Norman D. Smith

The Braided Stream Depositional Environment: Comparison of the Platte River with Some Silurian Clastic Rocks, North-Central Appalachians

Studies of the South Platte-Platte River in Colorado and Nebraska substantiate Ore9s (1964) conclusion that braided patterns in streams are created mainly by accretion of longitudinal bars and dissection of transverse bars. Distribution of bars in the South Platte-Platte River depends on texture of the bed load. Coarse, poorly sorted sediment favors formation of longitudinal bars, and finer grained, better sorted materials form transverse bars. The relative proportion of transverse to longitudinal bars increases downstream, following the river9s tendency to fractionate its load into finer sizes downstream. This is accompanied by an increase in the ratio of planar cross-stratification to horizontal stratification and a decrease in cross-channel topographic relief expressed as a bed-relief index. Relative abundances of planar cross-stratification and horizontal stratification, as well as bed-relief indices were measured in sandstones and conglomerates of the Lower Silurian Shawangunk Conglomerate, Green Pond Conglomerate, and Tuscarora Sandstone in New Jersey and Pennsylvania. These formations display downslope trends similar to those of the South Platte and Platte Rivers, and, combined with paleocurrent, grain-size distribution, and other data, suggest that the coarse eastern facies (Green Pond, Shawangunk) represent proximal braided stream deposits with longitudinal bars that grade westward and northwestward into finer grained distal braided stream sediments (Tuscarora) characterized by transverse bars.

Sedimentation in anastomosed river systems; examples from alluvial valleys near Banff, Alberta

ABSTRACT Three anastomosed river systems are described. Each reach consists of an interconnected network of low-slope, narrow and deep, straight to sinuous, stable channels that transport coarse sand and gravel. Channels are separated by levees and wetlands composed of silt/mud and vegetation. Gravel-bed braided channels occur upstream from each anastomosed system, joined by a transitional reach comprising stable, elongate, silt islands within braided channels. The three anastomosed reaches have formed upstream from elevating base levels caused by deposition of alluvial fans across trunk valleys. Rapid aggradation of floodplain alluvium is confirmed by buried volcanic ash layers. Channel migration is inhibited by root-stabilized banks which, combined with rapid vertical aggradation, results in production of stringer-like, coarse-grained channel deposits surrounded by overbank fines in stratigraphic cross sections. Although it is unlikely that such small base-level controls (alluvial fans) could produce extensive anastomosed deposits, other mechanisms such as glacial moraines, isostatic rebound, or marine transgressions could provide plausible controls for yielding important contributions to the stratigraphic record.

Transverse Bars and Braiding in the Lower Platte River, Nebraska

The Platte is a wide, shallow river which flows eastward from the Rocky Mountains across the Great Plains of Nebraska. Its lower reaches carry a dominantly sandy load and during intermediate and low discharges display a pronounced braided character accomplished primarily through dissection of tabular, flat-topped transverse bars. Transverse bars form by sediment aggrading to a profile of equilibrium (Jopling, 1966) and grow by downcurrent extensions of avalanche faces. Depth, velocity, and grain size tend to decrease on active bar surfaces from their upstream mouths to the downstream and lateral margins. Active surfaces are covered with small-scale bed forms whose distributions are controlled by the flow characteristics. A typical mouth-to-margin bed form progression is dunes to diminished dunes to ripples, reflecting downcurrent reduction of stream power. Water-surface slopes over active bars tend to be greater than those of the channel segments which feed them. Under ideal conditions, transverse bars are essentially lobate; however, most bars, especially during low discharges, assume irregular or asymmetrical patterns due to any of several factors that include bar-mouth cross-sectional geometry, proximity to exposed banks, adjacent currents, steadiness of flow, and basin depth distribution. Braiding (bar dissection) begins during decreasing discharges when the flow passing through the bar mouth becomes unable to sustain active sediment transport over the entire bar surface. A single bar, examined closely over a five-day period of gradually decreasing discharge, documents the evolution from wholly active to dissected states.

Some Sedimentological Aspects of Planar Cross-stratification in a Sandy Braided River

ABSTRACT Tabular sets of planar cross-stratification are characteristic structures produced by transverse bars in the sand-bed Platte River in Nebraska. Certain sedimentologic characteristics of the Platte bars provide potentially useful information for interpreting the origin of planar cross-strata in ancient deposits. Alternating coarse and fine-graded foreset laminations result from continuous avalanching at bar margins of sediment previously sorted by small scale bed forms on the bar surface. Surface dune transport yields thicker and more distinct coarse-fine foreset laminae than ripples. Foreset angles of inclination tend to decrease with increasing flow regime. Several structures are produced at active bar margins swept by side currents, including ripples and dunes perpendicular to the bar slip face, foreset spurs, and straight-crested ripples on the slip face. Each of these structures produce small scale cross-stratification oriented at high angles to the associated planar cross-bed. Large differences between current and foreset dip azimuths frequently occur because of the irregular and lobate shapes of transverse bars. Only 30.5% of the bar foreset directions correspond to within 5° of the currents that formed them. Foreset dip azimuths in a straight 2.1-km reach of the lower Platte are widely dispersed, although their vector mean accurately identifies the direction of the main channel complex. Planar cross-bed orientations computed for the entire Platte plus the Nebraska portion of the South Platte yielded a variance of 6,129 arid a vector magnitude of 35.6%, indicating dispersions greater than commonly assumed for braided streams. Possibilities for distinguishing planar cross-stratification formed by braided stream transverse bars from similar stratification types produced in other environments are discussed.

Fine-Grained Splay Deposition in the Avulsion Belt of the Lower Saskatchewan River, Canada

ABSTRACT In or about 1883, a northward avulsion of the meandering lower Saskatchewan River at the Cumberland Marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present, and now covers some 500 km2 The avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes. The Windy Lake splay, a predominantly fine-grained feature typical of large parts of the avulsion belt, was examined to investigate the relationships between splay evolution and facies development. The splay, now largely inactive, mostly formed over a 35-year period, depositing nearly 2 m of sediment over a 7.2 km2 area. The splay surface is ch racterized by stable, well-defined anastomosed distributary channels that separate interchannel wetland basins. Heights of channel levees diminish downstream, and the levees merge with channel-mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition. The anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation. Vibracore and soil-probe profiles show the splay to be dominated by upward-coarsening lacustrine-prodeltaic mouth-bar facies sequences capped by levees and organic-rich interchannel wetland deposits. Channels, now containing sand only sporadically in narrowed or slightly migrated reaches, will eventually fill with fine sediment when abandonment s complete. The Windy Lake splay is typical of stage III splays (Smith et al. 1989), whose deposits and associated lacustrine fills form at least half of the avulsion belt. Thin, widespread, predominantly upward-coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record.

William River: An outstanding example of channel widening and braiding caused by bed-load addition

The lower William River in northwestern Saskatchewan, Canada, presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load. A relatively narrow and deep single-channel stream as it flows northward to Lake Athabasca, the river picks up a 40-fold increase of bed load over a 27-km reach as it encounters a large dune field just south of the lake. As a result of the large infusion of eolian sand, the channel develops a thoroughly braided pattern while undergoing a 5-fold increase in width and a 10-fold increase in width/depth ratio.

Holocene avulsion styles and sedimentation patterns of the Saskatchewan River, Cumberland Marshes, Canada

Abstract This paper describes the styles and sedimentation patterns of Holocene avulsions of the Saskatchewan River at the Cumberland Marshes predating the most recent (1870s) avulsion. Holocene avulsions involved two major styles: (1) progradation of lacustrine deltas and splay complexes, and (2) local reoccupation or appropriation of preavulsive channels. Most avulsions were progradational and occurred in floodplain lakes occupying floodbasin areas between elevated alluvial ridges. The two most recent avulsions (Old Channel and 1870s) diverted mainly into inundated peatland. Progradational avulsions were accompanied by deposition of avulsion-belt sediments associated with multiple channel networks. The characteristics of avulsion-belt deposits include: (1) sediments dominated by poorly sorted silt and very fine sand, (2) lateral continuity in transverse and longitudinal directions with subhorizontal upper and lower boundaries, (3) predominantly nonerosional contacts with underlying sediments, (4) coarsening-upward successions, (5) consistent radiocarbon dates for the tops of preavulsive substrates, and (6) occurrence beneath levee and overbank deposits of the associated major channel belt, indicating that avulsion-belt deposition occurs prior to the development of a new master channel. The width of ancient avulsion belts reaches 18 km, with typical thicknesses of 2–3 m. Avulsion belt deposition accompanied by many small channels was followed by flow reversion to a few larger channels and, eventually, to one single channel, accompanied by deposition of levee and channel deposits. Reoccupational avulsions were not accompanied by deposition of widespread avulsion-belt sediments and led mainly to the modification of pre-existing channel belts. The sedimentation patterns of Holocene avulsions of the Saskatchewan River reveal that avulsion-belt sediments represent major components of the floodplain deposits, supporting the model developed from observations of the 1870s avulsion.

VEGETATION, HYDROLOGY, AND SEDIMENTATION PATTERNS ON THE MAJOR DISTRIBUTARY SYSTEM OF THE OKAVANGO FAN, BOTSWANA

This study investigated the distribution and determinants of marsh vegetation along the major distributary channel system of southern Africa’s largest wetland, the Okavango Delta, using a large, inter-disciplinary data set. Eight communities were recognized, dominated respectively by Pennisetum glaucocladum, Phragmites mauritianus, Cyperus papyrus, Cyperus papyrus/Miscanthus junceus, Miscanthus junceus, Imperata cylindrica, Pycreus nitidus and a mixed bog community. The Pennisetum glaucocladum community is situated in the Panhandle (a narrow valley reach at the head of the fan) on elevated scroll bars that are flooded seasonally for short periods (days to weeks). The Phragmites mauritianus community occurs largely in the Panhandle where channel margins have high clay contents and where soils are seasonally flooded for moderate periods (months). Fires are widespread in the Panhandle, but both Pennisetum glaucocladum and Phragmites mauritianus are stoloniferous, and meristems occur below the soil surface and are protected from fire. In contrast, the rhizomatous sedge C. papyrus dominates in situations where meristems are permanently submerged, and therefore protected from fire, such as areas of open channel water where current velocities are sufficiently low to enable the extension of C. papyrus into the channel. This situation exists where channel avulsion has recently taken place or where discharge is reduced by water loss from the channel by overspill. The semi-floating habit of C. papyrus in the channel fringe results in high hydraulic conductivities, which promotes water loss from channels and leads to sediment deposition within channels. Miscanthus junceus occurs in areas where the nutrient status of water is low, seasonal changes in water level are small, and the average water level is approximately constant over decadal time scales. It occurs some distance from the channel on the upper reaches of the Delta and progressively closer to the channel downstream such that it is the dominant channel fringe species in the distal reaches. This pattern, where a community occurs progressively closer to the channel downstream is similar for communities dominated by Imperata cylindrica and Pycreus nitidus suggesting that environmental gradients (probably nutrient supply) perpendicular to the channel axis are steep and that they are mirrored by long-range environmental gradients downstream. An analysis of hydraulic characteristics of this distributary river system illustrates that channel width varies most with variation in discharge, while channel depth and current velocity are relatively constant over the range of discharges in the study area. Since channel width is primarily a consequence of vegetation processes in the channel margin, especially the growth of the giant sedge C. papyrus it is clear that channel hydraulics are affected largely by vegetation.

Geomorphic effectiveness, sandur development, and the pattern of landscape response during jökulhlaups: Skeiðarársandur, southeastern Iceland

By contrast with other historical outburst floods on SkeiTararsandur, the 1996 jokulhlaup was unprecedented in its magnitude and duration, attaining a peak discharge of f53,000 m 3 /s in <17 h. Using a combination of field sampling and remote sensing techniques (Landsat TM, SAR interferometry, airphotos, and laser altimetry), we document the sandur-wide geomorphic impacts of this event. These impacts varied widely across the SkeiTararsandur and cannot be singularly attributed to jokulhlaup magnitude because pre-jokulhlaup glacial dynamics and the extant setting largely conditioned the spatial pattern, type, and magnitude of these impacts. Topographic lowering and asymmetric retreat of the ice front during the late twentieth century has decoupled the ice sheet from the moraine/sandur complex along the central and western sandur. This glacial control, in combination with the convex topography of the proximal sandur, promoted a shift from a primarily diffuse-source braided outwash system to a more point-sourced, channelized discharge of water and sediment. Deposition dominated within the proglacial depression, with approximately 3.8*10 7 m 3 of sediment, and along channel systems that remained connected to subglacial sediment supplies. This shift to a laterally dissimilar, channelized routing system creates a more varied depositional pattern that is not explicitly controlled by the concave longitudinal profile down-sandur. Laterally contiguous units, therefore, may vary greatly in age and sediment character, suggesting that current facies models inadequately characterize sediment transfers when the ice front is decoupled from its sandur. Water was routed onto the sandur in a highly organized fashion; and this jokulhlaup generated major geomorphic changes, including sandur incision in normally aggradational distal settings and eradication of proximal glacial landforms dating to fA.D. 1892. D 2002 Elsevier Science B.V. All rights reserved.

A study of morphology and texture of natural levees—Cumberland Marshes, Saskatchewan, Canada

Abstract Field investigations of natural levees were conducted in the developing avulsion belt of the lower Saskatchewan River. Surveyed transects show that levees adjoining main-thread and crevasse channels vary considerably in size, shape, and slope away from the channel. A slope value of 0.01 was used to define the boundary between levees and adjoining backswamps. Morphometric analyses show that, in general, narrower levees tend to be steeper, and levees adjoining recently developed crevasse channels are considerably steeper and narrower than those adjoining main-thread active or abandoned channels. Levees along main-thread active channels display the largest variety of shapes and slopes. Levee deposits become finer grained from the channel towards the backswamp. This variation can be expressed quantitatively either as an exponential decrease of median diameter or as a linear increase of the percentage of sediment finer than 0.016 mm (fine silt to clay). Correlation between topographic inflections and lateral variations in the percentage of coarse sediment (mainly sand-sized), and relationships between the slope of the levee and percentage of proximally deposited sand, suggest that non-uniform deposition of coarse overbank sediment is primarily responsible for the shape and slope of the levee. Initially, banks along newly formed channels (e.g., crevasse channels) have low relief and are readily submerged by floods. Coarse sediment transferred out of the channel is deposited within a short distance from the channel margin and leads to formation of initially narrow and steep levees. As overbank deposition continues, the banks become higher, and it becomes increasingly difficult for the coarser fraction to be transferred out of channel. In contrast, finer suspended sediment is more easily transferred over the banks and across the entire floodplain; deposition in distal portions of the levee reduces the slope. Following abandonment, the channel no longer receives significant amounts of sediment; distal portions of the levee, however, continue to receive finer sediment supplied by nearby active channels during floods, and slope is further reduced. Thus, as overbank sedimentation proceeds, natural levees tend to become wider with more gentle slopes because of the different transport mechanisms and settling velocities of coarse and fine suspended sediment.