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

geomorphic and hydrologic aspects of monsoon floods on the narmada and tapi rivers in central india abstract high magnitude floods during the monsoon season are considered to be india s recurring and leading natural disaster such large flood are extremely important events not only in terms of human impact but also from the standpoint of geomorphic effectiveness and geomorphic work the narmada and tapi rivers in central india are characterized by one of the most intense flood regimes in the seasonal tropics modern historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the narmada and tapi rivers a catastrophic flood on tapi in july 1991 that occurred from heavy rains and a dam break represents one of the highest unit discharges yet recorded in any part of central india flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world the rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium and in choral river a 5000 year old record has been located examination of the synoptic conditions associated with the flood generating low pressure systems reveals that all but a few of them are the result of bay of bengal depressions the flooding in such large rivers profoundly challenges flood hazard management because of limited instrumental records and large spatio temporal variation it is therefore imperative that the design engineering community in india incorporates the geomorphic sedimentologic and botanic information in the planning and design of water resource development projects

a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

anomalous north pacific atmospheric circulation and large winter floods in the southwestern united states abstract specific anomalous atmospheric circulation conditions over the north pacific are conducive to the occurrence of the largest winter floods 10 yr return period on rivers in six hydroclimatic subregions of arizona and southern utah nevada and california composite maps of anomalies in daily 700 mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the california coast and a high pressure anomaly in the vicinity of either alaska or the aleutian islands of these two major circulation features the presence of the low is the controlling factor in determining whether large floods will occur shifts in the locations of the low and high pressure anomalies over the north pacific appear to control which subregions experience floods with high elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region concerning the int

holocene vegetation and climate in the puerto blanco mountains southwestern arizona abstract plant macrofossils from 21 pack rat neotoma sp middens at 535 605 m from the puerto blanco mountains southwestern arizona provide and excellent history of vegetation and climate for the last 14 120 yr b p in the sonoran desert a late wisconsin juniper joshua tree woodland gave way to a transitional early holocene desertscrub with sparse juniperus californica california juniper by 10 540 yr b p important sonoran desert plants including carnegiea gigantea saguaro and encelia farinosa brittle bush were dominants riparian trees such as acacia greggii catclaw acacia prosopis velutina velvet mesquite and cerdicium floridum blue palo verde grew on dry south facing slopes in a middle holocene sonoran desertscrub in a warm wet summer climate with frequent winter freezes modern subtropical sonoran desertscrub formed about 4000 yr b p as summer rainfall and winter freezes declined cercidium microphyllum foothills palo verde sapium biloculare mexican jumping bean olneya tesota ironwood and stenocereus thurberi organ pipe cactus became dominant as riparian trees retreated to wash habitats the inferences of a latest wisconsin early holocene summer monsoonal maximum by j e kutzbach 1983 modeling of holocene climates in late quaternary environments of the united states vol 2 the holocene h e wright ed pp 271 277 univ of minnesota press minneapolis are not supported for the southwest apparently the persistence of late wisconsin circulation patterns offset any increases in insolation

palaeoflood and floodplain records from spain evidence for long term climate variability and environmental changes abstract palaeoflood chronologies from seven spanish river basins and floodplain aggradation chronologies from thirteen rivers are analysed these fluvial records were divided in to two sub sets namely atlantic 10 ka record and mediterranean 3 ka record river basins which represent distinct modern hydroclimatic conditions in atlantic basins floods result from intense widespread rainfalls associated with atlantic frontal systems transported by westerly airflow mediterranean river flooding is related to heavy rainfall induced by mesoscale convective systems mcss during autumn months evidence from radiocarbon dates in slackwater flood deposits shows six periods of flood clusters at 10 750 10 240 9550 9130 4820 4440 2865 2350 960 790 and 520 290 cal bp despite the different flood producing weather conditions in atlantic and mediterranean rivers the radiocarbon sample clusters overlap and indicate changes in large scale atmospheric circulation and climatic conditions in the iberian peninsula comparison with proxy records of mean temperature for the northern hemisphere demonstrates a relationship between the period of slackwater flood deposition and cold climatic phases e g the 2650 yr bp climatic event or ad 1590 1650 period of the little ice age radiocarbon dates from aggraded floodplain sediments were clustered at 2710 2320 2000 1830 and 910 500 cal bp the first cluster period is in phase with the timing of slackwater deposition whereas the third 910 500 cal bp occurs in between two periods of increased flood frequency as indicated by the palaeoflood and documentary flood records it is argued that the 910 500 cal bp floodplain aggradation period reflects the first post roman evidence of environmental change related to generalised land use changes at the catchment scale which produced high sediment load transported to overbank areas during high flows

climate and ephemeral stream processes twentieth century geomorphology and alluvial stratigraphy of the little colorado river arizona during the first 40 years of the twentieth century erosion was the dominant geomorphic process affecting the morphology of the little colorado river channel the discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation in the 1940s and early 1950s average annual precipitation declined reducing annual discharge to about 57 of that of the preceding period as well as reducing the frequency of large floods the channel adjusted to the new hydrologic regimen by reducing its width parts of the channel were frequently dry and riparian vegetation primarily nonnative salt cedar became established on the higher channel surfaces precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process as indicated by accretion of 2 to 5 m of flood plain alluvium between 1952 and 1978 events of 1980 however suggest that the flood plain has ceased to accrete although climate has not fluctuated the flood plain has probably reached a critical height above the channel beyond which further accretion is unlikely under the existing discharge regimen the recent history of the little colorado broadly suggests that flood plain development was initiated by climatically induced hydrologic fluctuations flood plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions

sedimentary records of large holocene floods from the middle reaches of the yellow river china slackwater deposits have been used as evidence for the magnitude and frequency of palaeofloods which occurred prior to gauged and historical data sets e g holocene extreme flood reconstruction in north america and australia the slackwater deposits from xiaolangdi reach of the yellow river have been identified and related to floods according to both eyewitness accounts and the analysis of sediments the technique successfully delineated the peak flood level and estimated a discharge of the 1994 ad flood in the yellow river the discrepancy of the estimated discharge was less than 5 compared with the instrumental records when we applied the technique to palaeoflood analysis the highest slackwater deposits and flood indicators from the tributary mouths the low river terraces and the loess platforms and loess alcoves together with radiocarbon dating provided evidence of at least six floods with peak discharges greater than 27 300 m3 s 1 at xiaolangdi section over the last 8500 yr the largest flood in ca 7362 yr b p had an estimated discharge of 42 900 m3 s 1 which is the maximum flood event known in the yellow river the results provide a basis for a determining the floods which were comparable to the most cataclysmic flood during the last 2360 yr in 1843 ad and b estimating the flood magnitude for a 10 000 yr time scale relevant for building xiaolangdi dam one of the largest dams under construction in the world the reconstruction of palaeofloods also provides evidence for a phase between ca 6000 8000 yr b p which was much wetter than today as a result of increased summer precipitation and enhanced pacific monsoon conditions in eastern china

accuracy of post bomb 137cs and 14c in dating fluvial deposits abstract the accuracy and precision of 137cs and 14c for dating post 1950 alluvial deposits were evaluated for deposits from known floods on two rivers in arizona the presence of 137cs reliably indicates that deposition occurred after intensive above ground nuclear testing was initiated around 1950 there was a positive correlation between the measured level of 137cs activity and the clay content of the sediments although 137cs was detected even in sandy flood sediments with low clay content 137cs is a valuable dating tool in arid environments where organic materials for 14c or tree ring dating are scarce and observational records are limited the 14c activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit and the accuracy was species dependent however undifferentiated mixtures of fine organic materials from several post bomb deposits of various ages repeatedly yielded dates between 1958 and 1962 and detrital charcoal yielded a date range of 1676 1939 in semiarid environments the residence time of most types of organic debris precludes accurate annual resolution of post bomb 14c dates

a geochemical record of flooding on the upper river severn uk during the last 3750 years abstract projected future changes in climate call for more definite information on the magnitude frequency relationships of major floods than can be obtained from short instrumental or historical records alone floodplains represent a sedimentary archive of unrecorded flood events which can potentially be used to extend existing flood records by centuries or millennia in this study a c 3750 year flood record was produced using ln zr rb profiles from two 4 m deep cores through floodplain silts at the roundabout in the upper severn catchment uk sediment geochemical profiles were obtained using high resolution 500 m xrf core scanning ages were assigned to the flood record using 14c dating and contaminant pb profiles comparison of sediment ln zr rb with sediment grain size showed that ln zr rb increases with increasing grain size and indicates that it can be used as a grain size proxy at this site within the ln zr rb flood record two floods that occurred at c 235 and c 10 cal bc were probably the largest that have occurred at the site during the past 3750 years while the historical flood of a d 1795 appears to be the largest since c ad 200 the record shows that during the past c 3750 years multicentennial periods characterized by the occurrence of high magnitude floods return periods c 30 years have alternated with periods of similar length without such floods these periods correspond to large scale variations in hydroclimate recorded in the north atlantic region however the incidence of the highest magnitude floods appears to be unaffected by changes in catchment land use this study provides information about flood magnitude frequency relationships and their controls at a local level that could be used to help catchment managers in assessing future flood risk at a time of rapidly changing climate

climatic geomorphic and archaeological implications of a late quaternary alluvial chronology for the lower salt river arizona usa abstract recent archaeological excavations along the lower salt river arizona resulted in the unexpected discovery of buried late pleistocene soils and cultural features dating 5800 7100 cal ybp early archaic the latter representing the earliest evidence of human activity in the lower salt river floodplain thus far identified because the lower salt river floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures our understanding of the river s geological history is limited here we present a late quaternary alluvial chronology for a segment of the lower salt river based on 19 accelerator mass spectrometry 14 c and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations deposits are organized into allostratigraphic units and reveal a buried late pleistocene terrace inset into middle to late pleistocene terrace deposits holocene terrace fill deposits unconformably cap the late pleistocene terrace tread in the site area and the lower portion of this fill contains the early archaic archaeological features channel entrenchment and widening 900 cal ybp eroded much of the older terrace deposits leaving only a remnant of fill containing the buried latest pleistocene and middle to late holocene deposits preserved in the site area subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments our study confirms that the lower salt river is a complex mosaic of late quaternary alluvium formed through vertical and lateral accretion with isolated patches of buried soils preserved through channel avulsion although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages intrinsic geomorphic and local base level controls limit direct correlations of lower salt river stratigraphy to other large rivers in the north american southwest

holocene history of the el nino phenomenon as recorded in flood sediments of northern coastal peru significant precipitation along the north central coast of peru lat 5 10 s occurs exclusively during el nino incursions of warm water into the peruvian littoral flood deposits from this region therefore provide a proxy record of extreme el nino events i present a 3500 yr chronology of the extreme events based on radiocarbon dating of overbank flood sediments from the rio casma lat 9 2 s the flood plain stratigraphy suggests that the el nino phenomenon has occurred throughout the holocene and that flood events much larger than that which occurred during 1982 1983 occur here at least once every 1000 yr

hydrologic variability and the onset of modern el nino southern oscillation a 19 250 year record from lake elsinore southern california there are very few terrestrial palaeoclimate archives spanning the last glacial maximum through the holocene from coastal southern california yet knowledge of past climate dynamics is critical for assessing present and future constraints on the region s dwindling freshwater resources we present initial results from two drill cores extracted from the present day edge of lake elsinore southern california s largest natural lake using a multiproxy approach including lithologic description mass magnetic susceptibility loi 550 c total organic matter and loi 950 c total carbonate we infer first order long term climate change over the past 19 250 calendar years furthermore we suggest possible first order forcing mechanisms that drive this change which include presence absence of continental ice sheets insolation and complex ocean atmosphere interactions our results indicate four distinct millennial scale climate modes over the past 19 250 calendar years these modes include a wet last glacial maximum 19 250 17 120 cal yr bp a relatively dry late glacial holocene transition 17 120 9450 cal yr bp a wet early holocene 9450 7670 cal yr bp and a highly variable mid to late holocene climate i e alternating wet dry cycles 7670 cal yr bp present we attribute the mid to late holocene climate interval to the onset of el nino southern oscillation ca 7000 cal yr bp and a more vigorous hydrologic system these results are supported by a variety of regional terrestrial and marine palaeoclimate archives copyright 2005 john wiley sons ltd

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

short duration holocene lakes in the mojave river drainage basin southern california stratigraphic sedimentologic and pedologic studies of beach ridge and lacustrine deposits indicate that up to five times during the holocene shallow lakes covered silver lake playa in southeastern california for periods of years to decades the two youngest lacustrine events at about 390 90 yr b p and 3620 70 yr b p coincide with the early and late neoglacial episodes of north america increasing evidence in recent years from other nonglaciated areas leads us to conclude that the effects of these climatic episodes were much more widespread than previously thought the climate during these episodes was characterized by an increased frequency of winter storms in the southwestern united states causing wetter conditions that affected diverse hyperarid environments in the mojave desert and adjacent regions we propose that this wide areal coverage was caused by large scale winter atmospheric circulation patterns which are probably related to changes in sea surface temperatures and oceanic circulation in the eastern north pacific ocean

eolian landscape evolution and soil formation in the chaco dune field southern colorado plateau new mexico abstract eolian deposits of the chaco dune field cover a broad expanse of the southeastern colorado plateau in new mexico and comprise a large part of the late quaternary sedimentary record in this region the primary eolian landforms include sand sheets parabolic and modified parabolic dunes barchan barchanoid dune complexes and ridge dunes many of the eolian landforms in the chaco dune field have remained stable over a sufficiently long time to permit soil development weakly to moderately developed calcic soils which formed in eolian deposits of mid holocene and latest pleistocene age possess clay enriched weak argillic b horizons these soils have been buried progressively by younger eolian sediments forming regionally correlative bounding surfaces over the chaco dune field and recording relict latest pleistocene and middle holocene eolian landscapes soil geomorphic relations and radiocarbon dated material within the eolian deposits have been useful in 1 delineating three regionally correlative soil stratigraphic units and their associated landforms and 2 studying the types and rates of landscape evolution and climatic changes within this portion of the colorado plateau the most significant period of eolian landform construction is inferred to have occurred during the latest pleistocene glacial to interglacial climatic change forming widespread sand sheets and modified parabolic dunes eolian deposition during the middle and late holocene were dramatically influenced by the presence of previously developed latest pleistocene to early holocene buried soils which may have enhanced sand sheet and parabolic dune development due to the binding effect of the soils middle holocene eolian deposition may be related primarily to neoglacial climatic changes which enhanced ground surface moisture and promoted sand sheet and parabolic dune formation the youngest period of wolian deposition occurred after 1 5 ka and resulted in the formation of the most diverse suite of eolian landforms and deposition of volumetrically more eolian sediments than the mid holocene event in contrast to earlier interpretations the classic mid holocene or altithermal event apparently did not play a major role in the formation of eolian landscapes in the chaco dune field

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

palaeoflood record of the tagus river central spain during the late pleistocene and holocene abstract palaeoflood hydrology of the tagus river central spain was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at el puente del arzobispo in the central part of the catchment 35 000 km 2 in drainage area and in alcantara at the lower part of the catchment 52 000 km 2 in drainage area near the portuguese border the palaeoflood record with more than 80 flood events shows clusters of floods at specific periods from 9440 to 9210 14 c yr bp 8540 8110 bc 8500 to 8000 14 c yr bp 7500 7000 bc 6750 14 c yr bp 5000 bc 1200 to 950 14 c yr bp ad 785 1205 410 14 c yr bp ad 1450 1500 170 to 80 14 c yr bp ad 1670 1950 the largest flood s occurred during the periods 9440 to 9210 14 c yr bp 6750 14 c yr bp and 1200 to 950 14 c yr bp reaching minimum discharge estimates of between 4000 and 4100 m 3 s 1 in the el puente del arzobispo reach and 13 700 15 000 m 3 s 1 in the alcantara reach these periods with increased flood magnitude and or frequency in the tagus river are strongly related to increased moisture influx and winter precipitation in the iberian peninsula especially in the western zone proxy records sensitive to winter precipitation such as lake levels and vegetation changes indicated by pollen records are in good agreement with the clusters of floods found in the tagus river this flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the iberian peninsula

palaeoflood hydrology of the salt river arizona geomorphic evidence along bedrock confined reaches of the salt river in east central arizona provides a record of the river s largest flood events fine grained flood slackwater deposits accumulated at channel margin irregularities several metres above the low flow channel discharges associated with flow events responsible for the deposits were estimated by computer flow modelling these estimates document flood magnitudes in excess of gauged historic streamflows relative and radiocarbon dating suggest that a flood record in excess of 600 y is preserved in the slackwater sequences a prominent flood scar cut into grussy hillslope soils allows the extension of the prehistoric flood record to several thousand years a maximum discharge estimate of 4600 m3s 1 affixed to the flood scar represents the largest flood event in the record and is given a minimum recurrence interval of 1000 2000 y the 1952 flood is the largest historic flow event experienced along the study reach and is estimated at 2900 m3s 1 two palaeoflood events preserved in the slackwater stratigraphy exceed the 1952 event and are given recurrence intervals of 300 and 600 y the latter flood event had an estimated discharge of 3200 m3s 1 it is apparent that discharge estimates affixed to these infrequent large magnitude flood events approach a maximum with decreased probabilities large recurrence intervals this suggests that a physical limit on discharge may exist within the salt river drainage basin and is perhaps directly related to drainage basin size

late holocene flooding records from the floodplain deposits of the yugu river south korea abstract predicting responses of regional hydrological cycles to global warming requires an understanding of past hydrological events on various timescales this study investigated the variability of flooding as recorded in fluvial floodplain deposits the percentage of sand sized material in two cores collected from the floodplain of the yugu river one of the tributaries of the keum river central south korea has been tested as a proxy for the occurrence and magnitude of past flooding the sand fraction of the floodplain deposits in cores kl28 and kl29 which record deposition over the last 3500 years varied on multicentury timescales and suggests several past episodes of higher frequency large floods these clusters of large floods seem to correlate well with northern hemisphere temperature anomalies and suggest that more frequent large floods occurred during the medieval warm period mwp than during the little ice age lia the occurrence of more frequent large floods on the keum river tributary may have been driven by intensified northerly winds over east asia and resultant moisture convergence over korea and or a weakened thermal gradient between the tropical ocean and asian continent furthermore the centennial scale changes in the sand fraction of the floodplain deposits were partly similar to remote el nino southern oscillation enso activity implying a possible link between flooding in central south korea and enso activity this study has demonstrated that floodplain deposits have much potential for recording paleofloods though floodplains can be and quite often are very complex places in terms of their depositional history in regions where natural lakes suitable for the study of paleohydrology and paleoclimate are lacking well controlled time series data for the sand content at multiple sites in fluvial floodplains may be used to understand paleoflooding and extreme precipitation variability in the past

radiocarbon dating of flood events katherine gorge northern territory australia radiocarbon analyses of organic materials provide accurate dating of flood events in the katherine gorge northern territory australia selected flood events in the period 1955 1980 can be very precisely dated essentially to the calendar year through the use of curves showing the time variation of anomalously high 14 c activity in the troposphere that was generated by nuclear testing conventional radiocarbon analysis of alluvial charcoal and organic litter is used to establish dates for the largest katherine river floods of the past several centuries the improved geochronologic techniques demonstrated here will help improve paleoflood hydrologic interpretations

paleoflood analysis of an alluvial channel in the south central great plains black bear creek oklahoma abstract late holocene floodplain of the black bear basin in north central oklahoma was chronologically developed based upon slackwater deposition and radiocarbon dating of associated paleosols major paleoflood events occurred 3590 80 yr bp and 1150 100 yr bp assumptions developed for application in semi arid bedrock channels were tested and modified maximum accumulation of slackwater deposits were located some 400 m up tributaries rather than at the intersection of the mouths of present day tributary junctions these sites represent the intersection with tributaries during the extreme widths of floods on an alluvial floodplain physically tracing slackwater units up tributaries was possible in some cases 8 km an accurate estimate of flood stage was determined by measuring up terrace perpendicular to the tributary during floods aggradation and degradation of the main channel were insignificant because the broad floodplain is the most impacted feature in an alluvial setting the preflood landscapes determined from paleosol elevations were recreated to estimate the manning roughness coefficients and paleoflood discharges the hec 2 water surface profile hydrologic engineering center 1982 determined that paleodischarges of 4391 cm in 3590 bp and 8782 cm in 1150 bp emplaced the slackwater deposits along the tributaries of black bear creek

late quaternary lacustrine history and paleoclimatic significance of pluvial lake cochise southeastern arizona abstract during the latest quaternary freshwater pluvial lakes intermittently formed in the topographically closed willcox basin arizona a lacustrine sequence of six separate high stands of lake cochise is documented by stratigraphic studies 19 radiocarbon ages and supplementary evidence two stands of pluvial lake cochise older than 14 000 yr b p reached elevations above 1290 m the prominent 1274 m shoreline of lake cochise which circumscribes the basin was largely created during a high stand between 13 750 and 13 400 yr b p during the holocene water filled the willcox basin three times to an elevation slightly below the crest of the 1274 m shoreline this occurred once during the early holocene around or before 8900 yr b p and twice during the later part of the middle holocene since the middle holocene only shallow ephemeral lakes have occupied the deflated central portion of ancient lake cochise a depression known as the willcox playa the lacustrine sequence of lake cochise provides an independent evaluation of late quaternary paleoclimatic reconstructions for southern arizona and the american southwest

climatic solar oceanic and geomagnetic influences on late glacial and holocene atmospheric 14c 12c change late glacial and holocene 14c12c ratios of atmospheric co2 vary in magnitude from a few per mil for annual decadal pertubations to more than 10 for events lasting millennia a data set illuminating 10 to 104 yr variability refines our understanding of oceanic climatic versus geomagnetic or solar forcing of atmospheric 14c12c ratios most of the variance in the holocene atmospheric 14c12c record can be attributed to the geomagnetic millennia time scale and solar century time scale influence on the flux of primary cosmic rays entering the atmosphere attributing the observed atmospheric 14c12c changes to climate alone leads to ocean circulation and or global wind speed changes incompatible with proxy records climate ocean related 14c redistribution between carbon reservoirs while evidently playing a minor role during the holocene may have perturbed atmospheric 14c12c ratios measurably during the late glacial younger dryas event first order corrections to the radiocarbon time scale 12 000 30 000 14c yr b p are calculated from adjusted lake sediment and tree ring records and from geomagnetically defined model 14c histories paleosunspot numbers 100 9700 cal yr b p are derived from the relationship of model 14c production rates to sunspot observations the spectral interpretation of the 14c12c atmospheric record favors higher than average solar activity levels for the next century minimal evidence was found for a sun weather relationship

ecogeomorphology in the australian drylands and the role of biota in mediating the effects of climate change on landscape processes and evolution

implications of palaeohydrological proxies on the late holocene indian summer monsoon variability western india abstract in this paper we identify changes in the indian summer monsoon ism intensity during the late holocene as revealed by palaeohydrological proxies in the fluvial archives of western india proxy indices including the geomorphological sedimentological geochemical and biological evidence along with chronology have been employed to highlight the trend in ism intensity over the late holocene it is inferred that on a longer timescale the indian summer monsoon weakened during the late holocene however short pulses of ameliorated monsoon occurred around 3 1 6 and 0 3 cal ka bp clustering of flood events is seen to occur at 3 2 8 2 2 1 6 1 3 1 1 and 0 65 0 2 cal ka bp not necessarily coinciding with strengthening of the ism palaeoflood data with regard to the ism variability shows that the two most prevalent flood events at 0 5 and 1 7 cal ka bp coincide with the weak monsoon period climatic transition in this region the occurrence of high magnitude flood events during weak monsoon periods is also reflected in the modern flood and rainfall data it is surmised that high magnitude flood events may represent peak monsoon periods however cannot be singularly considered as indicators for enhanced monsoon conditions in a region only a more robust database of palaeohydrological proxies can further validate their implications on the monsoonal changes over western india

a holocene climatic record for the sonoran desert from pollen analysis of montezuma well arizona usa abstract pollen and macrofossil analyses of a radiocarbon dated core 1125 cm long from montezuma well elev 1125 m yavapai county arizona provide a record of summer precipitation similar to that of other monsoonal regions of the world precipitation was least from 4000 to 5000 yr b p and generally was greater than today before 8400 yr b p during the early holocene moist period oak and grass pollen are abundant and the best modern analogs are within the arizona monsoon boundary so increased summer precipitation is indicated climatic parameters are estimated with the technique of best modern analogs for fossil samples temperature was highest ca 3400 and 5700 yr b p and generally was cooler than today before 6800 yr b p brief cold intervals in the holocene match periods of global cooling and of positive 14 c and 10 be anomalies

holocene climatic events recorded in palaeoflood slackwater deposits along the middle yiluohe river valley middle yellow river basin china abstract palaeohydrological investigations were carried out in the middle reaches of the yiluohe river a major tributary in the lower middle yellow river basin typical palaeoflood slackwater deposits swds were identified in the holocene pedostratigraphy on the cliffy river banks analytical results including magnetic susceptibility and grain size distribution data indicated that these swds were deposited from the suspended sediment load in flood water these swds are different from eolian loess soils and aeolian sands in the riverbank profile they recorded several episodes of extraordinary palaeoflood events in the longmenxia reaches of the yihe river valley these flood events were dated at 3100 3000 a 1800 1700 a 770 610 a and 420 340 a using the optically stimulated luminescence method in combination with the pedostratigraphic correlations in the longhutan reaches of the luohe river valley the palaeoflood events were dated at 1975 1466 a i e from the han to wei dynasties ad 25 534 during which the capital city on the river banks was flooded many times as recorded in the literature these extraordinary flood events are well correlated chronologically with the known holocene climatic events that occurred in the northern hemisphere thus the monsoonal climate was highly variable with both floods and droughts occurring frequently during these episodes these results are important for understanding the response of river systems in eastern asia to global changes

stasis and change in holocene small mammal diversity during a period of aridification in southeastern utah biological conservation depends on understanding and disentangling the effects of decadal to centennial scale dynamics from the millennial scale dynamics documented in the fossil record the american southwest is expected to become increasingly arid over the next few decades and will continue to experience large scale human land use in various forms a primary question is whether the ecological fluctuations recorded over the past few decades fall outside the range of variation expected in the absence of recent land use and management i excavated and quantified mammal diversity change in two fossil bearing alcoves east canyon rims 2 and rone bailey alcove located in san juan county utah ams radiocarbon dates on 33 bone samples from these sites span 4 4 0 5 kyr and shed light on pre industrial faunal dynamics in the region over the course of environmental change localities with comparable small mammal diversity have not been reported from this region previously so these deposits provide novel insig

rapid climatic change in coastal southern california inferred from pollen analysis of san joaquin marsh pollen analysis and five radiocarbon dates of a 687 cm core provide a detailed chronology of environmental change in a marsh at the head of newport bay orange county california sediment deposition kept pace with sea level rise during the early history of the marsh from ca 7000 to 4500 yr b p the site was a freshwater marsh trees were more abundant than today and grassland was the regional vegetation as sea level rose salt marsh gradually invaded the site brief periods of freshwater marsh 3800 2800 2300 and after 560 yr b p correlate with episodes of global cooling during the neoglacial the historic period is marked by the appearance of exotic species particularly erodium cf cicutarium and eucalyptus and the spores of fungi sporormiella and thecaphora peak influx of pollen spores and charcoal probably reflect greater frequency of flooding and erosion ca 5000 yr b p and during the last 1000 yr

bayesian age modelling applied to palaeoflood geochronologies and the investigation of holocene flood magnitude and frequency a new methodology based on bayesian age modelling is presented for the analysis of palaeoflood geochronologies and palaeodischarge data bayesian age models were developed using the oxcal radiocarbon calibration software for the geochronologies of three palaeoflood sites in spain namely the gaudalentin tagus and llobregat rivers in se central and ne spain respectively the age depth plots resulting from the applied sequence models enabled the construction of flood magnitude frequency plots through substitution of the original stratigraphic depth data with the associated minimum discharge quantified by hydraulic modelling the age models presented demonstrate that a bayesian approach for analyzing holocene flood magnitude and frequency prevents the loss of geomorphic and hydrologic information inherent in radiocarbon frequency methods previously used in the analysis of palaeoflood data sets frequency approaches do not allow proper consideration of flood magnitude information and only incorporate t

paleoflood evidence for a natural upper bound to flood magnitudes in the colorado river basin the existence of an upper limit to the magnitude of floods in a region is a long standing and controversial hypothesis in flood hydrology regional envelope curves encompassing maximum flood magnitudes stabilize with progressive increases in the areal coverage and period of observation wolman and costa 1984 however the short lengths of conventional gaging records limit substantial advances in testing whether this stabilization is evidence of an upper limit in the colorado river basin there are 32 120 station years of gage data but the average period at a gaging station is only 20 years with most stations having less than 70 years of observation paleoflood magnitudes derived from sediments of large prehistoric floods from 25 sites on rivers in arizona and utah provide additional data to extend the records of the largest floods the paleoflood data identify the maximum flood discharges that have occurred on individual rivers over the last several hundred to several thousand years even with this increase in the observational period the largest paleoflood discharges do not exceed the upper bound of maximum peak discharges delineated by the envelope curve derived from the available gaged and historical records this result accords with the hypothesis of an upper physical limit for flood magnitudes and suggests that for the colorado river basin the upper limit can be approximated by existing systematic and historical data for large floods similar relationships also hold when paleofloods and gaged records are presented for the subregion of southern arizona

investigating extreme flood response to holocene palaeoclimate in the chinese monsoonal zone a palaeoflood case study from the hanjiang river abstract palaeoflood events recorded by slackwater deposits swds were investigated extensively by sedimentological criteria of palaeohydrology along the upper hanjiang river valley modern flood swds were collected for comparison with palaeoflood swd in the same reaches three typical palaeoflood swds were observed within holocene loess soil blanket on the first river terrace land the grain size distributions of palaeoflood swds are similar to modern flood swds whereas they are different from eolian loess and soil palaeoflood swd lies in three major pedo stratigraphic boundaries ts l 0 l 0 s 0 and s 0 l t in the holocene loess soil profiles the chronology of three palaeoflood episodes was established by osl dating and pedo stratigraphic correlation with the well dated holocene loess soil profiles in the upper hanjiang river basin holocene palaeoflood events were dated to 9500 8500 3200 2800 and 1800 1700 a b p respectively palaeoflood discharges were estimated by the palaeoflood model i e slope area method and step backwater method the highest discharges are 51 680 53 950 m 3 s 1 at the 11 500 time scale in the xunyang reach of the upper hanjiang river valley holocene extraordinary hydroclimatic events in the hanjiang river often result from abnormal atmospheric circulations from southwest monsoons in the chinese monsoonal zone these results provide a regional expression of extreme flood response to holocene palaeoclimate to understand the effects of global climatic variations on the river system dynamics

simulated changes in extreme temperature and precipitation events at 6 ka abstract we have employed a high resolution regional climate model rcm to test the sensitivity of extreme climate events to 6 ka orbital forcing using western north america as a case study calculated differences 6 ka control in annual extreme precipitation event frequency were mostly positive throughout the rcm domain as were differences in extreme event contribution to total annual precipitation these annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka which was in turn linked to changes in the daily variability of upper level zonal wind speeds over the northeast pacific positive differences in the frequency of extreme maximum daily temperature values occurred inland in the rcm domain with peak differences of 28 days year over the great basin likewise days in which the maximum daily temperature exceeded 32 c were 24 more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer mean first and last freeze dates were generally earlier in the 6 ka integration along the pacific coast and over the southern great basin and later over the northern great basin and interior of the pacific northwest we propose that by exerting controls on paleoclimate archives unique from changes in the mean climate state such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid holocene paleoclimate record of the region and that extreme climate events may have been similarly important in other regions and during other time periods

holocene glacier fluctuations in the american cordillera abstract in many areas of the american rocky mountains colorado wyoming utah idaho montana cascade range washington oregon and the sierra nevada california radiocarbon ages suggest that ice receded to near present limits before 10 ka bp a pre altithermal readvance or stillstand left moraines ca 1 3 km beyond and ca 50 300 m below present glacier margins at one locality on glacier peak in washington these deposits are perhaps dated to the early holocene but in general these deposits are probably at least 10 ka old glacial advances during the altithermal ca 8 to 5 ka bp elsewhere are rare radiocarbon evidence for altithermal glacial advances in the colorado front range is questionable the earliest radiocarbon dated neoglacial advances occurred about 5 ka bp in the washington cascades moraines and related deposits believed to be of early neoglacial age i e 5 to 3 ka bp elsewhere are poorly dated for example moraines believed to date to the early neoglacial in colorado triple lakes and wyoming temple lake are actually late pleistocene age based on radiocarbon ages derived from lake sediments at the type localities although relative age data i e lichens rock weathering soils from many areas suggest a three fold neoglacial sequence little supporting radiocarbon evidence is available however tephrochronology distinguishes three neoglacial advances near mount rainier in washington in most mountain ranges of the western united states fresh unweathered sharp crested moraines usually adjacent to present ice margins or near headwalls in empty cirques date to the little ice age of the last several centuries however detailed photographic or other historical records of glacial fluctuations during the last century are rare and detailed mass balance studies rarer still whether or not pre little ice age glacial fluctuations in the american cordillera are synchronous must await better radiometric dating of local moraine sequences

a 4500 year record of large floods on the colorado river in the grand canyon arizona a sequence of flood deposits left by the colorado river in the grand canyon arizona provides evidence of at least 15 floods with peak discharges greater than 5500 m 3 sec 1 over the last 4500 yr ten floods during the last 2000 2300 yr had discharges greater than 6800 m 3 sec 1 one flood 1600 1200 yr ago had a discharge exceeding 14 000 m 3 sec 1 a flow rate more than twice the largest gaged flood this record of flooding is one of the longest for a major u s river and combined with the gaged record of twentieth century floods allows determination of the frequency and history of large floods that have affected key aspects of colorado river geomorphology

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

flood geomorphology of the katherine gorge northern territory australia the katherine gorge is a narrow deep canyon system developed in resistant sandstone the tropical monsoonal climate results in rare flood events of very large magnitude these floods flush sand sized sediment as washload through the gorge but local accumulations are preserved as slack water deposits at tributary mouths paleoflood hydrologic analysis of the deposits employing step backwater flow modeling allows quantitative estimates to be made of geomorphically significant flows for an annual exceedence probability of 10 2 the gorge may experience discharges of 6 000 m 3 s mean velocities of 7 5 m s flow depths of 15 to 45 m stream powers per unit area as great as 1 10 4 watts m 2 and bed shear stresses of 1 5 10 3 n m 2 such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large scale riffles at maximum stage pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock probably through intense hydraulic action pool and riffle development thus reflects the channel boundary characteristics for extreme flow conditions additional indicators of especially intense flow phenomena include potholes flutes abraded facets on rock bar surfaces and scabland development on upland bedrock surfaces

late holocene vegetation changes in greenwater valley mojave desert california abstract small scale late holocene vegetation changes were determined from a series of 13 modern and fossil packrat middens collected from a site in the greenwater valley northern mojave desert california although the site is above the modern lower limit of coleogyne ramosissima black brush macrofossils of this shrub are only present in samples younger than 270 yr b p in order to measure changes more subtle than presence vs absence macrofossil concentrations were quantified and principal components and factor analyses were used to distinguish midden plant assemblages both the presence absence data and the statistical analyses suggest a downward shift of 50 to 100 m for coleogyne blackbrush communities between 1435 and 1795 a d

late quaternary geology of the middle gila river gila river indian reservation arizona abstract stratigraphy and dating provide an 18 000 yr record of erosion and deposition by the middle gila river prior to 18 000 cal yr b p the gila river cut into its floodplain and created a deep wide channel sand and gravel accumulated within this channel until 4250 4400 cal yr b p by this time the channel of the gila river was narrow and deep around 5000 cal yr b p fine grained sediments began to accumulate on the floodplain at 800 950 cal yr b p there was a major period of channel widening after this erosional episode the channel again narrowed as it filled with sand and gravel until around 200 cal yr b p on the floodplain deposition continued from about 5000 cal yr b p to 500 cal yr b p a period of stability and soil formation occurred on the floodplain between 500 and 200 cal yr b p after which overbank deposition resumed and buried the soil channel widening again occurred in the late 19th century the changes observed in the sedimentologic history of the gila river show that during the late pleistocene the gila river was a competent stream capable of carrying and depositing coarse sediment loads later deposition of sand and gravel during the first half of the holocene implies an increase in sediment yield from upstream watersheds changes during the last 4000 years reflect the response of the river to climatic perturbations the timing of large floods internal landscape thresholds and human impacts

the influence of bedrock weathering on the response of drainage basins and associated alluvial fans to holocene climates san bernardino mountains california usa the primary factors that control alluvial fan evolution still remain in question particularly for the holocene holocene centennial and millennial scale climate fluctuations are relatively subtle and more frequent than those of glacial interglacial transitions therefore intrinsic factors such as rock type or basin size are hypothesized to moderate significantly the influence of holocene climate and climate change on alluvial fan processes here we examine variability in styles and rates of alluvial fan aggradation along a single mountain front that is characterized by basins of varying size and rock type carbonate versus granite basin rock type is more closely correlated to variability in the episodic nature and magnitude of alluvial fan aggradation than is basin area bedrock physical and chemical weathering properties control sediment delivery to the piedmont and thus influence alluvial fan aggradation we suggest that the particle size of grus produced by weathering of granitic rocks fosters sedi

flood frequency of the mojave river and the formation of late holocene playa lakes southern california usa perennial lakes lasting several decades occupied silver lake playa the terminus of the mojave river as recently as 390 90 and 3620 70 bp flood routings from the mojave river headwaters to its terminal playa and water balance calculations for the resulting lakes indicate that floods comparable in magnitude to the largest floods in the modern record must occur almost annually to generate and maintain perennial lakes in contrast the modern floods of this magnitude have recurrence intervals greater than 18 years such an increase in flood frequency is required even if lake evaporation is reduced by as much as 50 modern extreme floods of the mojave river are related to meridional atmospheric circulation and a southeastward shift in the winter low pressure center over the north pacific ocean causing vigorous storms over the headwaters in the san bernardino mountains of southern california this winter pattern probably occurred more frequently to induce the large number of storms and floods needed

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

geobody architecture genesis and petrophysical characteristics of the budakalasz travertines buda hills hungary subsurface pre salt lower cretaceous reservoir systems encountered offshore brazil and angola that consist of porous continental carbonates are difficult to access because of the limited core material that is under concession therefore it is important to study comparable continental carbonate analogues such as travertines and lacustrine carbonates that potentially possess similar porosity and permeability characteristics the budakalasz pleistocene travertines buda hills hungary have been studied from a sedimentological geochemical and reservoir properties point of view in order to understand their formation petrophysical properties and 3d reservoir architecture the three dimensional geobody architecture of the budakalasz travertines illustrates the potential complexity of kilometre scale continental carbonate systems heterogeneous low angle sloping travertine lobes consisting of terrace slope smooth slope and reed facies are covered by a lime mudstone dominated gastropod and charophyte rich marsh pool facies the observed facies represent a change from a subaerial travertine to a palustrine to lacustrine system reflecting the ceasing of the system due to tectonic uplift the dominant paleo flow direction of the sloping lobes was towards the danube depression with several subordinate flow systems identified the srco isotope geochemical signatures reflect travertine precipitation at ambient to superambient temperatures from meteoric derived fluids with a mixed co2 origin of the marine carbonate substrate rocks a potential atmospheric and a soil derived component although the uniform signatures of the stable oxygen and carbon isotopes in addition to petrographic observations suggest partial diagenetic isotopic resetting the primary isotope trends seem to have been preserved a reservoir oriented facies evaluation in the three dimensional geobody frame of the budakalasz carbonates based on porosity and permeability core plug analyses reveals their heterogeneous nature terrace smooth slope and marsh pool facies have considerable porosities of 5 15 while the porosity of the reed facies may reach up to 27 permeability of all facies is strongly anisotropic mainly controlled by a better connectivity along layer parallel primary pores the reconstruction of the 3d geobody architecture of the budakalasz lobe complex coupled to its geochemical and petrophysical properties provides a unique dataset that contributes in understanding fossil travertine systems

paleofloods and a dam failure flood on the virgin river utah and arizona abstract flood deposits on the virgin river in southern utah and northern arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since ad 900 in the virgin river gorge several of the natural paleoflood discharges were comparable with or even greater than the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m3 s 1 in this reach of the virgin river to estimate the paleoflood discharges the heights of slackwater flood deposits in the lower bedrock gorge of the virgin river were compared with water surface elevations calculated with the hec 2 step backwater hydraulic model the timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits the estimates of both the 1989 flood and the paleofloods were carried out in the virgin river gorge about 60 km downstream of the dam break site no natural floods of this magnitude have occurred within the relatively short period of the gaged record this study illustrates how incorporating rare extreme events from the long term paleoflood record improves the database for flood frequency analysis the 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower virgin river

glacial interglacial changes in the isotopes of nitrate from the greenland ice sheet project 2 gisp2 ice core received 3 march 2005 revised 23 september 2005 accepted 26 october 2005 published 24 december 2005 1 the 15 n 14 n and 18 o 16 o ratios of nitrate in the greenland ice sheet project 2 gisp2 summit greenland ice core are much higher in ice from the last glacial period than in the pre industrial holocene despite the lack of a significant glacial interglacial change in nitrate concentration while both the 15 n 14 n and 18 o 16 o records are anticorrelated with snow accumulation rate neither is satisfactorily explained by accumulation changes or post depositional processes the similarity in the glacial interglacial change in 15 n 14 n from several different greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large scale glacial interglacial change in the isotopic composition of atmospheric nox the glacial interglacial change in 18 o 16 oi s best explained by a greater contribution of hno3 production from hydrolysis of n2o5 which has implications for reconstruction of past atmospheric oxidant levels although isotope effects associated with nox photochemistry and nitrate scavenging have not been fully characterized the 15 n 14 n data may indicate glacial interglacial changes in the relative contributions from different natural sources of nox on a hemispheric or global scale

relation of sediment load and flood plain formation to climatic variability paria river drainage basin utah and arizona suspended sediment load flow volume and flood characteristics of the paria river were analyzed to determine their relation to climate and flood plain alluviation between 1923 and 1986 flood plain alluviation began about 1940 at a time of decreasing magnitude and frequency of floods in winter summer and fall no floods with stages high enough to inundate the flood plain have occurred since 1980 and thus no flood plain alluviation has occurred since then the decrease in magnitude and frequency of floods appears to have resulted from a decrease in frequency of large storms particularly dissipating tropical cyclones and not from a decrease in annual or seasonal precipitation suspended sediment load is highest in summer and fall whereas flow volume is highest in winter fall shows the greatest interannual variability in suspended sediment load flow volume and flood size because climatic conditions are most variable in fall the relation between sediment load and discharge apparently did not change within the period of sediment sampling 1949 1976 even though the channel elevation and width changed significantly annual suspended sediment loads estimated for periods before and after 1949 1976 show that decrease in suspended sediment load caused by floodplain alluviation in the paria river and other tributaries could have been a significant part of the decrease of suspended sediment load in the colorado river in the early 1940s

the evolution of hypoxia off the changjiang estuary in the last 3000 years evidence from benthic foraminifera and elemental geochemistry abstract estuarine hypoxia has exerted great influence on the ecological system and fishery production in the past decades consequently attracting intensive concern in this study the upper 18 0 m of core yd0902 retrieved from the hypoxic area off the changjiang estuary was dated by ams 14c method and analyzed with lithology grain size benthic foraminifera and geochemical elements to decipher the paleoenvironmental change and hypoxic history in the last 3000 years the core location was in the neritic to prodelta transitional environment during 3000 2160 cal yr bp and the delta front environment after 2160 cal yr bp no hypoxia occurred during 3000 2600 cal yr bp potentially related to the contemporary occurrence of a rapid cold climate event in the northern hemisphere hypoxia has likely existed persistently since 2600 cal yr bp the period 2600 600 cal yr bp characterizes persistently severe hypoxia mainly linked with the warm climate strong east asian summer monsoon easm and rainfall intensified taiwan warm current intrusion and upwelling and high primary production lessened hypoxia followed in the period 600 260 cal yr bp because of the cold climate weak easm and minor precipitation the recent period post 260 cal yr bp was not disclosed by the core due to lost sediments in the top 4 5 m and could only be surmised to have severe hypoxia because of climate factors similar to the period 2600 600 cal yr bp our findings shed new light on long term hypoxic evolution and controlling factors in mega river estuaries

holocene changes in flooding frequency in south korea and their linkage to centennial to millennial scale el nino southern oscillation activity to trace past changes in flooding frequency we investigated fluvial sediments in the middle reach of the nakdong river south korea sediments with larger grain size lower total organic carbon percentage and depleted 13 c values in the recovered sediment cores were interpreted as periods of more frequent flooding patterns of decreased long term flooding frequency and vegetation changes during the early to late holocene were similar to the decreasing regional summer monsoon intensity multicentennial frequent flooding periods in the study area 2900 3400 cal yr bp 3600 3900 cal yr bp 4600 5300 cal yr bp and 5800 6400 cal yr bp corresponded to stronger el nino southern oscillation enso activity periods based on previous studies showing that high frequency tropical typhoon driven coastal inundation along the western coast of japan during the middle to late holocene was coupled with stronger enso activity it is likely that the observed centennial to millennial scale flood frequency change in south korea was influenced mainly by changes in the genesis and tracks of tropical typhoons at centennial to millennial time scales this suggests that the centennial to millennial scale hydrologic changes in east asia were linked to the remote atmospheric oceanic circulation changes represented by an enso like pattern

persistent millennial scale shifts in moisture regimes in western canada during the past six millennia inferences of past climatic conditions from a sedimentary record from big lake british columbia canada over the past 5 500 years show strong millennial scale patterns which oscillate between periods of wet and drier climatic conditions higher frequency decadal to centennial scale fluctuations also occur within the dominant millennial scale patterns these changes in climatic conditions are based on estimates of changes in lake depth and salinity inferred from diatom assemblages in a well dated sediment core after periods of relative stability abrupt shifts in diatom assemblages and inferred climatic conditions occur approximately every 1 220 years the correspondence of these shifts to millennial scale variations in records of glacial expansion recession and ice rafting events in the atlantic suggest that abrupt millennial scale shifts are important to understanding climatic variability in north america during the mid to late holocene unfortunately the spatial patterns and mechanisms behind these large and abrupt swings are poorly understood similar abrupt and prolonged changes in climatic conditions today could pose major societal challenges for many regions

extraordinary flood events and the response to monsoonal climatic change during the last 3000 years along the middle yangtze river valley china abstract well dated long term flood data series is very important in studying of hydrological response to past climatic change along the mainstream of the yangtze river five palaeoflood events recorded by slackwater deposits were identified by the sedimentological criteria and analytical results a detailed chronological framework of palaeoflood events during the late holocene was established by optically stimulated luminescence dating in combination with anthropogenic remains and with pedo stratigraphic correlations with well dated holocene loess soil profiles within the middle yangtze river basin the five palaeoflood events were dated to 2 43 0 18 ka 1 82 0 17 ka 0 94 0 07 ka 0 72 0 06 ka and 0 59 0 05 ka respectively these results are consistent with geological records at other palaeoflood sites and historical flood records along the middle yangtze river valley a regional synthesis of the palaeoflood chronology over the last 3000 years was compiled along the middle yangtze river extraordinary flood events generally seem to be correlated with late holocene climatic variability i e neoglacial cooling roman warm period dark ages cold period medieval climate anomaly and little ice age the high resolution climatic proxies from stalagmites of the dongge cave and heshang cave ice cores of the grip the total solar irradiance variations and sunspot number and enso activities suggest that these hydroclimatic events are possibly related to the weaker asia summer monsoon and the cooling climatic events as well as the stronger enso activities during the late holocene in the yangtze river valley these results provide insights into the response of hydroclimatic system to global change in the large rivers of asia

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

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern

a 1000 year record of temperature and precipitation in the sierra nevada abstract tree ring data from subalpine conifers in the southern sierra nevada were used to reconstruct temperature and precipitation back to a d 800 tree growth of foxtail pine pinus balfouriana and western juniper juniperus occidentalis ssp australis is influenced by nonlinear interactions between summer temperature and winter precipitation reconstruction of the separate histories of temperature and precipitation is feasible by explicitly modeling species and site differences in climatic response using response surfaces the summer temperature reconstruction shows fluctuations on centennial and longer time scales including a period with temperatures exceeding late 20th century values from ca 1100 to 1375 a d corresponding to the medieval warm period identified in other proxy data sources and a period of cold temperatures from ca 1450 to 1850 corresponding to the little ice age precipitation variation is dominated by shorter period decadal scale oscillations the long term record presented here indicates that the 20th century is anomalous with respect to precipitation variation a tabulation of 20 and 50 yr means indicates that precipitation equaling or exceeding 20th century levels occurred infrequently in the 1000 yr record