Channel Bar Research Articles

Microbial Community Structure and Metabolic Potential of the Hyporheic Zone of a Large Mid-Stream Channel Bar

To develop a greater understanding of hyporheic zone microbial biogeochemistry, we sampled pore fluids from a piezometer array associated with the McCarran Ranch channel bar (MRCB); a partially submerged cobble island in the Truckee River, NV, USA. Flowing surface water and pumped pore fluids were characterized by prokaryotic community structure, metabolic potential, and aqueous physicochemistry. Concentrations of potential respiratory electron acceptors were highest in surface water and riverbed porewater and sequentially depleted in porewaters along the inferred flowpath (O2, then NO3−, then SO42−). Correspondingly, cultivable nitrate reducers/denitrifiers were most abundant in surface water and riverbed porewater, despite oxic conditions. Cultivable sulfate reducers were overall most abundant in surface water. Prokaryotic community reconstruction from 16S rRNA gene sequences indicates that the surface water community was less diverse than that of porewater and supports a shift in metabolic strategy, from aerobic heterotrophy in surface water (e.g., Comamonadaceae and Sporichthyaceae) to chemolithotrophy and anaerobic metabolisms (e.g., Hydrogenophaga spp., Ferribacterium spp., Methanobacterium spp.) along the hyporheic flow path. These data indicate that prokaryotic communities within the MRCB are phylogenetically and metabolically diverse and contribute to biogeochemical cycling in this common yet relatively understudied habitat.

Sequence and sedimentary characteristics of upper Cretaceous Sifangtai Formation in northern Songliao Baisn, northeast China: Implications for sandstone-type uranium mineralization

Sequence and sedimentary characteristics play an important role in the mineralization of sandstone-type uranium deposit. Two third-order sequences, including four system tracts namely lowstand system tract (LST), transgression system tract (TST), highstand system tract (HST) and regression system tract (RST), and two sedimentary facies, including braided-meandering river and meandering river are identified in the Upper Cretaceous Sifangtai Formation in northern Songliao Basin. Sandstones with better porosity and permeability formed in LST are good uranium reservoir, and the compacted fine-grained deposits in TST are good confining layers. They form a reservoir-confining assemblage that is beneficial to uranium mineralization. The uranium is mainly hosted in the braided-meandering river depositional system, the channel bar ore-hosting sandstones are connective and have good reservoir property, the maximum porosity and permeability are 42% and 1140 mD, respectively. Suitable thickness and sand percentage of sand bodies are beneficial to uranium enrichment, the industrial grade uranium deposits mainly occur in areas with sand percentage of 0.6–0.8 and sand body thickness of 8–12 m. Organic matter and sulfur act as adsorbents and reducing agents in the progress of uranium mineralization, braided-meandering river with strong hydrodynamic conditions bring a large amount of terrestrial carbon debris, which creates conditions for the formation and adsorption of sulfur and uranium. A sequence and sedimentation uranium metallogenic model of Sifangtai Formation has been established, large and high grade uranium deposits are formed in braided-meandering river sand bodies of LST, which have good reservoir properties, suitable thickness, sufficient organic matter and reducing sulfur content, and regional confining bed.

Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation, Canada

The fluvial–tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial–tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley (“A” Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial–tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the “A” Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.

DINAMIKA BENTUKAN LAHAN FLUVIAL AKIBAT SEDIMENTASI DI SUNGAI GRINDULU, SEGMEN ARJOSARI-PACITAN (Fluvial landform dynamics caused by sedimentation of Grindulu River, Arjosari-Pacitan Segment)

Landforms experience dynamics during the geomorphological process. The fluvial process occurs due to the flow of water from erosion, erosion transportation, and sedimentation activities. Sedimentation material is resulted from erosion transported through the river. Later the deposited erosion will produce landform variations. Therefore, information of landform dynamic is important in controlling runoff and floods. The purpose of this study was to identify and determine the dynamics of the fluvial landform in Grindulu River, Arjosari-Pacitan Segment. The method used is combination of desk analysis and field surveys. The desk analysis is conducted to identify the types of landforms through visual interpretation of SPOT-7 Imagery, while field surveys are to determine sample units using purposive sampling. Based on landform analysis in the Arjosari-Pacitan Segment, there were five fluvial landforms processes due to sedimentation namely point bar, channel bar, flood plain, meandering, and natural levee. The dynamics that occur are dominated by the aggradation process, namely the addition of flood plain landform area of 0.872 ha (0.216%), channel bar of 0.091 ha (0.019%), and natural levee of 0.014 ha (0.002%). The identification of landforms can be used as a basis in spatial planning to optimize water system in watershed. Keywords: landform; fluvial; sedimentation; Grindulu Watershed ABSTRAK Bentuklahan mengalami dinamika selama terjadinya proses geomorfologi. Proses fluvial terjadi akibat aliran air dari aktivitas erosi, transportasi, dan sedimentasi. Material sedimentasi berasal dari hasil erosi yang terangkut melalui sungai yang kemudian diendapkan yang nantinya akan menghasilkan variasi bentuklahan. Dengan demikian informasi mengenai dinamika bentuklahan sangat penting untuk mengendalikan aliran permukaan dan banjir. Tujuan penelitian ini adalah untuk mengidentifikasi dan mengetahui dinamika bentuklahan hasil proses fluvial di Sungai Grindulu, Segmen Arjosari-Pacitan. Metode yang digunakan yaitu kombinasi antara desk analysis (dilakukan untuk mengidentifikasi jenis-jenis bentuklahan melalui interpretasi citra SPOT-7) dan survey lapangan (menentukan unit sampel secara purposive sampling). Hasil analisis bentuk lahan pada Segmen Arjosari-Pacitan diperoleh lima bentuklahan proses fluvial akibat sedimentasi yaitu point bar , channel bar , flood plain , meandering, dan natural levee . Dinamika yang terjadi didominasi oleh proses agradasi yaitu penambahan luasan bentuklahan flood plain sebesar 0,872 ha (0,216%), channel bar sebesar 0,091 ha (0,019%), dan natural levee sebesar 0,014 ha (0,002%). Identifikasi bentuklahan ini dapat dijadikan dasar dalam perencanaan tata ruang untuk mengoptimalkan tata air dalam DAS. Kata kunci: bentuklahan; fluvial; sedimentasi; DAS Grindulu

Numerical Study of Alternate Bars in Alluvial Channels With Nonuniform Sediment

AbstractRivers typically present heterogeneous bed material, but the effects of sediment nonuniformity on river bar characteristics are still unclear. This work investigates the impact of sediment size heterogeneity on alternate bars with a morphodynamic numerical model. The model is first used to reproduce a laboratory experiment showing alternate bar formation with nonuniform bed material. Subsequently, the influence of sediment size heterogeneity on alternate bars is investigated distinguishing hybrid from free bars, definition based on the presence/absence of morphodynamic forcing, considering the results of nine scenarios. In four of them, a transverse obstacle is used to generate forcing. The computations are carried out with the Telemac‐Mascaret system solving the two‐dimensional shallow‐water equations with a finite element approach, accounting for horizontal and vertical sediment sorting processes. The results show that sediment heterogeneity affects free migrating and hybrid bars in a different way. The difference lies in the presence/absence of a migration front, so that distinct relations between bed topography, bed shear stress, and sediment sorting are obtained. Sediment sorting and associated planform redistribution of bed roughness only slightly modify free migrating bar morphodynamics, whereas hybrid bars are greatly impacted, with decreased amplitude and increased wavelength. Increased sediment size heterogeneity increases the degree of sediment sorting, while the sorting pattern remains the same for both free and hybrid bars. Moreover, it produces averagely higher, longer, and faster free bars, while in the case of hybrid bars their wavelength is increased but no general trend can be determined for their amplitude.

Impact of Differential Densification on the Pore Structure of Tight Gas Sandstone: Evidence from the Permian Shihezi and Shanxi Formations, Eastern Sulige Gas Field, Ordos Basin, China

The tight sandstone reservoirs of the Permian Shihezi and Shanxi Formation with strong heterogeneity constitute the main producing zone of the eastern Sulige gas field. The process of differential densification results in various reservoir qualities. Mineral composition, structural characteristic, pore system, and diagenesis were investigated with analyses of well logs, thin sections, porosity, and horizontal permeability of the core plugs; environmental scanning electron microscopy (ESEM); nuclear magnetic resonance (NMR); X-ray computed tomography (X-CT); and fluid inclusion homogenization temperature. The results show that lithic sandstone reservoirs experienced complex and various diagenetic evolutions. Eight types of densification modes can be divided according to the diagenesis paths; these modes represent lithofacies with different densification times and reservoir qualities. Intense mechanical compaction is the main reason for the formation of lithofacies 1, 2, and 5. Lithofacies 4, 6, and 7 formed due to intense cementation, increasing the impermeability of the diagenetic system. The primary pore space in lithofacies 3 is preserved due to the overpressure and chlorite coatings. The dissolution and weak cementation of lithofacies 8 contribute to reservoir development. The middle-lower part of braided channel lags and channel bars, the middle part of meandering riverbed lags, and the middle part of point bars are favourable for reservoir development.

MORPHOLOGICAL CHARACTERISTICS OF THE GIANH RIVER (FROM CO CANG TO CUA GIANH) IN RELATION TO THE EROSION AND ACCUMULATION

On the basis of morphological characteristics and erosion - accumulation of sediment, it is possible to divide the stretch of the Gianh River from Co Cang to Cua Gianh (about 54km in length) into 3 sections as follows: Meandering channel (from Co Cang to Tien Xuan Isles): the length of the channel is 27.69km and the width of the channel is 80-250m. The channel is in the form of a meandering, narrow riverbed, flow plays a dominant role, deposition activities develop strongly at the convex side, while erosion occurs strongly in the concave side (cut side); Braided channel (from Tien Xuan Isles to Quang Phu): the length of the channel is 17.06km and the width of the channel is 800-2,200m. The channel is straight, the river bed is large and the depth of the river bed is 2-11m. Sedimentation occurs mainly at the bottom of the channel and creates bar in the middle of the channel; Straight channel (from Quang Phu to Cua Gianh): the length of the channel is 9.23km and the width of the channel is 800-1,000m. The channel is straight and the depth of the river bed is 8-12.5m. In addition to the role of river flow, it is strongly influenced by marine dynamics. The erosion and accretion activities occur mainly in estuaries. The results above show trend of river development: i) Meandering channel is the most vulnerable to changes for morphology of channel by erosion and accretion of sediment and can create 1-2 horseshoe pools by the river change line; ii) Braided channel mainly changes in the bottom of channel by the formation of channel bar; iii) Straight channel mainly changes in the estuary (the mouth of the river can be moved, enlarged or narrowed).

Extreme sediment fluxes in a dryland flash flood

A flash flood on 28th September, 2012, rose to a peak discharge of 2357 m3 s−1 from zero within one hour in the ephemeral Nogalte channel in SE Spain. Channel morphology and sediment sizes were measured at existing monitored sites before and after the flood and peak flow hydraulics calculated from surveyed floodmarks and cross-sections. Maximum peak sediment fluxes were calculated as ~600 kg s−1 m−1, exceeding maximum published, measured dryland channel values by 10 times and common perennial stream fluxes by 100 times. These high fluxes fit the established simple bedload flux - shear stress relations for dryland channels very well, but now extended over a much wider data range. The high sediment fluxes are corroborated by deposits at >1 m height in a channel-side tank, with 90 mm diameter sediment carried in suspension, by transport of large blocks and by massive net aggradation as extensive, structureless channel bars. Very high sediment supply and rapid hydrograph rise and recession produced the conditions for these exceptional sediment dynamics. The results demonstrate the extreme sediment loads that may occur in dryland flash floods and have major implications for catchment and channel management.

Island development in a mountain river subjected to passive restoration: The Raba River, Polish Carpathians

In the twentieth century the heavily channelized Raba River incised deeply in its mountain course. Abandonment of channelization structures in a 2.3-km-long reach within a forested corridor was followed by considerable channel widening during floods of 30- and 35-year recurrence interval, re-establishment of a multi-thread channel pattern and island development. Morphological and botanical surveys were conducted annually between 2011 and 2017 to determine the processes and patterns governing development of islands and their floristic complexity. Hydraulic conditions promoting establishment and persistence of islands were determined with one-dimensional hydraulic modelling of flood flows for 8 unmanaged river cross-sections with islands and 8 cross-sections in the adjacent channelized reaches. Average age, number of islands and their average and total area in the reach markedly increased over the study period. However, the increase was not steady but moderated by island erosion by flood flows, island establishment shortly after major floods and island coalescence in the years without such floods. Hydraulic modelling indicated that river cross-sections with islands are typified by significantly lower values of mean water depth, flow velocity, unit stream power and bed shear stress at flood flows than cross-sections in the adjacent, channelized reaches. Such conditions promote deposition of living driftwood on channel bars, initiating island development, and reduce the probability of erosion of existing islands. The total number of plant species on islands varied highly and either exceeded or was similar to that recorded on riparian forest plots in particular years. This study indicates that (i) island re-establishment in the river was initiated by substantial channel widening, (ii) variation in flood magnitudes exerts a considerable influence on the trajectory of island development, and (iii) the contribution of islands to the overall species richness of plant communities in the river corridor at early stages of island re-establishment may be highly varied.

Riverbed transformation as a result of a ferry crossing construction – case study

Riverbed under the influence of natural and anthropogenic factors is constantly being transformed. The changes of the channel parameters related to natural phenomena are rarely dynamic. Contrarily it is in the case of human activity. The introduction of river engineering structures to the riverbed aims to quickly adapt the river to the requirements. If construction of such forms is designed taking account of natural conditions prevailing in the riverbed, then negative effects cannot be observed. The situation is different when the river's characteristics were not considered in the design process. The article discusses the changes that the riverbed has undergone due to its intensive contraction at the ferry crossing at km 488 of Vistula, in Gassy. Current and archival aerial photographs were analyzed in terms of the transformation of the riverbed width, as well as the size of the mid-channel islands. The focus was concentrated on a section of the river 2 km above and 4 km below the ferry crossing. In particular, the parameters of the channel bar located directly below the narrowing and their change over the last decades were analyzed. Conclusions were drawn regarding the case study and recommendations for designing analogous solutions ensuing from the analysis were formulated.

River Morphodynamic Evolution Under Dam-Induced Backwater: An Example from the Po River (Italy)

Research Article| October 16, 2018 River Morphodynamic Evolution Under Dam-Induced Backwater: An Example from the Po River (Italy) Vittorio Maselli; Vittorio Maselli 1University of Aberdeen, School of Geosciences, Aberdeen, U.K. 2Institute of Marine Sciences, ISMAR-CNR, Bologna, Italy Search for other works by this author on: GSW Google Scholar Claudio Pellegrini; Claudio Pellegrini 2Institute of Marine Sciences, ISMAR-CNR, Bologna, Italy Search for other works by this author on: GSW Google Scholar Fabrizio Del Bianco; Fabrizio Del Bianco 3Consorzio ProAmbiente, Bologna, Italy Search for other works by this author on: GSW Google Scholar Alessandra Mercorella; Alessandra Mercorella 2Institute of Marine Sciences, ISMAR-CNR, Bologna, Italy Search for other works by this author on: GSW Google Scholar Michael Nones; Michael Nones 4Research Centre for Constructions, Fluid Dynamics Unit, University of Bologna, Bologna, Italy Search for other works by this author on: GSW Google Scholar Luca Crose; Luca Crose 5Agenzia Interregionale per il Fiume Po, AIPO, Settore Navigazione Interna, Parma, Italy Search for other works by this author on: GSW Google Scholar Massimo Guerrero; Massimo Guerrero 6Department of Civil, Chemical, Environmental and Material Engineering, University of Bologna, Bologna, Italy Search for other works by this author on: GSW Google Scholar Jeffrey A. Nittrouer Jeffrey A. Nittrouer 7Rice University, Department of Earth, Environmental, and Planetary Sciences, MS-126, Houston, Texas 77005, U.S.A. Search for other works by this author on: GSW Google Scholar Journal of Sedimentary Research (2018) 88 (10): 1190–1204. https://doi.org/10.2110/jsr.2018.61 Article history first online: 16 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Vittorio Maselli, Claudio Pellegrini, Fabrizio Del Bianco, Alessandra Mercorella, Michael Nones, Luca Crose, Massimo Guerrero, Jeffrey A. Nittrouer; River Morphodynamic Evolution Under Dam-Induced Backwater: An Example from the Po River (Italy). Journal of Sedimentary Research 2018;; 88 (10): 1190–1204. doi: https://doi.org/10.2110/jsr.2018.61 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyJournal of Sedimentary Research Search Advanced Search Abstract River systems evolve in response to the construction of dams and artificial reservoirs, offering the possibility to investigate the short-term effects of base level oscillations on fluvial architecture. A major effort has been dedicated to the understanding of river response downstream of large dams, where deep channel incisions occur in response to the removal of sediment that is sequestered in the upstream reservoir. Integrating field observations and numerical-modeling results, this work quantifies the sedimentary and morphological changes of the Po River (Italy) upstream of the Isola Serafini dam to investigate the impact of dam-induced backwater on river morphodynamics. The construction of a reservoir generates a new base level that forces an upstream shift of alluvial lithofacies and a change in the planform geometry of the river. The lateral migration rate of the channel is up to 45 m/yr upstream of the influence of backwater flow and ca. 10 m/yr at the transition from normal to backwater flow conditions (30 km from the dam). Within this reach, a reduction of the bed shear stress promotes deposition of coarse-grained sediment and the emergence of the gravel–sand transition of the river. The lateral migration of the channel continuously decreases over time, and rates < 5 m/yr can be observed in the reservoir backwater zone. This trend is accompanied by the drowning of channel bars, the reduction of river competence, and an increase in bedform spacing. Oscillatory backwater and drawdown surface water profiles can be observed closer to the dam, which are associated with varying low-discharge and high-discharge events, respectively. While low-flow conditions, persisting for much of the year, allow the deposition of fine-grained sediment, high-discharge events promote not only the resuspension and transport of fine material but also the progressive erosion of channel bars and the overall deepening of the thalweg. This study provides a clear picture of the river evolution in response to the construction of a hydropower dam that may be of help in predicting how other fluvial systems will respond to future human interventions. Moreover, the result of how changes in base level and oscillations in water surface profile (backwater and drawdown) control river hydro-morphodynamics and sediment transport may provide new insights when reconstructing ancient fluvial and deltaic sequences. You do not currently have access to this article.

Sedimentary facies identification of the Triassic in the Hongqiba region of the Northwest margin of Junggar Basin, West China

ABSTRACTThis study determined the lithological association and identified sedimentary facies of Triassic in the Hongqiba region through analyzing core samples, wireline log patterns, and seismic facies. The spatial variation of the sedimentary system in the Triassic was then determined based on source supply and hydrodynamic conditions. Results show that the Triassic in the Hongqiba region was dominated by coarse deposits derived from fan delta deposition system, including fan delta plain, fan delta front, and pre-fan delta. Primary microfacies are distributary channel, underwater distributary channel, inter-distributary bay, and distal bar. Climate changing from drought to humidity during the Triassic resulted in widespread lake transgression.

Aerial photo-based analysis of the hydromorphological changes of a mountain river over the last six decades: The Czarny Dunajec, Polish Carpathians

Changes in hydromorphological conditions in the mountainous Czarny Dunajec River over the last six decades were investigated through the analysis of archival aerial photos and contemporary orthophotos. The aerial images from 7 dates between 1954 and 2009 were used to evaluate changes in selected hydromorphological parameters in six reaches delimited along the river course. As a result of channelization works and in-stream gravel mining, most of the analysed river course experienced dramatic hydromorphological alterations in the active river zone: change from the multi-thread to a single-thread channel, river narrowing associated with a reduced occurrence or elimination of channel bars and islands, transformation of the gravelly bed to a bedrock–alluvial or bedrock bed, stabilization of river banks with engineering structures, and disruption of longitudinal river continuity by transverse hydraulic structures. These changes largely reduced habitat heterogeneity in the river and disrupted or limited three-dimensional connectivity of the river ecosystem. However, in the reach that had generally avoided these human pressures, such negative changes did not occur and the number of low-flow channels and the proportion of islands in the active river zone increased during the study period. In all study reaches, positive changes occurred in the riparian and floodplain areas: the proportion of wooded channel banks and forest cover in the floodplain area increased over the last six decades. The degradation of hydromorphological conditions in the active river zone was reflected in a significant impoverishment of fish and benthic invertebrate communities as well as of ground beetles inhabiting low river benches. The study confirmed the usefulness of archival aerial photos in reconstructing temporal changes in river hydromorphology. The number of features that can be evaluated with this approach is smaller than in field-based assessments of contemporary rivers but larger than in studies based on historical maps or paleohydrological analysis.

Remotely Sensed Analysis of Channel Bar Morphodynamics in the Middle Yangtze River in Response to a Major Monsoon Flood in 2002

Channel bars are a major depositional feature in channels, and are considered as an important part of the morphodynamics of an alluvial river. The long-term morphodynamics of bars have been intensively investigated. However, relatively little is known about the response of channel bars to a major river flood, which is considered to be the predominant force in shaping bar morphology. This is especially the case for the monsoon-affected Yangtze River, where fluvial geomorphic work is largely carried out during monsoon floods. In this study, multi-temporal satellite images and river stage data were used to examine the morphodynamics of four large channel bars in the middle Yangtze River in response to a major monsoon flood in 2002. Based on bar surface areas estimated with Landsat images at different river stages, a rating curve was developed for each of the four bars, which was used to estimate bar volume through an integral process. Our study shows that two of the bars tended to be stable, while the other two experienced severe erosion during the flood. The results reveal that the flood caused a total bar surface area decrease of 1,655,100 m2 (or 8.30%), and a total bar volume decline of 5.89 × 106 m3 (or 6.10%) between the river stages of 20.81 m and 25.75 m. The volume decrease is equivalent to a sediment loss of approximately 8.25 × 106 metric tons, based on an average bulk density of 1.4 metric tons per cubic meter. The results imply that channel bars in the middle Yangtze River can also be large sediment sources rather than depositional areas during the flood. The decrease of sediment load in the middle of Yangtze River was found to be responsible for the dramatic morphodynamics of channel bars, which could last for a long period of time, depending on the operation of the Three Gorges Dam, which opened in 2003. Hence, we suggest making management efforts to protect the bars from further erosion.

The impacts of the Three Gorges Dam upon dynamic adjustment mode alterations in the Jingjiang reach of the Yangtze River, China

Dam operation profoundly modifies downstream flow-sediment regimes, resulting in a variety of geomorphologic responses, without uniform laws. The Jingjiang reach of the middle Yangtze River has undergone significant adjustment mode alterations, with intensive channel erosion and major adjustment area changing from the bankfull channel to the medium flow channel at the time boundary of the Three Gorges Dam (TGD) impoundment. While these changes have received some attention, little work has been undertaken to reveal the reason. In this study, based on 172 cross sections measured in the post-flood period from 2003 to 2015, different river patterns, longitude and reach-averaged channel dimensions were assessed to reflect the channel adjustment characteristics of the Jingjiang reach. An index has been proposed to measure the cumulative water transport capacity of each discharge interval and to assess the relationship with reach-scale medium flow channel dimensions. The results indicate that the channel adjustment changes are directly driven by the alterations of flow duration and sediment transport capacity. With the deficiency of floods, medium flow channels respond quite well to small and medium discharges of under approximately 16,500 m3/s. A discharge of 16,500 m3/s which can be regarded as a critical threshold, corresponds to the average top levels of braided and meandering channel bars, which are of morphological significance in constraining the passing flow in the medium flow channel. Small and medium discharges under the critical threshold associate with relatively larger reductions in sediment loads and hold increased geomorphic impacts (QmJP), eventually resulting in adjustment mode alterations. The results present herein for the Jingjiang reach of the Yangtze River can provide a better understanding of dam operation and the corresponding impacts on downstream channel morphology.

Changing Pattern of Channel Morphology of Alaknanda River in Srinagar Valley (Garhwal Himalaya), India

The Alaknanda River is the most significant parental river of Ganga and forms an 11.5 km long and 2.5 km wide valley, locally known as the Srinagar Valley. The purpose of the present study is to highlight the recent landform changes in the Alaknanda channel course after the Kedarnath disaster, 2013. The Kedarnath flood completely changed the channel morphology of the Alaknanda river. The river changed its course at Srikot, SSB and Sriyantra Tapu with lower terraces being silted by sands at Ranihat, SSB, Bhaktiyana and Sriyantra Tapu. A new depositional terrace also formed opposite to Sriyantra Tapu. New lateral channel bars, braided channels, back swamp, rapids, pools and river souls were identified in the channel course of the river. Shifting of the channel course at Chauras still remains a serious problem for the Garhwal University Chauras Campus. About 2–5 m silt was deposited on the lower terrace at SSB, and ITI. The Srikot river bed was appended to 4.60 m. Shifting of channel course remains a serious threat to the Srinagar valley. Urbanization, sand and boulders mining, construction of dam, hydrological canal, road and settlements are the prominent example of anthropogenic activities which affect the shifting channel.

Sedimentary Structures and Lithofacies Found in a Channel Bar of Brahmaputra River in Panikhaiti, Kamrup District, Assam

Sedimentary Structures and Lithofacies Found in a Channel Bar of Brahmaputra River in Panikhaiti, Kamrup District, Assam

Evolution of river planforms downstream of dams: Effect of dam construction or earlier human‐induced changes?

AbstractWhen studying the evolution of landscape, it is difficult to discriminate the influence of anthropogenic from natural causes, or recognise changes caused by different sources of human action. This is especially challenging when the influence of certain sources is overprinted. For instance, although dam closure is the most common method of altering river courses, dam construction is often preceded by hydro‐technical works such as channel straightening, embankment construction or sediment mining. Both dam construction and the hydro‐technical works that precede dam closure can result in changes in the balance between sediment supply and transport capacity, and often, changes in river planform.The main objective of this study was to verify whether the works preceding dam closure are an important driver of river planform changes on the lower Drava River (Hungary). The case study is based on geological and geophysical surveys, as well as the analysis of historical maps covering an anabranching, 23 km long valley section. We show that channel straightening conducted prior to dam closure resulted in a transition from a meandering to sinuous planform with channel bars. Dam construction itself then caused enhanced incision, exposure of bar surfaces, vegetation encroachment and the formation of an anabranching planform. Based on this study, we developed models of alluvial island and channel planform evolution downstream of dams. Dam construction enhances channel incision, narrowing, and the reduction of flow caused by earlier hydro‐technical works. Many rivers downstream of dams experience episodes of anabranching or wandering, with a multi‐thread pattern replacing sinuous, braided and meandering courses. When incision continues, river patterns evolve from anabranching to sinuous via the attachment of alluvial islands to floodplains. However, the timing and sequence of these changes depend on hydrological and sediment supply regimes, geomorphic settings and anthropogenic actions accompanying dam construction. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Седиментология и стратиграфическая корреляция вендских отложений на юго-западе Сибирской платформы: выдающийся вклад внешнего источника кластического материала в образование осадочных систем

The study focuses on the sedimentation and stratigraphy of a large Vendian (Ediacaran) sedimentary basin in the southwestern Siberian craton, with implications for the origin of its units and subunits. Sediments are been correlated along the craton edge and inward into the craton interior using stratigraphic sequences, including glacial and post-glacial deposits. Clastic complexes in the Vendian: upper Chapa, Taseeva, Oselok, and Baikal groups composed of fluvial deposits are of special importance for correlations. The structure of the continental Aysa Fm. of the Oselok Group in the Sayany area has been characterized in detail, based on universal classification of lithofacies and their sequences, and correlated with sediments in the craton interior. There are two Ediacaran subunits: a continental-marine complex below and a continental complex above. The continental-marine subunit in the subsided craton margins was deposited mainly upon postglacial plains (sandurs) and marginal sea shelves, with contributions from intracratonic clastic sources. The deposits of the continental subunit belong to centripetal fluvial systems directed from distant craton-margin mountain upland toward the epicontinental sea in the central Siberian craton. The continental subunit is composed of large sections of red and variegated fluvial deposits from a foreland basin which was accumulated mainly in foredeeps in the over-compensation setting and partly reached the slopes of forebulges. Big deep sand rivers with large channel bars were formed at early and late during orogenic activity, while perennial anastomosing and braided mud-sand rivers with levees originate during passive stages of orogeny. The largest progradation of fluvial systems onto the Siberian craton is marked by channel deposits of the lower members from the Nemchanka, Moshakovka, Aysa, Khuzhir and Kachergat fms., as well as by the Bokhan sandstone member of the Chora Fm.

High Resolution Sequence Stratigraphy Correlation and Sedimentary Model of Braided Rivers: A Case on Paleogene Palogue Oilfield, South Sudan

Complicated fluvial sands correlation and braided river models are still challenges for heterogeneous anisotropic reservoir evaluation for petroleum development in oilfields. Based on high-resolution sequence stratigraphy and sedimentology, core observation, and the analysis of logging and seismic data, high-resolution sequence stratigraphy framework of Paleogene Yabus formation of Palogue Oilfield, South Sudan is established, and sedimentary characteristics of braided rivers are analyzed, (1) In braided river deposits, channel bars are usually pro-gradating seawards and going thicker upwards. And braided channels are retro-gradating landwards and going thinner upwards. Both of them can be subdivided into channel (bar) trunks and channels (bar) flanks, showing vertical stacking and lateral migration; (3) As the A/S (Accommodation space/sediments supply) increasing upwards, braided river sands are shifting frequently as a reciprocating migration in Yabus V/VI, and they are more stable as the A/S decreasing upwards as a vertical aggradation in Yabus VII/VIII; (4) With 77 wells sedimentary microfacies identification, sand thickness and sand/bed ratio distribution, sedimentary facies distribution of all members and zones are analyzed. According to the A/S change, three braided river models are concluded, including isolated channel type (high A/S period), crossed channels type (medium A/S period), and superposed channels type (low A/S period).