paleoclimatic interpretations of buried paleosols within the pre illinoian till sequence in northern missouri usa abstract northern missouri preserves shallow buried paleosols ranging in age from early to late pleistocene that are developed in six pre illinoian tills beneath illinoian mis 6 and wisconsinan mis 4 2 loess the morphology of these paleosols changes with age reflecting a changing climate during the pleistocene and cosmogenic nuclide burial dates of the respective sola provide age control on the timing of these changes the depth to secondary calcium carbonate nodules within the weathering profiles increases with younger age indicating a transition to moister conditions during the pleistocene and these nodules are absent entirely within the modern soils after approximately 0 4 ma the sola became distinctly redder even as the time available for pedogenesis became shorter culminating in the bright red sangamon geosol mis 5 this trend is consistent with increasing interglacial temperatures and or precipitation finally erosion rates determined from cosmogenic nuclide concentrations within the sola also increase systematically with younger age this increase may be due to some combination of changing climate more frequent glaciations and the deposition of a thick cover of unconsolidated glacial sediment above the stable residuum dominated preglacial landscape

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

a near synthesis of pre illinoian till stratigraphy in the central united states iowa nebraska and missouri abstract nearly identical sequences of pre illinoian glacial till are present throughout the north central united states in iowa nebraska and missouri in each of these states three tills with normal magnetic remanence have been found above two three tills with reversed remanence and the lithologic differences among successive tills are nearly the same each till is capped by a mature paleosol except for the two youngest reversed polarity tills which are nearly identical in lithology and separated by immature weathering profiles these results seem to indicate that deposits from five major pre illinoian glaciations are present throughout this entire region cosmogenic nuclide burial ages provide relatively precise ages for the three earliest glaciations that reached northeast missouri ca 2 4 1 3 and 0 76 ma ages of the youngest two glaciations in this area are less precise but both are between 0 2 and 0 4 ma the tills in nebraska and western iowa have not been dated directly but are constrained by the ages of interbedded tephra and these limits are consistent with the burial ages in missouri for tills in the same relative stratigraphic positions with one exception two of the three normal polarity tills in nebraska western iowa are present below the 0 64 ma lava creek b tephra and thus should be between 0 64 and 0 78 ma in age however only one of the normal polarity tills in northeast missouri is within this range this discrepancy indicates either a problem with the age constraints or that the till sequence is more complex than yet recognized

terex toolbox for semi automated selection of fluvial terrace and floodplain features from lidar terraces and floodplains are important indicators of near channel sediment dynamics serving as potential sediment sources and sinks increasing availability of high resolution topography data over large areas calls for development of semi automated techniques for identification and measurement of these features in this study we introduce a novel tool that accommodates user defined parameters including a local relief threshold selected by a variable size moving window minimum area threshold and maximum distance from the channel to identify and map discrete terrace and floodplain surfaces each of the parameters can easily be calibrated for a given watershed or reach subsequently the tool automatically measures planform area absolute elevation and height relative to the local river channel for each terrace polygon we validate the tool in two locations where terrace maps were previously developed via manual digitization from lidar and extensive field mapping campaigns the tool is also tested on six different types of rivers to provide examples of starting selection parameters and to test effectiveness of the tool across a wide range of landscapes generally the tool provides a high quality draft map of terrace and floodplain surfaces across the wide range of environmental conditions for which it has been tested we find that the tool functions best in catchments where the terraces are spatially extensive with distinct differences between the terrace and floodplain the most challenging environments for semi automated terrace and floodplain mapping include steep catchments with dense riparian vegetation and very small terraces 10 m2 in areal extent we then apply the tool to map terraces and floodplains in the root river watershed southeastern minnesota and generate exceedance plots for terrace heights these plots provide a first pass analysis to indicate the tributaries and reaches of the river where terraces constitute a significant source of sediment copyright 2013 john wiley sons ltd

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

large scale facies sequences in alluvial overbank environments abstract gradational coarsening upwards or fining upwards sequences order of meters thick in overbank environments can be produced by progressive crevasse splay levee progradation or abandonment while the channel belt remains unchanged in position in contrast meter scale facies sequences related to periodic channel bell avulsion may show relatively abrupt changes in facies and paleocurrent direction simulation models suggest that the number of such avulsion related facies changes in any overbank sequence bounded above and below by channel belt deposits is on the order of 1 to 10 this is confirmed using an ancient example in this example the average thickness of each avulsion bounded overbank facies sequence was 3 m assuming an average recurrence interval for avulsions of about 1 000 ye rs the mean overbank deposition rate was about 3 mm per year

a gis method to identify flat surfaces and restore relict fluvial long profiles from terrace remnants gives new clues on how large basins respond to endorheic exorheic transitions duero basin iberian peninsula this is an mite contribution spanish government plan estatal de investigacion cientifica y tecnica y de innovacion cgl2014 59516 pr2011 0044 gis technical support was funded by the spanish ministerio de economia y competitividad under grant pej 2014 a 93258 lrr is funded by the marie curie clarin cofund programme reference aca 17 19 ficyt principality of asturias

geomorphic mapping and taxonomy of fluvial landforms abstract fluvial geomorphologists use close to a 100 different terms to describe the landforms that make up riverscapes we identified 68 of these existing terms that describe truly distinctive landforms in which form is maintained under characteristic conditions and fluvial processes clear topographic definitions for these landforms to consistently identify and map them are lacking with the explosion of continuous high resolution topography and digital elevation models we have plenty of new basemaps in which these landforms are clearly visible but very few examples of manual or automated classification of fluvial landforms fluvial landforms are the building blocks of a river and are variously referred to as geomorphic units morphological units habitat units and channel units we present a tiered framework for describing geomorphic units with tier 1 differentiating units on the basis of their stage tier 2 separating shape e g concave convex or planar tier 3 using particular key attributes to narrow in on the likely specific geomorphic unit type and tier 4 differentiating those types on the basis of vegetative or roughness modifiers information on the assemblage and configuration of geomorphic units can be used to inform process based interpretations of the range of river behavior the accuracy and transferability of such analyses is fundamentally tied to the taxonomy we assign to these discrete building blocks in this paper we clarify the terminology and definitions relating to the identification and delineation of geomorphic units margins and structural elements we establish a set of procedures that can be used to manually map and identify these features the proposed framework provides a rigorous and repeatable approach to identification of topographically defined features of riverscapes we demonstrate the application of these systematic yet flexible procedures with a series of maps from rivers in differing valley settings

performance evaluation of three dem based fluvial terrace mapping methods the availability of high resolution digital elevation models dems derived from airborne light detection and ranging lidar surveys has spurred the development of several methods to identify and map fluvial terraces the post glacial landscape of the sheepscot river watershed maine where land use change has produced fill terraces upstream of historic dam sites was selected to implement a comparison between terrace mapping methodologies at four study sites within the watershed terraces were manually mapped on lidar dem derived hillshade images to facilitate the comparison among fully and semi automated dem based procedures including 1 spatial relationships between interpreted terraces and surrounding natural topography 2 feature classification algorithms and 3 the terex terrace mapping toolbox each method was evaluated based on its accuracy and ease of implementation the four study sites have varying longitudinal slope 0 0008 0 006 m m channel width 5 30 m surrounding landscape relief 20 80 m type and density of surrounding land use and mapped surficial geologic units all methods generally overestimate terrace areas average predicted area 210 of manually defined area with the most accurate results achieved within confined river valleys surrounded by the steep hillslopes accuracy generally decreases for study sites surrounded by low relief landscapes predicted areas ranged 4 953 of manual delineations we conclude with the advantages and drawbacks of each method tested and make recommendations for the scenarios where the use of each method is most appropriate copyright 2016 john wiley sons ltd

100 000 years of african monsoon variability recorded in sediments of the nile margin abstract multiproxy analyses were performed on core ms27pt recovered in hemipelagic sediments deposited on the nile margin in order to reconstruct nile river palaeohydrological fluctuations during the last 100 000 years the strontium and neodymium isotope composition of the terrigenous fraction and the major element distribution reveal large and abrupt changes in source oscillating between a dominant aeolian saharan contribution during arid periods and a dominant nile river contribution during pluvial periods iron content shows a strong correlation with strontium and neodymium isotopes this allows the use of a high resolution continuous fe record as a proxy of blue nile sediment input over the last 100 000 years the detailed fe record with approximately 10 years resolution during pluvial periods is consistent with subtropical african records of well dated lake level fluctuations and thus constitutes a first continuous high resolution record of the east african monsoon regime intensity over ethiopia the detailed fe record shows the two main known pluvial periods attributed to strengthening of the african monsoon over ethiopia the nabtian period from 14 to 8 ka cal bp and the saharan period from 98 to 72 ka bp for the first time the last glacial period marine isotope stage mis 2 3 and 4 is documented with a continuous record showing large oscillations between high and low east african palaeo monsoon regimes the end of the nabtian period occurred at 8 ka in core ms27pt i e much earlier than on the east equatorial african region where it ended around 5 5 ka we interpret this as evidence that the southward shift of the rain belt occurred 3000 years earlier over the eastern ethiopian highland and propose that the gradual southward migration of the rain belt was associated with highly variable intensity and longer rainy seasons from 8 to 5 ka during the last glacial period two wet periods are present around 60 50 ka bp and 38 30 ka bp these two humid periods are in phase with the rise of atmospheric ch4 concentrations suggesting that wetland tropical african area was one of the sources of the atmospheric ch4 during the mis 3 during the last glacial maximum and mis 4 high saharan aeolian influxes in phase with records of aeolian dust deposited in east antarctica are documented this study highlights the importance of reconstructions of monsoon rainfall fluctuation at high temporal resolution to better understand the link between low and high latitude climate variability at millennial timescales

hydrogeomorphological differentiation between floodplains and terraces floodplains and terraces in river valleys play important roles in the transport dynamics of water and sediment while flat areas in river valleys can be identified from lidar data directly characterizing them as either floodplain or terraces isnot yet possible to address this challenge we hypothesize that since geomorphic features are strongly coupled to hydrologic and hydraulic dynamics and their associated variability there exists a return frequency or possibly a narrow band of return frequencies of flow that are associated with floodplain formation and this association can provide a distinctive signature for distinguishing them from terraces based on this hypothesis we develop a novel approach for distinguishing between floodplains and terraces that involves transforming the transverse cross sectional geometry of a river valley into a curve named river valley hypsometric rvh curve and linking hydraulic inundation frequency with the features of this curve our approach establishes that the demarcation between floodplains and terraces can be established from the structure of steps and risers in the rvh curves which can be obtained from the dem data further it shows that these transitions may themselves be shaped by floods with 10 to 100 yr recurrence we additionally show that when floodplain width and height above channel bottom are normalized by bankfull width and depth the ratio lies in a narrow range independent of the scale of the river valley

modeling framework for sediment deposition storage and evacuation in the floodplain of a meandering river application to the clark fork river montana 1 a theory for the evolution of a river floodplain and the associated movement of tracer sediment is applied to the case of the upper clark fork river montana whose floodplain is contaminated with tailings derived from historical mining activities deposition of mine tailings raised the elevation of the floodplain which now contributes contaminated sediment to the channel the theory is capable of reproducing the basic aspects of the system s evolution including aggradation of the floodplain during the mining era and subsequent net supply of sediment from floodplain to channel after the cessation of mine input results indicate that the natural removal of tailings from the floodplain could take thousands of years a period longer than is required for the channel to simply rework the tailings the self stabilizing nature of the model results in an evolution toward a new postmine steady state with tailings eventually almost entirely removed from the system

late quaternary highstands at lake chilwa malawi frequency timing and possible forcing mechanisms in the last 44 ka the extensive shoreline deposits of lake chilwa southern malawi a shallow water body today covering 600 km2 of a basin of 7500 km2 are investigated for their record of late quaternary highstands osl dating applied to 36 samples from five sediment cores from the northern and western marginal sand ridges reveal a highstand record spanning 44 ka using two different grouping methods highstand phases are identified at 43 7 33 3 ka 26 2 21 0 ka and 17 9 12 0 ka total error method or 38 4 35 5 ka 24 3 22 3 ka 16 2 15 1 ka and 13 5 12 7 ka finite mixture model age components with two further discrete events recorded at 11 01 0 76 ka and 8 52 0 56 ka highstands are comparable to the timing of wet phases from other basins in east and southern africa demonstrating wet conditions in the region before the lgm which was dry and a wet lateglacial which commenced earlier in the southern compared to northern hemisphere in east africa we find no evidence that wet phases are insolation driven but analysis of the dataset and gcm modelling experiments suggest that heinrich events may be associated with enhanced monsoon activity in east africa in both timing and as a possible causal mechanism

fluvial terraces of the lower susquehanna river abstract fluvial terraces of the lower susquehanna river offer a unique opportunity to investigate the late stage geologic and geomorphic evolution of the u s atlantic passive margin petrography and elevation distinguish and provide a basis for correlation of two groups of terraces the upland terraces and lower terraces through the piedmont newark basin and great valley downstream correlation to dated upper coastal plain and fall zone fluvial deposits relative weathering and soil profile development characteristics establish terrace age upland terraces tg1 tg2 and tg3 middle to late miocene strath terraces 80 to 140 m above the present channel occur only along the piedmont reach they are underlain by unstratified texturally mature quartz dominated roundstone diamictons lower terraces qtg qt1 qt6 pliocene and pleistocene strath and thin aggradational terraces within 45 m of the present channel are underlain by stratified and unstratified texturally and compositionally immature sand gravel and pebbly silt terrace age and longitudinal profiles suggest complex interactions among relative base level long term flexural isostatic processes climate and river grade our model for terrace genesis requires the susquehanna river to attain and maintain a characteristics graded longitudinal profile over graded time for the u s atlantic margin we propose that straths are continually cut along this graded profile during periods of relative base level stability achieved by slow steady isostatic continental uplift acting in concert with eustatic rise change in an external modulating factor such as eustatic fall or climate change results in fluvial incision and subsequent genesis of strath terraces longitudinal profiles of lower susquehanna river terraces which converge at the river mouth diverge through the piedmont and reconverge north of the piedmont contrast with their hypothesized original concave up profiles progressive and cumulative flexural upwarping of the atlantic margin accounts for terrace profile deformation suggesting flexural isostasy as a first order regional deformation mechanism these results offer new interpretations of terrace age correlation and geologic significance that require modification of previous studies suggesting uplifted or anticlinically warped peneplains on the u s atlantic margin

paleohydrology of lake victoria east africa inferred from 18o 16o ratios in sediment cellulose delta 18 o values of aquatic cellulose preserved in a 7 6 m sediment core from near the center of lake victoria east africa record a 13 000 yr hydrological history of the basin relative to the present value 3 4 highly elevated lake water 18 o values of 8 2 from 13 200 to 7250 14 c yr b p indicate closed basin conditions and precipitation evaporation ratios much smaller than at present an abrupt drop in the lake water 18 o to 0 1 marks the rapid transition to an open basin between 7250 and 7150 14 c yr b p from 7140 to 5400 14 c yr b p increasing lake water 18 o values from 0 1 to 5 1 5 are proposed to reflect outlet downcutting and a likely decrease in relative humidity these data have unexpectedly shifted our view of the timing of changes in the lake victoria water balance and imply a period of at least 10 000 years 17 000 to 7000 14 c yr b p when the lake victoria basin did not contribute waters to the main nile river

integration of channel and floodplain suites i developmental sequence and lateral relations of alluvial paleosols abstract the lower eocene willwood formation of the bighorn basin northwest wyoming consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from eocene pedogenesis willwood paleosols vary considerably in their relative degrees of maturity maturity is defined as stage of development as a function of the amount of time required to form five arbitrary stages are proposed to distinguish these soils of different maturities in the willwood formation stage 1 soils the least mature are entisols stage 2 and stage 3 soils are intermediate in maturity and are probably alfisols and stage 4 and stage 5 soils the most mature are spodosols these stages are not only time progressive elements of an in situ maturation sequence for w llwood soil formation but in the lateral dimension they are also usually distributed sequentially study of willwood paleosols indicates that an inverse relationship exists between soil maturity and short term sediment accumulation rate the least mature willwood paleosols formed in areas of relatively high net rates of sediment accumulation on 1 channel levee and crevasse splay sediments of the proximal alluvial ridge and 2 deposits filling large and small paleovalleys formed by major episodes of gullying lowered baselevels in contrast the fine grained sediments of the distal floodplain where net sediment accumulation rates were relatively low experienced development of much more mature soils soils of intermediate maturities occur in the order of their stage on intervening proximal floodplain and distal alluvial ridge sediments adjacent bodies of sedimentary rock that differ in their ancient soil properties because of distance from areas of relatively high sediment accumulation are denoted by the new term pedofacies the remarkable sequence of paleosols in the willwood formation clearly illustrates several important principles of soil sediment interrelationships in aggrading alluvial systems that have broad application to other deposits this is especially true in view of the widespread distribution of paleosols in nearly all ancient fluvial rocks further study of willwood paleosols will not only enable precise lateral correlation of coeval alluvial sediments and thereby fluvial sedimentary events from the distal to the proximal realms of the floodplain but will also contribute to increasingly informative evaluations of the nature tempo and mode of alluvial succession

a revised 30 000 year paleoclimatic and paleohydrologic history of lake albert east africa abstract new geochemical and palynological data and a sedimentological reinterpretation of two cores from lake albert allow a revision of the paleoclimatic and paleohydrologic history of northwest uganda over the last 30 000 14 c yr bp our findings document one and possibly three periods of significantly reduced inflow to the nile river system from the white nile based on sediment fabric and mineralogy shifts in percentages of total organic carbon toc and total organic nitrogen ton hydrogen index hi and 13 c and 15 n as well as poor preservation of sporopollenin and siliceous microfossils we recognize two mature paleosols that formed between 18 000 and 12 500 14 c yr bp at the surface of exposed lacustrine muds a thin fossiliferous zone between the two soils documents a brief period of flooding lake albert was below the level of the coring site and thus a closed basin for several thousand years during the late pleistocene around 12 500 14 c yr bp the younger vertisol was flooded by a major rise in lake level relatively deep water conditions were established soon after this transgression but an apparent change in sedimentation rate sometime between 8 000 and 3 400 14 c yr bp may reflect another briefer period of low water which resulted in interruption of sedimentation at the coring site palynological data document a rapid and abrupt shift to drier conditions around lake albert from 11 400 to 9 900 14 c yr bp coincident with the cool younger dryas event of the northern hemisphere geochemical evidence suggests that throughout the terminal pleistocene and early holocene and again in the late holocene the water column was relatively well mixed with little tendency to persistent anoxia

controls over the strontium isotope composition of river water abstract strontium concentrations and isotope ratios have been measured in river and ground waters from the ganges orinoco and amazon river basins when compared with major element concentrations the data set has allowed a detailed examination of the controls over the strontium isotope systematics of riverine input to the oceans in the following environments 1 1 typical drainage basins containing limestones evaporites shales and alumino silicate metamorphic and igneous rocks 2 2 shield terrains containing no chemical or biogenic sediments and 3 3 the floodplains that constitute the largest areas of many large rivers the strontium concentration and isotope composition of river waters are largely defined by mixing of strontium derived from limestones and evaporites with strontium derived from silicate rocks the strontium isotope composition of the limestone endmember generally lies within the phanerozoic seawater range which buffers the 87 sr 86 sr ratios of major rivers a major exception is provided by the rivers draining the himalayas where widescale regional metamorphism appears to have led to an enrichment in limestones of radiogenic strontium derived from coexisting silicate rocks the strontium isotope systematics of rivers draining shield areas are controlled by the intense transportlimited nature of the weathering reactions and thereby limits variations in the strontium flux from these terrains floodplains are only a minor source of dissolved strontium to river waters and precipitation of soil salts in some floodplains can reduce the riverine flux of dissolved strontium to the oceans the most effective mechanisms for altering the isotope ratio and flux of riverine strontium to the oceans are increased glaciation and large scale regional metamorphism of the type produced during continental collision both mechanisms provide a means for increasing the 87 sr 86 sr ratio of the global riverine flux

late pleistocene desiccation of lake victoria and rapid evolution of cichlid fishes lake victoria is the largest lake in africa and harbors more than 300 endemic species of haplochromine cichlid fish seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the late pleistocene before 12 400 carbon 14 years before the present these results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid

strontium isotope evidence for late pleistocene reestablishment of an integrated nile drainage network evidence of late pleistocene closed basin conditions in equatorial african lakes has far reaching implications for the history of the nile river there has been widespread acceptance that overflow of lakes victoria and albert into the nile occurred some time during the terminal pleistocene but recent suggestions that lake victoria remained a closed basin until ca 7 2 ka imply a much longer period of greatly reduced white nile discharge this hypothesis has major consequences for current ideas about the river9s paleohydrology early human exploitation of riverine resources nile delta sedimentation and late quaternary genesis of sapropels in the eastern mediterranean here we resolve the timing of lake victoria9s overflow by using strontium isotopes as a tracer for water from the principal lakes and rivers of the upper nile our data demonstrate that overflow of lakes victoria and albert into the nile drainage network occurred no later than 11 5 14 c ka

a high resolution 11 400 yr diatom record from lake victoria east africa abstract fine interval 30 45 yr sampling of a core from lake victoria s damba channel shows that numerous abrupt changes in the lake s diatom assemblages have occurred in response to climatic fluctuations over the past 11 400 14 c yr four distinct climatic phases bounded by sudden transitions are inferred 1 variably dry 11 400 10 000 yr b p 2 humid 10 000 7200 yr b p 3 more seasonal 7200 2200 yr b p and 4 more arid 2200 0 yr b p with a dry little ice age event 600 200 yr b p the diatom inferred paleoclimatic history for northern lake victoria closely resembles that inferred from a well dated pollen record from pilkington bay spectral analysis of the diatom record reveals strong periodicities including globally distributed 2360 2550 1400 1030 1130 and 500 cal yr cycles repeated rapid shifts between aulacoseira and nitzschia dominated diatom assemblages suggest that post 1960 changes in the lake s phytoplankton communities have had earlier climate driven analogs

historical changes in channel network extent and channel planform in an intensively managed landscape natural versus human induced effects abstract humans have become major geomorphological agents effecting substantial change in the characteristics of earth s physical landscapes the agricultural midwest of the united states is a region marked by pronounced human influence at the landscape scale humans undoubtedly have strongly influenced critical zone processes including fluvial processes in intensively managed agricultural landscapes yet the exact nature of human alteration of these processes is unknown this study documents historical changes in the extent of the stream channel network and in channel planform within the upper sangamon river basin an intensively managed agricultural watershed in illinois results indicate that the modern channel network is nearly three times more extensive than the channel network in the 1820s most change in drainage density has occurred in headwater portions of the basin where numerous drainage ditches have been added to the network to drain flat uplands no detectable change in channel position is evident between 1940 and 2012 along about 60 of the total length of the sangamon river and its major tributaries nearly 30 of the total length exhibits change related to meander dynamics cutoffs and lateral migration whereas about 8 has changed as a result of channelization channelized sections typically remain straight for decades following human modification supporting the notion that humans produce long lasting catastrophic change in channel planform in this region the findings confirm that humans are effective agents of morphological change in fluvial systems in this intensively managed watershed documenting human induced versus natural changes in fluvial systems is important for evaluating how other critical zone processes in intensively managed landscapes have been affected by these changes human induced changes in channel extent and planform most likely have altered this landscape from one dominated by biogeochemical transformations and storage of water and sediment prior to agricultural development into one now characterized by enhanced fluxes of water sediment and nutrients

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

reassessment of lake victoria upper nile river paleohydrology from oxygen isotope records of lake sediment cellulose reconstruction of lake water 1 8 o from analysis of cellulose 1 8 o in two sediment cores from lake victoria east africa shows a large shift to lower values during the terminal pleistocene this shift records the transition from closed to open basin conditions following desiccation at the last glacial maximum although oxygen isotope analysis of cellulose from one core had placed this overflow at 8 ka 7200 1 4 c yr b p reevaluation of the age model for this core in addition to new stratigraphic and chronological evidence from a second core suggests that basin overflow was established much earlier ca 13 ka our refined view of the timing of lake victoria overflow inferred from the oxygen isotope records is consistent with other paleolimnological studies indicating that lake sediment cellulose is an effective and sensitive isotopic archive of major hydrologic events in this region

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels

numerical modeling of fluvial strath terrace formation in response to oscillating climate many river systems in western north america retain a fluvial strath terrace rec ord of discontinuous downcutting into bedrock through the quaternary their importance lies in their use to interpret climatic events in the headwaters and to determine long term incision rates terrace formation has been ascribed to changes in sediment supply and or water discharge produced by late quaternary climatic fluctuations we use a one dimensional channel evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences the model includes sediment transport vertical bedrock erosion limited by alluvial cover and lateral valley wall erosion we set limits on our modeling by using data collected from the terraced wind river basin two types of experiments were performed constant period sinusoidal input histories and variable period inputs scaled by the marine 18 o rec ord our simulations indicate that strath terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates straths are cut when the channel floor is protected from erosion by sediment and are abandoned and terraces formed when incision can resume following sediment cover thinning high sediment supply promotes wide valley floors that are abandoned as sediment supply decreases in contrast wide valleys are promoted by low effective water discharge and are abandoned as discharge increases widening of the valley floors that become terraces occurs over many thousands of years the transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment transport capacity formation of terraces lags by several thousand years the input changes that cause their formation our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation the incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the last glacial maximum this apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock uplift rate

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

response of the nile and its catchment to millennial scale climatic change since the lgm from sr isotopes and major elements of east mediterranean sediments abstract changes in 87 sr 86 sr and major element geochemistry from two sediment cores 9509 and 9501 in the eastern mediterranean em were used to resolve changes in sediment provenance and hence determine climate changes in the nile catchment and eastern sahara desert over the past 25 ka the sediment was described by a three end member system comprising blue nile bn 87 sr 86 sr 0 7506 sr 210 ppm white nile wn 87 sr 86 sr 0 7094 sr 72 5 ppm and saharan dust sd 87 sr 86 sr 0 7183 sr 99 ppm the sedimentary record of these cores represents the suspended load carried down the nile river and discharged into the s e levantine basin and thus records palaeoclimatically controlled changes in erosion and transport in the catchment during arid periods 0 5 ka bp and prior to 11 ka bp fluxes of bn sediment at 9509 6 g cm 2 yr 10 12 g cm 2 yr respectively were greater than during the peak of the african humid period ahp from 5 to 11 ka bp 2 yr this latter period witnessed the deposition of the youngest organic rich sediment termed sapropel s 1 in the em basin by contrast the flux of wn increased during the ahp from 5 g cm 2 yr at 13 ka bp to 15 g cm 2 yr in the ethiopian highlands bn catchment increases in the amount and duration of the monsoon during the ahp caused more vegetation to grow resulting in less soil erosion in the wn catchment increased rainfall caused more catchment erosion and higher sediment flux through the sudd marshes the sedimentation rate in core 9509 increased during the ahp because of the greater importance of the wn sediment flux relative to the bn sediment flux saharan dust flux also decreased during the ahp reaching a minimum at 6 ka bp core 9509 due to greening of the sahara desert at the onset of s 1 the changes in nile flow as determined by 87 sr 86 sr and climatic changes in the em basin determined by 18 o of planktonic foraminifera were simultaneous confirming that such isotopic tracers cannot be used directly to determine the cause of the circulation changes in the em at this time the increase in the proportion of bn sediment at 9509 with a somewhat higher grain size during the h 1 period 15 17 ka bp was caused by erosion and redistribution of sediment from the nile delta and or the israeli coast as sea level rose

revealing the natural complexity of fluvial morphology through 2d hydrodynamic delineation of river landforms abstract fluvial landforms at the morphological unit scale 1 10 channel widths are typically delineated and mapped either by breaking up the one dimensional longitudinal profile with no accounting of lateral variations or by manually classifying surface water patterns and two dimensional areal extents in situ or with aerial imagery mapping errors arise from user subjectivity varying surface water patterns when the same area is observed at different discharges and viewpoints and difficulty in creating a complete map with no gaps or overlaps in delineated polygons this study presents a new theory for delineating and mapping channel landforms at the morphological unit scale that eliminates in field subjective decision making adds full transparency for map users and enables future systemic alterations without having to remap in the field delineation is accomplished through a few basic steps first near census topographic and bathymetric data are used in a two dimensional hydrodynamic model to create meter scale depth and velocity rasters for a representative base flow second expert judgment and local knowledge determine the number and nomenclature of landform types as well as the range of base flow depth and velocity over each type this step does require subjectivity but it is transparent and adjustable at any time third the hydraulic landform classification is applied to hydraulic rasters to quickly completely and objectively map the planform pattern of laterally explicit landforms application of this theory will reveal the true natural complexity yet systematic organization of channel morphology

floodplain width adjustments in response to rapid base level fall and knickpoint migration abstract geomorphology has long been engaged in characterizing the form and understanding the evolution of floodplains this study primarily examines floodplain width in four tributaries to the minnesota river in southern minnesota namely the maple le sueur and blue earth rivers and seven mile creek the tributary systems are relatively young having formed after retreat of the wisconsinan ice sheet and are rapidly evolving in response to a base level fall of nearly 70 m 13 400 ybp as a result the lower reaches of all four river channels are aggressively incising this study applies a new freely available open code plug in for arcgis to measure floodplain width as a function of elevation above the geomorphic top of bank systematic changes are observed in all four systems above the steep incising knickzone reaches of each of the tributaries relatively wide and mostly unconfined floodplains are observed within the knickzone floodplains progressively narrow to between 25 60 of their width above the knickzone despite the fact that sediment loads increase significantly in the downstream direction a power law relationship is consistently observed between floodplain width and the ratio of local channel slope to contributing drainage area to about the negative one third power in all four rivers this same power law relationship is observed for two other rivers that differ significantly from our study systems in climatic and tectonic environment as well as base level history

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

strontium and its isotopes in canadian rivers fluxes and global implications systematic sampling of the 39 largest canadian rivers shows that the weighted average 87sr86sr ratio in the dissolved load is 0 7111 similar to previous measurements on such large rivers as the amazon and mississippi consequently we believe that the above estimate is likely representative of the global average this imposes a limit of 6 5 1011g yr 1 on the amount of sr exchanged with basalts in hydrothermal cells on mid oceanic ridges evaluation of geological information suggests that sr from silicate sources is of considerable importance for all but the largest canadian rivers the latter have chemical and isotopic composition consistent with 4 1 carbonate vs silicate derivation of sr but such interpretation is not unique in terms of their water discharge the 39 canadian rivers studied account for 4 2 of the world total and their weighted average concentrations for other dissolved solutes are tds 176 ppm ca 18 ppm cl 6 8 ppm and sr 84 ppb

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

landscape evolution valley excavation and terrace development following abrupt postglacial base level fall many high latitude fluvial systems are adjusting to base level changes since the last glaciation channels that experienced base level fall may still be incising often through glacial diamictons tills these tills can be quite competent behaving more like weak bedrock than unconsolidated sediment and erode at a fast pace thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems here we integrate light detection and ranging lidar topography strath terrace chronology and numerical modeling to determine knickpoint migration and incision history of the le sueur river in central minnesota usa results indicate that the le sueur river is best modeled as a detachment limited channel with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development the le sueur river meanders as it incises so we coupled the best fit incision model to a meander model to determine valley excavation history the excavation history was used to determine a natural background erosion rate prior to land use changes associated with settlement and agricultural expansion in the mid 1800s we compared background fine sediment silt and clay erosion rates with historic decadal average annual suspended loads results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads recent changes in hydrology associated with land use and climate change have increased flows in rivers leading to higher sediment loads not just from field erosion but from increased bank and bluff erosion in the deeply incised valleys

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

nile river sediment fluctuations over the past 7000 yr and their key role in sapropel development the provenance pattern of nile river sediments can be used as a proxy for paleoclimatic changes in east africa the 87sr 86sr ratios are particularly appropriate for such provenance investigations because the white nile drains predominantly crystalline basement rocks whereas the blue nile and atbara flow off the ethiopian highlands which consist of tertiary volcanic rocks a high resolution profile of 87sr 86sr and ti al ratios from a well dated core in the nile delta shows a close correspondence with known changes in nile flow over the past 7000 yr at times of higher river flow there was markedly decreased input of blue nile derived and total sediment this change was caused by northward movement of the inter tropical convergence zone resulting in increased vegetative cover in the ethiopian highlands due to higher rainfall and a longer wet season this inverse relationship between nile river flow and sediment flux may have had important implications in the development of agricultural technology in ancient egypt the marked minimum in 87sr 86sr at 4200 4500 yr b p is coincident with the end of the old kingdom in egypt and provides independent evidence that demise of the old kingdom might have been associated with an extended period of catastrophic low floods during the quaternary and late neogene there was periodic deposition of organic rich sediments sapropels in the eastern mediterranean that represent important indicators of major environmental change evidence from the ti al ratio suggests that the pattern of erosion and sediment supply from the nile catchment observed in this study also occurred throughout much of the neogene and quaternary the reduced inputs of blue nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to n limitation in the eastern mediterranean which are important characteristics of sapropel deposition