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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.

The Selenga River Delta, Lake Baikal, Russia, is ∼600 km2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge. The delta is positioned along the deep-water (∼1600 m) margin of Lake Baikal, a half-graben–styled rift basin, qualifying it as a modern analogue of a shelf-edge delta system. This study provides a detailed field survey of channel bed sediment composition, channel geometry, and water discharge. The data and analyses presented here indicate that the Selenga Delta exhibits downstream sediment fining over tens of kilometers, ranging from predominantly gravel (coarse pebble) and sand near its apex to silt and sand at the delta-lake interface. We developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels. The findings include the following. (1) The Selenga River Delta consists of at least eight orders of distributary channels. (2) With increasing channel order downstream, channel cross-sectional area, width-depth ratio, water discharge, boundary shear stress, and sediment flux systematically decrease. (3) The downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels. (4) Over longer time scales, gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation, so that coarse sediment cannot be transported to the axis of the Baikal Rift basin. The distribution of sediment grain size in deltaic channels, as related to hydrodynamics and sediment transport, plays a critical role in influencing stratigraphy, because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits. Therefore, the Selenga River Delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy.

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.

We documented arroyo evolution at the tree, trench, and arroyo scales along the lower Rio Puerco and Chaco Wash in northern New Mexico, USA. We excavated 29 buried living woody plants and used burial signatures in their annual rings to date stratigraphy in four trenches across the arroyos. Then, we reconstructed the history of arroyo evolution by combining trench data with arroyo-scale information from aerial imagery, light detection and ranging (LiDAR), longitudinal profiles, and repeat surveys of cross sections. Burial signatures in annual rings of salt cedar and willow dated sedimentary beds greater than 30 cm thick with annual precision. Along both arroyos, incision occurred until the 1930s in association with extreme high flows, and subsequent filling involved vegetation development, channel narrowing, increased sinuosity, and finally vertical aggradation. A strongly depositional sediment transport regime interacted with floodplain shrubs to produce a characteristic narrow, trapezoidal channel. The 55 km study reach along the Rio Puerco demonstrated upstream progression of arroyo widening and filling, but not of arroyo incision, channel narrowing, or floodplain vegetation development. We conclude that the occurrence of upstream progression within large basins like the Rio Puerco makes precise synchrony across basins impossible. Arroyo wall retreat is now mostly limited to locations where meanders impinge on the arroyo wall, forming hairpin bends, for which entry to and exit from the wall are stationary. Average annual sediment storage within the Rio Puerco study reach between 1955 and 2005 was 4.8 × 10 5 t/yr, 16% of the average annual suspended sediment yield, and 24% of the long-term bedrock denudation rate. At this rate, the arroyo would fill in 310 yr.

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.

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 (