avulsion processes at the terminus of low gradient semi arid fluvial systems lessons from the rio colorado altiplano endorheic basin bolivia abstract the rio colorado dryland river system in the southeast of the endorheic altiplano basin bolivia terminates on a very flat coastal plain at the edge of the salar de uyuni the world s largest salt pan with an area of ca 12 500 km 2 since the pleistocene the basin has experienced several lake expansion and contraction cycles in response to wetter and drier climate periods respectively at present the basin is in a dry climate period which results in a lake level lowstand and progradation of fluvial systems such as the rio colorado onto the former lake bottom the present field study of the terminus of the rio colorado shows that the river experiences a gradual downstream decrease of bankfull width and depth this bankfull decrease is caused by the combined effects of 1 extremely low gradient of the lake bottom and hence loss of flow energy and 2 downstream transmission losses due to high evaporation potential and river water percolation through the channel floor peak water discharge in seasonal short duration rain periods causes massive overbank flooding and floodplain inundation on satellite images the morphology of the river terminus has a divergent pattern and resembles a network of coeval sinuous distributary channels however field observations show that only one channel is active at low flow stage and at high flow stage an abandoned partially infilled channel may be active as well the active channel at its termination splits into narrow and shallow anastomosing streams before its demise on the lacustrine coastal plain the rest of the channels which form the divergent network are older sediment filled abandoned sinuous river courses with multiple random avulsion points these channel deposits together with extensive amalgamated crevasse splay deposits form an intricate network of fluvial sand deposits successive stages of progressively deeper crevasse channel incision into the floodplain are the result of waning stage return flow of floodwaters towards the main channel this results in increased crevasse channel width depth and length and redeposition of eroded lacustrine coastal plain sediment at the junction of the crevasse channel with the present day river a sedimentary model is proposed in which deep river bank incision by return flow in crevasse channels creates the preferential locus for river avulsion in a lowstand systems tract with little vertical accommodation increase the recurring avulsed river paths result in a thin but laterally extensive network of amalgamated channel fill point bar crevasse channel and crevasse splay deposits the resulting sedimentary architecture could easily be mistaken as produced by a distributary system of simultaneously active fluvial channels

climate and lake level history of the northern altiplano bolivia as recorded in holocene sediments of the rio desaguadero abstract strata exposed in terraces and modern cutbanks along the rio desaguadero contain a variety of lithofacies that were deposited in four distinct facies associations these facies associations document a history of aggradation and downcutting that is linked to holocene climate change on the altiplano braided stream meandering stream deltaic and shoreline and lacustrine sediments preserved in multi level terraces in the northern rio desaguadero valley record two high water intervals one between 4500 and 3900 yr bp and another between 2000 and 2200 yr bp these wet periods were interrupted by three periods of fluvial downcutting centered at approximately 4000 yr bp 3600 yr bp and after 2000 yr bp braided river sediments preserved in a single terrace level in the southern rio desaguadero valley record a history of nearly continuous fluvial sedimentation from at least 7000 yr bp until approximately 3200 yr bp that was followed by a single episode post 3210 yr bp of downcutting and lateral migration the deposition and subsequent fluvial downcutting of the northern strata was controlled by changes in effective moisture that can be correlated to holocene water level fluctuations of lake titicaca the deposition and dissection of braided stream sediments to the south are more likely controlled by a combination of base level change and sediment input from the rio mauri

evaluation of infrasound signals from the shuttle atlantis using a large seismic network inclement weather in florida forced the space shuttle atlantis to land at edwards air force base in southern california on june 22 2007 passing near three infrasound stations and several hundred seismic stations in northern mexico southern california and nevada the high signal to noise ratio broad receiver coverage and atlantis positional information allow for the testing of infrasound propagation modeling capabilities through the atmosphere to regional distances shadow zones and arrival times are predicted by tracing rays that are launched at right angles to the conical shock front surrounding the shuttle through a standard climatological model as well as a global ground to space model the predictions and observations compare favorably over much of the study area for both atmospheric specifications to the east of the shuttle trajectory there were no detections beyond the primary acoustic carpet infrasound energy was detected hundreds of kilometers to the west and northwest nw of the shuttle trajectory consistent with the predictions of ducting due to the westward summer time stratospheric jet both atmospheric models predict alternating regions of high and low ensonifications to the nw however infrasound energy was detected tens of kilometers beyond the predicted zones of ensonification possibly due to uncertainties in stratospheric wind speeds

early to middle holocene transition in the pastos chicos record dry puna of argentina abstract this paper presents a multi proxy record and its paleoenvironmental interpretation from pastos chicos 23 40 29 s 66 25 32 w 3781 m asl susques jujuy province argentina the analysis includes a study of fluvial sediments and geomorphology and the contained diatoms and pollen record at centenary resolution two main environmental phases characterize the 9000 4200 bp interval the first phase was a humid period between 9000 and 7300 bp circa 10 000 8100 cal bp which showed organic soils formation in a floodplain broadly vegetated by grasses with a high and stable water table level the second phase developed between 7300 and 6000 bp 8100 6800 cal bp and showed a moderately drier environment interrupted by punctuated humid events such as those around 7000 and 6300 bp this environment could be interpreted as a permanently existing meandering river with a floodplain with shallow oxbow lakes bordered by a diverse shrub steppe after 6000 bp the fluvial system turned into a braided river situated in a sandy floodplain with dunes and ephemeral ponds at the end of the sequence i e post 4200 bp salt crusts developed and the river lowered its base level by 8 m these results seem to show that the pastos chicos river basin evolved from low energy and more humid conditions established during the early and the first part of the middle holocene to drier ones in a system with more energy during some events of the second part of the latter period the start of this aridization process at 6000 bp in the basin seems to be substantially later than other observations in local records of the andean area this could be due to physiographic control of the moisture by the catchment area 1000 km2 that generates a catchment averaged regional scale signal of smaller amplitude which is delayed for individual tributaries these studies with other ongoing analysis will improve the accuracy of the models of resource structure in the area in several space time scales and consequently advance knowledge concerning the organizational pattern of human societies in the past

climate change and mass movements in the nw argentine andes abstract the chronology of multiple landslide deposits and related lake sediments in the eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary between 35 000 and 25 000 14 c yr bp and after 5000 14 c yr bp the older cluster may correspond to the minchin wet period 40 000 and 25 000 14 c yr bp identified in tropical and subtropical south america suggesting a causal relation between enhanced landslide activity and climate change the younger cluster predates the titicaca wet period that began at about 3900 14 c yr bp which also affected other regions in the andes and the amazon basin no landslide and associated lake sediments are documented during the tauca wet period between 16 000 and 8000 14 c yr bp however the two clusters correspond to periods where it assumed that the el nino southern oscillation enso and tropical atlantic sea surface temperature dipole tad were active the analysis of the present day precipitation patterns in nw argentina indicates significant spatial and temporal differences between the intra andean part of the study area and the andean foreland whereas the tad seems to consistently increase rainfall the intensity of precipitation during the el nino phase of the enso is reduced to only 25 of the mean annual average in the intra andean basins whereas the regions east of the andes receive more than 125 similar results but with an opposite sign characterize la nina events the comparison of this pattern with paleo precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and therefore increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds

application of strontium isotopes to understanding the hydrology and paleohydrology of the altiplano bolivia peru abstract strontium concentrations and strontium isotopic ratios were measured in natural waters and carbonate sediments from throughout the bolivian and peruvian altiplano in order to improve hydrologic and paleohydrologic mass balances with the ultimate goal of better understanding the paleoclimatic history of the central andes rivers flowing into lake titicaca have a wide range of strontium isotopic ratios that exhibit spatial patterns consistent with the lithologies of the different drainage basins because of the limited exchange of water between the two main sub basins of lake titicaca lago grande and lago huinaimarca and between the sub basins of lago huinaimarca there are significant differences in strontium isotopic ratios between the sub basins calculated elemental balances of strontium in lake titicaca are in reasonable agreement with previously published budgets of water and major elements however the strontium isotopic budget indicates that the lake is not in isotopic steady state this also implies that the major element budgets are unlikely to be in steady state lake titicaca had a higher than modern strontium isotopic ratio during the early and middle holocene elevated values persisted in lago grande until at least 2000 cal yr bp consistent with other evidence that modern hydrologic conditions namely overflow were not established until that time an isotopic budget calculated for late pleistocene paleolake tauca in the central altiplano suggests that between 70 and 83 of its riverine inputs were derived from lake titicaca overflow this calculated flow represents about a 30 fold increase over the average discharge of the modern rio desaguadero at desaguadero or a seven fold increase over its average discharge into lago poopo the strontium isotopic budget if complete precludes the possibility that decreased evaporation alone could have accounted for the existence of paleolake tauca

nature of environmental changes in south america at the last glacial maximum abstract a review is given of geomorphological evidence suggesting the nature of environmental changes in south america during the last glaciation maximum lgm the data are from glacial geocryogenic alluvial colluvial and aeolian systems and span 68 of latitude making the most complete land transect of major climatic systems in the southern hemisphere glaciers in the northern and central andes may have reached their maximal extent by 27 000 yr b p reduced precipitation at the lgm caused by lower temperatures and lower atmospheric humidity probably led to slight glacier recession in the tropical andes icefields in the southern andes were most expansive when global temperature and sea level were lowest because pacific westerlies supplied abundant moisture and western outlets advancing across the continental shelf were constrained by sea level low gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the lgm draw down of water tables possibly impacted the forest cover enhancing the drying influence of reduced sea surface temperature and atmospheric humidity as forest and grass covers diminished in extent effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation colluvial and aeolian processes became more active and widespread almost 25 of the continent is covered with palaeo aeolian features presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the lgm substantial changes in precipitation totals and incidence and in ground water availability probably eliminated modern type tropical rainforest except in areas currently receiving ca 5000 mm of annual rainfall equivalent changes in vegetation cover spread across much of the guyana and brazilian highlands and throughout the southern temperate regions

misr based passive optical bathymetry from orbit with few cm level of accuracy on the salar de uyuni bolivia abstract we demonstrate that under ideal circumstances passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters our target area is the salar de uyuni in bolivia it is a large active salt flat or playa which is maintained as an almost perfectly level and highly reflective surface by annual flooding to a mean depth of 30 50 cm we use misr data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle we use a single icesat laser altimetry profile to calibrate our water depth model though the salt surface is probably the smoothest surface of its size on earth with less that 30 cm rms height variations over an area of nearly 104 km2 it is not completely featureless topography there includes a peripheral depression or moat around the edge of the salt and several sets of prominent parallel ridges with 5 km wavelength and 30 cm amplitude the process by which these features form is still not well characterized

a 25 000 year record of earthquakes on the owens valley fault near lone pine california implications for recurrence intervals slip rates and segmentation models seven trenches in eastern california across the owens valley fault near lone pine expose two episodes of faulting since early holocene time in the form of 1 m throw in lacustrine beds with liquefaction that were buried and then faulted again 1 m by the m 7 5 to 7 75 a d 1872 owens valley earthquake geomorphic maps applications of sequence stratigraphy and analyses of radiocarbon from charcoal and tufa deposits indicate that the paleoearthquake the penultimate event here occurred between 10 200 200 and 8800 200 cal yr b p the cumulative vertical displacement from these last two earthquakes in three trenches averages 2 4 0 3 m 2 and the penultimate event has slightly larger displacements a synthesis of available data indicates that the antepenultimate event was probably as large and occurred between ca 24 000 and 14 000 cal yr b p 2 thus the two interseismic intervals between the last three surface faulting earthquakes on the southern owens valley fault are each 10 000 yr this 25 000 year record indicates that the two event normal oblique slip rate on the owens valley fault near lone pine is 1 0 0 5 m k y this result is similar to that of several previous geological studies here yet it is still slower than slip rates on the northern owens valley fault and several factors slower than contemporary geodetic measurements this study attempts to account for different dating methods and interpretational uncertainties to acknowledge how little is known about the slip history of the owens valley fault and adjacent faults and to consider the role of segmentation as well as splay and distributed faulting in comparisons of displacement data among different sites along the entire 100 10 km length of the owens valley fault

parabolic halite dunes on the salar de uyuni bolivia abstract palaeoenvironmental interpretation in desert and evaporitic sediments is often ambiguous consequently the recognition of primary sedimentary structures that indicate subaerial exposure or shallow marine conditions are therefore of crucial importance when interpreting marginal sedimentary evaporite facies parabolic halite dunes described here for the first time are observed on the salt crust of the largest salt flat in the world the salar de uyuni in bolivia the halite dunes are relatively small 0 7 1 3 m in width and length well defined lobate to hemicyclic parabolic dunes and are formed directly on the salt crust a model describing the formation and migration of the dunes in relation to brine level and climatic conditions is presented

high resolution seismic reflection profiles from lake titicaca peru bolivia evidence for holocene aridity in the tropical andes high resolution seismic reflection profiles of the sediments of lake titicaca peru bolivia suggest that lake levels in the recent past were considerably lower than today incised channels on the major deltas extend to depths of 85 m below modern lake level erosional truncation of onlapping seismic reflectors is found at similar depths this interpretation of the seismic data is supported by analyses of sediment cores from the lake which indicate that there was a significant drop in lake level during the early to mid holocene

multiple landslide clusters record quaternary climate changes in the northwestern argentine andes the chronology of multiple landslide deposits and related lake sediments in the semi arid eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary i e between 40 and 25 and after 5 14 c kyr bp these clusters may correspond to the minchin maximum at around 28 27 14 c kyr bp and titicaca wet periods after 3 9 14 c kyr bp the more humid conditions apparently caused enhanced landsliding in this environment in contrast no landslide related damming and associated lake sediments occurred during the coipasa 11 5 10 14 c yr bp and tauca wet periods 14 5 11 14 c yr bp the two clusters at 40 25 and after 5 14 c kyr bp may correspond to periods where the el nin o southern oscillation enso and tropical atlantic sea surface temperature variability tav were active this however was not the case during the coipasa and tauca wet periods lake balance modelling of a landslide dammed lake suggests a 10 15 increase in precipitation and a 3 4 c decrease in temperature at v30 14 c kyr bp as compared to the present in addition time series analysis reveals a strong enso and tav during that time the landslide clusters in northwestern argentina are therefore best explained by periods of more humid and more variable climates 2003 elsevier science b v all rights reserved

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

salinities and sediment transport in the bolivian highlands guyot j l roche m a noriega l calle h and quintanilla j 1990 salinities and sediment transport in the bolivian highlands j hydrol 113 147 162 salinities and sediment loads of the rivers of the entire interior drainage basin of the lake titicaca rio desaguadero lake poopo and salars as well as their evolution from upstream to downstream have been characterized on the basis of the results of four samplings during runoff periods on the other hand the regimes of dissolved and suspended matter transport are characterized with perioclic measurements and samplings at hydrometrical stations over several years the junction of lake poopo and the salar of coipasa since 1985 is noted which recalls that an unique terminal lake of large area existed during some parts of the quaternary mechanical and chemical erosion rates have been calculated for the basins that drain the western rio mauri and eastern rio suches cordilleras of the bolivian andes most of the sediment comes from the western cordillera with a mechanical erosion rate of 640 t kmz yr i for the basin of the rio mauri on the other hand lake titicaca is the main source of dissolvea matter to lake poopo via the rio desaguadero

dryland anabranching river morphodynamics rio capilla salar de uyuni bolivia the dryland anabranching river rio capilla is characterized by nonvegetated and vegetated reaches with prominent channel morphology to identify the morphodynamics of such dryland anabranching systems and their controls we investigated the rio capilla of the southern altiplano plateau using high resolution satellite imagery and field measurements comparison of high resolution satellite data reveals that erosion exceeds deposition for the main channel accompanied by changes in channel planform such as meander and channel morphology on site surveys combined with high precision gps and high resolution satellite imagery show that channels are characterized by shallowness and poor development of levees the study area of the rio capilla is divided into two zones of different slopes zone 1 with a high slope and zone 2 with a low slope zone 1 has a relatively straight main channel with few anabranches and grass covered banks that are stable despite the high gradient whereas zone 2 is typified by more anabranches with nonvegetated banks and the main channel experiences prominent bank accretion and erosion excavations show that point bar deposits are fine sand dominated in two reaches and that river banks primarily consist of silt and clay the limited vegetation cover and abundance of desiccation cracks and macropores make the river bank more erodible leading to pronounced lateral migration in this low gradient dryland river system shallow channels and poor development of levees in combination with in channel accretionary benches result in frequent overbank flooding which results in a high density of crevasse splays over unconsolidated river banks and accretionary benches connection of headcuts and crevasse channels together with lateral migration and chute channels and reactivation of partially abandoned meanders produces an anabranching pattern in such dryland river systems

a late quaternary diatom record of tropical climatic history from lake titicaca peru and bolivia abstract a composite high resolution diatom stratigraphy from three piston cores and one box core in the deep sub basin of lake titicaca reveals large moisture variations during the past 30 kyr in the altiplano region diatom sequences indicate orbital and millennial scale variability in water level and salinity the pelagic freshwater diatom species cyclotella andina and cyclotella stelligera dominate glacial age sediments suggesting that the lake was above its present outlet generally wet conditions continued until 11 000 cal yr bp as indicated by high percentages of freshwater planktonic diatoms large pulses of benthic diatom species between about 11 000 and 10 000 cal yr bp suggest brief intervals of large amplitude declines in lake level during the early holocene 10 000 8500 cal yr bp a freshwater diatom assemblage suggests overflowing conditions pelagic freshwater diatoms are replaced ca 8500 cal yr bp by the salinity indifferent species cyclotella meneghiniana and by benthic taxa indicating the beginning of lake regression during the mid holocene 6000 3500 cal yr bp the abundance of the saline taxon chaetoceros muelleri coupled with high abundances of epiphytic and epipelic diatoms indicates maximum salinity and lowest lake levels in the entire 30 000 year record lake transgression began ca 4000 cal yr bp and the lake achieved modern levels by about 1500 cal yr bp these water level changes imply changes in effective moisture most likely resulting from large precipitation changes precipitation was high throughout the last glacial maximum 21 000 18 000 cal yr bp likely due to an enhanced south american summer monsoon during peak summer insolation in the southern hemisphere in contrast the mid holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon

buoyancy arrest and shelf ocean exchange when steady flow in a stratified ocean passes between the continental slope and open ocean its ability to cross isobaths is potentially limited by buoyancy arrest if the bottom ekman transport vanishes and there are no interior stresses then steady linear flow on an f plane must be geostrophic and follow isobaths exactly the influence of arrest on cross shelf transport is investigated here to establish 1 whether there are substantial penetration asymmetries between cases with upwelling and downwelling in the bottom boundary layer 2 over what spatial scales hence in what parameter regime buoyancy arrest is important and 3 the effects of depth dependent interior flow the problem is approached using scalings and idealized numerical models the results show that there is little or no asymmetry introduced by bottom boundary layer behavior further if the stratification is weak or moderate as measured by a slope burger number s 5 an f where a is the bottom slope n is buoyancy frequency and f is the coriolis parameter buoyancy arrest does not exert a strong constraint on cross isobath exchange

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

climatic influence on sedimentology and geomorphology of the rio ramis valley peru abstract fluctuations in regional precipitation and in base level the level of lake titicaca trigger changes in fluvial erosion and deposition in the largest tributary valley of lake titicaca the rio ramis peru relationships between fluvial sedimentation and terrace formation demonstrate that large scale aggradational and downcutting events are associated with specific regional climatic events that have been previously documented in the lake titicaca basin and elsewhere on the altiplano five laterally extensive terrace tracts a through e are present within the valley downcutting of the ramis valley is correlative with base level lake titicaca level fall caused by a decrease in precipitation aggradation is correlative with base level rise during increasing precipitation sedimentary facies underlying the terraces record deposition in a large paleolake followed by deposition of both meandering and braided fluvial sediments the highest terrace in the valley e is underlain by a thick sequence of lacustrine strata that was deposited during an extremely wet period in the lake titicaca region that terminated at about 40 000 cal yr bp these lacustrine strata were subsequently downcut likely at a time of large drop in lake level resulting in a deeply incised valley the main phase of fluvial aggradation in the valley sediments underlying the d through b terraces was slow spanning about 25 000 years it occurred before during and after the last glacial maximum from before 35 140 790 cal yr bp until about 10 421 181 cal yr bp a time period that was mostly characterized in the lake titicaca basin by high precipitation rates and high base levels the aggradation resulted in almost 40 m of sediment accumulation in the valley and was followed by rapid about 2 000 years but episodic incision into the underlying fluvial sediments to form the c and b terraces as well as the erosional surface beneath the modern fill terrace a this erosional phase corresponded to several periods of local base level lake titicaca lowering between 10 241 181 and 1 668 76 cal yr bp the modern fill terrace has aggraded about 2 m above modern river level likely within the last 1 500 years

low holocene level 7700 to 3650 years ago of lake titicaca bolivia abstract preliminary results of sedimentological and paleontological analysis of two cores taken in lake titicaca bolivia indicate that between 7700 and 3650 yr b p the lake level was up to 50 m lower than at present evaporitic deposits occurred at the maximum low lake level

digital elevation models for topographic characterisation and flood flow modelling along low gradient terminal dryland rivers a comparison of spaceborne datasets for the rio colorado bolivia abstract many dryland rivers are terminal systems with small channels undergoing prominent downstream size reductions before ending on channelless floodplains in wetlands or at playa margins spaceborne digital elevation models dems provide potential for assessing subtle topographic and hydrodynamic changes in these low gradient low relief settings but challenges are posed by limitations in vertical and horizontal accuracy this study evaluates the use of different spaceborne dems for topographic characterisation and flood flow modelling of the low gradient

on the origin of the bolivian high and related circulation features of the south american climate abstract the climatological structure in the upper tropospheric summertime circulation over south america is diagnosed using a gcm with and without south american topography a linear model and observational data emphasis is placed on understanding the origin of observed features such as the bolivian high and the accompanying nordeste low to the east results from the linear model indicate that these two features are generated in response to precipitation over the amazon basin central andes and south atlantic convergence zone with african precipitation also playing a crucial role in the formation of the nordeste low the direct mechanical and sensible heating effects of the andes are minimal acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central andes in the gcm the effects of the andes include a strengthening of the bolivian high and northward shift of the nordeste low primarily through changes in the precipitation field the position of

late quaternary palaeolakes rivers and wetlands on the bolivian altiplano and their palaeoclimatic implications drill cores of sediments from the rio desaguadero valley bolivia provide new information about the climate of tropical south america over the past 50 000 years the modern rio desaguadero is fed by lake titicaca overflow and by local tributaries in the wetter northern altiplano and discharges into lake poopo in the more arid central altiplano during the late quaternary the rio desaguadero valley was the site of several generations of palaeolakes and wetlands that formed during periods of increased precipitation and local runoff augmented by increased overflow from lake titicaca sediments recovered by drilling in eight localities along the 390 km long valley of the rio desaguadero yield a regional history of lacustrine sedimentation and effective precipitation lacustrine strata in the drill cores record 12 distinct wet periods in the past 50 000 years four of these wet periods resulted in the formation of major palaeolakes in the rio desaguadero valley during the last glacial maximum from before 20 000 to 16 000 cal yr bp during the late glacial from about 14 000 to 12 000 cal yr bp in the early holocene from about 10 000 to 7900 cal yr bp and in the late holocene from 4500 cal yr bp to present the period that appears to have been most arid was between 7900 and 4500 cal yr bp the altiplano wet periods were generally synchronous with north atlantic cold events respectively the last glacial maximum the younger dryas the 8200 cal yr bp event and the neoglacial implying a relationship between past precipitation variability in tropical south america and north atlantic sea surface temperature copyright 2005 john wiley sons ltd

william river an outstanding example of channel widening and braiding caused by bed load addition the lower william river in northwestern saskatchewan canada presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load a relatively narrow and deep single channel stream as it flows northward to lake athabasca the river picks up a 40 fold increase of bed load over a 27 km reach as it encounters a large dune field just south of the lake as a result of the large infusion of eolian sand the channel develops a thoroughly braided pattern while undergoing a 5 fold increase in width and a 10 fold increase in width depth ratio

holocene vegetation history from fossil rodent middens near arequipa peru rodent abrocoma lagidium phyllotis middens collected from 2350 to 2750 m elevation near arequipa peru 16 s provide an 9600 yr vegetation history of the northern atacama desert based on identification of 50 species of plant macrofossils these midden floras show considerable stability throughout the holocene with slightly more mesophytic plant assemblages in the middle holocene unlike the southwestern united states rodent middens of mid holocene age are common in the arequipa area the midden record does not reflect any effects of a mid holocene mega drought proposed from the extreme lowstand 100 m below modern levels 6000 to 3500 yr b p of lake titicaca only 200 km east of arequipa this is perhaps not surprising given other evidence for wetter summers on the pacific slope of the andes during the middle holocene as well as the poor correlation of summer rainfall among modern weather stations in the central andes atacama desert the apparent difference in paleoclimatic reconstructions suggests that it is premature to relate changes observed during the holocene to changes in el nino southern oscillation modes

vegetation invasions into absolute desert a 45 th000 yr rodent midden record from the calama salar de atacama basins northern chile lat 22 24 s plant macrofossils percentage abundance of grass taxa fecal pellet d13 c and plant cuticle contents from 49 fossil rodent middens dated by 14 c record changes in local vegetation and precipitation since 45 ka calibrated or measured to thousands of calendar years before present in the central atacama desert lat 228 248s of northern chile the midden sites are along the hyperarid upper margin 2400 3100 m of the absolute desert in an extreme environment sparsely vegetated by annual herbs and halophytic shrubs conditions between 40 and 22 ka may have been at least intermittently dry and possibly cooler as implied by four middens with low species richness we infer a large increase in summer rainfall between 16 2 and 10 5 ka on the basis of the lowering of steppe grasses by as much as 1000 m prominence of c4 grasses and summer annuals high species richness and displacement of northern species at least 50 km south of their modern ranges the precipitation increase was greatest for a cluster of middens between 11 8 and 10 5 ka abrupt drying evident in a dramatic decrease in grass abundance occurred after 10 5 ka at all four midden localities increased percentages of