Abstract Several hundred papers on fluvial sedimentology have appeared since the first fluvial conference 9 years ago (Calgary, 1977). Many variants of existing models have been published, including several sub-models of high-sinuosity rivers based on sediment load grain size, and the anastomosed model has become well documented and widely known. The main advance in the study of low-sinuosity rivers is the recognition of mid-channel macroform bar complexes comparable in their medium- to long-term geomorphic significance to point bars. Increasing attention is gradually being paid to the overbank environment, with the development of floodplain facies models, and an improved understanding of the conditions necessary for coal formation. Few new syntheses of facies models have been attempted. The current research trend is toward documentation of three-dimensional architecture, both on the local scale (channels and complex bars) and on the regional scale, using two- and three-dimensional lateral outcrop profiles and employing chronostratigraphic markers to tie sections together.

Abstract The development of three separate devices at Birkbeck College, London, and their deployment in a gravel-bed stream allows us to pinpoint entrainment thresholds, to measure bedload continuously and to quantify the ingress of matrix fines. This provides a comprehensive picture of processes and gives clear indication of the reasons why gravel beds appear to react unpredictably to flood flows and why a universal bedload equation has yet to be developed. We show that ubiquitous bed microforms (e.g., pebble clusters) delay initial motion so that stream power is 5 times greater than it is when bedload transport ceases. We also show that low-flow ingress of matrix fines increases the difference to 16 times by increasing shear strength of the stream bed. Our continuous monitoring devices indicate a regular pulsation of bedload transport (mean wave period 1.7 hrs) that cannot be attributed to deficiencies in the samplers because it is corroborated by distinctly different and independent instruments; the pulses are not explained by changes in hydraulic parameters. We suggest that they are kinematic waves of mobile bed particles, the first dynamic manifestation of this phenomenon in a natural stream. Our data give a clear indication of those facets of water-sediment interaction that must be investigated in order to arrive at a general bedload transport equation for gravel-bed streams, and the data highlight the naive assumptions currently made in paleohydraulics when using clast size as an indicator of paleo-flow conditions.

Abstract An electromagnetic coil placed in the bed of a stream can provide instantaneous real-time electronic data on the motion of coarse, naturally-magnetic, bed material. Data collected in May 1982 show that sediment transport of material larger than 32 mm occurred on the bed during falling stage, began somewhat gradually, and ended nearly instantaneously. Because transport data were not averaged, transport waves of material larger than coarse pebbles can be seen on the record. During early parts of the transport event, the period of the waves was less than 1 min. By the end of the event, waves were occurring at approximately 6-8 min intervals. In 1985, the device detected the motion of coarse pebbles and cobbles in one part of a stream, while other parts of the stream showed no motion. If background noise problems can be overcome, the device has the potential to provide insight into the nature and location of pebble and cobble transport on the bed of gravelly streams and the conditions which lead to that transport.

Abstract Flow dynamics at river channel confluences can be characterized by six major regions of flow stagnation, flow deflection, flow separation, maximum velocity, flow recovery and distinct shear layers. The dominant controls upon the magnitude of these regions are shown to be the junction angle and the ratio of discharges between the confluent channels. Through the combined use of scaled laboratory modelling and an analysis of field evidence, the dynamics of flow are found to produce a confluence morphology which consists of avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone. Tracing of sediment in both laboratory and natural channels reveals distinct sediment pathways within the junction which can be explained through the model of flow dynamics. A knowledge of confluence flow dynamics is important when assessing channel design criteria, junction bed morphology and ancient confluence sediments.

Abstract Grain size sorting in channels of the Rio Grande varies with location within ripples and megaripples and within downstream progressions of bedforms. In the natural sand-dominated floodway channel, plane beds and ripples commonly have log-hyperbolic grain size distributions. Under waning flow with minor suspended sediment, late-forming ripples contain unimodal (0.15 mm) log-hyperbolic distributions. Where these ripples have overtaken and mixed with earlier formed migrating coarser grained ripples, bimodal (0.15 and 0.30 mm modes) ripples result. These mixed ripple distributions may be modelled mathematically by mixing hyperbolic distributions. In subjacent dunes formed from mixed ripples overtopping the dunes’ brinks, upper foresets are slightly bimodal. Toesets are also bimodal, but lower foresets are more poorly sorted and approximate one or more hyperbolic distributions. In a 50-m reach of dunes, upper foresets contain progressively more pebbles downstream. Waning flow in the artificial Rio Grande conveyance channel left gravel-dominated straight-crested megaripples with entirely different grain size distributions. Gravel armor developed from the troughs to the crests of the 2.8-m-wavelength megaripples. Both troughs and crests have distributions that exhibit broad “plateaus” from 0.25 to about 10 mm, and both have secondary modes at 0.063 mm. Upper slipfaces have roughly log-hyperbolic distributions, but contain irregular amounts of coarse and fine grains. Lower slipfaces have log-hyperbolic distributions with modes ranging from 1.4 mm upstream to 0.71 mm downstream. The slipfaces deposited during waning flow are better sorted and finer grained than the foresets of the megaripple interiors, which are composed of a mixture of upper and lower slipface grains along with even coarser clasts.

Abstract Significant attributes of the Kosi fan are its flat surface, fine-grained, highly bioturbated sediments, perennial water saturation, and abundant abandoned channel facies. Pre-existing drainages controlled fan shape, and disparate monsoon and dry season discharges cause some channels to be vegetated and muddy while simultaneously containing sandy point bars. Similar fans should be recognizable by isopach-defined wedges, tectonic position, bowing of grain-size isograds away from the orogen, clustering of large channels, and interfingering with interfan facies.

Abstract This study follows the pioneering work of Coleman (1969) and presents the results of a study of a 200-km-long reach of the Brahmaputra River in Bangladesh. Channel pattern and migration has been monitored using LANDSAT imagery and historic maps. Channel pattern in the Brahmaputra is varied. The river is mostly braided but some reaches are anastomosed or meandering. Channel movement is dominated by lateral migration with some minor channel switching and one major avulsion in the last 200 years. There is a hierarchy of channels in the Brahmaputra. First-order channels encompass the whole river and may comprise several second-order channels which, in turn, have third-order channels within them. The channels divide and rejoin around bars which scale with the bankfull width and depth of each channel. Bar types include lateral (point) bars, diagonal bars, medial bars and tributary bars. Bars sometimes coalesce to form semi-permanent bar assemblages. Channel cross sections observed by echo-sounding show a high degree of channel asymmetry and extremely complex cross sections which can be related to bar development. The pattern of deposition is divided into four styles: new mid-channel bars (13 percent), channel abandonment (15 percent), lateral accretion to bank (19 percent) and addition to bars (53 percent). The latter is the most important and is subdivided into three elements of deposition: upstream accretion (14 percent) downstream accretion (29 percent) and flank accretion (57 percent).

Abstract Accretion in meandering rivers does not exclusively occur in point bars. This is demonstrated by the River Tywi, a meandering gravel-bed stream in South Wales, in which deposition occurs at the concave sides of meander bends. These deposits, termed counterpoint bars in this paper, form at sites commonly assumed to be ones of erosion. There are two types of counterpoint bar in the Tywi; fine-grained examples, similar to the concave bank benches of Page and Nanson (1982), and large gravel bars. Gravel counterpoint bars are a little known type. They are bulbous to elongate in plan view, are as much as 30-40 m wide and 60-70 m long. They consist of a bar platform, secondary channel and riffle. Gravel counterpoint bars appear to have a relatively high preservation potential in the Tywi valley. This is suggested by historical maps and the presence of several abandoned examples on the floodplain. These form elongate and bulbous assemblages of two or more bars. Morphology and sedimentary structures are distinct from the more familiar fine-grained examples described from other rivers; however, gravel counterpoint bars may be more difficult to distinguish from gravel point bars if preserved in the geologic record.

Abstract Modern meandering river point bar deposits formed in fluvial and tidally-influenced environments were investigated to explain why large-scale epsilon cross-stratification (ECS) is common in ancient fluvial rocks but appears to be absent in modern deposits. To resolve this problem several modern meandering systems were studied; the Athabasca upper delta plain in northeast Alberta, Canada; the mesotidally-influenced reach of the Willapa River, southwest Washington State, U.S.A.; and the lower Daule and Babahovo Rivers, Ecuador, which have micro- and mesotidally-influenced depositional conditions. As well, exposed point bar facies in 18-m-high cut-banks were examined in the fluvial, lower Liard River (Middle Holocene), NWT, Canada, and tidally-influenced Willapa River (Late Pleistocene), Washington State. Based on sedimentologic results obtained from these areas, a threefold lithofacies classification of point bar deposits is proposed: (1) fluvial sandy point bar facies, (2) low-energy fluvial and microtidally-influenced (upper estuary) point bar facies, (3) mesotidally-influenced point bar facies deposited in upper and middle estuary settings. The latter two facies are very similar to many reported ancient meandering river point bar rocks. The three lithofacies models are compared with four ancient examples of point bar rocks selected from Alberta, Canada, the Lower Cretaceous middle McMurray Formation (Athabasca Oil Sands), the Upper Cretaceous Judith River and Horseshoe Canyon Formations and the Paleocene Paskapoo Formation.

Abstract Lateral accretion surfaces (LAS) are rarely seen in modern point-bar sequences. In point bars in the arroyo of the Rio Puerco west of Albuquerque, they are clearly seen because 1) beds of texturally and structurally distinct wind-blown sand are deposited on point bars during no-flow periods and not totally removed during floods, and 2) point bars are excised soon after they form because of rapid channel sweep and swing. LAS are readily identified texturally because eolian units have a marked paucity of coarse silt, apparent in number-frequency but not weight-frequency cumulative size distributions. The surfaces characteristically are rugged (notably like staircases) in plan and section, and separate overlapping tabular and wedge-shaped units locally truncated by scour in chute channels

Abstract Bella Coola River drains 5050 km 2 of glacierized mountains on the central coast of British Columbia. Spring snowmelt and autumn rainstorm floods occur up to 1000 m 3 s The valley-fill is composed of Quaternary glacial and fluvial sediments, with modern alluvium representing less than 3 percent of the total volume. Proportions of light and heavy minerals in alluvial deposits indicate that headwater volcanic terrain is the dominant sediment source; however, distal tributaries draining plutonic rocks are locally important. Sedimentation within the modern channel yields medial, lateral and point bars and vegetated channel islands. Avulsions within ‘sedimentation zones’ result in a network of slough channels. Floodplain development occurs in three ways: infilling of sloughs after channel avulsions, lateral accretion of channel bars, and overbank deposition. The most prominent facies assemblage is horizontally and trough cross-stratified sands over massive gravels. Intervening stable reaches are cobble-paved zones through which sediment is transported directly. Facies within these reaches comprise horizontally stratified sands and silts, indicating that floodplain development also occurs by vertical accretion. This association of channel zone and floodplain facies is widespread in mountain valley rivers in the northern Cordillera today. They are not aggrading and would not be prominent in the stratigraphic record.

Abstract This detailed study of the sedimentology and facies architecture of three overbank subenvironments which occur marginal to a meander belt in the lower Mississippi River Valley leads to the following conclusions: 1) backswamp, levee and splay deposits can be subdivided into units related to the establishment of the associated channel belt; 2) cycles related to avulsion, levee progradation, splay progradation and abandonment, and sheet-flood events are preserved in flood basin deposits; and 3) flood basin facies prograde basin ward as the levee builds upward over time along a channel belt margin. The establishment of the channel belt is divided into four phases: 1) a pre-avulsion stage, 2) an avulsion stage, 3) an early channel belt, and 4) a late channel belt stage. In the study area, the flood basin sequence affiliated with the avulsion and establishment of the channel belt (30 m) has a maximum thickness of about 10 m. The avulsion event is probably recorded in the sedimentary sequence as the lithologic change from blue clay with detrital organic debris deposited in standing water to sheet-flood silts and sands related to levee and splay progradation during incipient formation of the channel belt. Surficial levee deposits (silty sand unit) exhibit an overall coarsening-upward sequence (2.5 m) that reflects meander bend migration toward the sampling site during the late stage of channel belt development. Surficial splay deposits (silty sand unit) initially coarsen upward during the progradational phase and then fine upward as the splay is abandoned (3 m). Individual flood cycles (mm-cm) occur as small-scale fining-upward rhythmites with poorly preserved stratification in all sub-environments.

Abstract The sediment supply, deltaic deposits and processes of sedimentation in the glacier-fed lake downstream from Tunsberg-dalsbre in western Norway have been investigated. The depositional processes were analyzed by a computer model which is structurally different but of the same type as that of Bonham-Carter and Sutherland (1968). Particle trajectories are computed by numerical integration through the lake flow field and the zones of sedimentation of different particle sizes in the bottomset layers determined. The effect of turbulence was represented by the introduction of a stochastic element causing the particles to move in a random walk. Various test runs showed that the turbulent spread of particle trajectories increased with decreasing particle fall velocity. Application of the model to Lake Tunsbergdalsvatn involved investigations of the lake flow field during different sediment and water discharge situations. Calculations of the extension of the zones of sedimentation during this latter situation tallied with the results obtained from analysis of grain size distributions of bottomset beds. A process response model of the long-term delta development is presented. The present lake is a remnant of an 8-km-long and 100-m-deep fjord valley lake that existed after deglaciation 9000 BP. A major part of the volume of the delta sediments was deposited by flow divergence processes at the delta front. The deltaic deposits are regarded as the results of a series of different phases of delta development through time. During each phase, the magnitude and composition of sediment supply, and the basin flow fields, were decisive for delta formation processes. The morphology of former basin conditions, however, influences the flow field in a feedback loop, and future development is dependent upon past history.

Abstract Paleohydraulic data from outwash terrace sequences in New Zealand and Iceland suggest that they were formed by glacially controlled, high magnitude, incising flood events. Relic incision channels cut into the terrace surfaces provide evidence of steeper gradients, coarser sediment and thus greater bed-shear stresses, velocities, Froude numbers, and discharges than those of the braided channel systems on the terrace surfaces. The paper proposes that channel incision and terrace formation can be generated by catastrophic floods which exceed the thresholds for channel entrenchment. These thresholds are defined by the flow conditions predicted by paleohydraulic modeling of the relic channels.

Abstract Stratification and estimated paleoflow conditions for valley-fill deposits suggest that Ogallala Group streams in western Nebraska were similar to modern streams of south-central Alberta. Ogallala stratification includes medium-scale (0.5 to 2.0 m thick) trough crossbedded sand and gravel, tabular indistinctly horizontally bedded and imbricated gravel, and horizontally bedded sand and pebbly sand. Valley fills are 15 to 55 m thick and 800 to 1800 m wide at the top. Some are in bedrock-floored channels resembling the “inner channels “ of Shepherd and Schumm (1974). Gradients for three well exposed paleovalley floors range from 0.0014 to 0.0020 (m/m) after tectonic correction. This compares with 0.00135 for the modern North Platte River Valley in Nebraska. The average intermediate diameters of the 10 largest clasts from tabular gravel beds found at 17 sites varied between 0.077 and 0.15 m. The average median intermediate diameter for gravel from four well exposed tabular gravel beds is 0.024 m. Consistent paleodepth estimates of about 2 m correlate well with the scale of cross-stratification observed in the valley fills. Paleo-velocities are estimated at about 2 m/sec, and Froude numbers of about 0.4 are consistent with a lower flow regime in the stability field of dunes. Two-dimensional specific in-channel paleodischarges were 3 to 4 m 2 /sec. Total paleodischarge estimates based on slope-discharge relationships for gravel-bed rivers range from 340 to 1240 m 3 /sec and are comparable to average annual peak discharges on the North Platte River reported 80 to 90 years ago. Ogallala streams were probably dominated by macroforms similar to the “crescent-shaped bars” of the North Saskatchewan River. Deposition also took place on longitudinal bars in deeper channels. Shallow upper-flow regime transport and deposition is recorded by horizontally bedded sand and pebbly sand.

Abstract Taphonomy is concerned with reconstruction of biological and sedimentary processes leading to preservation of faunal remains in the stratigraphic record; traditionally, it has been viewed as a specialized branch of paleontology. Recognition of the value of taphonomic work on skeletal remains in fluvial sediments is a fairly new development. The fact that many units of Late Paleozoic to Tertiary alluvium apparently lack vertebrate remains implies either surficial or diagenetic destruction, adverse paleoecological conditions, or oversight in the course of sedimentological studies. In geologic terms, mortality, postmortem developments and burial are stages that develop quickly within channel or overbank settings. Reworking of skeletal debris into younger stratigraphic intervals also occurs, particularly in multistory channel sequences. Remains are classified in terms of their degree of articulation and concentration. Each mode of preservation has a fairly characteristic set of background conditions relating to the cause and location of mortality in the context of concurrent sedimentary activity. Taphonomic studies of well exposed units should adopt a standard scheme of site notation, record locations precisely, and make a comprehensive log of paleontological and sedimentary features. Depositional systems near base level with lowland taxa, i.e., meander plains and estuarine complexes, are the main target for taphonomic studies. Sedimentary regimes particularly conducive to vertebrate preservation include pedogenesis, standing-water bodies, bankfull and above-bankfull discharge, channel-base lags, and amalgamation surfaces in multistory sequences. As well as assisting in facies and architectural studies of fluvial systems, skeletal remains can provide valuable insight into the hydrologic conditions of paleochannels.

Abstract The deposits of the Potwar Plateau in northern Pakistan show large-scale changes in fluvial deposition during the period between 14 and 6 my BP in three superimposed formations (Chinji, Nagri, Dhok Pathan) approximately 2700 m in total thickness. Five cross sections spaced throughout this sequence document intervals 30-80 m thick over lateral distances of 3-5 km. These show the architectural relationships of sand bodies, fine-grained facies, pedogenic horizons, and vertebrate fossil occurrences within floodplain deposits between major channel belt sandstones. Vertebrate fossil localities are particularly frequent within the fine-grained fill of different scales of abandoned floodplain channels (crevasse-splay and/or tributary channels). Such channels are larger and more common in the lower part of the sequence where average rates of sediment accumulation were 0.14-0.32 m/1000 yrs. as determined from the Siwalik magnetostratigraphic time framework. Higher average accumulation rates of 0.46-0.48 m/1000 yrs for the upper part of the sequence correspond to fewer fine-grained channel fills and lower fossil abundance. Changes in type and frequency of floodplain channels are attributed to a combination of differences in the fluvial regimen and in sediment accumulation rates. The quality of the vertebrate record is linked to the frequency and/or magnitude of abandoned channels on the sub-Himalayan alluvial plain and thus to large-scale tectonic and climatic controls on the fluvial systems.

Abstract Foxe’s Flat, on the south bank of the Saskatchewan River, is underlain by the gravel, sand and silt of a meander lobe. It was produced by northeasterly migration of the river when it was flowing about 5 to 10 m higher than its present level. The 10 to 12 m of deposits in Foxe’s Flat comprise four facies. Facies 1, at the base, consists of crudely stratified pebble and cobble gravels and minor sands with lateral accretion bedding dipping toward the paleochannel at less than 5°. It is overlain by, and interbedded with, discontinuous lenses of sands and gravelly sands (facies 2) which contain trough and planar crossbedding. Deposits of facies 3 are interbedded sand, silt and clay with inclined bedding dipping toward the paleochannel at angles of as much as 12°; the beds of facies 3 may interfinger downdip with either facies 1 or facies 2. The sequence is capped by horizontally layered clayey silt and fine sand of facies 4. The four facies represent deposition in the channel and on a point bar surface (facies 1), on the top of the gravelly platform of the point bar (facies 2) in chute and slough channels on the point bar (facies 2), on lower parts of the point bar during falling stage (facies 2), on an inner accretionary bank (facies 3), and on a floodplain (facies 4). Individual accretion surfaces are not generally traceable across all four facies but are prominent in both the upper and lower parts of the sequence. Preservation of lateral accretion bedding in the gravels may have been favored by transport of the gravels in low-relief sheets. The more steeply dipping bedding of the inner accretionary bank was scoured repeatedly by strong flows. Wide variations in stream discharge transported gravels onto the flood plain and deposited mud drapes low on the point bar. Some boulders were deposited from rafted ice. Vertical sequences in Foxe’s Flat are similar to those described from other gravelly meandering stream deposits and from some braided stream deposits. The lateral transition from lateral accretion bedding in the gravels (facies 1) through lenticular sand deposits (facies 2) to the lateral accretion bedding of silts and sands (facies 3) indicates systematic migration of a stream with only one major channel.

Abstract Three-dimensional exposures provided by quarrying of aggregate have been used to determine the geometry of genetic units within a sequence of outwash gravels. The genetic units represent lithofacies associations and are of three types. Association A is predominantly composed of gravelly lithofacies interpreted to have been deposited as sheets by laterally migrating bar complexes in the active channel belt of a low sinuosity river. Association B comprises thin, discontinuous sheets of sandy lithofacies interpreted to have been deposited from shallow flows as floodplain or bar top sediments. Association C consists of sands and fine gravels deposited as channel fills. The distribution of units of these various associations indicates that the active channel belt, at any one moment in time, only occupied part of the valley floor and was flanked by areas of overbank deposition. These latter areas were traversed by single thread channels infilled by vertically aggraded sands and gravels.