valley fill alluviation during the little ice age ca a d 1400 1880 paria river basin and southern colorado plateau united states valley fill alluvium deposited from ca a d 1400 to 1880 is widespread in tributaries of the paria river and is largely coincident with the little ice age epoch of global climate variability previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern colorado plateau the alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca a d 1200 1400 and 1860 1910 respectively the fill forms a terrace in the axial valleys of major through flowing streams this terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of hillslopes of sparsely vegetated weakly consolidated bedrock suggesting that eroded bedrock was an important source of alluvium along with in channel and other sources paleoclimatic and high resolution paleoflood studies indicate that valley fill alluviation occurred during a long term decrease in the frequency of large destructive floods aggradation of the valleys ended about a d 1880 if not two decades earlier with the beginning of historic arroyo cutting this shift from deposition to valley entrenchment near the close of the little ice age generally coincided with the beginning of an episode of the largest floods in the preceding 400 500 yr which was probably caused by an increased recurrence and intensity of flood producing el nino events beginning at ca a d 1870

late quaternary arroyo formation and climate change in the american southwest arroyos entrenched ephemeral streams that form in desert environments first appeared in the arid and semiarid american southwest after 8000 14 c yr b p for at least 7 k y prior to that time climate vegetation and groundwater conditions were not conducive for arroyo formation along the floors of desert valleys after a hiatus in arroyo formation the frequency of arroyo cutting and filling increased dramatically after 4000 14 c yr b p the early holocene arroyos and increased frequency of arroyo incision after 4000 14 c yr b p are related to the establishment and changes in postglacial vegetation climate and groundwater conditions as a result arroyo sequences preserve a record of large scale climate change and small scale climatic perturbations that occurred during the holocene human modification of valley flood plains is an additional factor that contributed to mid nineteenth and early twentieth century arroyo cutting

the relation of holocene fluvial terraces to changes in climate and sediment supply south fork payette river idaho abstract well preserved holocene terraces along the south fork payette river in central idaho provide a record of fluvial system behavior in a steep mountain watershed characterized by weathered and erodible idaho batholith granitic rocks terrace deposit ages were provided by 14c dating of charcoal fragments and optically stimulated luminescence osl dating of sandy sediments along with pairing of many terrace tread heights these data indicate episodic downcutting during the holocene with a mean incision rate of 0 9 m ka from 7 ka to present prior to 7 ka the river incised to within 3 m of current bankfull but then aggraded by 5 m over at least a 10 km long reach in an episode centered 7 6 ka aggradation may relate to 1 increased hillslope sediment input from landslides and debris flows in steep tributary basins with abundant grussified granitic bedrock 2 possible local landslide damming of the channel 3 decreased peak discharge or 4 a combination of these factors middle holocene channel aggradation ca 7 6 ka corresponds with a period of prolonged and widespread aridity in the northern rocky mountains between 5 and 1 3 ka the river aggraded slightly and then remained stable forming a prominent terrace tread at 3 m above current bankfull modest aggradation to vertical stability of the south fork payette river at the 1 5 m terrace level 1 0 0 7 ka corresponds with large fire related debris flows in tributaries during medieval droughts three intervals of incision 5 5 5 ka 1 3 1 0 ka and 0 5 ka correspond with frequent but small fire related sedimentation events and generally cooler wetter conditions suggesting increased snowmelt runoff discharges other possible drivers of channel incision include an increase in stochastic or climate modulated large storms and floods and a reduction in delivery of hillslope sediment to the channel aggradation is more confidently tied to climate through increases in hillslope sediment delivery and or decreased stream power both likely related to warmer drier conditions including high severity fires that reduce snowmelt and decrease vegetation cover on steep slopes thus the holocene terraces of the south fork payette river do not reflect simple stepwise incision with periods of vertical stability and lateral migration but record substantial episodes of aggradation as well we infer that increases in hillslope erosion and mass movements combined with reduced discharges during prolonged droughts episodically reverse the post glacial trend of downcutting in particular during the middle holocene the present bedrock dominated channel implies a strong tendency toward incision in the late holocene

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

late quaternary environmental change in the bonneville basin western usa abstract excavation and analyses of small animal remains from stratified raptor deposits spanning the last 11 5 ka together with collection and analysis of over 60 dated fossil woodrat midden samples spanning the last 50 ka provide a detailed record of changing climate in the eastern great basin during the late pleistocene and holocene sagebrush steppe dominated the northern bonneville basin during the full glacial suggesting that conditions were cold and relatively dry in contrast to the southern basin which was also cold but moister limber pine woodlands dominated 13 11 5 ka indicating increased dryness and summer temperatures 6 7 c cooler than present this drying trend accelerated after 11 5 ka causing lake bonneville to drop rapidly eliminating 11 species of fish from the lake from 11 5 8 2 ka xerophytic sagebrush and shadscale scrub replaced more mesophilic shrubs in a step wise fashion a variety of small mammals and plants indicate the early holocene was 3 c cooler and moister than at present not warmer as suggested by a number of climatic models the diversity of plants and animals changed dramatically after 8 2 ka as many species disappeared from the record some of the upland species returned after 4 ka and great salt lake became fresh enough at 3 4 and 1 2 ka to support populations of utah chub

quaternary palaeolakes in the evolution of semidesert basins with special emphasis on lake bonneville and the great basin u s a abstract this paper reviews attributes of quaternary lakes and lake basins which are often important in the environmental prehistory of semideserts basin floor and basin closure morphometry have set limits on palaeolake sizes lake morphometry and basin drainage patterns have influenced lacustrine processes and water and sediment loads have influenced basin neotectonics information regarding inundated runoff producing and extra basin spatial domains is acquired directly from the palaeolake record including the littoral morphostratigraphic record and indirectly by reconstruction increasingly detailed hypotheses regarding lake bonneville the largest late pleistocene palaeolake in the great basin are subjects for further testing and refinement oscillating transgression of lake bonneville began about 28 ka the highest stage occurred about 15 ka and termination occurred abruptly about 13 ka a final resurgence of perennial lakes probably occurred in many subbasins of the great basin between 11 and 10 ka when the highest stage of great salt lake successor to lake bonneville developed the gilbert shoreline and when the russell shoreline formed 500 600 km away in the carson desert of the lake lahontan basin the highest post gilbert stage of great salt lake which has been one of the few permanent lakes in the great basin during holocene time probably occurred between 3 and 2 ka a set of eighteen general observations regarding geologic aspects of the palaeolake record in semidesert basins is helping to guide quaternary studies in the great basin

lithologically influenced geomorphic responses to holocene climatic changes in the southern colorado plateau arizona a soil geomorphic and ecologic perspective abstract the semiarid landscape occupied by the navajo and hopi peoples on the colorado plateau in northeastern arizona in the southwestern united states is characterized by an extensive network of deeply incised arroyos since the early 20th century many researchers have proposed that the recent formation of arroyos in this region and also many attributes of modern vegetation communities were caused directly by overgrazing of vegetation by domestic livestock of the navajo other researchers however have proposed other causes for such features such as climatic change we believe that the landforms soils and vegetation of a small area located on antelope mesa in this region underlain by the highly erodible materials of the miocene bidahochi formation may have been more sensitive to minor climatic changes of the holocene than landscapes of massive mesozoic sandstones that dominate the colorado plateau in the antelope mesa area the presence of actively filling channels rather than arroyos in the upper parts of many drainage basins and associated soils and ecologic patterns indicate that the aggradation 1 was initiated in downstream reaches and within the past two centuries 2 may be linked to recently accelerated slope erosion and 3 is unrelated to past or ongoing grazing this suggests the ongoing aggradation may be related to recent minor climatic changes geochronologic and soil geomorphic evidence indicate that the most recent cycle of arroyo incision and filling may be a small scale analogue for larger magnitude older cycles that produced regionally recognizable paired terraces that are attributable to previous holocene climate changes we propose that climatic change and more specifically increases in precipitation caused an acceleration in the erosion of the steep typically minimally vegetated slopes of the bidahochi formation the beginning of the neoglacial period ca 2 3 ka effects of which are documented by other proxy records in this region may be the climatic change that triggered widespread deposition of a large mid holocene alluvial unit in this region alternating episodes of eolian activity and soil formation in the study area and in this general region provide independent evidence of holocene climatic changes our interpretations of the origin of the cut and fill cycles that emphasize the role of drainage basin lithology and differ significantly from previous interpretations emphasize the linkage of climatic changes groundwater levels and arroyo incision this study reveals the importance of integrating soil and ecologic studies with geomorphologic research such an approach may be critical in helping understand how anthropogenically induced climatic changes of the next century could impact geomorphic processes and the ecology of arid and semiarid regions

age and paleoclimatic significance of late holocene lakes in the carson sink nv usa abstract new dating in the carson sink at the termini of the humboldt and carson rivers in the great basin of the western united states indicates that lakes reached elevations of 1204 and 1198 m between 915 and 652 and between 1519 and 1308 cal yr b p respectively these dates confirm morrison s original interpretation lake lahontan geology of the southern carson desert professional paper 40 u s geol survey 1964 that these shorelines are late holocene features rather than late pleistocene as interpreted by later researchers paleohydrologic modeling suggests that discharge into the carson sink must have been increased by a factor of about four and maintained for decades to account for the 1204 m lake stand the hydrologic effects of diversions of the walker river to the carson sink were probably not sufficient by themselves to account for the late holocene lake level rises the decadal long period of increased runoff represented by the 1204 m lake is also reflected in other lake records and in tree ring records from the western united states

enhanced sediment delivery in a changing climate in semi arid mountain basins implications for water resource management and aquatic habitat in the northern rocky mountains the delivery and transport of sediment through mountain rivers affects aquatic habitat and water resource infrastructure while climate change is widely expected to produce significant changes in hydrology and stream temperature the effects of climate change on sediment yield have received less attention in the northern rocky mountains we expect climate change to increase sediment yield primarily through changes in temperature and hydrology that promote vegetation disturbances i e wildfire insect pathogen outbreak drought related die off here we synthesize existing data from central idaho to explore 1 how sediment yields are likely to respond to climate change in semi arid basins influenced by wildfire 2 the potential consequences for aquatic habitat and water resource infrastructure and 3 prospects for mitigating sediment yields in forest basins recent climate driven increases in the severity and extent of wildfire suggest that basin scale sediment yields within the next few years to decades could be greater than the long term average rate of 146 t km 2 year 1 observed for central idaho these elevated sediment yields will likely impact downstream reservoirs which were designed under conditions of historically lower sediment yield episodic erosional events massive debris flows that dominate post fire sediment yields are impractical to mitigate leaving road restoration as the most viable management opportunity for offsetting climate related increases in sediment yield however short term sediment yields from experimental basins with roads are three orders of magnitude smaller than those from individual fire related events on the order of 101 t km 2 year 1 compared to 104 t km 2 year 1 respectively for similar contributing areas suggesting that road restoration would provide a relatively minor reduction in sediment loads at the basin scale nevertheless the ecologically damaging effects of fine sediment material

anthropogenic strath terrace formation caused by reduced sediment retention across north america human activities have been shown to cause river incision into unconsolidated alluvium human caused erosion through bedrock however has only been observed in local and isolated outcrops here we test whether splash dam logging which decreased in stream alluvial cover by removing much of the alluvium trapping wood caused basin wide bedrock river incision in a forested mountain catchment in washington state we date incision of the youngest of four strath terraces using dendrochronology and radiocarbon to between 1893 ce and 1937 ce in the middle fork teanaway river and 1900 ce and 1970 ce in the west fork teanaway river coincident with timber harvesting and splash damming in the basins other potential drivers of river incision lack a recognized mechanism to cause t1 incision or are not synchronous with t1 incision hence the close temporal correspondence suggests that reduced sediment retention triggered by splash damming led to the observed 1 1 mm y 1 to 23 mm y 1 of bedrock river incision and reduction of the active floodplain to 20 and 53 of its preincision extent on the middle and west forks respectively the development of such anthropogenic bedrock terraces may be an emerging globally widespread physiographic signature of the anthropocene

interpreting climate modulated processes of terrace development along the colorado front range using a landscape evolution model flights of terraces that flank range fronts throughout the rocky mountains record episodic stream incision over the past 1 5 ma studies dating terraces in the denver basin along the colorado front range suggest that these high surfaces were formed during glacial intervals and were rapidly incised and abandoned during interglacials modulation of sediment supply or transport capacity associated with climate change related to glacial interglacial cycles have been suggested as possible drivers for the repeated aggradation and abandonment of these high surfaces potential mechanisms for increasing sediment supply and transport in rivers include variations over time in sediment flux from intermittently glaciated major valleys the efficiency of hillslope sediment transport and the magnitude and timing of rainfall intensity and stream flow in this study we use a landscape evolution model to determine whether any of these processes in isolation or in combination is sufficient to explain the observed rates and patterns of terrace formation and abandonment along the colorado piedmont cycles of channel aggradation and incision are apparent in the models when either mean rainfall intensity is varied or a glacial source of sediment is added the models suggest that in the absence of a large addition of sediment to the streams changes in stream power are necessary to allow channel aggradation and the planation of bedrock surfaces and increased sediment flux from hillslopes is necessary to match observations of increased denudation rates that coincide with the deposition of terrace capping gravels

interplay between tectonics climate and fluvial transport during the cenozoic evolution of the ebro basin ne iberia 1 three dimensional modeling that integrates fluvial sediment transport crustal scale tectonic deformation and lithospheric flexural subsidence is carried out to simulate the landscape and drainage evolution of the ebro sedimentary basin ne iberia the ebro basin underwent a long period of closed intramountain drainage as a result of tectonic topography generation at the pyrenees the iberian range and the catalan coastal range in the late oligocene the catalan coastal range underwent extension leading to the formation of the valencia trough nw mediterranean but the ebro basin remained closed for nearly 15 myr more before the ebro river cut through the remnants of the topographic barrier this drainage opening caused widespread basin incision that shaped spectacular outcrops of the syntectonic and posttectonic infill here we investigate the processes controlling these major drainage changes the modeling results constrained by a large data set on the tectonic and transport evolution of the area predict a closed phase characterized by a large lake in the central eastern ebro basin dry climatic conditions probably lowered the lake level and contributed together with rift flank uplift to prolong this endorheic basin stage the age and amount of reworked sediment after the opening are consistent with an onset of basin incision between 13 and 8 5 ma as a result of lake capture by escarpment erosion and lake level rise associated with sediment accumulation and wetter climatic conditions sea level changes in the mediterranean had no major impact in the large scale drainage evolution of the ebro basin index terms 1625 global change geomorphology and weathering 1824 1886 1815 hydrology erosion and sedimentation 3210 mathematical geophysics modeling 3344 meteorology and atmospheric dynamics paleoclimatology 8102 tectonophysics continental contractional orogenic belts keywords pyrenees drainage evolution lake sediment budget erosion flexure

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

development of an osl chronology for late pleistocene channel belts in the lower mississippi valley usa abstract the broad alluvial valley of the lower mississippi river contains large channel belts for which age control has been limited by the lack of organic material for radiocarbon dating and deposit ages greater than the limit of radiocarbon in order to produce a detailed chronology of fluvial evolution samples for optical dating were collected from late pleistocene channel belt deposits quartz sand was analyzed using the coarse grained single aliquot regenerative dose sar technique samples showed little evidence of partial bleaching and optical ages are for the most part consistent with the available radiocarbon and relative age constraints optical ages range from 85 to 11 ka and along with geomorphic and stratigraphic relationships indicate that the mississippi river was meandering during the end of the last interglacial 85 83 ka and switched to a braided regime by 64 50 ka during initial marine oxygen isotope stage ois 4 3 glaciation of the upper mississippi drainage basin the mississippi river switched back to a meandering regime after formation of the youngest braid belt at 11 ka the 69 optical ages presented here provide the first detailed chronology of lower mississippi valley braid belt and pleistocene meander belt formation and provide an example of the utility of optical dating in fluvial settings where material for radiocarbon dating is scarce

geomorphology of the burnt river eastern oregon usa topographic adjustments to tectonic and dynamic deformation abstract eastern oregon contains the deepest gorge in north america where the snake river cuts vertically down 2300 m this deep gorge is known as hells canyon a landscape containing such a topographic feature is likely undergoing relatively recent deformation study of the burnt river a tributary to the snake river at the upstream end of hells canyon yields data on active river incision in eastern oregon indicating that quaternary faults are a first order control on regional landscape development through 1 24 000 scale geologic mapping a 500 000 year record of fluvial incision along the burnt river was constructed and is chronologically anchored by optically stimulated luminescence dating and tephrochronology analyses a conceptual model of fluvial terrace formation was developed using these ages and likely applies to other non glaciated catchments in eastern oregon mapped terraces inferred to have formed during glacial interglacial cycles provide constraints on rates of incision of the burnt river incision through these terraces indicates that the burnt river is down cutting at 0 15 to 0 57 m kyr 1 this incision appears to reflect a combination of local base level adjustments tied to movement along the newly mapped durkee fault and regional base level control imposed by the downcutting of the snake river deformation of terraces as young as 38 7 5 1 ka indicates quaternary activity along the durkee fault and when combined with topographic metrics slope relief hypsometry and stream steepness reveals a landscape in disequilibrium longer wavelength lithospheric dynamics delamination and crustal foundering that initiated in the miocene may also be responsible for continued regional deformation of the earth s surface

3400 years of agricultural engineering in mesoamerica lama bordos of the mixteca alta oaxaca mexico the origins of early mesoamerican agricultural techniques are not well established our charcoal derived radiocarbon chronology dates cross valley check dams or lama bordos buried by up to 11 5 m of sediment in arroyos near coixtlahuaca oaxaca mexico now it is clear that people in highland mexico built lama bordos at least 3400 3500 years ago several hundred years earlier than previously dated engineering systems in mesoamerica lama bordo systems evolved as intensively and extensively managed landscapes coeval with climate shifting to more arid conditions they provide clear examples of human produced stratigraphy and artificial landscapes stair stepped valleys more importantly these lama bordo systems signal a major cultural tipping point toward sedentary agricultural life and solidify our understanding of the neolithic transition in mesoamerica

fire storms and erosional events in the idaho batholith in late december 1996 the south fork payette river basin in west central idaho experienced a prolonged storm that culminated on january 1 1997 with intense rain on melting snow that triggered slide failures producing debris flows and sediment charged floods failures occurred in saturated cohesionless grussy colluvium derived from weathered idaho batholith granitic rocks many failures along the south fork payette river originated in ponderosa pine forests burned in the 1989 stand replacing lowman fire an example is the 0 49 km 2 jughead creek basin where a single large colluvial failure produced almost 40 of the total volume eroded from the basin and generated a massive and rapid debris flow failures also occurred in steep unburned and unforested drainages such as hopkins creek in this south facing 0 58 km 2 basin 15 colluvial hollows failed but no single failure produced more than 10 of the total eroded volume sediment transport in hopkins creek occurred by prolonged sediment charged sheetflooding despite vegetation differences sediment yields from the geomorphically similar hopkins creek 42 000 mg km 2 and jughead creek 44 000 mg km 2 basins were quite similar these 1997 erosion events are equivalent to several thousand years of sediment yield at low rates 2 7 30 mg km 2 year 1 measured by short term sediment trapping and gauging in idaho batholith watersheds if similar large events were solely responsible for sediment export recurrence intervals ris of several hundred years would account for higher sediment yields averaged over 10 4 year from idaho batholith watersheds dating of small fire induced sheetflooding events in an early holocene tributary junction fan of jughead creek indicates that frequent small sedimentation events ri 33 80 year occurred between 7400 and 6600 cal year bp with an average yield not greatly exceeding 16 mg km 2 year 1 compared with the holocene average erosion rates during that 800 year period were unusually low suggesting that sediment yields have not been constant over time and that climatic variations and related fire regime changes may exert a strong influence on the probability of major erosional events copyright 2001 john wiley sons ltd

holocene strath terraces climate change and active tectonics the clearwater river basin olympic peninsula washington state the 400 km 2 clearwater river basin located on the pacific flank of the actively uplifting olympic mountains of western washington state contains a well preserved flight of holocene fluvial terraces we have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic fluvial active tectonic and climatic processes that operate at holocene spatial and temporal scales detailed field mapping reveals three prominent holocene straths and their overlying terrace deposits terrace ages fall into three broad ranges ca 9000 11 000 yr b p qt4 4000 8000 yr b p qt5 and 0 3000 yr b p qt6 terrace deposit stratigraphy sedimentology and age distributions allow us to consider two alternative models for their genesis the favored model states that the terrace ages are coincident with lateral incision of the clearwater channel emplacement of the terrace alluvium and the carving of the straths vertical incision of the clearwater channel was primarily relegated to the brief 1000 yr intervals when we have no record of terraces alternatively the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision in both models we envision a clearwater river channel at or near capacity with a temporally variable rate of both lateral and vertical incision small deviations from this at capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel we consider several causes for variable hillslope sediment flux in this tectonically active setting including holocene climate change and ground accelerations related to earthquakes holocene rates of vertical incision are reconstructed along nearly the entire clearwater valley from the wide distribution of dated terraces incision rates clearly increase upstream mimicking a pattern documented for pleistocene terraces in the same basin however the rates are 2 3 times those determined for the pleistocene terraces the faster holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift however we favor an alternative explanation in which the holocene rates represent a channel rapidly reacquiring its stable graded concavity following protracted periods of time in the pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation these results illustrate how even in tectonically active settings representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial interglacial cycle

a 2200 yr record of hydrologic variability from foy lake montana usa inferred from diatom and geochemical data abstract a 2200 yr long high resolution 5 yr record of drought variability in northwest montana is inferred from diatoms and 18o values of bio induced carbonate preserved in a varved lacustrine core from foy lake a previously developed model of the diatom response to lake level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data high frequency decadal fluctuations in the de trended 18o record mirror variations in wet dry cycles inferred from banff tree rings demonstrating the sensitivity of the oxygen isotope values to changes in regional moisture balance low frequency multi centennial isotopic changes may be associated with shifts in the seasonal distribution of precipitation from 200 b c to a d 800 both diatom and isotope records indicate that climate was dry and lake level low with poor diatom preservation and high organic carbon nitrogen ratios subsequently lake level rose slightly although the climate was drier and more stable than modern conditions at a d 1200 lake level increased to approximately 6 m below present elevation after which the lake fluctuated between this elevation and full stage with particularly cool and or wetter conditions after 1700 the hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake s outlet spectral analysis of the 18o data indicates that severe droughts occurred with multi decadal 50 to 70 yr frequency

fire and alluvial chronology in yellowstone national park climatic and intrinsic controls on holocene geomorphic processes we employed a systemwide approach a large and robust set of radiocarbon ages and modern process analogs to interpret the holocene history of forest fire related sedimentation and overall alluvial activity in northeastern yellowstone national park debris flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire related sedimentation within valley side alluvial fans of soda butte creek fire related deposits make up approximately 30 of the late holocene fan alluvium fifty 14 c ages on fire related events cluster within the intervals of 3300 2900 2600 2400 2200 1800 and 1400 800 yr b p and suggest earlier episodes of large fires and fan aggradation around 7500 5500 and 4600 4000 yr b p a major pulse of fire related debris flow activity between 950 and 800 yr b p coincided with the height of the widely recognized medieval warm period ca a d 1050 1200 instrumental climate records over the last 100 yr in yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods enhancing the potential for severe short term drought major forest fires and storm generated fan deposition along lower soda butte creek fill cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca 8000 yr b p terrace level t1a 7000 5600 t1b 3100 2600 t2 2000 1300 t3 and post 800 yr b p t4 these periods coincide with overbank sedimentation on slough creek and the lamar river but alternate with intervals of fire related fan deposition implying a strong climatic control local paleoclimatic data suggest cooler effectively wetter conditions during terrace tread formation in warmer drier intervals reduced average runoff in axial streams results in meander belt narrowing concurrent channel incision may be caused by infrequent large floods greater resistance to downcutting however allowed fewer terraces to be formed along slough creek and the lamar river alluvial systems in northeastern yellowstone show a clear response to millennial scale climatic cycles wherein alluvial fans aggrade and prograde over flood plains in drier periods axial streams widen their flood plains and trim back the fans during wetter periods small scale climatic fluctuations of the holocene thus had substantial impact on postglacial landscapes in northeastern yellowstone

stable isotope characterization of the miocene lacustrine systems of los monegros ebro basin spain palaeogeographic and palaeoclimatic implications the lacustrine carbonate facies from three allostratigraphic units n1 n2 and n3 of miocene age in the los monegros region ebro basin are characterized isotopically the dominant facies are marls laminated limestones stromatolitic limestones massive limestones and bioturbated limestones these are associated with lacustrine sulphate deposits and distal alluvial facies palaeohydrological reconstructions of the study area are supported by the isotopic and the mineralogical composition of the carbonates massive and bioturbated limestones occupy a very similar compositional domain and have the lowest isotopic compositions of all the facies 9 18opdb 4 5 and 6 4 13cpdb 0 4 reflecting a short residence time of the water and variable commonly high influence of biogenic co2 laminated limestones and stromatolites define a comparatively enriched domain with 18o values ranging from 6 to 0 and 13c values ranging from 3 5 to 0 these values indicate stronger evaporation and enhanced 12c assimilation due to intense biological activity during periods of longer water residence time marls have isotopic values intermediate between these two domains and correspond to periods of lacustrine dilution dolomite bearing samples mainly laminated and stromatolitic facies show a 18odol cal 7 5 with respect to calcite from the same facies while 13c values show very little enrichment up to 1 the positive correlation r 0 85 between 18o and dolomite contents suggests that dolomite resulted from the progressive evaporative concentration of a single water mass and not from the mixing of waters of different compositions the dolomite is thought to be primary or at most the product of very early diagenetic processes the isotopic composition of the facies and their spatial and temporal variations depend on the depositional environment and were constrained by changing regional paleogeography isotopic trends from units n1 to n3 upper agenian vallesian period display a regular depletion in heavy isotopes that indicates a climatic change towards wetter conditions

holocene fire and vegetation along environmental gradients in the northern rocky mountains abstract holocene records of fire vegetation and climate were reconstructed from four sites in the bitterroot range region of the northern rocky mountains in order to examine the vegetation and fire histories and evaluate the hypothesis proposed by whitlock and bartlein 1993 regarding the effects of increased summer insolation on precipitation patterns vegetation history in the series of sites was broadly similar in the late glacial period the pollen data suggest open parkland dominated by picea or alpine meadow which reflect conditions cooler and drier than present these open forests were replaced in the early to middle holocene by forests composed mainly of pinus and pseudotsuga which suggest conditions warmer than present modern forest compositions were in place by ca 3000 cal yr bp and small variations in the timing of the vegetation shifts reflect local differences among sites the long term trends in fire occurrence support the hypothesis proposed by whitlock and bartlein 1993 that precipitation regimes were sharpened during the early holocene summer insolation maximum but their location has remained unchanged as a result of topographic constraints sites located in areas currently summer dry were drier than present during the early holocene and fires were more frequent conversely sites located in the areas that are summer wet at present were wetter than present in the early holocene and fires were less frequent on millennial time scales it appears that the climate boundary is controlled by topography and does not shift

latest pleistocene alpine glacier advances in the sawtooth mountains idaho usa reflections of midlatitude moisture transport at the close of the last glaciation mountain glaciers in the sawtooth mountains responded strongly to reinvigorated atmospheric moisture transport following the last ice sheet maximum the glaciers constructed an extensive moraine belt in the southeastern sawtooth mountains allowing determination of a detailed radiocarbon chronology from lake and marsh cores moraine morphometry and soil data indicate construction of seven to nine moraines in each of four valleys during two main late pleistocene ice advances a basal radiocarbon date from marsh cores in the alturas valley documents that the maximum advance during marine oxygen isotope stage 2 occurred shortly before ca 16 900 calibrated cal yr b p minimum limiting dates on glacial lacustrine sediment in lake and marsh cores from three valleys clustered around 13 950 cal yr b p document the maintenance or reestablishment of extensive ice volume during early late glacial time the ca 16 900 cal yr b p advance postdates the last ice sheet maximum by 4000 yr and is broadly correlative with maximum advances of the north yellowstone outlet glacier and the puget lobe of the cordilleran ice sheet and a near maximum advance of wallowa mountain glaciers these synchronous advances indicate response of a variety of ice systems to reinvigorated moisture transport following the ice sheet maximum as does the subsequent sawtooth advance during early late glacial time together these responses indicate strong sensitivity of certain ice systems to moisture delivery fluctuations

latest pleistocene and holocene glacier fluctuations in western canada we summarize evidence of the latest pleistocene and holocene glacier fluctuations in the canadian cordillera our review focuses primarily on studies completed after 1988 when the first comprehensive review of such evidence was published the cordilleran ice sheet reached its maximum extent about 16 ka and then rapidly decayed some lobes of the ice sheet valley glaciers and cirque glaciers advanced one or more times between 15 and 11 ka by 11 ka or soon thereafter glacier cover in the cordillera was no more extensive than at the end of the 20th century glaciers were least extensive between 11 and 7 ka a general expansion of glaciers began as early as 8 4 ka when glaciers overrode forests in the southern coast mountains it culminated with the climactic advances of the little ice age holocene glacier expansion was not continuous but rather was punctuated by advances and retreats on a variety of timescales radiocarbon ages of wood collected from glacier forefields reveal six major periods of glacier advance 8 59 8 18 7 36 6 45 4 40 3 97 3 54 2 77 1 71 1 30 ka and the past millennium tree ring and lichenometric dating shows that glaciers began their little ice age advances as early as the 11th century and reached their maximum holocene positions during the early 18th or mid 19th century our data confirm a previously suggested pattern of episodic but successively greater holocene glacier expansion from the early holocene to the climactic advances of the little ice age presumably driven by decreasing summer insolation throughout the holocene proxy climate records indicate that glaciers advanced during the little ice age in response to cold conditions that coincided with times of sunspot minima priority research required to further advance our understanding of late pleistocene and holocene glaciation in western canada includes constraining the age of late pleistocene moraines in northern british columbia and yukon territory expanding the use of cosmogenic surface exposure dating techniques using multi proxy paleoclimate approaches and directing more of the research effort to the northern canadian cordillera

glacial sequence near mccall idaho weathering rinds soil development morphology and other relative age criteria abstract the sequence of glacial deposits near mccall idaho previously assigned to the pinedale and bull lake glaciations contains deposits of four different ages these ages are defined by multiple relative age criteria including weathering rinds soil development surface rock weathering morainal morphology and loess stratigraphy the thickness of weathering rinds on basaltic clasts is statistically representative and reproducible and can be used to estimate numerical ages following in order of decreasing relation to age are soil development surface rock weathering and moraine morphology the glacial deposits near mccall appear to correspond to times of high worldwide ice volume indicated by the marine oxygen isotope record pilgrim cove and mccall deposits both assigned to the pinedale glaciation are late wisconsin in age perhaps 14 000 and 20 000 years respectively they represent a rare case in which deposits of pinedale age can be separated by relative age data timber ridge deposits assigned to the bull lake glaciation have subdued but well preserved morainal morphology relative age data indicate that they are pre wisconsin in age probably about 140 000 150 000 years old although we cannot exclude an older age williams creek deposits are clearly distinct from and intermediate in age between mccall and timber ridge deposits weathering rinds and the inferred ages of the other deposits suggest an early wisconsin age for williams creek deposits

mountain erosion over 10 yr 10 k y and 10 m y time scales we used cosmogenic 10 be to measure erosion rates over 10 k y time scales at 32 idaho mountain catchments ranging from small experimental watersheds 0 2 km 2 t o large river basins 35 000 km 2 these long term sediment yields are on average 17 times higher than stream sediment fluxes measured over 10 84 yr but are consistent with 10 m y erosion rates measured by apatite fission tracks our results imply that conventional sediment yield measurements even those made over decades can greatly underestimate long term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs our observations also suggest that sediment delivery from mountainous terrain is extremely episodic sporadically subjecting mountain stream ecosystems to extensive disturbance

pleistocene incision rates in the western united states calibrated using lava creek b tephra the lava creek b ash bed erupted from the yellowstone caldera ca 0 64 ma provides a datum for measuring long term fluvial incision west of the mississippi river the ash is widely preserved due to its substantial volume broad initial dispersal and the aggrading environment into which the ash fell drainages incised soon after lava creek b deposition isolating the ash from fluvial erosion and preserving it in fill terraces calculated rates of incision since ca 0 60 ma range from 2 to 30 cm k y 1 rates are high in most areas near the rocky mountains and downstream along rivers draining mountainous terrain and are lowest east of the high plains and along the snake river incision rates along many rivers decrease downstream rates of downcutting increased in the late pleistocene along several major rivers indicating that climate change altered sediment budgets regional and temporal data suggest that fluvial incision records increased middle and late pleistocene runoff from the southern rocky mountains rather than epeirogenic uplift but regional rock uplift cannot be excluded as a significant factor

timing and nature of late quaternary mountain glaciation compilations of regional to subcontinental scale mountain glacier chronologies from five continents plus hawaii examine the temporal and spatial patterns of mountain glaciation the compilations yield key insights into the nature of past fluctuations in temperature and precipitation in mountains and adjacent regions and provide baseline data for examining the dynamics of glacier responses to climate change key insights from these compilations include the wide variability of glacier responses to fluctuations in insolation temperature and precipitation and the regionally specific variations in climatic variables through late quaternary time the compilations highlight the need to improve the density and quality of geochronological data and to enhance the understanding of the links between climate and glaciation copyright 2008 john wiley sons ltd

erosional and chemical denudation rates in the southwestern idaho batholith annual erosional denudation ed and chemical denudation cd rates are determined on four forested watersheds located in the idaho batholith idaho u s a bedrock in the study area is a coarse grained quartz monzonite annual sediment yields are computed from sediment collected in settling ponds at the watershed mouths and corrected for trapping efficiency solution loads for denudation components na k ca mg sio2 a12o3 and feo are computed using equations relating concentration to flow and by integration of mean daily flow values eleven years of water chemistry data and 19 to 23 years of sediment yield data are used in the denudation estimates strong relationships exist between cd ed and annual water yield so probability distributions of annual water yield are used in monte carlo simulations to establish frequency distributions of annual cd and ed rates mean annual total denudation for the four watersheds cd ed equals 235 kg ha 1 yr 1 or 8 9 bubnoffs the 11 year mean ed rates exceed cd rates for three of the four watersheds although it is more probable in any given year that cd will exceed ed for three of the watersheds ed is temporally more variable than cd for all watersheds simulations of denudation over 1000 year periods indicate that the frequency of occurrence of ed is skewed toward higher denudation values and the modal ed rate is substantially lower than the mean value cd rates are less skewed with mean and mode in the same class ninety seven per cent of cd is composed of na ca and sio2 fluxes episodic high ed rates are related to high peak flows from snowmelt runoff associated with winters of large snow accumulation perturbations such as fire or road construction would be expected to accelerate ed more than cd

an optical age chronology of late pleistocene fluvial deposits in the northern lower mississippi valley the lower mississippi valley lmv contains many large braided channel belts that are preserved west of the holocene floodplain previous efforts to establish geochronologic control on channel belt construction have been hindered by the lack of organic material for radiocarbon dating luminescence techniques provide a burial date for the sediment itself and may prove useful in this context samples from three channel belts in the northern lmv were analyzed using the single aliquot regenerative technique on 90 125mm quartz optical ages 19 7 17 8 16 1 15 0 and 12 5 12 1 ka are consistent with geomorphic relationships and indicate that channel belts were formed in the late pleistocene under glacial conditions these optical ages provide the first detailed chronology of lmv channel belt formation and are the first step towards developing a chronology for the entire lmv r 2003 elsevier science ltd all rights reserved

lithospheric folding in iberia integration of stress indicator data gravity data crustal kinematics data and analysis of topography and recent vertical motions demonstrates the occurrence of consistently oriented spatial patterns of large scale alpine to recent intraplate deformation in iberia the inferred upper crustal and lithospheric deformation patterns and the timing of the associated expressions at or near the surface support the existence of a close coupling with plate boundary processes operating at the margins of iberia patterns of lithosphere and upper crustal folds are oriented perpendicular to the main axis of present day intraplate compression in iberia inferred from structural analysis of stress indicator data and focal mechanism solutions these findings suggest the presence of lithospheric folds with wavelengths compatible with theoretical predictions of folding wavelengths of variscan lithosphere stress induced intraplate deformation set up by plate interactions is compatible with indications for the absence of present day deep mantle lithosphere interactions inferred from seismic tomography

expansion of alpine glaciers in pacific north america in the first millennium a d radiocarbon ages and lichen dated moraines from 17 glaciers in coastal and near coastal british columbia and alaska document a widespread glacier advance during the first millennium a d glaciers at several sites began advancing ca a d 200 300 based on radiocarbon dated overridden forests the advance is centered on a d 400 700 when glaciers along an 2000 km transect of the pacific north american cordillera overrode forests impounded lakes and deposited moraines the synchroneity of this glacier ad vance and inferred cooling over a large area suggest a regional climate forcing and to gether with other proxy evidence for late holocene environmental change during the me dieval warm period and little ice age provide support for millennial scale climate variability in the north pacific region

implications of burial alterations on luminescence dating of archaeological ceramics recent mineralogical studies on archaeological pottery samples report significant variations in alkali metal concentrations due to environmental alterations during burial here we examine the effects of potassium k leaching on luminescence dating the effect on the estimation of the dose rate is studied by considering four models of leaching exponential linear early and late and their impact on fine and coarse grain dating are calculated the modeling approaches are applied to two cases of pottery in which evidence for alteration was found additionally tl dating performed on pottery of one of the studied cases indicates the importance of leaching effects on absolute dating measurements

stream terrace genesis implications for soil development genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting base level of erosion complex response threshold of critical power diachronous and synchronous response times and static and dynamic equilibrium climatic and tectonic stream terraces are major terraces below which flights of minor complex response degradation terraces can form these three types of terraces can be summarized by describing a downcutting aggradation renewed downcutting sequence for streams with gravell bedload by tectonically induced downcutting streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream terrace landform aggradation events record brief reversals of long term tectonically induced downcutting because they raise active channels they may be considered as major the result of climatic perturbations or minor the result of complex response model types of perturbations climatically controlled aggradation followed by degradation leaves an aggradation surface this type of fill terrace tread is the fundamental climatic stream terrace landform aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form first in alluvial fill and then in underlying bedrock along tectonically active streams these are complex response terraces because they result from interactions of dependent variables within a given fluvial system pauses in degradation to a new base level of erosion and or minor episodes of backfilling lead to formation of complex response fill cut and strath or of fill terraces fill cut terraces are formed in alluvium they are complex response terraces because they are higher than the base level of erosion good exposures and dating are needed to distinguish static equilibrium complex response minor strath terraces from dynamic equilibrium tectonic major straths strath terraces may be regarded as complex response terraces where degradation rates between times terrace tread formation exceed the long term uplift rate for the reach based on ages and positions of tectonic terraces late quaternary global climatic changes control aggradation events and even the times of cutting of major tectonic straths because the base level of erosion can not be attained during times of climatically driven aggradation degradation events most terrace soils form on treads of climatic and complex response terraces aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors climatic terraces of adjacent watersheds also should be roughly synchronous correlatable variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full glacial times and interglacial times soils on some complex response terraces may be diachronous within a given fluvial system and typically are diachronous between watersheds

the little ice age in the canadian rockies abstract this paper reviews the evidence and history of glacier fluctuations during the little ice age lia in the canadian rockies episodes of synchronous glacier advance occurred in the 12th 13th early 18th and throughout the 19th centuries regional ice cover was probably greatest in the mid 19th century although in places the early 18th century advance was more extensive glaciers have lost over 25 of their area in the 20th century selective preservation of the glacier record furnishes an incomplete chronology of events through the 14th 17th centuries in contrast varve sequences provide continuous annually resolved records of sediments for at least the last millennium in some highly glacierized catchments such records have been used to infer glacier fluctuations evaluation of recent proxy climate reconstructions derived from tree rings provides independent evidence of climate fluctuations over the last millennium most regional glacier advances follow periods of reduced summer temperatures reconstructed from tree rings particularly ca 1190 1250 1280 1340 1690s and the 1800s reconstructed periods of higher precipitation at banff alberta since 1500 are 1515 1550 1585 1610 1660 1680 and the 1880s glacier advances in the early 1700s the late 1800s and in places the 1950 1970s reflect both increased precipitation and reduced summer temperatures negative glacier mass balances from 1976 to 1995 were caused by decreased winter balances the glacier fluctuation record does not contain a simple climate signal it is a complex response to several interacting factors that operate at different timescales evaluation of climate proxies over the last millennium indicates continuous variability at several superimposed timescales dominated by decade century patterns only the 19th century shows a long interval of sustained cold summers this suggests that simplistic concepts of climate over this period should be abandoned and replaced with more realistic records based on continuous proxy data series the use of the term lia should be restricted to describing a period of extended glacier cover rather than being used to define a period with specific climate conditions

pleistocene geomorphology and geochronology of eastern grand canyon linkages of landscape components during climate changes we report new mapping soils survey and geochronologic luminescence u series and cosmogenic nuclide data from pleistocene deposits in the arid setting of eastern grand canyon the result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes tributary streams and the colorado river to the last 400 kyr of glacial interglacial climate change the best preserved last 80 kyr of this record indicates a stratigraphic chronologic disconnect between both deposition and incision along the colorado river versus along the trunks of local tributaries for example the colorado river finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during marine isotope stage 3 and then tributary incision followed during the millennial scale fluctuations of the last glacial epoch potentially concurrent with mainstem aggradation the mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic hydrologic changes in its mountain headwaters with aggradation beginning during full glacial times and continuing into subsequent interglacials the contrasting lag time in responses of the dryland catchments within grand canyon may be largely a function of the weathering limited nature of hillslope sediment supply r 2005 elsevier ltd all rights reserved

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

response of bankfull flood magnitudes to holocene climate change uinta mountains northeastern utah long term variations in holocene fl ood magnitude were quantifi ed from the bankfull dimensions of abandoned channels preserved on flsurfaces in the northern uinta mountains of northeastern utah cross sectional areas of abandoned channels were reconstructed and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross section areas the results indicate systematic nonrandom variations of bankfull fl oods in the northern uinta mountains large fl oods as much as 10 15 greater than modern dominated from 8500 to 5000 calendar yr b p and again from 2800 to 1000 cal yr b p small fl oods as much as 15 20 less than modern characterize the periods from 5000 to 2800 cal yr b p and from 1000 cal yr b p to near present the middle and late holocene record of bankfull fl ood magnitude compares well with independent evidence for climatic variation in the area the early holocene record indicates that larger than modern bankfull fl oods coincide with warmer than modern mean annual temperature we hypothesize that an increased range of magnitude for seasonal solar radiation during the early holocene favored the accumulation and rapid melting of deep snowpacks in the high uinta mountains thus producing large fl oods despite warmer mean annual temperatures the episode of smaller than modern bankfull fl oods between 5000 and 2800 cal yr b p coincides with records of increased forest fi re frequency in the northern uintas larger than modern fl oods from 2800 to 1000 cal yr b p coincide with a local decrease in forest fi re frequency and evidence for minor local glacial readvances the decrease in fl magnitudes following 1000 cal yr b p corresponds to numerous local and regional records of warming during the medieval climatic anomaly

26al and10be dating of late pleistocene and holocene fill terraces a record of fluvial deposition and incision colorado front range cosmogenic 26al 10be and 14c dating of fluvial fill terraces in steep canyons of the colorado front range provides a temporal framework for analysing episodic aggradation and incision results from boulder canyon show that terrace heights above the modern channel grade can be divided into 1 bull lake 100 ka 20 15 m above grade 2 pinedale 32 10 ka 15 4 m above grade and 3 holocene age 15 m above grade record bull lake deposition well preserved terraces of pinedale age suggest that the range of terrace height above grade reflects short term fluctuations in the river profile during periods of rapidly changing stream load and power net river incision apparently occurred during transitions to interglacial periods soil development and stratigraphic position along with limited cosmogenic and 14c dating suggest that 130 ka terraces in boulder canyon correlate with the louviers alluvium and that 32 to 10 ka fills in the canyon correlate with the broadway alluvium on the adjacent high plains late pleistocene incision rates 0 15 m ka 1 along boulder canyon exceed pre late pleistocene incision rates and are higher than middle to late pleistocene incision rates 0 04 m ka 1 on the high plains this study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments copyright 2002 john wiley sons ltd