Home About us Contact
|
Home About us Contact | ||
|
|
||
Aggradation
Kinds of Aggradation Selected AbstractsThe micro-topography of the wetlands of the Okavango Delta, BotswanaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2005T. Gumbricht Abstract The surface of the 40 000 km2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This micro-topography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro-topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro-topography is, therefore, an expression of the non-random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro-topography, combined with estimates of current sediment in,ux, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near-perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro-topography has several useful hydrological applications. Copyright © 2004 John Wiley & Sons, Ltd. [source] Paleoindian geoarchaeology and paleoenvironments of the western Killpecker Dunes, Wyoming, U.S.A.GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 1 2003James H. Mayer The Killpecker Dunes in southwestern Wyoming have long been known to contain evidence for Paleoindian occupation. This paper presents the results of geoarchaeological investigations in the western, dormant portion of the Killpecker Dunes. Five localities, including the Krmpotich Folsom and Finley Cody Complex sites, were examined in order to better understand the Paleoindian geochronology of the dune field, and to facilitate a late Quaternary paleoenvironmental reconstruction. Age control is provided by radiocarbon, optical, and artifact ages. Four late Quaternary eolian units, designated as strata 1,4, were recognized; strata 1 and 2 are relevant to the Paleoindian record. Aggradation of stratum 1 began by at least 14,690 cal yr (,12,550 14C yr B.P.), probably in the form of a sand sheet under cool, dry conditions. Redoximorphic features are ubiquitous in stratum 1, indicating a rise in the water table during the latest Pleistocene. A buried soil (Calcid) at the top of this unit indicates a period of stability between ca. 12,000 and 11,000 14C yr B.P. and has the potential to yield Clovis (11,200,10,900 14C yr B.P.) artifacts. Unconformably overlying stratum 1 is stratum 2, a latest Pleistocene to early Holocene sand containing Folsom (10,900,10,200 14C yr B.P.) through Cody Complex (9000,8500 14C yr B.P.) occupations. A buried soil characterized by the accumulation of illuvial clay and sodium (Natrargid) occurs at the top of stratum 2 and probably formed in and around interdunal ponds. The association of Paleoindian material with the stratum 2 soil suggests the use of interdunal areas as natural traps for hunting extinct bison. The accumulation of soluble salts indicates increased rates of evaporation during the early Holocene, probably from an increase in summer insolation. Although conditions in the dune field during the early Holocene became increasingly dry, they were probably moister than at present. © 2003 Wiley Periodicals, Inc. [source] The spatial and temporal patterns of aggradation in a temperate, upland, gravel-bed riverEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2009Emma K. Raven Abstract Intensive field monitoring of a reach of upland gravel-bed river illustrates the temporal and spatial variability of in-channel sedimentation. Over the six-year monitoring period, the mean bed level in the channel has risen by 0·17 m with a maximum bed level rise of 0·5 m noted at one location over a five month winter period. These rapid levels of aggradation have a profound impact on the number and duration of overbank flows with flood frequency increasing on average 2·6 times and overbank flow time increasing by 12·8 hours. This work raises the profile of coarse sediment transfer in the design and operation of river management, specifically engineering schemes. It emphasizes the need for the implementation of strategic monitoring programmes before engineering work occurs to identify zones where aggradation is likely to be problematic. Exploration of the sediment supply and transfer system can explain patterns of channel sedimentation. The complex spatial, seasonal and annual variability in sediment supply and transfer raise uncertainties into the system's response to potential changes in climate and land-use. Thus, there is a demand for schemes that monitor coarse sediment transfer and channel response. Copyright © 2009 John Wiley & Sons, Ltd. [source] Dating young geomorphic surfaces using age of colonizing Douglas fir in southwestern Washington and northwestern Oregon, USA,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2007Thomas C. Pierson Abstract Dating of dynamic, young (<500 years) geomorphic landforms, particularly volcanofluvial features, requires higher precision than is possible with radiocarbon dating. Minimum ages of recently created landforms have long been obtained from tree-ring ages of the oldest trees growing on new surfaces. But to estimate the year of landform creation requires that two time corrections be added to tree ages obtained from increment cores: (1) the time interval between stabilization of the new landform surface and germination of the sampled trees (germination lag time or GLT); and (2) the interval between seedling germination and growth to sampling height, if the trees are not cored at ground level. The sum of these two time intervals is the colonization time gap (CTG). Such time corrections have been needed for more precise dating of terraces and floodplains in lowland river valleys in the Cascade Range, where significant eruption-induced lateral shifting and vertical aggradation of channels can occur over years to decades, and where timing of such geomorphic changes can be critical to emergency planning. Earliest colonizing Douglas fir (Pseudotsuga menziesii) were sampled for tree-ring dating at eight sites on lowland (<750 m a.s.l.), recently formed surfaces of known age near three Cascade volcanoes , Mount Rainier, Mount St. Helens and Mount Hood , in southwestern Washington and northwestern Oregon. Increment cores or stem sections were taken at breast height and, where possible, at ground level from the largest, oldest-looking trees at each study site. At least ten trees were sampled at each site unless the total of early colonizers was less. Results indicate that a correction of four years should be used for GLT and 10 years for CTG if the single largest (and presumed oldest) Douglas fir growing on a surface of unknown age is sampled. This approach would have a potential error of up to 20 years. Error can be reduced by sampling the five largest Douglas fir instead of the single largest. A GLT correction of 5 years should be added to the mean ring-count age of the five largest trees growing on the surface being dated, if the trees are cored at ground level. This correction would have an approximate error of ±5 years. If the trees are cored at about 1·4 m above the ground surface (breast height), a CTG correction of 11 years should be added to the mean age of the five sampled trees (with an error of about ±7 years). Published in 2006 by John Wiley & Sons, Ltd. [source] Holocene valley aggradation driven by river mouth progradation: examples from AustraliaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2006Paul Rustomji Abstract Since the end of the post-glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south-eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid-Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid-Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd. [source] Overbank deposition along the concave side of the Red River meanders, Manitoba, and its geomorphic significanceEARTH SURFACE PROCESSES AND LANDFORMS, Issue 13 2005Gregory R. Brooks Abstract Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low-angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada. [source] Dating floodplain sediments using tree-ring response to burial,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2005Jonathan M. Friedman Abstract Floodplain sediments can be dated precisely based on the change in anatomy of tree rings upon burial. When a stem of tamarisk (Tamarix ramosissima) or sandbar willow (Salix exigua) is buried, subsequent annual rings in the buried section resemble the rings of roots: rings become narrower, vessels within the rings become larger, and transitions between rings become less distinct. We combined observations of these changes with tree-ring counts to determine the year of deposition of sedimentary beds exposed in a 150-m-long trench across the floodplain of the Rio Puerco, a rapidly filling arroyo in New Mexico. This method reliably dated most beds thicker than about 30 cm to within a year of deposition. Floodplain aggradation rates varied dramatically through time and space. Sediment deposition was mostly limited to brief overbank flows occurring every few years. The most rapid deposition occurred on channel-margin levees, which migrated laterally during channel narrowing. At the decadal timescale, the cross-section-average sediment deposition rate was steady, but there was a shift in the spatial pattern of deposition in the 1980s. From 1936 to 1986, sediment deposition occurred by channel narrowing, with little change in elevation of the thalweg. After 1986 sediment deposition occurred by vertical aggradation. From 1936 to 2000 about 27 per cent of the arroyo cross-section filled with sediment. The rate of filling from 1962 to 2000 was 0·8 vertical m/decade or 85 m2/decade. Published in 2005 by John Wiley & Sons, Ltd. [source] The micro-topography of the wetlands of the Okavango Delta, BotswanaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2005T. Gumbricht Abstract The surface of the 40 000 km2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This micro-topography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro-topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro-topography is, therefore, an expression of the non-random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro-topography, combined with estimates of current sediment in,ux, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near-perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro-topography has several useful hydrological applications. Copyright © 2004 John Wiley & Sons, Ltd. [source] Airborne dust deposition in the Okavango Delta, Botswana, and its impact on landformsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2004M. Krah Abstract This study investigated the local-scale generation and movement of dust in the seasonal swamps of the Okavango Delta, Botswana, with a view to examining possible transfer of material between ,ood plains and islands. It was found that most of the dust load was carried in the lowest 3 m of the air column, and consisted mainly of amorphous silica, indicating that dust was generated largely on the ,ood plains. Dust loads were found to be highest above the ,ood plains and lowest over the interiors of islands, probably due to the baf,ing effect of the island trees on wind velocity. The contrast in dust loads between islands and ,ood plains suggests that there is a net transfer of dust from ,ood plains to islands, but it was not possible to quantify this transfer. It is evident, however, that ,ood plains experience net erosion and islands net aggradation. A strong seasonality in dust loads was observed, with the maximum dust loads coinciding with maximum wind velocity in October. This also coincides with peak seasonal ,ooding in the delta, and only non-inundated ,ood plains are capable of generating dust. Years of low ,ood therefore appear to be more dusty. There may also be transfer of material from higher-lying to lower-lying ,ood plains, which may reduce the topographic contrast on the ,ood plains. Copyright © 2004 John Wiley & Sons, Ltd. [source] Flume experiments on the horizontal stream offset by strike-slip faultsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2004Shunji OuchiArticle first published online: 4 FEB 200 Abstract Flume experiments, in which the middle section of an erosion channel is displaced horizontally, have been conducted to assess the response of streams to horizontal displacement by a strike-slip fault. The experimental erosion channel was developed in a mixture of sand and clay, which provided relatively stable banks with its cohesiveness. Horizontal displacement of a strike-slip fault perpendicular to the channel is expected to add a ,at section to its longitudinal pro,le along the fault line. The experimental stream eliminated this ,at section with downstream degradation, upstream aggradation, and lateral channel shift. As a result, a roughly continuous longitudinal pro,le was maintained. This maintenance of a continuous longitudinal pro,le along channel is considered to be the principle of stream response to horizontal displacement by a strike-slip fault. Downstream degradation was the dominant process of this stream response in the overall tendency of erosion without sand supply. When the rate of fault displacement was low (long recurrence interval), the experimental stream eroded the fault surface, jutting laterally into the channel like a scarp, and de,ected the channel within the recurrence interval. This lateral channel shift gave some gradient to the reach created by fault displacement (offset reach), and the downstream degradation occurred as much as completing the remaining longitudinal pro,le adjustment. When the rate of fault displacement was high (short recurrence interval), the lateral erosion on the ,rst fault surface was interrupted by the next fault displacement. The displacement was then added incrementally to the existing channel offset making channel shift by lateral erosion increasingly dif,cult. The channel offset with sharp bends persisted without much modi,cation, and downstream degradation and upstream aggradation became evident with the effect of the offset channel course, which worked like a dam. In this case, a slight local convexity, which was incidentally formed by downstream degradation and upstream aggradation, tended to remain in the roughly continuous longitudinal pro,le, as long as the horizontal channel offset persisted. In either case, once the experimental stream obtained a roughly continuous gradient, further channel adjustment seemed to halt. Horizontal channel offset remained to a greater or lesser extent at the end of each run long after the last fault displacement. Copyright © 2004 John Wiley & Sons, Ltd. [source] Modelling the effect of form and profile adjustments on channel equilibrium timescalesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2003Martin W. Doyle Abstract A model for describing river channel pro,le adjustments through time is developed and applied to a river responding to base-level lowering in order to examine the effect of channel widening and downstream aggradation on equilibrium timescales. Across a range of boundary conditions, downstream aggradation controlled how quickly a channel reached equilibrium. Channel widening either increased or decreased the equilibrium timescale, depending on whether or not sediment derived from widening was deposited downstream. Results suggest that pro,le adjustments are more important than channel width adjustments in controlling equilibrium timescales for a channel responding to base-level lowering. Copyright © 2003 John Wiley & Sons, Ltd. [source] Geomorphic changes in a complex gully system measured from sequential digital elevation models, and implications for managementEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2003Harley D. Betts Abstract High-resolution digital elevation models (DEMs) were derived from sequential aerial photography of an active ,uvio-mass movement (gully) complex in New Zealand's North Island East Coast region, to measure geomorphic changes over approximately one year. The gully showed a complex behaviour, combining ,uvial and mass movement erosion, deposition, and reworking of materials stored in an active debris fan. During the measurement period 5200 ± 1700 m3 of material were eroded from the 8·7 ha gully complex and 670 ± 180 m3 from the 0·8 ha depositional fan, giving a total of 5870 ± 1710 m3 for the entire gully complex,fan system. The results provide a high-resolution description of gully behaviour over a short time period, and also demonstrate that mass movement (slumping and debris ,ows) accounted for almost 90 per cent (4660 ± 200 m3) of the sediment generated. This erosional response is described in terms of gully evolution by comparing the gully complex to other systems in the region in various stages of development. The effect of gully evolution on geomorphic coupling between the gully complex and channel system is described, and coupling is also shown to vary with the magnitude and frequency of rainfall events. From a land management perspective the success of strategies, such as tree planting, to mitigate against gully erosion depends on the stage of gully development , particularly on whether or not mass movement erosion has begun. In contrast to gully rehabilitation efforts elsewhere, basin-wide afforestation in the early stages of gully incision is favoured over riparian planting, given that mass movement assisted by excessive groundwater pressure is the main process leading to uncontrollable gully expansion. To protect land effectively against continuing gully erosion of headwater catchments and resulting downstream aggradation, it is necessary for land managers to understand the spatial and temporal variability of gully development fully so that mitigation efforts can be targeted appropriately. Copyright © 2003 John Wiley & Sons, Ltd. [source] Stratigraphic landscape analysis: Charting the Holocene movements of the Nile at Karnak through ancient Egyptian timeGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 3 2008J. M. Bunbury Geological analysis of 5,10-m-long sediment cores in the context of the anthropologically derived materials within them has allowed us to identify ancient landscape features in the Theban area around Luxor, Egypt. From these observations we propose a sequence of island formation and northwestward movement of the Nile from the Middle Kingdom onward in the area of the temple complexes of Karnak. The geoarchaeological techniques used appear to document the Holocene lateral migration and vertical aggradation of the Nile. Our method can be used to test postulated movements and is applicable to sites in river or coastal plains where sediments were being deposited during the occupation of the site. The sediments were sieved to retrieve sherds and numerous other small items (2 mm and larger), which included worked stone fragments, rootlet concretions (rhizocretions), desert polished sand grains, and occasionally beads. The small stone fragments can be correlated with buildings and sherds of known age within the site, while the rhizocretions and desert sand grains indicate environmental conditions prevailing at the time of deposition. © 2008 Wiley Periodicals, Inc. [source] Stratigraphic and geochronological context of human habitation along the Galana River, KenyaGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 7 2007David K. Wright Geoarchaeological investigations along the Galana River, eastern Kenya, document a pattern of channel aggradation and then degradation from the Middle to Late Holocene. Archaeological occupations at six sites in fluvial terraces along a ,100 km stretch of the Galana River in Tsavo East National Park correspond with fluvial aggradation beginning ca. 6000 years ago. Artifact analyses indicate that the inhabitants of these sites utilized ceramics and stone tools similar to Pastoral Neolithic traditions detected at other penecontemporaneous archaeological sites in East Africa and possessed domesticated cattle from ca. 3700 years ago. The site occupations that occur during this period have dense artifact concentrations of predominantly locally procured items. The Galana River incised after ca. 900 years ago and there is a noticeable paucity of archaeological material, reflecting more peripatetic lifestyles. This shift in settlement pattern may reflect a decrease in reliance on riverine food sources during the Late Holocene, with diminished riparian environments associated with channel incision. © 2007 Wiley Periodicals, Inc. [source] Paleoindian environmental change and landscape response in Barger Gulch, Middle Park, ColoradoGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 6 2005James H. Mayer Middle Park, a high-altitude basin in the Southern Rocky Mountains of north-central Colorado, contains at least 59 known Paleoindian localities. At Barger Gulch Locality B, an extensive Folsom assemblage (,10,500 14C yr B.P.) occurs within a buried soil. Radiocarbon ages of charcoal and soil organic matter, as well as stratigraphic positions of artifacts, indicate the soil is a composite of a truncated, latest-Pleistocene soil and a younger mollic epipedon formed between ,6000 and 5200 14C yr B.P. and partially welded onto the older soil following erosion and truncation. Radiocarbon ages from an alluvial terrace adjacent to the excavation area indicate that erosion followed by aggradation occurred between ,10,200 and 9700 14C yr B.P., and that the erosion is likely related to truncation of the latest-Pleistocene soil. Erosion along the main axis of Barger Gulch occurring between ,10,000 and 9700 14C yr B.P. was followed by rapid aggradation between ,9700 and 9550 14C yr B.P., which, along with the erosion at Locality B, coincides with the abrupt onset of monsoonal precipitation following cooling in the region ,11,000,10,000 14C yr B.P. during the Younger Dryas oscillation. Buried soils dated between ,9500 and 8000 14C yr B.P. indicate relative landscape stability and soil formation throughout Middle Park. Morphological characteristics displayed by early Holocene soils suggest pedogenesis under parkland vegetation in areas currently characterized by sagebrush steppe. The expansion of forest cover into lower elevations during the early Holocene may have resulted in lower productivity in regards to mammalian fauna, and may partly explain the abundance of early Paleoindian sites (,11,000,10,000 14C yr B.P., 76%) relative to late Paleoindian sites (,10,000,8000 14C yr B.P., 24%) documented in Middle Park. © 2005 Wiley Periodicals, Inc. [source] Landforms, sediments, soil development, and prehistoric site settings on the Madaba-Dhiban Plateau, JordanGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 1 2005Carlos E. Cordova This paper examines recurrent spatial patterns of prehistoric sites in relation to landforms, alluvial fills, and soil development in the uplands and valleys of the Madaba and Dhiban Plateaus of Jordan. Mousterian lithics (Middle Paleolithic) are largely found on high strath terraces plateaus, where they are associated with red Mediterranean soils. In valleys, Upper Paleolithic sites are often associated with reworked loess deposits of the Dalala allostratigraphic unit. Epipaleolithic occupations are found stratified in deposits of the Thamad Terrace, and Pre-Pottery Neolithic and Pottery Neolithic occupations are associated with colluvium mantling the Thamad Terrace. The Tur al-Abyad Terrace and the Iskanderite alluvial inset are the remnants of middle Holocene floodplains, which were attractive areas for Chalcolithic and Early Bronze Age settlements. Sometime around 4000 B.C., stream incision and further lateral erosion destroyed these floodplains. These historic terraces are underlain by alluvial deposits ranging in age from Roman to Early Islamic periods. The sequence of allostratigraphic units, paleosols, and terraces are the basis for reconstructing phases of fluvial aggradation and stream incision during the past 20,000 years. © 2005 Wiley Periodicals, Inc. [source] Alluvial records of medieval and prehistoric tin mining on Dartmoor, southwest EnglandGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 3 2004Varyl R. Thorndycraft The role of tin mining in the society of prehistoric Dartmoor and its impact on the local landscape have long been discussed despite equivocal evidence for prehistoric mine sites. A fluvial geomorphological approach, using floodplain stratigraphy, combined with sediment geochemistry and mineralogy, was employed to identify prehistoric tin mining at the catchment scale. Waste sediment, released during hydraulic mining of alluvial tin deposits, caused downstream floodplain aggradation of sands with a diagnostic signature of elevated Sn concentration within the silt fraction. At a palaeochannel site in the Erme Valley, sediment aggradation buried datable peat deposits. A period of aggradation postdating cal. A.D. 1288,1389 is consistent with the 13th century peak in tin production identified in the documentary record. An earlier phase of aggradation, however, occurred between the 4th and 7th centuries A.D., providing evidence of late Roman or early Post Roman tin mining activity on Dartmoor. © 2004 Wiley Periodicals, Inc. [source] The effects of temporal and spatial patterns of Holocene erosion and alluviation on the archaeological record of the Central and Eastern Great Plains, U.S.A.GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 2 2002E. Arthur Bettis III Patterns of erosion and deposition act as a filter that strongly influences the disposition of the archaeological record of the Central and Eastern Plains of the North American Midcontinent. Detailed studies of alluvial valley stratigraphy in four drainage basins in the region reveal temporal and spatial patterns of fluvial system behavior that control the preservation and visibility of past human activity. These basins are located on a 600-km-long longitudinal gradient extending from semiarid southwestern Kansas to moist-subhumid central Iowa. Despite significant environmental variability along this transect, basin-wide patterns of Holocene erosion and deposition are similar across the study area. From ca. 11,000 to 8000 yr B.P., aggradation, punctuated by slow alluviation and/or stability around 10,000 yr B.P., was the dominant process in large and some small valleys. The early and middle Holocene (ca. 8000,5000 yr B.P.) was a period of net erosion and sediment movement in small valleys, sediment storage in large valleys, and episodic aggradation on alluvial fans. During the late Holocene (post-5000 yr B.P.), alluvial fans stabilized, small valleys became zones of net sediment storage, and aggradation slowed in large valleys. Basin-wide aggradation followed by entrenchment and channel migration characterizes fluvial activity during the Historic period. Consideration of the effects of these temporal and spatial patterns of Holocene erosion and alluviation on the archaeological record is crucial for developing efficient cultural resource sampling strategies and for accurately interpreting the archaeological record. © 2002 John Wiley & Sons, Inc. [source] Hydrometeorological controls and erosive response of an extreme alpine debris flowHYDROLOGICAL PROCESSES, Issue 19 2009Lorenzo Marchi Abstract On 29 August, 2003, an intense convective storm system affected the Fella River basin, in the eastern Italian Alps, producing rainfall peaks of approximately 390 mm in 12 h. The storm triggered an unusually large debris flow in the ungauged Rio Cucco basin (0·65 km2), with a volume of approximately 78 000 m3. The analysis of the time evolution of the rainstorm over the basin has been based on rainfall estimates from radar observations and data recorded by a raingauge network. Detailed geomorphological field surveys, carried out both before and after the flood of August 2003, and the application of a distributed hydrological model have enabled assessment of flood response, estimation of erosion volumes and sediment supply to the channel network. The accounts of two eyewitnesses have provided useful elements for reconstructing the time evolution and the flow processes involved in the event. Liquid peak discharge estimates cluster around 20 m3 s,1 km,2, placing this event on the flood envelope curve for the eastern Italian Alps. The hydrological analysis has shown that the major controls of the flood response were the exceptional cumulated rainfall amount, required to exceed the large initial losses, and the large rainfall intensities at hourly temporal scales, required to generate high flood response at the considered basin scale. Observations on the deposits accumulated on the alluvial fan indicate that, although the dominant flow process was a debris flow, sheetflood also contributed to fan aggradation and fluvial reworking had an important role in winnowing debris-flow lobes and redistributing sediment on the fan surface. This points out to the large discharge values during the recession phase of the flood, implying an important role for subsurface flow on runoff generation of this extreme flash flood event. Copyright © 2009 John Wiley & Sons, Ltd. [source] Developing a post-fire flood chronology and recurrence probability from alluvial stratigraphy in the Buffalo Creek watershed, Colorado, USA,HYDROLOGICAL PROCESSES, Issue 15 2001John G. Elliott Abstract Stratigraphic and geomorphic evidence indicate floods that occur soon after forest fires have been intermittent but common events in many mountainous areas during the past several thousand years. The magnitude and recurrence of these post-fire flood events reflects the joint probability between the recurrence of fires and the recurrence of subsequent rainfall events of varying magnitude and intensity. Following the May 1996 Buffalo Creek, Colorado, forest fire, precipitation amounts and intensities that generated very little surface runoff outside of the burned area resulted in severe hillslope erosion, floods, and streambed sediment entrainment in the rugged, severely burned, 48 km2 area. These floods added sediment to many existing alluvial fans, while simultaneously incising other fans and alluvial deposits. Incision of older fans revealed multiple sequences of fluvially transported sandy gravel that grade upward into charcoal-rich, loamy horizons. We interpret these sequences to represent periods of high sediment transport and aggradation during floods, followed by intervals of quiescence and relative stability in the watershed until a subsequent fire occurred. An alluvial sequence near the mouth of a tributary draining a 0·82 km2 area indicated several previous post-fire flood cycles in the watershed. Dendrochronologic and radiocarbon ages of material in this deposit span approximately 2900 years, and define three aggradational periods. The three general aggradational periods are separated by intervals of approximately nine to ten centuries and reflect a ,millennium-scale' geomorphic response to a closely timed sequence of events: severe and intense, watershed-scale, stand-replacing fires and subsequent rainstorms and flooding. Millennium-scale aggradational units at the study site may have resulted from a scenario in which the initial runoff from the burned watershed transported and deposited large volumes of sediment on downstream alluvial surfaces and tributary fans. Subsequent storm runoff may have produced localized incision and channelization, preventing additional vertical aggradation on the sampled alluvial deposit for several centuries. Two of the millennium-scale aggradational periods at the study site consist of multiple gravel and loam sequences with similar radiocarbon ages. These closely dated sequences may reflect a ,multidecade-scale' geomorphic response to more frequent, but aerially limited and less severe fires, followed by rainstorms of relatively common recurrence. Published in 2001 John Wiley & Sons, Ltd. [source] Recent modelling of sedimentation of suspended particles: a survey,IRRIGATION AND DRAINAGE, Issue 2 2001P. Boogerd modélisation de la sédimentation; dépostition granulaire; transport sédimentaire Abstract Recent literature on modelling of sedimentation was studied. Attention was paid to hydrodynamics, numerical simulation, settling velocity models, sediment and velocity distribution functions, and sediment transport equations. Many popular theories, e.g. those regarding stratification and preferential sweeping, are under discussion. The traditional view that large-scale, energy-containing fluid motions dominate the transport of particles is found to be under attack, as is the modification of the von Karman coefficient to account for the presence of sediment. It is unclear which model for hindered settling should be used under what circumstances, and the effect of particle distribution cannot yet be calculated. Even the most basic problems, such as settling of multiple and/or non-spherical particles in a quiescent liquid, still require research. In the field of sediment distribution functions new solutions are still not entirely satisfactory. Furthermore, the predictive value of transport rate models is still rather low, and several popular sediment transport functions consistently allow more degradation than aggradation. Copyright © 2001 John Wiley & Sons, Ltd. Ce document est une étude de la littérature récente de la modélisation de la sédimentation, en prêtant attention à la hydrodynamique, les simulations numériques, les modèles de la vitesse de déposer, les fonctions de la répartition du sédiment et de la vitesse, et les équations du transport sédimentaire. Beaucoup de théories modernes, celles concernant la stratification et l'entraînage préférentiel par exemple, sont en cours de discussion. L'idée traditionnelle que les mouvements fluides au champ extensif et contenant de l'energie dominent le transport granulaire est attaquée, ainsi que la modification du coefficient von Karman pour tenir compte de la présence du sédiment. Quel modèle à user pour la déposition gênée, et en quelle situation, n'est pas évident, et l'effet de la répartition granulométrique est incalculable. Même les problèmes les plus fondamentaux, comme la déposition d'e multiples granules ou des granules non-sphériques dans un liquide quiescent, ont besoin de recherche. Dans le domaine des fonctions de la répartition du sédiment, les nouvelles solutions ne sont pas encore entièrement satisfaisantes. En outre la valeur prédictionnaire des modèles de la vitesse de transport est encore assez basse, et plusieurs fonctions de transport populaires permettent constamment plus de dégradation que d'accroissement. Copyright © 2001 John Wiley & Sons, Ltd. [source] Holocene pollen records from the central Arctic Foothills, northern Alaska: testing the role of substrate in the response of tundra to climate changeJOURNAL OF ECOLOGY, Issue 6 2003W. Wyatt Oswald Summary 1To explore the role of edaphic controls in the response of arctic tundra to climate change, we analysed Holocene pollen records from lakes in northern Alaska located on glaciated surfaces with contrasting soil texture, topography and tundra communities. Using indicator taxa, pollen accumulation rates (PARs) and multivariate comparison of fossil and modern pollen assemblages, we reconstructed the vegetational changes at Upper Capsule Lake (Sagavanirktok surface) and Red Green Lake (Itkillik II surface) in response to increased effective moisture between the early and middle Holocene. 2In the Red Green record, low PARs and the continuous presence of taxa indicative of prostrate-shrub tundra (PST; Equisetum, Polypodiaceae, Thalictrum and Rosaceae) indicate that the vegetation resembled PST throughout the Holocene. During the warm, dry early Holocene (11 300,10 000 cal years BP), PST also occurred on Sagavanirktok surfaces, as evidenced by PST indicators (Bryidae, Polypodiaceae, Equisetum and Rosaceae) in this interval of the Upper Capsule record. However, PARs increased, suggesting increased vegetation cover, PST taxa declined and taxa indicative of dwarf-shrub tundra (DST; Rubus chamaemorus and Lycopodium annotinum) increased between 10 000 and 7500 cal years BP. 3We hypothesize that between the early and middle Holocene the fine-textured soils and smooth topography of Sagavanirktok surfaces led to increased soil moisture, greater vegetation cover, permafrost aggradation, anoxic and acidic soil conditions, slower decomposition and the development of a thick organic layer. In contrast, soil moisture remained low on the better-drained Itkillik II surface, and vegetational changes were minor. 4Landscape-scale substrate variations have an effect on how tundra responds to climate change, suggesting that the response of arctic ecosystems to future variability may be spatially heterogeneous. [source] Fluvial response to Holocene climate change in low-order streams of central Mexico,JOURNAL OF QUATERNARY SCIENCE, Issue 5 2010Aleksander Borejsza Abstract Alluvial sequences constitute a recognised source of information on past environmental change, but one that has scarcely been tapped in central Mexico. This paper reviews what is currently known about the Holocene alluvial stratigraphy of the region, focusing on the interplay between climate and the pace and style of sedimentation in the incised headwater reaches of stream networks. The records obtained in five different drainage basins , four in the state of Tlaxcala and one in Guanajuato , are presented and compared to published reconstructions of climate change. A near-synchronous incision of all stream networks occurred close to 10 200 14C a BP in response to an increase in precipitation and stream discharge. A spell of very humid but markedly seasonal conditions ensued, resulting in the formation of wet meadows along streams and the accumulation of thick fine-textured valley fills dominated by cumulic soil A horizons. After 9100 14C a BP a transition to a warmer and more arid climate provoked the thinning of vegetation cover on slopes, accelerated runoff and increased sediment delivery to streams. The aggradation of coarser-textured valley fills poor in organic matter set in. It ceased or slowed down significantly after a few millennia as the studied stream reaches achieved a near-graded condition adjusted to the relatively stable climate. Arid mid Holocene conditions are also reflected in the abundant precipitation of secondary carbonates in Guanajuato. At 3100 14C a BP higher precipitation caused more frequent flooding and a resumption of aggradation. Shortly after that date sedentary farmers colonised Tlaxcala. Agriculture altered runoff and sediment delivery to streams and accelerated cut-and-fill cycles on a scale that masked the impact of any climatic fluctuations. Guanajuato was colonised later and its alluvial record suggests the persistence of a humid climate at least until 1000 14C a BP. Copyright © 2009 John Wiley & Sons, Ltd. [source] A multi-proxy palaeoecological and palaeoclimatic record within full glacial lacustrine deposits, western Tennessee, USA,JOURNAL OF QUATERNARY SCIENCE, Issue 8 2009David A. Grimley Abstract The Fulton Section, along the Mississippi River in western Tennessee, USA, is a 1,km continuous exposure (,20,m vertically) of Quaternary fluvial and lacustrine deposits, inset within Eocene sediments and buried by thick loess. Fossiliferous slackwater lake sediments record maximum aggradation during the last two major glaciations, with deposition between ca. 190,140,ka and 24, 18 14C ka BP, based on amino acid and radiocarbon chronology, respectively. During the onset of full glacial conditions (ca. 24,22 14C ka BP), a relatively permanent shallow lake environment is indicated by ostracods, aquatic molluscs, and both pollen and macrofossils of aquatic plants. By 21.8 14C ka BP, increasing emergent plants, amphibious gastropods (Pomatiopsis) and heavier ,18O compositions suggest marsh-like conditions in a periodically drying lake. The surrounding uplands consisted of Picea,Pinus woodlands mixed with cool-temperate hardwoods (e.g. Quercus, Populus, Carya), grasses and herbs. More open conditions ensued ca. 20 14C ka BP, with loess and slopewash gradually infilling the former lake by 18 14C ka BP. Modern analogue analyses of ostracods and palaeontological evidence imply a full glacial climate similar to today's mixed-boreal zone in central Minnesota, USA, about 9°C cooler in mean annual temperature than present-day western Tennessee. Copyright © 2009 John Wiley & Sons, Ltd. [source] Preservation potential for Late Quaternary river alluviumJOURNAL OF QUATERNARY SCIENCE, Issue 2 2003John Lewin Abstract Valley sequences of Late Quaternary alluvial units reflect alluvial preservation as well as alluvial production factors. Effects of lateral channel migration, incision, aggradation and channel stability on preservation potential are explored and then considered in the light of 14 available data sets: cartographically dated and model data based on lateral channel migration; well-mapped and dated Late Quaternary valley unit surveys; and composite age,frequency plots for dated alluvial units and flood sediments. Despite much expectable variation between sites, and the complex effects of river-activity combinations, a common characteristic of the data sets examined is the significance of preservation factors. Lateral migration tends to eliminate older units as it creates new alluvial deposits, whereas incision may lead to the preferential preservation of older units beyond the incision slot. Aggradational environments are likely to preserve more complete records, although simultaneous lateral migration may eliminate, possibly repeatedly, the upper parts of alluvial units. The common pattern of inset and incised streams within Pleistocene and early Holocene fills or bedrock gives finite extent to later units within narrowing valleys so that development of new valley-floor units is necessarily at the expense of reprocessing earlier ones. Floods associated with both slack water deposits and berms are also responsible for the removal of accessible earlier materials, thus limiting the preserved record of earlier events. In light of these censoring effects of river activities, the sequence of preserved Late Quaternary units within UK sequences is reconsidered. It is concluded that preservation potential factors have led to spatial and temporal bias in the alluvial record, and that both here and elsewhere preservation potential needs to be considered systematically alongside variable sedimentation resulting from allogenic environmental factors when interpreting the alluvial archive. Copyright © 2003 John Wiley & Sons, Ltd. [source] MODELING THE LONG TERM IMPACTS OF USING RIGID STRUCTURES IN STREAM CHANNEL RESTORATION1JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 6 2006Sue L. Niezgoda Abstract: Natural channel designs often incorporate rigid instream structures to protect channel banks, provide grade control, promote flow deflection, or otherwise improve channel stability. The long term impact of rigid structures on natural stream processes is relatively unknown. The objective of this study was to use long term alluvial channel modeling to evaluate the effect of rigid structures on channel processes and assess current and future stream channel stability. The study was conducted on Oliver Run, a small stream in Pennsylvania relocated due to highway construction. Field data were collected for one year along the 107 m reach to characterize the stream and provide model input, calibration, and verification data. FLUVIAL-12 was used to evaluate the long term impacts of rigid structures on natural channel adjustment, overall channel stability, and changing form and processes. Based on a consideration of model limitations and results, it was concluded that the presence of rigid structures reduced channel width-to-depth ratios, minimized bed elevation changes due to long term aggradation and degradation, limited lateral channel migration, and increased the mean bed material particle size throughout the reach. Results also showed how alluvial channel modeling can be used to improve the stream restoration design effort. [source] An improved method for determination of Holocene coastline changes around two ancient settlements in southern Anatolia: a geoarchaeological approach to historical land degradation studiesLAND DEGRADATION AND DEVELOPMENT, Issue 4 2003Y. Bal Abstract Two well-known ancient sites in southern Anatolia were selected to investigate and quantify the impact of historical land degradation on the Mediterranean coast of Turkey. These sites are the Luwian settlements of Kelenderis (modern Ayd,nc,k) and nearby Nagidos (Bozyaz,), both in Mersin Province and both occupied since around 4000,BP. Changes in local climatic conditions over this period have produced variations in the rates of fluvial transport of sediment/soil from the hinterland into the relevant deltaic regions, thus influencing rates of coastal progradation and aggradation. In addition, both eustatic and neotectonic movements have contributed to deltaic subsidence and/or hinterland uplift, with consequential impact on coastal evolution (positive or negative). The novel geoarchaeological methodology adopted in this study involves the creation of a graphical archive from detailed and standardised measurements taken from rectified mono- and stereoscopic aerial photographs. These archival data were then integrated with data from several types of historical map and field measurements in order to develop a geographical information system (GIS) database that could be interrogated, enabling graphical models of past coastal change to be constructed and calculations then made of the coastal configurations at successive historical periods. These calculations reveal that over the past 6000 years there has been only limited erosion/degradation in the karstic hinterland supplying the sediment to these two study sites (contrary to some previous statements concerning the high degradation risk of Mediterranean karst terrains). Furthermore, rates of progradation in each delta appear to have become diminished or even reversed in the past several decades as a result of both natural and anthropogenic factors. The precise contribution of neotectonic movements in this seismically active zone remains unquantified and is a topic requiring further interdisciplinary study. Copyright © 2003 John Wiley & Sons, Ltd. [source] Observations on the relationship between lake formation, permafrost activity and lithalsa development during the last 20,000 years in the Tso Kar basin, Ladakh, IndiaPERMAFROST AND PERIGLACIAL PROCESSES, Issue 4 2008Bernd Wünnemann Abstract A close relationship has existed between high mountain permafrost and lake history in the Tso Kar basin, Ladakh, India throughout the last 20,kyr BP. Analyses of sediment cores indicate phases of permafrost growth during low lake levels between >20,10,cal. kyr BP and after 5,cal. kyr BP. Palaeo-shorelines and lake carbonate outcrops indicate a maximum lake size at ca. 8,5,cal. kyr BP when permafrost is believed to have been absent in the basin. Regression of the lake with accompanying salinisation since about 4,cal. kyr BP allowed permafrost aggradation. Permafrost mounds and thermokarst features are inferred to have continued to develop in the last 60 years. Sections in two 6,10,m high unvegetated mounds revealed thick ice lenses and reticulate cryostructures leading to their classification as lithalsas. Mound collapse is induced by surface erosion and slumping, while sediment reworking by wave action and other processes appears to prevent rampart formation. If lake levels continue to fall, further permafrost aggradation is expected in this high elevation enclosed basin. Copyright © 2008 John Wiley & Sons, Ltd. [source] The Devensian periglacial record on Thanet, Kent, UKPERMAFROST AND PERIGLACIAL PROCESSES, Issue 3 2003J. B. Murton Abstract The Devensian periglacial record on Thanet, Kent, is traced from c. 88 to 74,ka and from c. 24 to 12,ka by optical luminescence dating of aeolian sand and silt in the periglacial stratigraphy. The record commences before 88,ka with valley cutting at Pegwell Bay. Valley filling had begun by c. 88,ka and continued to at least 74,ka, coinciding with a major episode of loess deposition in Europe. Permafrost aggradation commenced before c. 21,ka, brecciating near-surface chalk by ice segregation in permafrost and the overlying active layer. Deposition of aeolian sand (coversand) occurred at c. 24,21,ka, correlating with the Older Coversand I in mainland Europe. Permafrost degradation commenced at c. 21,ka, probably due to climate warming during Greenland Interstadial 2. The resulting active-layer deepening through ice-rich permafrost initiated soft-sediment deformation and formation of large-scale patterned ground in an active layer c. 2,m deep. Renewed permafrost aggradation between c. 21.25 and 18,ka coincided with climate cooling during Greenland Stadial 2c and led to cryoturbation in a thinner active layer. Final permafrost degradation commenced no later than c. 14.7,ka, that is, the start of Greenland Interstadial 1e, and may have occurred to some extent during the climate warming associated with Greenland Stadial 2b (c. 19.5,16.9,ka). Renewed deposition of aeolian sand took place at c. 15.5,ka, coincident with loess deposition on Thanet. A final episode of aeolian sand deposition occurred at 12,ka, correlating with the Younger Coversand deposits that are widespread in northwest Europe and formed during Greenland Stadial 1. Copyright © 2003 John Wiley & Sons, Ltd. [source] Distinguishing tectonic from climatic controls on range-front sedimentationBASIN RESEARCH, Issue 4 2007M. C. Quigley ABSTRACT Geologic and chronometric studies of alluvial fan sequences in south-central Australia provide insights into the roles of tectonics and climate in continental landscape evolution. The most voluminous alluvial fans in the Flinders Ranges region have developed adjacent to catchments uplifted by Plio-Quaternary reverse faults, implying that young tectonic activity has exerted a first-order control on long-term sediment accumulation rates along the range front. However, optically stimulated luminescence (OSL) dating of alluvial fan sequences indicates that late Quaternary facies changes and intervals of sediment aggradation and dissection are not directly correlated with individual faulting events. Fan sequences record a transition from debris flow deposition and soil formation to clast-supported conglomeritic sedimentation by ,30 ka. This transition is interpreted to reflect a landscape response to increasing climatic aridity, coupled with large flood events that episodically stripped previously weathered regolith from the landscape. Late Pleistocene to Holocene cycles of fan incision and aggradation post-date the youngest-dated surface ruptures and are interpreted to reflect changes in the frequency and magnitude of large floods. These datasets indicate that tectonic activity controlled long-term sediment supply but climate governed the spatial and temporal patterns of range-front sedimentation. Mild intraplate tectonism appears to have influenced Plio-Quaternary sedimentation patterns across much of the southern Australian continent, including the geometry and extent of alluvial fans and sea-level incursions. [source] | ||||||||||||||||