Aggradation Processes Research Articles

Late Pleistocene and Holocene sedimentary record within the Jade Bay, Lower Saxony, Northwest Germany – New aspects for the palaeo-ecological record

The present-day tidal flat sediment body of the Jade Bay in Lower Saxony, Northwest Germany, has been examined using pile-driving core data. A total of 43 cores have been drilled in the tidal flats and 36 radiocarbon samples have been incorporated into the lithological record. Additionally, pollen and macro remain samples have been analysed.The onset of the basal peat started earliest 6225 cal. BC and latest 4045 cal. BC. Pollen analysis of the basal peat show, that for the western and central Jade Bay the corresponding palaeo-environment may started with an alder carr to Cyperaceae fen peat and locally developed to Sphagnum moss peat and, before marine conditions became dominant, ending under soggy conditions. From ∼3000 to ∼2800 cal. BC, the aggradation process stopped and gave way to marine inundation advancing southwards and being accompanied by the deposition of tidal flat sediments. The sedimentary record shows that the ancient Jade Bay may be subdivided into two different facies zones, (i) a western and central part with a pronounced basal peat being up to 1.16 m thick and spanning at maximum ∼3000 years of sedentation and (ii) an eastern part representing a tidal flat to brackish-lagoonal palaeo-environment with no pronounced basal peat, but with intercalated peat. The Holocene palaeo-environment of the Jade area under discussion was strongly influenced by at least two marine ingressions, each of which left a distinct marker in the sedimentary record.

Evolution and facies architecture of Paleogene Surtseyan volcanoes on Chatham Islands, New Zealand, Southwest Pacific Ocean

This paper reports for the first time phreatomagmatic deposits and preserved Surtseyan tuff cones in the Chatham Islands, New Zealand. Fieldwork has located the relicts of at least six, closely-spaced, Paleogene Surtseyan-cones and associated volcaniclastic sediments within the Red Bluff Tuff Formation. The complete stratigraphic section of the cones consists of two parts: 1) the lower part represents volcanic aggradational processes that constructed tuff cones in a short period of time, and is composed of a bedded interval of explosively fragmented, vesicular glassy basaltic pyroclasts (ash and lapilli sizes) as well as feeder-dykes, pillow-lavas and pillow-sills and 2) the upper part represents the rapid denudation of these cones by shallow marine currents or gravity-flows reflecting the instability of the tephra-pile forming the cones, and a much later marine faunal colonization stage (e.g. corals, brachiopods, molluscs, etc.). Erosion could have occurred almost immediately after (or even during) the emplacement of the volcanic pile, similar to what occurred at Surtla vent, a satellite submerged cone of the basaltic island volcano Surtsey, Iceland; the Waiareka–Deborah Volcanics Bridge Point, Aorere Point, and Lookout Bluff Surtseyan-cones (Otago, New Zealand); and Marion and Prince Edward islands (Southwest Indian Ocean), amongst others. By contrast, the complete faunal colonization and stabilization of a diverse marine community could have taken hundreds, or perhaps even thousands of years to reach their acme following the volcanic pulses. The structural, textural and compositional characteristics of the Red Bluff Tuff Formation support a phreatomagmatic mode of fragmentation similar to that at Surtsey Volcano, Iceland. The Red Bluff Tuff sequence represents one of the most complete marine tuff cones described in the geologic record.

Investigation of the Reaction Time and the Process of Synthesis of GaN Nanopowders by Ammoniating Ga<sub>2</sub>O<sub>3</sub>

GaN nanopowders were synthesized by ammoniating pure Ga2O3 powders at 1000°C for different soaking time. The results show that the growth of GaN at this temperature is based on vapor-solid mechanism. The ammoniating reaction finished and the rearrangement process started when the soaking time is 0.5 h. When the soaking time is 1.5 h, the process of high-temperature aggradation transformed to thermal decomposition-re-crystallization process. Further increasing of soaking time, the fabricated GaN particles with large size aggregated by directional fixed nanorods are substituted by self-governed particles, which agglomerate as honeycomb. Photolumine- scence measurement (PL) test reveals that as prepared sample with 1.5 h possesses the best luminescent property.

Mid-late Holocene environments of Agua Buena locality (34°50'S; 69°56'W), Mendoza, Argentina

In southern South America the acquisition of high-quality Holocene paleoclimate data is a priority due to the paucity of complete, continuous and well dated records. Here we report preliminary results from a combined sedimentological and palynological study of an alluvial fan sequence and the laterally connected sedimentary deposits of the Vega de la Cueva profile at Agua Buena east of the Andes in central Argentina. The main geomorphological units of the area were identified and mapped based on satellite image analysis and multiple field surveys. The sedimentological and pollen results allowed us to reconstruct the development of some environments. The Agua Buena record corresponds to the distal facies of the Arroyo Bayo alluvial fan starting the aggradation process prior to ca. 4100 cal yr BP. The organic-rich levels found were formed during the development of wetlands (vegas) dominated by Cyperaceae, Juncaceae and Poaceae. These highly productive environments with almost permanent water saturation were important between 4100 and 2800 cal yr BP, indicating more stable conditions. After 2800 cal yr BP, the organic content was comparatively lower with increasing sedimentation rates that are indicative of higher fluvial discharges. This information is fundamental to interpret both the pollen and charcoal records of the area and to evaluate their representativeness and potential to reconstruct past local and/or regional vegetation.

Reconstructing middle to late Holocene palaeogeographies of the lower Messenian plain (southwestern Peloponnese, Greece): Coastline migration, vegetation history and sea level change

The glacio-eustatic sea level rise after the Last Glacial Maximum (LGM) led to a worldwide flooding of shelf areas and the evolution of marine embayments and indentations. Its significant deceleration in mid-Holocene times resulted in the overcompensation by sediment yields and shoreline progradation in many areas. At the southeastern part of the lower Messenian plain (southwestern Peloponnese, Greece) detailed investigations of the Holocene stratigraphy revealed a maximum landward shoreline displacement around 3000 BC. Subsequently, aggradational processes started to form a prominent beach ridge in the late 3rd millennium BC. This is the substratum on which the early Iron Age Poseidon Sanctuary of Akovitika was founded approx. 900–850 BC. Palaeogeographic reconstructions based on 18 corings in the sanctuary and its surroundings revealed the later extension of adjacent marshland and gradual surface levelling due to seasonal inundations. These findings represent probable reasons for the abandonment of the ceremonial site around 380–350 BC. Palynological studies of the vegetational changes provide evidence for early land use and reflect regional settlement activities. Initial degradation and burning of woodland during late Neolithic times (c. 3500–3100 BC) as well as significantly reduced human impact during the Protogeometric Dark Ages (c. 1060–900 BC) are proven. Additionally, local relative mean sea level (RSL) change since 5000 BC was reconstructed based on 14C dated paralic peats.

Tectonic controls on the geomorphic evolution of alluvial fans in the Piedmont Zone of Ganga Plain, Uttarakhand, India

The Piedmont Zone is the least studied part of the Ganga Plain. The northern limit of the Piedmont Zone is defined by the Himalayan Frontal Thrust (HFT) along which the Himalaya is being thrust over the alluvium of the Ganga Plain. Interpretation of satellite imagery, Digital Terrain Models (DTMs) and field data has helped in the identification and mapping of various morphotectonic features in the densely forested and cultivated Piedmont Zone in the Kumaun region of the Uttarakhand state of India. The Piedmont Zone has formed as a result of coalescing alluvial fans, alluvial aprons and talus deposits. The fans have differential morphologies and aggradation processes within a common climatic zone and similar litho-tectonic setting of the catchment area. Morphotectonic analysis reveals that the fan morphologies and aggradation processes in the area are mainly controlled by the ongoing tectonic activities. Such activities along the HFT and transverse faults have controlled the accommodation space by causing differential subsidence of the basin, and aggradation processes by causing channel migration, channel incision and shifting of depocentres. The active tectonic movements have further modified the landscape of the area in the form of tilted alluvial fan, gravel ridges, terraces and uplifted gravels.

Use geopedological methods for the evaluation of sedimentation rates on river floodplains, southern Québec, Canada

Abstract This article pertains to the fluvial dynamics of rivers in southern Quebec, in particular to the aggradation and pedogenetic processes observed in floodplains affected by periodic floods. The frequency of flood events, notably along the Saint-Francois River and its main tributaries, leads to fine materials being frequently deposited on floodplains and affected development of alluvial soils. Particle size and physical and chemical analyses have led to a better understanding of the dynamics involved in the formation of floodplains and the development of soils related to this fluvial environment. Also, sedimentological analyses (layer texture and thickness, microstructure) combined with radiocarbon dating (14C) and isotopic methods (210Pb, 226Rd) enabled the determination of sedimentation rates along the floodplains. The radiocarbon dating results obtained from the organic layers buried in alluvial soils show relatively variable ages, i.e. between 2210 ± 60 and 30 ± 70 years BP. The data gathered reveal an active overbank deposition, which shows evidence of the high flood recurrence in many rivers of the basin. It is estimated that the annual floodplain aggradation ranges from 1.0 to 7.6 mm yr− 1 on average, which causes increases in floodplain elevations (vertical accretion). The sedimentation rates obtained using the constant rate supply dating model (210Pb) show average values that range from 2.1 to 10.7 mm yr− 1. Also, the presence of contaminated layers at the lower level (> 100 cm) of the floodplains suggested an active sedimentation rates along the rivers affected by floods.

Ice, moraine, and landslide dams in mountainous terrain

We review recent work on ice, moraine, and landslide dams in mountainous terrain, thus complementing several comprehensive summaries on glacier dams in intracontinental and Arctic areas of low relief. We discuss the roles of tectonic and climatic forcing on ice-, moraine-, and landslide-dam formation and sudden drainage, and focus on similarities and differences between their geomorphic impacts on confined valleys drained by steep bedrock and gravel-bed rivers.Despite numerous reported failures of natural dams in mountain belts throughout the world, their relevance to long-term dynamics of mountain rivers remains poorly quantified. All types of dams exert local base-level controls, thus trapping incoming sediment and inhibiting fluvial bedrock incision. Pervasive geomorphic and sedimentary evidence of outburst events is preserved even in areas of high erosion rates, suggesting that sudden dam failures are characterized by processes of catastrophic valley-floor aggradation, active-channel widening, and downstream dispersion of sediment, during which little bedrock erosion seems to be achieved.We find that, in the absence of direct evidence of former dams, a number of similarities among the geomorphic and sedimentologic characteristics of catastrophic outburst flows may give rise to ambiguous inferences on the dam-forming process. This is especially the case for tectonically active mountain belts where there is ample and comparable potential for the formation and failure of ice, moraine, landslide, and polygenetic dams concomitant with climatic oscillations or earthquake disturbance. Hence, the palaeoclimatic implications of erroneously inferring the cause of dam formation may be significant.We recommend that future research on natural dams in mountainous terrain addresses (a) climate- and earthquake-controlled systematics in the pattern of formation and failure; (b) quantification of response of mountain rivers to catastrophic outburst events and their concomitant process sequences; (c) elaboration of a comprehensive classification of natural dams in mountainous terrain with special attention to polygenetic dams; (d) physical-based modelling of dam formation, failure, and routing of water and sediment outbursts; and (e) quantitative controls on the contribution of natural dams to sediment budgets in mountainous terrain.

Stream response to Quaternary tectonic and climatic change: Evidence from the upper Weihe River, central China

Seven terraces of upper Weihe River in the Longxi Basin on the north flank of the Qinling Mountains are capped with a succession of loess units and paleosols that correlate with the standard marine isotope chronology. Based on fieldwork and terrace ages, which were determined using loess-paleosol stratigraphy, paleomagnetic, optical stimulated luminescence (OSL) and radiocarbon dating, the results show that the fluvial fills were deposited during marine oxygen isotope stages (MISs) 22, 20, 18/16, 12, 6, 4 and 2, respectively, whereas terracing occurred during glacial–interglacial transitions. This, in turn, points to eccentricity-related climatic changes that may have driven recurrent incision and aggradation processes and significantly influenced the formation of staircase terraces of Weihe River, through the direct and indirect influence of both temperature and precipitation on fluviatile activity. On the basis of terrace ages and its heights, incision rates were enhanced from 865 to 412 ka (∼0.27 m/ka), decreased from 412 to 128 ka (∼0.09 m/ka), and increased from 128 ka to present (∼0.32 m/ka).

Productivity increase in Northern Austria Norway spruce forests due to changes in nitrogen cycling and climate

AbstractWe investigated the long‐term temporal trend in growth rate, soil chemical status, and nutrient content of needles of two Norway spruce (Picea abies) stands in the Bohemian Massif, Austria. The aim was to quantify changes in the site productivity over the last four decades as a consequence of the enriching effect of N deposition, rising CO2 levels in the atmosphere, and changes in forest‐management practices. We used the data records of control plots from forest‐amelioration experiments that have been monitored for more than four decades. Both stands showed increased growth rates and a large deviation from the growth pattern of earlier applicable yield tables. The nutrient levels in the foliage remained unchanged and neither suggested luxury consumption nor nutrient imbalances. Results from soil chemistry analysis were inconclusive in respect of changes in soil conditions: an enrichment of the mineral soil with N and a decrease in the C : N ratio. Changes were confined to the uppermost part of the soil profile. Our data support the hypothesis that the sites are in a steady process of aggradation and that site productivity is rising.

Environmental changes and human settlement in the central Marches (Italy) during the early-middle Holocene

In the Early Holocene, the central Marches (Italy) underwent a widespread development of forest cover and soils which drastically reduced slope degradation and related aggradation processes in riverbeds. In such conditions, streams deeply incised the previous alluvial sediments. In the Apennine sectors, linear erosion was locally prevented by the growth of travertine dams in correspondence with river channel knickpoints and waterfalls. Fluvial deposition prevailed in the peri-Adriatic hilly belt, where river valleys still extended from the present coastline to the uprising Adriatic Sea, as testified by a 50-m-deep cored log, drilled near the Potenza River mouth. Several archaeological sites, ranging from the Mesolithic to the Aeneolithic, testify the recurrent presence of small-scale human groups on the alluvial plains of the lower valley sectors. Sandy-clayey sediments, emplaced by flooding episodes, repeatedly buried these settlements which were commonly located on the riversides. Small Aeneolithic communities were also present in the mountain sectors, around travertine-dammed swampy-lacustrine basins. On top of the sequences, Bronze Age sites were locally found. Widespread deforestation started in the early Iron Age (about 3000 yr BP), when alluvial plains and terraces were permanently occupied by large-scale human settlements. Geo-archaeological evidence of systematic deforestation at 2950 ± 50 14C yr BP, consisting of numerous round-shaped 1.5-3 m wide hollows, filled with soil sediments and upturned blocks of alluvial gravels, were found on top of a fluvial terrace in the Esino River basin. From the Iron Age to recent historical times, notwithstanding the progressive increase of debris supply to the drainage systems due to the spreading agricultural-pastoral activities, erosion dominated almost everywhere in the Marches rivers, likely induced by climatic factors.

Analisis fisiográfico para el levantamiento pedológico semidetallado

Dentro de un levantamiento edafológico semidetallado, se realizó un análisis fisiográfico del área entre los municipios de La Unión, La Victoria, Toro y Obando (Departamento Valle del Cauca, Colombia). El objetivo del trabajo fue la evaluación de la utilización de la metodología del análisis fisiográfico y su clasificación por el sistema CIAF. En el sector se encuentran dos provincias fisiográficas: la Depresión Cauca-Patía y la Cordillera Occidental. En la primera dominan los procesos agradacionales, mientras en la segunda predominan los degradacionales. En la zona plana, los elementos que permitieron el reconocimiento de las unidades de paisaje son: canales de drenaje, uso de la tierra, condición de drenaje y procesos pedogenéticos, que en algunas unidades se manifestaron a partir de cambios de tonalidades observadas por fotointerpretación. En estos sectores el relieve tuvo muy poca importancia debido a que los desniveles en el terreno son de baja significación. En el sector de mayor relieve los elementos que permitieron diferenciar las distintas unidades de paisaje fueron: la configuración de la superficie (relieve y pendientes compuestas), vegetación natural, rocas, patrón de drenaje, material parental y denudación. Se concluye que la metodología aplicada y la clasificación fisiográfica adoptada contribuyeron a entender los procesos que originaron estos paisajes.

STREAMBANK EROSION ASSOCIATED WITH GRAZING PRACTICES IN THE HUMID REGION

The effects of cattle grazing on stream stability have been well documented for the western portion of the U.S.,but are lacking for the east. Stream and riparian damage resulting from grazing can include alterations in watershedhydrology, changes to stream morphology, soil compaction and erosion, destruction of vegetation, and water qualityimpairments. However, few studies have examined the successes of best management practices (BMPs) for mitigating theseeffects. The objective of this project was to assess the ability of two common BMPs to reduce streambank erosion along acentral Kentucky stream. The project site consisted of two replications of three treatments: (1) an alternate water source anda fenced riparian area to exclude cattle from the stream except at a 3.7 m wide stream ford, (2) an alternate water source withfree stream access, and (3) free stream access without an alternate water source (i.e., control). Fifty permanent cross-sectionswere established throughout the project site. Each cross-section was surveyed monthly from April 2002 until November 2003.Results from the project indicated that the incorporation of an alternate water source and/or fenced riparian area did notsignificantly alter stream cross-sectional area over the treatment reaches. Rather than exhibiting a global effect, cattleactivity resulted in streambank erosion in localized areas. As for the riparian exclosures, changes in cross-sectional areavaried by location, indicating that localized site differences influenced the processes of aggradation and/or erosion. Hence,riparian recovery within the exclosures from pretreatment grazing practices may require decades, or even intervention(i.e., stream restoration), before a substantial reduction in streambank erosion is noted.

The Amspoort Silts, northern Namib desert (Namibia): formation, age and palaeoclimatic evidence of river-end deposits

Detailed geomorphological and chronological investigations of the NW-Namibian Amspoort Silt formation show that the sediments are typical river-end deposits. This type of endoreic sediment, occuring only in desert margin areas, provides valuable information about the palaeo-environment. In the Hoanib valley, the fine-grained deposits have buried riverine trees. Radiocarbon dating of the wood and luminescence dating of the sediments allow a detailed reconstruction of the aggradation processes. Accumulation started ∼10 km downstream of Amspoort around the beginning of the 15th century and ended in the 19th century, some kilometres upstream of Amspoort. This upstream shift of sedimentation during the Little Ice Age was caused by gradually decreasing runoff resulting from aridification of the upper part of the Hoanib river catchment lying east of the Namib desert margin ≥1.200 m a.s.l. The Amspoort Silt terrace is evidence of palaeo-hydrological fluctuations in NW-Namibia. At present, the Hoanib river erodes deeply into the silty deposits, indicating that NW-Namibia receives more monsoonal rainfall today than during the Little Ice Age. However, this contradicts the hypothesis of a (continual) natural aridification of NW-Namibia (Damaraland, Kaokoveld) since the mid-19th century in the course of global climatic change. Rather, deposition and erosion of the Amspoort Silts indicate that landscape degradation in NW-Namibia is primarily anthropogenically induced and most probably not accelerated by a decrease in precipitation.

Fluvial dynamics of an anabranching river system in Himalayan foreland basin, Baghmati river, north Bihar plains, India

Anabranching river systems are now regarded as a separate class in river classifications owing to their distinctive morphological/hydrological characteristics and fluvial processes. A better understanding of anabranching rivers still needs detailed data from different environmental and geographical settings. This paper presents a detailed account of an anabranching river system from the Himalayan foreland basin. The Baghmati river system from north Bihar Plains, eastern India provides a typical example of an anabranching river system located in the interfan area between the Kosi and the Gandak megafans. The river system is braided in upstream reaches and meandering in downstream reaches, but the midstream anabranching reach is characterized by low width–depth ratio (11–16), gentle gradient (0.00018–0.00015), variable peak discharge, frequent flooding and high sediment load. The anabranching in the midstream reaches is a response to its inability to transport high sediment load due to gentle channel slope and dominance of aggradation process. The development of anabranches is related to rapid and frequent avulsions of the river channels with eight major avulsions observed in the 30-km-wide floodplain in the last 230 years. The decadal scale avulsion history of the Baghmati river system makes it ‘hyperavulsive’ and the major causative factors for such channel instability are sedimentological readjustments and active tectonics in the basin area.

Floodplain aggradation and pedogenesis in a semiarid environment

Alluvial valley fills in the upper Republican River basin, southwestern Nebraska, provide evidence for processes of aggradation and pedogenesis on semiarid cut-and-fill floodplains. Sheetflow deposition on unchanneled valley floors represents an important mechanism of valley aggradation. Sheetflow deposits contain parallel to sub-parallel laminae and discontinuous lobate sedimentary structures. Rates of sediment deposition on cut-and-fill floodplains strongly determine the degree to which pedogenic features develop within aggrading alluvium. Three pedofacies common to semiarid alluvial deposits correspond with increasing aggradation rates: (1) cumulic soils; (2) multiple buried soils; and (3) no pedogenic features. Radiocarbon age determinations from alluvial fills indicate that floodplain aggradation greater than approximately 0.5 cm year −1 limits soil formation: this represents a threshold rate of pedogenic assimilation. Floodplain soils formed under aggradation rates lower than this exhibit a strong positive relationship between aggradation rate and total CaCO 3 percent, and a negative relationship between aggradation rate and organic carbon percent. Recognizing the associations between pedogenic/sedimentologic features and floodplain aggradation rates can help interpret past rates of sediment transport, storage and deposition.

La Nappe alluviale de Soucy-Les-Grandes-Pièces (Yonne, France) : reconstitution pluridisciplinaire du fonctionnement d’un cours d’eau du Pléistocène moyen

La Nappe alluviale de Soucy (Yonne, France) a livré six gisements du Paléolithique inférieur. Sa mise en place peut être rapportée, d'après l'aminochronologie et les datations RPE sur quartz, aux stades isotopiques 10 et 9. Le rang interglaciaire des formations fluviatiles supérieures fines est souligné par les malacofaunes qui révèlent un milieu tempéré continental. Une reconstitution des différentes étapes de la construction de la plaine alluviale est proposée à partir de la morphologie des dépôts, l'image C-M, l'analyse des traits pédosédimentaires en lames minces et l'étude des malacofaunes. Le remblaiement s'organise à partir de plusieurs chenaux dont les fonctionnements sont distincts dans le temps et l'espace. Le chenal A, le plus ancien, est bordé en rive occidentale par un banc de convexité. Au cours d'une étape ultérieure, un lit fluvial à chenaux multiples, parcouru par des écoulements énergiques mais intermittents, s'installe sur la rive orientale du chenal A. L'incision du chenal B se produit à l'issue de la migration du cours d'eau vers l'est. Le milieu est légèrement boisé. Le développement d'un sol alluvial grisâtre marque ensuite une phase de stabilisation, qui s'accompagne de la régression des malacofaunes forestières. L'étape suivante est caractérisée par des dépôts de plaine d'inondation qui recouvrent l'ensemble de la plaine. Le développement de mollusques xérophiles et la puissante aggradation sédimentaire indiquent l'ouverture du milieu qui pourrait correspondre à la dégradation des conditions climatiques dans le cadre de la fin d'un interglaciaire.

Facies architecture of a submarine fan channel–levee complex: the Juniper Ridge Conglomerate, Coalinga, California

The Upper Cretaceous Juniper Ridge Conglomerate (JRC) near Coalinga, California, provides a rare, high‐quality exposure of a submarine channel to overbank transition. The facies architecture of the JRC comprises a thick, predominantly mudstone sequence overlain by a channellized conglomerate package. Conglomeratic bounding surfaces truncate successions of interbedded turbiditic sandstones and mudstones both vertically and laterally. Thick‐bedded, massive sandstones are interbedded with conglomerates. Facies architecture, palaeocurrent indicators, slump features, sandstone percentages and sandstone bed thickness trends lead to the interpretation that these elements comprise channel and overbank facies. A vertical sequence with conglomerate at the base, followed by thick‐bedded sandstone, and capped by interbedded turbiditic sandstone and mudstone form a fining‐upward lithofacies association that is interpreted as a single channel‐fill/overbank system. Three similar lithofacies associations can be related to autocyclic processes of thalweg migration and submarine fan aggradation or to allocyclically driven changes in sediment calibre.

Coupled and Decoupled Numerical Modeling of Flow and Morphological Evolution in Alluvial Rivers

Existing numerical river models are mostly built upon asynchronous solution of simplified governing equations. The strong coupling between water flow, sediment transport, and morphological evolution is thus ignored to a certain extent. An earlier study led to the development of a fully coupled model and identified the impacts of simplifications in the water-sediment mixture and global bed material continuity equations as well as of the asynchronous solution procedure for aggradation processes. This paper presents the results of an extended study along this line, highlighting the impacts on both aggradation and degradation processes. Simplifications in the continuity equations for the water-sediment mixture and bed material are found to have negligible effects on degradation. This is, however, in contrast to aggradation processes, in which the errors purely due to simplified continuity equations can be significant transiently. The asynchronous solution procedure is found to entail appreciable inaccuracy for both aggradation and degradation processes. Further, the asynchronous solution procedure can render the physical problem mathematically ill posed by invoking an extra upstream boundary condition in the supercritical flow regime. Finally, the impacts of simplified continuity equations and an asynchronous solution procedure are shown to be comparable with those of largely tuned friction factors, indicating their significance in calibrating numerical river models. It is concluded that the coupled system of complete governing equations needs to be synchronously solved for refined modeling of alluvial rivers.

Sources, distribution and storage of heavy metals in the Rı́o Pilcomayo, Bolivia

The Rı́o Pilcomayo rises on the Cerro Rico de Potosı́ precious metal-polymetallic tin deposits of Bolivia, and flows in a southeasterly direction for ca. 600 km to Bolivia's southern border with Argentina. Mining of the Potosı́ deposits has occurred continuously since 1545, generating large quantities of waste materials in the headwater of the basin. In addition, a tailings dam breach at the Porco mine in 1996 released an estimated 235 000 m 3 of tailings and fluid into the upper reaches of the Rı́o Pilaya, the largest tributary to the Pilcomayo. Concentrations of As, Sb, Cd, Cu, Pb, Hg, Ag, Tl and Zn in contemporary channel sediments upstream of the Pilaya confluence are significantly elevated above background values. Elevated levels appear to be associated with pyrite- and other sulphide mineral-bearing tailings materials transported more than 200 km downstream of the Potosı́ mines. Significant downstream declines in elemental concentrations occur within 15 km, and again between 150 and 200 km, from the mines. The initial decrease in concentrations is due to the rapid dilution of nearly pure tailings effluent released to the river from milling facilities near Potosı́. The latter decrease results from a combination of geomorphic processes including the storage of sediment-borne metals within the channel bed and the influx of ‘clean’ sediment from several large tributaries. Downstream of the Pilaya confluence, concentrations of Cu, Pb, Hg and Zn are only slightly elevated above background values, and Ag, Cd, Sb and Tl cannot be distinguished from background levels. These data suggest that while the Porco tailings spill may have had a significant short-term impact on sediment and water quality along the lower reaches of the Rı́o Pilcomayo, its longer-term impacts were limited. Metals stored and eroded from alluvial deposits of historical age in upstream reaches appear to be an important source of metals to the river today. An additional, and perhaps more significant source, is the release of tailings effluent to the river from modern milling operations. The transport of these contaminants downstream of Icla (203 km from Potosı́) appears to be restricted by aggradational processes occurring in the vicinity of Puente Sucre. In addition, downstream of the confluence of the Rı́o Pilaya, inputs of large amounts of ‘clean’ sediment have caused dilution of the metal contaminants. Data from other studies where similar geomorphic processes have occurred suggest that the metals in the upper Pilcomayo may eventually be moved downvalley as the aggradational processes are reversed and channel stabilisation occurs. Thus, the most significant impacts of metal contamination may not be realised in downstream areas for decades.