Channel Belt Research Articles

The influence of subsurface architecture on scour hole formation in the Rhine–Meuse delta, the Netherlands

Abstract Scour holes are common features in deltaic rivers which can destabilise embankments through oversteepening of the river bed. Their development has been studied extensively from the hydraulic perspective, but another important control is the erodibility of the river bed which varies considerably due to thickening of heterogeneous deltaic substrate towards the coast. Therefore, we assessed the influence of delta-scale geological heterogeneity and local subsurface architecture on scour hole formation in addition to the hydrodynamic controls. We (1) created an inventory of 165 scour hole locations in the Rhine–Meuse delta, (2) assessed the hydrodynamic conditions at the locations, (3) extracted geometric characteristics and (4) determined the subsurface architecture from geological data. Central and lower delta branches have 0.6–0.7 scours per km while upper delta branches have less than 0.2. Downstream, 58% of scour holes were related to architectural elements, notably sand bodies from former Holocene channel belts and Early Holocene cohesive beds. These scours have steeper slopes due to higher proportions of cohesive sediments near the river bed. Furthermore, scours related to channel belt sand bodies are limited in downstream length and depth, up to maximum of approximately two times the water depth. From our results, we provide a delta-scale explanatory framework that relates the position of present-day river channels with respect to Pleistocene river deposits and Holocene fluvio-deltaic deposits to scour hole formation. Upstream rivers are incised in Pleistocene deposits showing less local variation in erodibility. The majority of scour holes here relate to engineering works. In central and lower delta branches, geologically inherited heterogeneity of the Holocene substrate at critical depths near the channel bottom adds to anthropogenic induced scours and results in high abundances. This demonstrates that downstream variation in subsurface architecture should be considered as a key control on scour locations and characteristics for management purposes.

Reconstructing late Miocene depositional environments in the central High Plains, USA: Lithofacies and architectural elements of the Ogallala Formation

The late Miocene Ogallala Formation hosts much of the High Plains aquifer of North America, though it is poorly studied in the central High Plains region where only a small fraction of its total thickness is exposed. One exception is in western Kansas, where up to 40 m of the Ogallala Formation crop out along the bluffs of Ladder Creek Canyon. These deposits consist of stacked and laterally extensive, multistory channel bodies dominated by sandy bedforms with little or no intervening floodplain mudstone. We interpret each ~2–4 m-thick story as a sandstone bed that formed during a single depositional episode and infilled a broad and relatively shallow braided river channel. Each story preserves elements suggesting high discharge conditions, followed by low discharge or the complete abandonment of the river channel after active streamflow either migrated away from the study area or ceased regionally for a relatively long period of time. High discharge conditions are characterized by channel-filling, current-formed transverse bars, gravel-rich longitudinal bars, and sand sheets deposited while the channel was largely flooded and nearly the entire riverbed mobilized. Low discharge conditions are indicated by lens-shaped, trough cross-stratified channels scoured into previously deposited bars. Channel-fill deposits were formed when the remaining flow was confined to one or more narrow streams that braided across the exposed channel belt. Persistent low discharge conditions are suggested by channel-belt-wide subaerial exposure of bar surfaces for a period of time sufficient to promote colonization by soil-burrowing organisms and moderate soil development. Abandonment of the broader channel belt is indicated by advanced calcretization within a story and the presence of aerially restricted and fossiliferous mud-filled channel pools where the final repositories of surface water attracted local paleofauna before infilling or drying completely. The depositional environment of the Ogallala Formation in the study area is most similar to “Platte type” fluvial systems characterized as shallow, perennial, and sand-dominated braided rivers. Comparisons with previously studied localities in the northern and southern High Plains reveal differences in fluvial style, sediment source areas, cut-and-fill geometries, and eolian input.

Distribution of landforms and buried sedimentary deposits during the growth of the Aceh River delta (Sumatra, Indonesia)

ABSTRACT Fluvial and coastal landforms are indicative of landscape river delta evolution over time and provide clues for understanding coastal adjustments to sea-level and fluvial dynamics fluctuations, tectonic displacements, and extreme waves. We have mapped the surface and sub-surface footprints of fluvial and coastal geomorphological features in the Aceh River delta, northern Sumatra, using imagery dataset, vertical facies logging and helicopter electromagnetic surveys. The result is a geomorphological map at the scale of 1:75.000 which outlines the main features of the deltaic plain, including rivers, tidal and buried channels, fluvial levees, beach ridges, swales, tidal flats and lagoons. We compare their spatial distribution to the geometry of buried sediment bodies, revealed by boreholes and resistivity maps. Buried channel belts and floodplain deposits document the former locations of the distributary channels of the Aceh River. Coastal-parallel beach ridges evidence 7–8 km of asymmetric delta progradation since the mid-Holocene sea-level high stand.

Seismic expression and stratigraphic preservation of a coastal plain fluvial channel belt and floodplain channels on the Gulf of Mexico inner continental shelf

AbstractSubsurface fluvial deposits in coastline‐proximal settings record the spatiotemporal evolution of the coastal landscape and may be viable repositories of sediment for future coastal restoration projects. However, quantitative linkages between the geomorphic form and stratigraphic expression of coastal plain fluvial elements remain lacking, complicating coastal stratigraphic interpretations and subsurface resource assessment. This study explores the expression and preservation of fluvial coastal plain geomorphic features through outcrop‐scale seismic stratigraphic analysis of ultra‐high‐resolution chirp acoustic reflection data from the north‐western Gulf of Mexico inner continental shelf, offshore the Brazos River, Texas, USA. The chirp data exhibit decimetre‐scale vertical resolution within the upper 35 m of the shelf, evincing the preservation of two distinct types of coexisting fluvial channel‐forms: Type 1, a 1 km wide and 9 m deep single‐storey channel belt filled with sand‐rich lateral accretion deposits and channel fills; and Type 2, numerous 10 to 800 m wide and 1 to 20 m deep incisional floodplain channels filled with extensive drapes of overbank‐derived sediments. The chirp data resolve fluvial geomorphic elements including lateral accretion surfaces, levées, floodplain deposits and abandoned channel fills. Rollover of lateral accretion surfaces, widespread draping of seismic reflectors within channel fills, and quantitative comparison between the interpreted stratigraphic forms and analogous features on the nearby Texas Coastal Plain suggest near‐complete stratigraphic preservation of the geomorphic form of both channel belt and floodplain channel elements. Rapid aggradation of the coastal plain by the Brazos River in this region during the Holocene transgression is hypothesized as a mechanism for the high degree of preservation achieved. This study proposes the first recognition criteria for the seismic stratigraphic expression of coastal floodplain channels and provides direct linkages between the geomorphic and stratigraphic expression of a coastal plain fluvial landscape, promoting improved coastal stratigraphic interpretations and assessment of subsurface lithofacies distributions.

Controls on the morphology of braided rivers and braid bars: An empirical characterization of numerical models

ABSTRACTBraided rivers exhibit highly variable morphologies, morphodynamic behaviours and resulting depositional records. To evaluate relationships between characteristics of braided‐river channel belts and river depth, water discharge and streambed gradient, 39 numerical modelling experiments were conducted with the software Delft3D to simulate braided‐river evolution under a broad range of boundary conditions. Data from model outputs were integrated with observations from 63 natural braided rivers differing with respect to river depth and streambed gradient. The modelled rivers each underwent similar evolutions, yet each culminated in markedly different final river morphologies, dependent on discharge and riverbed gradient. The rivers underwent evolutionary stages of: (i) formation of transverse unit bars with limited relief from an initially featureless bed; (ii) channel development around bars and in some cases dissecting transverse unit bars; (iii) formation of relatively simpler compound bars; and (iv) amalgamation of these simpler compound bars into more complex compound bars. Quantitative relationships relating to braided‐river channel‐belt morphology and organization are established, and the following results are noted: (i) bar elongation (length‐to‐width ratio) is correlated positively with riverbed gradient; (ii) bar height and area are correlated positively with discharge, and negatively with riverbed gradient; (iii) the river depth is the main predictor of mean braid‐bar area; and (iv) the degree of braiding is primarily associated with river width‐to‐depth ratio and riverbed gradient. Results arising from this research improve our understanding of controls on the morphology and architectures of braided fluvial channel belts; they provide a novel empirical characterization that can be applied for predicting channel depth, bar morphology, streambed gradient, and degree of braiding of modern fluvial systems and of the formative rivers of ancient preserved successions.

Geomorphically mediated carbon dynamics of floodplain soils and implications for net effect of carbon erosion

AbstractThe fate of organic carbon deposited in floodplain sediments is an important control on the magnitude and direction of the carbon flux from anthropogenically accelerated erosion and channelization of the riverine network. Globally, carbon deposition rates and mean residence time (MRT) within different geomorphic settings remains poorly constrained. We sampled soil profiles to 0.8 m depth from two geomorphic zones: active channel belt (ACB) and lowland floodplain, under long‐term pasture adjacent to the river Culm in SW England, UK. We evaluated sedimentation rates and carbon storage using fallout radionuclide 137Cs, particle size and total carbon analyses. Variation in decomposition was assessed via empirical (soil aggregate size, density fractionation combined with natural abundance 13C analysis) and modelling simulation (using the RothC model and catchment implications explored using a floodplain evolution model). Sedimentation and carbon accumulation rates were 5–6 times greater in the ACB than the floodplain. Carbon decomposition rates also varied with geomorphic setting. In floodplain cores, faster decomposition rates were indicated by greater 13C‐enrichment and subsoils dominated by mineral‐associated soil organic carbon. Whereas, in the ACB, carbon was less processed and 13C‐depleted, with light fraction and macroaggregate‐carbon throughout the cores, and RothC modelled decomposition rates were 4‐fold less than lowland floodplain cores. Including the ACB in floodplain carbon MRT calculations increased overall MRT by 10%. The major differences in the balance of sedimentation and decomposition rates between active and inactive floodplains suggests the relative extent of these contrasting zones is critical to the overall carbon balance. Restoration projects could enhance soil carbon storage by maximizing active floodplain areas by increasing river channel complexity.

Planform architecture, meander evolution and grain‐size variability of a deltaic channel belt in the Rhine‐Meuse delta, The Netherlands

AbstractIn the unconstrained, low gradient setting of major delta plains, individual meander belts tend to function for relatively short periods of time due to repeated channel avulsion. Their short lifetime makes ‘deltaic’ channel belts suitable to study the products of steady meander evolution as the deposits and internal architectural elements preserved are often without complications of repeated bend cutoffs. This study investigated how sedimentary characteristics as preserved from subsequent stages of activity differ within the Stuivenberg channel belt. The Stuivenberg channel belt is a long‐studied example in the Holocene Rhine‐Meuse delta, The Netherlands, for which the meander evolution, i.e. geometry and dimensions through initiation, main activity and abandonment stages and durations of activity involved, was reconstructed. Mapping of the channel belt started from established coring based and LiDAR‐aided methods. Explorative approaches were used to assess active channel width at the onset of abandonment, and to reconstruct meander positions during earlier stages. The migration history of five consecutive meanders is revealed from convex and concave ridge‐and‐swale scrolls. Analysis identifies them to be the product of translation, expansion and rotation trajectories during a period of 800 years. Grain‐size composition throughout the sand body is interpreted given these positional contexts, with special attention to local coarsening of upper point bar facies in deltaic meander belts. The findings emphasize: (i) the need to separate abandonment stage from main activity stage when analysing three‐dimensional channel belt architectures; and (ii) the influence of meander evolution on lithological characteristics of initial stage versus mature stage parts of deltaic channel belt sands.

Differences of sedimentary triggers and depositional architecture of lacustrine turbidites from normal regression to forced regression: Eocene Dongying depression, Bohai Bay Basin, East China

Sedimentary processes of marine turbidite systems under sea-level changes have been the focus of deep-water sedimentology. Compared with well-studied marine basins, the products of deep-water sediment gravity flows responding to lake-level changes in closed lacustrine basins are still poorly understood. In this study, we integrate cores of 10 exploratory wells, logs of 280 development wells, 3D seismic data and geochemical elements from the Eocene Dongying Depression, Bohai Bay Basin, East China to investigate sedimentary mechanisms and depositional architecture of lacustrine turbidite systems under lake-level changes. The studied strata are divided into three parasequence sets (PSS4 ~ PSS2), corresponding to early highstand normal regression with ascending trajectories, late highstand normal regression with flat trajectories, and forced regression with descending trajectories, respectively. From early highstand normal regression to forced regression, the ratios of Fe/Mn and U/Th reveal that the humid climate shifted to the arid climate. More lithofacies clues indicate that turbidites in highstand normal regression were triggered by river floods, whereas counterparts in forced regression were related to sediment failures. There are different architectural features of turbidite systems in three regression stages. PSS4 developed a series of channel belts within which individual channel elements are aggraded vertically. PSS3 is composed of a channelized lobe formed by individual distributary channels migrating laterally. PSS2 is characterized by a suite of compensational stacking debrite tongues. Climate forcing is a crucial factor controlling depositional architecture of the turbidite systems. A relatively humid climate during highstand normal regression led to rising lake-level and frequent floods, which are conducive to the formation of hyperpycnal-fed channel-lobe systems. Nevertheless, during forced regression under an arid climate, the strong progradational clinothems were prone to failure and sediment remobilization, resulting in debrite tongue complexes. This study highlights depositional differences of lacustrine turbidites during regression stages under the control of climate. Meanwhile, it also provides a new predictive model for deep-water hydrocarbon exploration and production in lacustrine basins worldwide.

Evolution of a stratigraphic model in a fluvial deltaic wedge: implications for groundwater resource protection in an oil and gas-bearing region in the Upper Cretaceous Dunvegan Formation, Northwest Territories, Canada

The Liard Basin, in northwestern Canada, contains one of Canada’s largest natural gas reserves. There are concerns about degradation of Dunvegan Formation groundwater quality, which is the basin's main shallow freshwater aquifer, as a consequence of oil and gas development. Previous lithostratigraphic studies interpreted the Dunvegan as conglomeratic alluvial fans. In this study, newly collected cores and surface resistivity surveys are integrated with legacy hydrocarbon well data and measured sections for improved sedimentary bedrock characterization. Rather than alluvial fans, we interpret the upper Dunvegan as a coarse-sand to gravel bed fluvial system that was likely tributive to incised valleys associated with linked Alberta Basin base-level falls. Correlation of well logs and measured sections suggest the lower Dunvegan comprises a prograding delta complex with off-lapping clinoforms fed by smaller sand-bed rivers that grade into the upper Dunvegan conglomeratic system, which built southeast towards the Cretaceous Western Interior Seaway. This sequence stratigraphic and facies architectural approach is employed to inform the aquifer–aquitard flow system and aid in understanding the 3D groundwater flow system, once hydrogeologic characterisation confirms position and thickness of important hydrogeologic units. Resistivity surveys suggest upper Dunvegan channel-belt facies may be well-connected and laterally extensive, potentially acting as interconnected aquifer units; whereas lower Dunvegan channel belts may be discontinuous, indicating smaller-scale flow units separated by floodplain and marine shale. This revised geologic understanding provides opportunities for determining impacts or resilience from shallow and deep sources of contamination associated with human activities, including oil and gas development, land-use, and (or) climate change.

Quantitative relationships between river and channel-belt planform patterns

Abstract Channel planform patterns arise from internal dynamics of sediment transport and fluid flow in rivers and are affected by external controls such as valley confinement. Understanding whether these channel patterns are preserved in the rock record has critical implications for our ability to constrain past environmental conditions. Rivers are preserved as channel belts, which are one of the most ubiquitous and accessible parts of the sedimentary record, yet the relationship between river and channel-belt planform patterns remains unquantified. We analyzed planform patterns of rivers and channel belts from 30 systems globally. Channel patterns were classified using a graph theory-based metric, the Entropic Braided Index (eBI), which quantifies the number of river channels by considering the partitioning of water and sediment discharge. We find that, after normalizing by river size, channel-belt width and wavelength, amplitude, and curvature of the belt edges decrease with increasing river channel number (eBI). Active flow in single-channel rivers occupies as little as 1% of the channel belt, while in multichannel rivers it can occupy >50% of the channel belt. Moreover, we find that channel patterns lie along a continuum of channel numbers. Our findings have implications for studies on river and floodplain interaction, storage timescales of floodplain sediment, and paleoenvironmental reconstruction.

Alluvial ichnofacies from Upper Triassic red beds in India: Implications for palaeoenvironment and palaeoclimate

Trace fossils are highly sensitive to the hydrodynamic conditions of depositional systems, substrate, and climate and hence yield information about spatio-temporal changes in palaeoenvironment and palaeoclimate. In this paper, we report alluvial trace fossil assemblages from the Upper Triassic Tiki Formation of the Rewa Basin, central India. Based on detailed facies analysis and architecture, two depositional environments are identified in this formation: (1) perennial braided channel belts and (2) extensive and complex floodplains containing ephemeral small fluvial channels, lakes, and crevasse splay deposits. Upper flow regime primary structures in laterally extensive sandstones encased in the floodplain deposits indicate short-lived high magnitude flood discharge, suggestive of seasonal climates. Trace fossils are sporadically distributed through these facies, generally in suites of low abundance, although in places the bioturbation index is higher. Three major ichnofacies, namely Skolithos, Scoyenia and Mermia are recognized, based on the association of different trace fossils preserved in the channel, crevasse splay of proximal floodplain and pond/lake of distal floodplain deposits, respectively. Vertical Beaconites in palaeosols and an abrupt change from Skolithos to Scoyenia ichnofacies indicate a highly variable water table, controlled by seasonal precipitation. The transitions between the Skolithos, Scoyenia and Mermia ichnofacies reveal that trace fossil preservation was variously controlled by channel abandonment, floodplain dewatering, substrate desiccation or periodic waterlogging of overbank areas. Specifically, increased water discharge over the floodplain during high precipitation, followed by post-flood recovery of hot, dry, low-water table conditions may have been a key controlling factor in shaping the ichnological characteristics of these Upper Triassic red beds.

Chronology constraints on the complex sedimentary stratigraphy of the paleo-Yangtze incised valley in China

Numerous studies have documented the sedimentary sequences of the Yangtze Delta in response to relative sea-level changes since the Last Glacial Maximum (LGM). However, the pre-LGM sedimentary history of the paleo-Yangtze Delta remains unknown, especially for the formation timing of the basal fluvial sedimentary succession in the paleo-Yangtze incised valley. Here, based on luminescence dating techniques, the sedimentary stratigraphy of three sediment cores (YD016, YD014 and YD006) were constrained and the sedimentary successions were analyzed. Combining with previously published sedimentary sequences, we infer spatiotemporal variation of the stratigraphic architectures over the Yangtze Delta through the time spanning from the late Pleistocene to the Holocene (∼50–0 ka). Our reconstruction for the paleogeography and environmental evolution of the paleo-Yangtze incised valley showed that the basal fluvial channel belts in the paleovalley associated with coeval laterally extensive paleosol-bearing stratigraphy at the interfluves occurred in the transition between marine isotope stage (MIS) 3 and MIS 2; the overlying sedimentary successions are characterized by the post-glacial floodplain deposits formed between 19 and 12 ka, and estuary to shallow marine deposits during early Holocene (11–8 ka); and then the upper most is the Yangtze Delta depositional systems dated to mid-to-late Holocene (since ∼8 ka). Correlations of the chronostratigraphy of sediment cores over the cross-sections in this region suggest that a superimposed compound fluvial channel complex has probably characterized the paleo-Yangtze incised valley systems since MIS 3. Apart from the post-LGM infilling of the LGM incised valley, the basal MIS 3 fluvial deposits might represent the earlier infilling over another incised valley, which might be formed during MIS 4 although our cores have not reached yet. Such hierarchical incised valley systems in the paleo-Yangtze Delta were likely related to the late Quaternary sea-level changes. In addition, the relict inherited topography since the last glacial period could have played an important role in the stacking patterns of the paleo-Yangtze incised valleys.

A quantitative depositional model of a large distributive fluvial system (Megafan) with terminal aeolian interaction: The Upper Jurassic Guará DFS in southwestern Gondwana

ABSTRACT Recent studies have shown that distributive fluvial systems are the dominant fluvial forms in modern continental sedimentary basins, thus composing a large part of the stratigraphic record. This study provides a basin-scale architectural analysis of the Guará Formation, from the Upper Jurassic record of southwestern Gondwana, and attempts to compare the formation's depositional model to those developed for distributive fluvial system (DFS) successions. This time interval is significant because it was a period of intense tectonic activity related to the Paraná–Etendeka plume and the Gondwana breakup. Quantitative analyses were performed on stratigraphic sections at 17 locations (exposing a total of 720 m of stratigraphy) located in southern Brazil and northern Uruguay, from a larger dataset of 64 locations (comprising a total of 1070 m of stratigraphy). Four facies associations were identified: perennial fluvial channel fills, ephemeral fluvial channel fills, floodplain deposits, and aeolian deposits, indicating a dryland climate. Spatial trends were analyzed along a downstream-oriented transect (NNE–SSW) across the system. Grain size, channel-body thickness, number of stories, and bar thickness decrease downstream, indicating a reduction in channel depth, flow capacity, and channelization of the fluvial system, interpreted to be associated with downstream-increasing bifurcation, infiltration, and evapotranspiration. Based on spatial trends and distribution of facies associations, the deposits are interpreted to have been accumulated from a large DFS which can be divided into four zones, from proximal to distal: Zone 1, dominated by perennial fluvial channels; Zone 2, a mixture of perennial and ephemeral channels; Zones 3 and 4, deposits situated externally of the fluvial channel belts dominated by aeolian and floodplain deposits prevailing in each zone, respectively. The Guará Formation likely records the stratigraphic signature of the largest distributive fluvial systems reconstructed from both modern and ancient datasets, and one of the first where fluvio–aeolian interaction is quantified. The Guará Formation DFS model presented herein is key to understanding paleoenvironmental, paleoclimatic, and geotectonic changes related to Gondwanan fragmentation.

Declassified intelligence satellite imagery as a tool to reconstruct past landforms and surface processes: The submerged riverscape of the Tigris River below the Mosul Dam Lake, Iraq

AbstractLocated along the Tigris River in the Kurdistan Region of Iraq, the Mosul Dam Reservoir is the second biggest dam of the Near East and represents an important water storage for local human activities. The dam was built between 1981 and 1988 north of the village of Eski Mosul, submerging the course of the Tigris River for c. 100 km. The analysis of historical images derived from declassified Corona satellite imagery acquired between December 1967 and August 1968 reveals the pristine pattern of the Tigris River, including the seasonal changes of its riverbed, shifting across the hydrological year from meandering to anastomosing patterns. Geomorphological mapping based on Corona images allowed us to estimate the seasonal modification of fluvial elements such as the floodplain and point, middle and longitudinal bars. The comparison with Landsat data collected since the 1990s showed the first phases of the basin filling and the control on the present‐day aspect of the lake inherited from the setting of the Tigris channel belt and, more in general, the litho‐structural control over the evolution of the local hydrographic network; we also document the influence of the original Tigris River course on its recent insertion into the lake. Our work allowed reconstruction of the ancient fluvial landscape below the Mosul Dam Lake and the evolution of its riverscape controlled by litho‐structural factors and seasonal variations of the river discharge. Finally, this contribution highlights the relevance of declassified intelligence satellite imagery in interpreting natural geomorphic processes and landforms, today altered by human agency.

Channel trajectories control deep‐water stratigraphic architecture

AbstractInterpretation of deep‐water channel deposits is challenging because the spatial arrangement of their constituent lithologies is highly variable. This variability is often thought to be a signature of complex interactions between controlling boundary conditions and processes. A three‐dimensional forward stratigraphic model of a sinuous meandering channel is used to explore the production of channelised deep‐water stratigraphy. This model highlights three stages of stratigraphic evolution for channel belts: (1) an initial phase of rapid growth in mean belt width and variability in belt width driven by increasing channel sinuosity; (2) a subsequent phase of reduced belt‐width growth rate because of cutoff processes; and (3) a mature phase during which repeated bend lifecycles act to produce a statistically stable channel‐belt width. When a trajectory defining the vertical movement of a channel over time is added to the model, commonly recognised patterns of deep‐water channel‐belt stratigraphy are produced. These results demonstrate how forward stratigraphic models provide insights into processes governing the evolution of deep‐water stratigraphy that elude interpretations of static outcrops and seismic images of subsurface examples.

Hydrothermal Features of a Sandstone Geothermal Reservoir in the North Shandong Plain, China

Abstract There are 1,000 geothermal wells with depths of 1500–2000 m drilled into the upper Tertiary sandstone geothermal reservoir in the Guantao Formation in the North Shandong Plain, which support about 61 million square meters of domestic space heating. Concerning the large-scale exploitation of the Guantao Formation geothermal reservoir and its important role in clean space heating, it is essential to fully understand the characteristics of the geotemperature, geothermal water flow, and hydrochemical features, especially the heat accumulation and geothermal water enrichment mechanism. Based on the data from pumping tests, temperature logging, water level measurements, and hydrochemistry and isotope sampling and analysis, in this study, the heat sources and accumulation, water sources, and enrichment mechanism of the sandstone geothermal reservoir in the Guantao Formation were explored. The geothermal gradient of the cap rocks and the reservoir’s temperature revealed that the geothermal temperature fields are controlled by fault structures, and the relative high temperature zones are distributed in the area where the uplifts and deep heat-controlled faults converge. The geothermal water-rich area is located in the ancient river channel zone. The closer the water-rich area is to the ancient river channel belt, the deeper and coarser the buried conglomerate aquifer is, and the better the water-rich condition is. The best water-rich areas are found in the alluvial fan of the ancient channel belt.

Groundwater arsenic content in quaternary aquifers of the Red River delta, Vietnam, controlled by the hydrogeological processes

The relation between arsenic groundwater concentrations and hydrogeological processes was investigated in the proximal part of the Red River delta, Vietnam, west of the depression cone formed by the exploitation of groundwater in Hanoi. Flow paths in the Quaternary aquifers were modeled based on previously interpreted geological structure and hydrogeological data gathered during field work in 2014–2017. Sedimentary structures and simulated flow patterns were compared with the spatial distribution of the groundwater arsenic concentration.The regression of the sea in the area started 4 ka BP in the Holocene. The low tectonic subsidence rate of the Red River delta led to intensive erosion and replacement of fine grained sediments of the sea level high stand by sandy channel belts, resulting in hydraulic connections between the Pleistocene and Holocene aquifers. The Pleistocene aquifer is recharged by both regional flow paths and naturally occurring vertical recharge through Holocene sand and clay layers. Young groundwater (<40 a) in the shallow Holocene aquifer generally discharges to surface water bodies. The shallow flow system is also seasonally recharged with surface water, as indicated by δ18O enrichment of groundwater and oscillating groundwater ages in wells in the vicinity of water channels. The deeper flow system discharges into the Red River and Day River or flows parallel to the rivers, toward the sea.The overall pattern of arsenic groundwater concentrations (decreasing with increasing sediment age) is modified by groundwater flow. The arsenic contamination of the Pleistocene aquifer of the Red River delta is not only caused by the intensive groundwater abstraction in Hanoi, as reported previously, but also by the natural flow of high arsenic groundwater from Holocene to Pleistocene aquifers in areas located outside of the depression cone. Groundwater with < 50 µg L-1 arsenic is found in the Pleistocene aquifer close to the recharge zone in the mountains bordering the Red River delta and in the Holocene and Pleistocene aquifers where clay deposits were eroded. Close to the recent Red River channel, recharge of older Holocene and Pleistocene sediments occurs partially by arsenic-contaminated groundwater from the youngest Holocene aquifers, and here arsenic concentrations exceed 50 µg L-1. A high arsenic concentration is also present in the early Holocene-Pleistocene aquifer, beneath thick clay layers, indicating a limited extent of flushing and the inflow of fresh organic matter.

Variations in water discharge at different temporal scales in a mud-prone alluvial succession: The Paleocene-Eocene of the Tremp-Graus Basin, Spain

The late Paleocene to earliest Eocene sedimentary record in the eastern Tremp-Graus Basin (southern Pyrenees, Spain) consists of a mud-prone fluvial succession informally known as ‘Upper Red Garumnian’, which is dominantly composed of non-channelized deposits interspersed with fluvial channel bodies. The succession records evidence of variations in water discharge at multiple temporal scales. One-hundred-and-eighty sand-prone and conglomeratic bodies present within otherwise fine-grained deposits have been examined by facies and architectural-element analyses. These have been assigned to four distinctive types: (i) simple and (ii) compound channelized deposits, (iii) multilateral and multistorey elements, and (iv) sediment gravity-flow deposits. Simple and compound channel bodies represent deposits of channelized high-energy flows associated respectively with single floods or multiple events occurring in the same channel belt. Multilateral and multistorey elements are associated with sedimentation in longer-lived channel belts. An increase in the occurrence of these latter elements through the stratigraphy can be related to changes in tectonic and/or climatic drivers, heralding a change in facies architecture that signifies a shift to perennial discharge conditions and incised-valley backfilling. This is itself followed by a marked change in architectural style in a part of stratigraphy recording the Paleocene-Eocene Thermal Maximum (PETM), when variations in precipitation regime and/or intrabasinal characteristics (e.g., vegetation) related to this event favoured the lateral migration of channels, causing the development of laterally extensive coarse-grained bodies. Floodplain sediments comprise (i) heterolithic deposits accumulated in the vicinities of the fluvial channels where riparian ecosystems were developed, and (ii) overbank mudstones with variable degree of pedogenesis, especially associated with wetting and drying cycles in areas of the floodplain located distally away from fluvial channels. This succession records the importance of water-discharge variations in the evolution of alluvial systems: it displays facies arrangements recording the effects of highly variable discharge in ephemeral to perennial channels, as well as variations in stratigraphic architecture testifying to longer-term hydrological changes in the late Paleocene and earliest Eocene.

Conditions to Preserve the Sedimentary Record of Channel Planforms in Temperate Rivers of the Northern Hemisphere

AbstractWe aim to identify conditions that influence the preservation of a complete record of channel planforms in the topmost layer of floodplains, prior to the maintenance in the rock record. We have tested a hypothesis that a successive decrease of stream power and channel belt width are necessary to preserve the record of channel planforms in the topmost floodplain layer over 103 to 104‐year time scales. A literature review was conducted for rivers of the temperate zone of the Northern Hemisphere. Stream power, valley, and channel belt widths, paleodischarges, sediment grain‐size, and age of paleochannels were used to identify four groups of rivers with preservation potential ranging from tens of thousands years to annual time scales. The decrease in stream power followed by sustained low stream power, and successive decrease of channel belt width were identified in rivers preserving a 103 to 104‐year record of channel planforms. River valleys with the record of at least two generations of paleochannels, and valley width/channel belt width ratios between 6 and 12, potentially preserve fluvial records over 103 to 104‐year time scales. We analyzed unusually well‐preserved records of channel planforms from the Obra and Sió Rivers (central Europe). A determination of trends in changes of stream power and channel belt widths based on an extensive set of geophysical, geological data, and sediment dating from earlier studies, confirmed the tested hypothesis. The proposed framework can be extended by fluvial records preserved by large, and coastal rivers, with the potential to include ancient fluvial records.

Channel morphodynamics and sediment budget of the Lower Ganga River using a hydrogeomorphological approach

AbstractThe Ganga River is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga River is vulnerable to siltation because of: (a) the naturally low slope in the alluvial stretch; (b) the confluence of several highly sediment‐charged rivers such as the Ghaghra, Gandak, and Kosi; and (c) the reduction in non‐monsoon flows because of upstream abstractions of both surface and groundwater. Additionally, the Farakka barrage has impacted the morphology of the Ganga River significantly, both upstream and downstream of the barrage. Large‐scale siltation in several reaches has reduced the channel capacity, leading to catastrophic floods in this region even at low discharges. This work has utilized historical remote sensing data and UAV surveys to reconstruct channel morphodynamics and compute sediment volumes accumulated in the channel belt along the Lower Ganga River between Buxar and Farakka. The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar–Gandhighat (GW1, 160 km); (b) Gandhighat–Hathidah (GW2, 106 km); (c) Hathidah–Azmabad (GW3, 182 km); and (d) Azmabad–Farakka (GW4, 132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga during the last four to five decades. Sediment budgeting using planform maps provides estimates of ‘extractable’ volumes of sediment in GW1, GW2, GW3, and GW4 as 656 ± 48, 706 ± 52, 876 ± 71, and 200 ± 85 Mm3, respectively. These estimates are considerably lower than those computed from the hydrological approach using observed suspended sediment load data, which assumes uniform sedimentation between two stations. Further, our approach provides reach‐scale hotspots of aggradation and estimates of extractable sediment volumes, and this can be very useful for river managers to develop a strategic sediment management plan for the given stretch of the Ganga River.