Swale Topography Research Articles

Backed‐Up, Saturated, and Stagnant: Effect of Milldams on Upstream Riparian Groundwater Hydrologic and Mixing Regimes

AbstractHow milldams alter riparian hydrologic and groundwater mixing regimes is not well understood. Understanding the effects of milldams and their legacies on riparian hydrology is key to assessing riparian pollution buffering potential and for making appropriate watershed management decisions. We examined the spatiotemporal effects of milldams on groundwater gradients, flow directions, and mixing regime for two dammed sites on Chiques Creek, Pennsylvania (2.4 m tall milldam), and Christina River, Delaware (4 m tall dam), USA. Riparian groundwater levels were recorded every 30 min for multiple wells and transects. Groundwater mixing regime was characterized using 30‐min specific conductance data and selected chemical tracers measured monthly for about 2 years. Three distinct regimes were identified for riparian groundwaters—wet, dry, and storm. Riparian groundwater gradients above the dam were low but were typically from the riparian zone to the stream. These flow directions were reversed (stream to riparian) during dry periods due to riparian evapotranspiration losses and during peak stream flows. Longitudinal (parallel to the stream) riparian flow gradients and directions also varied across the hydrologic regimes. Groundwater mixing varied spatially and temporally between storms and seasons. Near‐stream groundwater was poorly flushed or mixed during storms whereas that in the adjacent swales revealed greater mixing. This differential groundwater behavior was attributed to milldam legacies that include: berm and swale topography that influenced the routing of surface waters, varying riparian legacy sediment depths and hydraulic conductivities, evapotranspiration losses from riparian vegetation, and runoff input from adjoining roads.

Sub-centennially resolved behavior of an accreting sandy shoreline over the past ∼ 1000 years

ABSTRACT Coastal ridge plains represent a valuable record of past shoreline deposition. However, there remain questions regarding shoreline behavior on intermediate timescales (sub-centennial), the impact of storms, and process of ridge genesis. We address these questions through high-resolution reconstruction of the sandy-beach progradation at Boydtown Beach in Twofold Bay, southeastern Australia, over the past 1000 years using ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating. GPR profiles are dominated by seaward-dipping reflections that result from beach and dune progradation. Prominent reflections with heavy-mineral concentrations are also preserved resulting from storm erosion. OSL ages reveal alternative phases of steady and episodic accretion, rather than a constant progradation. We hypothesize that steady phases may result from moderate storm events where each successive storm only partially erodes the recovery of the previous event. This results in incremental seaward accretion of the active beach. Phases of episodic accretion could be the result of larger storm events or storm clusters when large post-storm recovery rapidly shifts the active shoreline seaward. The two modes of shoreline progradation (steady and episodic) appear broadly associated with a change in ridge-and-swale morphology whereby subdued ridge swale topography is associated with steady or incremental progradation and higher, better-defined ridges with episodic accretion. These results suggest that a single coastal ridge plain experiences variable intermediate-scale shoreline behavior in response to storm events which then lead to multiple modes of ridge genesis.

Mid-late Holocene anthropogenic and natural variations in geochemistry and sedimentation in the Yazoo Basin: Clark Lake, Sharkey County, Mississippi

ABSTRACTA 4000-yr history at Clark Lake reveals natural variations in sediment geochemistry and nutrient levels and anthropogenic influence on twentieth century sedimentation rates. Sediment texture and ridge and swale topography indicate that the channel system creating Clark Lake is now occupied by the Yazoo River. Between 1.24 and 0.60 m (2522–865 cal yr BP), total carbon percentages and the C/N ratio average 42% and 17, respectively. The base of the 1650-yr interval and onset of high organic activity in the lake coincides with the abandonment of the Yazoo Meander Belt. The top of the interval is marked by a drop in C/N ratio to 11 and a geochemical transition zone. No change in the rate of sediment accumulation or clay mineralogy was observed despite increased cumulative percentages of Al, Fe, K, Mg, Mn, Ca, and Na. The decrease in organic activity at 865 cal yr BP is attributed to unfavorable growth conditions related to the entrenchment of the Yazoo River and changes in the hydroperiod of the area. Significant floodplain alteration was completed in 1978, but the only major effect was increased sedimentation in the late twentieth century due to the completion of the Whittington Auxiliary Channel.

Exploring the role of trees in the evolution of meander bends: The Tagliamento River, Italy

AbstractTo date, the role of riparian trees in the formation of scroll bars, ridges, and swales during the evolution of meandering channels has been inferred largely from field observations with support from air photographs. In situ field observations are usually limited to relatively short periods of time, whereas the evolution of these morphological features may take decades. By combining field observations of inner bank morphology and overlying riparian woodland structure with a detailed historical analysis of airborne LiDAR data, panchromatic, and color images, we reconstruct the spatial and temporal evolution of the morphology and vegetation across four meander bends of the Tagliamento River, Italy. Specifically we reveal (i) the appearance of deposited trees and elongated vegetated patches on the inner bank of meander bends following flood events; (ii) temporal progression from deposited trees, through small to larger elongated vegetated patches (pioneer islands), to their coalescence into long, linear vegetated features that eventually become absorbed into the continuous vegetation cover of the riparian forest; and (iii) a spatial correspondence between the resulting scrolls and ridge and swale topography, and tree cover development and persistence. We provide a conceptual model of the mechanisms by which vegetation can contribute to the formation of sequence of ridges and swales on the convex bank of meander bends. We discuss how these insights into the biomorphological processes that control meander bends advance can inform modeling activities that aim to describe the lateral and vertical accretion of the floodplain during the evolution of vegetated river meanders.

Beach ridge patterns in West Aceh, Indonesia, and their response to large earthquakes along the northern Sunda trench

The morphology of beach ridge plains along active margins can be used to reconstruct coastal subsidence during large megathrust earthquakes. Here we use satellite imagery and automatic level surveys to reconstruct the build-up of a new beach ridge along a 10 km long stretch of the western Acehnese coast after the complete destruction of the beach during the great Sumatra -Andaman earthquake and successive tsunami in December 2004. The western Acehnese coast is characterized by ridge and swale topography reflecting the long-term progradation of the coastline. Radiocarbon dates obtained from marshy deposits in between ridges indicate an average progradation rate of 1.3–1.8 m per year over the last 1000 years. As a result of coseismic subsidence of 0.5–1 m and tsunami inundation in 2004, the most seaward beach ridge was destroyed and the coastline receded on average 110 m landward representing 65–85 years of average progradation. However, by 2006 a new 22 m wide ridge had formed. In the following years the coast prograded by an additional 30 m, but has not yet recovered to its pre-December 2004 position. In addition to the spatial data, topographic surveys conducted in 2009, 2012 and 2013 indicate that the crest of the newly formed beach ridge is 0.8–1.3 m higher than the crests of older beach ridges further inland. The source material for the new ridge is most likely sand transported seaward by the back flow of the 2004 tsunami and stored on the upper shoreface. In the months and years after the tsunami, this sediment is reworked by regular coastal processes and transported back to shore, leading to the reconstruction of a higher beach ridge in equilibrium with the vertical displacement of the coast and the resulting higher relative sea level. The preservation potential of the newly formed ridge depends on sediment availability within the coastal system to balance coastal profile adjustments due to rapid postseismic uplift. In Aceh, the preservation of seismically modified beach ridge morphology seems likely and another prominent ridge can be found in 640 m distance to the shoreline. It most likely formed in the aftermath of a previous megathrust earthquake and tsunami about 600 years ago matching sediment and coral records for this region.

Geoarchaeology of the Falls of the Ohio River: Quaternary landforms at the Falls

At the narrowest point in the lower part of its valley, the Ohio channel falls through a length of rapids between Louisville, KY and New Albany IN, known as the Falls of the Ohio. Below and above the Falls area, a ridge and swale topography characterizes the late Pleistocene to Late Holocene alluvial landforms, recording the shifting pattern of the channel and dictating the location of age based cultural materials. This geoarchaeological study looks at four distinct landforms at the Falls of the Ohio, on both the northern and southern banks, in order to ascertain the development of Holocene alluvial landforms in this area. Investigations undertaken recently within the Falls of the Ohio region, at the Newcomb (12CL2), M. Kraft (12CL935), and Shippingport (15JF702) sites, as well as information gained from prior excavations at the Old Clarkesville Site (12CL1), have provided the necessary insight into the development of the individual bars that these four sites where found lying upon. Texture, elevation, and temporal comparisons of these four alluvial bars have allowed for a generalized sequence of bar development to be formulated at the Falls. Forming as a mid-channel bar, the Old Clarkesville is the oldest landform, dating to the early Holocene. This was followed by the middle Holocene M. Kraft Bar, forming a side channel bar to the north and the similarly aged Shippingport Bar on the southern bank of the channel. The Newcomb Bar formed last in the late Holocene, filling in the swale or channel between the Old Clarksville and M. Kraft bars and allowing the formation of a single floodplain surface. This study shows a general conformity to other geoarchaeological models and patterns in the region and, as such, it appears that the Falls of the Ohio has had little effect on the overall natural and cultural deposition in the region. The exploration of these landscapes helps to expand on the regional synthesis of the understanding of the prehistoric cultural period deposits they hold.

Dynamics of a Community of Dominant Woody Riparian Species Along a Coastal River of Texas

Abstract Little is known about the structure of riparian communities within the central Gulf coast of Texas. We present a phytosociological description of the riparian corridor within two contiguous sites along the Mission River in Refugio County, Texas, that differ in respect to tidal influence along the river, and compare this woody riparian community to others in Texas. In summer 2009 and 2010, we established 33 transects perpendicular to the river after reviewing digital data from ArcMap 9.3.1 on elevation, edaphic characteristics, uses of land, and cover of land and wetland. We included sites above and below the tidal zone to ensure that the diversity of the community was documented. Transects traversed the edge of the river, ridge and swale topography, and up to the transitional zone between the riparian zone and upland habitat. We sampled a total of 137 plots and documented all species of trees, shrubs, and vines with diameter at breast height ≥3 cm. The important species were Ulmus crassifolia, Ce...

Co‐development of alluvial fan surfaces and arid botanical communities, Stonewall Flat, Nevada, USA

ABSTRACTArid alluvial fan and fluvial dry wash surfaces in Stonewall Flat, Nevada, USA, are characterized using surface geomorphic surveys, soil pits, botanical line surveys, and varnish microlamination dating techniques. Active and abandoned washes, and active fan surfaces are dominated by primary geomorphic processes of high‐energy sedimentation from flash floods. These surfaces are characterized by bar and swale topography, a lack of stone pavements, soil horizons, and rock varnish. Younger terraces and slightly older intermediate fan surfaces are in transition from primary sedimentation processes to lower energy secondary surface‐modifying processes of sheet wash and eolian transport and deposition. These surfaces are characterized by faint to no bar and swale topography, incipient to moderately well‐developed pavements and soil horizons, and abundant coppices. Old and stable fan surfaces are dominated by lower energy secondary processes and manifest well‐developed pavements, soils, and sparse coppices around widely distributed shrubs. Varnish microlamination dating yields ages of 13·15 ka for intermediate fan surfaces and 25·55 to 86·75 ka for stable fan surfaces. Plant communities co‐developing with these surfaces affect and are affected by both primary and secondary geomorphic fan processes. Relatively active surfaces contain few woody species. Co‐dominance of shrubs and annuals with abundant annuals between the shrubs is characteristic of surfaces transitional from primary processes to secondary processes. Stable surfaces dominated by secondary processes are characterized by woody perennials, with long‐lived woody species inhabiting the oldest surfaces. Feedback mechanisms between early botanical communities and eolian deposition affect coppice and pavement development. In turn, these surface features control both the composition and distribution of botanical communities on older, more stable surfaces. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

Mapping coastal morphodynamics with geospatial techniques, Cape Henry, Virginia, USA

The advent and proliferation of digital terrain technologies have spawned concomitant advances in coastal geomorphology. Airborne topographic Light Detection and Ranging (LiDAR) has stimulated a renaissance in coastal mapping, and field-based mapping techniques have benefitted from improvements in real-time kinematic (RTK) Global Positioning System (GPS). Varied methodologies for mapping suggest a need to match geospatial products to geomorphic forms and processes, a task that should consider product and process ontologies from each perspective. Towards such synthesis, coastal morphodynamics on a cuspate foreland are reconstructed using spatial analysis. Sequential beach ridge and swale topography are mapped using photogrammetric spot heights and airborne LiDAR data and integrated with digital bathymetry and large-scale vector shoreline data. Isobaths from bathymetric charts were digitized to determine slope and toe depth of the modern shoreface and a reconstructed three-dimensional antecedent shoreface. Triangulated irregular networks were created for the subaerial cape and subaqueous shoreface models of the cape beach ridges and sets for volumetric analyses. Results provide estimates of relative age and progradation rate and corroborate other paleogeologic sea-level rise data from the region. Swale height elevations and other measurements quantifiable in these data provide several parameters suitable for studying coastal geomorphic evolution. Mapped paleoshorelines and volumes suggest the Virginia Beach coastal compartment is related to embryonic spit development from a late Holocene shoreline located some 5 km east of the current beach.

Vegetation of Hooper Branch Nature Preserve, Iroquois County, Illinois

The Hooper Branch Nature Preserve is located in the extensive dune and swale topography of the sand deposits of northeastern Illinois. The plant communities present at the time of this survey were similar to those of pre-settlement times. The vegetation associated with the dunes had not been disturbed except for past grazing; the swales, in contrast, were drained and farmed before the area was dedicated in 1986. Dry and dry-mesic sand savanna was associated with the dunes where Quercus velutina (Black Oak) accounted for 75% to 97% of the importance value in the savanna. Carex pensylvanica (Pennsylvania Sedge) was the dominant ground cover species usually followed by Schizachyrium scoparium (Little Bluestem). In parts of the preserve that had been most recently burned, woody species were more common in the ground layer, with the most important being Q. velutina seedlings, Rhus copallina (Dwarf Sumac), and Rubus allegheniensis (Common Blackberry). A 3-ha flatwoods in the Preserve was dominated by Q. palustris (Pin Oak), which accounted for nearly 95% of the overstory.

York River Geology

Abstract The four separate sites of the Chesapeake Bay National Estuarine Research Reserve in Virginia are within the Coastal Plain province of the mid-Atlantic. The surficial geology at each site is of Quaternary age, primarily Holocene wetlands. The site at Taskinas Creek is set into Tertiary age strata. The underlying strata increase in age up-stream. Regionally, the Late Tertiary and Quaternary geology is a function of the series of major transgressions and regressions, during which the successively more recent high stands of sea level generally have not reached the level of the preceding high stand. As a consequence, stratigraphically higher, younger deposits occur topographically below exposures of the older strata. The two down-stream reserve sites are within the area of the Eocene age Chesapeake Bay Impact Crater. Also at these two sites, the tidal marshes are superimposed on a ridge and swale topography. The local rate of sea-level rise, approximately 4 mm/yr, is the underlying process driving ch...

A protocol for classifying ecologically relevant marine zones, a statistical approach

Mapping ecologically relevant zones in the marine environment has become increasingly important. Biological data are however often scarce and alternatives are being sought in optimal classifications of abiotic variables. The concept of ‘marine landscapes’ is based on a hierarchical classification of geological, hydrographic and other physical data. This approach is however subject to many assumptions and subjective decisions. An objective protocol for zonation is being proposed here where abiotic variables are subjected to a statistical approach, using principal components analysis (PCA) and a cluster analysis. The optimal number of clusters (or zones) is being defined using the Calinski–Harabasz criterion. The methodology has been applied on datasets of the Belgian part of the North Sea (BPNS), a shallow sandy shelf environment with a sandbank–swale topography. The BPNS was classified into 8 zones that represent well the natural variability of the seafloor. The internal cluster consistency was validated with a split-run procedure, with more than 99% correspondence between the validation and the original dataset. The ecological relevance of 6 out of the 8 zones was demonstrated, using indicator species analysis. The proposed protocol, as exemplified for the BPNS, can easily be applied to other areas and provides a strong knowledge basis for environmental protection and management of the marine environment. A SWOT-analysis, showing the strengths, weaknesses, opportunities and threats of the protocol was performed.

The Impact of Rapid Sea Level Changes on Recent Azerbaijan Beach Ridges

A ground-penetrating radar survey of a newly formed series of beach ridges along the southern Azerbaijan coast, Caspian Sea, illustrates rapid coastal response to the most recent sea level fall of 0.8 m in the Caspian Sea between 1995 and 1999. Effects of seasonal sea level fluctuations as well as individual storm occurrences can be linked to depositional beds on the ground-penetrating radar profiles. The beach ridge system is swash built and formed primarily under fair weather conditions. Ridge and swale topography can be related to seasonal sea level change. The rapid sea level change in the Caspian Sea combined with surface and subsurface data on coastal beach ridges provides a unique opportunity to observe and reconstruct coastal evolution at a resolution not possible along other oceanic coasts.

Luminescence ages for alluvial-fan deposits in Southern Death Valley: Implications for climate-driven sedimentation along a tectonically active mountain front

Controversy exists over whether alluvial-fan sedimentation along tectonically active mountain fronts is driven by climatic changes or tectonics. Knowing the age of sedimentation is the key to understanding the relationship between sedimentation and its cause. Alluvial-fan deposits in Death Valley and throughout the arid southwestern United States have long been the subjects of study, but their ages have generally eluded researchers until recently. Most mapping efforts have recognized at least four major relative-age groupings (Q1 (oldest), Q2, Q3, and Q4 (youngest)), using observed changes in surface soils and morphology, relation to the drainage net, and development of desert pavement. Obtaining numerical age determinations for these morphologic stages has proven challenging. We report the first optically stimulated luminescence (OSL) ages for three of these four stages deposited within alluvial-fans along the tectonically active Black Mountains of Death Valley. Deposits showing distinct, remnant bar and swale topography (Q3b) have OSL ages from 7 to 4 ka., whereas those with moderate to poorly developed desert pavement and located farther above the active channel (Q3a) have OSL ages from 17 to 11 ka. Geomorphically older deposits with well-developed desert pavement (Q2d) have OSL ages ∼25 ka. Using this OSL-based chronology, we note that alluvial-fan deposition along this tectonically active mountain front corresponds to both wet-to-dry and dry-to-wet climate changes recorded globally and regionally. These findings underscore the influence of climate change on alluvial fan deposition in arid and semi-arid regions.

Natural Maintenance of Sand Ridges and Linear Shoals on the U.S. Gulf and Atlantic Continental Shelves and the Potential Impacts of Dredging

Ridge and swale topography is exceptionally well developed on the continental shelves of the Mid-Atlantic Bight and the northeastern Gulf of Mexico. In both cases, these linear ridges are oriented parallel to the predominant wave approach direction, suggesting a common process for both their origin and maintenance. Most researchers have concluded that ridges were derived from shorefaces of barrier islands as they retreated across the shelf in response to rising sea level and tides or storm-driven currents maintain them.The widely cited ridge formation theory of Huthnance (1982) requires a sufficient sand source, currents to move the sand, and an irregularity on the sea floor around which the ridges are initiated. McBride and Moslow (1991) postulated that one of the initial irregularities is a segment of an ebb-tidal delta abandoned by inlet migration. However, the search for other precursors continues. These theories of origin provide little information on how these features maintain their form once they are detached from the shore yet remain in a zone of active wave attack (i.e. in depths less than 20 m). Snedden et al. (1999) indicate that shoals in water depths less than approximately 20 m are migrating shoreward through the influence of Stokes Drift under fair-weather waves based on the work of McHone (1973). However, this model does not explain the maintenance of the form of linear shoal and ridge features.To assess the impacts of dredging on these features it is essential that a better understanding of the processes that maintain these features be developed. A new conceptual model presented in this paper demonstrates how waves shoaling and refracting up either side of a ridge off the coast of Maryland and Delaware result in convergence of sand transport over the crest of the ridge, thus maintaining the ridge even after it is detached from shoreface processes.The possibility that these ridges might deflate or disappear as a consequence of dredging, resulting in dramatic changes in wave conditions along the shore, is a major concern. The application of a spectral or phase-resolving wave model combined with two-dimensional hydrodynamic and sand transport models as applied in this paper represents a method to evaluate this potential impact of dredging.

Modelling spatial variability using soil profiles in the Riverland of South Australia

The soils of the Riverland vary significantly throughout the region. Soils from a vineyard near Loxton, whilst having only half the topsoil depth of two near Waikerie, are comparable in water-holding capacity because the soils are less sandy. Topographic difference can cause an additional variation between areas in the same neighbourhood. Highland soils typically develop a dune swale topography that gives rise to anisotropic semivariograms. First slope soils on the other hand give isotropic semivariograms. Semivariograms for topsoil depth and readily available water (RAW) for the three vineyards are used in the calculation of the efficiency of the sampling grids employed to gain an understanding of the soils. Kriging variance values have been calculated for 1, 2, 3 and 4 hectare square irrigation valve areas (IVAs) using the industry standard grid 75×75 m as well as grids 50×50 m and 100×100 m. Choice of the appropriate sampling grid to achieve a specified accuracy can thus be made.

Fluvial geomorphic features of the Lower Mississippi alluvial valley

The Lower Mississippi Valley (LMV) has been one of the most intensively studied alluvial valleys in the world in terms of it's geological and geomorphic framework and history. A brief outline of the history of the major geological and geomorphological investigations of the LMV is provided. The results of these investigations are discussed in terms of the fluvial geomorphic framework of the valley and the apparent significant changes in the regime of the Mississippi River during the Late Wisconsinan and Holocene stages. The LMV occupies the broad deep synclinal trough of the Mississippi Embayment which extends from Cairo, Illinois, to the Gulf of Mexico in a slightly sinuous north-south trend. The embayment is filled with a north to south thickening wedge of non-marine and marine sediment ranging in age from Jurassic to Holocene. The major landscapes of the LMV may be considered in four regions: (1) a narrow active meander belt in a broad valley of Late Pleistocene valley train in the northern third; (2) a wide mosaic of interwoven Holocene meander belts in the middle third; (3) a relatively narrow valley of the Atchafalaya Basin bounded on each side by narrow meander belts in the upper part of the lower third; and (4) the broad distributary wedge of the deltaic plain in the southernmost region of the valley. The valley trains vary in age and landform with the oldest occurring as slightly dissected low ridges and the youngest as broad flats separated by shallow interwoven former braided channels. Meander belts formed throughout the Holocene are comprised of low natural levee ridges flanking abandoned courses and bordered by crescent-shaped oxbow lakes and ridge and swale topography. In the middle third of the valley, meander belts are separated by expansive backswamps of very little relief. The deltaic plain is also exceptionally flat, interrupted by the low natural levee ridges of the abandoned deltaic distributaries. The floodplain of the LMV is a complex mosaic of fluvial features and landscapes within the four landscape regions. Included in this mosaic are abandoned channels and courses, lateral accretion topography of ridges and swales, natural levees, crevasses and crevasse channels, distributary channels, backswamps and rimswamps, alluvial fans and aprons, valley trains (braided stream terraces), lakes and lacustrine deltas, terraces, and the alluvial valley bluff. Changes in the hydraulic regime of the Lower Mississippi River (LMR) since the Late Pleistocene have played a major role in the development of the landscape of the valley. The most important regime change was the diminishment of the influence of Wisconsinan glaciation in the upper Midwest and the resultant evolution of the Mississippi River from a broad braided outwash channel to a more narrow but sinuous meandering channel at the end of the Pleistocene. During the Holocene, the Mississippi River undoubtedly responded to major climatic changes, rising sea level, tributary stream influence, and possibly tectonism, diapirism, and subsidence through the growth and evolution, and abandonment of it's meander belts and deltas.

Woody vegetation and channel morphogenesis in low-gradient, gravel-bed streams in the Ozark Plateaus, Missouri and Arkansas

Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM 2, Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes. Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities. Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.

Geoecology of the Norra Storfjallet Area, Northern Sweden

The geoecology of the Norra Storfjillet region is reconstructed from the record of deglaciation beginning about 8000 yr BP with the first break-up of the Fenno-Scandian ice sheet (Stage 1). By Stage 2 valleys below 600 m a.s.1. (the Tirna Valley was an exception) had become free of ice. Stage 3 involved a retreat of small valley glaciers to about 700-800 m, major nunataks appeared on the high plateau, and outlet glaciers were considerably reduced in size. Stage 4 was characterized by a warmer and wetter climate (Mid-Atlantic chronozone, 6000-5000 yr BP) when timberline rose to about 900 m. This led to the development of Spodosols (podzols) under stands of dwarf pine in swale topography. This paleo-podzolic signature was not erased during alpine conditions that typified the Sub-Boreal (Stage 5) and Sub-Atlantic (Stage 6) chronozones that followed. These were periods of lower temperatures and with reduced precipitation initially during Stage 5. Precipitation increased during the Sub-Atlantic and, together with temperature, remained similar to the present. Human settlement began about 6000 yr BP and had a recognizable impact on the landscape. This activity accelerated during the Sub-Boreal and Sub-Atlantic, with more intensive use of the mountains for reindeer procurement. The reconstructions are based upon glacial, geomorphological, pedological, and archaeological evidence.

Defining archaeological features with electromagnetic surveys at the Cahokia Mounds State Historic Site

Electromagnetic (EM) surveys have been used at the Cahokia Mounds State Historic Site in southwestern Illinois to locate and define a number of buried archaeological features. Two instruments, Geonics EM31-D and EM38 conductivity meters, were employed to locate portions of a wooden stockade known as the Central Palisade; delineate a number of leveled earthen mounds; and explore a broad, flat area in the central portion of the site known as the Grand Plaza. EM surveys, together with limited excavation, provide a cost effective and nondestructive means of exploring a site as large and complex as Cahokia. Archaeological excavations confirmed that EM surveys were able to locate the Central Palisade, and more importantly, that they provided information on anthropogenically modified terrain within the Grand Plaza. The EM survey documented buried ridge and swale topography and borrow pits within this area. This evidence of landscape modification challenges previous conceptions about the extent of earthmoving at this important Mississippian center and suggests a promising area of application for EM surveys in archaeology.