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The process of river avulsion builds floodplains and fills alluvial basins. We report on a new style of river avulsion identified in the Landsat satellite record. We found 69 examples of retrogradational avulsions on rivers of densely forested fluvial fans in the Andean and New Guinean alluvial basins. Retrogradational avulsions are initiated by a channel blockage, e.g., a logjam, that fills the channel with sediment and forces water overbank (dechannelization), which creates a chevron-shaped flooding pattern. Dechannelization waves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; many rivers show multiple dechannelizing events on the same reach. Dechannelization ends and the avulsion is complete when the river finds a new flow path. We simulate upstream-migrating dechannelization with a one-dimensional morphodynamic model for open channel flow. Observations are consistent with model results and show that channel blockages can cause dechannelization on steep (10−2 to 10−3), low-discharge (~101 m3 s−1) rivers. This illustrates a new style of floodplain sedimentation that is unaccounted for in ecologic and stratigraphic models.

Pores within organic matter (OM) are a significant contributor to the total pore system in gas shales. These pores contribute most of the storage capacity in gas shales. Here we present a novel approach to characterize the OM pore structure (including the porosity, specific surface area, pore size distribution, and water accessibility) in Marcellus shale. By using ultrasmall and small-angle neutron scattering, and by exploiting the contrast matching of the shale matrix with suitable mixtures of deuterated and protonated water, both total and water-accessible porosity were measured on centimeter-sized samples from two boreholes from the nanometer to micrometer scale with good statistical coverage. Samples were also measured after combustion at 450 °C. Analysis of scattering data from these procedures allowed quantification of OM porosity and water accessibility. OM hosts 24–47% of the total porosity for both organic-rich and -poor samples. This porosity occupies as much as 29% of the OM volume. In contrast...

A regional high-resolution CHIRP seismic survey in the Gulf of Papua (GoP) extends the geographic scope of previous studies by 125 km and reveals that the Holocene clinothem along-margin geometries, surfaces of lap, and steepness of foreset slopes appear less dependent on eustatic sea level changes and sediment supply than previously suggested. Clinothems imaged by CHIRP profiles and sampled by sediment cores include two older, relict clinothems and a younger Holocene clinothem divided into three units by two surfaces of lap. New radiocarbon ages from this study establish that depositional timing is more recent than previously proposed for the transgressive deposits, the Holocene clinothem units, and surfaces of lap. While previous ages suggested that observed stratal relationships could coincide with meltwater pulses or sea level stillstands, the younger ages from this study question these results and suggest the dominant control on Holocene clinothem architecture may be inherited physiography, SW-NE differential subsidence, along-margin currents, and seasonal variations in wave energy. Inherited physiography underlying the Holocene clinothem consists of valleys incised during relative lowstands of Marine Isotope Stages (MIS) 4 and 2. Valleys in the relict clinothem potentially served as across-margin conduits for an older Holocene clinothem unit. After the older Holocene unit filled valleys and accommodation, younger Holocene units preferentially grew along- rather than across-margin as bottom shear stresses increased. Relict clinothems have undergone differential subsidence since deposition due to foreland basin loading, which has engendered more accommodation in the northeast GoP that systematically decreases toward the southwest. Areas with less accommodation are more likely to experience toplap as they are exposed to increased shear stresses. Therefore, the surfaces of lap are caused by interactions between accommodation and oceanographic currents. Examining stratal relationships along the Holocene clinothem has broad implications for sequence stratigraphy, as many clinothems exhibit a three-dimensional stacking pattern. Measurements of seafloor slopes yield complicated results, illustrating that seafloor steepness within lobes is influenced by a complex interplay of accommodation, sediment supply, underlying physiography, and oceanographic currents. This suggests that clinothem architecture in the GoP cannot simply be interpreted as a rate-related problem between rates of sediment supply and relative sea level.

Debate continues over paleoenvironmental conditions that prevail during deposition of organic-carbon (C)-rich marine source rocks in foreland basins and epicontinental seas. The focus of disagreement centers largely on paleowater depth and the prevalence of anoxia/euxinia. The issues of paleodepth and water column conditions are important for prediction of lateral variations in source quality within a basin because the viability of a hydrocarbon play depends on a thorough understanding of the distribution of source rock quality and depositional environments. We used inorganic geochemical data from the Middle Devonian Marcellus Shale in the Appalachian Basin to illustrate interpretive strategies that provided constraints on conditions during deposition. Source evaluation typically relies on the analysis and interpretation of organic geochemical indicators, potentially also providing evidence of the degree of thermal maturity and conditions of the preservation of the organic matter. The Marcellus Formation is thermally mature, making the evaluation of the organic-carbon fraction for geologic interpretation inadequate. Because most labile organic matter has largely been destroyed in the Marcellus Formation, analysis of inorganic elements may be used as an alternative interpretative technique. Several inorganic elements have been correlated to varying depositional settings, allowing for their use as proxies for understanding the paleodepositional environments of formations. A high-resolution geochemical data set has been constructed for the Union Springs Member along a transect of cores from proximal to distal in the Appalachian Basin in central Pennsylvania using major, minor, and trace elemental data. Our results suggested that during deposition, the sediment-water interface, and a portion of the water column, was anoxic to euxinic. As deposition continued, euxinia was periodically interrupted by dysoxia and even oxic conditions, and a greater influx of clastic material occurred. Such variations were likely related to fluctuations in water depth and progradation of deltaic complexes from the eastern margin of the Appalachian Basin.

Bars and subaqueous levees often form at river mouths due to high sediment availability. Once these deposits emerge and develop into islands, they become important elements of the coastal landscape, hosting rich ecosystems. Sea level rise and sediment starvation are jeopardizing these landforms, motivating a thorough analysis of the mechanisms responsible for their formation and evolution. Here we present recent studies on the dynamics of mouth bars and subaqueous levees. The review encompasses both hydrodynamic and morphological results. We first analyze the hydrodynamics of the water jet exiting a river mouth. We then show how this dynamics coupled to sediment transport leads to the formation of mouth bars and levees. Specifically, we discuss the role of sediment eddy diffusivity and potential vorticity on sediment redistribution and related deposits. The effect of waves, tides, sediment characteristics, and vegetation on river mouth deposits is included in our analysis, thus accounting for the inherent complexity of the coastal environment where these landforms are common. Based on the results presented herein, we discuss in detail how river mouth deposits can be used to build new land or restore deltaic shorelines threatened by erosion.

Understanding river response to changes in relative sea level (RSL) is essential for predicting fluvial stratigraphy and source-to-sink dynamics. Recent theoretical work has suggested that rivers can remain aggradational during RSL fall, but field data are needed to verify this response and investigate sediment deposition processes. We show with field work and modeling that fluvio-deltaic systems can remain aggradational or at grade during RSL fall, leading to superelevation and continuation of delta lobe avulsions. The field site is the Goose River, Newfoundland-Labrador, Canada, which has experienced steady RSL fall of around 3–4 mm yr –1 in the past 5 k.y. from post-glacial isostatic rebound. Elevation analysis and optically stimulated luminescence dating suggest that the Goose River avulsed and deposited three delta lobes during RSL fall. Simulation results from Delft3D software show that if the characteristic fluvial response time is longer than the duration of RSL fall, then fluvial systems remain aggradational or at grade, and continue to avulse during RSL fall due to superelevation. Intriguingly, we find that avulsions become more frequent at faster rates of RSL fall, provided the system response time remains longer than the duration of RSL fall. This work suggests that RSL fall rate may influence the architecture of falling-stage or forced regression deposits by controlling the number of deposited delta lobes.

Predictions of a delta's morphology, facies, and stratigraphy are typically derived from its relative wave, tide, and river energies, with sediment type playing a lesser role. Here we test the hypothesis that, all other factors being equal, the topset of a relatively noncohesive, sandy delta will have more active distributaries, a less rugose shoreline morphology, less topographic variation in its topset, and less variability in foreset dip directions than a highly cohesive, muddy delta. As a consequence its stratigraphy will have greater clinoform dip magnitudes and clinoform concavity, a greater percentage of channel facies, and less rugose sand bodies than a highly cohesive, muddy delta. Nine self-formed deltas having different sediment grain sizes and critical shear stresses required for re-entrainment of mud are simulated using Deflt3D, a 2D flow and sediment-transport model. Model results indicate that sand-dominated deltas are more fan-shaped while mud-dominated deltas are more birdsfoot in planform, because the sand-dominated deltas have more active distributaries and a smaller variance of topset elevations, and thereby experience a more equitable distribution of sediment to their perimeters. This results in a larger proportion of channel facies in sand-dominated deltas, and more uniformly distributed clinoform dip directions, steeper dips, and greater clinoform concavity. These conclusions are consistent with data collected from the Goose River Delta, a coarse-grained fan delta prograding into Goose Bay, Labrador, Canada. A reinterpretation of the Kf-1 parasequence set of the Cretaceous Last Chance Delta, a unit of the Ferron Sandstone near Emery, Utah, USA uses Ferron grain-size data, clinoform-dip data, clinoform concavity, and variance of dip directions to hindcast the delta's planform. The Kf-1 Last Chance Delta is predicted to have been more like a fan delta in planform than a birdsfoot delta.

We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal-fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1-D governing equation. The results show that with steady fluvial and tidal forcings, an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth while the river-dominated portion of the estuary experiences an increasing bed degradation. The fluvial-marine transition is identified by a “tidal length” that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel-shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening, and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hypersynchronous alluvial estuaries cannot be in equilibrium.

Organic-carbon–rich shales of the lower Marcellus Formation were deposited at the toe and basinward of a prograding clinothem associated with a Mahantango Formation delta complex centered near Harrisburg, Pennsylvania. Distribution of these organic-carbon–rich shales was influenced by shifts in the delta complex driven by changes in rates of accommodation creation and by a topographically high carbonate bank that formed along the Findlay-Algonquin arch during deposition of the Onondaga Formation. Specifically, we interpret the Union Springs member (Shamokin Member of the Marcellus Formation) and the Onondaga Formation as comprising a single third-order depositional sequence. The Onondaga Formation was deposited in the lowstand to transgressive systems tract, and the Union Springs member was deposited in the transgressive, highstand, and falling-stage systems tract. The regional extent of parasequences, systems tracts, and the interpreted depositional sequence suggest that base-level fluctuations were primarily caused by allogenic forcing—eustasy, climate, or regional thermal uplift or subsidence—instead of basement fault reactivation as argued by previous workers. Paleowater depths in the region of Marcellus Formation black mudrock accumulation were at least 330 ft (100 m) as estimated by differences in strata thickness between the northwestern carbonate bank and basinal facies to the southeast. Geochemical analysis indicates anoxic to euxinic bottom-water conditions. These conditions were supported by a deep, stratified basin with a lack of circulation.

The Critical Zone (CZ) incorporates all aspects of the earth's environment from the vegetation canopy to the bottom of groundwater. CZ researchers target processes that cross timescales from that of water fluxes (milliseconds to decades) to that of the evolution of landforms (thousands to tens of millions of years). Conceptual and numerical models are used to investigate the important fluxes: water, energy, solutes, carbon, nitrogen, and sediments. Depending upon the questions addressed, these models must calculate the distribution of landforms, regolith structure and chemistry, biota, and the chemistry of water, solutes, sediments, and soil atmospheres. No single model can accomplish all these objectives. We are designing a group of models or model capabilities to explore the CZ and testing them at the Susquehanna Shale Hills CZ Observatory. To examine processes over different timescales, we establish the core hydrologic fluxes using the Penn State Integrated Hydrologic Model (PIHM) – and then augment PIHM with simulation modules. For example, most land-atmosphere models currently do not incorporate an accurate representation of the geologic subsurface. We are exploring what aspects of subsurface structure must be accurately modelled to simulate water, carbon, energy, and sediment fluxes accurately. Only with a suite of modeling tools will we learn to forecast – earthcast -- the future CZ.

[1] In this work, extensive numerical simulations have been performed to assess the hydrodynamic and morphodynamic behavior of a river jet debouching in a large quiescent water body. A refined three-dimensional grid has been used to capture the transition zone between a stable jet and an unstable meandering jet. The model results show that the stability number S, which is a function of friction and river mouth aspect ratio, and the mouth Reynolds number are the two parameters that describe the stable/unstable character of the jet. From a morphodynamic point of view, a stable jet always tends to form a mouth bar. However, a decrease of the stability number together with jet instability increase the delivery of sediments to the jet margins, favoring the formation of subaerial levees and elongated channels. Frictional effects play a major role to set the distance at which the mouth bar becomes stagnant. The importance of the stability number in setting depositional patterns at the river mouth is larger than other variables (i.e., momentum of the jet and potential vorticity) and therefore should be considered in the design of restoration schemes for deltaic land.

[1] Regolith-mantled hillslopes are ubiquitous features of most temperate landscapes, and their morphology reflects the climatically, biologically, and tectonically mediated interplay between regolith production and downslope transport. Despite intensive research, few studies have quantified both of these mass fluxes in the same field site. Here we present an analysis of 87 meteoric 10Be measurements from regolith and bedrock within the Susquehanna Shale Hills Critical Zone Observatory (SSHO), in central Pennsylvania. Meteoric 10Be concentrations in bulk regolith samples (n = 73) decrease with regolith depth. Comparison of hillslope meteoric 10Be inventories with analyses of rock chip samples (n = 14) from a 24 m bedrock core confirms that >80% of the total inventory is retained in the regolith. The systematic downslope increase of meteoric 10Be inventories observed at SSHO is consistent with 10Be accumulation in slowly creeping regolith (~ 0.2 cm yr−1). Regolith flux inferred from meteoric 10Be varies linearly with topographic gradient (determined from high-resolution light detection and ranging-based topography) along the upper portions of hillslopes at SSHO. However, regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick, near the base of hillslopes. Meteoric 10Be inventories at the north and south ridgetops indicate minimum regolith residence times of 10.5 ± 3.7 and 9.1 ± 2.9 ky, respectively, similar to residence times inferred from U-series isotopes in Ma et al. (2013). The combination of our results with U-series-derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on SSHO hillcrests.

Delta morphology is thought to be controlled by factors such as river discharge, tides and waves. Numerical modelling shows that sediment cohesion also strongly influences the development of a delta’s characteristics. The morphologies of the world’s deltas are thought to be determined by river discharge, tidal range and wave action 1. More recently, sea-level rise 2,3 and human engineering4 have been shown to shape delta evolution. The effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4,5,6. In particular, the role of sediment cohesion, which is controlled by sediment size and type of vegetation, is unclear. Here we use a numerical flow and transport model7,8,9,10 to show that sediment cohesiveness also strongly influences the morphology of deltas. We find that, holding all other factors constant, highly cohesive sediments form bird’s-foot deltas with rugose shorelines and highly complex floodplains, whereas less cohesive sediments result in fan-like deltas with smooth shorelines and flat floodplains. In our simulations, sediment cohesiveness also controls the number of channels that form within the deltas, and the average angle of bifurcation of those channels. As vegetation generally acts as a cohesive agent, we suggest that deltas that formed before the expansion of land plants in the Devonian period should show fan-like characteristics, a finding consistent with the limited data from the sedimentological record11.

River deltas create new wetlands through a continuous cycle of delta lobe extension, avulsion, and abandonment, but the mechanics and timing of this cycle are poorly understood. Here we use physical experiments to quantitatively define one type of cycle for river-dominated deltas. The cycle begins as a distributary channel and its river mouth bar prograde basinward. Eventually the mouth bar reaches a critical size and stops prograding. The stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees. An avulsion occurs as a time-dependent failure of the levee, where the largest average bed shear stress has been applied for the longest time (R 2 = 0.93). These results provide a guide for predicting the growth of intradelta lobes, which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas.

Natural levees control the exchange of water between an alluvial channel and its floodplain, but little is known about the spatial distribution and evolution of levee heights. The summer 2005 flood of the Saskatchewan River (Cumberland Marshes, east-central Saskatchewan) inundated large areas of floodplain for up to seven weeks, forming prominent new deposits on natural levees along main-stem channels. Measurements of flood-deposit thickness and crest heights of 61 levee pairs show that the thickest deposits occur on the lower pre-flood levee in 80% of the sites, though no clear relationship exists between deposit thickness and magnitude of height difference. Only 16% of the pairs displayed thicker deposits on the higher levee, half of which occurred at sites where relatively clear floodbasin waters re-entered turbid channels during general flooding. Difference in crest elevation (ΔE) between paired levees is approximately log-normally distributed, both before and after the flood, though with different mean values. Supplemental observations from tank experiments indicate that during near-bankfull flows, temporally and spatially variable deposition and erosion occur on levees due to backwater effects associated with nearby channel bars and irregular rises of the channel bed forced by channel extension. During floods, preferential deposition in lows tends to even out crest heights. Copyright © 2009 John Wiley & Sons, Ltd.

Natural levees of the Columbia River near Golden, British Columbia, were investigated to identify the mechanisms that control levee development and morphology. Topographic profiles of 12 levee pairs were surveyed, and measurements of water-surface elevation, flow velocity, flow direction and turbidity were obtained during an average magnitude flood (1AE2 years recurrence interval). Sedimentation rates and grain-size distributions were measured from sediment traps placed along levee-to-floodbasin transects. Results show that water and sediment exchange between the channel and floodbasin was mainly by advection. During flooding, local floodbasins behave more as efficient water pathways than water storage features, resulting in down-valley floodbasin flows capable of limiting basinward growth of levees. Levee shape results primarily from two independent factors: (1) maximum channel water stage, which limits levee height; and (2) floodbasin hydraulics, which control width. In the Columbia River, the competence of floodbasin flows results in relatively narrow and steep levees. Natural levees grow under two general conditions of deposition as governed by flood-stage elevation relative to levee-crest elevation: front loading and back loading. During large floods when crests are inundated, front loading preferentially aggrades the proximal portions of levees with sediment directly from the channel, thus increasing levee slope. During average or below-average floods when many levee crests are not overtopped, back loading preferentially aggrades the distal levee areas and floodbasin floor, reducing levee slope. In the study area, a balance between front and back loading sustains these narrow and steep levee shapes for long periods, reflecting an equilibrium between hydraulic regime, floodplain morphology and deposition.

We here investigate the sedimentology of the early Danian (ca. 66-64 Ma) Salamanca Fm. in the north-central San Jorge Basin, southern Chubut Province, Patagonia, Argentina, in order to place the outstandingly diverse and well-preserved fossil floras it contains into specific environmental settings. These assemblages are among very few of Danian age from the entire Southern Hemisphere and thus provide critical data about geographic variation in recovery from the end-Cretaceous extinction. Understanding the depositional context of the Salamanca floras is necessary for comparison with other assemblages and for interpreting their exceptional preservation. The Salamanca Fm. was deposited above a widespread erosional sequence boundary (SB-1) resulting from a relative base level rise and widespread marine transgression during the early Danian (Chron C29n). In response to this increase in accommodation space, a broad, shallow estuary formed that most likely extended westward at least as far as the San Bernardo belt. A transgressive systems tract was deposited in this estuary, consisting of bioturbated sand fining upwards to silt. The maximum marine flooding surface at the beginning of the highstand systems tract is defined by well laminated,unburrowed, clay deposits of a low energy, deep shelf. The Salamanca highstand systems tract (HST) consists of sandy and silty facies capped by accreting subtidal bars and sandy shoals containing an abundance of tidal indicators, suggesting deposition proximal to the San Jorge paleo-estuary head. A second sequence boundary (SB-2), formed during Chron C28r and early C28n, separates the older highstand deposits from younger lowstand and transgressive deposits. These consist of estuarine sand shoals, trough cross-bedded sands deposited in aggrading, fluvially-influenced tidal channels, tidal flat muds, and bayhead deltas. The best preservation of compression floras and petrified trees occurred near the tops of subtidal bars below SB-2; at the end of the shallowing-upward cycle that caps the second HST; and in fluvially-influenced tidal channels, tidal flat mudstones, and bayhead deltas of the lowstand and transgressive systems 48 tracts that lie above SB-2. These settings were proximal to the source forests and had rapid rates of burial. We interpret the dark muds of the Banco Negro Inferior, which cap the Salamanca Formation, as a late transgressive and highstand systems tract deposited during a time of rising groundwater table and declining river slopes in a widespread, lowland coastal forest. Fil: Comer, Emily E.. State University of Pennsylvania; Estados Unidos Fil: Slingerland, Rudy L.. State University of Pennsylvania; Estados Unidos Fil: Krause, Javier Marcelo. Museo Paleontológico Egidio Feruglio; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Iglesias, Ari. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina Fil: Clyde, William. University of New Hampshire; Estados Unidos Fil: Raigemborn, María Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina Fil: Wilf, Peter. State University of Pennsylvania; Estados Unidos

We performed sensitivity experiments using an ocean general circulation model at two stages of the Late Ordovician (Caradoc, ∼454 Ma; Ashgill, ∼446 Ma) under a range of atmospheric pCO2 values (8–18× PAL; pre-industrial atmospheric level) at high and low sea level. The model results indicate that the long-term cooling trend during the Late Ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pCO2, sea level change, and paleogeographic change. These results also explain the occurrence of low latitude cool-water carbonates in North America. In all simulations, a drop in sea level led to a reduction in poleward ocean heat transport. This indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the Late Ordovician. Alterations in poleward ocean heat transport linked to changes of atmospheric pCO2 also indicate that there is a threshold of 10× PAL, above which ocean current change cannot be responsible for glaciation in the Late Ordovician. Continental drift could explain the observed global cooling trend in the Late Ordovician through a combined poleward ocean heat transport feedback and increased ice-albedo effect if atmospheric pCO2 was low during the entire Late Ordovician. The model results further indicate that the response of meridional overturning to changes in paleogeography, atmospheric pCO2, and sea level is stronger than the response of surface circulation to these perturbations. Because the overturning circulation is so strong, meridional overturning was the dominant mechanism for described changes in heat transport in the Late Ordovician.

Natural levees are common features of alluvial river systems, yet their origin and evolution are poorly understood. In this paper, we present morphologic and sedimentologic data from two anastomosed rivers and offer a hypothesis of natural levee growth in these systems based on mechanisms of sediment transport. In settings where floodbasins fill at the same rate as the channel, levees form by turbulent diffusion of suspended sediment away from a high-velocity thread into a floodbasin of relatively stagnant water. The theory of diffusive transport suggests that these levees should be narrow, steep, and display abrupt decreases in grain size due to rapid decreases in carrying capacity with distance into the floodbasin. In environments where an appreciable water surface slope is maintained between the main channel and the floodbasin, levees form by advection of sediment out of the channel and into the floodbasin. Advective transport theory indicates that these levees should be broad and gently sloped, with grain sizes gradually decreasing away from the main channel. Natural levees occurring in the anastomosed reach of the upper Columbia River in SE British Columbia display significantly different morphology from levees in the Cumberland Marshes region of the lower Saskatchewan River in east-central Saskatchewan. At the upper Columbia site, the rise in stage of the floodbasin water nearly keeps up with that of the channel water because of good communication between channel and floodbasins through crevasses. This inhibits the establishment of a water surface slope, and the bulk of the escaping sediment is deposited close to the channel. In contrast, the Cumberland Marshes region is characterized by wide and volumetrically large floodbasins. These conditions keep floodbasin water surface elevations relatively low and maintain an appreciable elevation head between the channel and its floodbasin, fostering levee growth by advective transport.

▪ Abstract Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.

The forces governing marine circulation of a meridional transcontinental seaway is explored with the Princeton Ocean Model. The Jurassic Laurasian Seaway, which connected the low-latitude Tethys Ocean with the Arctic Sea is modeled quantitatively. The global ocean is found to have a profound influence on seaway dynamics. A north-south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow. When the Tethys waters were the denser water, the net seaway flow was southward, and conversely, it was northward for denser Arctic waters. Marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by Boreal faunal groups and reduced salinities several times in the Jurassic. The model results suggest that this can be explained by southward flowing seaway currents, which may have been related to an oceanic thermohaline circulation where no northern high-latitude deep convection occurred.

Several major avulsions of the Saskatchewan River have occurred in the Cumberland Marshes (east-central Saskatchewan) during the past few thousand years. The most recent avulsion occurred in about the 1870s, converting over 500 km2 of floodplain into a belt of anastomosing channels, splay complexes, and small lakes, a region that is still evolving today. The avulsion began near the tip of a large meander bend by following a small outflowing creek (Sturgeon River) which in turn followed an abandoned former channel of the Saskatchewan River. Flow began to permanently divert out of the Saskatchewan when a narrow strip of floodplain separating the Sturgeon River from the nearby Torch River became breached. Diversion into the connected Sturgeon-Torch began to increase sometime in the 1870s and probably culminated around 1882. The triggering event for the avulsion may have been a chute cutoff of the meander bend, shown by numerical modeling experiments to have significantly raised water-surface elevations at the avulsion site. Increasing flow diversion soon overwhelmed the smaller Sturgeon-Torch channel (now known as the New Channel), and several crevasse splays formed to help accommodate avulsive discharge. Sixteen kilometres downstream, most of the avulsive flow spilled out of the New Channel to form a shallow (~1 m), marshy floodplain lake which flowed eastward down the regional floodplain gradient to the basin presently occupied by Cumberland Lake. Since its inception, the avulsion-generated lake has become gradually infilled by prograding splay complexes fed by networks of anastomosing channels to characterize most of the present-day avulsion belt.

Recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion. The sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution. The GOLEM model simulates basin evolution under the action of weathering processes, hillslope transport, and fluvial bedrock erosion and sediment transport. Results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change. An increase in runoff intensity (or a decrease in vegetation cover) will lead to a rapid expansion of the channel network, with the resulting increase in sediment supply initially generating aggradation along the main network, followed by downcutting as the sediment supply tapers off. By contrast, a decrease in runoff intensity (or an increase in the erosion threshold) will lead to a retraction of the active channel network and a much more gradual geomorphic response. Cyclic changes in runoff intensity are shown to produce aggradational-degradational cycles that resemble those observed in the field. Cyclic variations in runoff also lead to highly punctuated denudation rates, with denudation concentrated during periods of increasing runoff intensity and/or decreasing vegetation cover. The sediment yield from threshold-dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes, a finding which underscores the need for caution in interpreting modern sediment-yield data.

The rate of sediment influx to a basin exerts a first-order control on stratal architecture. Despite its importance, however, little is known about how sediment flux varies as a function of morphotectonic processes in the source terrain, such as fold and thrust growth, variations in bedrock lithology, drainage pattern changes and temporary sediment storage in intermontane basins. In this study, these factors are explored with a mathematical model of topographic evolution which couples fluvial erosion with fold and thrust kinematics. The model is calibrated by comparing predicted topographic relief with relief measured from a DEM of the Central Zagros Mountains fold belt. The sediment-flux curve produced by the Zagros fold belt simulation shows a delay between the onset of uplift and the ensuing sediment flux response. This delay is a combination of the natural response time of the geomorphic system and a time lag associated with filling, and then subsequently uplifting and re-eroding, the proximal part of the basin. Because deformation typically propagates toward the foreland, the latter time lag may be common to many ancient foreland basins. Model results further suggest that the response time of the bedrock fluvial system is a function of rock resistance, of the width of the region subject to uplift and erosion, and, assuming a nonlinear dependence of fluvial erosion upon channel gradient, of uplift rate. The geomorphic response time for the calibrated Zagros model is on the order of a few million years, which is commensurate with, or somewhat larger than, typical recurrence intervals for episodes of thrusting. However, model experiments also highlight the potential for significant variations in both geomorphic response time and in sediment flux as a function of varying rock resistance. Given a reasonable erodibility contrast between resistant and erodible lithologies, model sediment flux curves show significant sediment flux variations that are related solely to changes in rock resistance as the outcrop pattern changes. An additional control on sediment flux to a basin is drainage diversion in response to folding or thrusting, which can produce major shifts in the location and magnitude of sediment source points. Finally, these models illustrate the potential for a significant mismatch between tectonic events and sediment influx to a basin in cases where sediment is temporarily ponded in an intermontane basin and later remobilized.

Many storm beds are constructed of silt/sand layers interbedded with mud. The coarse sediment fraction originates from reworking of marine sands and/or erosion of the coastal active zone, which extends from fair-weather wave base to the beach berm or coastal dune. Observations and modelling studies show that some sand is removed from the active zone to the inner shelf during extratropical and tropical cyclones. On continental shelves that have large wave events superimposed on offshore nearbottom flow, this coarse material is incrementally transported across the shelf. Storm waves and swell sort this sediment during transport and thus produce storm deposits in waterdepthsof5–80m.ObservationsofstormbedsintheGulfofMexicoindicateinitial stormbedthicknessesofmillimetrestodecimetres.Theseobservationsaresupportedby event-scale numerical models, which also reveal the interaction of oceanographic and geological factors in generating storm beds. Historical records for hurricanes in the Gulf ofMexicosuggestrecurrenceintervalsontheorderof10yearsforstorm-beddeposition. For typical Gulf of Mexico environments, a storm bed must exceed 10cm in initial thickness in order to survive physical and biological reworking. These results are compared toastorm-dominatedsequencefrom theCretaceoussystemofUtahforwhich the preservation interval for storm beds is estimated to be 266 years. By using the recurrence interval for great storms from the Gulf of Mexico, a preservation rate of less than 20% is estimated for storm beds from the past.

Developing a unified, predictive science of surface processes requires a quantitative understanding of critical surface-dynamics processes. An efficient approach to acquire this understanding is community modeling, defined here as the collective efforts of individuals to code, debug, test, document, run, and apply a suite of modeling components coupled in a framework or community modeling system. The modeling components each consist of modular code, commonly with a standardized interface to allow different modules to communicate with other components written in a different programming language. The framework is a set of agreed-upon protocols that allow the components to function together. Because of the framework, users can assemble components coded and vetted by specialists into complex models tuned to their specific objectives. The advantages of community modeling are efficient use of community resources and more effective integration of scientists and software specialists.

Erosional escarpments common features of high-elevation rifted continets. Fission track data suffest that these escarpments form by base level lowering and/or marginal uplift during rifting, followed by lateral retreat of an erosion front across tens to hundreds of kioometers. Previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins. Yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat. In this study we present a nonlinear, two-dimensional landscape evolution model tha tis used to asses the necessary and sufficient conditions for long-term retreat of a rift-generated escarpment. The model represents topography as a grid of cells, with drainage networkds evolving as water flows across the grid in the direction of steepest descent. The model accounts for sediment production by weathering, fluvial sediment transport, bedrock channel erosion, and hillslope sediment transport by diffusive mechanisms and by mass failure. Numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift. Model results suggest that the necessary and sufficient conditions for long-term escarpment retreat are (1) incising bedrock channels in which the erosion rate increases with increasing drainage area, so that the channels steepen and propagate headward; (2) a low rate of sediment production relative to sediment transport efficiency, which promotes relief-generating processes over diffusive ones; (3) high continental elevation, which allows greater freedom for fluvial dissection; and (4) any process, including flexural isostatic uplift, that helps to maintain a drainage divide near an escarpment crest. Flexural isostatic uplift also facilitates escarpment, thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift. Of all the above conditions, high continental elevation is common ot most rift margin escarpments and may ultimately be the most important factor.

Measurements of fluvial bedrock incision were made with submillimeter precision in the East Central Range of Taiwan, where long-term exhumation rates and precipitation-driven river discharge are independently known. They indicate that valley lowering is driven by relatively frequent flows of moderate intensity, abrasion by suspended sediment is an important fluvial wear process, and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style.

Some event beds (tempestites) are thought to be emplaced on shallow marine shelves by the combined action of strong coastal currents and high waves during fairly short-lived storms. To test this hypothesis, a storm sedimentation system has been constructed from six numerical models describing a cyclonic wind field, three-dimensional coastal circulation, wind waves generated over the continental shelf, the combined effect of steady and oscillatory currents within the benthic boundary layer, suspended and bed load transport of sediment, and conservation of the seafloor. This model system is used to hindcast winds, currents, waves, and resulting sedimentation during Tropical Storm Delia, which passed over the Texas-Louisiana shelf on September 3–4, 1973. Sensitivity to the initial substrate is investigated in four experiments using uniform silt, uniform sand, a mud line at the 20-m isobath, and a simplified modern sediment distribution. Modeled coastal currents are vertically uniform and do not reveal the structure predicted by the mid-latitude geostrophic storm circulation model, because the predicted depth of the wind-mixed layer is greater than the water depth over the shelf. Shelf currents in excess of 2 m/s flow predominantly along the coast to the southwest during most of the storm, driven by the wind stress and the trapped coastal wave which peaks at 180 cm near Galveston. Significant wave heights reach 8 m on the outer shelf but are less than 4 m over the inner shelf. These waves combine with steady currents to produce bed shear velocities which locally exceed 20 cm/s. The region of highest stresses always lies to the right of the storm track (viewed down the path) and moves across the shelf with the eye of the storm. Three general sediment transport paths are evident: (1) onshore transport of finer sediment over the outer shelf to the right of the storm track, (2) westward-directed along-shelf transport of predominantly fine sediment between approximately 40-m and 20-m water depths, and (3) minor offshore transport of sand from the shoreface to depths less than 30m. The resulting event bed has a ragged appearance with a maximum thickness of about 20 cm in region 1, and covers an area of approximately 3×104 km2 to the right of the storm track. Aside from local transport associated with finer sediments, these results are relatively insensitive to initial sediment type. Comparison of model results to observed data from Buccaneer platform shows that the different models performed adequately during the peak of the storm, except for a significant underprediction of the significant wave height by the wind sea model. The estimated uncertainty in the calculated combined shear stresses u*, based on errors produced by the individual model components, is most dependent on the wave bottom orbital amplitude. The total uncertainty in u* is estimated to be approximately 7%.

A one‐dimensional numerical model of sediment routing is derived to simulate erosion, transport, and deposition of individual size‐density fractions in the bed material within a relatively straight, nonbifurcating alluvial channel. The reach of interest is subdivided into a number of longitudinal elements of varying width‐averaged properties. During each time step, flow depths and velocities in each element are determined from the gradually varied flow equation using the standard step method for backwater calculations. The bedload transport rate of each size fraction is calculated from a modified Bagnold equation implementing a novel approach that takes into consideration the effects of turbulent fluctuations in the bed shear stress. Critical shear stresses for entrainment of particles from the mixed bed are determined using relationships that treat grain protrusion and hiding. The transport of particles in suspension is modeled using a convection‐diffusion sediment continuity equation, either explicitly ...