surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

history and isostatic effects of the last ice sheet in southern british columbia abstract the late wisconsinan cordilleran ice sheet covered british columbia southern yukon territory and parts of alaska washington idaho and montana its major source areas were the high mountain ranges of the canadian cordillera and flow was strongly controlled by topography the last ice sheet began to develop about 30 000 25 000 14c yr ago but it did not achieve its maximum extent until 15 000 14 000 14c yr bp ice sheet growth was interrupted locally by stillstands and retreat ice sheet decay was rapid and was characterized by complex frontal retreat at the periphery accompanied locally by brief readvances and by downwasting and stagnation by 10 000 14c yr bp

soil erosion in relation to climate change and vegetation cover over the past 2000 years as inferred from the tianchi lake in the chinese loess plateau abstract understanding the response of past soil erosion to climate change and human disturbances is quite essential for managing and preventing soil erosion in the future here we report a high resolution soil erosion sequence during the past 2000 years based on grain size data from a sediment core gs07a in tianchi lake in the chinese loess plateau these grain size data are analysed by end member modelling and further deciphered by detailed modern investigations of surface sediments surface soils and modern dust samples aided by published grain size distribution characteristics of different depositional provenances and processes six strong soil erosion periods including more than 20 serious soil erosion events are inferred at 150 300 ce 450 750 ce 900 1100 ce 1200 1400 ce 1650 1800 ce and 1950 2000 ce these soil erosion records correspond well with intensive flood events in downstream rivers and the neighbouring longxi area documented from the historical literature suggesting that regional high intensity rainfall or flood events are the main cause of soil erosion vegetation cover in the catchment also influenced soil erosion light vegetation cover aggravated soil erosion even in the periods with a low frequency of flooding while heavy cover reduced soil erosion even in the periods with a high frequency of flooding our results highlight the role of sustainable land use and vegetation restoration around the catchment in controlling soil erosion in the chinese loess plateau

a transferable method for the automated grain sizing of river gravels 1 the spatial and temporal resolution of surface grain size characterization is constrained by the limitations of traditional measurement techniques in this paper we present an extremely rapid image processing based procedure for the measurement of exposed fluvial gravels and other coarse grained sediments defining the steps required to minimize the errors in the derived grain size distribution this procedure differs significantly from those used previously it is based around a robust object detection algorithm that produces excellent results on images exhibiting a wide range of sedimentary conditions crucially without any user intervention or site specific parameterization the procedure is tested using a data set comprising 39 images from three rivers with contrasting grain lithology shape roundness and packing configuration and representing a very wide range of textures it is shown to perform more consistently than the best existing automated method achieving a precision equivalent to that obtainable by wolman sampling but taking between one sixth and one twentieth of the time the error in area by number grain size distribution percentiles is typically less than 0 05

anthropogenic influences on changes in the sediment load of the yellow river china during the holocene using data on formation of the loess plateau of central china loess palaeosol sequences pollen analysis historical geography and long term experimentation on rates of soil erosion it is demonstrated that current serious erosion on the loess plateau is mainly due to human destruction of the natural vegetation and irrational land use these two modifications greatly reduce soil infiltration capacity and resistance of the soil to erosion the increase of soil erosion began ca 5000 bp at which time a hunting and pastoral life began to change gradually to a sedentary life centred around cultivation using the volume of holocene loessic sediments on the continental shelf of the bohai yellow and east china seas it is estimated that the present sediment load of the lower yellow river is at least three times greater than it was during the holocene although human induced soil erosion in the loess plateau began earlier it was not until the tang dynasty about 1000 bp that it began to increase rapidly ac

effective discharge in small formerly glaciated mountain streams of british columbia limitations and implications episodic sediment supply past glaciation and slow responses to disturbance make small mountain streams transitional alluvial regimes in which nonequilibrium conditions are common bed load effective discharge in these streams is on average a low magnitude high frequency event but is highly variable using a two phase sediment transport model and long term discharge records we distinguish between three types of streams streams in which gravel sediment 8 mm diameter moves frequently and effective discharge occurs during gravel transport frequently mobile gravel fmg streams in which gravel moves infrequently but effective discharge nonetheless occurs during gravel transport infrequently mobile gravel img and streams in which sand sediment 8 mm diameter moves over largely immobile gravel and effective discharge occurs frequently during sand phase transport sand over immobile gravel sg using only effective discharge frequency or magnitude to characterize a stream without information on mobile sediment type is insufficient to distinguish between fmg and sg streams only the img streams have large rare effective discharges that approximate the bankfull discharge in fmg and sg streams the effective discharge is much more frequent and smaller than the bankfull only in the img streams does the effective discharge approximate a channel forming discharge in fmg and sg streams the effective discharge bears little relation to the size or dimensions of the channel and is at best a channel maintaining flow at worst it is geomorphically meaningless effective discharge should not therefore be used in isolation as a proxy for channel forming discharge for mountain stream channel design or management

changes in sand content of loess deposits along a north south transect of the chinese loess plateau and the implications for desert variations geological records have shown that the deserts east of the helan mountains in northern china were covered by grass during the holocene optimum whereas during marine oxygen isotope stages 2 and 4 distribution of the deserts was almost the same as at present the wide advance retreat cycles of the deserts may have exerted an important control on grain size changes in the loess of the loess plateau by altering the distance between the source and the accumulation zone of the loess this challenges the widely accepted model that winter monsoon winds were the sole factor responsible for spatial and temporal changes in loess texture to observe spatial changes in sedimentological characteristics of loess during the last glacial interglacial cycle the texture of loess was measured along a north south transect of the loess plateau this transect consists of nine loess sections starting at yulin in the transitional region between the loess plateau and the mu us desert and ending at weinan in the southernmost part of the loess plateau southward changes in sand 63 m content along the transect suggest that variations in desert extent have indeed played a significant role in loess grain size distributions particularly in the northern part of the loess plateau it is proposed that sand content 63 m of loess in the loess desert transitional zone may be used as a proxy indicator for proximity to the desert margin

effective discharge recurrence intervals of illinois streams abstract effective discharge values are routinely used to characterize the geomorphic impact work that different flows have on streams and watersheds effective discharge values are also routinely employed in stream restoration design reliable flow frequency and sediment rating curves are needed to compute effective discharge values but are not available for many streams in lieu of regional effective discharge data researchers engineers often assume that a stream s effective discharge is the same as the 1 5 year flow event for that stream however research shows that this is not always true hence demand exists for developing regional effective discharge values that can be applied at ungaged stream locations a regional study of effective discharge values within illinois is performed eighty eight monitoring sites were identified as locations where effective discharge values could potentially be computed twenty of these sites were determined to have sufficient data to compute effective discharge values for these stations effective discharge values were computed using two different sediment discharge relationships a power curve and an alternative mean approach the mean approach estimates effective discharge values that are on average 1 9 times larger than those estimated using the power curve the sediment load carried by flows less than or equal to the effective discharge value are on average 2 7 times larger when the mean approach is used both the power curve and mean approaches typically yield effective discharge values greater than the stream s mean flow and less than its 1 1 year annual maximum series flow event some of the computed effective discharge values are less than the stream s mean flow computing effective discharge values remains a subjective process with large uncertainties involved class interval size sediment rating curve fitting techniques and available sediment data influence effective discharge results a paucity of monitoring data for illinois streams draining 2 where many geomorphic analyses and stream restoration projects take place hinders the regional applicability of effective discharge results the uncertainties associated with effective discharge values and the inability to validate effective discharge values as a stream s dominant discharge makes channel design using effective and dominant discharge theory problematic suggestions for improving regional effective discharge studies and effective discharge estimates are provided

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

the formation of a tidal intrusion front at the mouth of a macrotidal partially mixed estuary a field study of the dart estuary uk high resolution measurements of density and velocity were made before during and after the formation of a transient tidal intrusion front at the mouth of a narrow partially mixed estuary during spring tides detailed acoustic doppler current profiler adcp and density measurements throughout 1 km longitudinal transects at hourly intervals during spring tides indicated that the front formed when the barotropic inflow was sufficient to arrest the buoyant outflow from the estuary this has been shown to be driven by a constriction of the channel at the mouth and a depression in the seabed just landward of the mouth with these changes in topography shown to interact with the flow in a similar manner to theoretical two layer flow over the lee side of a sill a detailed analysis of the flow and stratification across the narrow frontal interface found a high degree of shear and mixing just prior to the plunge point which together with a hydraulic jump at the seabed depression eventually led to the destruction of the density interface and the total intrusion of seawater as the flooding tide strengthened there was no surface manifestation of this front during neap tides and further measurements have confirmed that the relatively weak barotropic flow at this time was not sufficient to overcome the stratification in a two layer regime an evaluation of the key frontal discriminators has confirmed the validity of their use in such an estuary

erosion caused by hyperconcentrated flow on the loess plateau of china abstract hyperconcentrated flows are widely observed in the middle yellow river basin they occur as a result of erosion but once they form on slopes they operate as a strong eroding and sediment carrying force and play an important role in drainage basin erosion and sediment production processes since the energy expenditure is less and sediment carrying capacity is greater hyperconcentrated flows have very high eroding and transporting abilities there exists a strong coupling relation between hyperconcentrated flow transport and gravitational erosion which ensures a rapid removal of mass wasted materials hyperconcentrated flows are an important factor responsible for the high intensity erosion and sediment yield in the middle yellow river basin many previous studies have indicated that the sediment delivery ratio on the loess plateau approaches 1 this can be explained by the action of hyperconcentrated flows as hyperconcentrated flows exhibit some particular behaviors in erosion and sediment production processes the present study suggests that the erosion by hyperconcentrated flows be regarded as a special type of erosion accordingly some countermeasures are proposed to control hyperconcentrated flows and the resulting erosion

holocene climate changes over the desert loess transition of north central china a 2 m trench section at the desert loess transition of north central china was analyzed at 2 cm intervals for grain size distribution md m and total organic carbon toc ams 14c time series of the md and toc records provides direct information on variations of the east asian monsoon climate during the last 10 000 yr md is considered to represent the wind strength and high md reflects strong winter monsoon winds toc denotes the degree of vegetation cover a function of the intensity of summer monsoon circulations from ca 10 000 to 7000 cal yr bp the climate was dominated by a strong winter monsoon and a very weak summer monsoon as indicated by high md and low toc values subsequently climatic conditions became warmer gradually from ca 7000 to 5500 cal yr bp the period between ca 5500 and 2700 cal yr bp displayed the highest toc and lowest md values over the last 10 000 yr marking a stable state of a warm humid climate i e the holocene climatic optimum of china during the interval of ca 2700 1500 cal yr bp the expansion and intensification of agrarian activities could have led to desertification and acceleration of deposition of coarse grained particles resulting in a decreased organic carbon accumulation rate however the toc during this interval still has values as high as during the episode of ca 7000 5500 cal yr bp reflecting a relatively stronger summer monsoon circulation since ca 1500 cal yr ago high amplitude fluctuations of toc that fluctuate in phase with the clay size fraction imply that the climate experienced three major oscillations with a prominent peak of enhanced summer monsoon circulation at ca 1300 cal yr ago and with two intervals of increased winter monsoon strength occurring around ca 1000 and 300 cal yr ago the md and toc records over the past 5000 yr demonstrate a good correlation with the archaeological phenological and historical records of china climate changes in the east asian monsoon region were not only related to the changing seasonality of solar insolation resulting from progressive changes in the earth s orbital parameters but were also controlled by the pattern and intensity of the atmospheric circulation

paleorainfall reconstructions from pedogenic magnetic susceptibility variations in the chinese loess and paleosols the rock magnetic properties of the chinese loess and paleosols constitute a unique and sensitive record of east asian paleoclimate through the quaternary period systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal which can be easily and rapidly measured at many sites across the loess plateau variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages as well as glaciations and interglaciations past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations the susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the loess plateau our rainfall reconstructions identify enhanced summer monsoonal activity in the chinese loess plateau region in the early holocene and the last interglaciation in the presently semiarid western area of the plateau annual precipitation in interglacial times was up to 80 higher than at present in the more humid southern and eastern areas values were up to 20 higher than today s levels during the last glaciation precipitation decreased across the entire plateau typically by 25 the relationship between pedogenic susceptibility climate and weathering age was examined over the northern hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate rather than soil age is the predominant factor that controls pedogenic susceptibility enhancement in loess soils

intratidal variations in stratification and mixing in the hudson estuary this study represents a detailed examination of intratidal variations in boundary layer structure in a relatively narrow straight section of the hudson river estuary during high river discharge the presence of active mixing is inferred from estimates of the richardson number the maximum height above bed to which mixing occurs is comparable during flood and ebb however the regions of mixing differ in density structure during the flood active mixing is confined to a well mixed near bed layer whereas during the ebb active mixing occurs throughout regions of significant stratification tidal straining maintains stratification during the ebb while it promotes the growth of a uniform bottom mixed layer during the flood in general maximum and minimum near bed stratification occur during late ebb and flood respectively reflecting the dominant role played by tidal straining in determining intratidal variation vertical stretching was also found to be an important term in the stratification balance during the ebb

hydrological and sedimentary controls over fluvial thermal erosion the lena river central yakutia abstract water regime and sedimentary features of the middle lena river reach near yakutsk central yakutia were studied to assess their control over fluvial thermal erosion the lena river floodplain in the studied reach has complex structure and embodies multiple levels varying in height and origin two key sites corresponding to high and medium floodplain levels were surveyed in 2008 to describe major sedimentary units and properties of bank material three units are present in both profiles corresponding to topsoil overbank cohesive and channel fill noncohesive deposits thermoerosional activity is mostly confined to a basal layer of frozen channel fill deposits and in general occurs within a certain water level interval magnitude frequency analysis of water level data from tabaga gauging station shows that a single interval can be deemed responsible for the initiation of thermal action and development of thermoerosional notches this interval corresponds to the discharges between 21 000 and 31 000 m 3 s 1 observed normally during spring meltwater peak and summer floods competence of fluvial thermal erosion depends on the height of floodplain level being eroded as it acts preferentially in high floodplain banks in medium floodplain banks thermal erosion during spring flood is constrained by insufficient bank height and erosion is essentially mechanical during summer flood season bank retreat rate is argued to be positively linked with bank height under periglacial conditions

holocene environmental change inferred from the loess palaeosol sequences adjacent to the floodplain of the yellow river china abstract chinese loess deposits are generally considered to be the product of dust storms and dust falls from the central asia arid zones that were transported across china by the northwesterly continental monsoon in contrast the zhengzhou loess found southeast of the loess plateau adjacent to the floodplain of the yellow river records a different eolian regime and dust source the zhengzhou loess was investigated by field observations measurements of magnetic susceptibility particle size distribution loss on ignition caco3 and chemical contents both field observations and the laboratory results indicate that during the last glacial the zhengzhou loess was supplied by two different eolian regimes and dust sources one was from the fresh flood deposits of the yellow river driven by the northeast winds from the low lying floodplain and the other was from the dust storms and dust falls that traveled across the loess plateau driven by the northwesterly continental monsoon from the central asian arid lands the early holocene 11 500 8500 a bp was a transition during the change in eolian regime and dust source because of the weakened northwesterly monsoon along with the global climatic amelioration following the retreat of the northwesterly monsoon from the onset of the mid holocene climatic optimum at 8500 a bp dust supply from the drifting sand zone on the yellow river floodplain became dominant because of the intensified strength of the northeast winds from the bohai sea from 3100 a bp onwards climatic aridity and extensive human disturbance have resulted in intensive eolian processes causing the incursion of the drifting sand into the zhengzhou loess zone these results show that loess accumulation is more complex than traditionally assumed the origin of loess deposits elsewhere outside the loess plateau may be related to dust sources derived from alluvial sediments of major river systems

field studies of velocity salinity and suspended solids concentration in a shallow tidal channel near tidal flap gates abstract the design and operation of mathematical models of solute mixing and sediment transport in estuaries rely heavily on the provision of good quality field data we present some observations of salinity suspended sediment concentration and velocity at one of the tidal limits of a semi enclosed tidal lagoon in southern england pagham harbour west sussex uk where the natural processes of tidal incursion and solute mixing have been heavily modified as a result of the construction of sea walls dating back to the 18th century these observations made immediately downstream of two parallel tidal flap gates by conductivity temperature depth ctd profiler and also using bed mounted sensor frames to measure velocity at 2 fixed depths have yielded a set of results covering 11 tidal cycles over the period 2002 04 it is clear from the results obtained that over a typical tidal cycle the greatest vertical salinity gradients occur in the 1 2 h immediately after the onset of the flood tide and that subsequently energetic mixing acts to rapidly break down this stratification under moderate to high fresh water flows 0 5 m 3 s the break down in vertical salinity gradient is more gradual while under low fresh water flows 3 s the vertical salinity gradient is generally less pronounced estimates of richardson number during the early flood tide period reveal values that vary rapidly between

linking spatial patterns of bed surface texture bed mobility and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout abstract in this paper we examined spatial patterns of bed texture bed mobility and channel hydraulics in four reaches of a mountain stream with moderate gradient

impacts of global warming on permo triassic terrestrial ecosystems geologists and palaeontologists have expressed mixed views about the effects of the end permian mass extinction on continental habitats and on terrestrial life current work suggests that the effects on land were substantial with massive erosion following the stripping of vegetation associated with long term aridification and short term bursts of warming and acid rain wildfires at the permo triassic boundary contributed to the removal of forests and the prolonged absence of forests from the earth s surface for up to 10 myr these physical crises on land impinged on the oceans suggesting tight interlocking of terrestrial and marine crises levels of extinction on land may well have been as high as in the sea and this is certainly the case for tetrapods the mass extinction seems to have been less profound for plants and insects but it is hard at present to disentangle issues of data quality from reductions in abundance and diversity several killing agents have been proposed and of these tetrapods may have succumbed primarily to acid rain mass wasting and aridification plants may have been more affected by the sudden effects of heating and wildfires and the crisis for insects has yet to be explored

holocene erosion triggered by climate change in the central loess plateau of china abstract understanding changes in holocene erosion is essential for predicting soil erosion in the future however the quantitative response of natural erosion to holocene climate change is limited for the loess plateau of china in this study two soil profiles were investigated on the luochuan and yanchang sites in the central loess plateau of china and four climate indicators i e magnetic susceptibility calcium carbonate content total organic carbon content and clay content

high resolution studies of the oldest cultivated soils in the southern loess plateau of china abstract the holocene loess soil sequence on the chinese loess plateau constitutes an excellent record of evolution of soil formation monsoonal climate eolian dust accumulation and influences of arable farming of over 8000 years a high resolution soil profile on the zhouyuan loess tableland to the west of xi an was studied using particle size analysis measurement of magnetic susceptibility total fe toc and caco3 the results provide new insights into the roles of eolian dust deposition and human impact of cultivation on soil development in this environmentally sensitive zone several loess layers are found interbedded with cinnamon soils in the profile this has been caused by changes in rainfall and intensity of dust deposition that is forced by changes in balance between the northwest continental monsoon and southeast maritime monsoon of east asia during the holocene the processes of soil formation were interrupted several times by intensified dust fall and climatic aridity in the last 11 500 years the initial clearance of forest for arable farming occurred at ca 7500 years bp on the loess tableland it was contemporaneous with the arrival of the wetter and warmer climate of the holocene megathermal 8500 3100 years bp thereafter arable farming has been maintained during both the time of soil formation and that of intensified dust accumulation though there could be temporary cessation or reduced intensity of cultivation in the drier intervals because of lack of rainfall and soil moisture increased intensity of dust accumulation has been ongoing since ca 3100 years bp in the loess plateau a loess deposit of 40 80 cm has overlaid the multiple or single welded cinnamon soils on the flat land over the plateau

a two fraction model for the transport of sand gravel mixtures 1 a mixed size sediment transport model using only two fractions sand and gravel offers practical advantages in terms of the required input and conceptual advantages by readily incorporating the nonlinear effects of sand content on transport rate a two fraction model is developed from flume and field transport observations sand and gravel transport rates are collapsed to a single transport function using a scaling parameter representing incipient motion for each fraction the incipient motion function is consistent with established values in the limit of pure sand and gravel beds and shows a sharp change with increasing sand content over the transition from a clast to matrix supported bed a two fraction model captures an important and previously unmodeled nonlinear effect of sand content on gravel transport rate and suggests important implications for a number of fluvial processes including the response of channels to sediment supply and the mechanism for abrupt sorting in gravel sand transitions

sedimentation and diagenesis of chinese loess implications for the preservation of continuous high resolution climate records chinese loess has been extensively utilized to produce continuous and high resolution climate records of the late cenozoic such work assumes uninterrupted loess deposition and limited diagenesis here closely spaced optically stimulated luminescence osl dates are used to characterize the holocene and late pleistocene sedimentation histories of three sites across a nw se transect of the chinese loess plateau the results suggest that sedimentation is episodic at subglacial interglacial time scales with rates rapidly varying within units and between sites unconformities noneolian deposition and mixing of sediments also appear to be common existing understanding of loess deposition therefore requires reexamination while previous reconstructions of rapid climate change not dated using absolute methods should be regarded with caution loess deposits may still yield detailed climate records from specific high sedimentation rate strata and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating the rapid changes in sedimentation presented here indicate the east asian monsoon has the capacity to vary on millennial scales

geomorphic impacts of flash flooding in a forested headwater basin abstract geomorphic impacts of a july 2001 flood in the hungry mother basin near marion virginia were examined to shed light on the relationships between channel characteristics and the frequency of channel modifying discharges creeks in the study area have been observed for many years with no significant channel changes since at least 1985 the 2001 flood had a recurrence interval of 200 years and caused the only channel change bank erosion and transport of coarse channel material observed in recent decades the paucity of fines in channels before or after the flood and the absence of sub sand sized material in the flood deposits indicates that normal frequent well below bankfull flows are sufficient to transport the apparently limited supply of fines the large particles transported during the 2001 flood after years of inactivity indicate that relatively rare floods are necessary to mobilize this material this suggests the notion of a bimodal dominant discharge on the one hand frequent flows considerably below bankfull levels are sufficient to maintain the channel and prevent significant accumulation of fine sediments and organic matter on the other hand rare floods are necessary to transport the coarser bed material and erode channel banks in the hungry mother area bed material has no relationship to normal flows or to flows with recurrence intervals on the order of 1 3 years bankfull discharge is apparently not related to either the maintenance or channel changing dominant discharges these results suggest that the use of channel dimensions and or bed material as surrogate indicators of hydrologic regimes can be quite complicated and that in some streams bankfull flow has no particular significance in terms of sediment transport and channel modifications

use of high water marks and effective discharge calculation to optimize the height of bank revetments in an incised river channel abstract in deeply incised rivers bankfull discharge i e the flow filling the channel to the top of the banks does not represent channel forming flow and increasingly large flows are associated with increasingly large boundary shear stress in such rivers solid bank revetments rip rap gabions retaining wall are usually constructed to the top of the banks similarly as in vertically stable rivers despite the fact that the upper parts of the banks may never be flooded to optimize the height of solid bank revetments in deeply incised channels it is thus important to determine whether a flood magnitude can be identified for which the combination of flow duration and bedload transport rate results in the highest river efficiency to transport bedload and perform geomorphic work this question was explored in the moravka river czech carpathians which deeply incised into non resistant flysch bedrock over the past few decades observations of high water marks e g trash lines wash lines after a flood in 2014 enabled reconstruction of the peak flood stage in the deeply incised reach and the adjacent vertically stable reach these observations together with post flood measurements of cross sectional channel geometry distances between consecutive cross sections and estimates of channel roughness were used in one dimensional hydraulic modelling aimed to determine a peak discharge of the flood in a number of cross sections in both reaches despite the close proximity of both reaches markedly higher discharge values were obtained for the incised reach and the discrepancy was used to calibrate roughness coefficients for the incised reach a flow duration curve determined on the basis of a 25 year series of daily discharges in the upstream gauging station together with data about channel geometry and roughness in the incised cross sections were used to simulate bedload transport at successive discharges with the bags sediment transport model the calculations indicated the effective discharge for bedload transport in the incised reach to have the recurrence interval of 7 12 years the upper limit of the reach average effective discharge is associated with the stage slightly higher than a half of the bankfull depth this highlights the disparity between effective and bankfull discharges and indicates that in incised channels channel forming flow should be linked with the former the vertical extent of the reach average effective discharge is proposed as an indicator of the optimal cost effective height of solid bank protection structures in the deeply incised channel and the suitability of this approach is discussed

observations of a tidal intrusion front in a tidal channel abstract a visible front indicated by a surface colour change and sometimes associated with foam or debris lines was observed in a tidal channel during neap tide this is an example of a tidal intrusion front occurring in the absence of sudden topographical changes or reversing flows typically reported to be associated with such fronts detailed acoustic doppler current profiler and conductivity temperature depth measurements were taken on repeated transects both with fronts apparent and with fronts absent the results indicated that the front occurred as a result of stratification which was sustained by the buoyancy flux and the weak tide induced mixing during neap ebb tide the stronger tide induced mixing during spring tide restrained stratification leading to the absence of a front the mechanism of the frontogenesis was the density gradient between the stratified water formed during neap ebb tide and the more mixed seawater during neap flood tide thus the water on the landward southwestern side of the front was stratified and that on the seaward side northeastern of the front was vertically well mixed gradient richardson number estimates suggest that the flow between the stratified and mixed water was near the threshold 0 25 for shear instability meanwhile the density gradient would provide an initial baroclinic contribution to velocity convergence which is indicated by the accumulation of buoyant matter such as foam grass and debris into a sharply defined line along the surface the front migrates with the flood current with a local maximum towards the eastern side of the channel leading to an asymmetrical shape with the eastern side of the front driven further into the tiaozhoumen tidal channel

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

high resolution measurements made across a tidal intrusion front high spatial resolution current and hydrographic measurements have been made across a tidal intrusion front in the james river estuary virginia during the first few hours of flood the current measurements were made with a surface towed acoustic doppler current profiler which was used in both profile and side scan configurations the profile data show a 30 50 cm s 1 inflow of water from the chesapeake bay a time evolving across front density gradient and downward velocity beneath the front of about 15 cm s 1 the side scan observations directly measure the across front change in current and provide a view of the three dimensional character of the frontal interface the near surface horizontal strain rate a critical parameter for understanding the modulation of surface waves across the front is estimated to be about 0 04 s 1 using phillips 1984 theory as a guide fronts with such large strain rates as these should be discernible in imagery from the ers 1 c band synthetic aperture radar for wind speeds of 4 m s 1

quantitative estimates of palaeoprecipitation at xifeng in the loess plateau of china abstract using10be recent studies heller et al 1993 beer et al 1993 have tried to reconstruct palaeoclimatic records by separating the secondary pedogenic and primary aeolian detrital contributions to the magnetic susceptibility of chinese loess here we use a technique which monitors the temperature dependence of magnetic remanence to estimate the magnetisation and relative contribution of the superparamagnetic sp single domain sd and multidomain md ferrimagnetic components in the xifeng loess section on the assumption that the sp ferrimagnetic component is dominantly of pedogenic origin the magnetisation of this component has also been determined for samples taken from the topmost palaeosol s0 at six sites over the loess plateau and directly correlated with present day precipitation at these sites this correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the xifeng section which covers about the last 140 000 years the data indicate a considerably reduced precipitation during the glacial loess forming periods and a significantly higher level of precipitation than today during the last interglacial period

regional variation of mechanical and chemical denudation upper colorado river basin u s a the variation of mechanical and chemical denudation is investigated using discharge and sediment yield data from the upper colorado river system annual precipitation ranges from approximately 150 mm to 1500 mm mean specific yield ranges from 0 2 1 s km2 6 mm p a to 151 s km2 475 mm p a the hydrological geomorphological system adjusts itself to these varying climatic conditions in some areas however the effects of lithology or land use seem to override the climatic controls it is demonstrated that the increase in the absolute and particularly the relative amount of suspended sediment is closely related to a decrease in annual runoff and to an increase in the importance of high magnitude low frequency events this indicates that in areas of low annual runoff and high runoff variability soluble rocks are more resistant than in more humid areas during high magnitude low frequency events suspended sediment concentrations and loads are very high in semiarid areas due to sparse vegetation cover and dominance of direct runoff events of moderate magnitude and frequency which in more humid areas transport most of the dissolved load seldom occur the trend towards increasing mechanical denudation is even observed in areas of very low runoff 0 221 s km2 7 mm p a the peak of sediment yield in dry areas seems to approximate the point of no runoff very closely mechanical and chemical denudation are of equal importance at a runoff of about 300 mm per year

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

changes in sediment yield of the yellow river basin of china during the holocene this research reconstructs the changes in sediment yield of the yellow river based mainly on a large number of 14 c dates collected from the literature the total volume of sediment yield of the basin during the holocene is estimated to be 8 0 x 10 12 tons the annual sediment yield had an increasing trend over the holocene from 0 68 0 72 0 79 to 1 0 1 x 10 9 tons over each 2500 year period from the early holocene to the present the changes in sediment yield are ascribed to both natural and anthropogenic reasons in which the latter became important in the late holocene the mean rate of increase in natural sediment yield throughout the holocene is estimated to be about 0 027 x 10 6 ton year and the natural annual sediment yield is projected to be 0 95 x 10 9 tons at the present the increasing trend of natural sediment yield is reasoned to be the consequence of evolution of landforms enhanced by tectonic movement and climatic change in the loess plateau the principal sediment source of the river towards a condition favorable to soil erosion comparing the estimated annual natural sediment yield with the current annual sediment load of 1 6 x 10 9 tons of the river it is clear that human activities have augmented natural soil erosion by about 41 in the yellow river basin furthermore an estimate of the overall pattern of anthropogenic sediment yield shows an accelerated increasing trend which is approximately correspondent with that of population in the loess plateau area c 2002 elsevier science b v all rights reserved

dependence of near surface magnetic susceptibility on dust accumulation rate and precipitation on the chinese loess plateau magnetic susceptibility ms of surface sediment varies systematically across the loess plateau in central china decreasing exponentially from 200 10 8 m3 kg at the northern margin of the qinling shan to 30 10 8 m3 kg near the southern margin of the mu us desert ms correlates highly with loess median grain size r2 0 79 which decreases south southeastward across the plateau it also correlates with mean annual temperature mat and mean annual precipitation map r2 0 58 and 0 60 respectively and with their product mat map r2 0 83 which is considered a measure of potential pedogenic activity because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel it is difficult to determine their relative influence on ms a simple ms model based on the observed spatial variation in loess thickness permits quantitative assessment of the effect of the dust accumulation rate on the ms signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals the model suggests that 84 of the loess ms variance is dictated by the diluting effect of dust and 10 11 is associated with meteorological factors primarily precipitation the observed and modeled relationships support hypotheses that attribute variations in ms in the loess paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils both of which are controlled by monsoon climate

flow resistance under conditions of intense gravel transport a study of flow resistance was undertaken in a channelized reach of the north fork toutle river downstream of mount st helens washington hydraulic and sediment transport data were collected in flows with velocities up to 3 m s and shear stresses up to 7 times the critical value needed for bed load transport details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance the plane gravel bed persists up to stresses 3 times critical at which point irregular bed forms appear bed forms greatly increase flow resistance and cause velocity profiles to become distorted the latter arises as an effect of flows becoming depth limited as bed form amplitude increases at very high rates of bed load transport an upper stage plane bed appeared velocity profiles measured in these flows match the law of the wall closely with the equivalent roughness being well represented by ks 3d84 of the bed load the effects noted here will be important in very large floods or in rivers that are not free to widen such as those cut into bedrock

did an extensive forest ever develop on the chinese loess plateau during the past 130 ka a test using soil carbon isotopic signatures abstract pleistocene vegetation history on the chinese loess plateau has been traditionally investigated using palynological methods and questions remain regarding whether an extensive broadleaf deciduous forest ever developed on the loess table under favorable climatic conditions the authors have employed a c isotope approach to address this question by comparing 13 c values in soil organic matter from different loess ecological domains with known source vegetation to the c isotope values obtained from a paleosol section that can be dated back to 130 ka the c isotopic compositions of modern soils from the loess table and the loess desert transition gave 13 c values of 24 5 to 18 2 and 25 7 to 20 7 respectively these c isotopic ratios are consistent with the standing modern vegetation that is dominated by a mixture of c 3 and c 4 plants and can be distinguished from that in the patchy forest areas where exclusive c 3 trees yield a narrow 13 c value range from 26 9 to 25 average 26 1 obtained 13 c compositions from paleosols and loess sediments in the lantian and the luochuan profiles vary from 24 to 16 9 indicating a grass dominated steppe with shifting c 3 and c 4 contributions controlled mainly by paleoclimatic changes during the late pleistocene the present results suggest no extensive forest coverage on the loess table during the past 130 ka even under the most suitable conditions for forest development this conclusion supports the explanation of natural causes for the development of only patchy forests on the modern loess table and provides critical historical information toward the vegetation restoration project that is currently underway on the chinese loess plateau

the dynamics of a bathymetrically arrested estuarine front abstract the mixing and circulation associated with a bathymetrically arrested estuarine front was studied using hydrographic and current data a quasisteady front exhibiting strongly convergent surface flows is formed along the steeply sloping inner margins of the flood tide delta during each semidiurnal tide cycle this front separates the brackish ambient water within a deep estuarine basin from the incoming oceanic tidal water the position of the front is dependent on the local water depth and the difference in density between the two water masses beneath the surface there is an inclined frontal interface where static stability is very low and vertical mixing intense a vertically integrated horizontal momentum equation was derived for flow in the upper layer and an estimate made as to the value of the associated entrainment coefficient

the dry holocene megathermal in inner mongolia the paleoclimate since 14 kyr bp 14c age was reconstructed based on a 16 22 m long sediment core collected from lake yanhaizi a saline lake located near the northern limit of the east asian summer monsoon in inner mongolia coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated these sediments have low organic carbon contents but high maturity indices indicating that they were deposited in an arid environment by contrast based on high organic contents and low maturity indices fine sediments were deposited during periods of high lake stand in a humid environment it was in general dry between 8 0 and 4 3 kyr bp the above dry and wet phases are consistent with those recovered from the arid semiarid transition zone elsewhere but are unlike the widely perceived humid holocene megathermal reported in east china and the newly reconstructed record in the alpine retreat lake in taiwan the discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon on the other hand much enhanced evaporation over higher monsoon precipitation at lake yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon this also accounts for the fact the high temperature holocene megathermal as revealed in the okinawa trough and the northern south china sea is correlated to the dry phases at lake yanhaizi conversely the 4 2 kyr bp coldest period in the holocene corresponds to a wet phase at lake yanhaizi

the hydraulics of density currents over estuarine sills it is demonstrated here that density currents being sensitive to the underlying topographies are subject to topographic control in estuaries indented with mild sills as a density current flows over a ridge that current shoals downstream but deepens upstream rendering the upstream flow more subcritical the earth s rotation weakens the topographic influence upstream and downstream and the vertical mixing of momentum and density also weakens the influence conventionally it was thought that the two layer estuarine circulation characterized by the upward entrainment of a landward undercurrent will be set up following the seaward passage of the density front in the presence of bottom topographies however there should be a final phase of adjustment during this phase there is a downward entrainment of the flow upstream of sills and a consequent modulation of the estuarine circulation

effective sediment transporting discharge from magnitude frequency analysis the long term effectiveness of a geomorphic event of a particular magnitude is the product of the effect of a single event of that magnitude times the frequency with which it recurs magnitude frequency analysis if discharge frequency of a stream is assumed to be log normally distributed and if sediment transport rate is assumed to be a power function of discharge then transport effectiveness attains a maximum value at some discharge termed here the effective discharge that moves the most sediment over the long term the effective discharge and its recurrence interval may be predicted if both assumptions are correct analysis of 55 u s streams demonstrates that the observed and the predicted recurrence intervals of the effective discharge are in poor agreement this lack of agreement results from the failure of the power function of discharge to predict adequately the sediment transport rate above some high discharge threshold or thresholds the observed recurrence interval of the effective discha

the long term paleomonsoon variation recorded by the loess paleosol sequence in central china abstract the loess paleosol sequence in central china during the last 2 5 ma is a good record of the asian monsoon variation which could be considered as the concrete realization of global paleoclimatic cycles the geological and biological evidence of the loess paleosol sequence indicates that the loess was mainly the dust deposits transported by the northerly wind of winter monsoons and also suffered to some extent from pedogenesis caused by weakened summer monsoons and that the paleosol accreted by the rather slow dust accumulation was closely related to strengthened summer monsoons an alternating occurrence of loess and paleosol in the luochuan section implies the variation history of warm humid climate with a summer monsoon dominance and cold dry climate with a winter monsoon dominance the magnetic susceptibility of loess and paleosol can be regarded as a proxy index of asian monsoon variation an analysis of the susceptibility curve and structure characters of luochuan section shows that the asian monsoon circulation was strengthened from early pleistocene to late pleistocene and the 500 ka bp point and 1200 ka bp point seem to be a threshold and a turning point respectively in terms of the long term paleomonsoon variation in asia

tidal fronts in estuaries abstract three estuaries in wales were carefully surveyed during flood conditions they are characterized by the intrusion of a tidal front in one estuary the front retreats after maximum flood velocities and stratification persists during the whole tidal cycle in the other estuaries the flood erases the initial stratification causing partially or totally mixed situations a tidal froude number is employed to characterize the evolution of stratification during the flood it is shown that gravity current theory approximately holds under well stratified conditions in a region near to but not too close to the head of the front

effective measures of effective discharge abstract the concept of effective discharge qe was introduced by wolman and miller 1960 as that stream discharge that transports the most sediment over time recently the validity of qe as an overall descriptor of sediment transport has been questioned because of its various interpretations and methods of calculation a new discharge index termed the half load discharge q1 2 was introduced as the discharge above and below which half of the total sediment load has been transported over time existing methods along with long records of continuous daily suspended sediment and discharge data are used to clarify what a discharge index can accomplish we also offer an objective and consistent empirical methodology for calculating qe based on a kernel density function generalized graphical relationships are introduced for summarizing the frequency and magnitude of both stream discharge and sediment loads we document how qe and q1 2 are related to standard descriptors of river discharge as well as gener

spatial and temporal reconstructions of changes in the asian palaeomonsoon a new mineral magnetic approach abstract a new quantitative approach is proposed for estimating palaeoprecipitation across the chinese loess plateau at present there is a strong rainfall gradient across the plateau from 300 mm yr in the near desert conditions in the northwest to over 750 mm yr in the southeast just 700 km distant we find that the concentration of ferrimagnetic iron oxide minerals in nine modern soil types represented by 37 individual soil profiles is strongly correlated with this contemporary rainfall gradient the ferrimagnetic concentration rises along this gradient from 0 01 in the northwest to over 0 2 in the southeast the nine modern soil types have been used in the construction of a rainfall vs magnetic susceptibility least squares regression climofunction past variations of loess soil iron oxide content are easily established through magnetic susceptibility measurements and so can be used to reconstruct the rainfall of former interglacial and glacial periods the physical and pedological basis of the rainfall vs susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variations in the structure of the asian monsoon the rainfall variations are about four times greater than has been suggested for this region by atmospheric general circulation modelling our data indicate increased rainfall throughout central china both in interglacial periods and in the early holocene the increases in monsoonal rain were particularly pronounced at our westernmost sites adjacent to the northeastern edge of the tibetan plateau in contrast for glacial periods a reduction in rainfall is found across the whole loess area with the greatest decreases in the southeast

high resolution multi proxy climate records from chinese loess evidence for rapid climatic changes over the last 75 kyr the loess paleosol sequence in loess layer l1 malan loess is investigated in three regions of the western chinese loess plateau nine pedogenic layers are found in l1 and three proxy climate indices magnetic susceptibility ms grain size gs and caco3 content are measured at intervals of 0 2 kyr in order to recover records of monsoon climate variations time series of ms gs and caco3 content document the high resolution history of summer and winter monsoon climate variations over the last 75 kyr the records show a high degree of similarity to the warm interstadials recorded in ice cores from greenland and the antarctic and with heinrich events in the north atlantic during the last glacial cycle bond cycles are also recorded by the chinese loess records overall our results indicate that numerous rapid changes in climate occurred in china during the last glacial cycle but that the range of climate variations was smaller than recorded in greenland

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

glacial erosion evolution of river long profiles and the organization of process domains in mountain drainage basins of coastal british columbia 1 in glaciated british columbia canada quaternary climate changes are responsible for profound spatial reorganization of earth surface processes these changes have left a landscape characterized by topographic anisotropy associated with a hierarchy of glacial troughs the evolution of glaciated landscapes is examined by analyzing the structure of geomorphic process domains and channel long profiles to identify process domains we use channel surveys and gis analysis to construct slope area transects of the channel network this analysis reveals generalized process form disequilibrium with a mismatch between topographic signatures and currently active geomorphic process domains at the landscape scale of source colluvial channels contributing area 1 km2 the glacial paraglacial signature commonly overrides that produced by contemporary debris flows along the axis of former ice flows relict glacial cirques introduce a hanging fluvial domain at contributing areas as small as 8 10 2 km2 and produce complex channel long profiles similar to those observed for rivers responding to tectonic forcing slope area relations typical of unglaciated equilibrium environments do not apply here the concept of process domains appears to hold however some major glacially forced modifications in the alluvial colluvial transition are observed and the definition of a depositional colluvial subdomain is proposed comparison between field and gis measured slopes reveals that gis associated error is not uniform between process domains and that gis based plots do not successfully discriminate field based process domains the combination of glacial and post glacial fingerprints and the effects of ongoing earth surface processes generate a complex landscape whose glacial signatures may persist until the onset of the next ice age

the legacy of pleistocene glaciation and the organization of lowland alluvial process domains in the puget sound region article i nfo alluvial rivers of the eastern puget lowland a landscape shaped by scour and fill from the cordilleran ice sheet continue to respond to patterns of deposition and scour by the last glacial age ice sheet 18 000 years after deglaciation topography revealed by valley cross sections created from high resolution lidar digital elevation models shows that rivers are aggrading in valleys eroded by subglacial runoff and degrading in valleys incised by rivers post glaciation slope area analysis of river profiles shows that profile concavity varies systematically between river segments in the two valley contexts concavity indices in mountain headwaters 0 3b b0 9 compare to those of many world rivers 0 2b b1 0 but in the lowlands these indices differ between valleys created by subglacial fluvial erosion 5b b45 and post glacially incised river valleys that grade to base levels set by these relict glacial valleys or by post glacial sea levels 1b b7 dramatic differences in river pattern landforms and dynamics occur in valleys having contrasting aggrading vs degrading and incomplete responses to pleistocene glaciation creating discrete valley scale hetero geneities in fluvial process domains along and between rivers these results point to the importance of valley scale organization of alluvial process domains along and between rivers having profiles remaining in disequilibrium from pleistocene glaciation they also point to the potential usefulness of slope area analysis of longitudinal profiles in distinguishing among different river valley process domains in lowland alluvial landscapes

a model study of density intrusions into and circulation within a deep silled estuary puget sound a laterally averaged hydrodynamical model is used to investigate episodic intrusions of denser more oceanic water across the shallow double silled entrance of puget sound and the resulting circulation within the interior deep basin tidal freshwater discharge and wind effects are also encompassed in model results model simulations were made of the 4 month period from december 1983 through march 1984 during that period hourly measurements of currents and salinity were made at the estuary entrance and on four moorings in the interior basin the first 60 days of salinity data were used with the model to choose rates of vertical turbulent mixing and the lag times between fresh water appearance at river gauges and its arrival in the main basin of the sound comparison of model results with the full 120 days of salinity and velocity data shows that the model incorporates the important circulation features of this estuary the model was then used to study intrusions and mixing in the sill region the model identifies these factors as controls on the occurrence and strength of intrusions the trans sill salinity gradient the intensity of tidal stirring and mixing over the sills the supply of fresher surface water resulting from river runoff and the winds time series of salinities in the interior basin show local maxima following local maxima in the trans sill salinity difference vertical transport in the sill region at both tidal and subtidal time scales is dominated by advection rather than diffusion though much of the vertical exchange occurs over the shallower sill a depth depression between the two entrance sills appears to be the site of significant overturning downwelling of surface water occurs on the landward side of both sills substantial freshening of the interior basin occurs during the winter months because river discharges are large and are stirred downward in the sill region to be entrained into landward bottom flows extraordinarily large river discharges can suppress the effect of large trans sill salinity differences with the effect of weakening potential intrusions winds can indirectly influence intrusions by longitudinally positioning fresher river influenced surface water in the sill region winds can directly influence intrusions via bottom compensation flows in admiralty inlet with northward winds encouraging intrusions and by changing salinities at the entrance to the sound and thus the trans sill salinity gradient

the tidal intrusion front an estuarine convergence zone freshwater discharge in a small estuary flows seaward as a density current which is forced back into the estuary during the flood phase of the tidal streams the development of the two layer flow and the associated frontal structure is described the front exhibits a v configuration with an isolated point convergence at its apex and an associated gyre system

palaeoclimatic and palaeoenvironmental implications of late pleistocene aeolian sand in the jin shaan gorges of the yellow river valley revealed by luminescence chronology river valleys are critical water gas rock interactions zone where close interaction between fluvial and aeolian deposition are a pronounced feature in arid semi arid and semi humid environments

sediment transporting flows in headwater streams the equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed at the same time the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it a prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives this study describes the bed load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel bed streams in snowmelt dominated parts of central and northern idaho at sites drainage areas range from 1 29 to 381 km 2 stream gradients range from 0 0042 to 0 0747 and median bed surface particle sizes range from 4 to 207 mm stream architecture includes riffle pool planar and step pool beds the bed load is much finer than the surface and subsurface material suggesting selective transport of the finer sizes nonetheless the majority of the load is sand at all flow discharges progressively coarser sediment was collected as flow discharge increased and painted rock experiments documented the transport of coarser particles at higher discharges the supply of sediment to the streams appears limited as indicated by observed clockwise hysteresis in bed load transport rates during each spring snowmelt and by the coarse surface armor observed at sites flows above bankfull discharge move 37 of the bed load whereas flows between mean annual discharge and bankfull move 57 of the bed load the bed load effective discharge has a recurrence interval that averages 1 4 yr and the magnitude of effective discharge averages 80 of bankfull discharge the recurrence interval of bankfull discharge averages 2 0 yr the ratio of effective discharge to bankfull discharge is independent of basin size grain size and gradient although the ratio increases with the relative magnitude of large infrequent events

maximal two layer exchange over a sill and through the combination of a sill and contraction with barotropic flow the analysis of two layer exchange flow through contractions with a barotropic component treated by armi farmer 1986 is extended to include exchange flows over sills and through a combination of a sill and contraction it is shown that exchange over a sill is fundamentally different from exchange through a contraction control at the sill crest acts primarily through the deeper layer into which the sill projects and only indirectly controls the surface layer this asymmetry in the control results in asymmetrical flows the interface depth above the crest is not one half the total depth as assumed in other studies by analogy with flow through contractions but is somewhat deeper the maximal exchange rate is less than for flow through a contraction of equal depth when both a sill and a contraction are present the contraction influences control at the sill crest only if it lies between the sill and the source of denser water the response to barotropic flow is also asymmetrical the transition to single layer flow occurs at much lower speeds for a barotropic component in one direction than the other results of the analysis are applied to exchange flow through the strait of gibraltar which includes both a sill and a contraction it is shown that maximal exchange conditions apply throughout part of the tidal cycle and observations illustrate several of the analytical predictions for barotropic flows including the formation of fronts single layer flow submaximal exchange and reverse flow

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

holocene vegetation variations and the associated environmental changes in the western part of the chinese loess plateau abstract the western part of the chinese loess plateau has experienced a series of environmental changes during the holocene a desert steppe of late glacial was succeeded by a forest steppe from 8850 to 7540 14 c years bp and a pinus dominated forest occupied the landscape from 7540 to 6560 14 c years bp a deciduous forest of high density and diversity existed from 6560 to 5790 14 c years bp afterwards the vegetation changed to a pinus dominated forest steppe 5790 4950 14 c years bp and then to an ulmus dominated forest steppe 4950 to 4000 14 c years bp the vegetation subsequently changed to a steppe from 4000 to 3120 14 c years bp and further to a desert steppe from 3120 to 2900 14 c years bp after a period of vegetation improvement steppe from 2900 to 2460 14 c years bp a desert steppe resumed 2460 2020 14 c years bp steppe vegetation was re established around 2020 14 c years bp and seems to have then deteriorated again around 1000 14 c years bp the general trend of aforementioned climatic changes is proposed here to have been modulated by the insolation changes specifically the insolation started to increase around 15 000 years bp and peaked around 9000 years bp when the obliquity driven seasonality reached a maximum with the summer insolation being about 8 more than the present as a result tropical holocene sst increased steadily from 10 000 to 6 000 years bp thus effectively strengthening the east asian summer monsoon in addition the documented high temperatures from 10 000 to 4000 years bp in high northern latitudes might have weakened the strength of the winter monsoon and thus enhanced the strength of the summer monsoon probably via lengthening of the rainy season directly contributing to the megahumid climate between 10 000 and 4000 years bp in the western part of the chinese loess plateau

holocene variability of the east asian summer monsoon from chinese cave records a re assessment oxygen isotope records from stalagmites in caves in southern china interpreted as proxy rainfall records reflecting the intensity of the east asian summer monsoon indicate gradual monsoon weakening for the last 9000 years as also documented for the indian monsoon coupled with high precision dating the speleothem proxy records have been used to test monsoon links with orbital forcing solar changes iceberg discharges in the north atlantic ocean currents and atmospheric methane however these benchmark cave records do not match other published dated e asian proxy rainfall record specifically here independently calibrated rainfall records from loess palaeosol magnetic properties and cave oxygen isotope intercomparisons which show variable e asian monsoon intensity through the entire holocene the strong correlation of the cave records with the extraregional indian monsoon record yet their mismatch with these other dated chinese rainfall records might be reconciled if the speleothem isotope variations reflect not changes in holocene rainfall amount but in rainfall source declining holocene influence of isotopically lighter indian monsoon sourced moisture over china would have resulted in increasing proportions of isotopically heavier rainfall sourced from the more oceanic e asian monsoon individual speleothems may thus regionally record holocene changes in indian monsoon intensity and isotopic influence conversely the other chinese proxy records described here reflect changes in rainfall amount and thus in e asian summer monsoon intensity for the holocene the e asian and the indian monsoon responses to orbital forcing are likely to have differed specifically due to e asian internal feedbacks and the seasonal contrasts between the two monsoon systems

flow convergence and stability at a tidal estuarine front acoustic doppler current observations characteristics of the flow field in an estuarine frontal zone have been investigated in a field study in the lower james river estuary underway sampling with an acoustic doppler current profiler adcp on repeated transects across the front provided information on the structure of the flow field near the front and its evolution in time as this tidal intrusion front advanced up the estuary during the flooding tide prominent and consistent features in the velocity field included a localized zone of convergent flow beneath the visible surface line and a stratified shear layer just upriver of the front within the shear layer between the buoyant surface water and the faster higher salinity undercurrent gradient richardson number estimates suggest that the flow was at or near the threshold for shear instability another shear type gradient in the flow field the across front variation of the along front velocity component strengthened over a sequence of transects with intensity increasing toward the surface tracking of the front was then interrupted when the identifying line of foam and accumulated material on the surface previously sharp and well defined broke up and dispersed to such an extent that the visible signature of the front was lost temporarily a visible frontal expression later reappeared and propagation upriver continued lower bound estimates of downwelling flow in the frontal zone were determined by continuity considerations

holocene loess accumulation and soil development at the western edge of the chinese loess plateau implications for magnetic proxies of palaeorainfall a high resolution holocene sequence of loess palaeosols and incipient soils at the western edge of the chinese loess plateau dated usingoptically stimulated luminescence techniques is used to identify the respective influence of dust accumulation rate time and climate on soil magnetic properties dust deposition and soil formation have both been quasi continuous through the holocene at this site the soils are thus accretionary in nature the degree of soil development as indicated both by geochemical and magnetic properties which correlate strongly varied through the holocene compared with the less weathered loess units each palaeosol is enriched in nitrogen and organic carbon depleted of carbonate due to leaching and displays higher values of pedogenic magnetic susceptibility frequency dependent susceptibility remanences and magnetisation magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine of single domain and superparamagnetic dimensions sediment accumulation rates were lowest from b12 to 2 5 ka providing ample time 100 s of years for weathering and soil formation to proceed yet pedogenesis through this interval was relatively weak conversely soil development was stronger during later intervals when loess accumulation rates were higher and soil forming intervals were correspondingly shorter in terms of jenny s 1941 soil forming equation accumulation rate and thus time sediment source parent material and topography show no systematic variation between the loess and the palaeosols therefore climate seems to be the key soil forming factor which has controlled the geochemical and magnetic properties of these holocene palaeosols r 2002 elsevier science ltd all rights reserved

frequency analysis of annual maximum suspended sediment concentrations in abiod wadi biskra algeria sediment transport processes in the mediterranean semi arid areas have interested a great number of hydrologists and statisticians frequency analysis fa procedures are commonly applied for several hydrological events such as floods and droughts however in general fa is not widely applied to treat suspended sediment concentration ssc especially in semi arid regions in the present study an fa was performed on ssc data from 1979 to 1991 observed upstream from the foum el gherza dam which is located at biskra in the south east of algeria this dam is problematic in terms of silting therefore this study is mainly motivated by providing an fa model and risk evaluation in order to assist dam managers to better evaluate the potential of silting resulting from ssc transport to the reservoir probability distributions commonly used in hydrology were tested to ssc data recorded at the m chounech station on abiod wadi located upstream the dam all the fa steps were considered including classical techniques e g goodness of fit tests statistical criteria as well as recently developed tools tail distribution classification their application led to the selection of the lognormal distribution ln2 to fit the considered data and hence an accurate risk assessment could be obtained because of the presence of a slight trend non stationary models are also considered in the present study copyright 2013 john wiley sons ltd

detailed measurements during a transient front in a small subtropical estuary high resolution measurements of velocity and physio chemistry were conducted before during and after the passage of a transient front in a small subtropical system about 2 1 km upstream of the river mouth detailed acoustic doppler velocimetry measurements conducted continuously at 25 hz showed the existence of transverse turbulent shear between 300 s prior to the front passage and 1300 s after this was associated with an increased level of suspended sediment concentration fluctuations some transverse shear next to the bed and some surface temperature anomaly

hydraulically controlled front trapping on a tidal flat 1 we report observations of baroclinic surface fronts in skagit bay the fronts formed repeatedly in very shallow 0 7 m deep water along the edge of a tidal channel shortly after the incoming tide flooded the adjacent tidal flat and eventually departed from channel edge as the flood tide progressed the fronts were surface convergence zones trapping gps drifters and other flotsam across front salinity temperature jumps were a few practical salinity units a few degrees with freshwater on the channel side across front transects of velocity from mobile and fixed acoustic doppler current profilers and density from mobile and fixed conductivity temperature depth profilers are presented composite froude numbers indicate that supercritical flow over the flats initially held fronts near the channel edge as flood tide progressed depth averaged flow speeds decreased while internal wave speeds increased owing to increasing depths and density gradients flow over the flats eventually became subcritical at which point fronts departed from the channel edge the front then formed the leading edge of a shallow around 0 3 m thick freshwater plume which propagated across the flats against the incoming tidal flow water was then exchanged between the channel and flats with a channel edge composite froude number near 0 5 consistent with mixing induced reduction in channel flats exchange the across channel velocity shear was correlated with the across channel wind consistent with a simple eddy viscosity model strong winds prevented plume propagation

disequilibrium of holocene sediment yield in glaciated british columbia it is generally supposed that specific sediment yield the quantity of sediment passing a monitored river cross section per unit area drained upstream of that section declines as the area drained increases1 3 part of the sediment mobilized from the land surface is supposed to go back into storage at field edges and on footslopes and floodplains in contrast we show here that data from british columbian rivers reveal a pattern of increasing specific sediment yield at all spatial scales up to 3 104 km2 this results from the dominance of secondary remobilization of quaternary sediments along river valleys over primary denudation of the land surface the result controverts the conventional model which has been derived from studies of small highly disturbed agricultural catch ments the rivers are still responding to the last glaciation giving a landscape relaxation time greater than 10 kyr this holds pro found implications for geomorphological theory and for studies of erosion

dry or humid mid holocene humidity changes in arid and semi arid china abstract spatial changes of effective moisture during the mid holocene have been reconstructed based on the compilation of recently published paleoclimate records including ice core lake level pollen assemblage and loess paleosol records both geological data and the spatial pattern indicate that it was dry in deserts during the mid holocene but the timing of the beginning and end of dry intervals differs from place to place deserts having higher aridity index values remain dry longer dry intervals during the mid holocene are more possibly asynchronous than synchronous in arid and semi arid china in the xinjiang region except in sites located in deserts the climate is generally wet during 7000 5000 a bp in the northern qinghai tibetan plateau effective moisture in most sites began to decrease after 5000 a bp the climate became dry after 4000 a bp except in the deserts in the loess plateau and the inner mongolia plateau