Phases Of Incision Research Articles

Navigating the unexpected: Insights into bladder injuries during emergency cesarean deliveries

Bladder injury is a recognized complication of emergency cesarean sections, yet comprehensive studies elucidating its incidence and associated risk factors are limited. This retrospective study analyzed data from emergency cesarean deliveries performed over a three-year period at a tertiary care center. Cases with documented bladder injury were identified, and relevant clinical variables including patient demographics, surgical details, various risk factors, and obstetric history were collected and analyzed.Among the emergency cesarean cases reviewed (n= 12218), bladder injury was identified in 22 cases making a incidence rate of 0.18%. The majority of injuries occurred during the initial phase of uterine incision. Significant risk factors associated with bladder injury included a history of previous cesarean sections, presence of adherent placenta, and intraoperative adhesions. Distribution analysis revealed varying prevalence rates across different clinical variables.Bladder injury represents a notable complication of emergency cesarean deliveries, necessitating a comprehensive understanding of associated risk factors and preventive measures. This study contributes valuable insights into the epidemiology and clinical correlates of bladder trauma in urgent obstetric interventions, emphasizing the importance of tailored interventions to enhance patient safety and reduce maternal morbidity.

Holocene alluvial dynamics, soil erosion and settlement in the uplands of Macedonia (Greece): New geoarchaeological insights from Xerolakkos in Grevena

This paper addresses the interplay between Holocene landscape evolution and human settlement dynamics, drawing new evidence from the alluvial history of Xerolakkos, a continental stream in Grevena (Western Macedonia, Greece). We developed an integrated geoarchaeological survey combining remote sensing geomorphological mapping, litho-stratigraphic analysis and radiocarbon dating with the site evidence of a new archaeological survey. Results revealed four major alluviation phases, corresponding to 1) the beginning of the Holocene until the Early Neolithic (∼6300/6200 BCE), 2) the end of the Early and the Middle Neolithic (∼6000–5400 BCE), 3) from the Middle Bronze Age to the Late Roman period (∼1800 BCE – 500 CE), and 4) during the Byzantine and Ottoman eras (∼500–1800 CE), all separated by phases of floodplain incision. Furthermore, the effects of several Holocene Rapid Climatic Changes (RCC) are traced and discussed together with potential human responses; we also provide the first alluvial sequence recording the ∼6200 BCE (8.2 kyr BP) event in the Balkans. While the climate and the local geomorphological setting are considered the primary drivers behind instability and erosion during the Early and Middle Holocene, a landscape change starting in the Middle Bronze Age (after ∼1800 BCE) followed by a re-organisation of the rural economy in the Roman period suggests the increasing involvement of anthropogenic forcing which, by the Ottoman period, evolved into a dynamic situation between climatic variability and adaptive land management. Finally, we demonstrate how soil erosion in the upper catchment constitutes a serious taphonomic bias when studying the regional archaeological record.

Stacking pattern transition caused by submarine channel inception and internal levée development

ABSTRACTThe shift from highly amalgamated and laterally offset to aggradational and vertically stacked submarine channels is a pattern that has been widely recognized in seismic reflection datasets as well as in outcropping successions worldwide. However, the sedimentary and stratigraphic details of such an important part of channel evolution and its implications have not been discussed extensively. That gap is addressed here by characterizing a previously undocumented, seismic‐scale outcrop of the Tres Pasos Formation (Magallanes Basin, Chile) that records the transition from a laterally offset/low‐aggradational channel complex to a vertically stacked aggradational complex associated with the development of an internal levée that enhanced channel aggradation. The boundary between the two complexes is defined by a composite erosional surface (up to ca 35 m relief) with two adjacent element‐scale channelized incisions with contrasting sedimentary infills that provide unprecedented insight into this key phase of submarine channel evolution. Observations indicate that, in between these two depositional architecture styles, there is a significant phase of deep incision and bypass. The relief achieved via this deep incision of one or multiple simultaneously active conduits is interpreted to be necessary to set up the conditions for flow stripping and, subsequently, overbank deposition. Finally, the siltstone‐rich intra‐channel lithofacies association observed directly overlying part of the incision is interpreted to represent the stratigraphic expression of the earliest deposits of an active submarine channel preserved due to the abrupt abandonment of this pathway and the development of an adjacent conduit, which resulted in the former being covered with fine‐grained overbank deposits.

Fluvial Response to Environmental Change in Sub-Tropical Australia over the Past 220 Ka

This paper uses a 30 m record of valley alluviation in the Lockyer Creek, a major tributary of the mid-Brisbane River in Southeast Queensland, to document the timing and nature of Quaternary fluvial response. A combination of radiocarbon and optically stimulated luminescence dating reveals a sequence of major cut and fill episodes. The earliest aggradation phase is represented by a basal gravel unit, dating to ~220 ka (marine isotope sub-stage 7d), and although little evidence supports higher fluvial discharges during MIS 5, a MIS 3 fluvial episode characterised by incision and aggradation dates to ~60 ka. A penultimate phase of incision to a depth of 30 m prior to ~14 ka saw the lower Lockyer occupy its current position within the valley floor. The Lockyer Creek shows evidence of only minor fluvial activity during MIS 2, suggesting a drier LGM climate. The appearance of alternating fine- and coarse-grained units at about 2 ka is notable and may represent higher-energy flood conditions associated with a strengthening of El Niño Southern Oscillation activity as observed in the flood of 2011. The aggradation rate for this Holocene floodplain unit is ~11 times higher than the long-term rate.

Ongoing India–Asia convergence controlled differential growth of the eastern Tibetan Plateau

The eastern Tibetan Plateau is known for its unique topography and active tectonics, serves as a crucial region for investigating geodynamics and lithosphere–atmosphere interaction. Despite its importance, the formation process and geodynamics of this region remain controversial. We integrated new and reported thermochronological data, and paleo-crustal thickness reconstruction to explore the tectonic deformation, climate–tectonic coupling, and geodynamics of the area. New apatite fission track data, collected from the upstream areas of major rivers in the eastern Tibetan Plateau, revealed multiple rapid river incision phases during the Eocene and Neogene–Quaternary. Apatite (U-Th)/He data obtained from a borehole near the Gaoligong shear zone indicates that the Yingjiang fault experienced two rapid exhumation phases, which initiated at ∼22 Ma and ∼18 Ma, respectively. Thermo-kinematic modeling (Pecube), based on multiple thermochronometers, predicted three phases of thrusting along the Longmenshan fault, which were initiated at ∼34 Ma, ∼17 Ma and ∼14 Ma, respectively. During the late Eocene period, Sr/Y and (La/Yb)n ratios suggest that the eastern Qiangtang and Songpan–Ganzi terranes underwent considerable uplift, followed by outward uplift in the collisional front zone and the southeastern margin of the plateau during the late Oligocene. Paleo-crustal thickness reconstruction also indicate another ∼1 km surface uplift during the Neogene–Quaternary. Our findings, when combined with reported thermochronological data, suggest that during the Neogene–Quaternary, the collisional front zone and the eastern margin of the plateau underwent greater exhumation compared to the eastern Qiangtang and Songpan–Ganzi terranes. The temporally and spatially varied exhumation shaped the landscape of the eastern Tibetan Plateau, driven mainly by precipitation-related erosion and promoted by fold-thrust and strike-slip thrust. This was controlled primarily by eastward lithosphere/asthenosphere extrusion associated with ongoing India–Asia convergence.

Incised valleys on the Algarve inner shelf, northern Gulf of Cadiz margin: Stratigraphic architecture and controlling factors in a low fluvial supply setting

A network of cross-shelf paleovalleys has been recognized over the paleo-inner shelf off the Gilão-Almargem Estuary, a small fluvial drainage system that presently receives minor sediment supply in the eastern Algarve shelf, northern margin of the Gulf of Cadiz (SW Iberian Peninsula). This study is aimed at determining the driving controls that triggered substantially different paleohydrological conditions and sedimentary dynamics of ancient fluvial systems in this margin. We focus on evidences of secondary controls on valley genesis and evolution, superimposed to primary glacio-eustatic control such as antecedent geology, low fluvial supply and changing hydrodynamic regimes. The architecture and spatial distribution of these paleovalleys were interpreted based on a grid of seismic profiles with different resolutions. Likewise, a sediment core obtained in a distal position of the paleovalley system provided information about sedimentary processes during the most recent stage of valley infilling. The chronostratigraphic framework was constructed based on regional seismic horizons defined in previous studies and complemented with two AMS 14C dates obtained from bivalve shells.The inner shelf paleovalley system is composed of several incised valley features which exhibit a remarkable similar internal architecture. These inner valley features exhibit two major incision phases (from oldest to youngest; IP 2 and IP 1) that are thought to constitute a simple paleovalley system formed during the last glacial cycle. The origins of the incision are considered to be different. The older one is related to fluvial incision during the sea-level fall leading into the Last Glacial Maximum, whereas the recent one is interpreted as the result of tidal scour during the postglacial transgression. Their corresponding infillings are interpreted, respectively, as estuarine bay-fill deposits and estuary-mouth sands. Overlying the paleovalley infilling, a distinctive reflective unit is in agreement with the generation of coastal barriers and related depositional systems.The formation of the paleo-inner-shelf paleovalley system was strongly conditioned by antecedent geology, which strongly limited the generation of wide incised valleys and determined the amount of incision landward of a well-defined break of slope. Its postglacial infilling was mainly estuarine in nature, likely involving the development of a dendritic system, with numerous barriers interrupted by tidal inlets and channels in a mixed estuarine system with low fluvial supply.

Comment on “Luminescence chronology of the Sankosh river terraces in the Assam–Buthan foothills of the Himalayas: Implications to climate and tectonics” by Quaternary Geochronology 72, 101364

In a study of which the main objective was to assess the impact of climate change and tectonics on the formation of river terraces along the Sankosh River, the eastern foreland of the Himalayas, the authors obtained geochronological data using luminescence technique. Four strath river terraces (T4–T1) were distinguished within the valley bottom, and alluvial sediments from three terraces (T4, T2 and T1) were dated to the age range from 143 to 14 ka. The alluvial mantels of river terraces were then linked to the scheme of glacial-interglacial cycle. The paper, however, suffers from many inconsistent and missing information, and the assessment on the landscape evolution of the river valley is incomprehensive. For instance, the authors relied on feldspars and therefore the IRSL method was used, but “OSL ages” are discussed at the end. They state that (i) tectonics creates space for sediment accommodation and (ii) the luminescence ages flank deposition and incision phases. Despite the fact that the statements are questionable, the ways these happen are not elucidated by the authors. Due to the lack of geochronological data for T3 terrace, its formation is very enigmatic. It is even more mysterious in the light of the data for the other terraces, but the authors made no attempt to explain this riddle. However, that certain inability of reconstructing the history of T3 would shed a shadow on the robustness of the ages obtained.

Incision history of the Mekong River valley revealed by spatially differential exhumation

The evolution of the Mekong River valley offers a unique record of the rock uplift and climate history of southeastern Tibet. Faster erosion rates along the valley than those on the flanks caused valley incision and local incision rates may reflect a mix of driving mechanisms, migrating knickpoints linked to a change in base level, and large-scale surface uplift. The thermochronometric-constrained erosion rates in the 2–3 km deep river channel compared to rates outside of the channels provide unique constraints on the driving mechanisms of the river incision. In particular, the onset of river incision at various points along the Mekong River helps identify whether exhumation signals are the result of knickpoint migration that would generate a younger incision upstream, or the outgrowth of the plateau, which would produce younger incision downstream. We analyze new and published apatite (UTh)/He ages collected along two elevation transects in the upper Mekong River: downstream at Tu'Er and upstream at Deqin. The data and thermokinematic modeling predict that the exhumation rates of the valley near Tu'Er and Deqin increased more rapidly than those of adjacent high-elevation, low-relief surfaces at ~11 Ma and ~ 4 Ma, respectively. Since then, exhumation variations have caused an increase of >2 km in the depth of the valley, indicating that the major incision of the Mekong River valley began at ~11 Ma near Tu'Er and approximately 4 Ma near Deqin. The results indicate that a major phase of the Mekong River valley incision may have developed and progressed upstream since the Miocene.

Geomorphological evolution of the western piedmont of Cumbres Calchaquíes (Tucumán Province, NW Argentina)

ABSTRACT The aim of this paper is to present a detailed geomorphological map of the western piedmont of Cumbres Calchaquíes (Tucumán Province, NW Argentina) bounded by the Amaicha River to the south and the administrative border with Salta Province to the north. It contains information about geomorphological units, their relative ages, and morphogenetic processes. The main map shows the presence of seven aggradation stages separated by incision phases. The first three stages (St1, St2, and St3) were modeled on a thick accumulation located in the upper piedmont from the Late Pliocene and Early Pleistocene. Intermediate stages (St4 and St5) are alluvial fans and pediments with less thickness and gradient. They cover the Early Pleistocene (St4) and the Late Pleistocene-Middle Holocene (St5). Finally, stages St6 and St7 are from the Upper Holocene.

Morphosedimentary and geoarchaeological records during the last 1400 years in the Ebro depression (NE Spain) and their paleoenvironmental interpretation

Most studies on the geomorphological evolution of the Holocene from the Ebro depression (NE Spain) are focused on the period up to the Roman Epoch (218 BC–476 AD) while some references to medieval (476 AD–15th century) deposits are also occasionally dated. This paper focuses on the establishment of aggradation units on the valley bottoms and slopes of the Ebro depression after 1400 BP, their origin, comparison with other areas, and relationship with global paleoenvironmental changes. These units were produced after the incision phase that marked the end of the large Holocene accumulation (unit H1) around 1400 BP (at the end of the Late Roman Epoch). Morphosedimentary records enable us to establish three aggradation units during the last 1400 years: unit H2 ( ca. 1400–650 cal BP); unit H3 – with two H3A subunits ( ca. 650–500 cal BP) and H3B ( ca. 500–320 cal BP); and the H4 unit (after 320 cal BP). These units are organized following five types of aggradation/incision arrangements with differing complexities. There are also connections among these units and two slope stages in the region. Finally, the genetic relationships between these units and global paleoenvironmental changes are shown (LALIA, MCA, and LIA) and related to anthropic activity. This contribution is the first detailed and systematic approach to the study of morphosedimentary units and sedimentary arrangements during the Recent Holocene in the Mediterranean area.

The largest Quaternary inland eolian system in Brazil: Eolian landforms and activation/stabilization phases of the Xique-Xique dune field

Eolian systems are sensitive to vegetation and climate changes, but also to the sediment supplied by neighbor depositional systems. This sensitivity is manifested in the alternation between periods of sand dune mobility and stability. In the Xique-Xique eolian system, the largest Quaternary interior dune field in Brazil (∼8000 km2), the neighboring depositional system responsible for the sand supply is the São Francisco River. We interpreted periods of dune activity and stabilization by mapping eolian landforms of this system, describing depositional facies under them and dating by optically stimulated luminescence (OSL-SAR). The stabilized parabolic landforms characterized and mapped (all formed by SE to E winds) were simple megadunes (25.5 ± 4.4 to 5.8 ± 2.0 ka), compound dunes (15.8 ± 1.2 to 5.2 ± 1.4 ka) and perched dunes (14.0 ± 1.0 and 1.2 ± 0.1 ka). We also recognized eolian plains (14.0 ± 2.1 to 6.5 ± 0.7 ka) and active parabolic dunes. Eolian deposits of undifferentiated landforms have also been described and dated (253.8 ± 19.0 to 8.2 ± 1.3 ka). The ages obtained allowed identification of two main phases of dune activity (∼60 to 18 ka and ∼16 to 5 ka) separated by a short period of stabilization marked by a paleosol (∼18 ka). The main periods of eolian activity are associated with phases of fluvial aggradation and high sediment supply, whereas dune stabilization since 5 ka is correlated with low sediment supply during the Late Holocene fluvial incision phase. The periods of eolian activity and stabilization respond not only to climate changes in the area occupied by the eolian system but mainly to local fluvial sediment supply by the river, which in turn can respond to more regional climatic controls. The Xique-Xique eolian system results from the coupling of abundant sediment supply from the fluvial system, high eastern wind speeds, and a significant wind deceleration caused by mountains on the western border of the dune field.

Climate changes and the formation of fluvial terraces in central Amazonia inferred from landscape evolution modeling

Abstract. Climate changes have been considered an essential factor controlling the shaping of the recent alluvial landscapes in central Amazonia, with implications for explaining the biogeographic patterns in the region. This landscape is characterized by wide floodplains and various terrace levels at different elevations. A set of older terraces with ages between 50 and >200 ka occupy the higher portions of central Amazonia, whereas multiple terraces next to floodplains occur at lower elevations and display ages of a few thousand years. These lower terraces, referred to as middle–lower terraces, reveal what can be perceived as a stochastic pattern both in space and time. Despite the widespread occurrence of these geomorphic features, no process-oriented analysis has been conducted to explain their formation. Here, we develop a landscape evolution model referred to as SPASE (Sedimentary Processes and Alluvial Systems Evolution) to explicitly account for fluvial erosion and deposition in combination with lateral channel migration to explore the controls on terrace development. The model results show that the higher terraces were deposited under the condition of a higher base level for the basins upstream of the confluence between the Solimões and Negro rivers. The subsequent decrease in the base level initiated a phase of gradual incision, thereby resulting in the current fluvial configuration. The model also predicts that high-frequency climate changes resulted in the construction of middle–lower terraces at various elevations which, however, are all situated at lower elevation than the higher terrace levels. Our model shows that dry-to-wet shifts in climate, in relation to the modern situation, yield a landscape architecture where middle–lower terrace levels are better preserved than wet-to-dry changes in climate, again if the current situation is considered as reference. Finally, our results show that fast and widespread landscape changes possibly occurred in response to high-frequency climate changes in central Amazonia, at least since the Late Pleistocene, with great implications for the distribution and connectivity of different biotic environments in the region. Because of this short timescale of response to external perturbations, we suggest that the streams in central Amazonia possibly also respond in rapid and sensitive ways to human perturbations.

Early Pleistocene complex cut-and-fill sequences in the Alps

Cut-and-fill sequences are the result of climatically or tectonically induced alternating aggradation and incision phases of a fluvial system. A recently established cosmogenic nuclide chronology of the Cover Gravels (Deckenschotter in German) in the northern Alpine Foreland, which are the oldest Quaternary glaciofluvial gravels and comprise evidence of early Pleistocene glaciations, suggests a cut-and-fill build-up. This suggested cut-and-fill architecture challenges the morphostratigraphy. The Deckenschotter deposits represent a suitable archive for reconstructing drainage patterns, base level changes, and the landscape evolution of the northern Alpine Foreland during the early Pleistocene. In this study, we focused on the highest morphostratigraphic Deckenschotter sites: three at Irchel and one in the area around Lake Constance. Sediment analyses were performed to determine their provenance and depositional environments. The geochronology was established using isochron-burial dating. The results indicate that the sediments were transported from the Central and eastern Central Alps, as well as from the Molasse, to the foreland and deposited in a proximal glaciofluvial environment. Based on these findings, we propose that the Deckenschotter are cut-and-fill sequences that accumulated in three stages during the early Pleistocene at ca. 2.5 Ma, ca. 1.5 Ma, and ca. 1 Ma. The presence of a cut-and-fill system implies that the regional base level was relatively constant during the early Pleistocene. In addition, the ca. 2.5 Ma glaciofluvial gravels document the first evidence of glaciers in the northern Alpine Foreland. This timing is synchronous with the onset of Quaternary glaciation in the northern hemisphere at ca. 2.7 Ma.

Geomorphic imprints of lithospheric flexure in central Australia

The Finke River in central Australia is counted among the world's oldest drainage systems; this raises the prospect of investigating how (tectonic and sub-lithospheric) processes have shaped the landscape in the long term. The Finke's upper reaches display an enigmatic set of intertwined active and relict bedrock gorges that suggest a complex history of incision, aggradation and re-incision. We measured cosmogenic 10Be and 26Al in fluvial gravels preserved in the gorges, and we applied a Markov chain-Monte Carlo-based inversion model to test two limiting-case hypotheses about the timing of gravel deposition and exhumation. Our results suggest that the nuclide memory contained within the gravels was essentially erased during sediment storage exceeding several million years. Previous studies attribute central Australia's geomorphic development to intensified post-Miocene aridity in tune with the perception of a quiescent tectonic regime. However, the close correlation between drainage patterns and gravity anomalies leads us to propose, instead, that cyclical in-plane loading in the presence of extreme lithospheric density anomalies caused to-and-fro tilting of the upper Finke. We show that relief variations of order 100 m accompany mild in-plane loading of ∼1–2 x 1012 N m−1, and we speculate that loading variations are likely the result of tractions imposed at the base of the plate rather than at plate boundaries. Acting together, these tectonic and sub-lithospheric processes triggered the phases of incision and aggradation that created the Finke's unusual intertwined bedrock gorges over a multi-million-year timescale.

Climate-tectonic imprints on the Late Quaternary Ravi River Valley Terraces of the Chamba region in the NW Himalaya

• The present manuscript provides pristine OSL ages of the Ravi River Valley terraces, a tributary of the Indus River. • Aggradation events were mainly induced by climate, whereas the incisions phases were governed by both tectonic and climate. • Synchronous incision phases in Hinterland (Ravi River) and Indus plain suggest akin Late Quaternary climatic conditions. • Unlike Ganga River system, Ravi River system does not show any time lag for incision between the hinterland and the plain. • The first report on reactivation age (46 ka) of the MBT and the Chamba Thrust in NW Himalaya. An attempt has been made to reconstruct the landform evolution in monsoon-dominated the Ravi River Valley (a major tributary of the Indus River), Chamba Nappe, Lesser Himalaya based on detailed field mapping, supported by the Optically Stimulated Luminescence (OSL) dates. In the upper Ravi River Valley sediments are generated by glaciation-paraglaciation processes and accumulated in terraces and alluvial fan deposits in lower reaches north of the Main Central Thrust during deglaciation and periods of intensified monsoonal phases. Field study and OSL dating demonstrate aggradation of river terraces and debris-flow terraces occurred in two major phases: 73–46 ka and 39–23 ka and at least six minor phases between <23 ka and 7 ka. The incision was initiated shortly after 7 ka and ascribed to the decrease in overall monsoon intensity and simultaneous reduction in sediment flux. The first phase (73–46 ka) of aggradation was regional and occurred in a pre-existing river valley, whereas the later phases (39 ka onward) occurred as cut-and-fill and degraded terraces. The incision of the first major phase (220 m thick) accelerated between ∼46 ka and 39 ka due to reactivation of the Chamba Thrust and the Main Boundary Thrust (MBT). The study suggests that the fluvial dynamics and aggradation phases in the Ravi River Valley were regulated by monsoon variability, whereas both tectonic activity and climatic variables governed the incision of these deposits. The contemporaneous aggradation and incision in the hinterland and the Indo-Gangetic plains suggest that the Late Quaternary climate variability was the main factor in the evolution of fluvial landforms.

Pleistocene Megaflood Discharge in Grand Coulee, Channeled Scabland, USA

AbstractBedrock erosion and canyon formation during extreme floods have dramatically altered landscapes on Earth and Mars. Grand Coulee was carved by outburst floods from Pleistocene glacial Lake Missoula and is the largest canyon in the Channeled Scabland, a megaflood‐scoured landscape in the northwestern USA. Quantifying paleo‐discharge is required to understand how landscapes evolve in response to extreme events, but there are few constraints on the magnitude of the floods that incised Grand Coulee; hence, we used hydraulic modeling and geologic evidence to quantify paleo‐flood discharges during different phases of canyon incision. When upper Grand Coulee was incising by headward waterfall retreat, the paleo‐discharge was 2.6 × 106 m3 s−1, which produced shear stresses great enough to cause the waterfall to retreat via toppling of basalt columns. The largest possible flood through upper Grand Coulee, a Missoula flood which raised glacial Lake Columbia to a stage of 750 m, produced a modeled discharge of 7.6 × 106 m3 s−1. The discharges associated with waterfall retreat and drainage of glacial Lake Columbia are &gt;80% and ∼50% lower, respectively, than the 14–17 × 106 m3 s−1 discharge predicted by assuming the present‐day topography was inundated to the elevation of high‐water marks. Due to bedrock incision, high‐water marks may overestimate paleo‐flow depth in canyons carved by floods, hence bedrock erosion should be considered when estimating paleo‐discharge in flood‐carved canyons. Our results indicate that outburst floods with discharges and flow depths much lower than those required to inundate high‐water marks are capable of carving deep canyons.

Geomorphic characterization of a seasonal river network in semi-arid western India using the River Styles Framework

The imprint of geologic, climatic and anthropogenic memory as controls on geomorphic river diversity is assessed for twelve River Styles in the Sabarmati Catchment. Geologic controls are the primary determinant of river character and behavior in the hinterland and pediment landscape units, where headwater streams transition to partly confined valleys and gorges with distinct structural lineations in a rectangular drainage network. Climate memory imprints entrenched rivers that make up alluvial fan and alluvial plain landscape units of the mid-lower catchment, shaped by phases of sediment aggradation and subsequent incision in response to enhanced monsoonal rains around 10 ka. Terraces constrain channels within confined and partly confined valleys with occasional and discontinuous floodplains respectively. Laterally unconfined channels are only found in the lower parts of the catchment, immediately upstream of the estuary/delta. Limited space for adjustment, impacts of flow regulation and ephemeral conditions restrict the range of contemporary river morphodynamics, but ridges and swales and abandoned channels on floodplains indicate more dynamic conditions in the past. Impacts of anthropogenic memory are most pronounced in the stopbank-controlled, barrage- and dam-impacted reaches, especially in the cities of Gandhinagar and Ahmedabad. An explanation of controls upon geomorphic river diversity, including an assessment of the role of stream power, presents a coherent platform to develop geomorphologically-informed approaches to river management.

Incision and aggradation phases of the Amazon River in central-eastern Amazonia during the late Neogene and Quaternary

The development of the Amazon River as a transcontinental drainage has been narrated in terms of the sedimentary record in western Amazonia and in marine offshore areas. However, little is known about the landscape changes in central-eastern Amazonia, which is considered a former water divide prior to the establishment of the Amazon as a transcontinental drainage. In this study, we carried out geomorphological and sedimentary facies analyses of fluvial deposits associated with the Amazon River in central-eastern Amazonia. Additionally, extended range luminescence ages and sediment provenance data are used to help to improve the chronostratigraphic framework of the study area. Erosive scarps bound the Amazon River valley in the study area, where higher elevation upland terrains (locally called ‘terra firme’) are sustained by older and resistant bauxitic-lateritic paleosols that cap the Cretaceous sandstone bedrock. Two abandoned fluvial terraces (T3 and T2), representing lower elevation terra firme, and the active floodplain (T1), collectively record the incision and aggradation phases of the Amazon River in a NE-SW oriented valley since the Late Miocene. The T3 is intensely dissected and has weathered deposits with surface elevations between 30 and 50 m above sea level (a.s.l.). These deposits are composed of gravel with boulders, coarse sand, and locally grey mud rich in organic matter. Quartz thermally-transferred optically stimulated luminescence (TT-OSL) dating allows us to determine the oldest Quaternary ages reported for fluvial terraces associated with the Amazon River. Depositional ages for the T3 deposits range from 944 ka to 2.3 Ma. The T2 deposits form a flat area with surface elevations between 25 and 30 m a.s.l. These deposits comprise fine to medium sand interbedded with mud and locally coarse sand and mud alternations forming inclined heterolithic stratification. Feldspar post-infrared infrared (pIRIR290) luminescence ages indicate an incision and aggradation phase leading to deposition of the T2 from 557 to 140 ka. The T1 represents the active floodplain with a surface elevation lower than 25 m a.s.l. These deposits are composed of moderately to well-sorted silty fine sand and greyish mud. OSL sensitivity measurements suggest that deposits from all terraces are formed by a mixture of Andean and shield-cratonic sediments, which are similar to the sediments transported by the modern Amazon River. The fractures in the Paleozoic to Cretaceous bedrock that comprises the higher elevation terrain incised by the Amazon River in central-eastern Amazonia lead to differential fluvial erosion, shaping the current NE-SW and NW-SE valley orientation. The T3 and T2 terraces record the progressive incision of the Amazon River valley since the Late Miocene, leading to poor preservation of fluvial deposits in comparison with western Amazonia. Here in central-eastern Amazonia, fluvial activity has been locked into a narrow fluvial valley in comparison with a wide area in western Amazonia. The stratigraphic scenario with a limited accommodation space favoured the recycling of terraces within the Amazon River valley. The restricted distribution of deposits associated with the Amazon River in central-eastern Amazonia is attributed to the uplift behaviour and high erosional resistance of the Paleozoic and Mesozoic bedrock forming a high elevation plateau. During the Pleistocene, the T3, T2 and T1 deposits record at least three main phases of incision, which promoted the abandonment of floodplains and formation of new lowland terra firme areas.

Tributary fans of the Middle Teesta Basin in Sikkim‐Darjeeling Himalaya, NE India: Their contribution to valley‐filling processes

Tributary fan deposits in the Sikkim Himalaya are well preserved in the Teesta Valley in the tectonic zone bounded by the Main Central Thrust and the Main Boundary Thrust. The lithofacies characteristics and assemblage patterns of these deposits bear imprints of the linked effects of tectonic and climatic factors on the sedimentation processes in the river valley. Two tributary streams, namely Turung Khola and Bembung Khola have been studied in detail. Three major fanlobes are preserved at Turung Khola and two fanlobes are preserved at the Bembung Khola. Terraces, floodplains, channel bars, and chute bars are associated with geomorphic features in this part of the Teesta Basin. Longitudinal profiles of the tributaries and that of the trunk river indicate a relationship between the knick points and high SL‐Index values, where fault/thrust intersections are present. Sediment characteristics of these fan deposits indicate that four types of flows viz. debris flows (DF), hyperconcentrated flows (HF), sheet flows (SF), and channel flows (CF) deposited these sequences. The CF deposits are dominant in the fan sequence of the Turung Khola followed by SF deposits, HF deposits, and debris flow deposits respectively. HF deposits are dominant in the F1 fanlobe sequence, whereas the active channel fanlobe is dominated by the CF deposits. The rest of the active channel sequence is composed of SF deposits. On the other hand, the F1 fanlobe of Bembung Khola is made up of DF deposits and F0 fanlobe is composed of channel flow deposits and flood sediment. Interactions between the tributary fans and the main Teesta River are studied using lithofacies and the associated morphological context. From the above analysis, an evolutionary model for depositional and incision processes at the tributary stream fan confluence allows the reconstruction of five phases of incision and aggradation through the last ~45 ka.

Middle to late Holocene alluvial history of the northeast monsoon dominated coastal tropical rivers of south India

The region around the coastal Chennai area, India, is drained by three major rivers that rise from the remnants of the Eastern Ghats and receive dominantly the northeast rains. These rivers meander short distances, deposit sediments into the Bay of Bengal and preserve signatures of past phases of sedimentation and incision. In this study, detailed mapping using satellite imageries and extensive field work, logging of the river cliff bank sections was supported by sediment texture and structure and optically stimulated luminescence (OSL) dates. The main objective of this study was to infer the major periods of aggradation and incision in coastal drainage basins receiving dominantly the northeast rains. Integration of all the earlier available dated alluvial sequences (14C ages) and the present study OSL dates reveal that a major period of sedimentation occurred during the early Holocene (9 to 8.3 ka), mid Holocene (7.6–5.5 ka) and late Holocene to recent (2.4 and 1.6 ka, 0.9–0.6 ka and around 0.2 ka) periods. The Koratallaiyar river has preserved older middle Holocene terraces. Younger ages of the south easterly and east flowing rivers such as the Palar and the Coovum signify that they have incised and eroded to the present riverbed levels by catastrophic floods during the late Holocene period. This has resulted in an imbalance between sediment supply and sediment transporting power thus laterally stacking sediments rather than vertically.