Tectonic controls on the morphodynamics of the Brahmaputra River system in the upper Assam valley, India (original) (raw)
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Morphodynamics in the Upper Assam Part of the Brahmaputra River- A Planform GIS Based Study
Three big rivers of the eastern Himalayas namely, the Lohit, the Dibang, and the Siang confluence in the extreme NE part of India to form the Brahmaputra River. A 240 km long stretch of the Brahmaputra River extending from its 1915 confluence point at Kobo to the end part of the old Majuli Island in the downstream direction was divided into three units. The basis of the morphological divisions was the presence of exceptionally big river islands termed locally as the ‘Majuli’ the literary meaning of which is the land locked between two rivers. ‘Majuli’ Islands differ from the other sandbars in the sense that the later features develop directly as the consequence of the sediment load redistribution whereas the former features representing much older floodplains are due to the sudden diversions like anabranching or avulsion bringing thereby older floodplains inside a braided regime. Formation of Majuli-like landforms is thus a part of river dynamics that might be related either purely to the variability in the sediment dispersal pattern or neotectonic influences and of course there might be interplay of both. Some of the major questions dealt with in this paper are: What is the exact nature of temporal variability of the bankline shift? Is there any relationship between the temporal variability of bankline shifts and changes in channel belt width? Is there any correlation among the intra geomorphic parameters at different locations of the river? To find out proper explanations for these questions, the parameters measured were respectively, bankline shifts, channelbelt width, channelbelt area (CHB), Braid bar area (BB), the Interfluve area (IF) and the ratio BB/CH and changes in BB/CH (from 1915-1975 and 1975-2005) .Measurements were done for 37 numbers of 2nd order reaches covering three units mentioned above. It was observed that during 1915-2005, the Brahmaputra River banks made overall positive excursions causing thereby continuous widening of the river. The variability in the widths of the channel belt seems to be controlled at some of the reaches by the south bankline shift and in the other reaches by the north bankline shift. Both unit 1 and unit 3 represent major sites of aggradation. However, lately, the site of aggradation seems to be switching over from unit 1 to unit 3.
Braided Rivers, 2006
Chapter 1 Introduction 1 1.1 Setting the scene 1 1.2 Rationale of the Research 8 1.3 Working Hypothesis 1.4 Assam earthquake 11 1.5 Objectives and scope of work 13 1.6 Approach 16 1.7 Structure of the thesis Chapter 2 Evolution and Geomorphologic Development of the Jamuna, Padma and Lower Meghna Rivers 23 2.1 Introduction to the Jamuna, Padma and Lower Meghna Rivers 2.2 Geological setting 2.3 Tectonics and their influence on fluvial processes in the Bengal basin 2.4 Recent Evolution of the Bengal Basin and its river system 2.5 Avulsion of the Brahmaputra River to form the Jamuna River 2.6 Morphological development of the Jamuna River .40 2.7 Morphological development of the Padma River 2.8 Morphological development of the Lower Meghna River Chapter 3 River response 75 3.1 Introduction .., 3.2 Timescale and variables 3.3 Role ofvariables 3.3.1 Discharge ' 3.3.2 Size and input of bed material 3.3.3 Characteristics of the bank material 3.3.4 Valley slope 3.4 Natural and anthropogenic causes of channel change 90 3.4.1 Tectonics 90 3.4.2 Seismic events 93 3.4.3 Anthropogenic interventions 95 3.5 Models of channel response to disturbance 99 3.5.1 Lane (1955) 99 3.5.2 Schumm (1969} 3.5.3 Bettess and White (1983) 3.5.4 Simon (1989) 3.5.5 Jiongxin (1996) 3.5.6 A scheme to resolve disagreements between the models ll1 Chapter 4 Data Used in this Study 115 4.1 Introduction 4.2 Satellite Images 4.2.1 Types of satellite images used 4.2.2 Band Selection 123 4.3 Corona photographs and map of 1953 124 4.4 Cross-sectional survey 125 4.5 Water Level 128 4.6 Discharge 130 4.7 Bed material samples 132 4.8 Sediment concentration Chapter 5 Methodology 137 5.1 Introduction
Response of active tectonics on the alluvial Baghmati River, Himalayan foreland basin, eastern India
Geomorphology, 2005
Active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope. The Baghmati, an anabranching, foothills-fed river system, draining the plains of north Bihar in eastern India has responded to ongoing tectonic deformation in the basin. The relatively flat alluvial plains are traversed by several active subsurface faults, which divide the area in four tectonic blocks. Each tectonic block is characterized by association of fluvial anomalies viz. compressed meanders, knick point in longitudinal profiles, channel incision, anomalous sinuosity variations, sudden change in river flow direction, river flow against the local gradient and distribution of overbank flooding, lakes, and waterlogged area. Such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through DEM and field observations to understand the nature of vertical movements in the area. The sub-surface faults in the Baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes. D
Structural controls on topography and river morphodynamics in Upper Assam Valley, India
Geodinamica Acta
The basement in Upper Assam foreland basin exhibits a typical flexure pattern. An elongated continuous subsurface basement high has developed between Himalaya and Naga-Patkai ranges. A few prominent basement lows have developed adjacent to the foothill regions. It is noticed that the general topographic slope near the foothill regions is not only controlled by aggradation process, but also influenced by the flexured basement of the region. The basement lows have influenced the overlying topography significantly. Rivers flowing over those regions have shown unidirectional lateral migration. North bank tributaries like Subansiri, Jiadhal and Dikrang have been affected by the Subansiri basement low. Coseismic subsidence of sediments over Subansiri basement low had resulted subsidence of the North Lakhimpur-Ranga Nadi region in 1950 Assam earthquake. Some south bank tributaries like Disang and Dikhow have been affected by Nazira basement low. Topographic elevation along the Subansiri river is lower than that part of the Brahmaputra located south of Majuli. This typical topographic setting of the region makes the mouth of the Subansiri river and Majuli region highly susceptible to erosion.
Issues Concerning Basin Evolution & Fluvial Dynamics Associated with the Brahmaputra Valley, Assam
The Brahmaputra River is a great conduit through which a huge mass transfer of sediments takes place. The source is broadly the active zone of continent to continent collision between the Indian and the Eurasian plate. And, the sink is the Bay of Bengal where the sedimentary architecture of the great Bengal fan is going on since late Eocene that is, not less than the last 35 million years. The catchment responsible for supplying vast quantities of sediment includes erosion of actively uplifting mountains of the Himalayas, slope erosion of the Himalayan foothills and movement of alluvial deposits stored in the Assam valley. The present day Brahmaputra valley, a NE-SW trending intermountain alluvial relief was earlier a part of the Assam-Arakan basin and to be more precise, it constituted mainly the shelf part of the basin. Interestingly, basins undergoing active tectonic adjustments are not considered suitable for hydrocarbon prospects- however, the Brahmaputra valley, in spite of be...
Geological Society of America Bulletin, 2016
Tectonic displacement of drainage divides and the consequent deformation of river networks during crustal shortening have been proposed for a number of mountain ranges, but never tested. In order to preserve crustal strain in surface topography, surface displacements across thrust faults must be retained without being recovered by consequent erosion. Quantification of these competing processes and the implications for catchment topography have not previously been demonstrated. Here, we use structural mapping combined with dating of terrace sediments to measure Quaternary shortening across the Indus River valley in Ladakh, NW Himalaya. We demonstrate ~0.21 m k.y.-1 of horizontal displacement since ca. 45 ka on the Stok thrust in Ladakh, which defines the southwestern margin of the Indus Valley catchment and is the major back thrust to the Tethyan Himalaya in this region. We use normalized river channel gradients of the tributaries that drain into the Indus River to show that the lateral continuation of the Stok thrust was active for at least 70 km along strike. Shortening rates combined with fault geometries yield vertical displacement rates that are compared to time-equivalent erosion rates in the hanging wall derived from published detrital 10 Be analyses. The results demonstrate that vertical displacement rates across the Stok thrust were approximately twice that of the time-equivalent erosion rates, implying a net horizontal displacement of the surface topography, and hence narrowing of the Indus Valley at ~0.1 m k.y.-1. A fill terrace records debris-flow emplacement linked to thrust activity, resulting in damming of the valley and extensive lake development. Conglomerates beneath some of the modern alluvial fans indicate a northeastward shift of the Indus River channel since ca. 45 ka to its present course against the oppo site side of the valley from the Stok thrust. The integration of structural, topographic, erosional, and sedimentological data provides the first demonstration of the tectonic convergence of drainage divides in a mountain range and yields a model of the surface processes involved.
Chaliyar River, a west-flowing river, originates at about 2300 m elevation in the Western Ghat hill ranges in the southern part of India. We have studied geomorphic aspects of this river by examining longitudinal profiles and drainage pattern in order to understand the rock uplift and river incision. Chaliyar River and its tributaries display uneven longitudinal profiles with numerous knickpoints along the profiles. River concavity and river morphology were analysed to better understand the influence of tectonics and rock uplift on the fluvial and topographic system in the Chaliyar River basin. Wide variability in the concavity index of the tributaries of the Chaliyar River reflects the role of tectonism in carving the present river profiles. Steepness and concavity indices computed for the longitudinal profiles suggest that the rate of uplift is exceeding the rate of incision and are independent of lithology. River incision is not uniform in the Chaliyar River. The streams become graded (absence of knickpoints) towards the river mouth, suggesting that the uplift and incision are in equilibrium.
Geomorphic evolution of a non-glaciated river catchment in Lesser Himalaya: Response to tectonics
Quaternary International, 2017
The study discusses detail valley formation and sedimentation processes in the monsoon dominated non-glaciated catchment of the Ramganga river in the Lesser Himalaya. The geomorphic and sedimentological studies in this basin indicates phases of massive aggradation that was controlled mainly by channel bound processes and debris flows/landslides. The luminescence chronology of the fill sequences suggests that the valley filling occurred mainly in response to the enhanced monsoon after the Last Glacial Maxima (LGM), during Medieval Warm Period (MWP) and Little Ice Age (LIA). This phase is common in both glaciated and the non-glaciated catchments of Himalaya. The Ramganga River that flows through various tectonic structures of the Lesser Himalaya shows development of wide valleys with thick fill deposits in the fault zones. Chaukhutiya Fault (CF) and Binau-Bhikiyasain-Naurar Fault (BBNF) are the two main transverse faults where the evolved geomorphology pertains to their tectonic activity. The computed morphometric variables such as Ratio of valley floor width to valley height (Vf) and Stream Gradient Index (SL) show higher values in the transverse fault zones. Basin asymmetry vectors along the South Almora Thrust and BBNF are characterized by preferred stream migration in NE and SW direction suggesting BBNF with dip slip movement. Thick clay deposits at different sites along the Ramganga River resulting from blocking of the river, particularly along the BBNF, also point towards tectonically induced landslide and channel blockage. Later phase of tectonic activity, bracketed between 27 and 24 ka, is evident from deformed fluvial deposits in the form of folds and faults. Evidences of tectonic activity in the form of soft sediment deformation structures (SSDS) generic to seismic activity in layers comprising alternation of clay and sand are observed in the Himalayan Frontal Thrust (HFT) zone. The diagnostic features such as dykes, faults and folds suggests that the shaking event took place between 38 ka and 30 ka.
Drainage Basin Dynamics: An Introduction to Morphology, Landscape and Modelling, 2022
The Bengal Basin is one of the largest peripheral collisional foreland basins at the juncture of three converging lithospheric plates (Indian Plate, Eurasian Plate and Burma Plate), carrying signatures of regional scale sesimogenic faults, which is conducive for the frequent and recurrent earthquakes. The recent earthquakes occurred with a magnitude range (M w) of 3.6-5.7 in the western shelf zone of the Bengal Basin and the seismologists have predicted an event of potential earthquake of M w 8.2-9.0 due to locked mega thrust. Such activeness of seismic events reflects the vulnerability of densely populated towns and cities located in the Ganga-Brahmaputra-Megna Delta. The main point of research interest is the anomalous fluvial responses to active tectonics in the shelf zone of the Bengal Basin to explore the relative perkiness of regional tectonic uplift or subsidence. Since Palaeogene time, the peninsular river system (viz., Brahmani, Dwarka, Mayurakshi, Ajay, Damodar, Dwarkeswar, Silai and Kasai river basins) was directly influenced by the underlying structure and en echelon faults, and several landforms, channel morphology and morphostratigraphical units were distorted and deformed due to seismic shocks. The present study tries to document and understand the significant tectonic elements, geomorphometric anomalies and soft-sediment deformation structures on the alluvial river valleys and Quaternary floodplains using seismic information, proxy data, geomorphic indices of active tectonics, thematic mapping and stratigraphic analysis of depositional facies.
Journal of Hydroinformatics
Environmental catastrophes on a global scale have prompted a thorough evaluation of river morphology for sustainable basin development methods. Geomorphological investigations of river basins can provide significant information regarding quaternary tectonic deformations. The present investigation intends to reveal tectonic imprints in the Bearma River Basin (BRB). Bearma is a significant river in central India which flows through Vindhyan Supergroup, Lameta and Deccan Trap and contributes to developing the marginal Gangetic plain's architecture. The digital elevation data have been utilized to obtain the morphotectonic indices, tectonic activity classes and topographic characteristics. Bearma is an elongated basin with uplifted topography, continuously migrating channels, high hypsometric integral, and several stream length-gradient anomalies, indicating tectonic control over the basin. According to the tectonic activity index, 15.33, 38.99, and 46.55% areas of the BRB have high...