tectonic forcing we favor a climatically forced drop in external base level as driver of the incision pattern we observe plio pleistocene glacial damming and diversion of the teays river to form the modern ohio river lowered regional base level for the study area potentially forcing the paleo landscape developed during the teays era to adjust to the modern drainage pattern the upper new river may therefore represent the potential for glacially driven drainage rearrangement to drive transient topographic evolution hundreds of kilometers away from the ice margin long after the disappearance of ice sheets copyright 2013 john wiley sons ltd

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

holocene space time succession of the middle atrato wetlands choco biogeographic region colombia abstract successional sequences from forested wetlands in the middle atrato river basin were reconstructed using characterisation of present vegetation communities and palynological analysis a 4 8 km transect drawn across a river meander and two 6 and 8 m deep sediment cores san martin and villanueva were collected in the floodplain within two different vegetation assemblages based on the floristic and environmental characteristics of the local vegetation communities ecological changes spanning the last 4 ka cal years bp were analysed in san martin and villanueva cores present vegetation is dominated by four communities determined by flood tolerance and drainage conditions we found euterpe oleraceae mauritiella macroclada campnosperma panamensis and oenocarpus bataua forests and mixed forest and open vegetation in a gradient from poor to improved drainage conditions vegetation changes in the palynological record suggest that sedimentation and erosion processes on flood basins are due to changes in drainage conditions and to variable flooding levels a wet period in the 4 to 2 7 ka interval is postulated which might be related to sea level rise or local subsidence lower flooding levels and improved drainage conditions dominated the 2 7 to 1 6 ka interval whereas a flooding event and a hiatus occurred between 1 5 and 0 5 ka this flooding event might be synchronous with analogous events as recorded in the colombian amazonia between 1 6 and 1 45 ka forest disturbance probably of anthropogenic origin is recorded in both sites since 0 5 ka

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient

controls on the spacing of first order valleys 1 many landscapes are composed of ridges and valleys that are uniformly spaced even where valley locations are not controlled by bedrock structure models of long term landscape evolution have reproduced this phenomenon yet the process by which uniformly spaced valleys develop is not well understood and there is no quantitative framework for predicting valley spacing here we use a numerical landscape evolution model to investigate the development of uniform valley spacing we find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area a proxy for water flux and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium valley spacing is most sensitive to the relative rates of advective erosion processes such as stream incision and diffusion like mass transport such as soil creep and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a peclet number we use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape the observed scaling relationships also provide insight into the cause of transitions from rill like drainage networks to branching networks the spatial scale of first order drainage basins the contributing area at which hillslopes transition into valleys and the narrow range of width to length ratios of first order basins

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

geomorphic and paleoclimatic implications of latest pleistocene radiocarbon dates from colluvium mantled hollows california radiocarbon analyses of charcoal from basal colluvium in 11 california hollows show a clustering of dates between 9000 and 15 000 b p an indication that changes in the storage and discharge of colluvium from hillslopes accompanied the pleistocene holocene transition hollows are sites of topographically induced convergence and deposition of colluvial debris and evacuation of this debris was apparently more thorough and possibly more frequent in the latest pleistocene perhaps due to a combination of changes in vegetation and rainfall characteristics one hypothesis is that greater storm intensities occurred in the latest pleistocene and induced a higher frequency of landslides in hollows and a regional extension of channel heads upslope relative to the holocene during the last pleistocene california hollows apparently contained smaller volumes of colluvium in storage the increased storage of debris during the holocene may have resulted in a diminished supply of sediment to stream channels

holocene climatic variations in the western cordillera of colombia a multiproxy high resolution record unravels the dual influence of enso and itcz abstract the paramo de frontino 3460 m elevation in colombia is located approximately halfway between the pacific and atlantic oceans it contains a 17 kyr long stratigraphically continuous sedimentary sequence dated by 30 ams 14c ages our study covers the last 11 500 cal yr and focuses on the biotic pollen and abiotic microfluorescence x or xrf components of this high mountain ecosystem the pollen record provides a proxy for temperature and humidity with a resolution of 20 35 yr and xrf of ti and fe is a proxy for rainfall with a sub annual ca 6 month resolution temperature and humidity display rapid and significant changes over the holocene the rapid transition from a cold mean annual temperature mat 3 5 c lower than today and wet younger dryas to a warm and dry early holocene is dated at 11 410 cal yr bp during the holocene mat varied from ca 2 5 c below to 3 5 above present day temperature warm periods 11 410 10 700 9700 6900 4000 2400 cal yr bp were separated by colder intervals the last 2 4 kyr of the record is affected by human impact the holocene remained dry until 7500 cal yr bp then precipitations increased to reach a maximum between 5000 and 4500 cal yr bp a rapid decrease occurred until 3500 cal yr bp and the late holocene was dry spectral analysis of xrf data show rainfall cyclicity at millennial scale throughout the holocene and at centennial down to enso scale in more specific time intervals the highest rainfall intervals correlate with the highest activity of enso variability in solar output is possibly the main cause for this millennial to decadal cyclicity we interpret enso and itcz as the main climate change driving mechanisms in frontino comparison with high resolution xrf data from the caribbean cariaco basin a proxy for rainfall in the coastal venezuelian cordilleras demonstrates that climate in frontino was pacific driven enso dominated during the yd and early holocene whereas it was atlantic driven in cariaco itcz dominated from ca 8000 cal yr bp climate in both areas was under the dual influence of enso and itcz thereby showing existing teleconnections between the tropical pacific and atlantic oceans the frontino record is to date the highest resolution holocene study in nw colombia an implication of these results is that new records should be analyzed with multiproxy tools in particular those providing high resolution time series such as xrf

comparison of gully erosion estimates using airborne and ground based lidar on santa cruz island california abstract gully erosion removes comparatively large volumes of soil from small areas it is often difficult to quantify the loss of soil because the footprint of individual gullies is too small to be captured by most generally available digital elevation models dems such as the usgs national elevation dataset airborne lidar light detection and ranging has the potential to provide the required data density but an even newer class of ground based sensors may provide better local resolution at lower cost in this study we compared digital elevation models produced by airborne and ground based lidar systems with ground based geomorphic and geodetic survey data to determine their utility in quantifying volumetric soil loss due to gully erosion in a heavily degraded watershed 7 55 10 2 km 2 on southwestern santa cruz island in southern california volumetric estimates of the eroded sediment were produced by comparing the lidar derived dems of the gully system to a modeled pre erosion surface average point densities were significantly higher for the ground based lidar system and provided more detailed information however its limited scanning footprint and side looking orientation presented serious challenges in collecting continuous data from deeply incised gullies making the airborne system preferable for this type of investigation and likely for most applications where heavy topographic shadowing is prevalent

channel head locations in forested watersheds across the mid atlantic united states a physiographic analysis abstract channel heads are the beginning of river networks and thus knowing their location is important in assessing water resources and health threats to fluvial ecosystems despite their importance most channel heads are unmapped remote sensing technologies have not yet proven effective under forested canopies suggesting that predictive models of channel head locations are the best solution to the impracticality of field mapping the millions of these features that exist in the u s alone in this study we compared the locations of 253 field mapped channel heads in forested watersheds across five different physiographic provinces to a suite of landscape attributes to develop statistical relationships and explanations for the occurrence of channel heads topographic attributes were best correlated to catchment source areas of channel heads with local slope being the primary explanatory variable in one physiographic province local plan curvature in another and average profile curvature in the other three we also found an approximate 1 1 relationship between average plan curvature and average profile curvature for channel heads suggesting that channel heads in the mid atlantic u s tend to occur at the outlet of bowl shaped catchments although our results are region specific the insight gained by investigating site specific processes across a large physiographically diverse area will help promote more general and robust models that can be applied in a range of landscapes

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

late glacial and holocene environmental and climatic changes from a limnological transect through colombia northern south america abstract we present a synthesis of the palaeolimnological and palaeoclimatic reconstructions of four sites in colombia the record from lake el caimito the westernmost site on the pacific coast dates from the late holocene and shows lacustrine sedimentation frequently interrupted by fluvial pulses these pulses probably reflect periods of increased precipitation related to la nina phases east of el caimito site is the patia swamp situated between the western and central andean cordilleras the patia records the dynamics of forest expansion reduction and changes in water levels although the climatic signal of the patia core is difficult to reconstruct there is a clear increase in humidity in the mid holocene the fuquene lake record on the eastern andean cordillera records dry and cold conditions during the late pleistocene very humid conditions for the early mid holocene and dry conditions during the mid late holocene las margaritas site on the eastern savannas records dry conditions during the early holocene and overall humid conditions for the mid and late holocene climate conditions from the fuquene and las margaritas sites seem to reflect the holocene movements of the intertropical convergence zone itcz the latter site being more affected by humidity coming from the amazon region

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

late quaternary vegetation and climate change in the panama basin palynological evidence from marine cores odp 677b and tr 163 38 abstract the late quaternary paleoenvironmental history from pacific slopes of the western andes is reconstructed by pollen analysis of 32 samples from two marine sediment cores from the panama basin eastern equatorial pacific core odp 677b 83 44 2200 w 1 12 1440 n 3473 m water depth is 185 cm long and spans the last 39 410 years core tr 163 38 81 583 w 1 337 n 2200 m water depth is 103 cm long and covers the last 17 380 years six ecological groups were established mangrove brackish and fresh water swamps terra firma lowland forests broad range taxa andean forests and open vegetation a good correspondence was found between the changes of these ecological groups in the two cores the records evidence the continuous presence of all vegetation types during the last 39 410 years and specially the uninterrupted occurrence of tropical rain forest they record a development from 1 a cold and humid phase 39 410 28 120 yr cal bp with moderately high sea levels 2 the coldest and driest phase in the record 28 120 14 500 yr cal bp accompanied by the lowest sea levels 3 a transitional phase when sea level rose and humid conditions dominated 4 a stage 11 300 5600 yr cal bp of the highest sea levels and moisture conditions including a drier period 7000 yr bp to 5 a final period 5600 yr cal bp present when sea level reached its present height humidity persisted and indicators of disturbance expanded peaks in pollen and spore concentration associated with high river discharge periods indicate periods of higher precipitation around 33 500 28 000 and 12 000 9000 yr cal bp although main vegetation responses seem to reflect rainfall and moisture variations a good correspondence was found between 18o values and percentages of andean and lowland pollen suggesting that vegetation also responded to tempearture changes

late holocene environmental history of southern choco region pacific colombia sediment diatom and pollen analysis of core el caimito abstract we present a multi proxy study of pollen diatoms sediment characteristics and major elements of a 610 cm sediment core from lake el caimito located in the humid rain forest of southern choco pacific colombia we propose an integrated reconstruction of the local basin development and of the regional vegetation which is possibly related to the tectonic activity of an unstable coastal area time control is based on 7 ams 14 c dates that show that the record represents the last 3850 cal yr bp from 3850 to 2700 cal yr bp sandy deposits low carbon content absence of diatoms and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system erosive event s removed part of the sediment record and we observed a hiatus representing 700 years after that the basin became more isolated from the river drainage system from 2010 to 1430 cal yr bp mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record benthic and littoral benthic diatom species indicate a shallow water body and a stable water chemistry mangrove forest was close to the lake apparently growing along the close by inlets regionally the main vegetation elements were palms and taxa in the families of moraceae urticaceae melastomataceae fabaceae and from a number of other families and genera characteristic of tropical lowland rain forest from 1430 to 810 cal yr bp the river impact gradually diminished each fluvial event that affected the local forest is shown in the pollen record by an expansion of cecropia dominated pioneer forest decreasing intensity of forest disturbance coincides with an increase in the diversity of fossil pollen taxa possibly reflecting an increasing plant diversity of the forest mangrove pollen declined indicating that the coastline moved seaward and suggesting tectonic uplift of the coastal area between 810 and 580 cal yr bp mangrove forest was closer to the lake again reflecting an inland migration of the coastline suggesting tectonic subsidence from 580 to 300 cal yr bp the last fluvial events were recorded diatom associations indicate oligotrophic and acidic water the mangrove belt moved seaward again suggesting tectonic uplift palms and cecropia became more abundant suggesting increased human impact in the near shore lowland forest during the last 300 years stable lacustrine conditions and lowland rain forest with the highest floral diversity is registered

holocene paleolimnological reconstruction of a high altitude colombian tropical lake abstract aquatic environments in the high andean may be quite vulnerable to climatic conditions and hydrological processes precipitation evaporation that have the potential to alter water levels and chemistry this study reconstructs the paleohydrological history of lake tota colombia to provide a record of environmental changes in the eastern flank of the colombian andes during the late holocene a 54 cm core was collected at the margin of the lake at a depth of 1 5 m and the paleohydrological reconstructions were based on changes in diatom assemblages granulometry and bulk organic geochemical parameters the core was subdivided into 5 biozones by assessing the significance of the variance among the coniss clusters biozone t5 base 4000 calibrated years bp was dominated by the planktonic diatom discostella stelligera and was deposited in a period of high water level sedimentation during this period was rich in carbon due to phytoplanktonic production the granulometry data suggests that the lake was under high to moderate hydrological dynamic conditions indicating active deposition at the river delta front biozone t4 beginning at 3600 cal yrs bp was dominated by three species d stelligera cocconeis placentula var lineata and fragilaria sensu lato and this association could indicate that the lake was under the influence of large fluctuations in water level this interpretation is supported by granulometry and geochemical data particularly by the low levels of total organic carbon toc in biozone t3 corresponding to 3000 cal yrs bp we found a lowering of the water level suggested by low levels of toc poor granulometric sorting and higher carbon nitrogen c n ratios suggest a macrophyte influence on the sedimentation avoiding direct deltaic influence in this biozone the benthic diatom f sensu lato group was dominant in biozone t2 starting 2400 cal yrs bp the proxies indicate that the lake experienced increasing water levels and that stronger circulation patterns occurred in the water column than in the preceding periods as inferred by the presence of aulacoseira granulata a ticoplanktonic species the sediment in this biozone is well sorted and finer indicating a greater depth biozone t1 at the top of the core and since 2000 cal yrs bp to the present evidence for a recent reduction in water level of lake tota can be deduced by the presence of achnanthidium minutissimum var minutissimum

global systematics of arc volcano position global systematics in the location of volcanic arcs above subduction zones are widely considered to be a clue to the melting processes that occur at depth and the locations of the arcs have often been explained in terms of the release of hydrous fluids near the top of the subducting slab see for example refs 3 6 grove et al conclude that arc volcano location is controlled by melting in the mantle at temperatures above the water saturated upper mantle solidus and below the upper limit of stability of the mineral chlorite and in particular that the arc fronts lie directly above the shallowest point of such melt regions in the mantle here we show that this conclusion is incorrect because the calculated arc locations of grove et al are in error owing to the inadequate spatial resolution of their numerical models and because the agreement that they find between predicted and observed systematics arises from a spurious correlation between calculated arc location and slab dip a more informative conclusion to draw from their experiments is that the limits of chlorite stability figure 1b of ref 7 cannot explain the global systematics in the depth to the slab beneath the sharply localized arc fronts

objective extraction of channel heads from high resolution topographic data fluvial landscapes are dissected by channels and at their upstream termini are channel heads accurate reconstruction of the fluvial domain is fundamental to understanding runoff generation storm hydrology sediment transport biogeochemical cycling and landscape evolution many methods have been proposed for predicting channel head locations using topographic data yet none have been tested against a robust field data set of mapped channel heads across multiple landscapes in this study four methods of channel head prediction were tested against field data from four sites with high resolution dems slope area scaling relationships two techniques based on landscape tangential curvature and a new method presented here which identifies the change from channel to hillslope topography along a profile using a transformed longitudinal coordinate system our method requires only two user defined parameters determined via independent statistical analysis slope area plots are traditionally used to identify the fluvial hillslope transition but we observe no clear relationship between this transition and field mapped channel heads of the four methods assessed one of the tangential curvature methods and our new method most accurately reproduce the measured channel heads in all four field sites feather river ca mid bailey run oh indian creek oh piedmont va with mean errors of 11 7 5 and 24 m and 34 3 12 and 58 m respectively negative values indicate channel heads located upslope of those mapped in the field importantly these two independent methods produce mutually consistent estimates providing two tests of channel head locations based on independent topographic signatures

a statistical framework to quantify spatial variation in channel gradients using the integral method of channel profile analysis 1 we present a statistical technique for analyzing longitudinal channel profiles our technique is based on the integral approach to channel analysis drainage area is integrated over flow distance to produce a transformed coordinate which has dimensions of length assuming that profile geometry is conditioned by the stream power law defined as e kamsn where e is erosion rate k is erodibility a is drainage area s is channel gradient and m and n are constants the slope of a transformed profile in elevation space should reflect the ratio of erosion rate to channel erodibility raised to a power 1 n this quantity is often referred to as the channel steepness and represents channel slope normalized for drainage area our technique tests all possible contiguous segments in the channel network to identify the most likely locations where channel steepness changes and also identifies the most likely m n ratio the technique identifies locations where either erodibility or erosion rates are most likely to be changing tests on a simulated landscape demonstrate that the technique can accurately retrieve both the m n ratio and the correct number and location of segments eroding at different rates where model assumptions apply tests on natural landscapes illustrate how the method can distinguish between spurious channel convexities due to incorrect selection of the m n ratio from those which are candidates for changing erodibility or erosion rates we also show how given erosion or uplift rate constraints the method can be used to constrain the slope exponent n

ecological effects of natural hazards and human activities on the ecuadorian pacific coast during the late holocene the la tolita archeological site is located in the province of esmeraldas in the northwestern part of ecuador at the border with colombia this area comprises one of the world s wettest coastal regions with mangrove and swamps along the coast and has one of the richest tropical rainforest of the planet extending inland today this region is seriously affected by natural hazards including earthquakes deposition of thick layers of volcanic ash tsunamis and el nino flooding the region also hosted one of the most important and famous cultures of the late holocene the la tolita tumaco which dominated northern south america between 2900 and 1100 yr bp with the aim of characterizing the influence on the environment of the different factors of natural hazards climate changes and human activities we drilled a 4 m long sediment core in a swamp close to the la tolita site the record dates back to 5000 years bp multiproxy analyses of pollen microcharcoal xrf based geochemical data and geochronology were performed on the sediment to distinguish the different drivers of change at 3000 yr bp an earthquake dramatically modified the landscape elevating the ground and changing the course of the rivers in the following two millennia until 1100 yr bp raised field agricultural activities dominated the site providing evidence for an increase in the local population human activity progressively declined after 1100 yr bp with the loss of the regional influence of the tolita culture the climate remained permanently moist throughout the sediment record both the rainforest and the mangrove remained well developed and marine incursions were short and frequent

the prediction of hillslope flow paths for distributed hydrological modelling using digital terrain models the accuracy of the predictions of distributed hydrological models must depend in part on the proper specification of flow pathways this paper examines some of the problems of deriving flow pathways from raster digital terrain data in the context of hydrological predictions using topmodel distributed moisture status is predicted in topmodel on the basis of spatial indices that depend on flow path definition the sensitivity of this index to flow path algorithm and grid size is examined for the case where the surface topography is a good indicator of local hydraulic gradients a strategy for the case where downslope subsurface flow pathways may deviate from those indicated by the surface topography is described with an example application

channel network extent in the context of historical land use flow generation processes and landscape evolution in the north carolina piedmont intensive agricultural land use in the 18th to early 20th centuries on the southeastern piedmont resulted in substantial soil erosion and gully development today many historically farmed areas have been abandoned and afforested and such landscapes are an opportunity to study channel network recovery from disturbance by gullying channel initiation mapping watershed area slope relationships and field monitoring of flow generation processes are used to identify channel network extent and place it in hydrologic historical and landscape evolution context in six study areas in the north carolina piedmont 100 channel heads were mapped in fully forested watersheds revealing a channel initiation relationship of 380 as1 27 where a is contributing area m2 and s is local slope m m flow in these channels is generated by subsurface and overland flow the measured relative slope exponent is lower than expected based on literature values of 2 for forested watersheds with subsurface and overland flow suggesting that the channel network extent may reflect a former hydrological regime however geomorphic evidence of recovery in channel heads within fully forested watersheds is greater than those with present day pasture present day channel heads lie within hollows or downslope of unchanneled valleys which may be remnants of historical gullies and area slope relationships provide evidence of colluvial aggradation within the valleys channel network extent appears to be sensitive to land use change with recovery beginning within decades of afforestation channel initiation mapping and area slope relationships are shown to be useful tools for interpreting geomorphic effects of land use change copyright 2012 john wiley sons ltd

a process based model for colluvial soil depth and shallow landsliding using digital elevation data a model is proposed for predicting the spatial variation in colluvial soil depth the results of which are used in a separate model to examine the effects of root strength and vertically varying saturated conductivity on slope stability the soil depth model solves for the mass balance between soil production from underlying bedrock and the divergence of diffusive soil transport this model is applied using high resolution digital elevation data of a well studied site in northern california and the evolving soil depth is solved using a finite difference model under varying initial conditions the field data support an exponential decline of soil production with increasing soil depth and a diffusivity of about 50 cm 2 yr the predicted pattern of thick and thin colluvium corresponds well with field observations soil thickness on ridges rapidly obtain an equilibrium depth which suggests that detailed field observations relating soil depth to local topographic curvature could further test this model bedrock emerges where the curvature causes divergent transport to exceed the soil production rate hence the spatial pattern of bedrock outcrops places constraints on the production law the infinite slope stability model uses the predicted soil depth to estimate the effects of root cohesion and vertically varying saturated conductivity low cohesion soils overlying low conductivity bedrock are shown to be least stable the model may be most useful in analyses of slope instability associated with vegetation changes from either land use or climate change although practical applications may be limited by the need to assign values to several spatially varying parameters although both the soil depth and slope stability models offer local mechanistic predictions that can be applied to large areas representation of the finest scale valleys in the digital terrain model significantly influences local model predictions this argues for preserving fine scale topographic detail and using relatively fine grid sizes even in analyses of large catchments

using hilltop curvature to derive the spatial distribution of erosion rates 1 erosion rates dictate the morphology of landscapes and therefore quantifying them is a critical part of many geomorphic studies methods to directly measure erosion rates are expensive and time consuming whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents if hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates in this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high resolution 1 m digital elevation data published and new cosmogenic radionuclide analyses in the feather river basin california suggest that erosion rates vary by over an order of magnitude 10 to 250 mm kyr 1 hilltop curvature increases with erosion rates allowing calibration of the hillslope sediment transport coefficient which controls the relationship between gradient and sediment flux having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature additionally we show that hilltop curvature continues to increase with rising erosion rates after gradient limited hillslopes have emerged hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient providing soil production on hilltops can keep pace with erosion finally hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels