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Predicting when turbidity currents are erosional or depositional (i.e., leaving no depositional record versus leaving a deposit) remains challenging. Here we combine observations from submarine channel morphology with a new sediment transport model to derive thresholds for net erosional, equilibrium, or net depositional flow to predict how far turbidity currents can transport different grain size classes down the channel. The approach was applied to the modern Madden and Omakere Channels, which traverse the Hikurangi subduction margin of the North Island of New Zealand. A bathymetric data set was used to establish the downstream change of channel geometry. Taking account of centripetal and Coriolis forces, the flow superelevation method was used to estimate variations in flow velocity and concentration along the channels. These parameters were used as model inputs to estimate the potential distribution of sand in the system assuming the separate cases of well-sorted and poorly-sorted sediment in suspension. The predicted sand distribution maps deposited by poorly-sorted flows in the channels show good agreement with root mean square amplitude mapping of the seafloor. These results confirm that thicker flows, and those carrying well-sorted suspensions, can bypass sediment over lower slopes than thinner flows and those carrying more poorly-sorted suspensions. The net erosion and net deposition thresholds derived from this study may help to guide and constrain predictions of potential sediment bypass zones in seafloor and subsurface systems and hence better constrain the predicted loci of deposition.

Submarine channels are the main conduits and intermediate stores for sediment transport into the deep sea, including organics, pollutants, and microplastics. Key drivers of morphological change in channels are upstream-migrating knickpoints whose initiation has typically been linked to episodic processes such as avulsion, bend cutoff, and tectonics. The initiation of knickpoints in submarine channels has never been described, and questions remain about their evolution. Sedimentary and flow processes enabling the maintenance of such features in non-lithified substrates are also poorly documented. Repeated high-resolution multibeam bathymetry between 2012 and 2018 in the Capbreton submarine canyon (southeastern Bay of Biscay, offshore France) demonstrates that knickpoints can initiate autogenically at meander bends over annual to multi-annual time scales. Partial channel clogging at tight bends is shown to predate the development of new knickpoints. We describe this initiation process and show a detailed morphological evolution of knickpoints over time. The gradients of knickpoint headwalls are sustained and can grow over time as they migrate through headward erosion. This morphology, associated plunge pools, and/or development of enhanced downstream erosion are linked herein to the formation and maintenance of hydraulic jumps. These insights of autogenically driven, temporally high-frequency knickpoints reveal that cut-and-fill cycles with depths of multiple meters can be the norm in submarine systems.

Water flowing over sand in fluvial and marine settings often results in the formation of current ripples. Found in modern and ancient deposits on Earth and Mars, ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records. Despite decades of observations of current ripples, there is no universal scaling relation to predict their size or to distinguish them from dunes. Here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows. Results show that ripples are larger with more viscous fluids, coarser grains, smaller bed shear stresses, and smaller specific gravity of sediment. The scaling relation also segregates ripples from dunes, highlighting a narrow regime of transitional bedforms that have morphologic properties and sediment transport conditions that overlap with both ripples and dunes. Our analysis shows that previous absolute size–based definitions of ripples and dunes only hold for certain conditions, such as water flows transporting siliciclastic grains on Earth. The new theory allows estimates of ripple sizes in foreign fluids and on other planets, including meter-scale ripples in methane flows on Titan or in viscous brines on Mars.

River deltas, low-lying landforms that host high concentrations of human population and ecosystem services, face a new, and mostly unknown, future over the coming decades and centuries. Even as some deltas experience decreased sediment supply from damming, others will see increased sediment discharge from land-use changes. There are proposals to actively use riverine sediment supply to build new land and counteract delta loss. We present a novel approach to understanding the morphology of deltas by quantifying the balance between river inputs and the largely overlooked ability of waves to spread sediments along the coast. Defining a fluvial dominance ratio—river sediment input versus the potential maximum alongshore sediment transport away from the delta mouth—allows a quantitative assessment of this sediment transport balance. For a series of deltas on Java, Indonesia, that exhibit a large range of sediment loads but have a homogeneous drainage lithology and wave climate, and for more eclectic global examples, shoreline deflection increases along with this fluvial dominance ratio. The fluvial dominance ratio also predicts the observed transition from cuspate, wave-dominated deltas to fluvially dominated deltas with protruding, crenulated shorelines. Not only does this approach provide a more quantitative foundation for paleoenvironmental reconstructions and delta management, perhaps more importantly, this simple metric of fluvial dominance has a predictive application in determining potential morphology of deltas created by engineered sediment diversions.

The relative importance of extreme events in the long-term exhumation of landscapes is difficult to determine; the degree to which modern records capture long-term mean process rates is therefore commonly unknown. In September 2013, a massive precipitation event in the Front Range of Colorado (USA) triggered over 1100 landslides and debris flows in a historically sedate landscape. Here we employ topographic differencing, utilizing repeat aerial lidar, to quantify net sediment transport volumes in 120 of these mass movements within an area of 102 km 2 west of Boulder, Colorado. Comparing our results against published long-term erosion rates, we find that these mass movements evacuated the equivalent of hundreds to thousands of years of hillslope weathering products. We conclude that (1) rare debris flows perform the majority of sediment transport and channel erosion within steep channels along the eastern edge of the Front Range, potentially explaining discrepancies between modern sediment yields and long-term erosion rates in such settings, and (2) the high spatial density of debris flows along the corridors bounding steep canyons suggests that the landscape switches to debris-flow dominance as knickpoints pass headward along the canyons.

Slopes in steady-state soil-mantled landscapes tend to increase downslope in a way that balances local transport capacity with the sediment supplied from progressively larger source areas. Most formulations of sediment transport due to hillslope processes scale transport rate with local slope, which produces convex-up forms that are independent of the properties of the underlying lithologies. In this study, we document soil-mantled hillslopes that show variations in slope that mimic the underlying stratigraphy. We present stratigraphic and soil-thickness measurements, topographic analyses, and numerical models to demonstrate that variations in rock type can impact the forms of these soil-mantled hillslopes if hillslope transport rates scale with local slope and soil thickness. This demonstrates that hillslope forms in soil-mantled landscapes can be influenced by the underlying lithology through a coupling between the processes that produce soil from rock and those that transport this soil downslope.

Sediment transport in mountain channels controls the evolution of mountainous terrain in response to climate and tectonics and presents major hazards to life and infrastructure worldwide. Despite its importance, we lack data on when sediment moves in steep channels and whether movement occurs by rivers or debris flows. We address this knowledge gap using laboratory experiments on initial sediment motion that cross the river to debris-flow sediment-transport transition. Results show that initial sediment motion by river processes requires heightened dimensionless bed shear stress (or critical Shields stress) with increasing channel-bed slope by as much as fivefold the conventional criterion established for lowland rivers. Beyond a threshold slope of ∼22°, the channel bed fails, initiating a debris flow prior to any fluvial transport, and the critical Shields stress within the debris-flow regime decreases with increasing channel-bed slope. Combining theories for both fluvial and debris-flow incipient transport results in a new phase space for sediment stability, with implications for predicting fluvial sediment transport rates, mitigating debris-flow hazards, and modeling channel form and landscape evolution.

The combination of large, temperate glaciers and rapid crustal convergence in the Saint Elias Mountains (southeastern Alaska, USA, and Yukon Territory and British Colombia, Canada) provides an exceptional opportunity to study the interactions between the tectonic and surface processes that have shaped most active orogens on Earth during much of the Quaternary. This research first provides a review of thermochronometric data sets recording exhumation under two major glacier systems of the Saint Elias Mountains, the Bagley-Bering and the Seward-Malaspina systems. These data sets are integrated over the single glacier systems and used in conjunction with glaciological data to investigate the interactions of glacial erosion and tectonics. Despite their proximity, the glaciological processes and geological settings of these two glacial systems differ significantly. On the east side of the orogen, sediments eroded from bedrock underneath the Malaspina Glacier reflect regions of rapid erosion under the slowly moving Seward Ice Field. Because the Seward Ice Field overlies a localized zone of major faulting and rapid exhumation, the strained and fractured bedrock is primed for erosion. On the west side, the Bering Glacier is the primary outlet for the Bagley Ice Field, which covers half of the crest of the orogen; however, few if any of the sediments at its terminus originate from under the Bagley Ice Field. Sediment transport is likely hindered by subglacial freeze-on processes that reduce the sediment-carrying capacity of subglacial rivers, though glacial surges are likely exceptions that deposit sediment far beyond the active margin of the glacier. Our study concludes that the widely invoked concepts of glacial erosion should be used with caution, as oversimplification can fail to account for important site-specific differences in geologic and glacial conditions.

Gravity-driven flows on the seafloor are the largest, yet least well understood, sediment transport agents on Earth. Recent exploration wells in ultradeep basins have revealed the presence of large sandy submarine fan systems of enigmatic facies types, many hundreds of kilometers from paleocoastlines. These sedimentary deposits often defy conventional turbidite or debrite interpretations, having a character suggestive of deposition from flows with transient turbulent-laminar rheologies. In the Wilcox Formation (Gulf of Mexico), inferred transitional flow deposits have distinctive stratigraphic stacking patterns, from fine-grained debrites to coarser grained turbidites. The vertical sequence of beds is here inferred to reflect the longitudinal bed distribution in response to lobe progradation, and demonstrates a transition from well-mixed turbulent flow, to progressively more rheologically stratified flow, and eventually to fully laminar flow. The progressive development of internal rheological boundaries resulted in a high-concentration but fluidal basal layer, and an upper quasi-laminar layer with an overriding sheared dilute turbidity current. The long runout of the flows is linked to their high silt and clay content; it is most likely flow expansion at the channel-lobe transition that drives flow transformation. This process-based model may be applicable to many deep-water settings and provides a framework within which to interpret the stratigraphic and spatial distribution of these complex deposits.

Clastic injectites are widely recognized in deep-water stratigraphic successions, although their sediment transport processes, propagation direction, and depth of injection are poorly constrained. Understanding how they form is important, as injectites are increasingly being recognised as significant components of sedimentary basin-fills, yet are not predicted by standard sedimentary facies models. Here, analysis of features on the margins of exhumed clastic sills and dykes, and clasts within them, enables their genesis to be determined. A diverse array of diagnostic structures is found on the margins of injectites in the Karoo Basin, South Africa, where the net direction of injection and position of the parent sand are well constrained. Injectite margin features include mudstone clast-rich surfaces, planar or smooth surfaces, blistered surfaces, and parallel and plumose ridged surfaces. Combined, these features are critical in distinguishing injected sands, where injectites are strata-concordant, from those of primary deposition. All features are indicative of propagation through brittle, very fine-grained sediments, at depths where the applied shear stress is at least four times the tensile strength of the host rock. Additionally, the presence of parallel ridges, plumose ridges, and steps allows local fracture propagation to be constrained, and in turn injection direction. The features described provide evidence that sands were injected at considerable depth in closed fractures with limited capacity for flow dilution and turbulence enhancement. Calculated Reynolds numbers, lack of erosion at injectite walls, and the presence of mud clasts at the top and base of sills, indicate that many flows were likely fully laminar during injection. The sedimentary features of these confined, relatively deep, laminar flow-induced injectites are very different from injectites that reach the surface and produce extrudites. Surface-linked injectites are associated with open conduits where a greater fraction of carrier fluid to particles can be accommodated, enabling highly turbulent, lower-concentration flows.

We investigate rocky coastline evolution over millennial time scales using exploratory analytical and numerical models based on interactions between beaches and sea cliffs. In the models, wave-driven sea-cliff retreat is a nonlinear function of beach width, where cliff retreat is maximized by sediment abrasion and minimized by either a lack of beach sediment or too much sediment (which prevents waves from reaching the sea cliff). As sea cliffs retreat, beach sediment is produced and distributed alongshore by wave-driven sediment transport, and local beach widths determine future cliff retreat rates. Numerical experiments indicate that through such interactions, rocky coastlines can reach an equilibrium configuration where headlands and embayments remain stable through time, even in the absence of alongshore variations in sea-cliff lithology. Furthermore, the equilibrium coastline configuration, or the alongshore proportion of rocky headland to cliff-backed pocket beach, can be predicted analytically. Initial tests suggest that predictions match well qualitatively with actual landscapes. © 2011 Geological Society of America.

Aeolian dune development is influenced by feedback between surface properties and sediment transport, yet little is known about the larger scale temporal and spatial natures of this relationship. Surface moisture is particularly influential, and is generally recognized in aeolian environments for its ability to increase the critical shear velocity required to entrain sediment in beach settings or, alternatively, to sustain vegetation and stabilize surfaces at a dune-field scale. However, conceptual models and field work have alluded to its importance in protodune initiation, while field observations infer that seasonal moisture input may contribute to residual dune ridge formation at the dune-field scale. This has the potential to reveal geomorphic adaptation to variations in climate, and identify a recognizable signature in the rock record. This article presents a simulation model that produces geomorphological features similar to field observations and is capable of examining the implications of surface and transport feedback at both scales. Results (1) reveal the control of surface moisture at different temporal scales, (2) display complexity in the development of multiple spatial scales within a cellular automaton framework, (3) highlight the importance of transient sand strips and sediment supply frequency in aeolian transport dynamics and protodune development, and (4) explore the relationship and significance of feedback duration, development time, and bedform spatial scale in the development of incipient dunes. This study illustrates the importance of considering geomorphic feedback when assessing the influence of surface moisture in aeolian process–dominated systems.

Recent studies show that paleoenvironmental (allogenic) signals preserved in the stratigraphic record may be contaminated or overprinted by internally generated (autogenic) sedimentation. This is problematic, but it is unclear over what temporal and spatial scales autogenic patterns are most prevalent. We propose that scale breaks in basin-fi lling trends can be used to identify the transition between allogenic and autogenic stratigraphy. Using data from numerical and physical experiments and an ancient outcrop, we explore how compensation, the tendency for sediment transport systems to preferentially fi ll topographic lows, varies with stratigraphic scale. Object-based models demonstrate the temporal scales at which stratigraphy changes from being partially infl uenced by autogenic processes to being completely determined by allo- genic forcings and suggest that this transition occurs at a time scale set by the maximum scale of surface roughness in a transport system divided by the long-term aggradation rate. This hypothesis is validated in a physical experiment where delta topography was monitored along fl ow-perpendicular transects at a high temporal resolution relative to channel kine matics. The strength of compensation in the experiment changes at the predicted time scale, where the maxi mum surface roughness is equal to the depth of the experimental channels. Above this compensation time scale deposits stack purely compensationally, but below this time scale deposits stack somewhere between randomly and deterministically. Similar scale-dependent stacking is also observed in the Ferris Formation (Cretaceous-Paleogene, Hanna Basin, Wyoming, United States). This study demonstrates that scale-dependent compensational stack- ing may be useful for isolating allogenic and autogenic stratigraphy in sedimentary basins.

Liquid water and ice are the dominant agents of erosion and sediment transport in most actively growing mountain belts. An exception is in the western Qaidam basin along the northeastern margin of the Tibetan Plateau, where wind and windblown sand have sculpted enormous yardang fields in actively folding sedimentary strata. Here, we present observations suggesting that since the late Pliocene, wind episodically (during glacial and stadial periods) removed strata from the western Qaidam basin at high rates (>0.12–1.1 mm/yr) and may have accelerated rates of tectonic folding. Severe wind erosion likely occurred during glacial and stadial periods when central Asia was drier and the main axis of the polar jet stream was located ~10° closer to the equator (over the Qaidam basin), as predicted by global climate models. Reconstructed wind patterns, the estimated volume of Qaidam basin material removed by wind, and numerical models of dust transport all support the hypothesis that the Qaidam basin was a major source of dust to the Loess Plateau.

Acoustic surveys collected six months before and after the passing of a major tropical cyclone over the Swatch of No Ground submarine canyon (Bay of Bengal) revealed the formation of widespread mass failures 30 km offshore of the Ganges-Brahmaputra river delta. Mass sediment flows several kilometers wide with minimal run-out distances ( 1 km wide × 5–60 m thick) developed in areas with preexisting subsurface sediment deformation and evidence for active submarine fluid discharge. In contrast, narrow, steep-walled (7°–10°) gullies present before the storm did not fail, suggesting that the gully walls may be shear hardened by the preferential funneling of gravity flows from the Bengal shelf. Combined, the widespread mass failures and gullies are part of a rapidly accreting (5–50 cm/yr), net-aggradational canyon system that supports multiple mechanisms for sediment transport from the active river mouth to the canyon.

Naturally occurring elongate bodies that are segmented or appear to be in the process of segmentation occur in a variety of environments and scales. A simple, process-based numerical model of planform shoreline evolution demonstrates that fetch controls on alongshore sediment transport can result in the segmentation of an elongate water body into smaller, rounded lakes or ponds. The shape of elongate water bodies leaves their long coasts prone to a high-angle-wave instability in shoreline shape that results in the formation of capes that grow through interactions with one another along the same coast. In a numerical model, as capes extend farther offshore, a new behavior emerges, whereby capes on opposing coasts attract one another laterally as they grow, suggesting a novel mechanism for large-scale shoreline self-organization through fetch-limiting interactions. We demonstrate these interactions through analysis of local net sediment flux and coastline stability. Ensemble model runs suggest that, for a symmetric wind distribution, the initial segmentation of a water body requires four lengths per initial width, yet water bodies with higher initial aspect ratios segment to one final round water body per factor of two of the initial aspect ratio. Wave-dominated elongate water bodies with coasts consisting of clastic sediment (and a lack of vegetation) are most likely to undergo this predicted segmentation.

Submarine mass failures triggered by energetic forcing events such as hurricanes and earthquakes are relatively well studied due to the potential for infrastructure damage and tsunami generation; such failures are common on heavily sedimented margins where underconsolidated deposits are preconditioned to fail. However, studies of seafloor sediment movement between large events remain scarce. Using repeat bathymetric surveys of the Mississippi River Delta Front (MRDF), we document substantial seafloor movement in absence of major hurricanes. About 1 m/yr of deepening was observed within preexisting failures, with downslope sediment transport on the order of 10 5 m 3 /yr. Outside failure features, seafloor depths remained stable or showed minor (

The Truckee River in western Nevada has severely incised in response to a net 20 m of lake-level lowering of Pyramid Lake over the past 120 yr, leaving a suite of cut terraces that extend ~15 km upstream from the lake where incision was arrested by bedrock in the channel. Channel planform changes and terrace development were mapped over a 70 yr period using rectified aerial imagery and light detection and ranging (LiDAR) data in ArcGIS. Discrete point elevations were extracted from the LiDAR data for channel remnants on each of the different-aged terraces and combined with channel distance measurements derived from the appropriate aerial imagery to derive true channel gradients for each of the photo years. These same point elevation measurements were also resolved to a common valley distance (CVD), as is often done when studying prehistorical terrace sequences. Comparison of the true channel distance (TCD) profiles to the CVD profiles demonstrates that the TCD profiles are longer and less steep than the CVD profiles. These differences exist because resolving terrace elevations to a CVD does not account for decreasing sinuosities and straighter channel planforms through time. As base level fell to its historical low elevation, the channel became steeper, straighter, and smoother, which increased the rate of sediment transport and delivery to Pyramid Lake. Over the past 120 yr, ~60,000,000 m 3 of sediment have been removed from the bed and banks of the lower river and redeposited in the lake, temporarily increasing the sedimentation rate. Larger trenches through deltaic and lacustrine deposits occupied by rivers and streams flowing into Pyramid Lake and other closed basins may reflect similar rapid responses to lowering base levels at times in the past. Therefore, lowering lake levels at Pyramid Lake and elsewhere during relatively dry times may be at least partly responsible for transient increases in sedimentation rates in lacustrine archives.

High-resolution multibeam bathymetry and chirp (compressed high-density radar pulse) seismic data acquired from an autonomous underwater vehicle outline in unprecedented detail the shape and near subbottom character of the axial channels within upper Monterey and Soquel Canyons (offshore California, USA). In Monterey Canyon, the bathymetric data span water depths from 100 m to >2100 m, and include the confluence with Carmel Canyon at ∼1900 m water depth. The bathymetric data for Soquel Canyon begin close to the canyon head at 100 m water depth and extend down to the intersection with Monterey Canyon. The seafloor within the axis of Monterey Canyon is covered with sediment fill out to 910 m water depth. Below this water depth exposures of underlying strata are common, presumably because of decreasing sediment drape and generally increased erosional resistance of the pre-canyon host strata. The seafloor within the axial channel of upper Soquel Canyon is smooth and contains horizontally layered sediment fill. In contrast, the sediment fill within the incised portions of the axial channel of Monterey Canyon is characterized by distinctive crescent-shaped bedforms down to the limit of the surveys. These differences in morphology and texture correspond with the contrasting cohesive strength of the sediments filling these canyons and the increased propensity for weakly cohesive sands and gravels in Monterey Canyon to fail. Episodic movement of coarse-grained sediments down Monterey Canyon maintains a longitudinal gradient of ∼1.6°. The more cohesive fine-grained sediments in Soquel Canyon stabilize the seafloor and maintain a substantially higher longitudinal gradient (3°–6°) than that measured in Monterey Canyon. The textural and lithologic data, plus previously published observations, indicate that upper Monterey Canyon is currently active, whereas upper Soquel Canyon appears to be inactive as a coarse sediment transport conduit. Episodic seabed sediment failures in active submarine canyons are hypothesized to control the gradient of the axial channel. The propensity for sediment failure in weakly cohesive coarse-grained sediments results in shallower horizontal gradients compared to submarine canyons stabilized by more cohesive fine-grained sediments.

Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feedbacks between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change.

River deposits, in which dune cross-bedding is widely preserved, reflect temporal and spatial variations in flow and/or sediment transport conditions that are in turn strongly coupled to climate and/or human impact. Understanding these flux variations is critical in reconstructing paleoenvironments and predicting future changes. I introduce a new approach to quantitative interpretation of fluvial deposits by linking two recent theories that relate: (1) the geometry of the preserved dune cross-sets to the formative dune-bed topography, and (2) the active dune-bed topography to sediment-transport rate. On the basis of previously obtained experimental data, I propose a method to reconstruct the probability distribution of dune-bed surface elevation from the cross-strata preserved under net aggrading conditions. This approach is applied to an ancient fluvial-dune deposit, where the curve expressing the predicted morphodynamic conditions fits within results from experimental and modern natural data. The findings are potentially important for a wide variety of geosciences studies.

Landscape evolution models are widely used to explore links between tectonics, climate, and hillslope morphology, yet mecha- nisms of hillslope erosion remain poorly understood. Here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion. In our experimental hillslope, disturbance-driven sediment transport rates increase nonlinearly with slope due to dilation-driven gran- ular creep, and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding. We use spectral analysis to quantify the variability of sediment flux and estimate the slope- dependent transition from creep to landsliding. The power spec- trum of sediment flux steepens with hillslope gradient, exhibiting fractal 1/f scaling just below the creep-landsliding transition. By evolving the experimental hillslope under fixed base-level boundary conditions, we demonstrate how disturbance-driven transport gen- erates hillslope convexity. The transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model, as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep-dominated transport.

Computer modeling suggests that thrusting order can be discriminated on the basis of foreland basin geometry and parasequence stacking patterns. The position of sequential thrust sheets relative to the basin affects the amount and distribution of subsidence, source terrain elevation and distance from the shoreline, and sediment transport into the basin. For this study, a forward numerical model was constructed to quantitatively simulate the interaction of geologic processes active in the foreland basin setting, including thrusting and orogenic development, lithospheric flexure, orogenic denudation, sediment transport across the coastal plain, and nearshore deposition. In order to isolate the effects of tectonics and sediment supply on nearshore deposition, eustatic sea level is held constant. Model runs simulate two different thrusting sequences: foreland stepping (in sequence) and hinterland stepping (out of sequence). When in-sequence thrusting is modeled, sediment is trapped in intra-orogen basins. Because sediment is prevented from reaching the shoreline, the basin is underfilled. When out-of-sequence thrusting is modeled, sediment is transported from the orogen to the shoreline. Episodes of thrusting result in subsidence and marine flooding surfaces, and resulting accommodation space allows for parasequence development. Comparing model output to stratigraphy observed in eastern Utah suggests that Late Cretaceous out-of-sequence uplift and thrust loading resulted in aggradational and progradational parasequence stacking in the Campanian Blackhawk Formation. This result supports the theory that the Pavant thrust system was active prior to deposition of the Blackhawk Formation and that the location of thrust shortening shifted toward the hinterland prior to or during Blackhawk Formation deposition.

Since the 1991 eruption of Mount Pinatubo, the specific sediment yields from watersheds draining its slopes have been the highest ever recorded. In spite of this overwhelming sediment load, rivers inundated by pyroclastic-flow deposits delivered almost half of the initial deposits to downslope alluvial and/or debris fans by 1996. Although most of this transport occurred by hyperconcentrated flows and debris flows (lahars), very high sediment transport rates also characterize low flow. Measured flow velocities and depths, channel properties, and the size of both mobile and stable clasts in the Sacobia and Pasig-Potrero Rivers indicate median bed-surface grain sizes of 2 to 8 mm, grain-size-independent dimensionless critical shear stresses ( τ*c) of 0.016 to 0.041 and Manning’s n values of 0.017 to 0.024, values well below those previously reported for steep mountain channels. The dramatic bed-surface fining, bed mobility, and selective transport in these extremely sediment-rich channels indicate that changes in grain size, τ*c values, and bed roughness can increase transport capacity in response to high sediment supply. Our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain-size‐dependent selective transport as end-member concepts that apply to channels with low (or intermittent) and high (or continuous) sediment supply, respectively.

Spacing is a time-varying characteristic of bedform fields deriving from the behavior of defects (ends of crest lines) in the bedform pattern. In a model based on this hypothesis, crest line length is lost and spacing increases because bedform defects, which are smaller in height and faster migrating than surrounding bedform crest lines, merge with larger bedforms as defects migrate through bedform fields. Spacing in large bedform fields asymptotically increases with the logarithm of time as pairs of oppositely facing defects meet and annihilate. Spacing in small bedform fields, such as flumes, exponentially approaches a fixed value as defects are eliminated at the boundaries of the field. Model predictions are compatible with observed spacing of transverse bedforms, ripples formed under waves and linear dunes, calling into question the widespread assumption that bedform spacing approaches a steady-state value characteristic of fluid flow and sediment transport over two-dimensional bedforms.

A systematic technique for sequential restoration of salt-related extensional structures incorporates Airy isostasy to quantify both the loading subsidence and the change in water depth during any time interval. The results are used to generate paleo-sea-floor templates for restoration and to reconstruct evolving salt and sediment geometries. Paleobathymetries calculated by the restoration technique are compatible with paleobathymetric environments derived from biofacies analysis of benthic foraminifers sampled in five wells along a reflection seismic profile from western Eugene Island, offshore Louisiana, northern Gulf of Mexico. Furthermore, both aggradation and progradation rates measured from the restorations are consistent with regional depositional patterns. The results demonstrate that the restoration method adequately determines paleobathymetry and sea-floor paleotopography. Restoration of multiple sections can provide a framework for the estimation of sediment transport and deposition geometries and facies development. 26 refs., 3 figs.

Results from laboratory experiments show that ice at its melting temperature regelated through dense, three-dimensional arrays off clasts abrading a rock bed. Regelation theory for an isolated, stationary array of clasts successfully approximates periods when ice separated from the bed, thermal gradients, and clast-bed contact forces, despite the relative motion of clasts and incoming heat from the bed. The process may entrain sediment at glacier beds and restrict deposition to zones where sediment in ice is concentrated, effective pressure is low, and basal melting is rapid.

Existing theories of off-bank sediment transport cannot account for rapid rates of sedimentation observed in Bahama bank and Florida shelf periplatform environments. Analysis of the physical processes operating during winter cold fronts suggests that accelerated off-bank transport of shallow-water mud may be achieved by sinking off-bank flows of sediment-charged hyperpycnal (super-dense) platform waters.

A short-duration, high-intensity rainfall on a 0.43-km 2 (105-acre) alpine drainage basin in the central Colorado mountains produced a peak discharge, including water and sediment, of 108 to 134 m 3 /s (3,810 to 4,730 cfs). This exceeds by an order of magnitude the discharge of the 100-yr flood as predicted by the procedures recommended by the U.S. Soil Conservation Service and exceeds by a factor of two the largest discharge ever reported in a basin of similar size. The Manning formula and superelevation calculations were two methods used to compute velocities and discharges. These two methods depend upon independent sets of assumptions, but they produced similar results. Calculations of the fluid-dynamic stresses required to transport boulders in the upper channel also suggest flood depths and velocities within the range of values computed through use of the Manning formula and superelevation calculations. In the lower channel, bed load moved as inertial grain flows. One flow studied in detail transported sediment onto a part of the alluvial fan not reached by deep flood water. The larger clasts were transported to the top and front of the grain flow. This type of sorting supports a theory of grain-flow deformation first suggested by Bagnold.

Field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin, migrating accumulations of sediment, and coarse grains cluster at their leading edge. These accumulations are one or two coarse grains high and are much longer (0.2-0.6 m long in sand; 0.5-2.0 m in fine gravel) than their height. The authors propose the term bedload sheet for these features, and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment. In essence, coarse particles in the bedload slow or stop each other, trap finer particles in their interstices, and thus cause the coarse particles to become mobile again. Bedload sheets develop on the stoss side of dunes, causing the dune to advance incrementally with the arrival of each sheet. Successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain-size sorting that distinguishes cross-bedding. Available flume experiments and field observations indicate that bedload sheets are a common, but generally unrecognized, feature of heterogeneous sediment transport.

An unusual stratigraphic unit (nicknamed “homogenite”) fills topographic lows in the complex ridge and trough bathymetry at two survey sites on the Western Mediterranean Ridge and the Calabrian Ridge. On near-bottom 4-kHz seismic-reflection profiles, this unit us an acoustically transparent, near-surface, flat-lying layer, whereas in cores, it is a homogeneous gray marl as much as 7 m thick. Grain size decreases upcore within the unit, implying that it was deposited in a single event controlled by gravitational settling. The stratigraphic position of the homogenite relative to a firmly dated sapropel bed suggests emplacement between 4400 and 3100 yr B.P. The source of the homogenite is inferred to be the nearby basin walls. Farther east, two other sites with similar rugged topography lack homogenite entirely. A triggering mechanism is required which is capable of initiating massive sediment transport simultaneously in many separate basins at the western two survey sites, but which is not effective at the eastern sites. A large archeologically recorded earthquake of the correct age is considered and rejected because its epicenter is closer to the nonhomogenite-bearing sites than to the sites where this sediment type was observed, and because several other earthquakes of comparable magnitude have since been recorded in the area, whereas the homogenite is unique. The 3,500 yr B.P. collapse of the caldera of the volcano of Santorini caused a huge tsunami which is recorded archeologically and geologically around the eastern Mediterranean. Because of refraction of the tsunami by the bathymetry, and because the caldera collapsed in its southwest corner, a disproportionate amount of tsunami energy was directed toward the western area where homogenite is observed. In contrast, the homogenite-free sites were relatively sheltered. An order-of-magnitude calculation shows that the near-bottom oscillating currents accompanying the Santorini tsunami were at or above the threshold erosion velocity at the homogenite-bearing sites. In addition, the near-bottom pressure pulse under the tsunami at the homogenite-bearing sites was sufficient to cause liquefaction of sediments. Neither mechanism was adequate to cause sediment transport or slope failure at the homogenite-free sites.

During experiments on sediment transport and resistance to flow with a uniform 0.33-mm sand, data were recorded on the movement of individual rocks having intermediate diameters from about 0.1 to 0.5 foot. The experiments were conducted in a flume 2 feet wide by 60 feet long and for most runs, depth was held constant at 0.5 foot. The experiments showed that rocks on the sand bed moved downstream consistently only if the flow was in the upper regime—that is, only if the bed forms were plane bed, standing waves, or antidunes. The rocks moved at velocities that were approximately half the average velocity of the water. On all bed forms in the lower flow regime (ripples, ripples superimposed upon dunes, and dunes), the rocks moved upstream and down into the bed. That is, the rocks moved into a scour pocket that formed at the upstream side of the rock. The movement upstream and down into the bed is limited by and approximately equal to the distance below the original rock position of the minimum bed elevation plus approximately half the rock diameter. The data indicate that cross-bedded sand deposits formed by the ripple or dune phases of transport would contain few, if any, pebbles or cobbles. Because the flow, in at least the downstream reaches, of most rivers is in the lower regime, the upstream movement and scour into the bed demonstrated in these experiments is an important factor in the sorting process.