channel responses to declining flow on the rio grande between ft quitman and presidio texas abstract a profound decline in the discharge of the upper rio grande following upstream water development set in motion a chain of responses in channel geometry and sediment budget the case of the rio grande provides a well documented example of the evolution of channel geometry in response to change in a single external variable in a regime of declining flow channel change was driven by an excess of sediment relative to the energy available to do work in the stream channel several different responses can be identified which operated at different rates responses can be ranked according to reaction time to define 3 stages in channel evolution stage i is identified as the shrinking of the channel as excess sediment was deposited within it during stage ii channel capacity began to stabilize as overbank flow carried sediment onto natural levees and floodplain valley wide aggradation began and avulsive migration resumed especially at the mouths of tributaries where stage iii is occurring lag deposits of bedload at tributary mouths are locally regrading the valley initial shrinking of the channel produced positive feedback to the decrease in discharge and sediment transport capacity not until later stages did subordinate responses appear which tended to balance the sediment budget and these involved lateral migration the river approached a balanced sediment budget both by increasing its capacity by shortening and steepening and by decreasing sediment inout by migrating away from tributaries

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

morphology and sedimentology of a tributary confluence bar in a regulated river north tyne u k the form and structure of a tributary confluence bar in a regulated channel are analysed and related to channel adjustment to the imposed flow regime the bar is characterized by a tripartite structure with two facies and by marked lateral coarsening characteristics which relate to the changing hydraulic conditions within the narrowing channel form

impact of flow regulation on near channel floodplain sedimentation article i nfo article history rates and the spatial extent of near channel floodplain sedimentation on regulated and unregulated reaches in two upland rivers in central vermont u s a are measured using the short lived fallout radionuclide 210 pb we find consistent profiles of 210 pb inventories across all sites inventories are low immediately next to the channel increase to a peak value as the inundation frequency decreases and then asymptotically diminish with distance from the channel to the equilibrium inventory associated with atmospheric deposition alone we infer from our data that flow regulation has impacted sediment deposition to floodplains below the dam total sediment deposition is less and it is constrained to a narrower band immediately along the active channel flow regulation does not appear however to impact the general form of the 210 pb inventory profile suggesting a uniformity of process across regulated and unregulated floodplains

the equilibrium alluvial river under variable flow and its channel forming discharge when the water discharge sediment supply and base level vary around stable values an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state i e a dynamic or statistical equilibrium state here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope width and bed surface texture the solution holds in river normal flow zones or outside both the hydrograph boundary layer and the backwater zone and accounts for grain size selective transport and particle abrasion we consider the variable flow rate as a series of continuously changing yet steady water discharges here termed an alternating steady discharge the analysis also provides a solution to the channel forming water discharge which is here defined as the steady water discharge that given the mean sediment supply provides the same equilibrium channel slope as the natural long term hydrograph the channel forming water discharge for the gravel load is larger than the one associated with the sand load the analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone which we term the normal flow load distribution the fact that the distribution of the imposed sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer the results quantify the findings by wolman and miller 1960 regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry

submarine palaeoenvironments during emeishan flood basalt volcanism sw china implications for plume lithosphere interaction during the capitanian middle permian end guadalupian extinction event abstract plume induced lithospheric uplift and erosion are widely regarded as key features of large igneous province lip emplacement as is the coincidence of lip eruption with major extinction and oceanic anoxic events oae the emeishan lip which erupted during the capitanian previously termed end guadalupian extinction event has provided the most widely discussed example of axisymmetric doming above a rising mantle plume advocates have argued that in excess of 500 m of uplift occurred over 30 000 km 2 causing extensive radially distributed erosion and alluvial fan formation however the recognition of submarine hydromagmatic and phreatomagmatic style volcanism as well as syn volcanic marine sediments interbedded in the eruptive succession now requires further examination to this simple plume uplift model here we present data from newly discovered sections from the center of the putative uplifted area around lake er hai sw yunnan province that provide a more complete history of the emeishan volcanism these reveal that platform carbonate deposition was terminated by rapid subsidence followed quickly by the onset of volcanism importantly these eruptions also coincide with widespread losses amongst fusulinacean foraminifera and calcareous algae for at least the lower two thirds of the 4 5 km thick lava pile eruptions continued at or below sea level as testified by the presence of voluminous mafic volcaniclastic deposits pillow lavas and development of syn volcanic reefal limestones in the emeishan inner zone only in the later stages of eruption did terrestrial lava flows become widely developed this onset of volcanism in a submarine setting and the consequent violent phreatomagmatic style eruptions would have had a profound effect on marine fauna and exacerbated any volcanically induced climate effects during the capitanian the late permian of sw china at the time of the emeishan was an extended area of thinned lithosphere with epeiric seas which appear to have been sustained through the onset of lip emplacement therefore while there remains substantial geochemical support of a plume origin for emeishan volcanism lip emplacement cannot be ubiquitously associated with regional pre eruption uplift particularly where complex lithospheric structure exists above a plume

age and duration of the emeishan flood volcanism sw china geochemistry and shrimp zircon u pb dating of silicic ignimbrites post volcanic xuanwei formation and clay tuff at the chaotian section abstract the age and duration of the emeishan basalts sw china remain poorly constrained largely due to the severe thermo tectonic overprinting of the ar ar system and failure to obtain zircon separates from erupted basalts in an attempt to solve this problem geochemical analyses and shrimp zircon u pb dating have been carried out on rare felsic ignimbrite in the uppermost of the emeishan lava succession the xuanwei formation which immediately overlies the emeishan basalts and a clay tuff at the middle late permian boundary at the chaotian section clastic rocks of the lowermost xuanwei formation group 1 in eastern emeishan large igneous province lip have a geochemical affinity to the emeishan felsic volcanic rocks whereas the overlying sediments group 2 are compositionally more akin to mafic components of the emeishan traps this is the reverse of volcanic sequence of the central emeishan lip where the felsic extrusives sit above predominant mafic rocks it is likely that the clastic rocks are water transported sediments resulting from erosion of the volcanic rocks in the center of the emeishan lip this interpretation is further supported by the general agreement between the age of the lowermost xuanwei formation 257 4 ma 260 5 ma and that of the silicic ignimbrite 263 4 ma and the clay tuff at the middle late permian boundary at chaotian 260 5 ma these ages interpreted as the termination age of the emeishan volcanism are indistinguishable within error from the middle late permian boundary age 260 4 0 4 ma and the main stage 259 262 3 ma of the emeishan volcanism inferred from dating of mafic and alkaline intrusions in the emeishan lip all these suggest that the emplacement of the emeishan volcanism took place over a very short interval moreover the temporal link and geochemical analyses suggest that the chaotian clay at the middle late permian boundary was genetically related to the emeishan silicic volcanism this together with the fact at that both the emeishan basalts and the chaotian clay rest on the maokou formation leads us to infer that the emeishan basalt was emplaced at the middle late permian boundary in this sense the emeishan volcanism can be regarded as a boundary event and its age is presumed at 260 ma although more precise dating is required both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the emeishan volcanism was one of likely causes of the end guadalupian mass extinction

changes in hydrologic regime by dams dams have major impacts on river hydrology primarily through changes in the timing magnitude and frequency of low and high flows ultimately producing a hydrologic regime differing significantly from the pre impoundment natural flow regime this paper presents the analysis of pre and post dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the united states our overall goals are to document the type magnitude and direction of hydrologic shifts because of impoundment using the entire database for the national inventory of dams nid for dams possessing longstanding u s geological survey usgs gages downstream we identified 21 gage stations that met length of record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area to assess hydrologic changes associated with dams we applied a hydrologic model the indicators of hydrologic alteration iha supplemented with orientation statistics for certain hydrograph parameters dams had significant impacts on the entire range of hydrologic characteristics measured by iha for many characteristics the direction and significance of effects were highly consistent across the 21 sites the most significant changes across these sites occurred in minimum and maximum flows over different durations for low flows the 1 day through 90 day minimum flows increased significantly following impoundment the 1 day through 7 day maximum flows decreased significantly across the sites at monthly scales mean flows in april and may tend to decline while mean flows in august and september increase other significant adjustments included changes in annual hydrograph conditions primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment the number of high pulses has increased following impoundment but the average length declines the mean rate of hydrograph rise and fall has declined significantly these results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale for large and small rivers

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

response of a gravel bed river to dam closure insights from sediment transport processes and channel morphodynamics this work deals with the impacts of dams on large gravel bed rivers in terms of altering coarse transport regimes and the relationship with river morphodynamics using data collected by a tracer based monitoring program carried out in a 4 km long study sector of the parma river italy located downstream from a relatively recently established dam we applied a virtual velocity approach to estimate the coarse bed material load at four river cross sections monitoring and calculation results provided new insights into the impacts of the dam on streambed material mobility and the sediment regime over the 17 month calculation period a longitudinal gradient of effects was observed along the study sector sections located closer to the dam are characterized by more evident impacts due to deficits in coarse sediment input from upstream sediment mobility here is strongly altered especially in the highly armored main channel and the overall bed material load is extremely low a partial recovery of sediment dynamics was observed at the sections located further from the dam where estimates indicate higher sediment yield the observed longitudinal trend in the coarse sediment transport regime matches the morphology as the river shifts downstream from a sinuous configuration with alternate bars to a wandering one the novel insights into alteration of coarse sediment dynamics and the relationship with river morphodynamics are potentially applicable to many other fluvial contexts affected by similar impoundments

downstream effects of flaming gorge reservoir on the green river colorado and utah the green river is one of the principal tributaries in the colorado river basin and drains 44 700 m 2 in wyoming colorado and utah since october 1962 flows of the green river have been regulated by flaming gorge reservoir which is located 412 river miles upstream from its confluence with the colorado river mean annual runoff has not been affected by the reservoir the duration of the relatively large discharges that transport most of the annual sediment load however has decreased significantly as a result the mean annual sediment discharge has decreased by 54 to 3 21 10 6 tons from 6 92 10 6 tons at the jensen gage located 105 river miles downstream from the reservoir and by 48 to 8 83 10 6 tons from 17 0 10 6 tons at the green river utah gage located 290 river miles downstream from the reservoir sediment supply to i he channel equals the annual transport within a relatively short distance 68 river miles downstream from the reservoir downstream from river mile 166 the supply of sediment from upstream plus tributary inflow exceeds the transport of sediment by 5 4 10 6 tons per year on an average the quasi equilibrium that appears to have existed prior to the reservoir no longer occurs along a majority of the green river in response to the reduced peak discharges the bankfull channel width of the green river has decreased by 10 adjustment of the channel to decreased peak flows and altered sediment loads is nowhere complete at present it appears that a century or more will be required for the green river to adjust to the effects of flaming gorge reservoir

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

streamflow regulation and multi level flood plain formation channel narrowing on the aggrading green river in the eastern uinta mountains colorado and utah abstract the style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river the magnitude of streamflow regulation and the post dam sediment transport regime we measured different magnitudes of channel adjustment on the green river downstream from flaming gorge dam ut usa that are related to these three factors bankfull channel width decreased by an average of about 20 in the study area in reaches with abundant debris fans and eddy deposited sand bars the amount of channel narrowing was proportional to the decrease in specific stream power the fan eddy dominated reach with the greatest decrease in stream power narrowed by 22 while the reach with the least decrease in stream power narrowed by 11 in reaches with the same magnitude of peak flow reduction meandering reaches narrowed by 15 to 22 and fan eddy dominated reaches narrowed by 11 to 12 specific stream power was not significantly affected by flow regulation in the meandering reaches in the diverse array of reach characteristics and deposit types found in the study area all pre and post dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre dam floods an intermediate bench that was inundated by rare post dam floods and a post dam floodplain that was inundated by the post dam mean annual flood analysis of historical photographs and tree ring dating of tamarix sp shows that the intermediate bench and post dam floodplain are post dam landforms in each reach type although these two surfaces occur at different levels they are forming simultaneously during flows of different magnitude and while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies both reach types contain deposits at all of these topographic levels the process of channel narrowing varied between fan eddy dominated and meandering reaches in the meandering reaches where stream power has not changed narrowing was accomplished by essentially the same depositional processes that operated prior to regulation in fan eddy dominated reaches where significant reductions in stream power have occurred channel narrowing has been accompanied by a change in dominant depositional processes mid channel sand deposits are aggrading on deposits that in the pre dam era were active gravel bars these deposits are creating new islands and decreasing the presence of open framework gravel bars in eddies bare sand bars are replaced with vegetated bars that have a simpler topography than the pre dam deposits

medium and short term channel planform changes on the rivers tay and tummel scotland abstract channel planform change was analysed using a variety of data sources for the medium term 25 years and

silicic magmas from the emeishan large igneous province southwest china petrogenesis and their link with the end guadalupian biological crisis abstract the emeishan large igneous province in sw china comprises a bimodal mafic silicic suite with the silicic rocks occurring at the uppermost part of the thick lava sequence the silicic rocks have an age of 257 263 ma and are thus roughly coeval with the guadalupian loping g l boundary event most silicic rocks trachyte and rhyolite from the emeishan province have rather uniform e nd values 1 to 2 9 that are comparable with the uncontaminated high ti basalts this fact and the remarkably narrow ranges of incompatible element ratios e g zr nb of the basalt silicic suite indicate a genetic relationship between basalt and silicic members the significant difference between the emeishan rocks and the experimental melts of hydrated basaltic crust suggests the fractional crystallization of basaltic magma rather than crustal melting as the major petrogenetic process for the formation of silicic rocks indeed their major and trace element trends can be modeled by fractionation of the observed mineral phases feldspar clinopyroxene fe ti oxide and apatite in contrast to the virtually closed system differentiation processes associated with trachytes the rhyolites may have experienced interaction with upper crustal material during ascent the geochemical characteristics and recent assessment of timing of the emeishan volcanism suggest the emeishan rhyolites as the potential source of the widespread clay bed at the g l boundary in south china this enhanced the causal link between the emeishan eruption and the end guadalupian biological crisis

temporal and spatial scales of geomorphic adjustments to reduced competency following flow regulation in bedload dominated systems abstract because of the combined effects of reduced sediment transport capacity and competency following flow regulation morphological changes are expected to occur in channels downstream from dams and specifically at tributary junctions where local inputs of water and sediment occur using a combination of historical aerial photographs mainstem and tributary channel pebble counts and hec ras flow modeling for two watersheds in south central vt one unregulated and the other regulated since 1961 we document the time series of post regulation channel narrowing and associated bar growth due to the influx of tributary sediment channel adjustments at regulated tributary junctions have been significant in ca 50 years following impoundment with channels downstream of the confluences narrowing over 15 after an initial ca 20 year lag before the onset of accelerated narrowing moreover flow modeling suggests that downstream of regulated confluences the modern median grain size d50 along the channel bed is immobile no significant channel narrowing has occurred either above or below unregulated tributary junctions or on the mainstem upstream of regulated confluences however greater channel sediment fining is observed upstream of regulated confluences than above unregulated confluences thus the primary mode of mainstem channel adjustment differs up and downstream of regulated tributaries these confluence effects have occurred where the tributary drainage area is only 0 2 times that of the mainstem well below the threshold ratio of 0 6 required for significant geomorphic effects at unregulated confluences highlighting the geomorphic scale shift of dams lastly we evaluate the downstream length required for a river to recover from the impacts of impoundment and demonstrate that even distal locations are impacted by flow regulation unlike the impacts of flow regulation in the western us where channel incision and bar erosion predominate following impoundment we find that in situations where bed incision is minimal and where sediment loads are low but bed caliber high bar growth and channel narrowing are significant adjustments at tributary junctions following impoundment therefore at our sites the effects of dams on reduced competency may be more profound than on reduced sediment transport capacity highlighting the importance of geologic and geomorphic settings in understanding fluvial responses to impoundment

colorado river sediment transport 2 systematic bed elevation and grain size effects of sand supply limitation the colorado river in marble and grand canyons displays evidence of annual supply limitation with respect to sand both prior to topping et al this issue and after the closure of glen canyon dam in 1963 systematic changes in bed elevation and systematic coupled changes in suspended sand concentration and grain size result from this supply limitation during floods sand supply limitation either causes or modifies a lag between the time of maximum discharge and the time of either maximum or minimum depending on reach geometry bed elevation if at a cross section where the bed aggrades with increasing flow the maximum bed elevation is observed to lead the peak or the receding limb of a flood then this observed response of the bed is due to sand supply limitation sand supply limitation also leads to the systematic evolution of sand grain size both on the bed and in suspension in the colorado river sand input during a tributary flood travels down the colorado river as an elongating sediment wave with the finest sizes because of their lower settling velocities traveling the fastest as the fine front of a sediment wave arrives at a given location the bed fines and suspended sand concentrations increase in response to the enhanced upstream supply of finer sand then as the front of the sediment wave passes that location the bed is winnowed and suspended sand concentrations decrease in response to the depletion of the upstream supply of finer sand the grain size effects of depletion of the upstream sand supply are most obvious during periods of higher dam releases e g the 1996 flood experiment and the 1997 test flow because of substantial changes in the grain size distribution of the bed stable relationships between the discharge of water and sand transport rates i e stable sand rating curves are precluded sand budgets in a supply limited river like the colorado river can only be constructed through inclusion of the physical processes that couple changes in bed sediment grain size to changes in sand transport rates in some rivers the upstream supply of sediment is in equi librium with the upstream supply of water whereas in others the upstream supply of sediment is decoupled either com pletely or partially from the upstream supply of water in the first type of river changes in sediment transport are controlled by changes in the discharge of water whereas in the second and perhaps more common type of river changes in sedi ment transport are also coupled to changes in sediment grain size in this paper we investigate the systematic changes in bed elevation sediment transport and sediment grain size that occur in response to changes in the upstream supply of sand in a river with an intermittent limited supply of sand specifically the colorado river in marble and grand canyons figure 1 to develop an intuitive understanding of the linkage be tween sediment grain size and the upstream supply of sediment in a river it is informative to first examine sediment transport

sedimentation within a regulated river channel change induced by tributary sediment injection to a regulated gravel bed river is described for the river rheidol u k eighteen years after dam closure morphological and sedimentological data are presented for an aggrading reach a primary lobate bar appears to have initiated flow separation and the subsequent construction of channel side berms hydraulic sorting has operated to produce both longitudinal and lateral particle size reduction and selective particle size dispersal appears to be an important process influencing channel change the channel is adjusting to a lower width depth ratio form of smaller capacity and with a steeper slope

reliability assessment of slopes with three dimensional rotated transverse anisotropy in soil properties the influence of soil variability on three dimensional 3d probabilistic slope stability analysis has been previously investigated for soils that display isotropic spatial variability features or

confluence effects in rivers interactions of basin scale network geometry and disturbance regimes 1 we reviewed 14 studies documenting the effects of tributaries on river morphology at 167 confluences along 730 km of river spanning seven orders of magnitude in drainage area in western united states and canada in both humid and semiarid environments the probability of observing significant confluence related changes in channel and valley morphology due to tributary influxes of sediment e g changes in gradient particle size and terraces etc increased with the size of the tributary relative to the main stem effects of confluences on river morphology are conditioned by basin shape and channel network patterns and they include the nonlinear separation of geomorphically significant confluences in river networks other modifying factors include local network geometry and drainage density confluence related landforms i e fans bars terraces etc are predicted to be dominated by older features in headwaters and younger features downstream a pattern driven by the frequency and magnitude of floods and punctuated sediment supply that scale with watershed size index terms 1824 hydrology geomorphology 1625 1815 hydrology erosion and sedimentation 1821 hydrology floods 1848 hydrology networks keywords confluences fluvial geomorphology river networks

geomorphology and endangered fish habitats of the upper colorado river 2 linking sediment transport to habitat maintenance alluvial reaches of the colorado river near grand junction colorado provide important habitat for the endangered colorado squawfish this paper examines recent changes in the geomorphology of the colorado river and addresses the question of what can be done to improve existing fish habitats observations of channel change during periods of above average runoff from 1993 through 1995 indicate minor scour and fill of the main channel but more significant scour and enlargement of side channel and backwater habitats and flushing of fine sediment from the bed modeled relations between discharge and dimensionless shear stress indicate that the threshold of bed load transport 0 03 occurs at about half the bank full discharge these flows are exceeded about 26 days per year on average the bank full discharge produces an average of 0 047 and is exceeded about 8 days per year the width and depth of the channel thus appear to be set by a discharge that produces an average boundary shear stress of 1 5 times greater than the critical shear stress the effective discharge is slightly less than the bank full discharge and approximately 80 of the total sediment load is carried by the highest 10 of all discharges

differentiating the effects of logging river engineering and hydropower dams on flooding in the skokomish river washington usa abstract a decades long progressive loss of channel capacity in the skokomish river a 622 km2 basin draining the southeast olympic mountains of washington state has caused increasing flooding with severe consequences to endangered salmon runs infrastructure and private property to differentiate among multiple potential drivers of the capacity loss we analyze the geomorphic evidence for the potential effects of flow regulation by two dams constituting the cushman hydroelectric project which began regulating flow in the river s north fork in 1925 and diverting water out of the basin in 1930 sediment production from mid twentieth century logging in the river s south fork basin and twentieth century river engineering in the mainstem bankfull channel capacity in the mainstem has steadily declined since about 1940 from 370 m3 s 1 to

origin of k feldspar megacrysts in rhyolites from the emeishan large igneous province southwest china abstract silicic rocks occur in the uppermost units of the longest volcanic succession 5000 m thick in the binchuan area of the permian emeishan flood basalt province of sw china they are predominantly rhyolites and to a lesser extent trachytes both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size these megacrysts contain domains of albite arranged in vein like networks likely formed by post magmatic alteration crystal size distributions csd suggest that these megacrysts grew in a stable magmatic system consistent with relatively uniform core to rim compositional k 2 o 14 16 wt and isotopic profiles 208 pb 206 pb ratios 2 06 2 08 0 005 both whole rock trace elements and pb isotope ratios of these silicic rocks are similar to the emeishan basalts suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units major and trace element models further indicate that these rocks could not have formed exclusively by re melting of old crust or solidified basaltic rock but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization k feldspar bearing rhyolites are also observed in the last stages of other large igneous provinces we suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

the rate and pattern of bed incision and bank adjustment on the colorado river in glen canyon downstream from glen canyon dam 1956 2000 closure of glen canyon dam in 1963 transformed the colorado river by reducing the magnitude and duration of spring floods increasing the magnitude of base flows and trapping fine sediment delivered from the upper watershed these changes caused the channel downstream in glen canyon to incise armor and narrow this study synthesizes over 45 yr of channel change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations although bed lowering in lower glen canyon began when the first cofferdam was installed in 1959 most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2 6 m of sediment from the center of the channel the average grain size of bed material has increased from 0 25 mm in 1956 to over 20 mm in 1999 the magnitude of incision at riffles decreases with distance downstream from the dam while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream analysis of bed material mobility indicates that the increase in bed material grain size and reduction in reach average gradient are consistent with the transformation of an adjustable bed alluvial river to a channel with a stable bed that is rarely mobilized decreased magnitude of peak discharges in the post dam regime coupled with channel incision and the associated downward shifts of stage discharge relations have caused sandbar and terrace erosion and the transformation of previously active sand bars and gravel bars to abandoned deposits that are no longer inundated erosion has been concentrated in a few pre dam terraces that eroded rapidly for brief periods and have since stabilized the abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage discharge relations in the downstream part of the study area where riffles controlling channel elevation have not incised channel narrowing has resulted from decreased magnitude of peak discharges and minor post dam deposition these physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non native species models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit the pre dam surface and subsurface grain size and the magnitude of post dam average flows these results suggest that it is also necessary to consider 1 the possibility of variable responses among different channel elements and 2 the potential importance of exceptional flows resulting from management decisions

channel narrowing by vertical accretion along the green river near green river utah the green river is the longest tributary of the colorado river near the town of green river utah the green river narrowed in two discrete phases during the twentieth century the first phase of narrowing decreased average width by about 5 and occurred between about 1930 and 1940 when the magnitude of 2 yr flood mean annual discharge and effective discharge decreased by about 30 28 and 37 respectively during this first phase of narrowing saltcedar tamarisk spp an invading non native tree began to establish itself in the study area but botanists of that time did not describe the tree as abundant channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river9s banks further narrowing of an additional 14 occurred after 1959 this latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1 5 yr recurrence flood and when saltcedar were already abundant along the river the deposits that comprise the banks of the narrowing green river are composed of the suspended load of the river and these alluvial deposits are characterized by horizontal layers which indicate that they formed by vertical accretion a mechanism is proposed to explain the coarsening upward sequence of beds found in these vertically accreted deposits these changes in the channel of the green river are based on analysis of more than 2600 discharge measurements made by the u s geological survey resurvey of an abandoned measurement site matches of historical ground level photography and analysis of historical aerial photography within a geographic information system we have developed analytical techniques that permit analysis of width data from u s geological survey discharge measurements where gaging cross sections have adjustable beds and banks these techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means

colorado river sediment transport 1 natural sediment supply limitation and the influence of glen canyon dam analyses of flow sediment transport bed topographic and sedimentologic data suggest that before the closure of glen canyon dam in 1963 the colorado river in marble and grand canyons was annually supply limited with respect to fine sediment i e sand and finer material furthermore these analyses suggest that the predam river in glen canyon was not supply limited to the same degree and that the degree of annual supply limitation increased near the head of marble canyon the predam colorado river in grand canyon displays evidence of four effects of supply limitation 1 seasonal hysteresis in sediment concentration 2 seasonal hysteresis in sediment grain size coupled to the seasonal hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation analyses of sediment budgets provide additional support for the interpretation that the predam river was annually supply limited with respect to fine sediment but it was not supply limited with respect to fine sediment during all seasons in the average predam year sand would accumulate and be stored in marble canyon and upper grand canyon for 9 months of the year from july through march when flows were dominantly below 200 300 m3 s this stored sand was then eroded during april through june when flows were typically higher after closure of glen canyon dam because of the large magnitudes of the uncertainties in the sediment budget no season of substantial sand accumulation is evident because most flows in the postdam river exceed 200 300 m3 s substantial sand accumulation in the postdam river is unlikely

sediment transport constraints on river response to regulation abstract we demonstrate that alluvial channel response to regulation depends on the characteristic mode of sediment transport in a channel and thus for a channel in supply transport equilibrium on the caliber of sediment mantling the channel bed as well as changes in characteristic water discharge and sediment load we find that regulated coarse gravel and cobble bed alluvial rivers dominated by bedload transport are predicted and observed to preserve the value of a dimensionless ratio of characteristic water discharge channel slope width and size of sedimentary particles mantling the bed associated with the critical conditions required for the onset of significant sediment transport the covariant geomorphic adjustment of such rivers to regulation is independent of the degree of sediment impoundment although the nature and rate of changes in any specific characteristic appear to be sensitive to changes in sediment supply in contrast covariant geomorphic changes in sand and fine gravel bed alluvial rivers dominated by weak suspension transport are predicted and observed to be dependent on the degree of sediment impoundment and commensurate with the change in the sediment concentration of characteristic formative flows in both cases specific geomorphic characteristics of alluvial rivers exhibit varied responses to regulation in different rivers but across all rivers a systematic covariation in channel slope width and grain size is observed that for a given mode of sediment transport preserves a characteristic value of the shields number

longitudinal variability in hydraulic geometry and substrate characteristics of a great plains sand bed river article i nfo downstream trends in hydraulic geometry and substrate characteristics were investigated along a 200 km reach of the ninnescah river in south central kansas usa the ninnescah river is a large sand bed perennial braided river located in the central plains physiographic province and is a tributary of the arkansas river hydraulic ge ometry characteristics were measured at eleven reaches and included slope sinuosity bankfull channel width and bankfull channel depth results indicated that the ninnescah river followed a predicted trend of decreasing slope and increasing depth and width downstream there were localized divergences in the central tendency most notability downstream of a substantial tributary that is impounded and at the end of the surveying reach where the ninnescah river approaches the arkansas river surface grain size samples were taken from the top 10 cm of the bed at five points across the wetted cross section within each of the 11 reaches sediment analyses demonstrated a significant trend in downstream fining of surface grain sizes d90 and d50 but unlike previous studies of sand bedded rivers we observed coarsening of substrates downstream of the major tributary conflu ence we propose that the overall low discharge from the tributary was the primary reason for coarsening of the beddownstreamof thetributary results of this studyprovidevaluable baseline information thatcanprovide insight in to how great plains sand bed systems may be conserved managed and restored in the future

historical geomorphic analysis 1932 2011 of a by passed river reach in process based restoration perspectives the old rhine downstream of the kembs diversion dam france germany the old rhine downstream of the kembs diversion dam is one of the largest by passed river reaches in the world 50 km it offers a unique opportunity to study the morphological effects of by passing and address physical and ecological restoration approaches in regulated rivers we conduct a space time analysis of channel adjustment over a period of 80 years 1932 to 2011 we examine planform changes from aerial photographs erosional and depositional patterns from vertical profiles sediment sizes within the active channel and the new established floodplain and we date riparian vegetation encroachment results show that the old rhine exhibited rapid response to the completion of the by passing scheme in the 1950s with a 26 narrowing in median active channel width between 1956 and 2008 from vegetation encroachment on dewatered channel margins mostly groyne fields the narrowing was accompanied by overbank fine sediment deposition 1 5 cm y 1 aggradation since 1950 as well as slight bed degradation 0 7 cm y 1 since 1950 we found no downstream propagation of active channel narrowing over time nor propagation of bed degradation the channel was already significantly adjusted prior to the diversion scheme following the nineteenth century river straightening and groyne construction by passing dewatering mainly provided new pioneer habitat for synchronous vegetation establishment and promoted channel stability by decreasing sediment transport owing to peak flow reduction the morphological budget calculated over the past 20 years estimated a downstream output for coarse sediments at 16 000 m3 y 1 with 80 originating from bed degradation and 20 from bank erosion without significant inputs from upstream the present day morphodynamics remain sensitive to changes because of dynamic bed armouring 2 1 this retrospective analysis permits us to discuss management strategies for altered rivers recommended activities are gravel reintroduction with particle sizes finer than the armoured river bed to enhance sediment transport and promote form dynamism under current flood magnitudes this should be coupled with river widening to allow for complex depositional patterns that benefit ecological habitats these actions should not be implemented without prior analysis of local sediment transfer processes based on in situ experiments and modelling

effects of tributary debris on the longitudinal profile of the colorado river in grand canyon 1 the colorado river in grand canyon has long been known as a rapids and pools river with the rapids owing their existence primarily to tributary debris flows the debris flows deposit subaerial debris fans that constrict the channel laterally and when they enter the river raise the bed elevation the rapids are short wavelength 0 1 to 1 km small amplitude 5 m convexities in the river s longitudinal profile arising from the shallow gradient in the upstream pool and the steep gradient through the rapid itself analysis of the entire longitudinal profile through grand canyon reveals two long wavelength 100 km large amplitude 15 30 m river profile convexities the eastern canyon convexity between river mile rm 30 and rm 80 and the western canyon convexity between rm 150 and rm 250 convexities of intermediate scale are also identified in the longitudinal profile these longer wavelength larger amplitude convexities have strong spatial correlations with high rates of debris flow occurrence high densities of holocene debris fans the largest debris fans along the river and alluvial thicknesses of 10 m or more river profile convexities are unstable and require an active and powerful geologic process to maintain them in this case the abundant frequent and voluminous holocene debris flow activity in grand canyon at all wavelengths the most likely cause for these river profile convexities is holocene aggradation of the riverbed beneath them driven by the coarse particles of tributary debris flows large enough debris flows will slow river flow for kilometers upstream causing it to drop much of its suspended load integrated over time and all of the tributary point source contributions this process will build short wavelength convexities into long wavelength convexities for most if not all of the holocene the colorado river has been dissipating most of its energy in the rapids and expending the remainder in transporting fine sediment through grand canyon with little or no regional incision of bedrock

petrogenesis of rhyolites and trachytes from the deccan trap sr nd and pb isotope and trace element evidence trachytes and rhyolites from salsette island north of bombay have distinctive trace element and isotope features which mark them out from typical crustal melts their highly incompatible trace element and sr nd and pb isotope ratios are similar to those of the associated deccan flood basalts thus the rhyolites and trachytes are closely related to the basalts and a striking compositional gap between 50 and 65 sio2 suggests that the high sio2 rocks evolved by 10 15 partial melting followed by variable amounts of fractional crystallisation the source material could have been basalt within the deccan trap or related gabbroic rocks in deep crustal sill complexes the rhyolites yield an rb sr whole rock age of 61 5 1 9 ma with a slightly high initial 87sr 86sr 0 7085 18 it is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks

short and long term changes to bed mobility and bed composition under altered sediment regimes altered flow and sediment transport regimes from impoundment can result in significant channel bed composition changes that exacerbate the geomorphic and ecological effects of flow regulation using long term discharge and cross sectional data in combination with a two fraction sediment transport model we assess changes in the downstream bed of two flow regulated rivers with equivalent dam induced changes in flow but opposite changes in sediment flux supply limitation has led to incision and armoring in one case while supply excess has led to aggradation and embeddedness in the other under limited sediment supply bed elevation variability decreases soon after impoundment while excess sediment supply results in a decades long gradual decrease in both bed elevation variability and depth of incision although the balance of sediment supply and transport differs between dam management styles in both cases both the immediate and more gradual changes can be explained within the framework of a two fraction sediment transport model our results demonstrate the importance of considering bed composition on sediment transport predictions and the development of management strategies for ecosystem maintenance

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

impacts of cascade reservoirs on the longitudinal variability of fine sediment characteristics a case study of the lancang and nu rivers abstract the lancang river and nu river are parallel rivers located in southwestern china while the lancang river has several dams the nu river has none to explore the impacts of cascade reservoirs on the grain size and mineral composition of fine sediments 9 fine sediment samples from the nu river and 29 samples from the lancang river were collected and tested the results indicate that in the free flowing reaches ffrs of the lancang river the sediment grain sizes fined systematically and the mineral compositions varied slightly and these results were similar to those of the nu river without dams along the whole mainstream in the regulated reaches rrs of the lancang river the sediment grain sizes and mineral compositions indicated discontinuous fluctuations due to the dams trapping in the reservoir head areas the sediments were composed of fine silt or medium silt with 50 clay minerals but the sediments immediately downstream of the dams were fine sand or medium sand with 71 quartz and feldspar the relative abundance of each mineral was altered differently by passing through the rrs quartz chlorite and illite increased by 13 33 and 20 respectively while feldspar and calcite decreased by 26 and 80 respectively and dolomite and pyrite were completely trapped in the cascade reservoirs all these common minerals may be replenished except pyrite which originates from mining areas in the lancang subbasin

understanding crystal populations and their significance through the magma plumbing system crystals are rarely composed of a single crystal population that have grown solely from the batch of magma in which they are resident on emplacement either by eruption or shallow intrusion close investigation of the majority of crystal populations reveal that they comprise up to four main components phenocrysts crystals co genetic with their magmatic host xenocrysts crystals wholly or in part foreign to the magmatic host and magma system antecrysts crystals which are recycled one or several times before inclusion in the host magma but have an origin within the magmatic system and microlites which represent small co genetic crystals which nucleate and grow rapidly on decompression and eruption textural analysis techniques are employed to quantify key aspects of the crystal population including crystal shape crystal size distributions spatial distribution patterns and textural modification using dihedral angles santorini provides a case study of an active volcanic system where a combined textural analysis study has been developed highlighting how the crystal population is being continuously modified by a series of replenishment and mixing events developing textural and microgeochemical techniques provides the next stage in the interrogation of crystal populations linking textures to isotopic heterogeneities and providing fingerprints of where crystals are sourced and re cycled

coupled changes in sand grain size and sand transport driven by changes in the upstream supply of sand in the colorado river relative importance of changes in bed sand grain size and bed sand area sand transport in the colorado river in marble and grand canyons was naturally limited by the upstream supply of sand prior to the 1963 closure of glen canyon dam the river exhibited the following four effects of sand supply limitation 1 hysteresis in sediment concentration 2 hysteresis in sediment grain size coupled to the hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident and has not affected the degree to which the last two effects of sand supply limitation are evident in the colorado river in marble and grand canyons the first three of the effects involve coupled changes in suspended sand concentration and grain size that are controlled by changes in the upstream supply of sand during tributary floods sand on the bed of the colorado river fines this causes the suspended sand to fine and the suspended sand concentration to increase even when the discharge of water remains constant subsequently the bed is winnowed of finer sand the suspended sand coarsens and the suspended sand concentration decreases independently of discharge also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand thus suspended sand concentration in the colorado river is likely regulated by both changes in the bed sand grain size and changes in the bed sand area a physically based flow and suspended sediment transport model is developed tested and applied to data from the colorado river to evaluate the relative importance of changes in the bed sand grain size and changes in the bed sand area in regulating suspended sand concentration although the model was developed using approximations for steady uniform flow and other simplifications that are not met in the colorado river the results nevertheless support the idea that changes in bed sand grain size are much more important than changes in bed sand area in regulating the concentration of suspended sand published by elsevier b v