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To understand the Holocene sedimentary evolution of the Pearl River associated delta–estuary–shelf system, high‐resolution seismic data were acquired from Lingdingyang Bay to the inner shelf. A Pearl‐derived Holocene subaqueous clinoform developed over the pre‐Holocene incised channel/valley network. Overlying the Holocene ravinement surface, progradational highstand and acoustically semi‐transparent or transparent transgressive system tracts are separated by a maximum flooding surface. Restricted and thick in‐channel/in‐valley retrogressive deposits represent an earlier transgressive deposit. Displaying a westward‐oriented along‐shore extent, the highstand sediment accumulated asymmetrically within 30 m water depths, corresponding to a modern hydrodynamic environment. The wedge‐shaped highstand unit thickened landward with a depocentre (>15 m) located in western Lingdingyang Bay, possibly because of higher sediment trap efficiency inside the salinity front. Preliminary analyses suggest that the Pearl River has been trapping sediment to fill its extensive estuarine system for most of the Holocene and its sediment trap efficiency is greater since the sea‐level highstand. Since ca 7.5 ka bp, the total volume of highstand sediment accumulated in the entire delta‐estuary‐shelf dispersal system was approximately 128.36 × 109 m3. The calculated highstand riverine sediment flux was 16.01 to 16.50 Mt/yr. This value is equivalent to approximately one‐fifth of the flux that occurred prior to widespread dam construction and likely related to late‐Holocene intensified anthropogenic influence in South China. Since ca 7.5 ka bp, only ca 35% of the Pearl‐derived sediment has been dispersed to offshore shelf areas from the delta‐estuary system. With the dramatic decrease in river sediment discharge in recent decades, a larger fraction of Pearl‐derived sediment transported to the shelf might have been interrupted and changed. [ABSTRACT FROM AUTHOR]

Understanding trends in estuarine sediment fluxes is of great interest to sustainable estuarine management addressing anthropogenic interferences and climate change. The long‐term sediment budget of the Western Scheldt estuary, Netherlands, is investigated by a detailed analysis of a unique and long‐term bathymetric data set and data of a three‐dimensional subsurface model ('GeoTop'). Different sediment types show contrasting transport behaviour. The Western Scheldt narrowed and deepened, while the estuary exported sand (1.5 to 2.5 million m3 per year) and imported mud (0.5 to 1.5 million m3 per year) over the 1860 to 1955 period. The eroded sand originated from the channels in the seaward part of the estuary and was dispersed in all directions. A significant amount of mud permanently deposited in the side branches, which were also gradually reclaimed. These results suggest that sediment characteristics potentially play a crucial role in deriving long‐term sediment budgets and morphodynamic behaviour of estuarine environments. Future morphodynamic sand–mud model studies may reproduce and further explain the underlying transport processes of the current study. [ABSTRACT FROM AUTHOR]

Sedimentary systems respond to environmental forcings in dissimilar ways over different timescales. The Lake Malawi (Nyasa) Rift is an ideal natural laboratory for evaluating the overall functioning of lacustrine source‐to‐sink systems on orbital or younger scales. These closed sedimentary systems exhibited high spatiotemporal climate variability, and their responses to two late Pleistocene lake‐level stillstands are evaluated. The coarse‐grained deposits documented a dramatic transgression from 350 to 200 m below present lake‐level (BPLL) developed during an important climate transition in tropical Africa. Based on an integrated analysis of a digital elevation model and high‐resolution single/multi‐channel seismic profiles, catchment geomorphology has been linked with sediment delivery in the sink area. The coarse‐grained deposition of each source‐to‐sink system is quantified through a sediment mass calculation. A modified empirically‐derived 'BQART' predictor with a bedload equation to assess the sediment discharge is employed based on a Monte Carlo simulation, considering temperature lapse rate and topographic effects. The river discharges are estimated by specific empirical relationships that associate catchment area to various climate systems, developed using a global modern river database. The results show that the total sediment discharge increases from 7.53 to 9.50 Mt year−1; likewise, the preserved coarse‐grained deposits also record a significant increase in deposition rate from 350 to 200 m BPLL stage, indicating that the short length‐scale source‐to‐sink systems are sensitive to the high‐amplitude lake transgression developed from the climate shift. The volume of upstream buffered deposits may decrease within the progressively wetter climate, while the buffering degree was substantially influenced by the pre‐existing landforms. Moreover, the substantial deep‐water mud dispersal is not well‐developed, despite the relatively higher lake‐level and slightly wetter climate. This quantitative source‐to‐sink analysis with the modified sediment predictor yields preliminary constraints for system functioning in response to high‐amplitude climate change in a closed sedimentary system. [ABSTRACT FROM AUTHOR]

Models relating sediment supply to catchment properties are important in order to use the geological record to deduce landscape evolution and interplay between tectonics and climate. Water discharge (Qw) is an important factor in the widely used 'BQART' model, which relates sediment load to a set of measurable catchment parameters. Although many of the factors in this equation may be independently estimated with some degree of certainty in ancient systems, water discharge (Qw) certainly cannot. An analysis of a world database of modern catchments with 1255 entries shows that the commonly applied equation relating catchment area (A) to water discharge (Qw = 0·075A0·8) does not predict water discharge from catchment area well in many cases (R2 = 0·5 and an error spanning about three orders of magnitude). This is because the method does not incorporate the effect of arid and wet climate on river water discharge. The inclusion of climate data into such estimations is an opportunity to refine these estimates, because generalized estimates of palaeoclimate can often be deduced on the basis of sedimentological data such as palaeosol types, mineralogy and palaeohydraulics. This paper investigates how the relationship between catchment area and river discharge varies with four runoff categories (arid, semi-arid, humid and wet), which are recognizable in the geological record, and modifies the coefficient and exponent of the above-mentioned equation according to these classes. This modified model yields improved results in relating discharge to catchment area (R2 = 0·95 and error spanning one order of magnitude) when core, outcrop or regional palaeoclimate reconstruction data are available in nonarid systems. Arid systems have an inherently variable water discharge, and catchment area is less important as a control due to downstream losses. The model here is sufficient for many geological applications and makes it possible to include variations in catchment humidity in mass-flux estimates in ancient settings. [ABSTRACT FROM AUTHOR]

The effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood. Using mountain catchment-alluvial fan systems as simple analogues for larger landscapes, a wide range of theoretical studies, numerical models and physical experiments have hypothesized that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux, grain size and down-system fining rate. However, this hypothesis remains largely untested in real landscapes. This study measures grain-size fining rates from apex to toe on two alluvial fan systems in northern Death Valley, California, USA, which each have well-exposed modern and ca 70 ka surfaces, and where the long-term tectonic boundary conditions can be constrained. Between them, these surfaces capture a well-constrained temporal gradient in climate. A grain-size fining model is adapted, based on self-similarity and selective deposition, for application to these alluvial fans. This model is then integrated with cosmogenic nuclide constraints on catchment erosion rates, and observed grain-size fining data from two catchment-fan systems, to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid-glacial and modern interglacial conditions. In a fan system with negligible sediment recycling, a ca 30% decrease in precipitation rate led to a 20% decrease in sediment flux and a clear increase in the down-fan rate of fining, supporting existing landscape evolution models. Consequently, this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over <105 year timescales. However, in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilized and recycled over a time period longer than the duration of the climatic perturbation. These analyses offer a new approach to quantitatively reconstructing the effects of past climate changes on sedimentation, using simple grain-size data measured in the field. [ABSTRACT FROM AUTHOR]

Bedform geometry is widely recognized to be a function of transport stage. Bedform aspect ratio (height/length) increases with transport stage, reaches a maximum, then decreases as bedforms washout to a plane bed. Bedform migration rates are also linked to bedform geometry, in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment. However, how bedform morphology (height, length and shape) and kinematics (translation and deformation) change with transport stage and suspension have not been examined. A series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated, mixed-load dominated and suspended-load dominated conditions. The results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage, against a continuously increasing bedform length. Bedform size variability increased with transport stage, leading to less uniform bedform fields at higher transport stage. Total translation-related and deformation-related sediment fluxes all increased with transport stage. However, the relative contribution to the total flux changed. At the bedload dominated stage, translation-related and deformation-related flux contributed equally to the total flux. As the transport stage increased, the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster. At the suspended-load dominated transport stage, the deformation flux increased and the translation flux decreased as a fraction of the total load, approaching one and zero, respectively, as bedforms washed out to a plane bed. [ABSTRACT FROM AUTHOR]

The ability to predict bedform migration in rivers is critical for estimating bed material load, yet there is no relation for predicting bedform migration (downstream translation) that covers the full range of conditions under which subcritical bedforms develop. Here, the relation between bedform migration rates and transport stage is explored using a field and several flume data sets. Transport stage is defined as the non-dimensional Shields stress divided by its value at the threshold for sediment entrainment. Statistically significant positive correlations between both ripple and dune migration rates and transport stage are found. Stratification of the data by the flow depth to grain-size ratio improved the amount of variability in migration rates that was explained by transport stage to ca 70%. As transport stage increases for a given depth to grain-size ratio, migration rates increase. For a given transport stage, the migration rate increases as the flow depth to grain-size ratio gets smaller. In coarser sediment, bedforms move faster than in finer sediment at the same transport stage. Normalization of dune migration rates by the settling velocity of bed sediment partially collapses the data. Given the large amount of variability that arises from combining data sets from different sources, using different equipment, the partial collapse is remarkable and warrants further testing in the laboratory and field. [ABSTRACT FROM AUTHOR]