Active Meandering Research Articles

Rates, distributions and mechanisms of change in meander morphology over decadal timescales, River Dane, UK

AbstractThis paper analyses types and rates of change in river meander morphology and the links between mechanisms of change and emergent behaviour of planform morphology. It uses evidence of four dates of aerial photography combined with annual field mapping and ground photography to examine the morphological changes and mechanisms of change in a series of bends on an active meandering river, the River Dane in NW England, over a 25 year period. This unique data set allows insight into the spatial and temporal variability of bank line movement and component processes. Bank lines were mapped photogrametrically from air photos of 1984, 1996, 2001 and 2007 and the digitised courses compared in ArcGIS to produce calculations of erosional and depositional areas and rates. Most bends exhibit morphological change that largely follows the autogenic sequence, identified in qualitative models of meander development, from low sinuosity curves through simple symmetric and asymmetric bends to compound forms with lobe development in the apex region. Rates of erosion and bankline movement increase through this sequence until the compound phase. Relationships of amounts of movement to various curvature measures of bend morphology are complex. Several new loops, distinct from compound bend behaviour, have developed during the study period in formerly straight sections. Mechanisms of morphological change are illustrated for four types of bends: new, rapid growth bend; sharp‐angled bend with mid‐channel bar development; symmetric migrating bend; and simple to compound bend development. The changes take place in phases that are not simply related to discharge but to inherent sequences and feedbacks in development of bars and bend morphology and timescales for these are identified. Overall, emergent behaviour of systematic planform change, moderated by channel confinement and boundary features, is produced from spatially and temporally varied channel processes. Copyright © 2010 John Wiley & Sons, Ltd.

Impact of the alluvial style on the geoarcheology of stream valleys

The extensive archeological record within river valleys provides an environment long favored for study by the archeologic community. This record of human occupation varies with the alluvial style of the river, resulting in each river valley having a unique archeological record controlled by three factors; location of occupation sites, preservation of those sites, and recognition of the sites. If problems of site preservation and recognition in a river valley can be recognized and addressed, a better insight into settlement patterns and complex human–environment interactions can result. This study represents a synthesis of extensive published and secondary data sets pertaining to archaeology and alluvial geomorphology of the lower Missouri, Red, and Mississippi Rivers, to examine the relationship of prehistoric site distribution with fluvial geomorphology. The archeologic data are from published literature and government agencies. The geomorphic data are from a variety of published and unpublished investigations, including maps of alluvial geomorphology, digital elevation models, aerial photographs, and topographic maps. The data were input into spreadsheets and a GIS for qualitative and quantitative analysis. Prehistoric human occupation of the narrow Missouri valley floor is limited, with only 0.061 recorded sites/km 2. Additional sites are likely buried by thick vertical accretion sediment on the limited prehistoric portions of the floodplain and within alluvial/colluvial fans along the valley margin. Non-random preservation patterns, such as the absence of sites from the youngest cultural period, are attributed to shifts in settlement patterns. In comparison to the lower Missouri River valley, prehistoric occupation of the lower Red and Mississippi rivers was much more extensive and the density of occupation was 0.274 sites/km 2 and 0.277 sites/km 2 respectively. Significant differences do exist in the age of sites and their relationship to different floodplain depositional environments. Many sites along the active meander belt of the Red River were likely lost due to high rates of channel migration. Nevertheless, intense occupation of Paleo-Indian through Caddoan groups assures that many significant sites have been preserved. The wide valley and high rates of sedimentation suggest that many sites are likely buried, particularly along natural levees flanking an older meander belt. Recorded sites are also present along streams that incise the terrace, along the terrace margin, and along small channels that cross the backswamp. The spatial and temporal site distribution along the lower Mississippi valley is distinct in comparison with the Red River because the Mississippi valley is wider, the terraces are younger and less dissected, and the floodplain has more relief and is not broadly inundated by floodwater with relatively low sediment concentrations. Most of the significant Archaic through Mississippian period floodplain sites are located along the surfaces of abandoned Mississippi and distributary meander belts which aggraded above the adjacent backswamp and have rarely been influenced by flood sedimentation since abandonment. Abundant Paleo-Indian through Mississippian period sites are also widely distributed across undissected terraces. Only a few significant Mississippian sites are present along the active meander belt and in the backswamp where flooding was common. In comparison with the Red River, the archaeological record of the floodplain is less likely to be influenced by burial or erosion due to the Mississippi River having lower rates of overbank sedimentation and channel migration, respectively. The variable alluvial styles of the lower Missouri River, lower Red River, and the lower Mississippi River valleys presented varying opportunities and constraints to occupation and sustained settlement, and resulted in widely varying archaeological patterns that are partly dependent upon the alluvial geomorphology.

Temporal variations in fluvial processes on an active meandering river over a 20-year period

An active meandering river in NW England has been monitored annually for more than 20 years. Data on the occurrence of erosion, deposition, calibre of sediment and morphological changes have been recorded for a sequence of > 50 bends in a 10 km length reach. The data on process activity have been analysed for patterns and variation over the period 1981–2002. Occurrence of floods is hypothesised to be the major control on activity and this paper aims to test that relationship and to identify whether any major phases of activity emerge at this timescale. Data on magnitude of annual peak discharges and on the frequency of peaks over a threshold show a phase of relatively low peaks in the early 1980s, a phase of variable discharge and some low flows from 1987–1997, and a phase of high peaks 1998–2002. Frequency of high flows has increased through the period. Analysis of the relationship of processes to discharge characteristics indicates that erosion takes place on a high proportion of bends every year and that coarse material (cobbles) is deposited on bars in most years. Process thresholds for bank erosion and coarse sediment transport and deposition are below bankfull and occur on average several times a year. A trend of severity of erosion activity to winter peak discharge during the 1980's did not continue into the period of high flows of the late 1990's but deposition was similar in both periods. It is hypothesised that vegetation may be influencing response. Vegetation growth on the bars increased from the late 1980's and is likely to have decreased erosion but enhanced deposition. Number of frost days has decreased over the period but other factors could also be influencing vegetation. The occurrence of the process activity is compared to overall changes in morphology on the river measured from photogrammetric mapping of the course in 1984, 1996 and 2001. These show differences in average channel width but the interannual variability of processes raises questions of the numbers and sequences of flood events needed for different phases and hydromorphological responses of the channel to be identifiable.

Shifts in sauger spawning locations after 40 years of reservoir ageing: influence of a novel delta ecosystem in the Missouri River, USA

AbstractTo determine how reservoir ageing affects sauger Sander canadense spawning locations in Lewis and Clark Reservoir, we quantified riverine habitat and determined sauger spawning locations during recent years (2003–2004) and compared these results to historic (1950–1966) habitat conditions and spawning locations. The riverine habitat above the reservoir, which was historically homogeneous, has developed into two distinct areas: the upstream remnant river reach, characterized by a distinct main channel, and cold and clear water from the hypolimnetic discharge from the upstream reservoir, and the downstream delta section, which has abundant side channels and backwater areas and is warmer and more turbid water than the remnant reach. Sauger spawning locations have shifted markedly in the last 50 years. Sauger historically spawned in the upper reaches of this system, near Fort Randall Dam. However, in 2003, we found that sauger spawned in the lower delta reach and avoided the remnant reach. This shift likely occurred concomitantly with the development of the novel delta habitat. The delta likely functions more similarly to the historic remnant reach of this system (warmer temperature, turbidity, active meandering, complex habitats, etc.), indicating that sauger prefer to spawn in areas with historic riverine function. Thus, future management activities intended to enhance sauger populations should focus on the restoration of riverine function, such as that provided by emerging reservoir deltas, which may mimic pre‐impoundment conditions on the Missouri River. Copyright © 2008 John Wiley & Sons, Ltd.

1002 大規模乱流中における熱拡散場の特性(2)(OS10-1 大気海洋の流体現象と環境流体,OS10 大気海洋の流体現象と環境流体,オーガナイズドセッション)

Thermal diffusion from a point source was experimentally investigated in uniform shear flows having velocity gradients of 2.6, 6.1 s^<-1> and in quasi-isotropic turbulence fields. Short-time dispersion accompanied by the plume's meandering motion was realized in the large-scale turbulence fields of R_λ=310, 410 and 490, with the spectra of temperature fluctuation had clear inertial ranges. For sheared small-scale turbulence of R_λ=50 and 100, the turbulent Prandtl number Pr_t, in almost unchanged in the streamwise direction. The Pr_t was larger than ordinary Pr_t≒1.0, and approached approximately Pr_t≒1.44〜1.88, in the region where the plume's active meandering motion was clearly observed in the sheared large-scale turbulence.

Fluvial Geomorphology Suggests Recent Tectonic Activity Near the Northern Barrier Range, western New South Wales

Large faults expressed at the ground surface often have clear effects on drainage networks. Smaller scale tectonic activity, or that which occurs beneath sedimentary cover, can have more subtle effects that arise from changes to down-valley slope. These include an increase meander sinuosity or the development of anabranching. Fowlers Creek arises in the northern Barrier Range (NSW) and its terminal floodout flows across the flat surface of the Bancannia Trough. In the floodout's proximal and medial reaches the channel is sinuous but generally without active meander development. Active meandering occurs in only in two reaches, and is associated with higher slopes and repeated avulsions, the most recent being during the 20thC. It is suggested that small-scale uplift may have occurred, increasing the down-valley slope and therefore promoting meander development and avulsion. More closely spaced topographic data is necessary to support this hypothesis, however Fowlers Creek is one of nine creeks flowing east from the Barrier Range which show planform irregularities several kilometres from the rangefront. This supports the possibility of a concealed tectonic feature, and suggests that it is large-scale and linear. Major structures (e.g. mines, waste dumps) may be built in sparsely monitored remote locations where earthquake hazard estimation can be difficult. In such places, maps of fluvial geomorphology can be useful indicators of neotectonic activity.

Spatial variability, mechanisms and propagation of change in an active meandering river

This paper addresses questions of the spatial pattern of instability and the mechanisms of change in an active meandering river, particularly whether and how change is propagated. More than 20 years of monitoring of a sequence of nearly 100 bends on one dynamic meandering river, combined with historical data and previous analyses of processes of change, provide a unique insight into the link between annual changes produced by erosion and deposition and the longer-term changes in planform. The study reach of the River Dane in NW England exhibits stable and unstable sections adjacent to one another. Rates of movement range up to 3 m a − 1 , with maxima occurring in high curvature, free bends. Stable reaches are due to factors of gradient, curvature and bank resistance. Analysis of the large amount of data on occurrence of erosion and deposition in each bend each year reveals no definite association of changes in one bend with another. The detailed evidence of the morphological features in the bends shows that changes do not take place by bars moving progressively through reaches. Case studies of bends upstream of constrained, stable reaches indicate an oscillation of widening and narrowing of the channel, over a period of a few years, producing a net rotation of the bend. These areas are zones of stalling of sediment and change takes place by absorption and lateral movement. Overall, changes tend to be localised and fit the bend theory of meanders, but with low sensitivity reaches pinning the planform for longer periods in certain locations.

Morphology and architecture of the present canyon and channel system of the Zaire deep-sea fan

The Zaire deep-sea fan is one of the largest fans in the world still affected by turbidite sedimentation along a unique active meandering turbidite channel. This active channel is fed by turbidity currents, which are generated at the Zaire River mouth, and flow via a deeply entrenched canyon across the shelf and the continental slope. Based on a detailed study of the morphology and architecture of the present Zaire Canyon/Channel, several main zones can be defined (the canyon, the upper-fan valley, the upper and the lower channel-levee system leading into distal lobes). They are characterised by different behaviours in terms of erosion, transport and sedimentation within the canyon/channel. An important characteristic of the Zaire Canyon/Channel is the deep incision of the thalweg, well below the regional sea floor along its whole path. The origin of this entrenchment may be linked to the incision of the Zaire Canyon back across the shelf during the last sea-level rise. This incision of the canyon has allowed the continuity of turbidite activity during the Holocene, in maintaining the connection between the canyon head and the river mouth (in contrast to most of other large deep-sea fans, which are generally inactive during highstands). The entrenchment of the Zaire Canyon/Channel limits the overflow of turbidity currents and the turbidite sedimentation over levees, and prevents avulsion along the upper part of the Zaire Channel. Most of the sediment transported in turbidity currents are probably led down to the lower channel-levee system and the distal lobes.

Site preservation along an active meandering and avulsing river: The Red River, Arkansas

The low-gradient Red River is a rapidly migrating, sinuous stream with easily erodible banks. Avulsion is common at many scales, from individual meander bends that are cut off to major sections of the river that form multiple, complex meander belts. The present meander belt can be subdivided into mappable landforms—termed phases—that are associated with river courses of different ages and thus associated with archeological sites of different ages. Within the study area two phases are present. The younger Modern meander belt phase has formed within the past 0.2–0.3 ky, precluding preservation of prehistoric archaeological sites. Any protohistoric artifacts that may have been preserved in this meander belt phase would be deeply buried because as much as 2 m of the vertical accretion sediment has accumulated between artificial levees in <0.1 ky and 1–2 m of sediment has accumulated beyond the artificial levees in <0.2 ky. Archeological site preservation in this highly mobile fluvial end member can be used as a predictor for other, similar streams. A large prehistoric site is preserved on an older (0.5–1 kya) Late Prehistoric meander belt phase associated with an abandoned river course. In the study area a Fourche Maline 7 period (A.D. 800–900) through Caddo IV period (ca. A.D. 1500–1700) archeological site (3MI3/30) is preserved on this slightly higher altitude portion of the flood plain. At locations proximal to the river, the site may be buried by overbank sediment 0.4 m thick, but at more distant locations the site is at the surface or only buried by thin overbank sediment because of low sedimentation rates (0.04 cm yr−1) over the span of a millennium. Sites, such as 3MI3/30, that are occupied contemporaneous with overbank sedimentation may be stratified; however, localized erosion and removal of some archeological material may occur where channelized flow crosses the natural levee. © 1998 John Wiley & Sons, Inc.

Holocene flood-plain soil formation in the southern lower Mississippi Valley: Implications for interpreting alluvial paleosols

Holocene Mississippi River flood-plain soils in central Louisiana provide important insights on alluvial pedogenesis in an aggradational setting, insights that are useful for interpreting alluvial paleosols. The flood-plain soils are younger than ca. 5400 yr B.P. and are grouped into two categories: (1) meander belt and (2) backswamp soils. Meander belt soils are Entisols, Inceptisols, and Alfisols developed on point-bar ridges and natural levee crests; they consist of 3–5-m-thick, brown, silty and sandy, moderately well drained profiles with multiple parent materials, common yellow-brown and gray mottles, and a few iron and calcite nodules. Backswamp soils are Vertisols formed in flood-plain depressions away from active and abandoned Mississippi River channels and consist of up to 10-m-thick, gray, clayey, poorly drained profiles that have abundant yellow-brown mottles, iron and calcite nodules, gypsum accumulations, and slickensides. The thickness of the soils and the presence of multiple parent materials show that as the flood-plain aggrades, concurrent or alternating episodes of sedimentation and pedogenesis produce cumulative profiles. Profile thickness is more closely related to sediment accumulation rates than to the depth of chemical weathering, and the cumulative nature of the soils explains the difficulty of recognizing discrete paleosol profiles in many ancient alluvial deposits. Comparisons of soils formed on the three youngest Holocene Mississippi River meander belts (Saucier and Snead, 1989) show that the soils record episodes of meander belt growth, abandonment, and relocation. Meander belt 1 soils form as the active meander belt develops and consist of either vertically stacked Entisols or a single Inceptisol. The soils contain few calcite nodules and form over intervals of a few hundred years. Meander belt 2 soils are Alfisols, lack free carbonates, and form over intervals of a few thousand years following meander-belt abandonment and soil leaching. Meander belt 3 soils are Alfisols or Vertisols that contain common calcite nodules, and the profiles are developed in 1 to 2 m of backswamp mud that overlies meander-belt silt and sand. The mud inhibits profile leaching and leads to poor drainage and calcite precipitation. This change in soil hydrology complicates interpretations of the effects of time on meander-belt soil development. Backswamp soils are strongly influenced by seasonal episodes of soil wetting and drying, as demonstrated by the gray colors and the abundance of mottles, nodules, and slickensides in the soils. The distribution of calcite nodules in the backswamp and meander-belt soils is more closely related to seasonal water-table position than to mean annual precipitation, which suggests that not all calcic paleosols in ancient alluvial deposits represent semiarid or arid climates. Slickensides attest to seasonal rainfall, but the composition of the backswamp soils primarily reflects source-area lithology and depositional influences, rather than the effects of chemical weathering. In general, these soils provide an incomplete record of the humid subtropical climate of Louisiana. Although short-term sediment-accumulation rates decrease away from Mississippi River channels, the backswamp soils are not better developed (i.e., more mature) than the meander-belt soils, because poor drainage and slow but continuous sedimentation inhibits weathering. Collectively, these observations suggest that flood-plain soils that developed in aggradational settings, such as that of the Mississippi River flood plain, are best suited for interpreting depositional and hydrologic histories of ancient alluvial deposits, rather than paleoclimates.

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.

ANABRANCHING RIVERS: THEIR CAUSE, CHARACTER AND CLASSIFICATION

Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges.

Holocene Meander-Belt Evolution in an Active Extensional Basin, Southwestern Montana

ABSTRACT Lateral migration fluvial meander belts preserves wider than normal coarse-sediment bodies, asymmetric meander belts, that have a complex architecture recording the history of channel movement. Detailed investigations of the morphology and deposits of asymmetric meander belts of the Madison River and its South Fork, southwestern Montana (using topographic mapping, ground-penetrating radar profiling, coring, sonar, and radiocarbon dating) have been undertaken in an attempt to show how they are influenced by tectonic tilting and faulting, base-level change, catchment evolution, and climate change. The asymmetrical form of the meander belts resulted from a regional tectonically induced lateral bias to channel position over at least the last 10 ky. Radiocarbon ages from abandoned channel ills increase progressively up-tilt away from the active meander belts. The South Fork meander loops decreased in wavelength and channel width through the Holocene, but no systematic changes in channel size have been recognized in the Madison meander belt. These results indicate that the changes in discharge patterns of the two rivers differed, and this difference may be related to local climate change and drainage evolution in their catchments. Episodes of incision during meander-belt evolution have isolated abandoned meander loops with restricted age ranges on terraces above the modern flood plains. The terraces are extensive on the up-tilt side of both meander belts and on the footwalls of intrabasinal faults, with only small terrace remnants on the down-tilt side of the meander belts The distribution of terraces and large abandoned meander loops in the South Fork shows that channel-belt asymmetry did not develop by progressive lateral migration of the meander-belt axis, as previously suggested, but rather that the flood plain episodically decreased in width, with the down-tilt margin fixed in space. Some of the Madison River terraces, however, do show evidence of down-tilt migration of the river. Preservation of diatomite in downstream parts of both meander belts suggests periodic transgression by an ancestral Lake Hebgen. Changing lake shore position may explain some of the episodes of incision, and is probably associated with large seismic events and surface deformation.

Influences of Relative Sea-Level Rise and Mississippi River Delta Plain Evolution on the Holocene Middle Amite River, Southeastern Louisiana

The Holocene geomorphic history of southeastern Louisiana's middle Amite River is recorded in the stratigraphy of three alloformations, identified in decreasing age as the Watson (WAT), Denham Springs (DS), and Magnolia Bridge (MAG). The WAT meander belt formed by at least 9000 yr B.P., when sea level was lower and the Amite River was tributary to a larger ancestral drainage basin. The DS became an active meander belt by at least 3000 yr B.P., in response to relative sea-level rise and eastward progradation of the Mississippi River delta plain. The MAG developed its meander belt, in part, during the European settlement of the drainage basin, and is now attempting to adjust to modern anthropogenic influences. Geomorphic influences on the middle Amite River floodplain have temporal and spatial components that induce regional- and local-scale effects. Regional extrinsic influences caused meander belt avulsion that produced alloformations. However, local influences produced intrinsic geomorphic thresholds that modified channel morphology within a meander belt but did not induce alloformation development. Base-level influences of the relative sea-level rise and the Mississippi River delta plain were so dominant that the effects of possible climate change were not recognized in the Holocene Amite River system.

Rasgos morfoestructurales de la llanura amázonica del Perú: efecto de la neotéctonica sobre los cambios fluviales y la delimitación de las provincias morfológicas

Morphostructural trends of the amazonian regions of Peru: the effect of neotectonics on fluvial changes and upland wetland boundaries Abstracts The Amazonian regions of Peru are discussed within the context of current knowledge on recent Andean tectonics and their impact on river drainage. The main morphostructural provinces are the foothills or Subandean Zone which are bordered on the east by large depressions in the Marañón Basin of northeastern Peru, the Madre de Dios region of southern Peru and further to the east the Brazilian craton (Iquitos Geanticline). The structural framework of the Subandean Zone consists of the Subandean Thrust and Fold Belt (STFB) West which crops out mostly in the Upper Foothills, and the Subandean Tilted Block Zone (STBZ) East which is concentrated mainly in the Lower Foothills. The main rivers which cross the STFB are antecedent or superimposed, but the secondary drainage channels conform to these structures and are subsequent. The STBZ river basins run parallel to the structural contour. The Marañón basin comprises a northern depression along the Pastaza River, and to the south is the large Ucamara depression with its extended floodplains and structurally controlled lakes. To the east, the Iquitos geanticline represents a positively epeirogenic structure on the western border of the South American craton. The rivers showing active meander belts (Marañón, Ucayali and Tapiche) are drained by white (silty) water from the Andes. Watersheds are drained by stable black water (from organic acids) streams with underfit patterns like the Samiria and Pacaya rivers. The underfit pattern is interpreted as the abandonment of formerly active large streams of white water. The position in the depression of the black water streams suggests successive shiftings of the Marañón and Ucayali rivers respectively northward (50 km) and southeastward (100 km) during the recent time, probably the Holocene. The geodynamical environment of the Ucamara depression suggests a close relation between the recent tectonic activity and the successive shifting of the Marañón and Ucayali rivers. The three large areas of swamp and lakes are presented: the Concordia, Puinahua and the Punga lakes. Their features are discussed in relation to deep faults and fold belts in the basement. Neotectonic data as well as data from earthquake focal mechanisms are compared to the present morphostructural scheme of the area, and discussed in term of quaternary tectonic evolution.

Soils, vegetation and permafrost across an active meander of Indian River, central Yukon, Canada

Examination of soil profiles on concentric point bars dated by tree ring counts within a meander loop of the Indian River in central Yukon has been used as the basis for the interpretation of soil evolution and river meander migration associated with the reestablishment of permafrost. The soils heve evolved from immature Regosols at the river's edge, into complexly horizonated Luvisols with permafrost on the oldest, measurable, 140 year old point bar. Permafrost re-entered the ground after about 100 years, contrasting with almost immediate re-entry of permafrost three degrees further north. This difference in latitude defines a narrow ecotone representing a very rapid transition from the Discontinuous into the Continuous Permafrost Zones.

The significance of mid‐channel bars in an active meandering river

Mid‐channel bars are common on many active meandering gravel‐bed rivers, but specific information on timescales of development and on the occurrence of bars is lacking. Data from the River Dane in Cheshire, NW England, are presented here which indicate that a sequence of formation from accumulations of coarse gravel on the centre of riffles through to attachment of bars to floodplains is common and takes a period of 5–15 years. The sedimentological and vegetation changes through the sequence are described and the r&amp;#x006f;&amp;#x030a;1e of flow events is discussed. About 10% of bends in this meandering reach contain mid‐channel bars at any time. Bars develop in steeper sections of the channel and downstream of sharp corners and rapidly eroding apices. Many of the bars are caused by excessive erosion of the banks which overwidens the channel. In the longer‐term the bars become incorporated in the developing meanders.