Sediment-laden River Research Articles

Sedimentary erosive processes and sediment dispersal in Kaoping submarine canyon

The Kaoping submarine canyon, connected to the Kaoping River in the coastal plain in SW Taiwan, continues the dispersal path of modern Kaoping River sediments, from an active small mountainous drain basin to the receiving basin of the South China Sea. Using seismic reflection sections, Chirp sonar profiles, and bathymetric mapping, we reveal characteristic erosive processes responsible for multiple cut-and-fill features, deeply entrenched thalweg, and sediment dispersal that are closely related to turbidity currents in the canyon. The river-canyon connection setting, along with extreme climatic conditions and active tectonism, is favorable for generation of turbidity currents at the canyon head. The upper reach of the Kaoping Canyon is distinguished into three distinct morpho/sedimentary features. The canyon head is characterized by V-shaped axial thalweg erosion. The sinuous segment of the upper reach is dominated by a deeply incised canyon pathway with trough-like morphology. Relatively small-scaled features of cut-and-fill associated with the dominant incision process are commonly along the canyon floor, resulting in a flat-floored pathway. Sliding and slumping dominated the steep canyon walls, producing and transporting sediments to canyon floor and partially filling up canyon thalweg. The meandering segment is characterized by erosive features where deeply down-cutting occurs in the outer bend of the major sea valley, forming V-shaped entrenched thalweg. The recurrences of turbidity currents have allowed continuous incision of the canyon head and have kept the connection between the canyon head and the river mouth during Holocene highstand of sea level. The upper reach of the Kaoping Canyon is linked to drainage area and maintains as a conduit and/or sink for terrigenous and shallow marine material. Sediment-laden river plume operates in the Kaoping River-Canyon system, with turbidity currents flushing river sediments into the canyon head where the canyon thalweg is the most erosive. Presently, the upper reach of the Kaoping Canyon can be considered as a temporal sediment sink.

Experiments on Wedge-Shaped Deep Sea Sedimentary Deposits in Minibasins and/or on Channel Levees Emplaced by Turbidity Currents. Part II. Morphodynamic Evolution of the Wedge and of the Associated Bedforms

Rapidly decelerating sediment-laden flows typically emplace confined sedimentary deposits. In the fluvial environment, when sediment-laden rivers reach lakes the decelerating flow emplaces a subaerial delta with distinctive topset, foreset, and bottomset deposits. In the submarine environment, turbidity currents undergoing rapid deceleration commonly emplace sedimentary wedges (i.e., deposits thinning in the downstream direction). Froude-supercritical turbidity currents have an intrinsic self-regulating mechanism for deceleration, in that the faster they flow, the more they incorporate ambient sea water through mixing at their interface. In addition, special topographic configurations, such as the entrance into a zone of much lower slope and/or lateral confinement, or the passage into a confined minibasin, may trigger sudden flow deceleration by forcing a transition to subcritical flow through an internal hydraulic jump. The present paper and its companion present experiments on a generic configuration aimed at studying the emplacement of wedge-shaped sedimentary deposits by continuous supercritical density currents. The deceleration is achieved both by natural entrainment of ambient water and by the presence of an obstructing barrier downstream. Lightweight plastic sediment was used as an analog for sand, and was transported mostly as bedload, but with some suspension, by a saline underflow. The saline underflow served as a surrogate for a turbidity current driven by fine mud that does not easily settle out. The companion paper is focused on the flow patterns associated with the decelerating current. The present paper focuses on the depositional sequences. The decelerating supercritical flows produced a wedge with a distinct pattern of aggradation and progradation. In addition, a foreset-like structure is attributed to the presence of an internal hydraulic jump forced by the downstream barrier. Although they do not reproduce any specific field-scale setting, the experiments are deemed a good generic model for several wedge-shaped submarine deposits in various settings, from slope aprons to deposits in minibasins or on the external flanks of channel levees. The paper also documents the regimes of bedforms associated with the diverse flow regions. It provides the first evidence for the formation of trains of the upstream-migrating sediment waves known as cyclic steps, similar to those commonly observed on channel levees and also along the thalwegs of some steep canyons. In addition, the experiments provide convincing evidence for the formation of downstream-migrating antidunes as well.

Tropical-cyclone-driven erosion of the terrestrial biosphere from mountains

The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle. Measurements of particulate organic carbon load and composition in the LiWu river, Taiwan, during cyclone-triggered floods suggest that tropical cyclones may facilitate the delivery of non-fossil particulate organic carbon to the ocean and its subsequent burial. The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle1,2,3,4. More than one third of this organic carbon flux comes from sediment-laden rivers that drain the mountains in the western Pacific region3,5. This region is prone to tropical cyclones, but their role in sourcing and transferring vegetation and soil is not well constrained. Here we measure particulate organic carbon load and composition in the LiWu River, Taiwan, during cyclone-triggered floods. We correct for fossil particulate organic carbon using radiocarbon, and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge. Floods have been shown to carry large amounts of clastic sediment6. Non-fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation7,8,9. We estimate that on decadal timescales, 77–92% of non-fossil particulate organic carbon eroded from the LiWu catchment is transported during large, cyclone-induced floods. We suggest that tropical cyclones, which affect many forested mountains within the Intertropical Convergence Zone10, may provide optimum conditions for the delivery and burial of non-fossil particulate organic carbon in the ocean. This carbon transfer is moderated by the frequency, intensity and duration of tropical cyclones.

Irrigation agriculture and the sedimentary record in the Palpa Valley, southern Peru

Abstract Irrigation agriculture is an ancient and widespread feature in regions where allogenic rivers flow through arid landscapes. This paper reports the results of sedimentological–pedological investigations in the Palpa Valley in the coastal desert of southern Peru where sediment-laden river water is used for irrigation. An adapted facies classification is used to deduce a generalised stratigraphical sequence of the valley bottom sediments. Chronological constraints from radiocarbon dating and archaeological finds are used to reconstruct sedimentation rates for fossil to recent irragric anthrosols. Actualistic case studies are used to quantify present-day irrigation sedimentation. The investigations result in a clarification of the sedimentological and geomorphic history of this valley and a quantification of irrigation-related sedimentation. Irragric anthrosols date back more than 3500 years and provide direct evidence for the beginning of irrigation agriculture in this region. It is concluded that: (i) the up to approximately 4 m thick fine-grained sediments in the Palpa Valley bottom totaling a volume of 21.4 Mio. m3 are a result of long-lasting human impact and have to be considered as artefacts of prehistoric and historic irrigation agriculture in the hyperarid coastal desert; (ii) the fine-grained sediments represent a geoarchive which holds information on the history of irrigation agriculture as well as information on possible natural disturbance by extreme fluvial events; (iii) the identification of irragric anthrosols has implications for archaeological and paleoenvironmental reconstructions.

Environmental factors that influence the distribution of coral reef fishes: modeling occurrence data for broad-scale conservation and management

To manage coral reef species, it is important to identify the factors that determine their distribution inexpensively. We identified the remotely measured environmental factors that are most influential in determining the distributions of coral reef fish species on a regional scale. Logistic regression models for 227 fish species related presence/absence data to 4 remotely determined environmental predictor variables: depth, presence of a land–sea interface, exposure, and the distance to the nearest estuary. We compared modeled Akaike information criterion (AIC) values with AIC values of randomly distributed species with different numbers of occurrences and levels of habitat specificity to evaluate model significance. Efficient species distribution models were identified for 118 predominantly habitat-specific fishes of the 227 species for which we had data. All 4 predictor variables significantly influenced the distributions of at least some fish species. Depth was the most frequently efficient variable for single variable models. For combinations of 2 predictor variables, depth and exposure, as well as depth and distance from the nearest estuary, were the prevalent predictors of fish distributions. Several fish species responded to the combination of variables distance from an estuary and presence of the terrestrial–marine interface, indicating that these species depend on intact coastal reef habitat, which is in decline near the main sediment-laden rivers. Statistically significant models were predominantly developed for habitat-specific species. These habitat-specific species are of greater conservation concern than widespread species because threats affect them more severely if the threats are selectively affecting their habitat. For this reason, species distribution modeling using remotely determined environmental data may be an efficient method to build models for habitat-specific species and inform marine reserve design.

Real-time sediment monitoring in hydropower plants

Sediments have posed great challenges in the operation of Run-of-River (RoR) hydropower plants mainly built on sediment-laden rivers. The relationship between characteristics of sediments and the features of a power plant is not well known. The knowledge and experience gained so far is not enough to deal with the challenges that exist in the operation of hydropower plants with respect to sediment handling. Therefore, the need for real-time sediment monitoring in RoR hydropower plants can hardly be exaggerated. In connection with the Optimum Sediment Handling research programme, the authors were involved in sediment measurement in two of the power plants in Nepal and one in India. The real-time sediment monitoring system that was developed at NTNU and the manual water sampling techniques were employed to monitor the sediment concentration just upstream of the turbine. The sediment monitoring system was calibrated up to the level of observed concentration during the measurement period. In this paper, the authors present the procedures, instrument setup and obtained results from the real-time sediment monitoring system that was employed in these power plants.

Application of neural network and MODIS 250m imagery for estimating suspended sediments concentration in Hangzhou Bay, China

Suspended sediments concentration (SSC) in surface water derived from bottom sediment resuspension or discharge of sediment-laden rivers is an important indication of coastal water quality and changes rapidly in high-energy coastal area. Since artificial neural networks (ANN) had been proven successful in modeling a variety of geophysical transfer functions, an ANN model to simulate the relationship between surface water SSC and satellite-received radiances was employed. In situ SSC measurements from the Hangzhou Bay and the Moderate-resolution Imaging Spectroradiometer (MODIS) 250m daily products were adopted in this study. Significant correlations were observed between in situ measurements and band 1–2 reflectance values of MODIS images, respectively. Results indicated that application of ANN model with one hidden layer appeared to yield superior simulation performance (r2=0.98; n=25) compared with regression analysis method. The RMSE for the ANN model was less than 10%, whereas the RMSE for the regression analysis was more than 25%. Results also showed that different tidal situations affect the model simulation results to some extent. The SSC of surface water in Hangzhou Bay is high and changes rapidly due to tidal flood and ebb during a tidal cycle. The combined utilization of Terra and Aqua MODIS data can capture the tidal cycle induced dynamic of surface water SSC. This study demonstrated that MODIS 250m daily products and ANN model are useful for monitoring surface SSC dynamic within high-energy coastal water environments.

Characteristics of sediments and their dispersal systems along the shelf and slope of an active forearc margin, eastern Hokkaido, northern Japan

This study revealed sediment distributions and their dispersal systems in a shelf-slope setting along an active forearc margin off eastern Hokkaido, northern Japan, where the Pacific Plate is being subducted beneath the Okhotsk (North American) Plate at the 7 km deep Kuril Trench. The findings were based on analyses of the textures and structures of surface sediments, components of the sampled sand fractions, high-resolution acoustic reflection records, and sediment budgets. The studied margin has a 20–30 km wide shelf and steep (5–10°) slope marked by faults and folds associated with transpressional tectonics. The shelf sediments are classified into gravelly coarse to fine sand from the shelf edge to the uppermost slope (110–1000 m water depth), gravel and coarse sand on the shelf off Hanasaki, fine sand on the middle-outer shelf off Kiritappu, and very fine sand on the shelf off Akkeshi. The coarse sediments on the eastern shelf off Hanasaki and the shelf margin are interpreted to represent lag deposits from coastal erosion associated with a post-glacial transgression across topographic highs. The two depocenters of shelf sediments subsequent to the Last Glacial Maximum are situated in the middle–outer shelf off Kiritappu and in the inner–middle shelf off Akkeshi; these are interpreted as transgressive–highstand sequences whose locations were controlled by ridges on the shelf margin and topographic highs on the shelf. The shelf sediment thickness reaches ∼ 20 m, and the total volume is estimated at ca. 6 km 3. The slope sediments are grouped into medium–fine sand on the uppermost and upper slopes (150–2000 m water depth) and mud-dominated sediments on the upper to middle slope (1000–3000 m water depth). The sandy sediments on the slope at water depths greater than 1000 m are regarded as being derived from the shelf break by gravitational redistribution. The occurrence of diamictite-like sediments around gullies that incise into the slope indicates that mass movement and sediment gravity flows play an important role in the redistribution of sediments upon the slope. On the upper–middle slope, hemipelagic fallout and advection of turbid water from the shelf edge provide a supply of suspended material. The main contributor of sediment supply to the area is coastal erosion (1.81 Mt y − 1 ), along with a lesser contribution from fluvial input (0.10 Mt y − 1 ). During the latest Quaternary, the rates of sediment accumulation upon the shelf are estimated to have been ∼ 0.47 Mt y − 1 . The remainder of the material transported into the sea is probably redistributed to deeper parts of the slope via gravitational sediment flows or transported outside of the study area by alongshore and tidal currents. The studied shelf that kept less than a quarter of the mass from land is comparable with other active margin shelves of New Zealand and California, although they are fed by high sediment-laden rivers.

Sediment yield computation of the sandy and gritty area based on the digital watershed model

The Yellow River is well known as a sediment-laden river, which is the main reason that it cannot be controlled as easily as other rivers. Many researchers, such as Qian Ning et al., have found that the sediment load of the Yellow River comes mainly from the sandy and gritty area of the Loess Plateau. Therefore, it is very important to simulate the sediment yield in this area. This paper proposes a method to compute the sediment production in the sandy and gritty area based on the digital watershed model. The suggested model is calibrated and validated in the Chabagou basin, which is a small catchment in the study area. Finally, the model simulates the sediment yield of the sandy and gritty area in 1967, 1978, 1983, 1994 and 1997, which represents a high water and high sediment year, a mean water and mean sediment year, a high water and low sediment year, a low water and high sediment year, and a low water and low sediment year separately. The simulation results, including the runoff depth and erosion modulus, can well explain the “low water and high sediment” phenomena in the Yellow River basin. The total amount of the sediment production and its distribution generated by the model is very useful for water and soil conservation in the sandy and gritty area of the Loess Plateau.

Numerical modeling of hyperpycnal plume

When the density of sediment laden river water exceeds that of the ambient ocean water, the river plunges to the ocean floor and generates a hyperpycnal plume. Hyperpycnal plumes can travel significant distances beyond the continental shelf and may be sustained for hours to weeks. There are several Apennine Rivers in Italy that are likely to develop hyperpycnal discharges on the Western Adriatic shelf. Among them, River Tronto is a moderately ‘dirty’ river capable of producing 64 hyperpycnal flow events (lasting ⩾ 6 h) during a 100 year period. Numerical simulations of hyperpycnal events have been conducted for the Adriatic shelf near the mouth of River Tronto using a two-dimensional depth-integrated finite volume model to study the spreading of the plume and its interaction with the alongshore current. Simulation results indicate that the alongshore current has great impact on the spreading and deposition pattern of the hyperpycnal flow. Sedimentary deposits generated from a series of simulated hyperpycnal flow events have developed undulating bed forms.

Using MODIS Terra 250 m imagery to map concentrations of total suspended matter in coastal waters

High concentrations of suspended particulate matter in coastal waters directly affect or govern numerous water column and benthic processes. The concentration of suspended sediments derived from bottom sediment resuspension or discharge of sediment-laden rivers is highly variable over a wide range of time and space scales. Although there has been considerable effort to use remotely sensed images to provide synoptic maps of suspended particulate matter, there are limited routine applications of this technology due in part to the low spatial resolution, long revisit period, or cost of most remotely sensed data. In contrast, near daily coverage of medium-resolution data is available from the Moderate-resolution Imaging Spectroradiometer (MODIS) Terra instrument without charge from several data distribution gateways. Equally important, several display and processing programs are available that operate on low cost computers. The utility of MODIS 250 m data for analyzing complex coastal waters was examined in the Northern Gulf of Mexico. Using simple processing procedures, MODIS images were used to map the concentration of Total Suspended Matter (TSM). A robust linear relationship was established between band 1 (620–670 nm) MODIS Terra 250 m data and in situ measurements of TSM ( r 2=0.89; n=52; MSE=4.74) acquired during six field campaigns. This study demonstrates that the moderately high resolution of MODIS 250 m data and the operating characteristics of the instrument provide data useful for examining the transport and fate of materials in coastal environments, particularly smaller bodies of water such as bays and estuaries.

Double-thresholds in scour–fill processes and some implications in channel adjustment

Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again. Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous.

Numerical modeling study of sediment dispersal by a river plume

The Princeton Ocean model was used as a tool to examine the plume dynamics of river effluent and associated sediment dispersal of a small mountain river in southern Taiwan. A module of sediment transport was added to the model to address the advection, settling, and resuspension of sediment grains of a given size. The model was driven by synoptically observed water level variations and wind field. At the river mouth open boundary, sediment-laden river flow is prescribed to initiate the buoyant plume. The model was run for a 12-day period from late July to early August 1996 which includes two typhoon events. In this paper, three 12-h segments of the simulation using medium-grained silt (15.6m) as the sediment size are presented and discussed. Each of these segments represents plume behavior under the influence of (a) longshore tidal currents alone, (b) additional influence of northerly typhoon winds, and (c) additional influence of southerly typhoon winds. The results show that the dispersal of Tseng-wen River plume is determined by the interplay of the buoyancy of the effluent, the turbulent mixing by tide-dominated coastal flows in the lower part of the water column, and the turbulent mixing by the wind field in the upper part of the water column. The coastal jet related to the plume facilitates the dispersal of river sediment to the coast on the right-hand side of the river mouth. The grain-size composition of the river effluent also plays a role in the plume-related sediment transport because it not only determines the total amount of sediment dispersal, but also the temporal and spatial characteristics of the sediment flux associated with the plume. Other mechanisms that this model does not address, including effects of surface and internal waves should also be considered in the future.

Effects of ambient currents and waves on gravity-driven sediment transport on continental shelves

Observations from several shelf environments show that down-slope gravity-driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity-induced sediment flux. The phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment-laden rivers. The field sites examined are the Gulf of Bohai off the mouth of the Yellow River (Huanghe), the northern California shelf off the mouth of the Eel River, and the Louisiana shelf west of the mouths of the Mississippi River. Off the Yellow River, rapid down-slope transport over distances of a few km occurred when frictional resistance, induced by strong along shelf currents, was temporarily relaxed. More prolonged down-slope motion over longer distances occurred following floods of the Eel River, when wave and current agitation provided turbulence to sustain gravity-driven transport of fluid mud. On the Louisiana inner shelf, the down-slope gravity force was much weaker, but observations suggest that thin gravity flows may still have occurred in the presence of waves. A simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity-driven transport of suspended sediment. This theory is quantitatively consistent with the observations from the three sites. If the supply of easily suspended sediment is less than the capacity of ambient currents (including waves) to carry sediment, then intense turbulence limits gravity-induced sediment transport by increasing the drag at the base of the flow. When ambient currents abruptly cease, rapid down-slope transport can then occur over short distances until the sediment settles. Such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow. The maximum sustained rate of gravity-induced sediment transport occurs when ambient currents are strong, but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents. Feedback then leads to values of the gradient Richardson number (Ri) within the flow that are near the critical value of 1/4. This partially damps bottom drag, but still allows the generation of sufficient turbulence to maintain sediment in suspension. Observations also indicate systematic relationships among Ri, the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow.

Simulation of turbid underflows generated by the plunging of a river

Research Article| July 01, 2001 Simulation of turbid underflows generated by the plunging of a river Ahmed Kassem; Ahmed Kassem 1Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Jasim Imran Jasim Imran 1Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Ahmed Kassem 1Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA Jasim Imran 1Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA Publisher: Geological Society of America Received: 16 Nov 2000 Revision Received: 01 Mar 2001 Accepted: 21 Mar 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (7): 655–658. https://doi.org/10.1130/0091-7613(2001)029<0655:SOTUGB>2.0.CO;2 Article history Received: 16 Nov 2000 Revision Received: 01 Mar 2001 Accepted: 21 Mar 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ahmed Kassem, Jasim Imran; Simulation of turbid underflows generated by the plunging of a river. Geology 2001;; 29 (7): 655–658. doi: https://doi.org/10.1130/0091-7613(2001)029<0655:SOTUGB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract When the density of sediment-laden river water exceeds that of the lake or ocean into which it discharges, the river plunges to the bottom of the receiving water body and continues to flow as a hyperpycnal flow. These particle-laden underflows, also known as turbidity currents, can travel remarkable distances and profoundly influence the seabed morphology from shoreline to abyss by depositing, eroding, and dispersing large quantities of sediment particles. Here we present a new approach to investigating the transformation of a plunging river flow into a turbidity current. Unlike previous workers using experimental and numerical treatments, we consider the evolution of a turbidity current from a river as different stages of a single flow process. From initial commotion to final stabilization, the transformation of a river (open channel flow) into a density-driven current (hyperpycnal flow) is captured in its entirety by a numerical model. Successful implementation of the model in laboratory and field cases has revealed the dynamics of a complex geophysical flow that is extremely difficult to observe in the field or model in the laboratory. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Production, preservation and prediction of source-rock facies in deltaic settings

This article hypothesizes that production and preservation of source-rock type facies in deltaic systems, both landward and seaward of a coastline, is mutually exclusive, time-successive and related to the dynamics of relative sea level. Sedimentologic research in the Holocene Mississippi and Rhône delta complexes suggests that rheotrophic peats and associated organic-rich beds preferably accumulate in the accommodation space created behind landward stepping shorelines in a transgressive systems tract (TST). Such a setting also allows for a sufficient supply of recharging fresh nutrient-rich groundwater into the peat forming mires [Kosters, E.C., Suter, J.R., 1993. Facies relationships and systems tracts in the late Holocene Mississippi Delta Plain. Journal of Sedimentary Petrology 63 (4) 727–733.]. Independently carried-out quantitative paleoecological studies in the same delta systems (and in addition in the Orinoco and Po deltas) suggest that in a progradational setting (highstand systems tract/HST), seasonally discharged nutrient- and sediment-laden river waters on the shelf may give rise to anoxia or dysoxia. Subsequent overfertilization of the shelf leads to accumulation of organic-rich mud belts on the shelf [VanderZwaan, G.J., Jorissen, F.J., 1991. Biofacial patterns in river-induced anoxia. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society of London, Special Publication no. 58, pp. 65–82.]. Thus, production and preservation of source-rock type facies landward of a shoreline (as peats and related sediments) is preferred in a TST, when accommodation space and nutrient supply are landward of the shoreline. Vice versa, production and preservation of such facies seaward of a shoreline (as organic-rich shelf muds) occurs preferentially in a highstand systems tract. In that situation, accommodation space is on the shelf, where river-fed nutrients are supplied as well. This hypothesis suggests further potential for application of sequence stratigraphic concepts for improved understanding of the occurrence of source-rock type facies.

Sedimentology of the Madumabisa Mudstone Formation (Late Permian), Lower Karoo Group, mid-Zambezi Valley Basin, southern Zambia

Sediments of the Upper Carboniferous to Lower Jurassic Karoo Supergroup (∼ 4.5 km thick) were deposited in the mid-Zambezi Valley Basin, southern Zambia. The Upper Palæozoic Lower Karoo Group in this area ends with a Late Permian sedimentary unit called the Madumabisa Mudstone Formation. The formation is 700 m thick and comprises four lithofacies grouped into two facies assemblages, collectively interpreted as lacustrine deposits. Sediments of a massive mudrock facies assemblage were deposited from suspension, probably from sediment-laden rivers entering a lake. Concretionary calcilutite beds probably mark the positions of palæosediment-water interfaces where calcite was precipitated. A laminated mudrock facies assemblage is attributed to lacustrine deposition from inflowing rivers at the lake margins and shallow parts of the lake. Repeated thickening-upward cycles are evidence of upward shallowing, interrupted by events of more abrupt deepening. Sandstone interbeds are interpreted as fluvial deposits laid down during low lake stands, with cross-lamination and asymmetrical ripples indicating current rather than wave deposition. A fossil assemblage of ostracods, bivalves, gastropods, fish scales, the alga Botryococcus sp. and fossil burrows is consistent with a lacustrine origin for the formation.

Problems of reservoir sedimentation in China

As there are many heavily sediment-laden rivers in China, with high sediment concentration and a large quantity of sediment load, the sedimentation problems of the reservoirs built on those rivers are so serious that the amount of sediment deposited in the reservoirs is great and the rate of sedimentation is accelerated. According to the statistics, up to the end of 1981, a total amount of 11.5×109m3 of sediment were accumulated in those reservoirs, i.e. 14.2% of the total designed capacity were lost. The average annual loss in storage capacity reached 2.3 percent, being the highest in the world. Silting of impounding lakes not only has an effect on the benefits of the reservoirs and seriously threatens the life of reservoirs, but also results in many environmental problems which were not fully estimated in the planning of the reservoirs. In this paper, the situation of reservoir deposition in China are described from the following aspects: 1) the characteristics of hydrology and sediment of the rivers; 2) the seriousness of reservoir sedimentation in China; 3) problems caused by reservoir deposition; 4) the methods of minimizing sediment deposition, etc.

Sedimentology of the Changuinola peat deposit: Organic and clastic sedimentary response to punctuated coastal subsidence

An extensive peat deposit on the Caribbean coast near Changuinola, Panama, has developed in an area subject to periodic earthquake-driven coseismic subsidence. Thick, low-ash, low-sulfur peat is accumulating immediately behind an aggrading and prograding barrier system and adjacent to a flood-prone, sediment-laden river. Measurements of changes in local sea level as a result of a recent (April 1991) earthquake reveal 30–50 cm of subsidence, greatest at the southeastern extent of the study area, where the peat surface is submerged to a depth of 3 m beneath the shallow waters of Almirante Bay. Transgression is proceeding from southeast to northwest, parallel to the trend of the coast and the long axes of both the peat deposit and the major sand bodies. In the eastern part of the deposit, the effects of sea-level rise are evident in the degree of humification, mineral matter, and sulfur content of mangrove and back-mangove peats offshore and immediately adjacent to the marine margin, and in peats associated with brackish tidal channels that drain the deposit. However, most of the deposit shows no indications of marine influence, even though ≈40% of the peat is below present sea level. The western section of the deposit has evolved from low-lying palm swamps, which originated in swales on the barrier bar, into an oligotrophic bog plain with a water table elevated 6.75 m above sea level. As the mire evolved, transitions in vegetation resulted in transitions in peat types. Highly humified forest-swamp and palm-swamp peats underlie and surround well-preserved, fibric sedge peats, and create a partial hydrological bounding surface that restricts subsurface drainage from the central bog. The high water table and elevated topography of the mire and the low permeability and erosion resistance of the dense, woody peat effectively insulate most of the deposit from both clastic influx and the extensive intrusion of rising marine waters. It is evident that thick peat, and hence coal, deposits can accumulate due to tectonically driven, punctuated subsidence without leaving a record of high clastic input within the peat, even immediately adjacent to environments of active clastic deposition.

Sediment Transport in the Yellow River

Yang's unit stream power formula is modified for the computation of total bed-material load in a sediment-laden river with a high concentration of fine suspended materials. Theoretical and laboratory studies indicate that modification can be made on the values of particle fall velocity, flow viscosity, and relative specific weight without any change of the coefficients used in Yang's original formula. Comparisons between computed results from the modified formula and field indicate that the modified formula can be used to compute total bed-material load in the Yellow River with accuracy.