Rift basins commonly host multiple coexisting and interacting sediment sources; however, most previous studies have focused on systems with a single upstream source, limiting our understanding of geomorphic and stratigraphic evolution in multi-sediment-source settings. To address this limitation-the inability of single-source based models to explain boundary migration, stratigraphic development in multi-source rift basin systems-we conducted three-dimensional tank experiments simulating two sediment sources and examined the evolution of a coupled alluvial fan-delta system under three different rates of sea-level change. The experimental results showthat the boundary between the two depositional systems oscillates in response to relative supply dominance and autogenic channel migration, with the magnitude of boundary migration increasing downstream. In addition, systematic variations in topset slope, boundary migration amplitude, and oscillation periodicity were observed as a function of sea-level change rate. These findings provide experimental constraints on the mechanisms controlling the stratigraphic development of interaction zones arising from multiple sediment sources and sea-level fluctuations, commonly observed in rift-basin depositional systems.

Alluvial fans evolve into distinct planform shapes and topographic surfaces with capricious channel migration, depending on their depositional environments. The plan view of alluvial fans is uniquely organized into a "semicircular" configuration based on their upstream boundary conditions: sediment supply, Qs, and water discharge, Qw. Specifically, the margin of the alluvial fan exhibits its own level of bumpiness, consisting of protrusions and reentrants. We define the degree of this irregularity as a "fan-margin roughness" and investigate whether it varies systematically with and can therefore constrain upstream boundary conditions. To examine the effect of discharge conditions on the planform roughness, we conducted alluvial-fan experiments using a range of sediment and water discharge rates with a bimodal sediment mixture of coarse quartz sand and fine crushed-walnut-shell sediment and quantified the margin roughness using root-mean-square deviation (RMSD). RMSD decreases when local progradation preferentially fills reentrants, while RMSD increases when local progradation preferentially generates margin protrusions. The downstream topography of alluvial fans can be classified into three hierarchical scales: bar, lobe, and basin scale. Our scaling analyses demonstrate that: 1) the characteristic roughness of the fan margin is comparable to the lobe size, 2) roughness tends to decrease exponentially with the Qs/Qw ratio, and 3) substantial spatiotemporal fluctuations of roughness are observed when the Qs/Qw ratio is low. These dependencies occur because upstream Qs and Qw influence the distribution and mobility of channels and distributary lobes. Specifically: 1) the mobility of the channel(s) increases with increasing Qs and decreasing Qw and 2) the reoccupation probability of the channel(s) increases with decreasing Qs and increasing Qw. Quantifying fan-margin roughness may provide a framework for disentangling the upstream boundary conditions of modern and ancient fans in nature.

Conventional interpretations of alluvial fan margins attribute their changes to environmental factors such as tectonic activity or climate variations. Under steady dynamic conditions, fan margin (s) is expected to grow continuously, following the time (t) dependence of s~t(1/3), based on the mass conservation. However, this study aims to propose a new concept that challenges this conventional understanding. A key finding of this research is that the alluvial fan margin can retreat even under constant upstream boundary conditions, a phenomenon significantly influenced by ‘groundwater infiltration’. This study focuses on investigating the role of infiltration process in alluvial fan evolution. Seven tank experiments with varying sediment and water discharge rates were conducted, enabling analysis of fan retreat under constant upstream boundary conditions. Fans typically exhibited continuous progradation, but a critical point was observed where runoff water no longer reached the fan margin, resulting in fan retreat. At this stage, all runoff water infiltrated into the sediment deposit. Applying Darcy’s Law, we found a strong correlation between deposit thickness (dh) and infiltration rate, assuming constant hydraulic conductivity (Ks). Based on these experimental results, a computational model was developed to simulate the alluvial fan trajectories under similar conditions. The findings provide insights into field-scale applications by accounting for infiltration processes on alluvial fans.

Lakes serve as one of the significant sinks for organic carbon. For lake deposits, it is generally accepted that water depth is a primary control on the spatial distribution of total organic carbon (TOC) accumulation because the deeper part of a lake potentially has a higher organic population to be settled. However, lake TOC distribution is often spatially variable regardless of water depth, and an exact mechanistic explanation of TOC hotspots that do not align with the deepest water depth remaines unsolved. We suggest that flocculation significantly influences the lake TOC distribution pattern. The flocculation of fine sediment is possibly a major contributor to the occurrence of high TOC in deposits, as organic material aggregates with flocculated sediment during settling. The process of flocculation positively correlates with water salinity and thus potentially enriches TOC in deposits under a high saline condition. Furthermore, depending on the extent of flocculation, a lake entering plume type is different, leading to different run-out distances. Therefore, to investigate the role of flocculation associated with the TOC pattern in a lake, we design settling and gravity flow experiments with varying salinity and sediment concentration. Considering the flocculated grain size is seldom recorded in deposits, the current study provides a possible mechanism for why sediments with similar grain size deposited at similar water depths could have varying TOC content. This leads to a better understanding of the spatial distribution of carbon storage and the formation of source rocks associated with hydrocarbon reservoirs.

Lateral channel migration is a fundamental process in natural alluvial rivers; however, the factors that control the rate of migration remain unclear. Despite its importance in shaping river morphology, the impact of water discharge on river mobility is still largely unexplored. Here, we leverage a dataset of 64 rivers across the globe to show that higher variability in river discharge and stage promotes higher rates of river migration. To reveal the physical processes behind this relationship, we focused analyses on the lowermost 500 kilometers of the Mississippi River, where a pronounced gradient in water stage variability and migration rate exists. We demonstrate that stage variability affects channel mobility by influencing the sediment size of riverbanks and thereby controlling riverbank erodibility. These results can be used to predict river responses to climate change and decipher past hydroclimates using stratigraphy from Earth and Mars.

It has been widely accepted that channel geometry on alluvial fans is predominantly controlled by upstream boundary conditions and remains stable if those conditions do not change. This study challenges that notion by examining how channel width on alluvial fans varies in space and time under constant upstream conditions. Experiments using a sediment mixture (sand and crushed walnut) with constant sediment and water discharge rates reveal distinct patterns in channel width. In lower water discharge runs, total channel width decreases over time, while in higher discharge runs, it increases. Theoretical principles suggest that channel width is inversely proportional to grain size. Sand results in a wider channel width compared to walnut sediment, making its proportion along the channel a key factor in controlling the average channel width. The disproportional advancement of sand reaches relative to the fan margin, which varies across different discharge runs, drives changes in slope and channel width over time. This study highlights that surface grain-size distribution can change as the fan grows, even when external conditions remain constant, leading to variations in channel geometry. Sedimentary records often attribute signal changes to upstream boundary variations. However, our findings highlight the critical role of intrinsic fan width dynamics, underscoring the need to consider this factor in fan evolution studies.

River avulsion is a key process in shaping landscapes, rapidly shifting channels and altering surface processes and sedimentary patterns, particularly in lowland alluvial environments. While avulsion mechanisms are well understood in humid and deltaic regions, many dryland rivers exhibit straight or concave-down long profiles in their lower reaches, with the mechanisms behind these transformations remaining unclear. In this study, the long profiles of rivers in the R & iacute;o Colorado terminus system near Salar de Uyuni, Bolivia, are investigated using high-resolution satellite imagery, in situ measurements and flume experiments. The results reveal that nodal avulsions in upstream sections are typically preceded by multiple local avulsions in the downstream reaches. Analyses of 10 river profiles, each over 10 km in length, show that three profiles transition from concave-up upstream to concave-down downstream through nodal avulsion, while the remaining seven maintain relatively straight profiles due to local avulsion. Slope analysis indicates a sharp increase in the final 4 km towards the river termini, associated with a decrease in sinuosity. A novel model is proposed in which rivers prograde basinward through cut-and-fill processes, with the landward migration of avulsion locations driven by in-channel aggradation and overbank flooding. This process divides the original concave-up long profiles into concave-up upstream and concave-down downstream segments, with avulsions occurring primarily at the upstream end of the concave-down section as the profile straightens. The flume experiments corroborate this model, demonstrating that avulsion locations in dryland river terminus systems tend to migrate landwards, driven by profile straightening and sediment redistribution. These findings offer new insights into the evolution of alluvial rivers in semi-arid and arid endorheic basins, with broader implications for understanding the formation of both terrestrial river systems and ancient fluvial landscapes on Mars.

ABSTRACT Mud deposition is acknowledged as a significant contributor to delta architecture, yet its role is often oversimplified as a constant parameter in models of delta formation. A better understanding of mud retention on deltas would resolve remaining questions regarding delta growth. This study explores how spatiotemporally varied mud retention facilitates sustained delta growth in defiance of the concept of autoretreat, that is, the idea that shoreline progradation rates decline as a delta grows due to the expansion of subaqueous and subaerial delta surfaces. This research is inspired by prior field observations of the river‐dominated Mississippi Delta, USA, where the shoreline of a ca 6000 to 8000 km 2 subdelta prograded at a constant rate for roughly a millennium, despite its expanding delta surface, compaction and sea‐level rise. For this, a laterally averaged one‐dimensional numerical model is leveraged to test hypotheses that enhanced mud retention with time in: (i) the delta bottomset; and (ii) the delta plain (floodplain) supports a constant rate of shoreline progradation in a maturing delta. Results demonstrate that enhanced mud retention in both the bottomset and delta plain facilitates sustained delta growth. Neither component by itself can replicate the case study. Yet, with these two integrated components, the model reproduces the cross‐section and linearly prograding pattern observed in the Mississippi Delta. The findings provide an autogenic mechanism for sustained delta growth and support the importance of mud as a fundamental building block of deltas that should be incorporated in delta‐growth models of engineered river diversions.

Nearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea-level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two-dimensional or unconfined three-dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea-level rise and sediment supply: progradation, autoretreat and post-autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised-valley geometries, under conditions of constant rates of relative sea-level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross-sectional profiles, valleys with triangular cross-sections tend to contain deltas that experience faster rates of progradation, autoretreat and post-autobreak transgression under rising sea level, and exhibit a more prominent convex-seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex-seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea-level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence-stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record.

For the Anthropocene to get recognized as a real geological era, first and foremost its strata must be identified.Several geological formations such as bogs, lakebeds, reefs, ice sheets, speleothems, river estuary deposits, and sea floors have been considered as potential candidates for the Anthropocene strata.This consideration arises from the emergence of novel materials associated with the Anthropocene, including radioactive isotopes, plastics, and aluminum, started to be discovered in their sediments and dramatically increased since the mid-20th century.Yet, these deposits are no longer considered 'natural' because human activities are largely controlling the transport and depositional processes from source to sink.The Municipal Solid Waste (MSW) in landfills has been also 'unnaturally' transported and deposited (landfilled) by humans.Since the 1950s, the controlled landfills have been made worldwide, and thus the opening time of the landfills is clear.The MSW layers of landfills, which appeared globally, contemporaneously, and with distinct characteristics, are indeed the 'artificial (anthropogenic)' strata showing a new and clear aspect of human influence, unprecedented in geological time.The MSW layers can be considered valuable indicators of the Anthropocene era because they not only preserve the history of human life but also sensitively demonstrate the scale of human activities like mass production, consumption, and disposal.The MSW layers can be expected to serve as a unique window into the Anthropocene.

ABSTRACT Autogenic feedbacks can produce large‐scale, organized stratigraphic patterns in alluvial fans, but autogenic depositional signatures of specific upstream boundary conditions remain challenging to interpret. Here, a combination of theory, experiment and field application is used to explore how autogenic lithofacies changes can be interpreted as stratigraphic indicators of upstream boundary conditions. Six experiments were conducted to test the effects of sediment supply and water discharge rates on autogenic advance and retreat of the lithofacies boundary (grain‐size transition) in an alluvial fan with two dominant grain sizes. Migration of the grain‐size transition caused a short‐term zigzag pattern in the grain‐size transition position in the dip‐directional deposit section. For each experiment, time‐lapse images and laser topographic scans of the fan surface and stratigraphic cross‐sections of the final deposits were used to quantify characteristic timescales of autogenic processes. Timescales for fan‐margin migration, surface wet‐fraction change and grain‐size transition migration generally shorten as sediment supply rate increases and water discharge rate decreases. Increasing the sediment supply rate shortens the duration of the fluvial sediment storage and release cycle, producing higher frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge tends to widen channels and lengthens the duration of the fluvial sediment storage and release cycle, constructing lower frequency zigzags in the grain‐size transition trajectory. Increasing the water discharge also enables more sediment to transport further downstream during release events, leading to higher magnitude zigzags in the grain‐size transition trajectory. These relationships between upstream boundary conditions and the grain‐size transition trajectory demonstrate how autogenic stratigraphic signals could be used as a tool to infer relative changes in boundary conditions.

Two contrasting sinuosity patterns were identified in lowland rivers on Earth and Mars. The channel sinuosity either substantially increases or remains constant towards the coast. These bimodal patterns reflect the age of the channels and their lateral migration rates, which are associated with sediment supply and discharge variability.

A delta is a depositional landform that is formed when sediment transported by a river is deposited in a relatively low-energy environment, such as a lake, sea, or a main channel.Among these, a delta formed at the confluence of rivers has a great importance in river management and research because it has a significant impact on the hydraulic and sedimentological characteristics of the river.Recently, the equilibrium state of the confluence area has been disrupted by large-scale dredging and construction of levees in the Nakdong River.However, due to the natural recovery of the river, the confluence area is returning to its pre-dredging natural state through ongoing sedimentation.The time-series data show that the confluence delta has been steadily growing since the dredging, but once it reaches a certain size, it repeats growth and retreat, and the overall size does not change significantly.In this study, we developed a model to explain the sedimentation-erosion processes in the confluence area based on the assumption that the confluence delta reaches a dynamic equilibrium.The model is based on two fundamental principles: sedimentation due to supply from the tributary and erosion due to the main channel.The erosion coefficient that represents the Nakdong River confluence areas, was obtained using data from the tributaries of the Nakdong River.Sensitivity analyses were conducted using the developed model to understand how the confluence delta responds to changes in the sediment and water discharges of the tributary and the main channel, respectively.We then used annual average discharge of the Nakdong River's tributaries to predict the dynamic equilibrium positions of the confluence deltas.Finally, we conducted a simulation experiment on the development of the Gamcheon-Nakdong River delta using recorded daily discharge.The results showed that even though it is a simple model, it accurately predicted the dynamic equilibrium positions of the confluence deltas in the Nakdong River, including the areas where the delta had not formed, and those where the delta had already formed and predicted the trend of the response of the Gamcheon-Nakdong River delta.However, the actual retreat in the Gamcheon-Nakdong River delta was not captured fully due to errors and limitations in the simplification process.The insights through this study provide basic information on the sediment supply of the Nakdong River through the confluence areas, which can be implemented as a basic model for river maintenance and management.

Meandering rivers have shaped the landscapes of Earth and Mars through the development of sinuous and migrating channels. River channel sinuosity reflects an interplay of primary agents including water discharge and sediment supply, information that is archived in the sedimentary record. Here we examine the spatial variability of the sinuosity of 21 lowland rivers on Earth and six ancient river systems on Mars using satellite imagery, and identify a dichotomy in spatial patterns: instead of decreasing downstream as previously suggested, we find that the sinuosity either increases or remains constant approaching the river outlet. We conduct numerical modelling of channel migration to show that these bimodal patterns can be explained as a competition between the timescale required for channels to establish steady-state sinuosity and the avulsion timescale. This highlights the role of varying water discharge on meander development and demonstrates how the planform morphology of modern and ancient fluvial systems may be used to interpret hydrological regimes of river systems, with implications for lowland river migration patterns under future shifting climate regimes. Spatial patterns of channel sinuosity near river outlets reflect the interplay between the channel migration rate and the avulsion timescale, according to sinuosity measurements of lowland rivers on Earth and Mars and channel evolution simulations.

Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation. ▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. ▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. ▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. ▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. ▪ River deltas globally might lose 5% (∼35,000 km 2 ) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario.

The loss of land in coastal regions is an emerging topic across the scientific community, as many countries struggle to minimize the consequences of an accelerating relative sea‐level rise. Various methods have been attempted to mitigate land loss, and river diversion for the Mississippi River Delta, which makes use of the natural river delta‐building process, has been proposed and significantly examined. Prior delta‐building models predicted possible ranges of new delta‐building rates and verified the feasibility of reduction in land loss on the Louisiana coast by river diversion by only considering the delta topset and foreset deposition without incorporating the muddy bottomsets because sand was regarded as the main delta‐building sediment, and mud was treated as washload. Since sand flux is significantly smaller than mud flux in most coastal rivers and muddy bottomsets are common in most deltas, it is critical to understand the depositional processes of mud in deltas. Here, we present the results of a coupled numerical modelling and flume experiment that includes a moving boundary at the foreset‐bottomset break in addition to the shoreline. We find that bottomset aggradation can accelerate the shoreline progradation by decreasing the foreset length (i.e., depth at the delta front). We also apply our model to a field scale based on parameters taken from the Wax Lake Delta. When 10%–50% of the mud supplied to the delta is retained in the bottomset, the subaerial delta area increases by 4.4%–25.4% compared to that in a delta with no bottomset accumulation. Therefore, considering the bottomset in land‐building modelling can provide more accurate predictions for a new land‐building area by river diversion.

A natural levee is a typical wedge-shaped deposit adjacent to a river channel. Given its location and distinctive features, the levee can serve as a key to revealing depositional processes of the coupled channel to floodplain system preserved in the rock record. Levee–floodplain topographic evolution is also closely linked to river avulsion processes which can cause catastrophic floods. Nonetheless, the levee geometry and its aggradation pattern on the floodplain have not been fully incorporated in the study of avulsion. Here, we present a levee-building model using advection settling of suspended sediment to reproduce the evolution of a fluvial levee over floods and to examine the effects of boundary conditions on levee geometry and the grain-size trend of the levee deposit. We further investigate river avulsion frequencies and potential channel reoccupation associated with the grain-size distribution of overbank sediment flux and the overflow velocity into the floodplain, both of which can control the levee geometry, especially the aggradation rate at the levee crest. In the modeling results, the levee develops (1) a concave-up profile, (2) an exponential decrease in grain size of the deposit away from the main channel, and (3) a relatively steeper shape for coarser sediment supply and vice versa. The subsequent scaling analysis supports that the input grain size to the floodplain and levee profile slope are positively correlated with the avulsion frequency, whereas the overflow velocity is inversely proportional to the avulsion frequency. In connection with the avulsion styles and levee geometry, we suggest that relatively steeper levee slopes tend to promote more reoccupations of preexisting floodplain channels as protecting abandoned channels from topographic healing, but relatively gentler levees are likely to create a new avulsion channel as their remnant channels are more vulnerable to the removal of topographic memory. The insights drawn from the current modeling work may thus have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood dynamics via levee deposits. Based on the roles of natural levees on the avulsion frequency and channel reoccupation, the flood hazards triggered by river avulsions as well as the alluvial architecture in sedimentary records can be better assessed.