Channel Thalweg Research Articles

Geomorphic and Hydrologic Assessment of Erosion Hazards at the Norman Municipal Landfill, Canadian River Floodplain, Central Oklahoma

The Norman, Oklahoma, municipal landfill closed in 1985 after 63 years of operation, because it was identified as a point source of hazardous leachate composed of organic and inorganic compounds. The landfill is located on the floodplain of the Canadian River, a sand-bed river characterized by erodible channel boundaries and by large variation in mean monthly discharges. In 1986, floodwaters eroded riprap protection at the southern end of the landfill and penetrated the landfill's clay cap, thereby exposing the landfill contents. The impact of this moderate-magnitude flood event (Q12) was the catalyst to investigate erosion hazards at the Norman landfill. This geomorphic investigation analyzed floodplain geomorphology and historical channel changes, flood-frequency distributions, an erosion threshold, the geomorphic effectiveness of discharge events, and other factors that influence erosion hazards at the landfill site. The erosion hazard at the Norman landfill is a function of the location of the landfill with respect to the channel thalweg, erosional resistance of the channel margins, magnitude and duration of discrete discharge events, channel form and hydraulic geometry, and cumulative effects related to a series of discharge events. Based on current climatic conditions and historical channel changes, a minimum erosion threshold is set at bankfull discharge (Q = 572 m3/s). The annual probability of exceeding this threshold is 0.53. In addition, this analysis indicates that peak stream power is less informative than total energy expenditures when estimating the erosion potential or geomorphic effectiveness of discrete discharge events. On the Canadian River, long-duration, moderate-magnitude floods can have larger total energy expenditures than shorter-duration, high-magnitude floods and therefore represent the most serious erosion hazard to floodplain structures.

The periodic nature of step-pool mountain streams

Rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers, but whether periodicities exist in the occurrence of step-pools has not been investigated in a comprehensive way, and the implications for such periodicities have not been explored. Spectral analysis was applied to bed elevation data from streams in the Santa Monica Mountains of southern California to investigate periodicity in the occurrence of step-pools. Results indicate significant step-pool periodicities, confirmed by independent field measurements, that vary from 0.43 channel widths to 2.40 channel widths. Spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step-pool spacing. Underlying periodicities are nevertheless de- tected, suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms. Periodicity in the occurrence of step-pools suggests that step-pools are bedforms that represent a fundamental mutual adjustment between flow, channel morphology, and energy expenditure. A rhythmic channel thalweg supports the notion that step- pools are analogous to meandering in the vertical dimension, and implies that, because step-pool channels are not able to adjust energy expenditure in the plane dimension, they instead adjust boundary roughness in the vertical dimension. A continuum of rhythmic gravel bedforms that displays the results of such adjustments, and that shows how step-pools merge into pools and riffles in the stream channel system, is discussed and illustrated with empirical data from the Santa Monica Mountains. Although many differences exist between steep mountain streams and broad alluvial rivers, the findings of this study suggest that generalized principles of uniform energy expendi- ture may also apply to rhythmic step-pool streams. These findings are potentially valuable in the management and design of steep channels.

Pool-Riffle Morphology in an Actively Migrating Alluvial Channel: The Lower Mississippi River

This study analyzes the pool-riffle morphology of the Lower Mississippi River within the context of an actively migrating meandering river. Historic hydrographic surveys (1:20,000) from the late 1800s and early 1900s document the natural channel morphology of the Lower Mississippi, and are used to construct a 1225-km profile of the channel thalweg. Previous studies suggest that pools and riffles are limited to gravel bed channels. This study documents their occurrence within a large fine-grained channel, and illustrates the importance of these features within the context of meandering processes. The channel width at riffles is significantly greater than at pools, and the spacing of pools is scale dependent, averaging 7.2 channel widths. Extreme values occur where heterogeneous channel deposits control the channel alignment. Meander-bend radius of curvature and pool depth is inversely related, which would occur because of the enhancement of helical flow and channel bed shear stress associated with sharper meander-bend curvature. The migration of meander bends increases with pool depth. However, the adjustment of pool depth is inversely related to meander-bend migration. Higher rates of lateral migration are associated with a reduction in pool depth, while meander bends that were less active laterally were more effective at scouring the channel and thereby increasing the depth of pools. The results of this investigation of a large and complex alluvial channel provides linkages with detailed studies that document the mechanisms of meander-bend processes within individual channel reaches and laboratory flumes. Findings suggest that the pool-riffle adjustment of meander bends occurs synchronously with planform adjustment, and as suggested by Keller and Melhorn (1978), represents meandering in the vertical dimension.

Modos de Transporte de Sedimentos Finos no Estuário do Rio Itajaí, SC

This paper presents an assessment about the modes of fine sediment transport in the Itajaí River estuary. ·The available information to approach a conceptual model is derived from three primary sources: (1) the river sediment delivery; (2) the distribution of salinity and particulate suspended mailer (MPS) along the estuary under influence of different river discharge conditions; and (3) near bed hydro-sedimentological processes. The river sediment delivery was achieved by daily monitoring the water and the suspended sediment discharge, since November 1998 at the Indaial lymnimetric station, which represents about 70 % of the drainage basin. The salinity and MPS distribution in the estuary were obtained through 47 weekly surveys along the estuary when vertical profiles of these variables were acquired every 1-1,5 km from the mouth to the upper limit of the salt intrusion. The surveys were carried out between November 1998 and November 1999, using an inflatable boat. The results allowed obtain the relationship between the salt wedge extension as function of the river discharge and the estuarine trapping efficiency. The near-bed hydro-sedimenlological processes were assessed through the deployment of an instrument tripod on the estuarine bed al the channel thalweg 4 km upstream from the mouth. The current speed, water level and MPS data were recorded hourly, 1 m above the bottom, from September to November 1999. Based on this data set, two main modes of fine sediment transport in the Itajaí River estuary were identified: a tidal driven and a fluvial driven mode. The tidal driven mode occurs when the river discharge is bellow 200 m3.s-1. Near bottom sediments arc in all erosion-deposition cycle following the semi-diurnal tides and the anomalistic cycle of spring and neap tides, with net sediment transport landwards. During such periods fine sediment is imported from the inner shelf. This mode occurs at time scales of weeks to months. The fluvial driven mode are fully developed when the river discharge exceeds 1,000 m3.s-1. accompanied by suspended sediment load as high as 10,000 ton.day-1. Fluvial advective process fully dominates the sediment transport resulting in zero trapping efficiency.

Monitoring and modelling morphological change in a braided gravel-bed river using high resolution GPS-based survey

Recent research in fluvial geomorphology has emphasized the direct monitoring of channel topography as a useful tool to better understand the interrelationship between river form and process, and in particular to estimate bedload transport rates and reach-scale sediment budgets. Until recently, however, this morphometric approach has been limited to comparison of repeatedly surveyed cross-sections with the associated problems of low frequency sampling and an emphasis on cross-stream as opposed to downstream morphological dynamics. Recent advances in analytical photogrammetry have, however, permitted the acquisition of data at sufficient resolutions to represent macroform (101 m) and microform (10° m) channel geometry, in fully three dimensions. While inherently spatially distributed, the photogrammetric approach to land-surveying is, however, restricted to observable areas of the channel and is thus of limited use in the subaqueous zone, especially if flows are turbid or highly coloured. This paper presents an alternative approach to the study of three-dimensional morphological dynamics of a divided reach of the gravelly River Feshie, Scotland, in which topographic survey of both exposed and submerged areas of the reach was undertaken using the Global Positioning System (GPS). Survey results from two field programmes in 1998 and 1999 are presented in which a Geotronics Geotracer 2000, real-time kinematic GPS receiving unit was used to survey a 200 × 80 m reach. Rapid radial survey methods were employed permitting the collection of 2000 + survey points per day, giving a mean density over the 13 ha reach of 1·1 points per square metre. A detailed assessment of survey errors and DEM quality is presented and a limit of detection of c. 10 cm proposed as an upper threshold for the identification of significant change between DEMs. Preliminary interpretation of channel dynamics in 1998–99 reveals relatively little change to the overall macroscale reach structure. DEM differencing techniques, however, highlight important local changes including the progradation of submerged bar fronts, bar head aggradation and scour along major channel thalwegs and the erosion of fines from sloughs. Copyright © 2000 John Wiley & Sons, Ltd.

Evidence to support near-steady flows (?) in an alluvial sandy system: the plateau formation Bako National Park, Sarawak

The sandstones at Balm are interpreted as having been deposited in a sandy braided stream environment. This system is characterized by variable discharge, high bedload, complex bar forms, high lateral mobility with low preservation of fine material. The dominant sediments, concurrent with this interpretation are large-scale cross-bedding and parallel laminations. However, other sediments, particularly thick units of structureless sands commonly liquified are thought to be anomalous to this type of setting. Three methods of 'massive' sand deposition are recognized (i) bed amalgamation, (ii ) 'n ear steady' turbulent flow , and (iii) liquifaction. A turbidity current is thought to only be able to form during the 100 to 1,000 yr. storm event where a high concentration flow with high turbulence passes over a hydraulic jump. The unison of these three factors may allow the process of 'continual aggradation' to occur. Although this is thought to be rare these mega-storms have a high capacity to deposit large amounts of sediment, highly preservable in the rock record. However , with the benefit of extensive outcrops it can be recognized that the majority of structureless sands located at the Bako Peninsula are formed from the amalgamation of sands in the channel thalweg and the liquifaction of climbing ripple destroying the primary depositional fabric.

Large woody debris in bull trout (Salvelinus confluentus) spawning streams of logged and wilderness watersheds in northwest Montana

We measured large woody debris (LWD) in 20 known bull trout (Salvelinus confluentus) spawning stream reaches from logged and wilderness watersheds in northwestern Montana. Mean bankfull width of stream reaches was 14.1 m ranging from 3.9 to 36.7 m. Streams were large enough to move LWD and form aggregates. We determined the characteristics of individual pieces of LWD that were interactive with the stream channel. Large, short pieces of LWD attached to the stream bank were the most likely to be positioned perpendicular to stream flow, while large, long pieces either tended to be parallel to the flow or, when attached, were most apt to extend across the channel thalweg. Observations indicated that the majority of pools were formed as scour pools by either very large LWD pieces that were perpendicular to the stream or multipiece LWD aggregates. Among reaches in wilderness watersheds, ratios of large to small LWD, attached to unattached LWD, and with and without rootwads were relatively consistent. However, among reaches with logging in the watershed, these ratios varied substantially. These results suggest that logging can alter the complex balance of delivery, storage, and transport of LWD in northern Rocky Mountain streams, and therefore, the likely substantive change in stream habitats.

Symbiosis, competition, and physical disturbance in the growth histories of Pliocene cheilostome bryoliths

Free-living (unattached) subspherical bryozoan masses (bryoliths) in Pliocene tidal channel deposits of the Imperial Formation of southeastern California show complex intra- and interspecific interactions during their accretionary growth. Ranging up to 10 cm in length, the bryoliths are composed almost exclusively of the anascan cheilostome Biflustra commensale. Approximately 50% of the bryoliths are nucleated on cerithiid or muricoid gastropod shells; secondary occupants (presumably pagurid crabs) determined the subspherical growth and associated epibionts of all of these specimens. Evidence for crab occupation includes the thick and relatively symmetrical bryozoan overgrowths that form short tubes extending from the aperture, thinning and pinchout of laminae on the undersides of bryoliths (wear facets), and the distinctive borings (Helicotaphrichnus) of symbiotic worms. In some instances, shells were infested by bryozoans and other encrusters before death of the gastropod, but these overgrowths are thin relative to hermit-associated bryozoan colonies. Episodic hermit abandonment, indicated by extensive erosion of the bryolith and/or its colonization by a more diverse epibiont assemblage including oysters and serpulids, was more frequent among bryoliths nucleated on the largest and most fouled gastropod shells; it was also more frequent among bryoliths in the relatively high-energy tidal channel thalweg than among those associated with oyster thickets on muddy channel margins. Bryoliths nucleated on other shell substrata are similarly thick, but have more irregular stratigraphies including more sedimentary inclusions, more borings, and fewer encrusting epibionts. Pebbles of crystalline basement rock are also encrusted by B. commensale, but only thinly. All of these bryoliths not inhabited by crabs are limited to the channel thalweg. As many as four distinct colonies of B. commensale could coexist on a single bryolith; lines of competitive standoff between colonies are marked by mineralized walls and topographic ridges on the bryolith exterior, and by teepee-like structures in cross-section. These standoffs were preferred sites of infestation by other epibionts and were remarkably stable in position on bryoliths with continuous hermit occupation. Bryoliths that suffered repeated abandonment by hermits, or that depended entirely upon chance reorientation, are characterized by highly unstable standoff positions, reflecting scramble competition under less predictable conditions. These circumstances were most common among large bryoliths and among those in channel thalwegs.

Critical flow constrains flow hydraulics in mobile‐bed streams: A new hypothesis

A new hypothesis predicts that in mobile‐bed river channels, interactions between the channel hydraulics and bed configuration prevent the Froude number (Fr) from exceeding 1 for more than short distances or periods of time. Flow conditions in many steep, competent streams appear to be close to critical. Froude numbers of steep (slope ≈ 0.01) sand‐bed streams with considerable freedom to adjust boundaries oscillate between 0.7 and 1.3 over 20‐ to 30‐s cycles, with an average of 1.0 at the channel thalweg. Critical flow in these streams is maintained by the interaction between the mobile bed and free water surface at high Fr, which results in a cyclical pattern of creation and destruction of bed forms. Field observations support that a similar mechanism of flow–bed form interaction constrains Fr ≤ 1 in active‐bed braided gravel rivers, step–pool streams, laboratory rills, lahar‐runout channels, and even some bedrock channels. Empirical and analytical results show that as slope increases, competent flows tend to asymptotically approach critical flow. An assumption of critical flow would dramatically simplify paleohydraulic flow reconstructions and modeling of flow hydraulics in high gradient streams.

Automated drainage extraction in mapping the Monterey submarine drainage system, California margin

Drainage-extraction algorithms traditionally used for extracting river networks and watersheds from gridded land topography are applied to gridded multibeam bathymetry of the mid-California margin. The algorithms are used to automatically map two regional tributary networks of submarine canyons and deepsea channels draining Monterey Bay, the principal conduits of which are Acension and Monterey Canyons. The algorithms reliably map subaqueous drainage areas, but are prone to error in mapping the extent of submarine canyon and channel thalwegs due to operator subjectivity and algorithm limitations. A geomorphic comparison of the Acension and Monterey Canyon networks, with 12 river networks in the continental U.S., illustrates both the potential and weaknesses of using drainage extraction algorithms to analyze sediment pathways in gridded bathymetry.

MID-CHANNEL BAR GROWTH AND ITS RELATIONSHIP TO LOCAL FLOW STRENGTH AND DIRECTION

Anabranches of braided rivers typically migrate and avulse across the floodplain to produce new channel junctions, scour and subsequent mid-channel bar growth immediately downstream. Few quantitative studies have been made of this bar development process and the link to change in channel geometry and local flow strength and direction. This paper provides data on the spatial and temporal pattern of surface velocity as mid-channel bar growth is initiated downstream of a fixed junction scour in a generic scale flume model. The sequence of channel changes is: (i) development of a confluence scour with flow convergence and maximum velocity in the channel centre; (ii) exceedance of the local transport capacity and initial stalling of coarse sediment in the channel thalweg downstream of the scour; (iii) bar growth through entrapment of all sizes of bedload; (iv) change from velocity maximum to minimum and flow convergence to divergence when the bar height is approximately 55 per cent of the thalweg depth; (v) broadening of the bartop platform, a drop in local competence and bankward migration of the two distributaries whose cross-section and velocity remains approximately constant. These flume data and interpretations are compared to descriptions in the literature of the braiding process with particular reference to the flume work of Leopold and Wolman (1957) and Ashmore (1991, 1993). A new model for mid-channel bar growth is presented which helps explain the long-term development of the confluence–diffluence unit.

Effects of Coarse Woody Debris and its Removal on a Channel Affected by the 1980 Eruption of Mount St. Helens, Washington

During the May 18, 1980, eruption of Mount St. Helens, Washington, a pyroclastic surge introduced large volumes of coarse woody debris (CWD) and fine‐grained sediment to Clearwater Creek, approximately 15 km northeast of the summit. Effects of controlled CWD removal on sediment storage, substrate, and pool frequency and volume were measured in four reaches, each with three 200‐m segments, from 1982 to 1990. In each reach the upstream segment served as a control with no debris removal, and CWD was totally and selectively removed from the other two segments. Unique among similar experiments are the large size and volume of CWD and the large inputs of fine‐grained sediment. Except for segments of two reaches that received debris torrents, the Clearwater channel thalweg scoured until 1985. In three reaches, total debris removal caused additional scour and coarsening of the bed surface compared to segments with no or partial debris removal. Pools contracted from 1982 to 1985 and expanded afterward, especially in control segments. Total debris removal apparently caused pools to become shallower and, in segments of low sinuosity, decreased the frequency of major pools. Habitat complexity decreased after total debris removal, as indicated by a decrease in the standard deviation of residual depth and an increase in the size of substrate patches.

Gravel-bed deposition and erosion by bedform migration observed ultrasonically during storm flow, North Fork Toutle River, Washington

Ultrasonic depth sounding provides useful and unexpected information about peak discharge and sediment transport when applied during storm flow in channels with erodible beds. Streambed elevation was measured with dual ultrasonic depth sounders during the rise, crest, and recession of a storm flow in the North Fork Toutle River, Washington, on 3 December 1987. The sounder transducers were held in the flow on a rigid pipe which was suspended from a boom over the channel thalweg. The 12 h episode of depth sounding detected the superposition of fine-gravel dunes on large bed waves, the depth-limited growth of mean dune heights from 13 to 25 cm, and bedform-related deposition and erosion in the channel thalweg. The streambed elevation rose 0.3 m in 2 h with increasing stream discharge. Dune heights diminished for about an hour before the peak river stage was attained. Scour of the streambed continued through the peak stage and recession, with 0.7 m of scour over 10 h. Rapid scour of the streambed produced a falling stage while discharge was still rising. Dune heights grew as flow depth increased after peak stage. Streambed elevation was lowered as large bed waves of fine gravel migrated along the thalweg with successively lower troughs leading each bed wave. Bed elevation records from the dual depth sounders were used to calculate dune celerities of 3–6 cm s −1 and bedform wavelengths of 2–11 m. The large bed waves were subtle, dune-like gravel bedforms with wavelengths of 25–30 m. The celerities and bedform dimensions yielded bedform transport weight rates between 3 and 20 kg s −1 m −1 and grain shear stresses between 40 and 100 N m −2 for the depth-sounding episode.

Contemporary sedimentary processes in the Monterey Canyon-fan system

A study of the Monterey Canyon and fan was conducted to investigate contemporary sedimentary activity using a camera sled, Alvin dives and Sea-Beam bathymetry. Physical characteristics and plan view morphology of the walls, terraces, thalweg floor and levees of the Monterey Canyon and fan, as well as the gullied and non-gullied regions of the adjacent continental slope have been studied. The canyon floor and fan valley thalweg channel from 2900 to 3500 m exhibited properties indicative of moderate to low energy conditions (weak currents, smoothed sea-bed, deposits of disintegrated kelp, bioturbated mud substrate, fecal pellets, absence of scour). Chemosynthetic communities consisting of clams and a pogonophoran, were found in the fan valley from 3000 to 3600 m. Local areas of rockfalls and slumps from the canyon and fan valley walls were not fresh (sedimented, abundantly colonized by benthic biota). Terraces and levee crests were mud-draped and bedrock exposures on canyon walls were encrusted by benthic organisms. Freshest disturbances were found in gullies on the adjacent continental slope where bedrock was scoured clean of sediment and loose debris. A major submarine slide detached from escarpments on the lower slope, and extends across the fan. The slide surface was mud-draped, hummocky, and contained bedforms with wavelengths up to 150 m and up to 10 m relief. The survey was conducted shortly before the Loma Prieta earthquake (October 17, 1989) that caused substantial ground motion in on-shore regions of Monterey Bay. Any substantial sub-sea disturbances generated by the ensuing earthquake should be discernable from the pre-earthquake state by comparison with the deep-sea photographs and observations.

Characteristics of Some Submarine Fan Channels, Permian Ecca Group, South Africa

The vary well exposed submarine fan complex in the southwestern part of the Karoo basin permits close examination of channel-fills and in places their associated overbank deposits. The complex comprises five arenaceous fan systems some of which attain 60 m in thickness. The fans are vertically stacked and separated by basinal shale deposits; each system with its own direction of growth. The association of channelized sandstone bodies and thin-bedded sandstone and shale packages in an updip position from predominantly stacked lobe deposits suggest preservation of middle fan settings. A 500 m wide, 20 m thick channel-fill consisting massive amalgamated sandstone beds occupying the channel thalweg occurs in a setting dominated by thin-bedded, ripple-laminated sandstone and shale. Gradual thinning of the channel-fill beds toward the channel edges, lack of internal lateral accretion, and a high width to depth ratio suggests a low sinuous to straight channel. The channel-fill is capped by an abandonment facies characterized by ripple-laminated sandstone and shale. Stacked, laterally offset channel-fill deposits with highly erosional contacts and typical well-bedded overbank deposits form channel-overbank complexes and characterize the mid-fan region of the uppermost fan system. Palaeocurrent directions and gradual diminishing of bed-thickness away from the generally massively bedded, amalgamatedmore » channel-fill sandstones confirm a simultaneous channel/overbank origin for these deposits. Levee morphology has not been recognized. Both examples of channel-fills cited reveal part of the complexity of the channelized portions of submarine fans and hence the implications thereof in exploring for hydrocarbon reservoirs.« less

A Chaos Model of Meandering Rivers

A meandering river is a nonlinear dynamic system, and fractal geometry describes well the meander bends of such rivers. Based on a qualitative, sedimentological model of the process of meandering, a chaos model is proposed, describing meandering as the outcome of two processes: the feedback interaction between river curvature and a high-velocity thalweg channel within the river; and the interaction between meander bends causing abandonment and straightening of the river course. The system, when initiated from a nearly straight river course, moves toward a dynamic equilibrium in which the meander bends are fractal. This development is a case of self-organized criticality. The equilibrium represents a state of optimal energy dissipation in a situation where two counteracting processes are balancing each other. Sedimentology may be seen as the science that describes how nonlinear dynamic processes interact to create a depositional system. As indicated by the example of meandering rivers, the use of chaos and fractal models may give sedimentology a new turn toward understanding sedimentary processes and the 3-D architecture of sediment bodies.

Proterozoic canyons of the Flinders Ranges (South Australia): submarine canyons or drowned river valleys?

A number of kilometer-sized erosional incisions (canyons) occur within a specific stratigraphic interval in the late Proterozoic succession of the Adelaide Geosyncline, Flinders Ranges, South Australia. This paper describes in detail four adjacent east-west orientated incisions in the northern Flinders Ranges which are interpreted to be part of an ancient sinuously meandering canyon system. At least 600 m of siltstones and fine sandstones of the Bunyeroo and the Brachina Formations were removed during the erosional event that produced the canyon. Infilling sediments, which exhibit a similar succession in each incision, were deposited during emplacement of the Wonoka Formation. Although an identical sedimentary succession occurs within each incision, palaeocurrents determined from flute casts at the bases of sandstone beds alternate by close to 180° from one erosional feature to the next. Basal infilling sediments commonly contain lenticular breccias, interpreted as slumps derived from the Brachina Formation, which forms the incision walls. It is significant that clasts in these breccias preserve evidence of lithification predating the erosional event that produced the incisions. Channel-filling debris-flow units which occur mainly above the basal slumps, along with associated sandstones, were most likely emplaced episodically. Asymmetric ripple cross-laminae in the sandstone beds above and within debris-flow units commonly are wave modified. Isolated occurrences of hummocky cross stratification are observed in most units of the incision-fill. A thin ( < 100 cm) but extensive limestone unit, developed above the erosional unconformity east of the canyon and on southern canyon walls, has a non-marine isotopic signature which suggests subaerial exposure of at least upper portions of canyon walls. The final position of what is interpreted as a laterally migrating thalweg channel is located adjacent to the north wall of each incision. Current debate centers on whether the incisions represent the meandering portion of an ancient submarine canyon system, or whether they were cut subaerially and were subsequently drowned and filled by coastal onlap. Both models are discussed in this paper. Sinuous meanders can form on a variety of scales, both in subaerial as well as in submarine environments. The abundance of wave-generated and wave-modified sedimentary structures within the canyon fill and the presence of what appears to be a non-marine limestone precipitated on the canyon walls favour the subaerial erosion model.

Backbarrier Dynamics of the East Friesian Islands

ABSTRACT The backbarrier area of the East Friesian Islands has undergone extensive changes during its recorded history (since 1650). The physical setting that contributes to the dynamic nature of the backbarrier region is the presence of strong tidal currents (mesotidal conditions), the lack of extensive marsh development, a wide backbarrier area and a strong westerly wind regime that parallels the trend of the island chain. The major channels dissecting the backbarrier have migrated eastward 6.0 to 14.0 km during a 310-year period, extensively reworking the tidal flats. Headward erosion of these channels has been in response to wind-generated currents, sediment suspension by wave action, and an eastward growth of the barrier islands. Eastward displacement of the backbarrier drainage divides h s produced inlet drainage networks that are asymmetrically situated behind the downdrift barrier island, resulting in hook-shaped main inlet channels. Highly hook-shaped channels occur at inlets that have been stable historically, due to their channel thalwegs having scoured into resistant sediments. Inlets that have had a long history of eastward migration have straight to slightly hooked channels.

Morphology and Evolution of Shale-Filled Paleochannel in Wilcox Group (Paleocene-Eocene), Southeast Texas: ABSTRACT

ABSTRACT Local extensive paleo-channels exist with the upper to lower Wilcox Group (Paleocene-Eocene) of the Gulf Coast of North America. Recent reports document paleo-channels of the Wilcox Group: Bejuco-La Laja, Chicontepec, DeSoto, Nautla, Ovejas, St. Landry, and Yoakum. An eighth channel, the Tyler (Hardin) channel suggests that passive submarine erosional forces shaped the channels prior to any active erosional forces such as those related to turbidity currents. The north-south trending Tyler (Hardin) channel is 24 mi (40 km) long and 12 mi (20 km) wide and displays over 1000 ft (300 m) of shale fill. The channel thalweg bifurcates updip into two meandering steep-walled channels which display numerous side gullies. The channel grew from the youthful to the mature stage through massive erosive mechanisms and during the mature stage may have been further enlarged by more active erosional agents such as turbidity currents. Old age began with increased peralic sedimentation and infilling of the channel with deep-marine shale. Eventually, progradation and eustatic drop in sea level dried the channel, its younger fill and older adjacent deposits. Paleo-channels are silient economic features. The deep-marine shale infill of the channel acts as a source of oil and gas and a seal for adjacent reservoir rock. Upward migration of oil through fractures, faults, and sediments fills reservoirs in overlying structures formed in nearshore deposits of the upper Wilcox. Most of the major oil and gas fields of the Wilcox Group in Tyler and Hardin Counties are localized in the structures above the underlying channel fill.

Sedimentology and morphology of a low‐sinuosity river: Calamus River, Nebraska Sand Hills

ABSTRACTA study reach of the Calamus River, Nebraska Sand Hills, has a low sinuosity (less than 1.3) and braiding parameter (less than 1). Depending on sinuosity, the channel is occupied by alternate bars and point bars, the emergent parts of which form nuclei for midstream bars (islands). Channel migration occurs by bend expansion and translation, downstream and lateral growth of islands, and by chute cutoff.Channel‐bed sediment is mainly medium‐grained sand, but gravel and coarser sand sizes occur in thalweg areas adjacent to cutbanks and upstream parts of bars and islands, and finer sands occur on the downstream parts of bars and filling channels. Curved‐crested dunes cover most of the channel bed at most flow stages, with ripples restricted to shallow areas near banks. Bed material is mostly large‐scale cross‐stratified, with small‐scale cross‐strata interbedded with plant debris occurring in topographically high areas near banks.Vibracores through channel bars show a basal erosion surface overlain by large‐scale cross‐stratified sands, in turn overlain by small‐scale cross‐stratified sand interbedded with plant debris. The overall sequence generally fines upwards, but the large‐scale cross‐stratified portion either fines upwards, coarsens upwards, or shows little grain size variation. Lithofacies distributions vary spatially within and between bars depending on position in the bar and local channel curvature/width, in a similar way to unbraided rivers elsewhere. Lithofacies of bar deposits are similar to those in the active channel, and the elevations of the basal erosion surface and adjacent channel thalweg correspond closely.Channels abandoned by chute cutoff are filled progressively from the upstream end, and comprise deposits similar to the downstream parts of bars (i.e. fining upwards). The downstream extremities of channel fills may contain large proportions of peat relative to sand, but little mud due to the paucity of such fine suspended load in the Calamus.